Flow Visualization inside Thermosyphon for Measuring Heat Transfer Limit

2017 ◽  
Vol 139 (2) ◽  
Author(s):  
Jungho Lee ◽  
Jaebum Park ◽  
Jinsub Kim ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Film ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Heat Input ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Heat Transfer Limit ◽  
Condensed Liquid

Heat pipe is a highly effective passive heat transfer device using phase change within small temperature difference. It is noted that heat pipe should be operated under heat transfer limit for practical heat pipe heat exchanger applications. The measurement in local and overall heat transfer coefficient is significant to anticipate the heat transfer limit. The wall temperatures and inner working fluid temperatures were measured to determine the heat transfer coefficient. The adiabatic part with transparent Pyrex glass was visualized to understand flow behaviors inside the thermosyphon. The dynamic behaviors of condensed working fluid were visualized for the specific tilted angle and power inputs at pseudo steady-state. At low heat input of 250W, the thin condensed liquid film is observed to be returned from condenser to evaporator. With increasing heat input of 500W, the nucleate boiling starts to occur in evaporator. More activated vapors turn to make wavy motion in free surface of the returned condensed liquid film which is thickened. In power input of 1,250W, the vigorous flow motion happens periodically and the interaction between vapor and liquid bursting reaches a maximum heat transfer which is led to the heat transfer limit in the thermosyphon. Over heat transfer limit (2,000 and 2,500W), the overall heat transfer is decreased when the degree of bursting motion between vapor and liquid is gradually reduced.

A Loop Thermosyphon With Hydrophobic Spots Evaporator Surface

2018 ◽  
Author(s):  
Hongbin He ◽  
Biao Shen ◽  
Sumitomo Hidaka ◽  
Koji Takahashi ◽  
Yasuyuki Takata
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Immersion Method ◽  
Two Phase ◽  
Coating Materials ◽  
High Heat Transfer ◽  
Coated Surface

Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling improves heat transfer coefficient (HTC) a lot compared to single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circle spots (non-electroplating Ni-PTFE, 0.5∼2 mm in diameter and 1.5–3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and lower the incipience temperature overshoot using water as the working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests with heat loads (30 W to 260 W) reveals the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation sites density. Hydrophobic circle spots coated surface with diameter 1 mm, pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. The comparison of three evaporator surfaces with same spot parameters but different coating materials is carried out experimentally. Ni-PTFE coated surface with immersion method performs the optimal performance of the thermosyphon.

Thermal Characteristics of a Cylindrical Heat Pipe using Multi-Layer Screen Mesh Wick

2018 ◽  
Vol 10 (5) ◽  
Author(s):  
R. Sankar Rao ◽  
S. Bhanu Prakash
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Thermal Resistance ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Thermal Characteristics ◽  
Wire Mesh ◽  
Working Fluid ◽  
Wick Structure ◽  
Screen Mesh

Heat pipe is the most widely used heat exchanging device in removal of heat from any given system at a faster rate. The thermal characteristics of heat pipe with single and multi-layered screen mesh wicks have been observed with two working fluids water and acetone. Heat pipe of length 250 mm and 12.7 mm outer diameter, made of copper material is used in all the trials of with and without wick structure. A 100 mesh stainless steel screen wire mesh is chosen as wick structure. Experiments were conducted at different heat loads and various inclinations with 100% fill ratio in evaporator. The performance is measured based on total thermal resistance and overall heat transfer coefficient. The heat pipe is found effective at 60o inclination with acetone as a working fluid and with four layered screen mesh wick. Uncertainty in thermal resistance and heat transfer coefficient is calculated for a heat input of 10W at 0 and 60 inclinations.

