A Design and Performance Test of the Visualization within a Capillary Pumped Loop (*).

Transcription

1 IL NUOVO CIMENTO VOL. 16 D, N. 7 Luglio 1994 A Design and Performance Test of the Visualization within a Capillary Pumped Loop (*). W. K. LIN (1), p. L. WONG(1), P. C. WONG(1) P. C. CHENG(1) and H. PAUL WANG(2) (1) Department of Nuclear Engineering, National Tsing-Hua University Hsinchu~ Taiwan) R.O.C. (2) Department of Environmental Engineering, National Cheng Kung University Taiwan~ R.O.C. (ricevuto il 28 Ottobre 1994) Summary. -- Capillary Pumped Loop (CPL) is a thermal management device for advanced spacecrafts. This paper presents the operating principles of a CPL and the design characteristics of the components. Another purpose of this experiment was to build up a transparent loop in order to better determine and observe the internal thermofluld dynamics. The results indicated that the CPL could be in normal operation under various heat modes. Observation also told Non-Condensed Gas (NCG) was moving and collecting to somewhere between the condenser section. PACS Diffusion and ionic conduction in liquids. PACS Studies of specific liquid structures. PACS Cc - Conference proceedings Introduction. Two-phase thermal transport systems offer significant performance advantages over convectional single-phase systems for various reasons, including orders-ofmagnitude less pumping power with orders-of magnitude greater heat transfer coefficients and hence smaller temperature gradients for isothermally improved and smaller radiators. Recently, a larger and more complex spacecraft has been developed that has higher-power requirements than spacecrafts of the past. During the past ten years, a two-phase heat exchanger has been under development in order to meet this need. One concept, the capillary pumped loop (CPL) has been developed to a high state of technology readiness and is a prime candidate for thermal control of advanced spacecrafts [1, 2]. The concept of CPL was initially proposed by Stenger of NASA/Lewis [3] in the mid-1960s; however, it was not until late 1970s that serious development effort, (*) Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4-8, 1994.

2 884 W. K. LIN, P. L. WONG, P. C. WONG, P. C. CHENG and H. PAUL WANG '-= [0 vapour header! ~",solator 9 r ~_~2 ~_ "7 ~ ~ ~ ] 8 F ~. 1 ~ liquid header ~= f/~ll [I~)! I I IIF ~ 27 1[I IIIl IIII 19 ~ 1.~ ~ ~ [,2 H25 reservoir line ~=~: ~ 7~2~[(~Y~) )] 139"~:~ C "1) 2-:s 9 3"0 3 subcooler reservoir Fig The schematic diagram of CPL transparent loop. aiming at future spacecraft applications, was underway in the States. The innovative technology utilized in the CPL is a wicking material that induces a pumping force on the working fluid via capillary action. The advantage that this system has over convectional thermal management systems is its ability to transfer large heat loads over long distances at a controlled temperature [4]. The basic design of a CPL, illustrated schematically in fig. 1 [5-7], consisted of two evaporator sections, a vapor header, a condenser/subcooled section, a liquid header, and a reservoir. The CPL was a continuous loop with vapour and liquid flow in the same direction. The basic CPL operating principles were: when heat was applied to the evaporator, liquid was pumped from the porous polyethylene material and made to flow through the wick to the vapour header, and the condenser section, where heat was removed. The liquid then passed through a subcooled section which collapsed any remaining vapour bubbles and provided enough subcooling to the fluid to enter the isolator. A saturated liquid was pumped from the reservoir to control the system temperature as well as the pressure; this saturated liquid flowed through the reservoir line which connected the condenser/subcooler section, the evaporator sections and the reservoir section. The unique feature of a CPL, in contrast to convectional heat pipes, was to permit the wick structure to be isolated from the evaporator section. The advantage is that the smooth-walled tubing may be applied to the remainder of the system, and this reduces the friction pressure drop in the system, hence resulting in a higher heat transport capability than that of the conventional heat pipes Design characteristics of CPL. Evaporator design The CPL evaporator design consists of a porous tubular wick that was force-fit within an axially grooved aluminium tubing (shown in fig. 2); the extruding alumi-

