The paper presents the results of development and investigation of a copper miniature loop heat pipe (LHP) with acetone as a working fluid. The device was equipped with a flat evaporator measuring 80 × 42 × 7 mm and vapor and liquid lines with an outside diameter of 3 mm, whose lengths were 145 mm and 175 mm, respectively. The LHP was tested at heat loads from 5 W to 60 W, different orientations in the gravity field and heat-sink temperatures from -40°C to +50°C. It is shown that the LHP retains its efficiency at all testing conditions. It is also mentioned that at a heat-sink temperature of +50°C the device operates in the mode of constant conductivity in the whole range of heat loads, and in this case a minimum thermal resistance of the “heat source-heat sink” system equal to 0.16°C/W is achieved, which is independent of the LHP orientation in the gravity field.
Loop heat pipes (LHPs) belong to passive heat-transfer devices operating on an evaporation-condensation cycle and using “a capillary mechanism” for the transportation of a working fluid [
Of particular interest are LHPs with flat evaporators, which can be well joined with heat sources that have a flat contact surface measuring up to 50 ´ 50 mm and possess a lower thermal resistance [
As a working fluid, acetone was first used in a loop heat pipe with a cylindrical evaporator 30 mm in diameter made of steel and equipped with a fine-pored nickel wick [
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The aim of the present work was the development and investigation of a copper LHP with a flat evaporator capable of maintaining the temperature of the object being cooled at a level that does not exceed 70˚C at heat loads from 5 W to 50 W in a wide range of varying ambient conditions. By such conditions are meant changes in the device orientation with respect to the gravity field vector and the heat sink temperature, which affect considerably the LHP thermal characteristics. Methanol and acetone were considered as alternative working fluids. The choice between them was realized on the basis of a large variety of criteria.
Methanol and acetone satisfy practically equally the initial criteria, such as the chemical compatibility with copper and the temperature range in which they may be used as working fluids for LHPs. As for thermophysical criteria, the choice here is not so unambiguous as for conventional heat pipes, where, as a rule, the working fluid is evaluated by the so-called quality criterion [
where rl is the liquid density, kg/m3; σl is the surface tension coefficient, N/m; L is the latent heat of evaporation, kJ/kg; ml is the coefficient of dynamic viscosity of the liquid, Pa×s.
This criterion makes it possible to evaluate a working fluid from the viewpoint of the maximum capacity that can be achieved in a heat pipe, where the main pressure losses take place in the capillary structure. The temperature dependence of the criterion Kl for methanol and acetone is shown in
The graphs clearly point to the fact that methanol has a considerable advantage in the temperature range above 5˚C as the value of the criterion Kl is directly proportional to that of the maximum capacity that can be achieved at a given temperature of the working fluid. In loop heat pipes, however, the main pressure losses, as a rule, take place in the vapor phase of the working fluid during the motion in the vapor line. Therefore a more impartial assessment of a working fluid for an LHP can be obtained with the use of the criterion that may be presented as follows:
where mv is the coefficient of dynamic viscosity of vapor, Pa×s; L is the latent heat of evaporation, kJ/kg; rv is the vapor density, kg/m3.
The smaller the value of this criterion, the smaller the pressure losses in the vapor phase of the working fluid.
Here it is seen that at vapor temperatures lower than 30˚C vapor pressure losses are smaller for acetone than for methanol. This is quite important because lower pressure losses make it possible for an LHP with acetone as
a working fluid to operate at a lower temperature level with respect to the temperature of the heat sink, or to decrease the diameter of the vapor line at the same temperature as that of methanol. In the latter case there appears a more favorable possibility for the configuration of lines if it is necessary to locate an LHP in straitened conditions.
There are also some other parameters that allow a more comprehensive assessment of a working fluid for an LHP. To such parameters belongs the value of dPs/dTs, which characterizes the slope of the saturation line of the working fluid at a given temperature T. This parameter is quite important for realizing one of the main conditions of the LHP serviceability [
where DT is the vapor temperature drop between the evaporating and the absorbing surface of the wick, ˚C; DP is the pressure losses in the loop from the evaporating surface of the wick to its absorbing surface, Pa.
The calculated value of dPs/dTs for acetone and methanol is presented in
Here acetone has a higher value of dPs/dTs in the temperature range up to 35˚C. This means that an LHP with acetone as a working fluid can be started and operate at lower temperatures as compared with the temperature of the heat sink and, consequently, have a lower thermal resistance.
