CN106885634A - Unsteady wall heating heat flux distribution measuring method based on infrared thermal imagery thermometry - Google Patents

Abstract

Heat flux distribution measuring method is heated the invention discloses a kind of unsteady wall based on infrared thermal imagery thermometry, the temperature field of the heating wall that the method is changed over time using Infrared Thermography Technology record, the object uniform wall thickness of selection and through-thickness thermal resistance is smaller, therefore wall heating problems can be simplified to Two Dimensional Unsteady heat transfer problem, obtain unsteady equation of heat balance, then the temperature field for obtaining will be measured to time and space derivation, ignore free convection and heat radiation, be calculated the heating hot-fluid of Two dimensional Distribution.A kind of non-contacting calorimetric stream method is this method provide, the Two dimensional Distribution situation of wall heat flux can be in real time measured.

Description

Measurement method of unsteady wall heating heat flux distribution based on infrared thermal imaging temperature measurement technology

technical field

The invention relates to a method for measuring the heat flow distribution of a wall surface, in particular to a method for measuring and recording the temperature field change of the wall surface when it is heated by a heating source (such as a flame, an electric heater, hot gas, etc.) by using infrared thermal image temperature measurement technology to calculate the heat flow method of distribution.

Background technique

At present, there are three main methods for measuring wall heat flow: (1) using a resistive temperature sensor, obtaining the temperature gradient of known resistance through the output electrical signal, and using Fourier's law to calculate the wall heat flow, such as a plunger heat flow meter; 2) Thermocouple temperature sensors are used to measure heat flow by measuring surface temperature differences, such as thin-film thermocouples; (3) calorimetric elements are used to absorb heat, and the average temperature change rate of calorimetric elements is measured, and then the surface heat flow rate is calculated, such as Plug-shaped copper foil calorimeter.

The existing wall heat flow measurement method is to measure the heat flow intensity of one point at a time, which cannot solve the problem of real-time heat flow distribution in the entire area; moreover, contact measurement is required, and its accuracy depends on the tightness of the heat flow sensor and the measured object, the thickness of the heat flow sensor 1. The side length of the heat flow sensor is determined. For some complex structures, the installation of the sensor is inconvenient.

Contents of the invention

The purpose of the present invention is to provide a non-contact method for measuring heat flow distribution of unsteady wall heating based on infrared thermal image temperature measurement technology in view of the deficiencies of the current technology.

The purpose of the present invention is achieved through the following technical solutions, a non-contact method for measuring the heat flow distribution of unsteady wall heating based on infrared thermal imaging temperature measurement technology, the specific steps are as follows,

(1) Select an object with known density, specific heat capacity and thermal conductivity. The wall thickness is uniform and can be approximated as a flat plate. The Bi number is much smaller than 1. Use infrared temperature measurement technology to record the temperature field of the heating wall surface that changes with time;

(2) Since the Bi number is much smaller than 1, the thermal resistance in the thickness direction of the wall is ignored, and the object heating problem is simplified into a two-dimensional unsteady heat transfer problem, and the unsteady heat balance equation is obtained;

(3) Differentiate the temperature field measured in step 1) with respect to time and space, calculate the unsteady term of thermal temperature rise and the term of heat conduction, ignore natural convection and heat radiation, and calculate the heating according to the unsteady heat balance equation in step 2) heat flow.

In the above technical solution, further, the step 1) specifically includes: using infrared temperature measurement technology to record the temperature field of the wall surface of the object changing with time, and the recording process is basically unchanged from the initial heating moment to the temperature of the object.

Further, the step 2) is specifically: since the Bi number of the object is much smaller than 1, the lumped parameter method can be used, ignoring the thermal resistance along the thickness direction, considering that there is basically no temperature difference between the two surfaces of the object along the thickness direction, and heating the object The problem is simplified into a two-dimensional unsteady heat transfer problem, and the unsteady heat balance equation is obtained, namely

Thermal temperature rise unsteady item = heat conduction item + heat source item + natural convection item + radiation item

The equation contains 5 terms, from left to right are the time-varying unsteady term of the wall temperature due to heating, the heat conduction term of the wall itself, the heating heat source term, the natural convection term and the radiation term.

