CN120466631A - Boiling heat transfer flow passage of sectional variable-section steam generator - Google Patents

Abstract

The invention relates to the technical field of steam generators and discloses a boiling heat transfer flow passage of a sectional variable-section steam generator, wherein the heat transfer flow passage comprises a primary side flow passage and a secondary side flow passage, and heat exchange is performed through a shared partition wall. The core of the method is that the secondary side flow is divided into at least two functional sections along the flowing direction of working medium, such as a single-phase heat transfer section, a saturated boiling section and a drying section, the geometric dimension of the flow section and/or the structure of the inner wall surface of each functional section are specially designed according to the main flow pattern and the heat transfer mechanism of the section, the single-phase section can adopt a smaller section to improve the flow velocity to strengthen the convection, the inner wall surface of the saturated boiling section can be derivatized into an arc shape, a wave shape, a zigzag shape or provided with fins and the like to strengthen the boiling, the drying section can adopt a larger section and is provided with rotary fins to delay drying and strengthen the heat transfer, and the different sections are smoothly transited through continuously-changing curved surfaces. The invention obviously improves the compactness and the comprehensive performance of the steam generator.

Description

Boiling heat transfer flow passage of sectional variable-section steam generator

Technical Field

The invention relates to the technical field of steam generators, in particular to a boiling heat transfer runner of a sectional variable-section steam generator.

Background

Steam generators are key devices in energy conversion systems, particularly playing a central role in nuclear power, thermal power generation and various industrial processes, and their main function is to heat and convert secondary side working fluids (usually water) into high-temperature and high-pressure steam through heat exchange. In the heat transfer tube of the steam generator, the secondary side working medium undergoes a complex boiling heat transfer process, during which the fluid form and the heat transfer mechanism can change significantly along the flow path, gradually transition from the single-phase liquid state of the inlet to bubble flow, bullet flow and annular flow, until a possible dry phenomenon occurs.

However, the current widely used boiling heat transfer technology for steam generators, the heat transfer flow path design of which tends to employ a homogeneous structure with constant or small variations in geometry and dimensions throughout its length. This conventional design has inherent disadvantages in accommodating the strongly varying physical characteristics of boiling heat transfer along the path. The failure to refine structural optimization for specific heat transfer mechanisms and flow pattern characteristics of different boiling stages (such as single-phase preheating, nucleate boiling, forced convection evaporation, near drying, etc.) results in a heat transfer tube with less than optimal overall heat transfer performance and possibly with heat transfer bottlenecks or inefficiencies in certain sections. In addition, this lack of targeted design also makes it more difficult to effectively suppress or retard flow instabilities and heat transfer degradation (e.g., premature drying), which in turn may impact the operational safety and economy of the steam generator. Although various enhanced heat transfer techniques have been proposed, they have focused mostly on the general enhancement of the overall flow path, and it is difficult to provide a customized solution to the staged nature of the boiling process, and therefore their performance enhancing potential is limited or accompanied by a significant increase in flow resistance. Meanwhile, the traditional manufacturing process limits the realization of a complex internal flow channel structure, and further prevents deep excavation of boiling heat transfer performance.

Therefore, a novel heat transfer flow channel structure which can adapt to the characteristic change of the whole boiling heat transfer process and realize the efficient collaborative heat exchange of each stage is developed, and has important theoretical significance and engineering application value for improving the overall performance of the steam generator and realizing the compactness and the light weight of the device.

Disclosure of Invention

Aiming at the problems of poor adaptability to flow patterns at different stages of boiling heat transfer, large equipment volume and weight, inflexible heat transfer area distribution and the like of the heat transfer element of the existing nuclear energy steam generator, the invention aims to provide a novel boiling heat transfer flow passage of the steam generator and application thereof, so as to realize fine regulation and control on the boiling heat transfer process through innovative design on the structure, thereby improving the heat transfer efficiency and being beneficial to miniaturization and light weight of the steam generator.

In order to achieve the above purpose, the invention is realized by the following technical scheme:

The first aspect of the invention provides a sectional variable cross-section steam generator boiling heat transfer flow path designed for efficient heat exchange between a primary side high temperature working medium and a secondary side boiling working medium. The core structure of the runner is characterized in that:

The device comprises at least one primary side runner for conducting a primary side high-temperature working medium and at least one secondary side runner for conducting a secondary side boiling working medium. The primary and secondary side flow channels are thermally and physically separated by a common partition wall.

In order to achieve a compact arrangement of the flow-path bundles, the secondary side flow has a non-circular geometric basic cross-sectional shape, such as, but not limited to, a quadrilateral or hexagonal shape. The cross-sectional shape of the tube is more effective than conventional circular tubes, and reduces ineffective gaps between the flow passages.

The secondary side flow is critically divided into at least two functional sections along its length, i.e. the flow direction of the secondary side working fluid. The division is set based on different heat transfer stages (such as single-phase heating, nucleate boiling, transition to film boiling, etc.) of the secondary side working medium in the boiling process and corresponding characteristic flow patterns (such as bubble flow, bullet flow, annular flow, dry flow, etc.).

In order to purposefully optimize the heat exchange performance of each heat transfer stage, at least one of the geometric dimension of the flow cross section, the surface morphology feature of the inner wall surface, or how the flow adjusting or heat transfer strengthening member is arranged in the secondary side flow channel in the at least two different functional sections is custom designed according to the specific flow pattern and heat transfer strengthening mechanism of the boiling heat transfer stage corresponding to each section, so that the different sections show significant differences in the structural features. The differential design ensures that each section of flow channel can better adapt to the flow and heat transfer conditions in the flow channel, and breaks through the limitation of the traditional heat transfer pipe with a uniform structure.

