Thermal Analysis of a Water-Filled Micro Heat Pipe With Phase-Change Interfacial Resistance

2012 ◽  
Vol 134 (11) ◽  
Author(s):  
Yew Mun Hung ◽  
Kek-Kiong Tio
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Phase Change ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Interfacial Resistance ◽  
Vapor Interface ◽  
Micro Heat Pipe ◽  
Liquid And Vapor Phases

The progressive evaporation and condensation processes in a micro heat pipe, with which high heat fluxes at the liquid–vapor interface are associated, render it a device of high thermal conductance. By coupling the phase-change interfacial resistance model with a mathematical model based on first principles for fluid flow and heat transfer, the axial temperature variations of the liquid and vapor phases as well as those of other field variables are characterized and analyzed. The findings provide a well-defined exposition of the validity of uniform-temperature assumption for the liquid and vapor phases in the analysis of micro heat pipes. In conjunction with the acquisition of liquid and vapor temperature profiles, the heat transfer characteristics of the evaporation process can be analyzed. The local evaporative heat transfer coefficient and heat flux are evaluated. The results indicate that both heat transfer coefficient and heat flux are of considerably high values, confirming that the heat transport capability of a micro heat pipe is dominated by the phase-change heat transfer at the liquid–vapor interface.

Effect of Liquid Properties on Phase-Change Heat Transfer in Porous Wick Structures

2015 ◽  
Vol 138 (3) ◽  
Author(s):  
Steve Q. Cai ◽  
Avijit Bhunia
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Phase Change ◽  
Transfer Coefficient ◽  
Mode Transition ◽  
Maximum Heat ◽  
Maximum Heat Flux ◽  
Phase Change Heat Transfer ◽  
The Heat Transfer Coefficient

In a heat pipe, operating fluid saturates wick structures system and establishes a capillary-driven circulation loop for heat transfer. Thus, the thermophysical properties of the operating fluid inevitably impact the transitions of phase-change mode and the capability of heat transfer, which determine both the design and development of the associated heat pipe systems. This article investigates the effect of liquid properties on phase-change heat transfer. Two different copper wick structures, cubic and cylindrical in cross section, 340 μm in height and 150 μm in diameter or width, are fabricated using an electroplating technique. The phase-change phenomena inside these wick structures are observed at various heat fluxes. The corresponding heat transfer characteristics are measured for three different working liquids: water, ethanol, and Novec 7200. Three distinct modes of the phase-change process are identified: (1) evaporation on liquid–vapor interface, (2) nucleate boiling with interfacial evaporation, and (3) boiling enhanced interface evaporation. Transitions between the three modes depend on heat flux and liquid properties. In addition to the mode transition, liquid properties also dictate the maximum heat flux and the heat transfer coefficient. A quantitative characterization shows that the maximum heat flux scales with Merit number, a dimensionless number connecting liquid density, surface tension, latent heat of vaporization, and viscosity. The heat transfer coefficient, on the other hand, is dictated by the thermal conductivity of the liquid. A complex interaction between the mode transition and liquid properties is reflected in Novec 7200. In spite of having the lowest thermal conductivity among the three liquids, an early transition to the mode of the boiling enhanced interface evaporation leads to a higher heat transfer coefficient at low heat flux.

Performance of a Flexible Evaporator for Loop Heat Pipe Applications

2008 ◽  
Author(s):  
Mukta S. Limaye ◽  
James F. Klausner
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Heat Fluxes ◽  
Loop Heat Pipe ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Maximum Heat Flux

A flat and flexible evaporator, which conforms to contoured surfaces, has been developed for loop heat pipe applications. A loop heat pipe (LHP) is a passive, two phase heat transfer device that uses a porous membrane in the evaporator to circulate fluid. A number of flexible membranes have been tested as evaporator wicks that have a length of 12.7 cm and heated area of 50.6 cm2. For cellulose, polyethylene, and blotting paper membranes, maximum heat fluxes of 0.43, 1.5 and 2.9 W/cm2 have been observed, respectively. The maximum heat transfer coefficients measured for these membranes are 551, 876, and 2100 W/m2-K, respectively. The best performance was observed by a membrane made of a fibrous cotton matrix, typically used as gauze. This material has a large pore size and high wettability with water. When tested in a rigid, brass evaporator, the maximum heat flux observed is 5.95 W/cm2, and the maximum heat transfer coefficient is 2865 W/m2-K. A flexible evaporator is fabricated using a heat sealable, flexible barrier pouch, and the cotton matrix membrane is sealed inside. The maximum measured heat flux for the flexible evaporator is 3.2 W/cm2 and maximum measured heat transfer coefficient is 1165 W/m2-K. The observed reduction in heat transfer as compared to the rigid evaporator is due to the poor contact between the evaporator and membrane. It is concluded that for the flexible evaporator membranes considered, the heat transfer mechanism is boiling and the maximum heat flux is limited by the wicking rate of the membrane. For a given membrane, the wicking rate increases with a reduction in the wicking length and decreases with an increasing rate of evaporation. To further improve the performance of the flexible evaporator, it is necessary to ensure efficient vapor removal from the evaporator as well as maintaining good contact between the membrane and the evaporator surface.

scholarly journals A numerical model and validation of phase change material integrated thermoelectric radiant cooling panel

2019 ◽  
Vol 111 ◽  
pp. 01001
Author(s):  
Hansol Lim ◽  
Hye-Jin Cho ◽  
Seong-Yong Cheon ◽  
Soo-Jin Lee ◽  
Jae-Weon Jeong
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Numerical Model ◽  
Surface Temperature ◽  
Phase Change ◽  
Transfer Coefficient ◽  
Phase Change Material ◽  
Overall Heat Transfer Coefficient ◽  
Radiant Cooling ◽  
Change Material

