EA022126B1 - Method of measuring radiant heat flux in vacuum - Google Patents

Abstract

The invention relates to heat engineering and can be used in thermal vacuum tests of space crafts, in orbital conditions, in vacuum technological processes. The technical result provides enhancement of accuracy and sensitiveness of measurements in low temperature zones and also widening functional abilities of a technological process of determining incident specific heat flow rate. A method comprises arrangement in a tested volume 12 of a thermal measuring receiver 1 with thermal sensors 3 and 4, spaced to a required base, measuring the thermal sensors signals and recording their temperature values in time. The incident specific heat flow rate is determined for time sequence τ(j=0, 1, 2, ...), set by two integer-valued parameters n and m as, and the specific heat flow rate qfor time τis determined by the formula

Description

The invention relates to the field of heat engineering and can be used in thermal vacuum tests (TWI) of spacecraft, in orbital conditions, in vacuum processes, etc.

A known method of measuring the intensity of radiant flux during thermal vacuum tests of spacecraft [1]. The method includes measuring the electrical resistance of an electrically conductive thermally sensitive element. According to the method, the electrical resistance of the electrically conductive receiving thermosensitive element, the temperature of its front receiving platform and the temperature of the back side of the supporting body of the special device with the thermosensitive element are determined, then the intensity of the specific incident radiant flux is calculated. The intensity of the radiant flux of the incident radiation to the controlled spacecraft (H _TVK ) and the effective radiation of the controlled spacecraft (h _KA ) is determined from the relations = k-o- ( _£ -T, ^L- T /), (1)

4 "= Κ · _Ο · [(^ 5) · Τ ₂ '- (1.8) · Τ, η, (2) max ^ Γπίη where - t / t is the relative optical parameter;

8 = 85/80 - relative geometric parameter - the ratio of the lateral surface area (8 _b ) of the device to the base area (8 _O );

(^ δΚΙ + δ)] is the dimensionless coefficient; σ is the Stefan-Boltzmann constant; σ = 5.67-10 ^-8 W / (m ² -K ⁴ ).

The disadvantages of the method are the large inertia, the inconvenience of using due to the need to obtain data in the steady state thermal regime for two positions of the special device that implements the method, as well as the presence of parasitic heat sinks through the attachment points of the device, which results in low measurement accuracy.

Also known are measurement methods that are used in calorimetric, thermoelectric and photometric radiant energy receivers [2]. In radiant energy calorimeters, a method for measuring radiant heat fluxes is used, in which the coolant flowing through the internal channel of the calorimeter is heated by absorbed radiant energy incident on the input area of the calorimeter. The inner surface of the calorimeter is blackened, which leads to the absence of selectivity for the wavelengths of the received radiation. The external thermal insulation of the receiver ensures that there is no influence of extraneous heat fluxes on its readings. The known heat carrier flow rate and its measured temperature at the inlet and outlet of the calorimeter determine the absorbed power. The intensity of the radiant flux is determined from the equality of power absorbed by the coolant and incident on the known input area of the calorimeter.

The disadvantages of these methods of measuring radiant heat fluxes are the large inertia, the inconvenience of use associated with the need to supply flexible tubes for supplying the coolant, as well as the limited temperature range of application, determined by the boiling and freezing points of the coolant used.

A known method of measuring radiant heat fluxes using bolometers, where a sensitive element is used, which is a conductive metal layer deposited on a dielectric that serves as an electrically insulating substrate [3]. The sensing element prepared in this way consists of a glass container in which a certain pressure of air or some inert gas is maintained. The cylinder has a window made of a material that is transparent to radiation from the spectral region for which the bolometer is intended. Wire bends from the ends of the conductive layer are brought out from the cylinder. Using sensitive equipment, the resistance of the sensitive element of the bolometer is measured, and the temperature acquired by the conductive layer (metal tape) due to the thermal radiation absorbed is determined by the value of this resistance. Thus, the intensity of the radiant flux is judged.