Specifically, the secondary side flow channel can be divided into a single-phase heat transfer section, a saturated boiling section and a dry section along the length direction of the secondary side flow channel so as to correspond to the main physical process from liquid phase heating to complete evaporation of the secondary side working medium.

1. In the single-phase heat transfer section, the secondary side working medium is mainly liquid phase. To enhance convective heat transfer at this stage and to improve flow stability, the flow cross-sectional geometry (e.g., hydraulic diameter or flow area) of the secondary side stream of the stage is designed to be relatively smaller than the corresponding dimensions of the subsequent saturated boiling stage. The smaller flow cross section helps to increase the flow rate of the working fluid, thereby enhancing turbulence, increasing the wall heat transfer coefficient, and possibly inhibiting undesirable premature boiling.

2. In the saturated boiling section, the secondary side working medium undergoes severe phase change and undergoes various complex flow patterns from bubble flow to annular flow. The secondary side flow passage can adopt the quadrilateral or hexagonal basic cross section shape. The surface morphology of the inner wall surface may be specifically designed to further enhance boiling heat transfer in this region. For example, the side walls of the flow channel may be derivatized to be curved, wavy, zigzag, or provided with longitudinal fin-like projections along the length of the flow channel, or the longitudinal cross-section of the flow channel itself may be designed as a wavy, periodic cross-rib or zigzag profile. These changes in surface morphology aim to significantly increase boiling heat transfer efficiency by increasing the number of vaporisation cores, promoting bubble detachment, enhancing liquid film turbulence, expanding effective heat transfer area, etc.

3. In the dry section, the liquid film may tend to crack, with heat transfer being at risk of deterioration. To address this problem, the flow cross-sectional geometry of the secondary side stream of the section is designed to be relatively larger than the corresponding dimensions of the saturated boiling section, which helps to reduce vapor phase flow rate and pressure drop. More importantly, specific flow-modifying or heat transfer enhancing elements, such as rotating fins, are provided within the secondary side flow passage. These rotating fins are designed to direct the secondary side working fluid to produce a strong rotational or swirling flow. The centrifugal force generated by the rotary flow can effectively throw the liquid drops which are carried in the air flow and not evaporated to the heated partition wall surface and rewet the heated partition wall surface, thereby delaying or relieving the occurrence of the phenomenon of dry, and strengthening the convection and evaporation heat transfer of the area. At the same time, the fins themselves act as expansion surfaces to increase the heat transfer area.

In order to ensure the smoothness of the flow of the secondary working medium when the secondary working medium flows through the functional sections with different characteristic structures, the extra flow resistance and local overheating possibly caused by the abrupt change of the section or the internal structure are reduced, and smooth transition connection is preferably carried out among the single-phase heat transfer section, the saturated boiling section and the dry section through continuously-changing curved surfaces. The gradual transition design is helpful for maintaining the stability of the flow field and reducing energy loss.

Given the fine surface features and the geometric complexity of the internals (e.g., rotating ribs) that may be present within the segmented variable cross-section flow passage, conventional subtractive manufacturing or tubing bending assembly processes are difficult to achieve accurately and integrally. Therefore, the runner structure is particularly suitable for layer-by-layer printing and integral molding by using metal powder materials by adopting advanced additive manufacturing technology, such as laser powder bed melting (L-PBF) technology. This manufacturing method provides technical feasibility for realizing a heat transfer flow channel with highly optimized and complex internal geometry, enabling an accurate realization of the design intent.

The second aspect of the invention provides a steam generator comprising the segmented variable cross-section boiling heat transfer flow passage described above:

The core heat transfer area of the steam generator is comprised of one or more segmented variable cross-section boiling heat transfer flow path arrays as described above. By adopting the novel runner, the steam generator is hopeful to realize the aims of more compact overall structure and lighter weight of the equipment while maintaining or even improving the heat exchange capacity, and the adaptability to different working conditions and the running stability can be possibly improved.

In summary, the invention provides an effective new way for improving the comprehensive performance of the nuclear energy steam generator by innovative sectional and variable cross-section design of the secondary side boiling heat transfer flow passage of the steam generator and fine structural optimization aiming at the characteristics of each heat transfer stage and combining with advanced manufacturing technology.

The invention provides a boiling heat transfer flow passage of a sectional variable-section steam generator. The beneficial effects are as follows:

1. The invention divides the secondary side flow into a single-phase heat transfer section, a saturated boiling section and a dry section along the flow direction of the working medium, and performs customized section geometry and internal structure design aiming at the flow pattern and heat transfer characteristics of each section (such as the reduction of the section of the single-phase section to improve the flow speed, the optimization of the wall surface of the saturated boiling section to increase the vaporization core and the arrangement of the dry section to strengthen the wetting of the wall surface of the liquid drop), so that each heat transfer stage can work under better heat exchange conditions, thereby obviously improving the boiling heat transfer coefficient and the overall energy transfer efficiency of the secondary side working medium as a whole.

2. The invention adopts optimized basic flow passage and surface derivatization structure to adapt to the conversion from bubble flow to annular flow through sectional design, for example, in the saturated boiling section, and manages the dispersion flow with high air content through increasing the flow passage size and internal rotary ribs in the dry section, thereby improving the heat transfer performance of the flow pattern conversion area, inhibiting the flow instability and improving the operation stability and safety of the steam generator under different working conditions.