3 A DESIGN AND PERFORMANCE TEST ETC. 885 ~ l~mln,,ra Fig The cross-section area of the evaporator. ~it: mm nium wick tube was 39 mm OD, and 25 mm ID, the wick structure consisted of 82 fins, the distance between two fins was 0.4 mm, the lengths of the fin tip and fin bottom were 2 mm and 1.5 ram, respectively, the depth of the fin was 5 mm, the length of the evaporator was 400 ram. Into the aluminium tube was inserted another porous material with 25 mm OD and 15 mm ID, the porous material was tightly attached to the tip of the fin, the effective porous diameter was m and the permeability was about am 2. The two evaporators were heated by an electric wire, which was wrapped around the aluminium tube, and then insulated by a fiber glass. Evaporator isolator design The purpose of the isolator was to prohibit vapour from backflowing from the wick of the evaporator; another important purpose of the wicked isolator was to prevent any non-condensible gas bubbles, formed in the remainder of the system, from ~ 100- f A 400 Fig The schematic diagram of the isolator. section A-A unit: mm

4 886 W. K. LIN~ P. L. WONG, P. C. WONG, P. C. CHENG and H. PAUL WANG migration into individual pumps causing a deprime condition. The construction of the wick isolator is shown in fig. 3, with two inverted,,t, tubes connected with two evaporators. Into the main aluminium tube was inserted a porous polythyene material to prevent the vapour fluid from backflowing to the condenser or the reservoir. Condenser design When the working fluid left the evaporators, the fluid was usually saturated or slightly superheated. The condenser was made of smooth transparent pyrex glass, the condenser section was sized as to dissipate all the heat input acquired in the evaporator. The schematic diagram is shown in fig. 4. Subcooler design A complete condensation of the vapour phase before the working fluid returns to the evaporator would result in the stable operation. So that a certain degree of subcooting was required for the liquid entering the evaporators. This was because a finite amount of heat, conduced through the end fitting of the evaporator inlet line, would cause bubbles to be generated from a saturated liquid, and the bubbles might accumulate and block the liquid header and result in the deprimes of the wick. So the design of the subcooler section also required adequate size for proper energy dissipation; the subcooler was also made of two stainless-steel tubes, the upper one connected with the condenser section, and the bottom one connected with the reservoir shown as in fig. 5. Vapour / liquid headers For reasons of better observation, both the condenser and the subcooler were immersed into a cool-water bath, thus one may observe the fluid dynamic from the transparent pyrex glass. However this could not be done because of the needed isolation on both headers. Hence, one of the typical features of these two headers was to design a dual-layer glass, the air between the mezzanine was evacuated to 10-5 Torr, thus the heat dissipation by means of heat conduction and convection could be reduced to a minimum, and through this kind of design, one might also observe the vapour/fluid dynamic from a dual-layer transparent tube. A [20 to liquid heater I ~J to reservoir line,l- cool-water bath 528 Fig The schematic diagram of the condenser. I to condenser to reservoir,,i urlit: toni

5 A DESIGN AND PERFORMANCE TEST ETC. 887,+ cool-water bath unit: mm * " Fig The schematic diagram of the subcooler. Reservoir design The primary function of the reservoir was liquid inventory management and temperature control; the design requirements of the reservoir were a) the minimum as possible pressure drop from the of the reservoir, b) an accurate temperature control, c) to ensure the working fluid from was liquid phase. The current design of the reservoir was shown in fig. 6. This was made of stainless steel with 100 mm OD, 922 mm ID and 600 mm length, one side was mounted with a view window. To ensure the fluid from the reservoir was liquid phase, a 10 mm diameter hole was dripped on the bottom of the another side of the reservoir, so that the fluid came out was liquid phase because of the gravitational effect Performance test modulus. Several test configurations were made and described as following: a) constant power test: various constant-power levels were applied to both "l view window (glass) B fluid level section B-B Fig The schematic diagram of the reservoir. B 600 ~ A n:o Ul -- reservoir ~ A section A-A