The disadvantage of this method is the low sensitivity and inconvenience of use associated with the need for thermal stabilization of the back element. The disadvantages of the method also include greater selectivity for a certain region of the radiation spectrum due to the limited transparency window of the material of the balloon, which leads to errors in the determination of radiant heat flux.

Closest to the proposed invention, a method for determining heat flux in gradient media, selected as a prototype [4]. The method includes placing a heat measuring device (receiver) in the desired area of the physical medium under study with temperature-sensitive elements - temperature sensors spaced at the required base, measuring the temperature sensor signal, recording the temperature difference over time, and then determining the heat flux. Various temperature-sensitive elements are used to measure temperatures. The principle of temperature measurement in order to determine the heat flux is as follows. Thermosensitive elements located at a certain distance from each other are placed in the studied area of the medium. In the process of observation, thermosensitive elements change their physical properties, taking the ambient temperature. Then, changes in the physical properties of thermosensitive elements are converted and recorded. Thus, currently used thermo-measuring devices contain two functional units: 1) a thermosensitive measuring probe (thermo-measuring receiver), inside of which are thermosensitive elements spaced a certain distance from each other; 2) conversion and registration unit.

The disadvantage of this method is the low sensitivity in the low temperature region, insufficient speed and reliability of the measurement process.

The objective of the invention is to remedy these disadvantages of increasing the efficiency of the process of determining the incident specific radiant heat fluxes.

The technical result of the invention is to increase the accuracy and sensitivity of the method in the field of low temperatures, as well as expanding the functionality of the technological process for determining incident specific radiant heat fluxes.

The technical result is achieved by the fact that in the method for measuring radiant heat flux in a vacuum, including placing in a test volume a thermo-measuring receiver with temperature sensors spaced at the required base, measuring the signals of temperature sensors and recording their temperature values over time, according to the invention, one temperature sensor is placed on the back of the receiving element of the heat receiver, shield this surface from the incident radiant heat flux by the body of the heat meter receiver On the other hand, the frontal flat surface of the receiving element is facing the incident radiant heat flux, and the second thermal sensor is placed in the housing of the heat-measuring receiver and the temperature values T ₁ are recorded at successive times ΐ ₁ (последовательн = 0, 1, 2, ...) ( ₁ ) a temperature sensor on the receiving element and temperatures T ₂ ( ₁ ) of the temperature sensor on the receiver body, with the hemispherical incident specific radiant heat flux o, for time η from the time sequence {τ} (ί = 0, 1, 2, ...), defined by two chains ramie n and m as the ~ ^{t + |} - calculated according to the formula where am I, = {p = –Σ ^? Λ 'O 2t + 1 ^{= Tt} '

C is the heat capacity of the receiving element of the thermo-measuring receiver;

δ is the area of the flat surface of the receiving element that receives radiation;

δ, π is the gap area between the receiving element and the bearing housing of the thermo-measuring receiver;

α is the thermal conductivity of the gap between the receiving element and the housing of the heat measuring receiver;

ε _ουί is the degree of blackness of the front surface of the receiving element;

⁸ erg is the effective degree of blackness in the gap between the receiving element and the housing of the heat-measuring receiver;

σ is the Stefan-Boltzmann constant.

The essence of the method is illustrated by drawings in FIG. 1-8, where in FIG. 1 shows a principal temperature measuring receiver, and FIG. 2-8 are parameters cyclograms illustrating the process of measuring a hemispherical incident specific radiant heat flux.

The temperature measuring receiver 1 comprises a housing 2 with a temperature sensor 4; receiving element 8 in the form of a plate with a frontal flat surface 9 and a temperature sensor 3 on the reverse surface 7, which is shielded by the housing 2 from the incident radiant heat flux c. from the surrounding test volume 12; racks 10 for spacing temperature sensors 3 and 4 to the required base; controller 5 connected by lead wires 6 to temperature sensors 3, 4.

- 2 022126

The principle of measuring a hemispherical incident specific radiant heat flux 11 in vacuum by the proposed method is as follows.