3. The application of the non-circular basic section (such as quadrangle and hexagon) of the invention ensures that the runner bundles can be arranged more tightly, thereby reducing the ineffective space between the traditional bundles. Meanwhile, the heat transfer of each section is pertinently enhanced, and the required total heat transfer area can be reduced on the premise of meeting the same heat transfer power. These factors act together to help reduce the overall size and weight of the steam generator, meeting the development demands of nuclear power plants for equipment compactness and weight reduction.

4. According to the invention, the primary side flow channel and the secondary side flow channel are directly separated by the partition wall, and the secondary side flow channel is subjected to fine variable cross section and internal structural design, so that the heat transfer interface can be more flexibly distributed and optimized as required, the effective utilization rate of materials is improved, and the potential waste of the heat transfer area is avoided.

5. The complex sectional variable-section runner structure provided by the invention, particularly the fine surface characteristics and the strengthening elements in the structure, is dependent on the realization of additive manufacturing technologies such as laser powder bed melting and the like. The method breaks through the limitation of the traditional manufacturing process, opens up a new thought and a new technical path for designing the complex flow channel heat exchange equipment with higher integration level and better performance in the future, and promotes the progress of the design and manufacturing technology of the heat exchange equipment.

Drawings

FIG. 1 is a schematic diagram of a typical boiling heat transfer fluid flow pattern;

FIG. 2 is a schematic view of a basic heat transfer flow path, wherein a) is a quadrilateral basic heat transfer flow path, b) is a hexagonal basic heat transfer flow path;

FIG. 3 is a schematic view of a basic heat transfer flow channel derivatization structure, wherein a) is a flow channel cross section, and b) is a flow channel longitudinal section;

FIG. 4 is a two-dimensional view of a segmented flow path of a steam generator of the present invention;

FIG. 5 is a three-dimensional view of a segmented flow path of the steam generator of the present invention;

FIG. 6 is a schematic diagram of a prototype of the principles of the present invention;

FIG. 7 is a photograph of a heat transfer performance test of a prototype of the principles of the present invention.

1, Edges of a flow channel; 2, partition walls; the flow channel comprises a primary side flow channel, a secondary side flow channel, a round corner at the joint of the flow channels, an oblique angle at the joint of the edges of the flow channels, an arc shape, a first wave shape, a first zigzag shape, a longitudinal fin, a transverse fin structure, a second wave shape, a transverse fin structure and a second zigzag shape.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. The singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Also, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

For clarity, certain terms will be used in the drawings and the following description, which terms are to be construed broadly and not limiting. For example, "primary side" and "secondary side" refer to the relative regions of fluid flow, while "upstream" and "downstream" refer to the relative positions along the direction of fluid flow.

The boiling heat transfer flow passage (hereinafter simply referred to as a "heat transfer flow passage") of the sectional variable cross-section steam generator provided by the embodiment of the invention has the following basic unit constitution and layout modes.

In a preferred embodiment, as shown in fig. 2, the base unit of the heat transfer flow path comprises at least one primary side flow path 3 for conducting a primary side high temperature working medium and at least one secondary side flow path 4 for conducting a secondary side boiling working medium. The primary side flow channel 3 and the secondary side flow channel 4 are adjacently disposed and physically separated and thermally transferred by a partition wall 2 shared therebetween. The partition wall 2 is used as a main heat transfer interface to transfer heat of the high-temperature working medium in the primary side flow channel 3 to the working medium in the secondary side flow channel 4 so as to cause the secondary side working medium to boil.

The secondary side flow path 4 in the embodiment of the present invention is designed in cross section to have a non-circular basic cross-sectional shape that enables efficient space utilization and close arrangement. As shown in fig. 2a, such a non-circular base cross-sectional shape may be embodied as a quadrilateral. In another embodiment, as shown in FIG. 2b, the base cross-sectional shape may be hexagonal. Compared with the traditional circular pipeline, the non-circular pipeline with the quadrangular or hexagonal cross section has the main advantages that when a plurality of flow channel units are arranged side by side to form a flow channel bundle, ineffective gaps among the flow channel units can be remarkably reduced, and higher space filling rate is realized. The edge 1 (1 in fig. 2) of the flow channel forms the outer boundary of the flow channel or shares a part of the partition wall with the adjacent flow channel.

This compact arrangement not only helps to reduce the overall heat exchange core volume of the steam generator, but also helps to reduce the overall weight of the device, which is of great significance for applications where space is limited or where weight is critical, such as steam generators in Small Modular Reactors (SMR).

A plurality of such base flow channel units may be regularly arranged in an array to form a heat transfer core assembly having a large-scale heat transfer capability. By the design of the common partition wall 2, adjacent flow passage units can effectively share the heat transfer surface, further optimizing the compactness of the structure and the utilization efficiency of materials. The primary side flow channel 3 and the secondary side flow channel 4 can be designed into different flow cross-sectional areas and geometric shapes according to specific heat transfer requirements so as to match the physical properties and flow characteristics of working media at two sides.

In an embodiment of the present invention, a critical segmentation strategy is employed for the design of the secondary side stream 4. The sectional design is based on the deep analysis of complex physical phenomena and changeable fluid flow patterns of the secondary side working medium in the heating and boiling process.

Referring to fig. 1, there is schematically illustrated various fluid flow patterns that may occur in a secondary side working fluid (e.g., water) as it flows along a heating channel during a typical boiling heat transfer process. Starting from the single-phase liquid water at the inlet, the temperature of the working medium rises and reaches a saturated state as heat is continuously added, and then boiling is started. During boiling, the gas phase gradually increases, and the fluid presents bubble flow (bubbles are dispersed in a continuous liquid phase), bullet flow (large bubbles occupy most of the channel section to form bullet-shaped air clusters), restricted bubbles, elongated bubbles, stirring-annular flow and annular flow (liquid phase is attached to the wall surface in a film form, the gas phase flows at a high speed in the center) until dry flow possibly appears finally (liquid film breaks, dry spots appear on the wall surface, and liquid drops are entrained in the gas phase). Each flow pattern corresponds to a specific gas-liquid two-phase distribution pattern and heat transfer mechanism.

