RU1778657C - Method and pilot plant for determining heat transfer coefficient - Google Patents

Abstract

Purpose: experimental equipment, heat transfer in a recuperative heat exchanger with a complex plate-fin surface with identical heat transfer surfaces along the heated and cooled sides. SUMMARY OF THE INVENTION: A test heat exchanger is installed in the circuit of an experimental setup between a process heat exchanger and a heater. Heat carrier - air is pumped through the circuit by a supercharger, heating is carried out using a heating element. By measuring the flow rate of the heat carrier by the flow meter and the parameters of the heat transfer medium at the inlet and outlet of the heat exchanger of the pressure and temperature sensor, the heat transfer coefficient is determined through the heat transfer coefficient taking into account the correction for the change in the properties of the heat transfer medium, while adjusting the power in the heating element and the amount of air delivered by the supercharger, they support the difference average temperatures on the hot and cold sides of the heat exchanger within 20-40 °. 2s and 1 z.p. crystals, 1 ill., 1 tab.

Description

The invention relates to an experimental technique for studying heat transfer and relates to the improvement of means for determining the thermal characteristics of heat exchangers, mainly of a plate-ribbed regenerative type.

When creating power plants and energy saving technologies with gas coolants, efficient heat exchangers with developed plate-fin surfaces were widely used.

The complexity of the thermal processes occurring in them does not allow one to calculate the characteristics under such conditions using theoretical models. This problem can only be solved by a calculation-experimental method.

Various methods are known for determining the heat transfer coefficient for a finned heat-exchange surface, the problems of cooling computer components are considered in detail, the main types of heat-exchanging surfaces of which are lamellar and lamellar

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about eating VI

shaved. In this case, the most important share in thermal resistance is determined by the heat transfer coefficient from the heat exchange surface to the coolant (or vice versa).

The value of the heat transfer coefficient depends on many factors, varies widely and requires experimental determination for a specific design of the heat transfer surface. In this case, the heat transfer coefficient is determined by calculation and experimental means. However, a method for determining the heat transfer coefficient known from this source gives a significant error of 10-12%, requiring a large amount of experimental work.

Also known are works devoted to the study of heat transfer in plate-fin regenerative heat exchangers, which show methods for determining the heat transfer coefficient specific to such heat transfer surfaces and the corresponding experimental setups.

A known method of research in heating air by condensing water vapor. However, it requires the creation of a complex steam supply system, an installation with precise control of pressure and steam content of the heating medium. The error of this method is up to 5%.

A heat exchanger method and an experimental apparatus for its implementation are known, where a methodology for calculating the experimental determination of the heat transfer coefficient is given. The determination method known from this source provides a methodological error of the order of 2%, due to the error in the determination of the heat transfer coefficient for water. This method is based on setting a sufficiently small thermal resistance on the water side with respect to the same value on the air side. As a result, the heat transfer coefficient of air tends to the heat transfer coefficient. Significant water consumption (up to 2 kg / s) is required to obtain large heat transfer coefficients of water. As a result, the experimental setup is carried out bypass with its measurement system. All this leads to significant material and energy costs for research. In addition, the disadvantage of this method is that it requires precision measurement of small differences in water temperatures (0.4-2.5 ° C) and the average integral values of the two-dimensional temperature field in the cross section of the air flow before and after the heat exchanger,

Of the known technical solutions, the closest object to the claimed is a thermal wind tunnel for testing a gas-liquid heat exchanger, taken as a prototype. The method for determining the heat transfer coefficient at this object is described in detail in the mentioned book by G.I. Voronin, E.V. Dubrovsky, with. 31-38 and consists in heating the heat carrier, pumping it through the studied side of the heat exchanger and cooling it on the other hand with high flow rate water, in measuring the flow rate, temperatures of the heat transfer medium and water at the inlet and outlet on each side, followed by calculation of the heat transfer coefficient over the surface under study.

The experimental setup adopted for the prototype contains two circuits with different heat carriers, equipped with a heater, heat exchangers, a circulator, a flow tank, pressure, temperature and flow meters, as well as pipelines and valves.