Experimental Study of Boiling Phenomena by Micro/Milli Hydrophobic Dot on the Silicon Surface in Pool Boiling

2009 ◽  
Author(s):  
Hang Jin Jo ◽  
Hyungmo Kim ◽  
Ho Seon Ahn ◽  
Seontae Kim ◽  
Soon Ho Kang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Silicon Surface ◽  
Room Temperature ◽  
Pool Boiling ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
Working Fluid

Many pool boiling experiments to enhance the nucleate boiling condition have been conducted and could get brilliant and challengeable results. A consensus was that CHF and heat transfer were affected by a modified heating surface. One of the efforts was the nanofluids experiments, and they have exhibited an incredible enhancement of CHF when nanofluids have been used as a working fluid in pool boiling. The results also have showed clearly that such large CHF enhancement came from the deposition of nanoparticles on the heating surface changing the surface condition. The surface covered by oxidized metal nanoparticles has a high wettability, and so it affects CHF. The fact that the wettability effect is significant to the enhancement of CHF is also supported by other kinds of boiling experiments. In addition, many researchers reported that wettability enhances not only CHF but also nucleate boiling heat transfer coefficient. In this regard, the excellent boiling performance (a high CHF and a high heat transfer coefficient) in pool boiling could be achieved through some favorable surface modification which satisfies the optimized wettability condition. For finding the optimized condition, we design the special heaters to examine how two materials, which have different wettabilities, affect the boiling phenomena. The special heaters have hydrophobic dots on the silicon surface. The hydrophobic dots lead to an early bubble inception. The bubble interface is bounded on the material boundary. The peculiar teflon(AF1600) is used as the hydrophobic material. The contact angle of the heating surface which is made by teflon is 120° to water at the room temperature. The contact angle of the silicon surface is 60° at the room temperature. The experiments using the micro hydrophobic dots and milli hydrophobic dot are performed, and the results are compared with the reference surface.

Effect of Wick Characteristics on the Thermal Performance of the Miniature Loop Heat Pipe

2009 ◽  
Vol 131 (8) ◽  
Author(s):  
Randeep Singh ◽  
Aliakbar Akbarzadeh ◽  
Masataka Mochizuki
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Thermal Performance ◽  
Working Fluid ◽  
Vapor Flow ◽  
Loop Heat Pipe ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics

Two phase heat transfer devices based on the miniature version of loop heat pipe (LHP) can provide very promising cooling solutions for the compact electronic devices due to their high heat flux management capability and long distance heat transfer with minimal temperature losses. This paper discusses the effect of the wick properties on the heat transfer characteristics of the miniature LHP. The miniature model of the LHP with disk-shaped evaporator, 10 mm thick and 30 mm disk diameter, was designed using copper containment vessel and water as the working fluid, which is the most acceptable combination in electronic cooling applications. In the investigation, wick structures with different physical properties including thermal conductivity, pore radius, porosity, and permeability and with different structural topology including monoporous or biporous evaporating face were used. It was experimentally observed that copper wicks are able to provide superior thermal performance than nickel wicks, particularly for low to moderate heat loads due to their low heat conducting resistance. With monoporous copper wick, maximum evaporator heat transfer coefficient (hev) of 26,270 W/m2 K and evaporator thermal resistance (Rev) of 0.06–0.10°C/W were achieved. For monoporous nickel wick, the corresponding values were 20,700 W/m2 K for hev and 0.08–0.21°C/W for Rev. Capillary structure with smaller pore size, high porosity, and high permeability showed better heat transfer characteristics due to sufficient capillary pumping capability, low heat leaks from evaporator to compensation chamber and larger surface area to volume ratio for heat exchange. In addition to this, biporous copper wick structure showed much higher heat transfer coefficient of 83,787 W/m2 K than monoporous copper wick due to improved evaporative heat transfer at wick wall interface and separated liquid and vapor flow pores. The present work was able to classify the importance of the wick properties in the improvement of the thermal characteristics for miniature loop heat pipes.

Experimental Investigation and Performance Evaluation of a Multi Turn Closed Loop Pulsating Heat Pipe

2014 ◽  
Vol 592-594 ◽  
pp. 1554-1558 ◽  
Author(s):  
N. Narendra Babu ◽  
Rudra Naik
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Thermal Resistance ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Closed Loop ◽  
Experimental Result ◽  
Working Fluid ◽  
Pulsating Heat Pipe ◽  
Working Fluids