The geometry and dimensions of conventional homogeneous structured heat transfer tubes remain the same or vary little throughout their length, making it difficult to optimally design for the specific heat transfer characteristics of the different flow stages described above. The invention therefore proposes that the secondary side flow 4 is divided into at least two, in the preferred embodiment three main functional sections, in its length direction, i.e. the flow direction of the secondary side working medium.

As shown in fig. 4 and 5, the three functional sections are embodied as a single-phase heat transfer section on the inlet side of the working medium, an immediately following saturated boiling section, and a dry section on the outlet side of the working medium. The dividing mode enables the structural design of each section to be matched with the flow pattern characteristics of the main flow in the area and the dominant heat transfer mechanism, so that the heat transfer performance of each stage is improved to the maximum extent, and the flow stability is improved.

The purpose of dividing these functional sections is to achieve a fine management and enhancement of the boiling heat transfer process. By adopting different geometric parameters or internal structural characteristics of the flow channels in different sections, the heat transfer bottleneck or flow problems possibly occurring in each stage can be solved in a targeted manner, such as heat convection enhancement in a single-phase region, bubble management in a nucleate boiling region, and liquid film maintenance and heat transfer deterioration inhibition in a dry region. The sectional design based on the physical characteristics of the boiling process is one of core technical means for realizing efficient heat exchange and a compact structure.

In the embodiment of the invention, the unique structural characteristics are respectively designed for the single-phase heat transfer section, the saturated boiling section and the dry section divided by the secondary side stream 4 so as to adapt and strengthen the physical process of the corresponding heat transfer stage.

First, the design of the single-phase heat transfer section is concerned. As shown in fig. 4 and 5, this section is located at the upstream portion of the secondary side working fluid entering the heat transfer flow passage. In the present embodiment, the flow cross-sectional geometry of the secondary side stream 4 of the single-phase heat transfer section, such as its hydraulic diameter or effective flow area, is designed to be relatively smaller than the corresponding dimensions of the saturated boiling section downstream thereof. The purpose of this design is to achieve a higher flow rate when the secondary side working fluid (typically liquid water) flows through the smaller cross section. The higher flow velocity helps to enhance the turbulence degree of the fluid and destroy the laminar bottom layer near the pipe wall, thereby enhancing the single-phase convection heat exchange effect, so that the liquid-state fluid can quickly and effectively absorb heat from the partition wall 2, and the preparation is made for the subsequent boiling process. At the same time, the higher flow rates and regular flow passages (usually without complex internals at this stage) also help to improve flow stability and suppress undesirable early boiling or uneven flow that may be caused by local overheating.

Next, the design of the saturated boiling section is followed. The section is a core area in which the secondary side working medium is subjected to severe phase change and is subjected to transition from bubble flow to annular flow and other complex flow patterns. In this embodiment, as shown in fig. 4 and 5, the secondary side stream 4 of the saturated boiling section may take the aforementioned non-circular base cross-sectional shape of a quadrilateral or hexagonal shape or the like. To further enhance the boiling heat transfer in this region, the inner wall surfaces, i.e., the surfaces of the partition walls 2 facing the secondary side flow channels 4 and the inner surfaces of the sides 1 of the flow channels may be subjected to specific geometric modifications or integrated enhanced heat transfer structures. These modifications or structures are intended to significantly increase boiling heat transfer coefficients by one or more means, such as by increasing effective vaporization core point density, promoting effective generation and timely detachment of bubbles, enhancing turbulent turbulence and macroscopic mixing of near wall zone fluids, increasing effective heat exchange area, or improving wetting characteristics of the walls to maintain effective liquid film coverage. The choice and design of specific inner wall geometry derivatization and reinforcement structures.

Finally, the design of the drying section is concerned. This section is located at the downstream end of the secondary side stream where the gas phase fraction of the working medium is very high and the liquid film may become very thin or even break, resulting in deteriorated heat transfer. To cope with this problem, as shown in fig. 4 and 5, the flow cross-sectional geometry of the secondary side stream 4 of the dry section in this embodiment is designed to be relatively larger than the corresponding dimension of the saturated boiling section upstream thereof. This increase in cross-sectional size helps to reduce the flow rate of the high velocity steam stream, reducing the flow resistance and associated pressure drop.

More critical is that inside the secondary side flow 4 of the drying section ribs are provided for generating a swirling flow and directing the droplets to the heat transfer wall. In the preferred embodiment, these ribs are rotating ribs (as shown inside the drying section in the three-dimensional view of fig. 5). These rotating ribs are usually arranged in the flow channel in a spiral or circumferentially or non-uniformly distributed manner at a specific inclination. The working mechanism is that when the high-speed gas-liquid two-phase fluid flows through the rotary ribs, the flow guiding function of the ribs can lead the fluid to generate strong rotation or vortex motion. In such a rotating flow field, droplets having a relatively high density are thrown against and reattach to the heat transfer wall surfaces (i.e., the inner surfaces of the partition walls 2 and the inner surfaces of the flow channel sides 1) having a relatively high temperature by centrifugal force. The forced wall rewetting effect can effectively delay or reduce the occurrence of drying phenomenon, improve the heat transfer condition, and enable the wall to continuously transfer heat through evaporating liquid drops, thereby remarkably improving the heat transfer efficiency and critical heat flow density of the drying section and even the whole flow channel. At the same time, the rotating fins themselves also serve as expansion surfaces, which increases the heat transfer area to a certain extent.