The object adopted as a prototype provides research with the subsequent determination of the heat transfer coefficient for the studied heat transfer surface. However, the object cited as a prototype has a significant drawback consisting in the large expenses for research work, caused by the need to create an additional circulation loop for the liquid coolant, the manufacture of accurate means for measuring small differences in liquid temperatures and the average integral two-dimensional temperature field in the cross section of the air flow, as well as . insufficiently accurate determination of the heat transfer coefficient (methodological error of about 2%).

The aim of the invention is, in the case of making the heat transfer surfaces of the test heat exchanger identical, to increase the accuracy of the method by determining the heat transfer coefficient with a methodical error of not more than 1% while reducing research costs.

In order to achieve the set goal when carrying out a method for determining the heat transfer coefficient, which consists in heating the heat carrier and pumping it through the recuperative heat exchanger under study, in measuring the temperature of the heat carrier at the inlet and outlet of each side of the heat exchanger with the subsequent determination of the heat transfer coefficient from the measurement results, the heat transfer agent is passed sequentially through cold and hot side of the heat exchanger, and its heating is carried out after leaving the cold side of the latter, under erzhiva average temperature difference for hot - hot and cold sides of the heat exchanger in the range of 20 to 40 degrees.

To implement this method in an experimental setup containing a circuit with a coolant, equipped with a heater, heat exchangers, blowers, a flow tank, pressure, temperature and flow meters, as well as pipelines and valves, the coolant outlet from the studied heat exchanger on the cold side connected to the outlet of the same heat exchanger on the hot side through the heater, in addition, in order to increase the temperature of the coolant at the inlet of the cold side of the heat exchanger, technologically heat exchanger hot side input of which is connected to the output of the test coil on the hot side and cold side are connected to them in series.

A distinctive feature of the proposed method for determining the heat transfer coefficient is that the coolant is passed at the same flow rate sequentially through the cold and hot sides of the heat exchanger, in which each side is made of heat exchange surfaces with an identical channel profile and the same flow cross section, at In this case, the heat carrier is heated to a predetermined temperature after exiting the cold and before entering it into the hot side of the heat exchanger in such a way that the difference in average temperatures p of the hot and cold sides is from 20 to 40 degrees. In this case, the generalization of the heat transfer coefficient is carried out in the same way as in other known methods in the form of a criterial dependence of Nu Nu (Re) according to experimentally obtained data. However, in contrast to the known method, the identity of the channel profiles and the equality of the bore sections on each side of the studied heat exchanger. the same flow rate of the coolant, and the same flowing sequentially through the cold and hot sides, allows the adoption of Re idem, taking into account the fact that with limited heating, the thermal properties are almost unchanged. Neglecting the small value of the thermal resistance of the heat transfer surface, the tolerance of the equality of the coefficients of thermal efficiency of the heat exchange surfaces on each side, taking into account the equality of the thermal resistance of the coolant on the cold and hot sides, we obtain a fairly simple expression (see example) for calculating the heat transfer coefficient.

The restriction on heating the coolant in the implementation of the claimed method, namely; heating with a difference in average temperatures on the hot and cold sides of the heat exchanger from 20 to 40 degrees, which is also one of the distinguishing features, allows not only to simplify the calculation1, but is also the main condition for achieving a higher accuracy than in the prototype.

The error in calculating the heat transfer coefficient in general is calculated as follows:

to-j a

(

It can be seen from the formula that the error in determining a depends on the error in measuring the difference between the average temperatures of the coolant and the hot side taken into account in the second term of DDT / AT, as well as the error in calculating the coefficient mp, which reflects the change in the thermophysical properties of the coolant as a function of temperature on different sides of the heat exchanger. It is calculated as follows;

- + fefe

The error of its determination:

V

+

P

7f +

PL .5

In the formula, n is the exponent with the Reynolds number (when processing the results n 0.7). D p is the difference between the exponent obtained by summarizing the experimental data from 0.7. It can be seen from the formula that with an increase in the temperature difference between the heat carriers on both sides of the heat exchanger, the methodical error Agr / tp increases. As it decreases, the total error in determining the heat transfer coefficient increases due to an increase in DD G / DT. It was found by calculation that in the range of DT 20-40 ° the methodical error does not exceed 1% with a sufficiently low error of determination of a (no more than 3%), which is also confirmed experimentally (see table).

