Saturated Pool Boiling Mechanisms During Single Bubble Heat Transfer: Comparison at Two Wall Superheats

2000 ◽  
Author(s):  
Jungho Kim ◽  
Fatih Demiray ◽  
Nagaraja Yaddanapudi
Keyword(s):  
Heat Transfer ◽  
Constant Temperature ◽  
Pool Boiling ◽  
Minor Role ◽  
Dominant Mechanism ◽  
Different Temperatures ◽  
Transient Conduction ◽  
Microlayer Evaporation ◽  
A Minor ◽  
Transfer Mechanisms

Abstract A study of single bubbles growing on a microscale heater array kept at nominally constant temperature was performed. The behavior of bubbles nucleating at a single site at two different temperatures (22.5 K and 27.5 K superheat) is compared for saturated pool boiling of FC-72 at 1 atm. It is concluded that energy is transferred from the surface through similar heat transfer mechanisms at both superheats. Microlayer evaporation was observed to play a minor role in the overall heat transfer, with microconvection/transient conduction being the dominant mechanism. Evaluation of various heat transfer models are made.

Nucleate Pool Boiling: The Dominant Bubble Heat Transfer Mechanisms

2009 ◽  
Author(s):  
Jungho Kim
Keyword(s):  
Heat Transfer ◽  
Contact Line ◽  
Pool Boiling ◽  
Nucleate Pool Boiling ◽  
Transfer Energy ◽  
Numerical Work ◽  
Transient Conduction ◽  
Microlayer Evaporation ◽  
Transfer Mechanisms ◽  
Fitting Parameters

Enhanced convection, transient conduction, microlayer evaporation, and contact line heat transfer have all been proposed as mechanisms by which bubbles transfer energy during boiling. Models based on these mechanisms contain fitting parameters that are used to fit them to the data, resulting a proliferation of “validated” models. A review of the recent experimental, analytical, and numerical work into single bubble heat transfer is presented to determine the contribution of each of the above mechanisms to the overall heat transfer. Transient conduction and microconvection are found to be the dominant heat transfer mechanisms.

scholarly journals Pool boiling of nanofluids on biphilic surfaces

2021 ◽  
Vol 2116 (1) ◽  
pp. 012006
Author(s):  
P Pontes ◽  
E Freitas ◽  
D Fernandes ◽  
A Teixeira ◽  
R Ferreira ◽  
...  
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Bubble Dynamics ◽  
Pool Boiling ◽  
Alumina Nanoparticles ◽  
Minor Role ◽  
Low Concentrations ◽  
Gold Silver ◽  
Fluid Convection ◽  
A Minor

Abstract This study addresses the combination of customized surface modification with the use of nanofluids, to infer on its potential to enhance pool boiling heat transfer. Hydrophilic surfaces patterned with superhydrophobic regions are prepared and used to act as surface interfaces with nanofluids (water with gold, silver and alumina nanoparticles) and infer on the effect of the nature and concentration of the nanoparticles in bubble dynamics and consequently in heat transfer processes. The main qualitative and quantitative analysis was based on extensive post-processing of synchronized high-speed and thermographic images. The results show an evident benefice of using biphilic patterns, but with well-stablished distances between the superhydrophobic regions. Such patterns allow a controlled bubble coalescence, which promotes fluid convection at the hydrophilic surface between the superhydrophobic regions, which clearly contributes to cool down the surface. The effect of the nanofluids, for the low concentrations used here, was observed to play a minor role.

Microscale Heat Transfer Measurements During Subcooled Pool Boiling of Pentane

2009 ◽  
Author(s):  
Payam Delgoshaei ◽  
Jungho Kim
Keyword(s):  
Heat Transfer ◽  
Pool Boiling ◽  
Superheated Liquid ◽  
Side View ◽  
Heat Transfer Mechanism ◽  
Time Resolved ◽  
Earth Gravity ◽  
Microscale Heat Transfer ◽  
Transient Conduction ◽  
Microlayer Evaporation

Measurements of space and time resolved subcooled pool boiling of pentane in earth gravity environments were made using a microscale heater array. Data from individual heater elements in the array were synchronized with bottom and side view images from two highspeed cameras. The bubble growth was primarily due to energy transfer from the superheated liquid layer. Transient conduction and/or microconvection was found to be the dominant heat transfer mechanism. A composite model consisting of microlayer evaporation and transient conduction was developed and compared with the experimental data.