Through the fine structural design aiming at each functional section, the heat transfer flow channel can be more effectively adapted to complex changes in the boiling heat transfer process, and the overall heat exchange performance is improved.

As previously described, embodiments of the present invention provide various flow channel geometry derivatization and optimization schemes for achieving enhanced heat transfer in specific sections of the secondary side flow channel (particularly saturated boiling sections) or for improving the overall hydrodynamic performance of the flow channel. Referring to fig. 3, a detailed illustration of various specific geometric derivatizations based on a basic heat transfer flow path (e.g., a quadrilateral or hexagonal flow path as shown in fig. 2) is shown. These derivatization structures may be used alone or in combination depending on the actual heat transfer requirements and fluid characteristics.

In terms of optimization of the cross-sectional geometry of the flow channel, the morphology of the flow channel junctions and flow channel edges can be fine-tuned as shown in fig. 3 a.

One aspect is the transition design at the corners of the flow channel. For example, the junction of the side 1 of the flow channel constituting the secondary side flow channel 4 and the partition wall 2 (or the side of the adjacent flow channel) may no longer be a sharp right angle, but rather be designed as a smooth flow channel junction rounded transition 5. Such rounded transitions help reduce localized pressure losses as the fluid flows through the corners, avoid creating flow dead zones or excessive turbulence, thereby making the flow field more uniform and potentially reducing the risk of deposition or corrosion. Similarly, a bevel transition 6 at the edge junction of the flow channels may also be used, with a smooth transition depending on the particular design requirements.

Another important aspect is the morphological derivatization of the edge 1 of the flow channel itself to actively influence the flow and heat transfer in the near-wall region.

One way of derivatization is to design the edges 1 of the flow channel as an arc 7 curving towards the inside or outside of the flow channel. The arc-shaped side wall can change the perimeter and area distribution of the cross section of the flow channel, and affects the development of the boundary layer.

Another way is to design the side 1 of the flow channel with a first wave-like shape 8 that periodically undulates. The wavy wall surface can effectively disturb the fluid near the wall surface, promote the mixing of the main flow and the fluid near the wall surface, destroy the thermal boundary layer and further strengthen the convective heat exchange. The amplitude and wavelength of the waves are important design parameters.

Yet another way is to design the edge 1 of the flow channel as a repeated first zigzag 9. The tips and valleys of the serrations may act as a generator of turbulence, also to strongly disturb the boundary layer and enhance heat transfer. The shape, angle and depth of the serrations also affect their effectiveness.

Furthermore, longitudinal fins 10 can also be provided on the side 1 of the flow channel in the flow direction of the secondary side working medium. These fins extend directly into the fluid as an extended heat transfer surface, significantly increasing the effective heat transfer area on the secondary side. At the same time, the longitudinal fins can also play a certain role in regulating and guiding the fluid or can be used for separating different flow areas in the flow channel under a specific design.

In terms of derivatization of the geometric features of the longitudinal sections of the flow channels, i.e. the flow channel profile along the flow direction of the secondary side working medium, as shown in fig. 3b, various optimization designs can also be carried out to enhance the heat transfer.

One way is to design the inner wall surface of the flow channel (e.g., the side of the partition wall 2 facing the secondary side flow channel 4, or the inner surface of the side 1 of the flow channel) to have a second wavy shape 11 that continuously and smoothly undulates in the longitudinal direction. When fluid flows through the wavy surface, the fluid can be subjected to periodical acceleration and deceleration and bending of the streamline, so that secondary flow and vortex street can be induced to be generated, macroscopic mixing of the fluid is enhanced, a thermal boundary layer is effectively destroyed and rebuilt, and the overall heat transfer coefficient is improved.

Another way is to provide the flow channel with a transverse fin structure 12 in longitudinal section. These transverse fins (or called ribs, turbulators) may be arranged perpendicular to the main flow direction or at an angle, they act as obstacles to forcibly alter the main flow path of the fluid, forming a recirculation zone and reattachment zone downstream of the fins, these zones generally having a high local heat transfer coefficient. The transverse fins also increase the heat transfer surface area.

Similarly, the flow channel longitudinal section can also be designed as a profile of the second zigzag 13. The zigzag wall surface also generates strong fluid disturbance and vortex by introducing periodical geometric mutation in the flowing direction, and enhances the mixing of gas phase and liquid phase and the heat exchange of the wall surface.

These geometric derivatization features, illustrated in fig. 3, provide the designer with a range of structural elements and configurations that can be used to optimize the design. The designer can flexibly select and combine these features according to specific heat transfer requirements (e.g., promoting bubble nucleation, enhancing liquid film turbulence, increasing turbulence intensity, expanding heat exchange area, etc.) of different sections of the secondary side flow channel, thereby achieving optimal heat transfer performance and hydrodynamic characteristics.

In the embodiment of the present invention, since the secondary side stream 4 is divided into a plurality of functional sections (e.g., a single-phase heat transfer section, a saturated boiling section, and a dry section) having different geometric characteristics and internal structures, how to achieve a smooth transition between these different sections is an important design consideration.

Referring to the three-dimensional schematic of fig. 5, it is clearly shown that in a preferred embodiment of the present invention, the single-phase heat transfer section, the saturated boiling section and the dry section are smoothly connected by a continuously changing curved surface. This means that at the junction of the different functional sections, the geometry of the inner wall surface of the flow channel (for example the cross-sectional flow area, the hydraulic diameter or the introduction of internal features such as ribs) is not changed abruptly, but rather by a transition region with a gradual profile.