6 888 W.K. LIN, P. L. WONG, P. C. WONG~ P. C. CHENG and H. PAUL WANG evaporators. The objective of these tests was to verify the system start-up and to be able to operate normally under the system operating range. b) Heat load sharing test: one evaporator was introduced 100W, and the another one was not. The purpose of this test was to demonstrate the ability of an individual pump to convert from the evaporator mode to the condenser mode. c) Low-power limit test: both of the two power levels were below 100 W per evaporator. The objective of these tests was to investigate the deprime phenomenon at lower power levels. d) Variable set point of the reservoir temperature after heat load sharing test: the reservoir set point was changed or allowed to vary. The objective of this test was to demonstrate that system saturation temperature could be varied via the reservoir controller. e) The natural prime after heat load sharing test: after the heat load sharing mode of operation was conf'u'med, power was applied to the unheated evaporator. The purpose of this test was to demonstrate the ability of the evaporator to prime with an inlet temperature level near the saturation temperature, and prove that the unheated evaporator could convert from the condenser mode to the evaporator mode again a ~ reservoir inlet 40 v 20 i 120 ~100 8O E 1 =200W E2ffi200W i B i i i T i b~ inlet 'P I 60 40,~- /~i/ F subcooler o reservoir EI=200W..ii, ~.-~ 0 I'0 2'0 E2 =200W ~] 3'0 4'0 5'0 60 7'0 80 time (min) Fig The temperature distribution of constant-power test for evaporators 1 (a)) and 2 (b)). I

7 A DESIGN AND PERFORMANCE TEST ETC Test results. Constant-power test. Figures 7a), b) show the case in which the constant power 200 W was applied to both evaporators. The temperatures of the evaporators started from 22 ~ and reached the saturated temperature very quickly (within 6 min). It was proved that the liquid fluid was pumped through the porous material and in the meantime the temperature of the reservoir was dropped because the vapour fluid in the vapour header pushed the cool liquid fluid into the reservoir, thus reducing the temperature by several degrees and then recovery to a normal operating system temperature again. Heat load sharing test. Figures 8a), b) show the case in which the evaporator II (E2) was applied 100 W, part of the vapour fluid flowed back to the evaporator I (El) causing the temperature of the latter to increase gradually and to finally reach the saturated temperature as well as evaporator II; it was implied that E1 works as a function of the condenser during this period a) L ~1' 60 reservoir!~ ~ 2o G) i00 N 8O EI=0W E2=100W I I I I b), ~ ~] inlet / ~ inlet subcooler to reservoir I I t EI=0W E2 = 100W 0 80 I ' time (rain) Fig The temperature distribution of heat load sharing test for evaporators 1 (a)) and 2 (b)).

8 890 W. K. LIN, P. L. WONG, P. C. WONG, P. C. CHENG and H. PAUL WANG a) reserv~ "~L-~ ~ ~ inlet ~ v :- '~x? ~ i / ~ isolator ~ ~ o 20 ~ 0 I EI=g0w subcooler to reservoir.1 I E2--SOW -I i I I I I, 60-~~reserv~ b) ~ 80 i i IIJll ~f-"-~ inlet ~'~'~ 40 ~ #, ] / ~ " ~ v isolator ~-~ q subcooler to reservoir EI=50W E2=50W 0 o lo do 70 8o time (rain) Fig The temperature distribution of low-power limit test for evaporators 1 (a)) and 2 (b)). Figure 8a) also shows that when the temperature of the E1 rose up, the temperature of the reservoir started to fall down. Low-power limit test. In fig. 9a), b) two evaporators are shown with a power below 100W. The reservoir temperature dropped down when both evaporators temperatures were boosted; however, the temperatures of the reservoir and the subcooler would oscillate periodically under such low power and the system was not stable at this moment. Variable set point of reservoir temperature after heat load sharing test. When the heat load sharing was confirmed, the power of the reservoir was stripped, fig. 10a, b), and the temperature of the reservoir was then dropped down to 42 ~ however, the temperatures of the two evaporators only changed slightly, say 73 ~ to 70 ~ the situation kept on going until the power of reservoir was restarted, then all the temperature rose again.

