SU1781563A1 - Method of determination of local connective heat transfer ratio - Google Patents

Description

The invention relates to heat engineering and can be used in the assessment of con-. heat transfer between the object; radiating heat, and a cooling medium, for example, in the automotive industry, metallurgy, aviation industry, energy, etc.:

Known methods for determining the heat transfer coefficient based on the use of the Newton-Richmann law, according to which a = Q / S (ti - g ₂ ) [W / (m ² · K], where Q is the heat flux on the wall of the investigated object, ti is the temperature object surface;

t2 is the temperature of the cooling medium,

S is the surface area of heat transfer,

The known methods are inherent in the lack of accuracy in determining the heat transfer coefficient due to errors in measuring the heat flux, the surface temperature of the object and the temperature of the cooling medium, as well as not; the need to account for surface area: heat transfer. The noted shortcomings limit the widespread use of investment; nyh methods for determining the coefficient: heat transfer,;

There is a method of determining the local heat transfer coefficient '<%, including heating the sensor, thermal i flow, installed directly; proximity to the object, and measuring the heat flux Ch, the ambient temperature Toil and the temperature T _H i of the surface of the heat flux sensor in the mo-; the moment ti of equality of its temperature Tm i to the surface of the object, with the subsequent determination of cc> by measured values.

The disadvantage of this method is the low accuracy under conditions of unsteady heat transfer, due to the influence of the measurement error T _n <.

1781563 A1

The purpose of the invention is to improve the accuracy of the Definition of Οι under conditions of unsteady heat transfer.

This goal is achieved by the fact that in the known method, the heat sensor 5 is heated to a temperature Tki ± (0.01 - 0.1) Tki by current pulses with a repetition period equal to (3-20) t, where r is the thermal constant time of the sensor, 'and the value "1 is determined by the formula: Yu _and Qi' __ * '(T _k1 -T _Hi ) - (Тo ·, - Κι)' where Κι is the value of the coefficient of pro-proportion. ovality, corresponding to the value ΔΤ ₂ ι = (Τοί - Τ ,, ί) at ΔΤϋ = (Tki - Thi) = 0 and obtained during the sensor calibration.

In FIG. 1 shows a block diagram of a heat flow sensor of a device ² θ implementing the proposed method; in FIG. 2 is a diagram of a device for determining a local heat transfer coefficient; on .. FIG. 3 change in temperature of the heat flow sensor.

The heat flow sensor I (see Fig. 1) includes a temperature sensor 2, a thermal power converter 3 to an electric signal and a heater 4. Moreover, a heater 4 is installed on one side of the surface of the thermal power converter 3, and a temperature sensor 2 on the other side. From the output of the sensor 2. the EMF is removed, and from the output of the converter 3 is a signal whose actual value is proportional to the thermal power Qi. The temperature of the heater 4 is controlled by changing the current of the input signal,

As a sensor 2, a thermocouple is used. To convert the thermal ⁴ θ power into an electrical signal, a chemical current source (CIT) is used, loaded on an exemplary resistor Ro. As block 4, an electric heater is used, powered from a controlled source.

The temperature T of the heater 4 is measured using a sensor 2 (thermocouple) located on the surface of the heated thermal power converter 3. The heat flow sensor 1 itself is located at the level of the object under study. Moreover, to prevent heat exchange between them, a heat insulator is installed. A cooling stream (or medium) with a temperature T _о is created using a controlled chiller. As a cooling medium, a substance in a liquid or gaseous state can be used, for example water or liquid nitrogen, etc. If, for example, water is used as a cooling medium, the test object, heat flow sensor, heat insulator, cooler and temperature sensor are isolated from the influence of the cooling medium by .; for example, confined spaces.

In FIG. 2 shows a block diagram of one embodiment of a practical realization of the proposed method of determining the heat transfer coefficient, wherein 1 - sensor heat flow, consisting of the sensor 2 temperature T _n heat converter 3 thermal power to an electrical signal th heater 4, 5 - the object under study, 6 temperature sensor Tk of the studied object, 7 - temperature sensor T _{about the} cooling medium, 8 - cooler, 9 - cooler control unit, 10 - first temperature meter To, 11 - second temperature meter Tk, 12 - first differential lnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnufo numerizennnnnnnnnnnn; 19 - heat insulator, 20 - chamber or pipeline.

