INVESTIGATION OF VAPOR GENERATION INTO CAPILLARY STRUCTURES OF MINIATURE LOOP HEAT PIPES

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1 Minsk International Seminar Heat Pipes, Heat Pumps, Refrigerators Minsk, Belarus, September 8-, INESTIGATION OF APOR GENERATION INTO CAPIARY STRUCTURES OF MINIATURE OOP HEAT PIPES.M. Kiseev, A.S. Nepomnyashy, N.. Gruzdova Ural State University Department of Thermophysics and Surface Phenomena enin av., 8, Ekaterinburg, Russia () -77 () -778 fax Abstract Extensive development oop Heat Pipes (HP) technology offers capabilities to play HP an increasingly important role for instrument thermal control on some space and earth applications. However, heat transport systems on basis of miniature HP (MHP) are interesting in electronics packaging for high heat flux in zones of heat input and transport. The applications could include thermal management of a various electronic devices such as computer processors (CPU), where the heat dissipation requirement is expected to be in the range of to Watts. This paper presents some configurations of MHP with flat plates of evaporators and examines the experimental data which is investigated for MHP due to gravity. The heat transfer coefficient by phase exchange in the HP capillary structures is investigated for different configurations of the vapor ducts, working fluids and capillary structures. Experimental tests verify that the Miniature HP operates successfully by high heat flux in zones of heat input and transport and loop heat pipe heat transfer mechanism is better for these conditions than classical heat pipes. KEYWORDS oop Heat Pipe, Steady-state behavior, heat transfer coefficient, capillary structure, flat evaporator. INTRODUCTION oop heat pipes are two-phase transport devices which utilize the capillary pressure developed in a fine capillary structure (FCS) (pore wick) to circulate the working fluid in a closed loop system connected heat source and heat sink. The HP are capable of operating effectively at any orientation (top heat, horizontal heat and bottom heat) in a gravitational field over large distances. In the HP the liquid return to the evaporator section is along a smooth-walled tube with low frictional resistance. The wick design for the loop scheme is small since it is only located in the evaporator section. In this case it is the basic difference beside other heat pipes. HP systems have the potential to transport large amounts of heat over large distance or unfavorable accelerations with minimal temperature drops. Due to the extensive development efforts in ground tests and flight experiments over the last few years, the HP have reached the state of technology readiness and commercialization. Theoretical studies are also being conducted to better understand the operational mechanism of the HP. It is anticipated that the HP will become a major player in the thermal management of space and terrestrial systems in the near future []. On the other hand, the HP are excellent heat transfer devices for electronics packaging for high heat flux in zones of heat input and transport, when heat transport zone may connect heat source and heat sink by the flexible tubes at any orientation. It is important for a miniaturization of electronic components and for development of electronics as a whole [-]. This paper presents some configurations of MHP with flat plates of evaporators and examines the experimental data which is investigated for MHP due to gravity. The experiment effort was supplemented by extensive hardware development efforts, analytical modeling development and ground test verification. A summary of hardware development include:

2 Metal wick development status including nickel and titanium with effective pumping in the range of. to micron and permeability in the range x - m to x - m. The development of MHP with flat evaporators. Condensers and heat sinks development. Analytical modeling development was focused on predicted wick material conductivity effects on MHP performance, heat and mass transfer between wick and compensation chamber and design/performance characterization of wicks. Theoretical calculations of MHP and MHP software are presented and compared with several experimental results. Extensive ground test verification has been performed by Ural State University (USU) during the past several years. Results that will be presented include: The heat transfer coefficient by phase exchange in the HP capillary structures as a function of heat input, different configurations the vapor ducts, working fluids and capillary structures. MHP performance as a function of heat input/heat sink, heat agent, orientation and accelerations. Power cycling and sink temperature cycling behavior. Temperature and pressure drop characteristic. EXPEREMENTA APPARATUS AND PROCEDURE In fact, in electronics packaging for high heat flux in zones of heat input where the HP capillary structure is a generator of vapor and capillary pump the heat transfer coefficient cannot be avoided. The external and internal heat transfer coefficient can be defined. The external heat transfer coefficient is provided a good contact between electronic components and HP evaporator wall and is not of interest of HP performance. The internal heat transfer coefficient is very important for HP performance by high heat flux and defines the possibility to lead out of heat by using phase exchange in the HP wick. In spite of a lot the theoretical and experimental studies in these conditions it is not a clear reply how to increase heat transfer coefficient by using capillary phenomena. The effects of the phase exchange in the HP wick were studied by using the open HP system (external pressure about Pa). The schematic diagrams of experimental apparatus are shown in Fig.. a) b) H Н О Tw in 9 Tv Tw ex Tv Fig.. Open HP systems: HP evaporator; capillary structure (wick); vapor ducts (grooves); compensation chamber (cavity); vapor line; liquid line; 7 condenser; 8 water cooler; 9 copper heater; Tw ex, Tw in, Tv thermocouples