9 A DESIGN AND PERFORMANCE TEST ETC O a) reservoir J-'-~ 4O ~ 20 t~ 100~ / /' isolator ~ ~ ~, ~ ~ ~_-~-~.~ subcooler to reservoir EI=0W, E2=150W N 80 6o reservoir isolator ~ 20 F I_ EI=0W subcooler to reservoir ~t E2=150W '0 4; time (rain) Fig The temperature distribution of variable set point of reservoir after heat load sharing test. a) Evaporator 1, b) evaporator 2. Natural priming after heat load sharing test. As in the previous case, when the heat load sharing was confirmed again, the power of evaporator I started to be introduced up to 100 W, fig. lla), b). Both the temperatures of evaporator I and the reservoir were stepped down first and then picked up very quickly; it was proved that the evaporator I works as a function of the condenser under heat load sharing, but when the power is added to the evaporator I, it seems working as a function of the evaporator again Conclusion. The capillary pumped loop has been shown to be a vary versatile and flexible system capable of transporting large heat loads over long distances and operating over a wide range of conditions. In this typical loop, one may observe the fluid dynamic from the vapour header to the liquid header. One of the interesting points is that during a period of operation, all the NCG accumulate somewhere between the condenser section; since these NCG

10 892 w.k. LIN, P, L. WONG, P. C. WONG, P. C. CHENG and H. PAUL WANG a) reservoir l!.,l, I ~ /f~ ~! J ~ isolator]- ~9 20 o 120 I I L EI=0W ~l _ EI=100W ~1 i - E2=100W - ~- " E2=IOOW - l b)!i inlet 80 ~ " ~ ~ ~,. ~. ~. ~... reservoir / r~..... ~ ~ /1- / ts---._.." I:i 40 ~ suocooierl ou~mt so reservoir /~' EI=0W... i. EI=100W =l I~ E2fl00W "I- E2=100W 0 o 1'o io 3`0 4o 5'o 'o s'o 90 time (rain) Fig The temperature distribution of natural priming after heat load sharing test. a) Evaporator 1, b) evaporator 2. affect the system temperature so considerably, one may consider in the future to device a gas strap on the condenser section to improve the transport ability of the CPL. REFERENCES [1] J. T. Ku and R. MCISTOSH: Recent developments in capillary pumped loop technology, VIII IHPC, September 14-18, [2] J. T. Ku and E. J. K~OLICZEK: Capillary pumped loop gas and hitch hiker hight experiments, AIAA/ASME IV Joint Thermophysics and Heat Transfer Conference. AIAA , June 2, 1986, Boston, Massachusetts. [3] F. J. STENGER: Experimental feasibility study of water-filled capillary-pumped heat-transfer loops, NASA TMX-1310, NaSA Lewis Research Center, Clevend, Ohio. [4] J. T. Ku and E. J. KROLICZEK: Capillary pumped loop technology development, VI International Heat Pipe Conference, May 25-29, Grenoble, France (1987), p [5] J. Ku, S. YUN and E. KROLICZEK: A prototype heat pipe heat exchanger for the capillary

11 A DESIGN AND PERFORMANCE TEST ETC. 893 pumped loop flight experiment, AIAA XXVII Thermophysics Conference, July 6-8, 1992, Nashville, Tennessee. [6] J. Ku and E. J. KROLICZEK: A high spacecraft thermal management system, AIAA Thermophysics, Plasmvxlynamics and Lasers Conference, June 27-29, San Antonio, Texas (1988). [7] E. J. KROLICZEK, Z. KU and S. OLLENDORF: Desig~ development and test of a capillary pump loop heat pipe, AIAA, XIX Thermophysics Conference, June 25-28, Showings, Colorado (1984), p. 84.