For example, when transitioning from a single-phase heat transfer section having a relatively small flow cross-section to a saturated boiling section having a relatively large flow cross-section and possibly having a complex internal wall surface morphology, the walls of the flow channels are joined by a smoothly expanding curved section. Similarly, when the transition from the saturated boiling section to the drying section with larger flow cross section and rotating ribs inside, a similar gradual transition design is adopted, and the initial ends of the ribs can be designed to gradually blend into the flow channel.

The main benefits of adopting the continuous curved surface to carry out smooth transition connection are as follows:

First, it can reduce the additional flow resistance caused by abrupt changes in the flow path geometry. Severe cross-sectional sudden expansion or contraction, or abrupt appearance of internal structures, may result in separation of the fluid boundary layer, generation of vortices, and large fluctuations in local pressure, which all increase the energy loss of the fluid, i.e., increase the flow pressure drop. The smooth transition helps to maintain flow field stability and reduce unnecessary energy dissipation.

Second, the smooth transition helps to avoid localized stress concentrations or hot spots at structural changes. Abrupt changes in geometry may result in uneven distribution of heat flux density or form stress-concentrating weaknesses in the mechanical structure. The transition design of the continuous curved surface ensures that the wall surface temperature distribution and the structural stress distribution are more uniform, thereby being beneficial to improving the operation reliability and the service life of the heat transfer flow channel.

Furthermore, for the secondary side working medium, particularly when the secondary side working medium is in a two-phase flow state, smooth transition is helpful to maintain stable development of flow patterns, and severe flow pattern conversion or unstable flow phenomena such as flow layering, backflow or pulsation caused by geometric mutation are avoided.

Thus, in the practice of the invention, although the features of each functional section are optimized for its particular heat transfer stage, the connection between the sections is preferably in a gradual, smooth manner to ensure that the overall heat transfer flow path has good hydrodynamic performance and structural integrity while achieving efficient heat transfer.

In the embodiment of the invention, the boiling heat transfer flow passage of the designed sectional variable-section steam generator has obvious complexity in structure. This complexity is manifested in the fact that the secondary side flow has a variable flow cross-sectional geometry along the flow direction, that each functional section may contain a finely geometrically derivatized form of the inner wall surface (e.g., arcuate edges, waves, zigzags, longitudinal or transverse fins, etc.), and that complex flow-modifying or heat-transfer enhancing members (e.g., rotating fins) are provided within a particular section (e.g., dry section). In addition, smooth transitions between different functional sections are required by continuously varying curved surfaces.

Conventional manufacturing processes, such as subtractive manufacturing or combinatorial manufacturing methods based on bending of tubing, welding, machining (e.g., drilling, milling), etc., face significant challenges, and even are not feasible in some cases, when accurately and integrally implementing such flow channels with complex internal geometries and fine structures. For example, to create a specific shape of rotating ribs or fine wall corrugation within an elongated non-circular channel and to ensure a smooth transition between segments, it is difficult to achieve the required accuracy and integrity of the conventional process.

Thus, the fabrication of such a complex structured flow channel in embodiments of the present invention is particularly well suited for use with additive manufacturing (Additive Manufacturing, AM) techniques, also commonly referred to as 3D printing techniques. Among the various additive manufacturing techniques, the laser powder bed fusion (Laser Powder Bed Fusion, L-PBF) technique is a preferred solution for implementing the heat transfer flow channels of the present invention because of its ability to manufacture dense, metal parts with good mechanical properties, and its high flexibility in shaping complex geometries.

When the L-PBF technology is adopted to manufacture the heat transfer runner, the basic process comprises the steps of firstly slicing a detailed three-dimensional CAD model of the heat transfer runner to generate section data of each layer, then paving a layer of thin metal powder material (such as high-temperature resistant and corrosion resistant nickel-based alloy, stainless steel and other materials suitable for the working condition of a steam generator) in a forming cavity filled with protective atmosphere (such as argon or nitrogen), then selectively melting metal powder by a high-energy laser beam according to the section data of the current layer to solidify and combine with the previous layer, and repeating the processes of paving powder and laser melting, stacking layer by layer until the whole heat transfer runner component (or a key part thereof) is integrally formed.

The additive manufacturing technology, in particular to L-PBF, brings the following key advantages for the design and realization of the heat transfer flow channel:

The high degree of freedom in geometry enables runner structures with arbitrarily complex internal channels, fine surface features (e.g., micro fins, corrugations), and internal components (e.g., integrally formed rotating ribs) to be fabricated directly from CAD models without the need for conventional molds or complex assembly processes.

The integrated forming can integrally print out a functional structure (such as a variable cross-section channel with internal ribs) which can be realized by combining a plurality of parts at one time, so that a connecting interface is reduced, the structural integrity and the sealing performance are improved, and the potential defect risk caused by a connecting process such as welding and the like is possibly reduced.

The flexibility of material selection L-PBF technology can be applied to various metal materials, and is convenient for selecting the most suitable material according to the specific working condition requirements (such as temperature, pressure and corrosion environment) of the steam generator.

And the rapid prototyping and iterative optimization can rapidly manufacture prototypes for testing and verifying, and design and rapid iteration can be conveniently modified according to test results, so that the research and development process of the novel high-performance heat exchanger is accelerated.

By adopting the additive manufacturing technology, the complex and exquisite design concept of the sectional variable-section boiling heat transfer flow passage provided by the invention can be accurately realized, and powerful technical support is provided for breaking through the limit of the traditional manufacturing technology on the structural optimization of the heat exchanger and developing an advanced steam generator with higher heat transfer efficiency and more compact structure.