An Experimental Study of Miniature-Scale Pool Boiling

2003 ◽  
Vol 125 (6) ◽  
pp. 1074-1086 ◽  
Author(s):  
Tailian Chen ◽  
Jacob N. Chung
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
Pool Boiling ◽  
Heat Fluxes ◽  
Boiling Curve ◽  
Feedback Control System ◽  
Electronic Feedback ◽  
Fundamental Understanding ◽  
Peak Heat Flux ◽  
Different Temperatures

By generating single bubbles on a micro-heater at different wall superheats, an experimental study of miniature-scale pool boiling heat transfer has been performed to provide a fundamental understanding of the heater size effect. In this study, the constant-temperature microheater is set at different temperatures by an electronic feedback control system. The heat transfer history during the lifetime of a single bubble which includes nucleation, growth, detachment and departure has been measured. The boiling curve obtained from the microheater is composed of two regimes which are separated by a peak heat flux. It is suggested that in the lower superheat regime, the boiling is dominated by liquid rewetting and micro-layer evaporation, while in the higher superheat regime, conduction through the vapor film and micro-convection plays the key heat transfer role as the heater is covered by vapor all the time. In general, boiling on microheaters is characterized by larger bubble departure sizes, smaller bubble growth rates due to the dryout of microlayer as the bubble grows, and higher bubble incipience superheat. As the heater size decreases, the boiling curve shifts towards higher heat fluxes with corresponding higher superheats.

Enhanced Macroconvection Mechanism With Separate Liquid–Vapor Pathways to Improve Pool Boiling Performance

2017 ◽  
Vol 139 (5) ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Bubble Diameter ◽  
Pool Boiling ◽  
Nucleation Site ◽  
Enhancement Mechanism ◽  
Performance Improvements ◽  
Wall Superheat ◽  
Significant Performance ◽  
Multiscale Structures ◽  
Microlayer Evaporation

Understanding heat transfer mechanisms is crucial in developing new enhancement techniques in pool boiling. In this paper, the available literature on fundamental mechanisms and their role in some of the outstanding enhancement techniques is critically evaluated. Such an understanding is essential in our quest to extend the critical heat flux (CHF) while maintaining low wall superheats. A new heat transfer mechanism related to macroconvection is introduced and its ability to simultaneously enhance both CHF and heat transfer coefficient (HTC) is presented. In the earlier works, increasing nucleation site density by coating a porous layer, providing hierarchical multiscale structures with different surface energies, and nanoscale surface modifications were some of the widely used techniques which relied on enhancing transient conduction, microconvection, microlayer evaporation, or contact line evaporation mechanisms. The microconvection around a bubble is related to convection currents in its immediate vicinity, referred to as the influence region (within one to two times the departing bubble diameter). Bubble-induced convection, which is active beyond the influence region on a heater surface, is introduced in this paper as a new macroconvection mechanism. It results from the macroconvection currents created by the motion of bubbles as they grow and depart from the nucleating sites along a specific trajectory. Directing these bubble-induced macroconvection currents so as to create separate vapor–liquid pathways provides a highly effective enhancement mechanism, improving both CHF and HTC. The incoming liquid as well as the departing bubbles in some cases play a major role in enhancing the heat transfer. Significant performance improvements have been reported in the literature based on enhanced macroconvection contribution. One such microstructure has yielded a CHF of 420 W/cm2 with a wall superheat of only 1.7 °C in pool boiling with water at atmospheric pressure. Further enhancements that can be expected through geometrical refinements and integration of different techniques with macroconvection enhancement mechanism are discussed here.

scholarly journals Towards a better understanding of the HTL process of lignin-rich feedstock

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Benedetta Ciuffi ◽  
Massimiliano Loppi ◽  
Andrea Maria Rizzo ◽  
David Chiaramonti ◽  
Luca Rosi
Keyword(s):  
Spectroscopic Analysis ◽  
Operating Conditions ◽  
Industrial Plant ◽  
Minor Role ◽  
Spectroscopic Techniques ◽  
Reaction Products ◽  
Batch Mode ◽  
H Nmr ◽  
Different Temperatures ◽  
A Minor

AbstractThe hydrothermal liquefaction reactions (HTL) in subcritical conditions of a lignin residue has been studied on a lab scale. The starting material was a lignin rich residue co-produced by an industrial plant situated in Northern Italy producing lignocellulosic bioethanol. The reactions were carried out in batch mode using stainless steel autoclaves. The experiments were under the following operating conditions: two different temperatures (300–350 °C), the presence of basis catalysts (NaOH, and NH4OH) in different concentrations and the presence/absence of capping agent 2,6-bis-(1,1-dimethylethyl)-4-methylphenol (BHT). Lignin residue and reaction products were characterized by analytical and spectroscopic techniques such as CHN-S, TGA, GC–MS, EPR, and 1H-NMR with (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (T.E.M.P.O.). The addition of BHT did not significantly affect the yield of char which is formed by radical way. Spectroscopic analysis indicated that the level of radicals during the reaction was negligible. Therefore, the results obtained experimentally suggest that the reaction takes place via an ionic route while radical species would play a minor role.

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