MINIATURE HEAT TRANSPORT SYSTEMS WITH LOOP HEAT PIPES

Transcription

1 IV Minsk International Seminar Heat Pipes, Heat Pumps, Refrigerators Minsk, Belarus, September 4-7, MINIATURE HEAT TRANSPORT SYSTEMS WITH LOOP HEAT PIPES V. Kiseev, A. Belonogov Ural State University, Ekaterinburg, Russia Abstract Extensive development of Loop Heat Pipes (LHP) technology and several successful flight experiments offer capabilities to play LHP an increasingly important role for instrument thermal control on flight spacecraft. In most LHP tests are considered the big LHP which have the heat transport length more than 1m, evaporator diameter more than 15 mm and the diameter of vapor and liquid line more than 3mm. However, miniature heat transport systems are interesting for some space and earth applications: nano-satellite technologies, aerial and ground vehicles. Proposed paper attempts to present some configurations of Miniature LHP (MLHP) [1] (total length 3-mm) with flat plate (diameter/length = /25mm) and cylindrical (diameter/length = 1/1mm) envelopes of evaporators. In this paper, heat transfer experiments and analysis of loop heat pipes are investigated for different linear and vibrating accelerations. Experimental tests verify that the Miniature LHP operates successfully by various acceleration effects and loop heat pipe heat transfer mechanism is of little sensitivity on acceleration fields. KEYWORDS heat transfer, loop heat pipe, acceleration, vibration, centrifuge. INTRODUCTION The Loop Heat Pipes (LHP) are heat transfer devices that are capable of operating effectively at any orientation (top heat, horizontal heat and bottom heat) in a gravitational field over long distances. In the loop heat pipes (or the capillary pumped loops) 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. LHP systems have the potential to transport large amounts of heat over long distance or unfavorable accelerations with minimal temperature drops. However, most of LHP experimental studies have been made in the absence of linear and vibrating accelerations, even though, almost all heat pipe applications involve ones. In fact, in applications where the LHP are an integral part of a machine, such as a flying apparatus or an electric motor, acceleration and vibration cannot be avoided. Therefore, some studies of the effects of acceleration and vibration on the performance of the MLHP must be carried out to determine if these effects will either aid or hinder the operation of the MLHP. The effects of the linear accelerations were studied on the LHP by using a 4.5-m diameter centrifuge table rotating with a time constant angular velocity. An experimental centrifuge table was developed to study the heat transfer mechanisms during acceleration field variations. The acceleration forces may be divided in two orthogonal components: a longitudinal one (parallel to the main length of the heat pipe) and a radial one. The longitudinal component is the most important because it influences directly the capillary heat transfer of the heat pipes. Transient acceleration fields were generated by using a variant radius on the spin table. The effects of vibrating acceleration were studied on the LHP by using vibrating table. The vibrating table was designed for testing heat pipes under vibrating acceleration up to 15g. Tests were conducted on this program for adverse (+) and favorable (-) longitudinal vibrating accelerations from g to 12g. 15

2 The main objective of this test program was to demonstrate the operational performance of MLHP heat transfer mechanisms during acceleration field variations. It is important to note that the dynamic of the acceleration change interests on the performance of MLHP. EXPEREMENTAL APPARATUS AND PROCEDURE Proposed paper attempts to present some experimental results including the influence of acceleration on the operation of the LHP with cylindrical and flat configuration of evaporators. Some typical schemes of the miniature LHPs (MLHP) with cylindrical and flat evaporators developed at the Ural State University (USU) (Heat mass transfer laboratory) are presented in Fig.1. MLHPs can compete with ordinary heat pipes when the heat-transfer distance exceeds.3-.5m, and it is necessary to ensure efficient operation at any orientation in 1-g conditions or when it is necessary to provide a flexible or complicated configuration of thermal coupling between the heat source and the heat sink. Table 1 presents the main specifications of MLHPs from stainless steel developed at USU. Figure 2 presents some variants of flat evaporators. Table 1. Main specifications of MLHPs (all data are experimental). Specifications MLHP-USU with cylindrical evaporator Total length, mm 3 Liquid/vapor line, mm 3/2 /2 Evaporator active-zone length, mm Condenser length/diameter, mm 85/9 85/9 Internal liquid/vapor line diameter, mm 2 x.5/2 x.5 2 x.5/2 x.5 Internal evaporator activezone diameter, mm 9 x x.7 Wick material Nickel Nickel Averaged pore radius, μm 1 1 Porosity, % Thermal conductivity, W/(m K) Permeability, 1-14 m Total mass of MLHP, g Maximum total heat input, W - horizontal position, ϕ = - vertical position (evaporator above), ϕ = +, 1g (ammonia) (acetone) (pentane) (freon 11) 1 (water) (ammonia) (acetone) (pentane) (freon 11) (water) MLHP-USU with flat evaporator (acetone) (pentane) (acetone) (pentane) Thermal resistance, K/W (acetone).1-.4 (acetone) At the condenser heat out was taken by means of water in the water jacket which were installed in cooling section. MLHP charging strategy was tested in order referred to above in Table 1. 16