In an embodiment of the present invention, the aforementioned boiling heat transfer flow path of the segmented variable cross-section steam generator is intended to be applied as a core heat exchange element in the steam generator to improve the overall performance thereof.

A steam generator comprising the heat transfer flow channels of the present invention will have a heat exchange area (i.e., evaporator core or heat transfer tube bundle area) of the core formed by one or more segmented variable area boiling heat transfer flow channel units as described in detail hereinabove, or by flow channel bundles formed by a regular array of such flow channel units. The arrangement mode of the heat transfer flow channel units in the steam generator can be flexibly adjusted according to the overall design requirements of the steam generator (such as the integrated design of a vertical type, a horizontal type or a specific compact type reactor), the inlet and outlet header designs of primary side and secondary side working media, the required total heat exchange power and other factors.

Specifically, in the design of the steam generator, a primary side hot working fluid (e.g., coolant from the nuclear reactor core) will be introduced and distributed into the individual primary side flow channels 3 that make up the heat transfer flow channel array. At the same time, secondary side feedwater (e.g., preheated liquid water) will be introduced and distributed to the inlet ends (i.e., the beginning of the single-phase heat transfer segments) of the respective secondary side streams 4. The primary-side high-temperature medium flows downward or upward in the primary-side flow passage 3 (depending on the specific design), and its heat is transferred to the secondary-side medium in the secondary-side flow passage 4 through the common partition wall 2.

The secondary side working fluid will undergo the aforementioned single-phase heating, saturated boiling and possibly superheating (if a superheating section is designed) in sequence as it flows in the secondary side flow path 4. Because the secondary side flow 4 adopts a sectional variable cross-section design and performs structural optimization (such as small cross section of a single-phase section, wall strengthening of a saturated boiling section, rotary ribs of a dry section and the like) aiming at the characteristics of each heat transfer stage, the heat transfer efficiency of the secondary side working medium in the whole boiling and evaporating process is obviously improved. Finally, at the outlet end of the secondary side stream 4, high temperature and high pressure steam (or a high dryness steam-water mixture, depending on whether it is a superheated steam generator) will be generated, which is then collected and led out of the steam generator for driving a steam turbine for power generation or for other industrial processes.

By adopting the sectional variable-section boiling heat transfer flow passage, the steam generator formed by the invention is expected to realize the performance improvement in one or more of the following aspects:

the heat exchange efficiency is higher, because each heat transfer stage is pertinently enhanced, the heat exchange capacity of the unit heat transfer area is improved, so that more heat can be transferred under the same temperature difference and flow rate conditions, or smaller temperature difference is needed on the premise of transferring the same heat.

The application of non-circular base sections (such as quadrangles and hexagons) and the improvement of heat transfer efficiency, which can reduce the total heat transfer area and the number of flow channels required or make the flow channels more compact when meeting the same heat transfer power requirement, thereby contributing to the significant reduction of the volume and weight of the steam generator core heat exchange components and even the whole device. This is particularly important for the miniaturization and modular development of nuclear power plants.

Improved operational stability and safety by better adaptation and management of the flow patterns at different stages of boiling heat transfer (e.g. delayed drying by rotating fins), operational stability of the steam generator at different conditions can be improved, safe operational boundaries can be widened, and risks of local overheating or flow instability due to heat transfer deterioration can be reduced.

Potential material savings-improvements in heat transfer efficiency and compactness of the structure may result in reduced amounts of high performance materials required to manufacture the steam generator, thereby reducing costs.

In summary, the sectional variable-section boiling heat transfer runner provided by the invention is applied to the design of the steam generator, and an effective technical approach can be provided for improving the economy and the safety of the nuclear energy device and the adaptability to future development demands through the innovative structure and the performance advantages brought by the sectional variable-section boiling heat transfer runner. The specific structural parameters of the novel steam generator, such as the number, arrangement mode, total length, length proportion of each section and the like of the flow channel units, can be designed and optimized in detail according to specific application scenes and performance indexes.

In order to further verify the beneficial effects brought by the sectional variable-section boiling heat transfer flow passage applied to the steam generator, a principle model machine based on the principle of the invention is developed, and experimental study is carried out on the heat transfer characteristics of the principle model machine. The design power of the principle prototype is about 1/30 of the function of a steam generator of a certain model in the prior art (hereinafter referred to as "prior art SG"). As shown in fig. 6, a schematic diagram of a schematic prototype is shown, and the main design parameters of the schematic prototype are shown in table 1.

Table 1 principle prototype parameters

Power (MW)	0.8
		Weight (kg)	29
Long (mm)	200
		Wide (mm)	100
High (mm)	1200
		Primary side heat exchange area (m 2)	0.747
Secondary side heat exchanging area (m 2)	3.9
		Material	AM-316LN

For this model machine, performance tests were performed under specific working conditions, as shown in fig. 7, which shows a heat transfer performance test picture of the model machine, and the target values of the tests, the measured values of some key parameters, and the converted values (such as heat exchange power) obtained by further processing based on the measured values are summarized in table 2.

Table 2 experimental results

Parameters (parameters)	Target value	Measurement value	Converted value
				Primary side inlet temperature (°c)	319.5	319.3	/
Primary side outlet temperature (°c)	/	286.6	/
				Primary side flow (m 3/h)	20.83	21.3	/
Heat exchange power (MW)	0.8	/	0.813
				Secondary side inlet temperature (°c)	140	139	/
Secondary side outlet temperature (°c)	290	291	/
				Secondary side outlet pressure (MPa)	4.5	4.6	/
Primary side resistance (kPa)	/	29	/

As can be seen from the experimental results in Table 2, the heat exchange power obtained by practical conversion of the inventive principle model machine reaches 0.813MW under the experimental working condition, and meets and is slightly higher than the design target power of 0.8 MW. Meanwhile, the temperature of the steam at the secondary side outlet reaches 291 ℃, the outlet pressure reaches 4.6MPa, and the design requirements are met, so that the steam generator adopting the flow channel design of the invention can effectively realize the expected energy conversion and steam generation functions. The flow resistance on the primary side was measured at 29kPa.

