Experimental Investigation of an Evaporator Enhanced With a Micro-Porous Structure in a Two-Phase Thermosyphon Loop

2008 ◽  
Author(s):  
Richard Furberg ◽  
Rahmatollah Khodabandeh ◽  
Bjo¨rn Palm ◽  
Shanghua Li ◽  
Muhammet Toprak ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Rectangular Channel ◽  
Large Diameter ◽  
Nucleate Boiling ◽  
Nano Particles ◽  
Working Fluid ◽  
Two Phase ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Following is an experimental study of six different evaporators in a closed two-phase thermosyphon loop system, where the influence of various evaporator dimensions and surfaces was investigated. The evaporators featured a 30 mm long rectangular channel with hydraulic diameters ranging from 1.2–2.7 mm. The heat transfer surface of one of the tested evaporators was enhanced with copper nano-particles, dendritically connected into an ordered micro-porous three dimensional network structure. To facilitate high speed video visualization of the two-phase flow in the evaporator channel, a transparent polycarbonate window was attached to the front of the evaporators. Refrigerant 134A was used as a working fluid and the tests were conducted at 6.5 bar. The tests showed that increasing channel diameters generally performed better. The three largest evaporator channels exhibited comparable performance, with a maximum heat transfer coefficient of about 2.2 W/(cm2K) at a heat flux of 30–35 W/cm2 and a critical heat flux of around 50 W/cm2. Isolated bubbles characterized the flow regime at peak performance for the large diameter channels, while confined bubbles and chaotic churn flow typified the evaporators with small diameters. In line with previous pool boiling experiments, the nucleate boiling mechanism was significantly enhanced, up to 4 times, by the nano- and micro-porous enhancement structure.

Heat Transfer From a Small Heated Region to R-113 and FC-72

1989 ◽  
Vol 111 (4) ◽  
pp. 1053-1059 ◽  
Author(s):  
K. R. Samant ◽  
T. W. Simon
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Quartz Substrate ◽  
Rectangular Channel ◽  
Nucleate Boiling ◽  
Decomposition Temperature ◽  
Large Temperature ◽  
Resistance Thermometer ◽  
Maximum Heat ◽  
Heater Element

An experimental investigation of heat transfer from a small heated patch to a subcooled, fully developed turbulent flow is conducted. The test patch, approximately 0.25 mm long and 2.0 mm wide, is located on the floor of a small rectangular channel through which a coolant (R-113 or FC-72) is circulated. A thin film of Nichrome deposited on a quartz substrate serves as an integrated heater element and resistance thermometer. The maximum achievable heat flux with R-113, limited by the thermal decomposition temperature of the fluid, is 2.04 MW/m2 at a bulk velocity of 1.8 m/s and a high wall superheat of 80° C. The results obtained with FC-72 show large temperature excursions at the onset of nucleate boiling and a boiling hysteresis near the onset of nucleate boiling. These effects decrease with increasing velocity and/or subcooling. The heat flux at departure from nucleate boiling increases with increasing velocity and/or subcooling. A maximum heat flux of 4.26 MW/m2 at departure from nucleate boiling is observed.

Correlation for Prediction of Peak Heat Flux Achieved With a Bi-Porous Wick

2009 ◽  
Author(s):  
Ladan Amouzegar ◽  
Ivan Catton ◽  
Aleksander Vadnjal
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Material Properties ◽  
Working Fluid ◽  
Maximum Heat ◽  
Porous Wick ◽  
Maximum Heat Transfer ◽  
Capillary Limit ◽  
Small Pore ◽  
Fluid Properties

In the past researchers noted three distinct stages of evaporative heat transfer in a bi-porous wick. The maximum heat transfer rate is postulated to occur at the end of the second stage when the maximum number of small pores interfaces the vapor space. For optimization purposes a reliable model that relates the maximum heat flux of a bi-porous wick to the wick material properties, wick geometry given with average large and small pore diameter, wick thickness, and working fluid properties is demanded. In this work, a semi-empirical model that relates the heat flux at the capillary limit to the wick material properties, working fluid properties and wick dimensions is further developed. The model is based on the hydrodynamics of the capillary limit. The result is employed to qualitatively and quantitatively optimize the wick parameters for some specific cases and the optimization can be further performed using the proposed model.

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).]

scholarly journals Determination of Limiting Heat Flux for The Inception of Nucleate Boiling Regime for Crude Oils

2021 ◽  
Vol 85 (2) ◽  
pp. 115-127
Author(s):  
Obaid ur Rehman ◽  
Marappa Gounder Ramasamy ◽  
Nor Erniza M Rozali ◽  
Umesh B. Deshannavar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Nucleate Boiling ◽  
Crude Oils ◽  
Bulk Temperature ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Accuracy Of Results

Finding the limiting heat flux above which nucleate boiling starts and below which forced convective heat transfer exists is a crucial task for the accuracy of results from crude oil fouling tests. In this study, crude oils from two sources were tested at bulk temperatures of 100, 120 and 140 °C and different velocities. Heat transfer coefficient increased gradually with bulk temperature indicated lowering of the viscosity at high temperatures which promoted turbulence and enhanced heat transfer. The velocity effects were similar to that of bulk temperatures on maximum heat transfer coefficient while less heat flux was required to achieve the same surface temperature at lower velocities. Deshannavar and Ramasamy’s model to predict maximum heat flux was compared with experimental results and a poor estimation was observed for the crude oils tested.