A temperature measuring receiver 1 with temperature sensors 3, 4 are placed in the test volume 12, which are carried relative to each other to the required base by means of load-bearing racks 10. In this case, one temperature sensor 3 is located on the reverse surface 7 of the receiving element 8 and shield the surface 7 from the radiant heat flux falling from the test volume 12 with | case 2 of the thermo-measuring receiver 1. The front flat surface 9 of the receiving element 8 faces the hemispherical radiant heat flux 11 being measured incident from the test volume 12. The second temperature sensor 4 is located in the case 2 of the thermo-measuring receiver 1. The temperature sensors 3 and 4 are connected to the controller 5 by lead wires 6, by means of which then, at successive times ΐ ₁ (ί = 0, 1, 2, ...), the values of Tcu temperatures at the receiving element 8 with temperature sensor 3 and temperatures T ₂ ( ₁ ) on the housing 2 with the term by the sensor 4 of the thermo-measuring receiver 1. The heat flux 11 of intensity d (1) incident on the receiving element 8 over time leads to time variations in temperature T ₁ (1) of the receiving element 8, as well as temperature T ₂ (1) of the housing 2. The temperature of the latter changes due to conductive and convective heat exchange with the receiving element 8 and with the environment, which is the test object (not shown in the drawing) and partially the sources of the measured radiant heat flux 11 (for the sides of the housing 2). The heat balance equations for the receiving element 8 and for the housing 2 under the assumption that they are made of highly heat-conducting material and the temperature inhomogeneities in them can be neglected, are written as

where C1 = C1t1 is the total heat capacity of the receiving element 8;

C ₂ = c ₂ t ₂ - full heat capacity of the housing 2;

t ₁ , t ₂ - the mass of the receiving element 8 and the housing 2, respectively;

with ₁ , with ₂ - specific heat of the receiving element 8 and the housing 2, respectively;

8ι is the surface area of the receiving element 8;

ε _1ου1 is the blackness coefficient of the front surface 9 of the receiving element 8;

α _{1; 2} - coefficient of thermal conductive coupling (reciprocal of thermal resistance) between the receiving element 8 and the housing 2;

βι, ₂ = θείϊδι - coefficient of thermal radiant communication between the receiving element 8 and the housing 2; σ is the Stefan-Boltzmann constant;

θείϊ is the effective blackness coefficient for radiant heat transfer of the receiving element 8 and the housing 2;

β ₂ , εην = θ ₂ , είτδ ₂ - coefficient of thermal radiative coupling between the housing 2 and the environment;

e ₂ , erg is the effective coefficient of blackness for radiant heat transfer between the housing 2 and the environment;

δ ₂ - the area of the outer surface of the housing 2;

T _ep „is the effective radiant temperature of the environment;

and ₂ , _about b_] - the coefficient of thermal conductive coupling between the housing 2 and the test object;

T _{about |} - temperature of the test object (not shown in the drawing).

If temperatures T ₂ (1) and T ₂ (1) for a certain sequence of time instants are known, then from [5] one can express the measured heat flux

All coefficients included in this expression can be calculated or measured. The time derivative of temperature is calculated, knowing its value in a number of subsequent time instants. The simplest way to calculate it is a finite-difference approximation.

However, with this method, significant errors can occur due to the fact that the temperatures at neighboring times do not differ significantly and their difference can be comparable with the error of temperature sensors 3 and 4. Therefore, in the proposed measurement method, the derivative is calculated as the linear regression coefficient T ₁ ( ₁ ) by 1; where 1; these are 2t + 1 consecutive times at which temperature is measured, and T ₁ ( ₁ ) is the measured temperature T ₁ at these times. This coefficient, designated K. _T1 , is the coefficient of the linear function T (1) = a + K _T1 1, which best approximates the dependence of the experimental data T ₁ ( ₁ ) on 1 ₁ .