3 MLHPs were successfully used for cooling elements in power electronics and laser engineering when a heat load with a high density is concentrated on a relatively small surface. On the whole MLHPs have demonstrated quite a satisfactory serviceability in very unfavourable conditions. Nevertheless, there are real possibilities for further decreasing thermal resistance and increasing heat transfer capacity of these devices. Fig.1. Miniature Loop Heat Pipe designs: 1 - charging tube; 2 evaporator; 3 wick; 4 vapor ducts; 5 compensation cavity; 6 vapor line (channel); 7 liquid line (channel); 8 condenser; 9 transistor or diode. C cylindrical, P - flat evaporator. 17

4 Fig.2. Variants of flat evaporators Heat was transferred to the working fluid through the tube wall by means of resistive electric heaters sandwiched around the outer wall of evaporator. The heat input was controlled through variable transformers and measured with wattmeter. At the condenser heat out was taken by means of water in the water jacket which were installed in cooling section. Water (or Air) flow rate and the inlet/outlet temperature were measured and heat transfer rates were calculated by calorimeter 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. Aircraft acceleration is an important factor in MLHP design. MLHP were designed and tested to demonstrate the ability of the Miniature Loop Heat Pipes to restart after dry-out due to high accelerations. The MLHP were tested at the Ural State University (Heat mass transfer laboratory) rotary table. The Ural State University rotary table was designed for testing heat pipes under acceleration up to 15g [2]. A schematic diagram of experimental apparatus is shown in Fig.3. Fig.3. Experimental centrifuge: 1 water cooling system; 2 thermostat and liquid pump; 3 automatic commutator of thermocouples; 4 heat pipe; 5 balance weight; 6 rotating speed electrical motor; 7 - overflow annular sinkhole; 8 collector. 18

5 It consists of a 4.5-m diameter centrifuge table rotating with a time constant angular velocity. The MLHP is mounted in a test box that is bolted to the rotary table. Heat is provided to the evaporator by an electric resistance heater. The maximum allowable power is roughly W, and is set by the limitations of the cooling water supply to the rotary table. An experimental centrifuge table was developed to study the heat transfer mechanisms during acceleration field variations. The using a variant radius on the spin table generated transient acceleration fields. There are three potential orientations for the MLHP tests: (1) Adverse acceleration (evaporator above condenser), (2) Favorable acceleration (condenser above evaporator), and (3) Transverse acceleration (MLHP at a constant radius on the spin table, acceleration directed normal to the MLHP length). Tests were conducted on this program for adverse and favorable longitudinal acceleration from g to 12g. Proper operation of the heat pipe in terrestrial and microgravity-based applications is well known. However, most of these experimental studies have been executed in the absence of vibrations. In fact, in applications where the heat pipe is an integral part of machine vibrations cannot be avoided. Therefore, some study of the effects of vibrations on the performance of MLHP must be carried out to determine if vibrations will either aid or hinder the operation of the MLHP. The MLHP (see Fig.1) was tested at the Ural State University (Heat mass transfer laboratory) vibrating table. The Ural State University vibrating table was designed for testing heat pipes under vibrating acceleration up to 15g. A schematic diagram of experimental apparatus is shown in Fig.4. Fig.4. Experimental vibrating table: 1 heat pipe; 2 case of electrical magnet; 3 winding of electrical magnet; 4 mobile coil; 5 alternator of low frequency; 6 - amplifier of low frequency; 7 - energy unit; 8 sensor of tensometry; 9 - energy unit of tensometry; 1 - frequency meter. Experimental parameter ranges were as follows: vibration (sinusoidal and random) frequency, to Hz (additional to16 khz), acceleration to 15g, amplitude of vibrations to 15 mm, inclination angle from horizontal to 1. 19