The invention consists in the following.

Change the temperature T _{n of the} heat flux from the surface of the sensor 1 according to a sawtooth law by heating the sensor 1 with periodic current pulses supplied to the heater 4 with a repetition period of (3-20) g, where τ is the thermal time constant of the heat flux sensor 1. Heating is carried out to a temperature. Tki ± (0.01-0.1) Tk.

The temperatures Tki, T _O i and T _H i of the surfaces of the test object, the cooling medium and the heat flux sensor are measured, respectively, and then the temperature differences are determined

ΔΤιι = Tn - You (1) and

DT ₂ | = That! -Tn ₁ (2)

At times Ti, ti and tn-ι (see Fig.

3) the equalities of the first temperature difference (1) of the values of + Δ T, O and -Δ T, respectively, measure the heat flux Qi proportional to the output of the heat flux sensor 1, the temperature values Tki and Τοί. Temperature changes

1781563. 6, heating is selected from the condition ΔΤ = (0.01 0.1) Tk, where Tk is the surface temperature of the investigated object. In other words, Δ T does not exceed (1-10)% of Tk. The intervals ± ΔΤ in FIG. 3 are indicated by dashed lines 5 parallel to the time axis.

It should be noted that the duration of the period (cycle) of the surface temperature change of the sensor 1 depends on the thermal conductivity 1C of the structural elements of the heat flow sensor 1, in particular, for example, a heat power converter. from the area of the radiating surface and from the temperature of the cooling medium, 15 and in general, from the thermal time constant g.

Suppose that at the ti-th moment of time the measured value of the heat flux is 5 Qi, 'and the values of the surface temperatures of the test object and the cooling medium are equal, respectively, Tki and Т _О |.

By the value Δ Τι of the second difference (2). temperature corresponding to the instant of the fact that the first temperature difference (1) is equal to zero, i.e. when Δ Tn - 0, choose the corresponding discrete value of the proportionality coefficient K). Assume that the value Δ Τ ₂ ι (at Tm = T _H i) 15 corresponds to the value Κι selected from the table of n values ΔΤι Κ ι:

δτ ₂ ΔΤ ₂ 1 ΔΤ22 ΔΤ23 ' i ... ΔΤ21 ΔΤ2π to K1 to ₂ KZ ... ·! ..,. Ki ... Κη

obtained when calibrating the characteristics of the sensor 1 of the heat flux and approximating it with n steps (discrete) of the curve. /

The true. Value of the heat transfer coefficient is judged by the expression _:, ^{=? = K |} ΔΊΊ | -Δτ2 | ^{= K |} PM -T n0 - (To! -TN |):

-. where Qi is the value of the heat flux at the nth time moment; '

Κιdiscrete value of the coefficient C of proportionality corresponding to the value ΔΤ ₂ ι; .

LTi ΔΤ ₂ ι - the first and second temperature differences;

Tki, T _H | and That,; temperature values from the surfaces of the studied object, the heat flux sensor, and the cooling medium at the ΐth moment in time.

Sensors 6.2 and 7 (Fig. 2) form electrical signals with it. Etn and E ₁₀ proportional to the temperatures of the surfaces of the studied object 5, the primary transformer of the 1st cooling medium. For this, sensors 6 and 7 are installed, respectively, on the test object 5 and cool t body 8.

The thermal power converter 3 generates a signal Et. proportional to the heat flux Qi.

The signal Etk of the sensor 6 is fed to the input E of the second temperature meter 11 and to the direct input of the first differential temperature meter 12. The signal Ε _Η ι of the sensor 2 is fed to the combined inverse inputs of the first and second E differential meters' tempera, round 12 and 13. ·. The Eto signal of the sensor 7 is fed to the input of the first temperature meter 10 and to the direct input of the second differential temperature meter 13. The signal E _p from the output of the thermal power converter 3 is fed to the input of a millivoltmeter 13 of effective values.

The current source 15, the generator 16 and the cooler control unit 9 are manually controlled.