3 The configuration and the geometrical sizes of vapor ducts in the HP open system (Fig.b) were optimized. A wick is connected with the cooper heater without the wall of evaporator. Fig. and Fig. give examined configurations of vapor ducts. a) Concentric vapor ducts Radial vapor ducts b) 7 Fig.. Configurations of vapor ducts ( 7): a) only radial vapor ducts; b) radial and concentric vapor ducts ) a b ) ) c Fig.. Geometry of concentric vapor ducts: rectangular vapor ducts; trapezoid vapor ducts; triangular vapor ducts Metal wick development status including nickel and titanium with effective pumping in the range of. to micron and permeability in the range x - m to x - m was based in this studying. Table shows the main properties of metal porous wick. Table. Main properties of metal wicks (all data are experimental) No Wick material Porosity ε, % Effective pore radius Rp, - m Permeability K, - m Effective thermal conductivity λ, W/m K. Titanium (Ti) Titanium (Ti) Nickel (Ni) Nickel chips of porous Ni.. 9.

4 Fig. shows the basic configuration and geometrical size MHP flat plates of evaporators..8..., aa 8. Fig.. The basic configuration and geometrical size MHP flat plates of evaporators case of evaporator; wick (capillary structure); vapor ducts The researching object (HP or system of it's modeling) was located in thermostatic box where with a help of automatic regulation system the temperature of the environment was Tenv = ±, o C or Tenv = ±, o C. The methodic of the experiment was oriented on measuring of the temperature fields of researching objects with different heat agent, orientations and temperatures of the environment and heat sink. The heat input was controlled through variable transformers and measured with wattmeter. Heat input rate was given by a resistive electric heater and measured by a calorimetric plate. At the condenser heat out was taken by means of water in the water jacket which was installed in cooling section. Water flow rate and the inlet/outlet temperature were measured and heat transfer rates were calculated by calorimetric method. Temperature was measured with copper constantan thermocouples located at appropriate points on the tube wall and surrounding insulation at the evaporator, condenser and adiabatic sections in each. The precision of measurements of the heat flow rate was no higher than %, and the precision of measurements of temperatures was no higher than,%. Before the beginning of the experiment was checked the connection between a wick and a case of evaporator by the air-bubble method. The experiments were realized for adverse g acceleration (evaporator above condenser +H ) and favorable g acceleration (condenser above evaporator -H ). EXPEREMENTA RESUTS At the first step of experiment was studied the influence of the wick thickness (δ) on a heat transfer coefficient (α) with heat agent of water and acetone. This data are presented in Fig. and Fig.. As can be seen from this comparison, at other conditions being equal, there is the increasing of the heat transfer coefficient by the thickness of the wick about δ = (-7) mm. The dependences α = f(q) have maximum that it is evidence of a competing phenomena into capillary structure between liquid and vapor flow. The internal heat transfer coefficient α (W/m K) is defined as α q =, T T w in v q = S tr inp () where q is heat flux (W/m ), tr is heat transfer rates calculated by calorimetric method in condenser, S inp is the square of the surface of heat input, T w in is temperature on the internal surface of a

5 evaporator wall (the surface of the contact between a case wall and a wick) and T v is temperature of vapor. м К a, W/,,,,, Water,,, q, W/м q, W/м Fig.. Dependence the heat transfer coefficient α on the heat flux q for Nickel wick No (Table ) δ = 7 mm; δ = mm; δ = mm; - δ = mm. Adverse g acceleration H = +. m. Open HP system by external pressure about Pa a, W/м K Water q, W/м, Вт,,,,,, q, W/м Fig.. Dependence the heat transfer coefficient α on the heat flux q for Titanium wick No (Table ) δ = 7 mm; δ = mm; δ = mm; - δ = mm. Adverse g acceleration H = +. m Open HP system by external pressure about Pa The next series of the experiments solved a problem of optimization of a configuration and the sizes of the vapor ducts. The models of the capillary structure (No, Table ) had the optimal thickness δ = mm. The vapor ducts (radial and concentric, Fig.) had quadratic profile ( x mm) on a surface of the wick and a different configuration. The effective diameter of the vapor duct D ef and their total

