Heat Transfer Correlations for Liquid Film in the Evaporator of Enclosed, Gravity-Assisted Thermosyphons

1998 ◽  
Vol 120 (2) ◽  
pp. 477-484 ◽  
Author(s):  
M. S. El-Genk ◽  
H. H. Saber
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Smooth Transition ◽  
Working Fluid ◽  
Two Phase ◽  
Data Set ◽  
Evaporator Section ◽  
Heat Transfer Correlations

Heat transfer correlations were developed for the liquid film region, in the evaporator section of closed, two-phase, gravity-assisted thermosyphons in the following regimes: (a) laminar convection, at low heat fluxes, (b) combined convection, at intermediate heat fluxes, and (c) nucleate boiling, at high heat fluxes. These correlations were based on a data set consisting of a total of 305 points for ethanol, acetone, R-11, and R-113 working fluids, wall heat fluxes of 0.99–52.62 kW/m2, working fluid filling ratios of 0.01–0.62, inner diameters of 6–37 mm, evaporator section lengths of 50–609.6 mm, and vapor temperatures of 261–352 K. The combined convention data were correlated by superimposing the correlations of laminar convention and nucleate boiling using a power law approach, to ensure smooth transition among the three heat transfer regimes. The three heat transfer correlations developed in this work are within ±15 percent of experimental data.

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).]

Effects of Varying Geometrical Parameters on Boiling From Microfabricated Enhanced Structures

2003 ◽  
Vol 125 (1) ◽  
pp. 103-109 ◽  
Author(s):  
C. Ramaswamy ◽  
Y. Joshi ◽  
W. Nakayama ◽  
W. B. Johnson
Keyword(s):  
Heat Dissipation ◽  
Layer Structure ◽  
Single Layer ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometrical Parameters ◽  
Small Range ◽  
Two Phase ◽  
Wall Superheat

The current study involves two-phase cooling from enhanced structures whose dimensions have been changed systematically using microfabrication techniques. The aim is to optimize the dimensions to maximize the heat transfer. The enhanced structure used in this study consists of a stacked network of interconnecting channels making it highly porous. The effect of varying the pore size, pitch and height on the boiling performance was studied, with fluorocarbon FC-72 as the working fluid. While most of the previous studies on the mechanism of enhanced nucleate boiling have focused on a small range of wall superheats (0–4 K), the present study covers a wider range (as high as 30 K). A larger pore and smaller pitch resulted in higher heat dissipation at all heat fluxes. The effect of stacking multiple layers showed a proportional increase in heat dissipation (with additional layers) in a certain range of wall superheat values only. In the wall superheat range 8–13 K, no appreciable difference was observed between a single layer structure and a three layer structure. A fin effect combined with change in the boiling phenomenon within the sub-surface layers is proposed to explain this effect.

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.

Analytical Model Quantifying the Net Coolant Flow Rate to a Microstructured Copper Surface validated with Two-Phase PIV Measurements

2021 ◽  
Author(s):  
Matt Harrison ◽  
Joshua Gess
Keyword(s):  
Heat Transfer ◽  
Analytical Model ◽  
Flow Rate ◽  
Circular Disk ◽  
Nucleate Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
Coolant Flow ◽  
Coolant Flow Rate ◽  
Two Phase

Abstract Using Particle Image Velocimetry (PIV), the amount of fluid required to sustain nucleate boiling was quantified to a microstructured copper circular disk. Having prepared the disk with preferential nucleation sites, an analytical model of the net coolant flow rate requirements to a single site has been produced and validated against experimental data. The model assumes that there are three primary phenomena contributing to the coolant flow rate requirements at the boiling surface; radial growth of vapor throughout incipience to departure, bubble rise, and natural convection around the periphery. The total mass flowrate is the sum of these contributing portions. The model accurately predicts the quenching fluid flow rate at low and high heat fluxes with 4% and 30% error of the measured value respectively. For the microstructured surface examined in this study, coolant flow rate requirements ranged from 0.1 to 0.16 kg/sec for a range of heat fluxes from 5.5 to 11.0 W/cm2. Under subcooled conditions, the coolant flow rate requirements plummeted to a nearly negligible value due to domination of transient conduction as the primary heat transfer mechanism at the liquid/vapor/surface interface. PIV and the validated analytical model could be used as a test standard where the amount of coolant the surface needs in relation to its heat transfer coefficient or thermal resistance is a benchmark for the efficacy of a standard surface or boiling enhancement coating/surface structure.

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.

Heat Conduction Analysis on the Evaporator in Gravity Heat Pipe of Heavy Oil Wellbore

2014 ◽  
Vol 608-609 ◽  
pp. 991-995
Author(s):  
Wei Li ◽  
Xin Yuan Tian
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Heat Pipe ◽  
Nucleate Boiling ◽  
Liquid Pool ◽  
Transfer Model ◽  
Two Phase ◽  
Extra Energy ◽  
Phase Change Heat Transfer ◽  
Bottom To Top

A technology for using petroleum deposit’s energy and the principle of medium’s phase change heat transfer to make hollow rod into heat pipe, which transferred heat from bottom to top in wellbore by using it without extra energy is proposed. It can improve the temprature distribution of the fluid at the upper part of wellbore; therefore paraffin deposition and flocculation are improved. In this paper, heat transfer model of liquid film and liquid pool is established by means of the equation of N-S.Based on the principle of micro unit in liquid film’s thermal equilibrium and liquid pool’s heat transfer.By analying the heat transfer coeffcients of this two part,it was found out that gravity heat pipe had better heat transfer performance with increasing the length of liquid film in evaporator,improving the flow rate of inner steam and strengthening nucleate boiling of liquid pool,when the requirement of the continuous circulation of two-phase flow was achieved.

