An experimental investigation of flow boiling heat transfer and pressure drop of R134a in a horizontal 2.168 mm tube under hypergravity. Part II: Heat transfer coefficient

2015 ◽  
Vol 80 ◽  
pp. 597-604 ◽  
Author(s):  
Yu Xu ◽  
Xiande Fang ◽  
Guohua Li ◽  
Dingkun Li
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Flow Boiling Heat Transfer

A Fundamental View of the Flow Boiling Heat Transfer Characteristics of Nano-Refrigerants

2012 ◽  
Author(s):  
Lorenzo Cremaschi
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Phase ◽  
Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Working Fluid ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Flow Boiling Heat Transfer

Driven by higher energy efficiency targets and industrial needs of process intensification and miniaturization, nanofluids have been proposed in energy conversion, power generation, chemical, electronic cooling, biological, and environmental systems. In space conditioning and in cooling systems for high power density electronics, vapor compression cycles provide cooling. The working fluid is a refrigerant and oil mixture. A small amount of lubricating oil is needed to lubricate and to seal the sliding parts of the compressors. In heat exchangers the oil in excess penalizes the heat transfer and increases the flow losses: both effects are highly undesired but yet unavoidable. This paper studies the heat transfer characteristics of nanorefrigerants, a new class of nanofluids defined as refrigerant and lubricant mixtures in which nano-size particles are dispersed in the high-viscosity liquid phase. The heat transfer coefficient is strongly governed by the viscous film excess layer that resides at the wall surface. In the state-of-the-art knowledge, while nanoparticles in the refrigerant and lubricant mixtures were recently experimentally studied and yielded convective in-tube flow boiling heat transfer enhancements by as much as 101%, the interactions of nanoparticles with the mixture still pose several open questions. The model developed in this work suggested that the nanoparticles in this excess layer generate a micro-convective mass flux transverse to the flow direction that augments the thermal energy transport within the oil film in addition to the macroscopic heat conduction and fluid convection effects. The nanoparticles motion in the shearing-induced and non-uniform shear rate field is added to the motion of the nanoparticles due to their own Brownian diffusion. The augmentation of the liquid phase thermal conductivity was predicted by the developed model but alone it did not fully explain the intensification on the two-phase flow boiling heat transfer coefficient reported in previous work in the literature. Thus, additional nano- and micro-scale heat transfer intensification mechanisms were proposed.

Flow Boiling Heat Transfer in a Silicon Microchannel Heat Sink Using Liquid Crystal Thermography

2008 ◽  
Author(s):  
Ayman Megahed ◽  
Ibrahim Hassan ◽  
Tariq Ahmad
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Heat Sink ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Microchannel Heat Sink

The present study focuses on the experimental investigation of boiling heat transfer characteristics and pressure drop in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 295 μm, width of 254 μm, and a length of 16 mm. Un-encapsulated Thermochromic liquid Crystals (TLC) are used in the present work to enable nonintrusive and high spatial resolution temperature measurements. This measuring technique is used to provide accurate full and local surface-temperature and heat transfer coefficient measurements. Experiments are carried out for mass velocities ranging between 290 to 457 kg/m2.s and heat fluxes from 6.04 to 13.06 W/cm2 using FC-72 as the working fluid. Experimental results show that the pressure drop increases as the exit quality and the flow rate increase. High values of heat transfer coefficient can be obtained at low exit quality (xe < 0.2). However, the heat transfer coefficient decreases sharply and remains almost constant as the quality increases for an exit quality higher than 0.2.

Experimental Study of Flow Boiling Heat Transfer Coefficient of FC-72 Over Micro-Pin-Finned Surfaces

2010 ◽  
Author(s):  
Y. F. Xue ◽  
M. Z. Yuan ◽  
J. J. Wei
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Fluid Velocity ◽  
Nucleate Boiling ◽  
Flow Boiling Heat Transfer ◽  
Nucleate Boiling Heat Transfer ◽  
Boiling Heat Transfer Coefficient

Experiments of flow boiling heat transfer coefficient of FC-72 were carried out over simulated silicon chip of 10×10×0.5 mm3 for electronic cooling. Four kinds of micro-pin-fins with the dimensions of 30×60, 30×120, 50×60, 50×120 μm2 (thickness, t × height, h) respectively, were fabricated on the chip surfaces by the dry etching technique to enhance boiling heat transfer. A smooth chip was also tested for comparison. The experiments were conducted at three different fluid velocities (0.5, 1 and 2m/s) and three different liquid subcoolings (15, 25 and 35K). All micro-pin-finned surfaces show a considerable heat transfer enhancement compared to the smooth surface. Both the forced convection and nucleate boiling heat transfer contribute to the total heat transfer performance. The contribution of each factor to the total heat transfer has been clearly presented in the flow boiling heat transfer coefficient curves. In a lower heat flux region, the heat transfer coefficient increases greatly with increasing fluid velocity, but increases slightly with increasing heat flux, indicating that the single-phase forced convection dominates the heat transfer process. With further increasing heat flux to the onset of nucleate boiling, the heat transfer coefficient increases remarkably. For a given liquid subcooling, the curves of flow boiling heat transfer coefficient at fluid velocities of 0.5 and 1 m/s almost follow one line for each surface, showing insensitivity of nucleate boiling heat transfer to fluid velocity. However, at the largest fluid velocity of 2 m/s, the slope of the flow boiling heat transfer coefficient curves for micro-pin-finned surfaces becomes smaller, indicating that the forced convection also plays an important role besides the nucleate boiling heat transfer. The curves of the flow boiling heat transfer coefficient can be used to determine the boiling incipience at different fluid velocities, which provides a basis for the suitable fluid velocity selection in designing highly efficient cooling scheme for electronic devices.

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