J. Cent. South Univ. (2012) 19: 2578-2583
DOI: 10.1007/s11771-012-1313-x
Characterizing pressure fluctuation into single-loop oscillating heat pipe
PARK Yong-ho1, Md. Riyad Tanshen2, Md. J. Nine2, CHUNG Han-shik3, JEONG Hyo-min3
1. Department of Mechanical Engineering, Koje College, Jangseungpo, Koje, Gyeongnam, Korea;
2. Department of Mechanical and Precision Engineering, Gyeongsang National University,
445 Inpyeong-dong, Tongyeong, Gyeongnam, 650-160, Korea;
3. Department of Energy and Mechanical Engineering, Institute of Marine Industry, Gyeongsang National University,
Cheondaegukchi-Gil 38, Tongyeong, Gyeongnam, 650-160, Korea
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract: The pressure characteristics inside single loop oscillating heat pipe (OHP) having 4.5 mm inner diameter copper tube with the loop height of 440 mm were addressed. Distilled water was used as working fluid inside the OHP with different filling ratios of 40%, 60% and 80% of total inside volume. Experimental results show that the thermal characteristics are significantly inter-related with pressure fluctuations as well as pressure frequency. And the pressure frequency also depends upon the evaporator temperature that is maintained in the range of 60-96 ℃. Piezoresistive absolute pressure sensor (Model-Kistler 4045A5) was used to take data. The investigation shows that the filling ratio of 60% gives the highest inside pressure magnitude at maximum number of pressure frequency at any of set evaporator temperature and the lowest heat flow resistance is achieved at 60% filling ratio.
Key words: oscillating heat pipe; pressure fluctuation; heat flow resistance; filling ratio
1 Introduction
It is well established that oscillating heat pipes (OHPs) are widely used to meet the great challenge of heat transfer inside sophisticated electronics. Oscillating and pulsating phenomena play a great role in the field of heat transfer to control heat transfer into micro cavity. The self exciting oscillation inside the OHP that is driven by the fluctuation of pressure waves occurs much more and heat transfers quickly from one end to another. The pressure fluctuation occurs because of nucleate boiling by evaporative section and condensation of the working fluid. Oscillating heat pipe can transfer heat in quick response in any orientation. This work shows all about vertical orientation of single loop oscillation heat pipe.
Lately, many experiments have been conducted in the field of oscillating heat pipe because of its specific features. Extended investigations of OHP have been conducted since the first OHP was developed by AKACHI in 1990 [1]. The mechanism occurring in OHP is the utilization of pressure change in volume expansion and contraction during phase change to excite oscillating motion of liquid plugs and vapor bubbles between evaporator and condenser. By comparing OHP with other conventional heat transfer pipes, the unique feature of OHPs is that there is no wick structure to return the condensate to the evaporator and therefore there is no countercurrent flow between the liquid and the vapor flows because they both operate in the same direction [2]. 1) The thermally-driven oscillating flow inside the capillary tube effectively produces some free surfaces, which significantly enhances evaporating and condensing of heat transfer. 2) The oscillating motion in the capillary tube significantly enhances the forced convection in addition to the phase-change heat transfer. These significant characters of OHP make itself very special heat transfer device in modern application.
Past works over OHPs can be included within several features, like heat transfer characteristics and capability with different filling ratios, flow visualization inside OHP, effects of length ratio and diameter on performance of OHP, nanofluid and other applicable fluids used as working fluid for developing OHP performance.
KHANDEKAR et al [3], RITTIDECH et al [4], TONG et al [5] discussed the effects of many parameters on thermal performance, such as internal diameter, number of turns, working fluid and inclination angle of the device. WANG and NISHIO [6] investigated the effect of length ratio of heating section to cooling section on the ultimate heat transport capability of OHP. The effect of gravity on slug flow and the effect of number of turns on spatial dynamic pressure influence the OHP performance. Besides, the input heat is also a strong parameter that affects dynamic instability, especially in density wave oscillation [7-8].
SAHA et al [9] conducted flow visualization for closed-loop PHP made from Teflon tube of 2 mm in internal diameter and partially filled with R142b. The PHP consists of 10 meandering turns and it is 400 mm from the evaporator to condenser. The evaporator was heated by a hot bath and the condenser was cooled by a cold bath. It was concluded that the best thermal performance for the PHP is achieved when the filling ratio is from 0.5 to 0.6.
In 1998, CHANDRATILLEKE et al [10] developed the cryogenic loop heat pipes. The development of cry cooler extended superconducting magnets applications, where heat transport distance is large, and the heat conduction by a copper block will be constrained by its cross section transport capacity. MO et al [11-12] show that the heat transport capacity of loop heat pipe with liquid nitrogen as working fluid is very low, only 26 W when it operates in horizontal direction and its lowest heat flow resistance reaches 1.3 K/W, which is too high for most of cryogenic heat transport system.