Further, the step 3) is specifically: deriving the temperature field of the wall surface measured by infrared temperature measurement technology with respect to time, and then multiplying the known density and specific heat capacity parameters of the wall surface material to obtain the unsteady item; The temperature field of the wall surface is derived from the space, multiplied by the heat conduction coefficient of the wall surface, and the heat conduction term is calculated. Experimental tests have found that in the early stage of heating, the wall temperature is not high, and the natural convection and radiation items are relatively small, which are at least 2 orders of magnitude different from the average heating heat flux intensity value, so the natural convection and radiation heat dissipation items can be ignored.

The beneficial effect of the present invention is that the present invention records the temperature field changing with time in the entire area of the wall through infrared thermal imaging technology, and then derivates the temperature field with respect to space and time, calculates the unsteady heat balance equation, and obtains the real-time temperature in the measurement area. Transient heating heat flow distribution. Different from the existing wall heat flow sensor which needs to contact the wall, the method realizes non-contact measurement and is applicable to the heat flow measurement of various complex walls. At the same time, the existing wall heat flow sensor can only measure the heat flow value of one point on the wall at the same time, but the present invention can simultaneously measure the heat flow distribution in a two-dimensional area, which greatly improves the measurement efficiency and is applicable to a wider range of engineering environments , has great engineering and production practical significance, and has great application prospects.

Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Figure 1 is a schematic diagram of the measuring device;

Fig. 2 is the flowchart of specific example method of the present invention;

Fig. 3 is the temperature field distribution at three moments when different heating sources are heated separately;

Figure 4 is a heat flow diagram corresponding to a certain moment when different heating sources are heated separately;

In the figure 1. Heating source 2. Heating plate (radius R, diameter d) 3. Infrared thermal imaging camera 4. Graphics processor.

detailed description

The present invention is an unsteady wall surface heating heat flow distribution measurement method based on infrared thermal image temperature measurement technology, and the specific steps are as follows:

1. Select an object with known density, specific heat capacity and thermal conductivity. The wall thickness is uniform and can be approximated as a flat plate. :

The infrared temperature measurement technology is used to record the temperature field of the wall surface of the object changing with time, and the recording process starts from the initial heating moment to the temperature of the object being basically unchanged.

2. Since the Bi number is much smaller than 1, the thermal resistance in the thickness direction of the wall is ignored, and the heating problem of the object is simplified into a two-dimensional unsteady heat transfer problem, and the unsteady heat balance equation is obtained;

If the Bi number of the object is much smaller than 1, the lumped parameter method can be used, ignoring the thermal resistance along the thickness direction, and considering that there is basically no temperature difference between the two surfaces of the object along the thickness direction, and simplifying the object heating problem into a two-dimensional unsteady heat transfer problem, we get The unsteady heat balance equation, namely

Thermal temperature rise unsteady item = heat conduction item + heat source item + natural convection item + radiation item

The equation contains 5 terms, from left to right are the time-varying unsteady term of the wall temperature due to heating, the heat conduction term of the wall itself, the heating heat source term, the natural convection term and the radiation term.

3. Deriving the temperature field measured in step 1 with respect to time and space, calculating the unsteady item of thermal temperature rise and heat conduction item, ignoring natural convection and thermal radiation, and calculating the heating heat flow according to the unsteady heat balance equation in step 2;

The wall temperature field measured by infrared temperature measurement technology is derived with respect to time, and then multiplied by the known density and specific heat capacity parameters of the wall material to calculate the unsteady item; the wall temperature field is derived with respect to space, multiplied by the wall surface The heat transfer coefficient is calculated to obtain the heat conduction term. Experimental tests have found that in the early stage of heating, the wall temperature is not high, and the natural convection and radiation items are relatively small, which are at least 2 orders of magnitude different from the average heating heat flux intensity value, so the natural convection and radiation heat dissipation items can be ignored.