In order to more intuitively embody the progress brought by the technical scheme of the invention, the experimental result of the principle prototype is compared with the performance parameters of the SG in the prior art. Since the design function of the principle prototype is 1/30 of that of the prior art SG, when in comparison, the corresponding parameters of the prior art SG are converted according to the proportion of 1:30 to be used as a comparison base number. The detailed performance pair is shown in table 3.

Table 3 comparison of performance

From the comparative data in Table 3, it can be clearly seen that, on the premise of achieving similar power output (1:30 conversion), a principal prototype employing the segmented variable cross-section boiling heat transfer flow passage of the present invention, compared to the scaled prior art SG:

The weight of the feed is obviously reduced from 59.2kg to 29kg, and the reduction range reaches 50.9%.

The external volume is obviously reduced from 0.076m ³ to 0.024m ³, and the reduction amplitude reaches 68.4 percent.

The primary side flow resistance of the air conditioner is obviously reduced from 81kPa to 29kPa after conversion, and the reduction amplitude reaches 64.0%.

Meanwhile, the volumetric heat exchange area (area efficiency, defined as total heat exchange area/outline volume) of the principle model machine reaches 464.7m ²/m³, which is far higher than 320m ²/m³ of the prior art SG (the area efficiency is compared with the original parameters based on the prior art SG, and the area efficiency is theoretically unchanged because the outline volume and the heat exchange area of the comparison base are reduced in proportion), so that the design of the invention further has the remarkable advantages in the aspects of structural compactness and space utilization.

The experimental data and the comparison result fully prove that by implementing the technical scheme of the boiling heat transfer flow channel of the sectional variable-section steam generator, the weight and the volume of the steam generator can be greatly reduced and the flow resistance of the primary side can be obviously reduced while the heat transfer performance is effectively ensured and even improved. The method has important practical significance and application value for improving the overall economy, compactness and operation efficiency of the nuclear power device.

Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims (10)

1. A segmented variable cross-section vapor generator boiling heat transfer flow passage comprising:

at least one primary side flow channel and at least one secondary side flow channel separated by a common partition wall, the secondary side flow channel having a non-circular base cross-sectional shape capable of achieving a tight arrangement;

the secondary side flow is divided into at least two sections along the length direction of the secondary side flow, namely the flow direction of the secondary side working medium, and the at least two sections respectively correspond to different boiling heat transfer stages of the secondary side working medium;

and, at least one of the geometric dimension of the flow cross section, the surface characteristics of the inner wall surface, or the flow disturbance or heat transfer reinforcing element provided inside the secondary side flow in the at least two stages is specifically configured according to the flow pattern and the heat transfer characteristics of the boiling heat transfer stage corresponding to each stage, so that the characteristics of the at least one of the stages are different from each other.

2. The segmented variable cross-section steam generator boiling heat transfer flow passage as claimed in claim 1, wherein the secondary side flow passage has a basic cross-sectional shape of a quadrilateral or hexagon.

3. The segmented variable cross-section steam generator boiling heat transfer flow path of claim 2, wherein the secondary side flow path is divided along its length into a single phase heat transfer segment, a saturated boiling segment and a dry segment.

4. The segmented variable cross-section steam generator boiling heat transfer flow passage of claim 3 wherein said single phase heat transfer segment is characterized by:

compared with the saturated boiling section, the flow cross section of the secondary side flow is relatively reduced, so that the flow velocity and the flow stability of the secondary side working medium are improved.

5. A segmented variable cross-section steam generator boiling heat transfer flow path as claimed in claim 3, wherein the saturated boiling section is characterized by:

The quadrangular or hexagonal basic cross section shape is adopted, and the inner wall surface of the quadrangular or hexagonal basic cross section shape is provided with an enhanced heat transfer structure selected from at least one of the following:

arc derivatization, wave derivatization, zigzag derivatization and longitudinal fin derivatization of the runner edge;

Or wavy derivatization of the longitudinal section of the runner, derivatization of the transverse fin structure and zigzag derivatization.

6. A segmented variable cross-section steam generator boiling heat transfer flow path as claimed in claim 3, wherein the dry section is characterized by:

the secondary side flow has a relatively large cross-sectional flow area compared to the saturated boiling section, and is provided with ribs inside thereof for generating a swirling flow and guiding the droplets to the heat transfer wall surface.

7. The segmented variable cross-section steam generator boiling heat transfer flow path of claim 6 wherein the fins are rotary fins configured to direct fluid to create a rotational flow to separate and direct droplets entrained in the secondary side working fluid to the heat transfer wall of the secondary side flow path using centrifugal force to retard drying and enhance heat transfer.

8. A segmented variable cross-section steam generator boiling heat transfer flow path as claimed in claim 3 wherein the single phase heat transfer section, saturated boiling section and dry section are joined by a smooth transition through a continuously varying curved surface.

9. The segmented variable cross-section vapor generator boiling heat transfer flow path of claim 1 wherein said flow path is made of a metallic material and is adapted for unitary fabrication by additive manufacturing techniques such as laser powder bed melting.

10. A steam generator, wherein the core heat transfer member comprises a plurality of segmented variable area steam generator boiling heat transfer flow passages as claimed in any one of claims 1 to 9.