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.

A Numerical Model of a Reciprocating-Mechanism Driven Heat Loop for Two-Phase High Heat Flux Cooling

2016 ◽  
Vol 8 (4) ◽  
Author(s):  
Olubunmi Popoola ◽  
Ayobami Bamgbade ◽  
Yiding Cao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Numerical Model ◽  
Performance Optimization ◽  
Numerical Study ◽  
High Heat ◽  
Working Fluid ◽  
High Heat Flux ◽  
Two Phase ◽  
Transfer Device

An effective design option for a cooling system is to use a two-phase pumped cooling loop to simultaneously satisfy the temperature uniformity and high heat flux requirements. A reciprocating-mechanism driven heat loop (RMDHL) is a novel heat transfer device that could attain a high heat transfer rate through a reciprocating flow of the two-phase working fluid inside the heat transfer device. Although the device has been tested and validated experimentally, analytical or numerical study has not been undertaken to understand its working mechanism and provide guidance for the device design. The objective of this paper is to develop a numerical model for the RMDHL to predict its operational performance under different working conditions. The developed numerical model has been successfully validated by the existing experimental data and will provide a powerful tool for the design and performance optimization of future RMDHLs. The study also reveals that the maximum velocity in the flow occurs near the wall rather than at the center of the pipe, as in the case of unidirectional steady flow. This higher velocity near the wall may help to explain the enhanced heat transfer of an RMDHL.

Flow Boiling Enhancement in Microchannels With Diffusion Bonded Wire Mesh

2007 ◽  
Author(s):  
Hailei Wang ◽  
Richard Peterson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Wire Mesh ◽  
Working Fluid ◽  
High Heat Flux ◽  
Vapor Quality ◽  
Heating Wall

Flow boiling and heat transfer enhancement in four parallel microchannels using a dielectric working fluid, HFE 7000, was investigated. Each channel was 1000 μm wide and 510 μm high. A unique channel surface enhancement technique via diffusion bonding a layer of conductive fine wire mesh onto the heating wall was developed. According to the obtained flow boiling curves for both the bare and mesh channels, the amount of wall superheat was significantly reduced for the mesh channel at all stream-wise locations. This indicated that the nucleate boiling in the mesh channel was enhanced due to the increase of nucleation sites the mesh introduced. Both the nucleate boiling dominated and convective evaporation dominated regimes were identified. In addition, the overall trend for the flow boiling heat transfer coefficient, with respect to vapor quality, was increasing until the vapor quality reached approximately 0.4. The critical heat flux (CHF) for the mesh channel was also significantly higher than that of the bare channel in the low vapor quality region. Due to the fact of how the mesh was incorporated into the channels, no pressure drop penalty was identified for the mesh channels. Potential applications for this kind of mesh channel include high heat-flux electronic cooling systems and various energy conversion systems.

Reciprocating-Mechanism Driven Heat Loops and Their Applications

2003 ◽  
Author(s):  
Yiding Cao ◽  
Mingcong Gao
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Electronics Cooling ◽  
Working Fluid ◽  
High Heat Flux ◽  
Two Phase ◽  
Condenser Section ◽  
Evaporator Section ◽  
Two Phase Heat Transfer ◽  
Transfer Device

This paper introduces a novel heat transfer mechanism that facilitates two-phase heat transfer while eliminating the so-called cavitation problem commonly encountered by a conventional pump. The heat transfer device is coined as the reciprocating-mechanism driven heat loop (RMDHL), which includes a hollow loop having an interior flow passage, an amount of working fluid filled within the loop, and a reciprocating driver. The hollow loop has an evaporator section, a condenser section, and a liquid reservoir. The reciprocating driver is integrated with the liquid reservoir and facilitates a reciprocating flow of the working fluid within the loop, so that liquid is supplied from the condenser section to the evaporator section under a substantially saturated condition and the so-called cavitation problem associated with a conventional pump is avoided. The reciprocating driver could be a solenoid-operated reciprocating driver for electronics cooling applications and a bellows-type reciprocating driver for high-temperature applications. Experimental study has been undertaken for a solenoid-operated heat loop in connection with high heat flux thermal management applications. Experimental results show that the heat loop worked very effectively and a heat flux as high as 300 W/cm2 in the evaporator section could be handled. The applications of the bellows-type reciprocating heat loop for gas turbine nozzle guide vanes and the leading edges of hypersonic vehicles are also illustrated. The new heat transfer device is expected to advance the current two-phase heat transfer device and open up a new frontier for further research and development.

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