6 The MLHP is mounted in a test box that is bolted to the vibrating table. Heat is provided to the evaporator by an electric resistance heater. The maximum allowable power is roughly W, and it set by the limitations of the cooling air supply to the vibrating table. Tests were conducted on this program for adverse (+) and favorable (-) longitudinal vibrating accelerations from g to 12g. The MLHP (with the cooling air supply) was first tested on the vibrating table under adverse acceleration (MLHP located at the mobile part of the table and evaporator above condenser). EXPEREMENTAL RESULTS Testing Results by Constant Accelerations The MLHP were first tested on the rotary table under transverse acceleration (MLHP located at the edge of the table, radial acceleration directed normal to the MLHP length and evaporator above condenser) 1g adverse acceleration due to gravity. Figure 5 shows the averaged temperature of the MLHP evaporator for an adverse 1g acceleration due to gravity by the rotating centrifuge. flat evaporator Q=25W, a=1g Q=W, a=1g Q=15W, a=1g Q=25W, a=1g Q=W, a=1g Q=15W, a=1g cylindrical evaporator Time, min Time, min Fig.5. MLHP tests with an adverse constant acceleration: heat agent acetone. As can be seen from Fig.5 the evaporator temperature increase rapidly by Q = W and it is connected with the limitations of the cooling water supply to the rotary table. 1 Flat evaporator, acetone Q=15W Q=25W Q=W Averaged Acceleration, g ("- " Favorable, "+" Adverse) Fig.6. The comparative analysis of the MLHP operating characteristics in acceleration field

7 Fig.6 gives the comparative analysis of the MLHP operating characteristics for different accelerations and heat mode. The favorable acceleration decreases the evaporator temperature up to % for different heat mode. It is important to note that the dynamic of the acceleration change interests on the performance of MLHP. This experiment is shown in Fig.7. flat evaporator cylindrical evaporator 1 a= - 6g a= g a=1g a=6g a=12g 1 a= - 6g a= g a=1g a=6g a=12g Time, min Time, min Fig.7. MLHP dynamic characteristics by the acceleration change (Q = 25 W), heat agent acetone. As can be seen from this behaviour of Miniature Loop Heat Pipe there was no dry-out of MLHP at the interval of the acceleration (-6g g). There is the increasing of the evaporator temperature about % by the increasing of the accelerations from 6g to +12g at other conditions being equal. The above discussed experiments make it possible to use Miniature Loop Heat Pipe during acceleration field variations and loop scheme heat transfer mechanism is of little sensitivity on acceleration fields. Testing Results by Vibrations Tests were conducted on this program for the adverse (ϕ = + ) and favorable (ϕ = - ) inclination angle against gravity. The frequency of the sinusoidal vibrations ranged from to 1 khz, the acceleration due to vibrations from g to 12g and the amplitude of vibrations from to 15 mm. Fig.8 shows the averaged temperature of the MLHP evaporator (ϕ = +, adverse) for the maximum constant vibration acceleration (a f =5g) by the frequency vibrations. T, C W 25W 15W flat evaporator 1 f, Hz T, C cylindrical evaporator W 25W 15W 1 f, Hz Fig.8. Dependence the averaged temperature of the MLHP evaporator for a f =5g, ϕ = + on the frequency, heat agent acetone 21

8 Fig.9 shows the averaged temperature of the MLHP evaporator (ϕ = -, favorable) for the maximum constant vibration acceleration (a f =5g) by the frequency vibrations. T, C W 25W 15W flat evaporator 1 f, Hz T, C cylindrical evaporator W 25W 15W 1 f, Hz Fig.9. Dependence the averaged temperature of the MLHP evaporator for a f =5g, ϕ = - on the frequency, heat agent acetone As can be seen from this comparison, at other conditions being equal, there is the increasing of the MLHP evaporator temperature in interval of frequency - Hz by adverse vibrating acceleration. The influence of vibrating acceleration is absent by favorable vibrating acceleration. This fact is very interesting and it needs an attention. Perhaps a resonance phenomenon exists in interval of frequency - Hz by adverse vibrating acceleration. CONCLUSIONS The results obtained in heat transfer experiments in acceleration field are summarized as follows: (1) In spite of the Miniature Loop Heat Pipe is able to transfer heat under different accelerations up to 12g. There is the increasing of the evaporator temperature, about % by the increasing of the accelerations from 6g to +12g. There is no dry-out of the Miniature Loop Heat Pipe in this interval of accelerations. (2) The Miniature Loop Heat Pipe is able to transfer heat under different vibrating acceleration up to 1g in interval of frequency -1 Hz. There is the increasing of the MLHP evaporator temperature about (1-15)% in interval of frequency - Hz by adverse vibrating acceleration. The influence of vibrating acceleration is absent by favorable vibrating acceleration. This fact is very interesting and it needs an attention. Perhaps a resonance phenomenon exists in interval of frequency - Hz by adverse vibrating acceleration. The above discussed experiments make it possible to use Miniature Loop Heat Pipe during acceleration field variations and loop scheme heat transfer mechanism is of little sensitivity on acceleration fields. References 1. Maidanik Yu.F., State-of The-Art of CPL and LHP Technology The Proceeding of 11 th International Heat Pipe Conference, Kiseev V., Belonogov A., Belyaev A., Influence of Adverse Acceleration on The Operation of An Anti-gravity Heat Pipe J. Engineering Physics, Vol., No 4, 1986, pp (in Russian). 22