It can be seen from the table that at D PM 20 K, the error in determining the temperature head and the total error in calculating the coefficient increase, while at D K the methodological error increases. The adoption of such a value of DT allows to remove the requirement for precision measurements of the temperature of the coolant characteristic of the prototype. In this case, it is sufficient to have a temperature sensor error of about 0.3 °.

The inventive method has been developed and is most effective for recuperative heat exchangers of the gas-gas type, which can significantly reduce the cost of heating the working fluid, eliminating the need to create a second circuit with its own measurement system, which reduces the cost of the experimental setup.

The effect of heat recovery in the experimental setup is realized due to the fact that the outlet of the working fluid from the test heat exchanger on the cold side is connected to the inlet of the same heat exchanger on the hot side. This effect can be enhanced, and energy costs for experiments can be reduced if heating of the working fluid at the inlet of the cold side of the studied heat exchanger is necessary, when an additional process heat exchanger is installed on the cold side of the test object, the hot side of which is connected to the outlet of the tested heat exchanger on the hot side.

Thus, the distinctive features of the claimed method and the experimental setup for determining the heat transfer coefficient allow, at reduced research costs, to obtain more accurate (with a methodical error of no more than 1%) heat transfer values.

The drawing shows a diagram of an experimental setup embodying the claimed method for determining the heat transfer coefficient.

The installation contains a circuit with a working fluid, an open loop, when air is used as a working fluid. The installation includes an air intake 1, a supercharger 2, a supply tank 3, flow meters 4, pressure 5, temperatures (sensors) 6–9, a test heat exchanger 10, a heater 11 with a heating element 12, a process heat exchanger

13, piping and fittings 14-17 for switching flows. In this case, the heater 11 is installed in the circuit so that the output of the worker is connected to the input to it

bodies from the test heat exchanger 10 along the cold side, and the outlet of the heater 11 is connected to the hot side entrance to the test heat exchanger 10.

Technological heat exchanger 13 is installed on the bypass in front of the test heat exchanger 10 and with the help of valves 16 and 17 it can be connected to the circuit or disconnected from it. The hot side of the heat exchanger 13 is connected to the output through

the hot side from the test heat exchanger 10, and using valves 14 and 15, air can be discharged directly into the atmosphere (through valve 14) or through the hot side of the heat exchanger 13.

The claimed method for determining the heat transfer coefficient is carried out as follows.

For testing, a heat exchanger 10 is made with the same channel profile

and with the same living cross-sectional area on each side; hot and cold. To do this, the studied heat exchanger is made according to a cross-flow scheme in which each side is drawn from

the same number of layers having the same geometric parameters.

The heat exchanger 10 is installed in the circuit of the experimental stand, as shown in the diagram (see drawing), include a supercharger 2 and a heater 12. The working body of the stand shown in FIG. 1, is the air that is taken through the air intake 1, and the supercharger 2 is pumped through the supply tank 3, which serves

to stabilize the flow rate measured by the flowmeter 4, and to damp the pressure fluctuations P, measured by the sensor 5, into the installation circuit.

Depending on the level of the operating temperature of the coolant at the inlet of the cold side of the object 10, one of the valves 16 or 17 is opened. When the heating of the coolant is not required, the valve 16 and air through the valve 7 are supplied on the cold side of the heat exchanger 10 to the heater 11, in which, using the heating element 12, the air is brought to operating temperature 13, measured by the sensor 8. Then, heated air is passed along the hot side of the heat exchanger 10 and discharged into the atmosphere through valve 14. The temperature at the outlet of the heat exchanger 10 is hot Oron T4 measured sensor 9. On the cold side inlet temperature Ti and the output T2

measured respectively by sensors 6 and 7, air pressure P by sensor 5.

When the operating temperature at the inlet of the cold side of the object is increased in order to avoid installing a heating element at the inlet and accordingly reduce energy consumption, the valve 17 is closed, the valve 16 is opened and air is pumped to the cold side of the tested heat exchanger 10 through the process heat exchanger 13. At the same time, on the hot side of the heat exchanger 13, air is passed from the outlet of the heat exchanger 10 on the hot side, for which the valve 14 is closed and the valve 15 is opened.

With such a scheme for switching on heat exchangers 13, 10 and heater 11, the heat spent on heating the working fluid is maximally regenerated, which reduces energy consumption, in addition, the difference in the average temperature along the hot and cold sides of the heat exchanger 10 is easily kept within specified limits (20, 40 °) at increased working fluid temperatures up to 800-1000 ° С.