Most recently, it has been found that when nanoparticles [13] or micro particles are added into the base fluid in an OHP, the heat transport capability can be increased. The thermally-excited oscillating motion in the OHP can make the particles suspend in the base fluid. Although the nanoparticles added in the base fluid cannot largely increase the thermal conductivity [13], the oscillating motion of particles in the fluids might have additional contribution to the heat transfer enhancement in addition to the thermal conductivity. MA et al [14] charged the nanofluid into OHP and found that the nanofluids significantly enhance the heat transport capability of OHP. The investigation shows that OHP charged with diamond nanofluids can reach a heat flow resistance of 0.03 oC/W at power input of 336 W.
In 2010, QU et al [15] also conducted an investigation of the effect of spherical 56 nm Al2O3 particles on the heat transport capability in an OHP, and found that the Al2O3 particles can enhance heat transfer and an optimal mass fraction exists. Although these investigations have demonstrated that the particles can enhance heat transfer in an OHP, it is not known whether there exists an optimum particle size for a given type of particles.
The present work focuses on pressure characteristics inside oscillating heat pipe. The relation with heat transfer mechanism is investigated. It is well known that the heat transfer by oscillating heat pipe occurs for quick fluctuation of pressure intensity; however, works can be hardly found over pressure distribution inside OHP. From this point of view, the present work carries a fundamental role of OHP.
2 Experimental setup
The experimental apparatus is shown in Fig. 1 and it consists of a single loop OHP, water heater, cooling bath and data acquisition system. The heat pipe is made by bending a 937 mm copper tube with an inner diameter of 4.5 mm and outer diameter of 6.2 mm having the loop length of 440 mm. The single loop copper OHP was oriented vertically and the downward loop section with the length of 100 mm was submerged into the evaporative section. Evaporative temperature was kept constant by flowing hot water heated by tube type heater with digital control system. Similarly, the 140 mm of upper loop section was inserted through the condensing tank and kept sealed. Condenser temperature was maintained at 16-17 ℃ by using isothermal cooling unit. In the middle of this OHP loop, 200 mm adiabatic section was covered by 2 cm-thick glass wool to insulate this section thermally. T-type thermocouple was soldered to the outer wall of OHP in both condensing and evaporative sections to measure the wall temperature of OHP. After setting temperature of evaporative section, the temperature of evaporative fluid got stable within 5 min. One piezoresistive absolute pressure sensor (Model-Kistler 4045A5) was set with another small bypass tube coming out from condensing section of OHP to take the pressure characteristics inside the tube. The sensor was perfectly sealed and tested several times. Using lab view program with the help of computer, all the pressure data were acquired after getting the system stable. Data acquisition rate was 100 s-1 and data taking duration was about 10 min. To find out the frequency distribution, FFT analysis has been done with 10 240 data. For the convenience of understanding, a part of real experimental setup is shown.
Fig. 1 Schematic diagram of experimental setup
3 Data reduction
After receiving thousands of raw data signal in the form of voltage, it has been converted into pressure and analyzed to find mean and RMS value in following way:
1) Mean pressure
(1)
where N is the total number of data recorded in a period of time (i=1, 2, 3, …, N).
2) Pressure fluctuation or RMS value of P component
(2)
where N=10 240.
3) Heat flow resistance (RT/T ) has been calculated for the system to evaluate the performance as the evaporator section is subjected under constant wall temperature:
(3)
where RT/T is the heat flow resistance, defined as the ratio of temperature obtained by subtracting average condenser wall temperature 
from average evaporator wall temperature 
to the hot water temperature 
supplied to evaporative tank.
4 Result and discussion
The result of OHP varies with heat input energy source of fluid in the evaporator and the filling ratio of working fluid. Besides the shape and diameter, angle of installation has great influences on performance of OHP.
Figure 2 presents the effect of different filling ratios on heat flow resistance at various operating evaporative temperatures. Here, it can be shown that the lowest heat flow resistance has been achieved at 60% filling ratio at any evaporative temperature. With increasing the evaporative temperature, the heat flow resistance decreases for 60% and 80% filling ratio. But in case of 40% filling ratio, heat flow resistance increases with increasing evaporative temperature because of low volume ratio that is not capable to carry more heat until the pick of condensing section of OHP. Besides, the mass of fluid is so small in amount that it brings only less amount of heat to condenser and that’s why the temperature difference between evaporator and condenser is so small. In the case of 80% filling ratio, more fluids make much vapor plug on the pick and do not have a feasible space to ease the heat transfer. Because of more vapor plug in the case of 80% filling ratio, the driving force is decreased for the oscillation and transportation of heat from the evaporator to the condenser. So, 60% filling ratio facilitates the optimum conditions in mass and in volume to transport more heat from evaporative section that minimizes heat flow resistance and maximizes heat transfer through OHP.