- 3 022126

For the case when the values of the radiant heat flux 11 are recorded after every η moments of temperature registration, the coefficient K _{Т £} for the _) th registration of the flow is calculated as [5] where atr)

O-g, ίΛ (6)

Since when calculating the time derivative, 2t + 1 values of the measured temperature are used, then to increase the accuracy of calculations in the remaining terms of formula (5), which includes the measured temperature values, the average of these 2t + 1 values is used, i.e. (Т ^ (according to the first of the relations (8)) and its analogue for the case temperature (Т ₂ ) _3. Substituting (6) into (5) and taking into account the last remark, we obtain the calculation formula for the values of the measured flow at time moments η, determined the first of the relations (7) = 7 '- [с4С „-а, ₂ (ЗД) -> _? ) - А, ₂ ^ (<г ₂ >; - &) /)] + σ (τ ^, (9) which exactly corresponds to the ratio (1) according to the claims.

An example implementation of the method

To this end, we simulate the operation of the thermo-measuring receiver 1 on a mathematical model using the heat balance equations (3) and (4).

Let the temperature measuring receiver 1 be made as shown in FIG. 1, and the housing 2 and the receiving element 8 of the thermo-measuring receiver 1 are made of aluminum and have masses respectively of t ₂ = 9 g and t ₁ = 3 g.

The specific heat capacity of aluminum with _AZ = 1256 J / (kg-K), which allows to calculate the heat capacity C1 = Sd1t1 = 3.77 J / K, and C ₂ = Sd1t ₂ = 11.3 J / K. The calculated coefficients of thermal bonds between the body and the plate for this design are α _{1> 2} = 1.49 · 10 ^-3 W / K, β1.2 = ε6 £ δ1 = 2 · 10 ^-4 m ² , and the area and degree of blackness the outer surface of the plate are 81 = 1.96 · 10 ^-3 m ² and 8 ^ = 0.9, respectively.

We assume that the temperature measuring receiver 1 is located on the test object (not shown in the drawing) with conductive thermal coupling with it equal to α ₂ , _{ο £} , _ι = 0.01 W / K and radiant thermal coupling with the environment equal to β _{2> 6ην} = 1.2 · 10 ^-3 m ² . We set the temperature of the test object T, _{) b |} (1) and the ambient temperature for the sensor housing 2 1 _6ην ( _ί ), as shown in FIG. 2.

Let the radiant flux 11 d (1), defined as shown in FIG. 3. All of the above parameters are typical for the conditions of thermal vacuum testing of spacecraft. So the three levels of heat flux specified in the sequence diagram of FIG. 3, correspond to the effective radiant temperature in the hemisphere, respectively, 20, 80 and -196 ° C, respectively. The time interval on the given cyclograms corresponds to the duration of one turn of the Earth’s remote sensing satellite.

The temperature regime of the thermo-measuring receiver 1, with the above parameters and cyclograms of external loads, was modeled in the Ma1ysa4-14 package. The system of heat balance equations (3), (4) was solved numerically using the gkheb procedure under the initial conditions Т ₁ (0) = Т ₂ (0) = 20 ° С. The resulting temperature patterns of the receiving element 8 and the housing 2 are shown in FIG.

4.

Further, the process of measuring the heat flux was modeled as follows. Using the temperature arrays T1 and T2 found numerically, the functions were constructed using the linear interpolation method to find these temperatures at any time. Using these functions, for a sequence of time instants equally spaced with a step Αίι = ίΔΐ, the true temperatures / о and% corresponding to these moments are calculated.

The temperature measurement process was simulated by noise using a random number generator of these true values by a random additive distributed randomly with dispersion corresponding to the error of temperature δΤ temperature sensors 3, 4. As a result, we obtain two arrays of experimental (inaccurate) temperatures T ₁ ( ₁ ) and T _hell modeling the results of field measurements. In FIG. 5 shows a fragment of cyclograms of experimental temperatures between 10 and 15 minutes for the true temperatures shown in FIG. 4. The data correspond to the time interval between measurements Δΐ = 4 s, the standard error of the measurement is 5T = 0.1 ° C.