During the tests, the power actuated by the heating element 12, as well as the air supply G by the supercharger 2 are controlled so that the average temperature difference between the hot and cold sides of the heat exchanger 10 is from 20 to 40 °.

The arithmetic mean temperature head in the heat exchanger 10 is equal to

(T3-T2) + (T4-Ti) -ЈjT.

Correction factor unit takes into account the cross current of the working fluid. Taking into account that the costs through the hot and cold sides of the heat exchanger 10 are the same and the heat capacities are approximately equal (there is a small difference in average temperatures between the hot and cold sides), then from the heat balance

Q Gr Срт (Тз-Т4) GxCpxOVTi)

it follows that the temperature difference

Tz-Ta T4-Ti.

The heat transfer coefficient is generally written as follows

K 4

1 (1 + d 2 + F NF Fx Fx + Fr

-1

OG-RG-POG

The thermal resistance of the wall of the heat transfer surface can be neglected, because value (5 / A is three orders of magnitude less than 1 / ax 2, taking into account that the living cross-sectional areas of each side and the profiles of their channels are equal and identical, then the heat exchange surface areas are equal (Fx Fr). Therefore,

TO

1

+ -x1

UZx T / ox "g-7 / og

-1

)

C. the equation t / oX-t / og - coefficients

thermal efficiency of heat exchange surfaces on cold and hot

equal to the sides (distinguishing in the third character

after that), then

20 K (1 + §) 1.

Representing the ratio of sx / ag in the following form

 Ax.dr.NU Ax fGx.dx.gr.SY «r YAG dv NUr Yag Gr dr SXJ

Taking into account the accepted assumptions, we obtain “x Yah (

 R | eleven

"G 27

Here n is the exponent in the dependence generalizing the values of the heat transfer coefficient as a power function of the Reynolds number. Taking into account the literature data in which similar heat-exchange surfaces were studied, n is taken to a first approximation as 0.7. The difference between the new value of the exponent and 0.7 determines the methodological error of the claimed method.

It is known that the exponent n in

depending on the operational parameters, it can vary from 0.33 to 0.85. Therefore, the maximum difference of the sought exponent from 0.7 does not exceed 0.37. Prin in the temperature head

The dates are 30 °, taking into account changes in thermophysical interactions and their errors on each side, the methodical error (see the formula for kip / ty) will be 0.98%, while the error in determining the heat transfer coefficient is 2.9%.

The final calculated dependence for calculating the heat transfer coefficient, taking into account the conversion, is as follows

-Јp + H f Thus, in comparison with the prototype, the accuracy requirements for measuring instruments are significantly reduced, the experimental setup is simplified, the installed power of the plant heaters is reduced, which reduces the cost of research work. At the same time, the representativeness of experiments is increased by reducing the methodological error to 1%.

Claims (3)

SUMMARY OF THE INVENTION 1. A method for determining a heat transfer coefficient by heating a heat carrier and pumping it through a recuperative heat exchanger under study, measuring a temperature of a heat carrier at the inlet and outlet of each side of a heat exchanger, and then setting a heat transfer coefficient from the measurement results, characterized in that, in order to increase accuracy and reduce energy consumption when performing heat transfer surfaces of the studied heat exchanger identical, the coolant is passed sequentially through the hall one and the hot sides of the heat exchanger, and it is heated after leaving the cold side of the heat exchanger, maintaining the difference in average temperatures between the hot and cold sides of the heat exchanger within 20–40 °. 2, An experimental apparatus for determining the heat transfer coefficient, comprising a circuit with a heat carrier, equipped with a heater, heat exchangers, a supercharger, a supply tank, pressure, temperature and flow meters, as well as pipelines and fittings, characterized in that, in order to improve accuracy and reduce energy consumption when performing the heat exchange surfaces of the studied heat exchanger are identical, the coolant outlet from the studied heat exchanger on the cold side is connected to the inlet of the same heat exchanger on the hot side through the heater. 3. Installation according to claim 2, with the exception that at the inlet of the cold side of the studied heat exchanger, an additional technological heat exchanger is installed, the input of the hot side of which is connected to the output from the hot side of the studied heat exchanger, and the cold The sides of both heat exchangers are connected in series. r 5 / YU Q