A phase change material based radiant cooling panel with thermoelectric module (PCM-TERCP) is proposed in this study. It consists of two aluminium panels, and phase change materials (PCMs) sandwiched between the two panels. Thermoelectric modules (TEMs) are attached to one of the aluminium panels, and heat sinks are attached to the top side of TEMs. PCM-TERCP is a thermal energy storage concept equipment, in which TEMs freeze the PCM during the night whose melting temperature is 16○C. Therefore, the radiant cooling panel can maintain a surface temperature of 16◦C without the operation of TEM during the day. Furthermore, it is necessary to design the PCM-TERCP in a way that it can maintain the panel surface temperature during the targeted operating time. Therefore, the numerical model was developed using finite difference method to evaluate the thermal behaviour of PCM-TERCP. Experiments were also conducted to validate the performance of the developed model. Using the developed model, the possible operation time was investigated to determine the overall heat transfer coefficient required between radiant cooling panel and TEM. Consequently, the results showed that a overall heat transfer coefficient of 394 W/m2K is required to maintain the surface temperature between 16○C to 18○C for a 3 hours operation.

scholarly journals Experimental Determination of the Heat Transfer Coefficient of Real Cooled Geometry Using Linear Regression Method

Energies ◽  
2020 ◽  
Vol 14 (1) ◽  
pp. 180
Author(s):  
Asif Ali ◽  
Lorenzo Cocchi ◽  
Alessio Picchi ◽  
Bruno Facchini
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Linear Regression ◽  
Surface Temperature ◽  
Transfer Coefficient ◽  
External Surface ◽  
Internal Surface ◽  
Regression Method ◽  
Thermal Transient

The scope of this work was to develop a technique based on the regression method and apply it on a real cooled geometry for measuring its internal heat transfer distribution. The proposed methodology is based upon an already available literature approach. For implementation of the methodology, the geometry is initially heated to a known steady temperature, followed by thermal transient, induced by injection of ambient air to its internal cooling system. During the thermal transient, external surface temperature of the geometry is recorded with the help of infrared camera. Then, a numerical procedure based upon a series of transient finite element analyses of the geometry is applied by using the obtained experimental data. The total test duration is divided into time steps, during which the heat flux on the internal surface is iteratively updated to target the measured external surface temperature. The final procured heat flux and internal surface temperature data of each time step is used to find the convective heat transfer coefficient via linear regression. This methodology is successfully implemented on three geometries: a circular duct, a blade with U-bend internal channel, and a cooled high pressure vane of real engine, with the help of a test rig developed at the University of Florence, Italy. The results are compared with the ones retrieved with similar approach available in the open literature, and the pros and cons of both methodologies are discussed in detail for each geometry.

Experimental and theoretical investigation of heat transfer characteristics of cylindrical heat pipe using Al2O3–SiO2/W-EG hybrid nanofluids by RSM modeling approach

2021 ◽  
Vol 68 (1) ◽  
Author(s):  
R. Vidhya ◽  
T. Balakrishnan ◽  
B. Suresh Kumar
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Heat Transfer Coefficient ◽  
Thermal Resistance ◽  
Transfer Coefficient ◽  
Heat Pipe ◽  
Diffraction Spectrum ◽  
Step Method ◽  
Ceramic Nanoparticles ◽  
Transfer Mechanisms

AbstractNanofluids are emerging two-phase thermal fluids that play a vital part in heat exchangers owing to its heat transfer features. Ceramic nanoparticles aluminium oxide (Al2O3) and silicon dioxide (SiO2) were produced by the sol-gel technique. Characterizations have been done through powder X-ray diffraction spectrum and scanning electron microscopy analysis. Subsequently, few volume concentrations (0.0125–0.1%) of hybrid Al2O3–SiO2 nanofluids were formulated via dispersing both ceramic nanoparticles considered at 50:50 ratio into base fluid combination of 60% distilled water (W) with 40% ethylene glycol (EG) using an ultrasonic-assisted two-step method. Thermal resistance besides heat transfer coefficient have been examined with cylindrical mesh heat pipe reveals that the rise of power input decreases the thermal resistance and inversely increases heat transfer coefficient about 5.54% and 43.16% respectively. Response surface methodology (RSM) has been employed for the investigation of heat pipe experimental data. The significant factors on the various convective heat transfer mechanisms have been identified using the analysis of variance (ANOVA) tool. Finally, the empirical models were developed to forecast the heat transfer mechanisms by regression analysis and validated with experimental data which exposed the models have the best agreement with experimental results.

Numerical Simulation of Convective Heat Transfer in a Spark Ignition Engine

2008 ◽  
Author(s):  
Arash Mohammadi ◽  
Seyed Ali Jazayeri ◽  
Masoud Ziabasharhagh
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Combustion Chamber ◽  
Spark Ignition Engine ◽  
Cylinder Wall ◽  
Single Cylinder Engine ◽  
Crank Angle ◽  
Exhaust Valves

A computational fluid dynamics code is applied to simulate fluid flow and combustion in a four-stroke single cylinder engine with flat combustion chamber geometry. Heat flux and heat transfer coefficient on the cylinder head, cylinder wall, piston, intake and exhaust valves are determined. Result for a certain condition is compared for total heat transfer coefficient of the cylinder engine with available correlation proposed by experimental measurement in the literature and close agreement is observed. It is observed that the value of heat flux and heat transfer coefficient varies considerably in different positions of the combustion chamber, but the trend with crank angle is almost the same.

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