Pulsating heat pipe (PHP) is a passive heat transfer device, which transfers heat from one region to another with exceptional heat transfer capacity. It utilizes the latent heat of vaporization of the working fluid as well as the sensible heat. As a result, the effective thermal conductivity is higher than that of the conductors. An experimental study on three turn closed loop pulsating heat pipe with three different working fluids viz., Acetone, Methanol, Heptane and distilled water were employed. The PHP is made up of brass material with an inner diameter of 1.95mm, with a total length of 1150 mm for different fill ratios (FR) was employed .The PHP is tested for the thermal resistance and the heat transfer coefficient. The experimental result strongly demonstrates that acetone is a better working fluid among the working fluids considered in terms of higher heat transfer coefficient and lower thermal resistance.

scholarly journals PERFORMANCE OF HEAT PIPE FOR DIFFERENT WORKING FLUIDS AND FILL RATIOS

2011 ◽  
Vol 41 (2) ◽  
pp. 96-102 ◽  
Author(s):  
A. K. Mozumder ◽  
A. F. Akon ◽  
M. S. H. Chowdhury ◽  
S. C. Banik
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Thermal Resistance ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Thermal Capacity ◽  
Working Fluid ◽  
Working Fluids ◽  
Overall Heat Transfer Coefficient ◽  
Lower Temperature

An attempt is made to design, fabricate and test a miniature heat pipe with 5 mm diameter and 150 mm length with a thermal capacity of 10 W. Experiments were conducted with and without working fluid for different thermal loads to assess the performance of heat pipe. The working fluids chosen for the study were same as those commonly used namely, water, methanol and acetone. The temperature distribution across the heat pipe was measured and recorded using thermocouples. The performance of the heat pipe was quantified in terms of thermal resistance and overall heat transfer coefficient. The amount of liquid filled was varied and the variation of the performance parameters for varying liquid inventory is observed. Finally, optimum liquid fill ratio is identified in terms of lower temperature difference and thermal resistance and higher heat transfer coefficient. The data reported in this study will serve as a good database for the researchers in this field. Overall heat transfer coefficient of the Miniature heat pipe is found to be the maximum for the Acetone as working fluid.DOI: http://dx.doi.org/10.3329/jme.v41i2.7473

Parametric Study of the Effect of the Vapor Chamber Characteristics on Its Performance

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
Hamdy Hassan ◽  
Souad Harmand
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Hydrodynamic Model ◽  
Pressure Difference ◽  
Power Input ◽  
Working Fluid ◽  
Vapor Chamber ◽  
Vapor Region

In this work, the effect of vapor chamber characteristics, the properties of its working fluid and the operating parameters on the vapor chamber performance are studied. Also, the effects of these parameters on the cooling process are considered. A three dimensional hydrodynamic model is used for solving the fluid flow through the liquid and vapor regions of the vapor chamber. The hydrodynamic model is coupled with a three dimensional thermal model to calculate the model temperature. The hydrodynamic model takes into consideration the circulation of liquid between the two wick regions. An implicit finite difference method is used to solve the numerical model and a validation of the numerical model is presented. The effect of porosity of the wick material, wick structure, solid wall material, working fluid, wick region thickness, vapor region thickness, power input, and heat transfer coefficient of the cooling fluid are taken in the study. Their effects on the heat pipe temperature, pressure difference of the heat pipe, liquid and vapor velocities and mass evaporated are studied. The results show that, to increase the cooling performance of the heat pipe, the porosity, wick thickness, power input, and vapor region thickness should be decreased and the heat transfer coefficient should be increased. To minimize the maximum pressure difference of the heat pipe, increase porosity, wick thickness, and vapor thickness and decrease heat transfer coefficient and power input. The study shows that the increase of wick thickness by a factor of four decreases the maximum pressure difference by about 75% and increases the maximum vapor chamber temperature 30%. It also shows that the vapor region thickness has an insignificant effect on the vapor chamber temperature and pressure. The increase of the heat transfer coefficient of the cooling liquid decreases its effect on heat pipe performance.