An important criterion for heat pipes, including those operating in the so-called “antigravitational” regime, when the motion of the liquid phase of a working fluid proceeds against gravity forces, is “the capillary pump parameter”, which is presented in the form of the relation:
where σl is the surface tension coefficient, N/m; rl is the liquid density, kg/m3.
A graphic dependence of this criterion on temperature is given in
It is seen that by this criterion acetone has an additional advantage over methanol, which is also confirmed by the experimental results obtained in [
Another factor that “plays” in favor of acetone is the vapor pressure, which does not exceed the normal ambient pressure at temperatures up to 56˚C. This allows the use of acetone in LHPs with flat evaporators, which are quite sensitive to the internal pressure, for cooling heat sources with maximum temperatures up to 60˚C - 70˚C, and in some cases even to 70˚C - 80˚C. Such a level of temperatures is quite usual, for instance, for many components of
electronics.
On the basis of the analysis presented above, the working fluid chosen was acetone.
The loop heat pipe was made of copper, including the main wick sintered from copper powder and the secondary wick in the compensation chamber made of a copper net. The evaporator had a flat shape and was equipped with a thermal interface in the form of a copper plate measuring 42 × 42 × 1 mm and corresponding to the dimensions of the heat source, which was soldered to the active zone of the evaporator. The serpentine condenser was soldered to a copper plate measuring 150 × 40 × 2 mm, which served as the condenser thermal interface. The LHP scheme is shown in
The LHP was tested in the range of heat loads from 5 W to 60 W at different orientations, which were characterized by the slope j with respect to the horizontal plane. The LHP positions are shown in
Parameter | Value |
---|---|
Evaporator (length × width × thickness), mm Vapor line (length/diameter), mm Liquid line (length/diameter), mm Condenser (length/diameter), mm | 80 × 42 × 7 145/3 175/3 353/3 |
The heat source was an aluminum block with two heating cartridges located in it. The heating surface of the heat source corresponded to the dimensions of the evaporator thermal interface. The thermal interface of the condenser was pressed to a heat sink made of aluminum in the form of “a cold plate”, through which a chiller pumped the liquid being thermostatted with temperatures of −40˚C, −20˚C, 0˚C, 20˚C and 50˚C. The block-diagram of the experimental setup is presented in
The heat load supplied to the heat source varied stepwise with a step of 10 W up to the critical values, at which the LHP temperature did not stabilize, or to the values at which the vapor temperature exceeded 60˚C. The exception was the first step from 5 W to 10 W. The value of the heat load was measured by a wattmeter with an accuracy of ±1%. The heat losses were not taken into account. The LHP operating temperature was measured at 6 points with the help of copper-constantan thermocouples “OMEGA” T-TT-30. The readings were registered by a data acquisition unit “Agilent” 34970A (accuracy ± 0.1˚C). The main temperature reference point (TRP) was the temperature of the heat source, which was measured by a thermocouple embedded in the heating surface of the aluminum block pressed to the evaporator thermal interface with the use of a heat-conducting paste. The temperature of the thermal interface of the condenser was measured at three points distributed over its surface.
The results of testing the LHP are given as graphic dependencies of the heat source temperature Ths and the
thermal resistance of the “heat source-heat sink” system Rsys on the heat load, slope and heat sink temperature. In this case the heat sink temperature Thk was determined as the average temperature of the thermal interface of the condenser. The value of Rsys was calculated by the formula:
where Ths is the heat source temperature, ˚C; Thk is the heat sink temperature, ˚C; Q is the heat load, W.
These results testify that a heat load of 50 W was achieved in all testing conditions. It should be mentioned that the maximum heat source temperature in this case did not exceed 68˚C even when the heat sink temperature was equal to +50˚C. The corresponding vapor temperature was about 60˚C, which is quite acceptable for an LHP with a flat evaporator, because the internal pressure exceeded the external atmosphere pressure only slightly. The LHP also demonstrated serviceability at a negative operating temperature, which was observed at heat loads below 20 W when the heat sink temperature was equal to −20˚C and −40˚C. A maximum heat load of 60 W was achieved at all slopes except j = +90˚ when the heat sink temperature was equal to +50˚C.
It should also be mentioned that the most interesting and unconventional results were obtained at a heat sink temperature of +50˚C.