When determining the heat transfer coefficient using the control unit 9, a mode of operation of the cooler 8 is established, which ensures that the temperature of the cooling medium is equal to, for example, T _o , '. . :

Using the modulating signal generator 16, a certain value of the period of the modulating signal and its amplitude are set, providing a change in the heating temperature of the heater 4 within. ± ΔΤ. It should be noted that the period of the modulating signal is selected taking into account the time constant τ of the establishment of transient thermal processes (T = (30-20) t).

The Δ value is set in the range (0.01-0.1) · Tk taking into account the following circumstances: firstly, the random component of the measurement error with a probability of 0.997-0.9999 should be in the range (3-4) σ, where σ is the mean square temperature measurement error: secondly, the inertia of the heater does not provide an instant change in temperature after. '1781563' 8 changes in control action. Therefore, taking into account the inertia of the heater, the interval Δ Τ is chosen deliberately larger than by the first condition.

Using a controlled current source, select this power value! _n heater 4, which provides heating it to a temperature Tn = Tk. Under the influence of the modulating signal of generator Ϊ6, a sawtooth law of changing the temperature of the heater relative to the temperature of the object under study is achieved (see Fig. 3). . ^: ·· To eliminate the influence of the temperature of the test object 5 on the heater 4, a heat insulator 19 / placed between the 5th blocks 1 can be used. In some cases, this is not necessary. On the-. For example, in the case of a low-power current source 15, additional thermal power of the test object 5 is used. In other cases, the heat insulator 19 is not used because of the difficulty of its installation and ensuring reliable thermal insulation.

After setting the set value of the supply current of the heater 4 and the amplitude of its change (Δ I), the button 17 is turned on. As a result, the surface temperature of the Heat flux Sensor 1 will change according to a sawtooth law, as shown in FIG. 3.

Using the first differential temperature meter 12 measure the value of the first temperature difference (1). Let us assume that as a result of measurements we have obtained '·;

Ni = (Etc - Eth) Si (1 +} '. Ι) + ΔΜί,' (5) / where ΔΝι is the additive component of the measurement error. <

(Etc - E _T h) Si) -h is the multiplicative component of the measurement error:

- the steepness of the transformation.

Etc - Ethn-difference of the signals of the sensors 6 and 2 supplied to the Differential temperature meter 12. Using the second, differential temperature meter 13 measure the value of the second temperature difference (2): *

N2 = (Eto - Eth) S ₂ (.1 + η) + ΔΝ2. ' (6) where LMg is the additive component of the measurement error;

(Eto - Eth) S2 Ϋ2. - the multiplicative component of the measurement error.

- the steepness of the transformation.

Eto - Etn — the difference of the signals of the sensors 7 and 2 supplied to the second differential temperature meter 13.

Since the control of the supply current of the current source 15 is carried out from the modulating signal generator 16, the measurement of the supply current from the ΔΤ values of the block 12 is not performed. At time ti, the first temperature difference is equal to zero, i.e. when ΔΤΐί = O, determined by the readings of the digital reading device of block 12, the millivoltmeter 14 measures the effective value of L /, the output signal of the thermal power converter 3; '. /. . ' N3 = ViS ₃ (1 + АМз, '(7) where Vi is the actual value of the converter output signal;

- the steepness of the transformation ;:

LMZ is the additive component of the measurement error;. 'Τ.

V1S3 Uz ~ the multiplicative component of the measurement error.

At the same time, using temperature gauges 11 and 10, determine the temperature values Tri and ίο уточ:; ''N4 = Е _TK S4 (1 + ”τ Δ N4..' (8) t

N5 - E _ro Ss (1 + J's) ΔΝ5, (9) where Етк and Ето are the output signals of sensors 6 35 and 7, respectively; ' and Ss is the steepness of the conversion of signals into code of blocks 11 and 10;

Δ N.4. and. ΔΝε - additive errors of temperature measurement using meters 11 and 10, respectively,

EtkS4 / 4 and EtoSs are the multiplicative components of the measurement error.

At the tjth moment of time, corresponding to the vanishing of the first temperature difference, i.e. DTc = 0, at the output of the first differential temperature meter 12 a short pulse is formed. This impulse arrives at. control inputs of the output registers of TO, 11, and 13. As a result, the measured values N4, N5, and N2 of the temperatures Tki and Ty and the second temperature difference ΔΤ2Ι are stored. Resetting is done manually by pressing the reset buttons (not 55 are shown).