Visualization of Two-phase Bursting Flow Effect on the Two-Phase Closed Thermosyphon

2019 ◽  
Vol 141 (10) ◽  
Author(s):  
Yeonghwan Kim ◽  
Dong Hwan Shin ◽  
Jin Sub Kim ◽  
Seung M. You ◽  
Jungho Lee
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Cylindrical Channel ◽  
Nucleate Boiling ◽  
Liquid Pool ◽  
Working Fluid ◽  
Two Phase ◽  
Evaporator Section

Abstract Two-phase flow inside the two-phase closed thermosyphon (TPCT) including evaporator, adiabatic and condenser sections was visually investigated in order to qualitatively analyze the complicated behaviors of both liquid film and vapor flows simultaneously. The semi-cylindrical channel which is 650 mm long was formed in the long copper block and the flat face of the channel was covered with a flat Pyrex glass for visual observation. The inner diameter of the semi-cylindrical channel was 25 mm and distilled water was used as a working fluid. The filling ratio of the thermosyphon was fixed at 0.5 and the inclination angle was set to 60º. As the heat flux increases, nucleate boiling becomes dominant and the bursting motion starts to begin in the liquid pool at the evaporator section. The bursting liquid flow reaches the condenser section and changes the condensation regime from dropwise to filmwise by flooding the condenser wall, which results in the decrease of condensation heat transfer coefficient. In addition, the vigorous vapor generation which occurs in the liquid pool at the evaporator section disturbs the circulation of the condensate film from the condenser to the evaporator section. As a result, the local dry-out occurs on the evaporator section with increasing heat flux, so the boiling heat transfer coefficient is decreased. [This research was supported by the Ministry of Science and ICT through the National Research Foundation of Korea (NRF-2018H1D3A2000929).]

Experimental Investigation on Performance of Pulsating Heat Pipe

2015 ◽  
Author(s):  
Tarigonda Hari Prasad ◽  
Pol Reddy Kukutla ◽  
P. Mallikarjuna Rao ◽  
R. Meenakshi Reddy
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Thermal Resistance ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Heat Pipes ◽  
Working Fluid ◽  
Pulsating Heat Pipe ◽  
The Given ◽  
Liquid Vapor

Pulsating heat pipes (PHP) receives heat from the working fluid distributes itself naturally in the form of liquid–vapor system, i.e., receiving heat from one end and transferring it to other end by a pulsating action of the liquid–vapor system. Pulsating heat pipes have more advantages than other heat pipes. The problem identified is, to calculate the performance of the pulsating heat pipes with respect to different inclinations using various parameters. In this paper, experiment on performance of closed single loop pulsating heat pipe (CLPHP) using water as a working fluid is considered. The parameters such as thermal resistance (Rth), heat transfer coefficient (h), and variation of temperature with respect to time for the given input at different inclinations such as 0°, 45°, and 90° are taken for the present work. Water is used as the working fluid and is subjected to 50% filling ratio and vacuumed at a pressure of 2300Pa. The performance is calculated at different inclinations of the CLPHP with single turn/loop. The factors such as heat transfer coefficient, thermal resistance, time taken for heating the pulsating heat pipe with the given input are calculated. Finally, it has been concluded that is preferable orientation for PHP and it was found be at vertical orientation i.e., at 90° inclination, because more pulsating action is taken place at this inclination and henceforth, heat transfer rate is faster at this inclination.

scholarly journals Characteristics of Heat Transfer for Heat Pipe and Its Correlation

2011 ◽  
Vol 2011 ◽  
pp. 1-7 ◽  
Author(s):  
Aloke Kumar Mozumder ◽  
Mohammed Shafiul Hasib Chowdhury ◽  
Abul Fahad Akon
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Cooling System ◽  
Thermal Capacity ◽  
Input Power ◽  
Working Fluid ◽  
Fill Ratio ◽  
The Heat Transfer Coefficient

In spite of wide application of heat pipe in microelectronics cooling system, a complete understanding of heat pipe mechanism has not yet been completed. An experimental investigation of heat transfer performance of a heat pipe for dry condition and with three different liquids as acetone, methanol, and water having four fill ratios, for each liquid has been conducted in the present study. The heat pipe was 5 mm in diameter and 150 mm long with a thermal capacity of 10 W. The evaporator and condenser's temperatures were measured with varying input power to estimate the heat transfer coefficient. This study reveals that the dominating parameters for the heat transfer coefficient are evaporator surface temperature, saturated boiling temperature of working fluid, latent heat of vaporization, and fill ratio. The investigation also shows that 85% fill ratio can be regarded as an optimum value for a heat pipe. A new correlation for the heat transfer coefficient has been proposed here which fairly agrees with the experimental results.

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