Fig. 2 Heat flow resistance at different filling ratios
For 60% filling ratio, Fig. 3 shows pressure distribution inside OHP where different evaporative temperatures have been maintained. With increasing evaporative temperature, inside pressure of OHP increases and at the temperature near boiling point, the value of pressure distribution becomes multiple of 60 ℃ or 80 ℃. Similarly, in Fig. 4, it can be shown that temperature distribution at condensing section gets higher with increasing evaporative temperature. Pressure and temperature increase with passing time and with increasing evaporative temperature. It takes almost 40 s to get thermal stability. Both of pressure and temperature give higher magnitude at 96 oC and it gets down at lower evaporative temperature. Pressure and temperature distribution profiles have a great similarity and temperature distribution depends upon pressure distribution inside OHP. It is recommended to use evaporative temperature near boiling point of working fluid to get high thermal efficiency for OHP.
Fig. 3 Pressure distribution at different evaporative temperatures at 60% filling ratio
Fig. 4 Temperature distribution at 60% filling ratio
In Fig. 5, the mean pressure for different filling ratios at different evaporative temperatures is shown. Among all three filling ratios, 60% filling ratio shows higher magnitude of mean pressure at all of set evaporative temperatures compared with other filling ratios of 40% and 80%. Mean pressure at 40% filling ratio results in flat value while 60 % and 80 % filling ratios show gradual inclination with increasing evaporative temperature. This happens because more liquid plugs and vapor bubbles are generated at 60 % filling ratio, which increases the mean pressure inside OHP.
Fig. 5 Comparison of mean pressure inside OHP among different filling ratios
Figure 6 shows that the RMS value of pressure gives higher magnitude, which implies that the intensity of pressure is large in 60% filling ratio. This happens because the liquid plug inside OHP reduces at 60% filling ratio compared with 80% filling ratio and makes it free to transport heat from evaporative section, as a result, the pressure near condenser increases. So, the fluctuation value from mean pressure is much more at 60% filling ratio because of creating more slug into heat pipe. In the case of 80% filling ratio, pressure fluctuation linearly increases until 80 ℃ evaporative temperature, but further increasing evaporative temperature does not make any change in performance. This occurs because of the mass of fluid at 80% filling ratio is large enough to create vapor plug inside the condensing section and discontinues the flow from evaporator to condensing section, which results in less heat transfer from evaporative section. So, 60% filling ratio is the optimum filling ratio that produces more pressure fluctuation inside OHP.
Figures 7-9 show individual frequency distribution for each applied filling ratio. For 40% filling ratio in Fig. 7, the magnitude and number of frequency are not so comprehensive. The magnitude increases with increasing evaporative temperature but the frequency is nearly zero at lowest applied temperature of 60 ℃. But at 80 ℃ and 96 ℃ it gives the number of frequency of about 2 Hz. Similarly, it can be shown for 60% filling ratio in Fig. 8 that both frequency and magnitude are strongly comprehensive and achieve higher values than those of 40% and 80% filling ratios. The best performance in pressure characteristics is driven by 60% filling ratio.
Fig. 6 RMS value of pressure inside OHP at different filling ratios
Fig. 7 Frequency distribution at 40% filling ratio: (a) TE=60 ℃; (b) TE=80 ℃; (c) TE=96 ℃
Fig. 8 Frequency distribution at 60% filling ratio: (a) TE=60 ℃; (b) TE=80 ℃; (c) TE=96 ℃
Fig. 9 Frequency distribution at 80% filling ratio: (a) TE=60 ℃; (b) TE=80 ℃; (c) TE=96 ℃
In oscillating heat pipe, heat transfer occurs because of repeated pressure fluctuation and the more repetition of pressure fluctuation means more heat transfer. Figure 10 shows the comparison of mean pressure frequency at various temperatures at three different filling ratios. The higher pressure frequency is achieved at 60% filling ratio at any evaporative temperature calculated by FFT analysis using at least 10 240 data for each case. Finally, from the above discussion of experimental result of pressure and pressure frequency behavior, heat flow resistance and temperature distribution, it can be concluded that 60% filling ratio gives optimum result among all of the variables.
Fig. 10 Comparison of frequency among different filling ratios varying with temperature
5 Conclusions
1) 60% filling ratio creates more slugs into OHP, which drives more heat from evaporative section to condensing section. So, the heat flow resistance surely is improved at 60% filling ratio. It is recommended that to get higher thermal efficiency, the heat input in evaporative section should be near the boiling point of working fluid at 60% filling ratio.
2) Temperature distribution totally depends upon the pressure distribution into OHP. Repeated pressure fluctuation reduces heat flow resistance, and high evaporative temperature and the optimum filling ratio cause high rate of pressure fluctuation.
Nomenclature
References
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(Edited by YANG Bing)
Foundation item: Project(2011-0009022) supported by Basic Science Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology of Korea
Received date: 2011-12-12; Accepted date: 2012-04-26
Corresponding author: JEONG Hyo-min, Professor: Tel.: +82-55-772-9114; Fax: +82-55-772-9119; E-mail: hmjeong@gnu.ac.kr