Then, the arrays of experimental temperatures and the array of moments of time were processed in accordance with the calculation algorithm (6) - (9) for the values η = 4 and m = 8, and the measured heat flux with | ^ex |. Calculation results in comparison with a given (or real) heat flux s. |

- 4 022126 are presented in FIG. 6.

For comparison, in FIG. 7 shows the results of processing the measurement data according to an algorithm that does not take into account the heat capacity of the receiving element 8 of the thermo-measuring receiver 1, i.e. without the first term in square brackets of relation (9) and calculations of the derivative of temperature with respect to time.

In FIG. 8 shows the simulation results of the measurement of heat flux for the standard error of the temperature measurement of the receiving element 8 and the housing 2 of the heat measuring receiver 1 of the standard error of the measurement 5T = 0.2 ° C, which, in comparison with FIG. 6 illustrate the effect of temperature error of temperature sensors 3, 4 on the accuracy of heat flow measurement.

The accuracy of the proposed measurement method is also influenced by how accurately calculated or measured the coefficients included in the calculation formula (9). The measurement accuracy can be improved by introducing the calibration procedure of the thermo-measuring receiver 1 by adjusting these parameters to best match the measured readings with the given values of the incident flow 11 under the conditions of a given heat load.

The developed method was tested in real conditions for measuring the hemispherical incident specific radiant heat flux in vacuum during the creation and design of space objects and provided the measurement of the incident specific heat flux with high accuracy and sensitivity in the range from 10 to 1400 W / m ² in the temperature range from -196 to + 120 ° C.

The method allows to control the actual specific heat fluxes of the studied objects with simultaneous indication and recording of data in a personal computer (PC), as well as to exchange data with a PC. The developed method for determining the incident specific radiant heat fluxes and the calculation model significantly expand the functionality of the method, which can also be used in various industrial vacuum processes.

Information sources.

1. KI No. 2354960, 05/10/2009.

2. Andreichuk O.B., Malakhov N.N. Thermal tests of spacecraft. - M.: Mechanical Engineering, 1982, p. 130.

3. Markov M.N. Infrared receivers. - M .: Nauka, 1968, p. 50, 56, 254.

4. KI No. 94025742 A1, 05/10/1996 (prototype).

5. Radchenko S.G. Regression Analysis Methodology: Monograph. - K .: Korniychuk, 2011.- p.

376.

Claims (2)

A method of measuring radiant heat flux in a vacuum, including placing a thermo-measuring receiver in the test volume with temperature sensors spaced at the required base, measuring the signals of temperature sensors and recording their temperature values over time, characterized in that one temperature sensor is located on the back surface of the receiving element of the temperature measuring receiver, screened this surface from the incident radiant heat flux by the housing of the heat-measuring receiver, the frontal flat surface of the receiver about the element is turned to the incident radiant heat flux, and the second temperature sensor is placed in the body of the heat measuring receiver and the temperature values T ₁ ( ₁ ) of the temperature sensor at the receiving element are recorded at consecutive times ΐ ₁ (ί = 0, 1, 2, ...) temperature T ₂ ( ₁ ) of the temperature sensor on the receiver’s body, with the hemispherical incident specific radiant heat flux s.], for time moment η from the time sequence {tD (] = 0, 1, 2, ...) given by two integer parameters pit like 1 / «+ 2yu Τ, = - ± - Σ / 2t + 1, calculated by the formula where 2t ('Ή - (//} * 2 / and - V //, 1 + 1 - 5 022126 Μ, = 2sh + 1 at + 2t Σν, | => and where C is the heat capacity of the receiving element; δ is the area of the site receiving the measured radiation; δ _ιη is the area of the gap between the receiving element and the bearing body; α is the thermal conductivity of the supporting racks between the receiving element and the housing; ε _ουΐ - the degree of blackness of the receiving area; ε ₆ ίτ is the effective degree of blackness in the gap between the receiving element and the housing; α is the Stefan-Boltzmann constant. FIG. 5 - 7 022126 FIG. 8