According to the testimony of the second differential temperature meter 13. corresponding to the value N2 of the second temperature difference (2), at the time equal to .1781563 zero of the first temperature difference, select the discrete value N. to! proportionality coefficient Ki according to the tabular data obtained during calibration of the heat flow sensor 1 ..

Using a calculator 18, determine the value of the heat transfer coefficient according to the expression of a sawtooth law by alternately changing the current supply to the heater within the specified limits. This reduces the time required to reach the set measurement mode.

The obtained results of determining the heat transfer coefficient (at Tk = const during the measurement time) can be processed according to the expression of a similar expression (3). , _. _ r. <where η is the number of dimensions

In contrast to the famous ones proposed. g— ..

. ^k ., 1 s in order to reduce by a factor of νη the random method for determining the local coefficient 15 ... _ ···.

„Sl. Tääläääää plgoyshipg ™ ıamapayiya. The heat transfer element is characterized by increased measurement accuracy. This is achieved by eliminating the additive component of the temperature measurement error and decreasing g. by taking it into account when choosing dis. coefficient values, proportionality Ki. When determining chastnogo'ot dividing provided susche- _t: governmental influence multiplicative decrease ^rate P ^and 25 _to tours, kativnoy component errors determine the temperature difference. Its complete exclusion is achieved only at ΔΤ?.! Values corresponding to the middle di ·. temperature ranges at F 'stepwise approximation of the calibration characteristic. In addition, improving the accuracy of determining the heat transfer coefficient is achieved due to exclusive. errors from the nonlinearity of the calibration curve of the sensor. 'by ·. selection of discrete values of the proportionality coefficients Ki for discrete _: values of the temperature difference ΔΤ21, corresponding to moments of time equal to 4 zero of the first temperature difference, i.e., at · .. · '. · ΔΤϋ- = 0. ''

In the proposed method, an increase in accuracy is also achieved through high-precision determination ............,

Unlike prior art, the proposed component measurement errors. · If unsteady temperature Tc can usefully be measured depending Oi (ti) of the multiplicative sostav- 20A ^eniya) heat transfer coefficient Issled. - p '* Gl zvr g »rch Rchp λι / το» 5 о ατλιι λληιιι лг η π ητι averaging the results over a certain time interval, taking into account the non-stationary (or measurement frequency) heat transfer element, differs during the whole measurement time (of the observed object) and then exercise

Claims (1)

'Claims A method for determining a local coefficient. heat transfer, which includes: heating the heat flow, 3Q installed in the immediate vicinity of the object, and measuring the heat flux. ka Qi, ambient temperature T _O i and temperature T _M i of the sensor surface tep. of a new flow at the time ti of equality of its temperature to the Tu surface of the object, with the exception of the fact that, in order to increase accuracy in conditions of unsteady heat transfer, the heat flow sensor is heated to a temperature Tki ± (0, 01-. 40 0.1) Tki by current pulses with a repetition period equal to (3-20) t, where t is the thermal time constant of the sensor, and the value of the local heat transfer coefficient is determined by the formula: temperature values Τηϊ. This is achieved at the expense of a highly defined time point of the Dimension. ’. Improving the speed of determining the heat transfer coefficient a is achieved by measuring the temperature T _{n of the} heat flux from the sensor surface by ^{; 0} . .. ^Ki (Tki - THiXToi -Μ * 'where Ki is the value of the proportionality coefficient corresponding to the value ΔΪ2! = (Toi - Т _Н |) at ΔΤΐί = (Тк) -Тн)) = 0 and. obtained during the calibration of the sensor. Ί | r.sj.c I '. I ..w J I 7 'I • bhodzhH / signem' ... I - I. T.S.o.e f I ^ 9- '_____ | 77 ^Guards * I 3jt. nyagre & zgsh g; ι: ι • I I ι ι> * ^a i- ~ * 4 ι ^^ l / learn / ⁴ ^ ·. I WMWWfV? • »3 tfSTQWW S »> vi shadows i i Figure 2