PERFORMANCE PARAMETERS OF A CLOSED LOOP THERMOSYPHON

Permanent development of electrotechnical devices aims to increase their performance, to efficiently use the space, to reduce their mass, to achieve higher efficiency and reliability as well as a higher technological level. Another trend in the development of electrotechnical components is miniaturization of dimensions which leads to the increase in local heat loading due to heat waste of electronic components. More intense heat production and its insufficient removal often cause deterioration of electronic system parameters and electronic components failures. To maintain suitable working conditions it is necessary to dissipate waste heat. From various cooling methods used in electronics the heat pipe seems to be one of highly efficient and reliable way of heat removal[1]. The closed loop thermosyphon works on the same principle as the standard gravitational heat pipe in which heat transfer occurs due to the flow of vapor and liquid phase of the working fluid between the evaporation and condensation sections of the heat pipe. The difference between them is in the way of the working fluid circulation. While in the standard gravitational heat pipe theworkingfluid flows between the evaporation and condensation sections in the same space, in the closed loop thermosyphon the working fluid flows in a closed loop between the evaporation and condensation sections. Due to the absence of interaction and reverse flow of vapor and liquid phase the closed loop thermosyphon features better ability to heat transfer between its evaporation and condensation sections than the standard gravitational heat pipe.


Introduction
Permanent development of electrotechnical devices aims to increase their performance, to efficiently use the space, to reduce their mass, to achieve higher efficiency and reliability as well as a higher technological level. Another trend in the development of electrotechnical components is miniaturization of dimensions which leads to the increase in local heat loading due to heat waste of electronic components. More intense heat production and its insufficient removal often cause deterioration of electronic system parameters and electronic components failures. To maintain suitable working conditions it is necessary to dissipate waste heat. From various cooling methods used in electronics the heat pipe seems to be one of highly efficient and reliable way of heat removal [1]. The closed loop thermosyphon works on the same principle as the standard gravitational heat pipe in which heat transfer occurs due to the flow of vapor and liquid phase of the working fluid between the evaporation and condensation sections of the heat pipe. The difference between them is in the way of the working fluid circulation. While in the standard gravitational heat pipe theworkingfluid flows between the evaporation and condensation sections in the same space, in the closed loop thermosyphon the working fluid flows in a closed loop between the evaporation and condensation sections. Due to the absence of interaction and reverse flow of vapor and liquid phase the closed loop thermosyphon features better ability to heat transfer between its evaporation and condensation sections than the standard gravitational heat pipe.

Construction of the closed loop thermosyphon
Main parts of the closed loop thermosyphon are: − evaporator, − condenser, − pipe system to transport the working fluid and − inlet and closing valves.
The evaporator (Fig. 1) enables on the base of a phase change (boiling) of the working fluid an intensive heat removal from its surface. It has to be constructed so that it will prevent the leakage of the working fluid, maintain pressure differences in all the walls and enable heat transfer from the electronic component into the working fluid as well as a suitable distribution of liquid and vapor phases of the working fluid. When choosing the material suitable for the construction of evaporator, it is necessary to pay attention to its thermokinetic characteristics. To provide a minimal temperature drop between a heat source and evaporator, the evaporator material must feature high thermal conductivity. To prevent escape of vapor, it should not be porous. The material should have high strength but, at the same time, it should be easily machineable and compatible with the working fluid [2]. Fig. 1 shows the closed loop thermosyphon from aluminum alloy designed with respect to the above requirements. The evaporator body is a plate with dimensions 116 ϫ 80 ϫ 30 mm. To provide the working fluid circulation there are two12 mmopeningsdrilled horizontally on the plate and connected with nine 6 mm vertical connecting channels. They provide the transport of heated fluid vapor from the bottom to the top section of the evaporator. On the outer contact surface of the evaporator and electronic component there is a groove with a mounted temperature sensor.
The closed loop thermosyphon condenser (Fig. 2) is a soldering plate heat exchanger Alfa Laval. This choice was determined by the effort to achieve a compact construction of the cooler providing the working fluid cooling in the heat pipe at defined temperatures of the cooling water and being able to withstand high Heat exchanger plates are arranged so that there are optimal channels among them into which heat carrying fluid is introduced through openings in the corners of the plates. Each plate is flown by the primary fluid from one side and by the secondary fluid from another side with simultaneous presence of heat transfer. A copper connector connects the plates not only along their circumference but also in all connecting places of neighboring plates. Brazed heat exchangers are therefore able to withstand high temperatures (up to 225 °C) and pressures (up to 49 bars) and have high efficiency of heat transfer even at low logarithmic mean temperature difference. The transport section of the closed loop thermosyphon provides the circulation of vapor and liquid phases between the evaporator and condenser of the heat pipe. The whole transport section consists of 10 mm copper connecting tubes. Transient glass tubes were mounted on the evaporation and condensation sides of the transport section of the heat pipe to visualize and check the working fluid flow. All connecting transient points of the whole heat pipe system are vacuum-tight. The intake and closing valves are located on the top of the evaporation transport section.