The content of the present invention will be further explained below by way of examples. The experiment uses an aluminum circular plate with a thickness of 2.5mm and a radius of 152mm. The constant pressure heat capacity c _p is 880J/(kg·K), the density ρ is 2700kg/m ³ , and the thermal conductivity κ _p is 237W/(m· K), since the wall thickness is only 2.5mm, and the plate radius is 152mm, its Bi number is far less than 1, and the thermal resistance along the thickness direction is very small and can be ignored. An infrared imager is placed 1m above the plate and connected to an image processor. The entire experimental setup is shown in Figure 1. The heating source is an electric heating chip, the heating source 1 is a square heating chip, and the heating source 2 is a circular heating chip, all of which are constant heat flow heating chips. Experiments and calculations are carried out according to the flow chart shown in Figure 2. Fig. 3 is the distribution of the temperature field of the plate at three moments of t=10s, t=15s and t=20s under different heating conditions of the heater. The first row of Fig. 4 is the unsteady term distribution diagram of the wall surface changing with time obtained by calculating the first derivative of the temperature field with respect to time. Calculate the second order derivative of the temperature field with respect to the two-dimensional space, and obtain the distribution diagram of the space heat conduction item shown in the middle of Figure 4. The boundary condition is uniform heat flow boundary condition, according to the natural convection empirical formula and the thermal radiation formula W=σ(ΔT) ⁴ , in the initial stage of heating, various parameters are substituted to calculate the natural convection and thermal radiation items, and it is found that they differ from the unsteady or heat conduction items by at least 2 orders of magnitude, so the natural convection can be ignored term and heat radiation term, so as to calculate the heat source distribution diagram of the unsteady heating plate shown in the bottom row of Fig. 4.

Claims (4)

1. a kind of unsteady wall based on infrared thermal imagery thermometry heats heat flux distribution measuring method, it is characterised in that tool Body step is as follows：

(1) selection density, specific heat capacity and object known to the coefficient of heat conduction, uniform wall thickness, and its Bi number are much smaller than 1, using red The temperature field of the heating wall that outer thermometry record is changed over time；

(2) ignore object through-thickness thermal resistance, object heating problems are simplified to Two Dimensional Unsteady heat transfer problem, obtain non-fixed Normal equation of heat balance；

(3) by step 1) in the temperature field that obtains of measurement to time and space difference derivation, calculate the hot unsteady item of temperature rise and heat Conduction item, ignore free convection and heat radiation, according to step 2) unsteady equation of heat balance calculate heating hot-fluid.

2. the unsteady wall based on infrared thermal imagery thermometry according to claim 1 heats heat flux distribution measurement side Method, it is characterized in that, described step 1) it is specially：The temperature changed over time using infrared temperature-test technology record object wall , recording process is basically unchanged since the initial heating moment to object temperature.

3. the unsteady wall based on infrared thermal imagery thermometry according to claim 1 heats heat flux distribution measurement side Method, it is characterized in that, described step 2) it is specially：Because object being measured through-thickness thermal resistance is smaller, ignore along wall thickness direction Thermal resistance, so as to heating problems are simplified into Two Dimensional Unsteady heat transfer problem, builds equation of heat balance, i.e.,：

The hot unsteady item of temperature rise=heat transfer item+thermal source item+free convection item+radiation term

Equation includes 5 items, is successively from left to right wall surface temperature due to the unsteady item changed over time caused by heating, The heat transfer of wall itself, heat source, free convection and radiation term.

4. the unsteady wall based on infrared thermal imagery thermometry according to claim 1 heats heat flux distribution measurement side Method, it is characterized in that, described step 3) it is specially：The wall surface temperature that will be obtained by infrared temperature-test technology measurement was asked the time Lead, multiplied by the density and specific heat capacity parameter of known wall surface material, be calculated unsteady item；By wall surface temperature to space Derivation, is multiplied by the coefficient of heat conduction of wall, is calculated heat transfer, and at the heating initial stage, wall surface temperature is relatively low, free convection and Heat loss through radiation can be ignored, and so as to be calculated heat source according to unsteady item and heat transfer item, that is, heat hot-fluid.