First, the heat load dependence of the heat source temperature has a quasi-linear character in the whole range, which points to the LHP operation in the regime of constant conductivity. This is caused by the fact that the working fluid is forced out of the condenser and fills the entire compensation chamber before the heat load supply. Thus, in the LHP operation the condensation surface does not change.
Second, it is seen that the dependence of the heat source temperature on the LHP slope is practically leveled. This fact is demonstrated more clearly in
Third, the thermal resistance of the “heat source-heat sink” system determined by Equation (5) reaches its minimum value of 0.16˚C/W at Thk = +50˚C, too, and depends only slightly on the LHP slope. As an example
The characteristic heat load dependence of thermal resistance at j = +90˚ for different heat sink temperature is presented in
As for the LHP operation at low heat sink temperatures of −20˚C and −40˚C, here the regime of variable conductivity is observed in the whole range of heat loads for all slopes except the case where j = +90˚*. Besides, the thermal resistance here has the highest values. This can be explained by the fact that the condensation surface at low temperatures frees itself to a considerably smaller degree than at high temperatures, and the heat rejection proceeds mainly at the expense of the increasing temperature difference between the condenser and the heat sink. Nevertheless, it should be mentioned that at low temperatures one can achieve a higher maximum heat load.
Unconventional are also the LHP operating characteristics at j = +90˚* when the compensation chamber is located below the heating zone of the evaporator. In the graphs (
It is also interesting that only at such an orientation the LHP operates at negative heat source temperatures when the temperature of the heat sink is equal to −20˚C and −40˚C, and the heat load is relatively small. At all other ambient conditions the LHP operation is realized at positive heat source temperatures.
The thermal resistance was calculated by Equation (5).
The standard uncertainty of the thermal resistance can be estimated as follows:
where DThs, DThk, and DQ are the errors of direct measurements of the temperature and the heat load.
An analysis of the accuracy of indirect measurements of the thermal resistance has shown that the maximum
Heat load, W | Uncertainties, % | ||||
---|---|---|---|---|---|
Thk = −40˚C | −20˚C | 0˚C | +20˚C | +50˚C | |
5 | 12.0 | 12.0 | 12.0 | 12.0 | 39.4 |
10 | 6.0 | 6.0 | 6.0 | 6.1 | 21.7 |
20 | 3.0 | 3.1 | 3.2 | 3.6 | 10.3 |
30 | 2.1 | 2.1 | 2.3 | 2.9 | 6.7 |
40 | 1.6 | 1.7 | 1.8 | 2.3 | 4.8 |
50 | 1.3 | 1.3 | 1.4 | 1.6 | 2.8 |
error was observed at low values of the heat load and small values of the temperature difference Ths - Thk.
It is seen that the maximum error is observed at a heat load of 5 - 10 W when the temperature of the cooling liquid is equal to +50˚C. At the same time, in the rated range of heat loads from 30 W to 50 W the maximum value of the error at all conditions is in the range from 1.3% to 6.7%.
A miniature copper loop heat pipe with a flat evaporator and acetone as a working fluid has been developed. The device has been tested in the range of heat loads from 5 W to 60 W at different orientations in the gravity field and heat sink temperatures from −40˚C to +50˚C. It is shown that the LHP is capable of operating efficiently at all the conditions indicated. It has also been mentioned that even at a heat sink temperature of +50˚C and a maximum heat load of 50 W the heat source temperature does not exceed 68˚C. In this case the LHP operates in the regime of constant conductivity, the thermal resistance of the “heat source-heat sink” system reaches a minimum value at a level of 0.16˚C/W and does not depend on the LHP orientation.
Yury F.Maydanik,Vladimir G.Pastukhov,Mariya A.Chernysheva, (2015) Development and Investigation of a Miniature Copper-Acetone Loop Heat Pipe with a Flat Evaporator. Journal of Electronics Cooling and Thermal Control,05,77-88. doi: 10.4236/jectc.2015.54006
g: gravitational constant, m/s2
K: criterion
L: latent heat of evaporation, kJ/kg
P: pressure, Pa
Q: heat load, W
R: thermal resistance, ˚C/W
T: temperature, K
σ: surface tension coefficient, N/m
m: coefficient of dynamic viscosity, Pa×s
r: density, kg/m3
j: slope, grad (˚)
S: sum
hs: heat source
hk: heat sink
l: liquid
s: saturation
v: vapor