Working fluid
The first criterion for a design of suitable working fluid is the range or operating temperature. As there can be more working fluids within the range of suitable operating temperatures, it is, therefore, necessary to observe and compare their further thermophysical characteristics when determining the most suitable one. The main requirements for working fluid characteristics are compatibility with the heat pipe material, good temperature stability, suitable vapor pressure, large latent heat of evaporation, high thermal conductivity, low viscosity of fluid and vapor, acceptable point of freezing and solidification from the point of cooling operation.
The choice of working fluid has also to be done on the basis of thermodynamic considerations concerning various limitations of heat transfer in heat pipes (viscose, sonic, capillary limits and limits of bubble boiling). Vapor pressure within the range of operating temperatures has to be sufficiently large to avoid high velocity of vapor which may cause instability of heat flows. The working fluid must feature high latent heat of evaporation which will enable to transfer the highest possible amount of heat with the least fluid flow provided the low pressure difference in the heat pipe is maintained. Thermal conductivity of the working fluid should be, if possible, high to minimize radial temperature gradient and decrease possibilities of film boiling on the surface walls. The resistance to the fluid flow is minimized through the choice of fluid with low values of viscosity of fluid and vapor phases [3]. In compliance with

Measurement of loop thermosyphon performance parameters
To determine the heat pipe performance parameters a measuring device was designed -its scheme can be seen in Fig. 3. Due to possible applications of the heat pipe system serving,for example, also heat transfer in a region with the temperature of 50 °C, measurements with the maximum input temperature of the cooling fluid up to 50 °C were made. The measurement was performed at the increasing waste heat performance of the electronic element from 20 to 370 W. The electronic element fixed in a standard way to the evaporation section of the heat pipe was connected to the unidirectional current source HEWLET PACKARD 6575A, DC POWER SUPLY 0-120 V/ 0-18.

Mathematical model for determination of performance parameters of the closed loop thermosyphon
The closed loop thermosyphon has an evaporation section separated from the condensation section, i.e., it has a separate section for a vapor and liquid phase of the working fluid. In the evaporation section of the closed loop thermosyphon the condensate is heated up to the boiling temperature of the working fluid. Boiling temperature is given by instantaneous absolute pressure in the circuit of the closed loop thermosyphon. It can be seen from the experiments that at the temperatures up to approx. 80 °C the evaporation or bubble boiling of the working fluid FC 72 takes place. The mathematical model of performance parameters of the closed loop thermosyphon was created on the basis of criterion equations (1a) and (1b) just for the bubble boiling [5]. The Prandtl number is determined from relation (2) where a is thermal conductivity Reynolds number for boiling is where B is the critical average of vapor bubble and q is the density of thermal fluid flow, , Pr a v l = From relation (1a) or (1b) the Nusselt criterion Nu B is determined and substituting relation (7), theheat transfer coefficient α is expressed The temperature on the contact area of the fluid and inner area of the evaporator chambers Ts v is calculated according to relation (8) The temperature on the contact area of the electronic component and the outer area of the evaporator of the closed loop thermosyph Ts is calculated from relation (9) that was derived from relations for heat conduction and transfer (10) and (11) (9) (10) (11) [6] Basic physical characteristics of FC 72 in dependence on temperature are given by the manufacturer in tables or analytical form from which it is possible to determine the required variables in dependence on temperature Comparison of calculated and measured temperature t s calculated. The calculation started from the condition t v ϭ t out (evaporation temperature of the evaporating working fluid FC 72). Table 2 and Fig. 6 show comparison of calculated and measured values of temperature tsof the closed loop thermosyphon in dependence on the measured evaporation temperature of the working fluid FC 72 tout in a range from30 °C to 65 °C. Fig. 6 shows calculated and measured temperatures tsof the heat pipe in dependence on temperature tout. It is obvious from the courses of temperatures that the measured values differ from the calculated ones only by approx. 3 to 6 °C. This closeness of the results approves the correct functionality and technological procedure of the heat pipe prototype as well as the option to use the simplified mathematical model for dimensioning of cooler with heat pipes for similar electronic components or systems.

Conclusion
The objective of experiment was to design and construct a prototype of the closed loop thermosyphone and verify its functionality at the cooling of electronic component used in real applications at the highest admissible temperature on the contact area with the cooler 100 °C and maximum admissible voltage and current 20 V and 20 A. Another objective of activities also was to show on a simple mathematical model the potential for the cooling of the heat pipe system and compare the resultant values of calculated and measured temperatures on the contact area of the cooled electronic component. According to the experimental measurements and calculations the closed loop thermosyphon cooling proved its high efficiency, which can be also seen at the full performance of the electronic component and also as the highest temperature of the cooling water 50 °C used to cool the evaporating working fluid in the condenser. The temperature on the contact area of the electronic component with the heat pipe evaporator was always below 80 °C. This experiment approves the cooling quality of the closed loop thermosyphon and justification of its use for the cooling of high efficiency electronic components and systems generating huge thermal flows of waste heat. The use of heat pipes for the cooling of output electronics, mainly electronic semiconductor elements, offers, together with reduced requirements for quantity of constructional material and saved space, also better cooling performance and improved cooling in the area of higher waste heat output above 100 W.