6 relative section ξ are defined as S S D ef =, ξ = Π S where S is the square of vapor duct section, П is vapor duct perimeter; S vd is the total square of the section of the vapor ducts on the surface of the contact between the wick and the wall of heat input. This data is presented in Fig.7. vd inp () a, W/м К,9,7,,,,,, Aceton Р = q, W/м 7,9,7,,,,,, Р = ( - ) q, W/м Fig.7. Dependence the heat transfer coefficient α on the heat flux q for Nickel wick No (Table ) No (Fig.), ξ = ; No (Fig.), ξ =.9; No (Fig.), ξ =.; - No (Fig.), ξ =.; - No (Fig.), ξ =.; - No (Fig.), ξ =.; 7 - No 7 (Fig.), ξ =.; Adverse g acceleration H = +. m; left: open HP system; right: close HP system As can be seen from Fig.7 the maximal heat transfer coefficient by the maximum of heat flux is achieved by ξ =. -. (No, Fig.) as for open HP system and for close HP system. Existence of an optimum in surface configuration of vapor grooves gives the evidence about the competition between hydraulic and thermal resistances in zone of vapor grooves. After optimization of the coefficient ξ that is determined practically of geometry of heat and mass input to surface of evaporation into capillary structure was made several series of experiments. It optimized the effective diameter of the concentric vapor ducts D ef. The radial vapor ducts (Conf. No, Fig.) had only quadratic profile ( x mm) on a surface of the wick. The concentric vapor ducts had a different profile (Fig.) and D ef. By this study we didn t observe the visible influence of concentric vapor ducts profile on heat transfer process. Therefore we used triangular concentric vapor ducts surface as more technological. They were located on the wick or on an internal surface of the wall of heat input. These data are shown in Fig.8. As can be seen in Fig.8 the decrease of the sizes and distance between concentric vapor ducts goes to intensification of heat transfer and the increasing of heat transfer coefficient. The placement of concentric vapor grooves on an internal surface of the wall increase the heat transfer coefficient more than ( ) %. Apparently, the decrease of distance between concentric vapor ducts decreases the distance of vapor exit from vapor pores to vapor grooves improves the vapor collection from pores and accordingly intensify heat transfer in fine porous structure.

7 a, W/м К,,,, Р = kpa q, W/м,8,,,,,, Р = kpa q, W/м Fig.8. Dependence the heat transfer coefficient α on the heat flux q for Nickel wick No (Table ) eft: concentric vapor ducts are located on the wick. D ef = mm; D ef =. mm; D ef = mm; - D ef =. mm; Right: concentric vapor ducts are located on a internal surface of the wall of heat input. - D ef =. mm; - D ef =. mm; Adverse g acceleration H = +. m; open HP system CONCUSIONS The results obtained in heat transfer experiments in MHP with flat plates of evaporators are summarized as follows: () There is an optimal thickness of the fine porous wick (about -7 mm) for concrete conditions of MHP operation. () It is showed that for the intensification of heat transfer in zone of heat input of MHP with flat plates of evaporators a special system of vapor ducts is necessary. There is the optimal surface square of vapor grooves not more than.. from heat input square. () The decreasing of the sizes and distance between concentric vapor ducts goes to intensification of heat transfer and the increase of heat transfer coefficient. The placement of concentric vapor grooves on an internal surface of the wall increase the heat transfer coefficient more than ( ) %. Apparently, the decrease of distance between concentric vapor ducts decreases the distance of vapor exit from vapor pores to vapor grooves improves the vapor collection from pores and accordingly intensify heat transfer in fine porous structure. References. Faghri A. Heat pipe science and technology, Taylor & Francis, Washington, 99, pp Kiseev., Belonogov A. Miniature heat transport systems with loop heat pipes, Proc. of th Minsk Int. Seminar Heat Pipes, Heat Pumps, Refrigerators, Minsk, Belarus,, pp. -.. Chernysheva M.A., ershinin S.., Maidanik Yu.F. Development and test results of loop heat pipes with a flat evaporator, Proc. of th Int. Heat Pipe Conference, Moscow, Russia,, A-.. Pastukhov.G., Maidanik Yu.F., ershinin S.., Korukov M.A. Miniature loop heat pipes for electronic cooling, Proc. of th Int. Heat Pipe Conference, Moscow, Russia,, F. 7