On Development of a Semi-Mechanistic Wall Boiling Model

2013 ◽  
Author(s):  
Saurish Das ◽  
Hemant Punekar
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Thermal Stresses ◽  
Cooling System ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Wall Heat Transfer ◽  
Cooling Systems ◽  
Transfer Phenomenon

In modern cooling systems the requirement of higher performance demands highest possible heat transfer rates, which can be achieved by controlled nucleate boiling. Boiling based cooling systems are gaining attention in several engineering applications as a potential replacement of conventional single-phase cooling system. Although the controlled nucleate boiling enhances the heat transfer, uncontrolled boiling may lead to Dry Out situation, adversely affecting the cooling performance and may also cause mechanical damage due to high thermal stresses. Designing boiling based cooling systems requires a modeling approach based on detailed fundamental understanding of this complex two-phase heat and mass transfer phenomenon. Such models can help analyze different cooling systems, detect potential design flaws and carry out design optimization. In the present work a new semi-mechanistic wall boiling model is developed within commercial CFD solver ANSYS FLUENT. A phase change mechanism and wall heat transfer augmentation due to nucleate boiling are implemented in mixture multiphase flow framework. The phase change phenomenon is modeled using mechanistic evaporation-condensation model. Enhancement of wall heat transfer due to nucleate boiling is captured using 1D empirical correlation, modified for 3D CFD environment. A new method is proposed to calculate the local suppression of nucleate boiling based on the flow velocity, and hence this model can be applied to any complex shaped coolant passage. For different wall superheat, the wall heat fluxes predicted by the present model are validated against experimental data, in which 50-50 volume mixture of aqueous ethylene glycol (a typical anti-freeze coolant mixture) is used as working fluid. The validation study is performed in ducts of different sizes and shapes with different inlet velocities, inlet sub-cooling and operating pressures. The results are in good agreement with the experiments. This model is applied to a typical automobile Exhaust Gas Recirculation (EGR) system to study boiling heat transfer phenomenon and the results are presented.

scholarly journals Modelling of Boiling Flows for Nuclear Thermal Hydraulics Applications—A Brief Review

Inventions ◽  
2020 ◽  
Vol 5 (3) ◽  
pp. 47
Author(s):  
Giovanni Giustini
Keyword(s):  
Heat Transfer ◽  
Liquid Water ◽  
Cfd Simulation ◽  
High Surface ◽  
Computational Modelling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Thermal Hydraulics ◽  
Two Phase ◽  
Water Reactors

The boiling process is utterly fundamental to the design and safety of water-cooled fission reactors. Both boiling water reactors and pressurised water reactors use boiling under high-pressure subcooled liquid flow conditions to achieve high surface heat fluxes required for their operation. Liquid water is an excellent coolant, which is why water-cooled reactors can have such small sizes and high-power densities, yet also have relatively low component temperatures. Steam is in contrast a very poor coolant. A good understanding of how liquid water coolant turns into steam is correspondingly vital. This need is particularly pressing because heat transfer by water when it is only partially steam (‘nucleate boiling’ regime) is particularly effective, providing a great incentive to operate a plant in this regime. Computational modelling of boiling, using computational fluid dynamics (CFD) simulation at the ‘component scale’ typical of nuclear subchannel analysis and at the scale of the single bubbles, is a core activity of current nuclear thermal hydraulics research. This paper gives an overview of recent literature on computational modelling of boiling. The knowledge and capabilities embodied in the surveyed literature entail theoretical, experimental and modelling work, and enabled the scientific community to improve its current understanding of the fundamental heat transfer phenomena in boiling fluids and to develop more accurate tools for the prediction of two-phase cooling in nuclear systems. Data and insights gathered on the fundamental heat transfer processes associated with the behaviour of single bubbles enabled us to develop and apply more capable modelling tools for engineering simulation and to obtain reliable estimates of the heat transfer rates associated with the growth and departure of steam bubbles from heated surfaces. While results so far are promising, much work is still needed in terms of development of fundamental understanding of the physical processes and application of improved modelling capabilities to industrially relevant flows.

Effect of Electric Fields on Two-Phase Impingement Heat Transfer

2007 ◽  
Author(s):  
Xin Feng ◽  
James E. Bryan
Keyword(s):  
Heat Transfer ◽  
Electric Fields ◽  
Capillary Flow ◽  
Heated Surface ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometrical Parameters ◽  
Two Phase ◽  
Transfer Rates ◽  
Impingement Heat Transfer

The effect of electric fields applied to two-phase impingement heat transfer is explored for the first time. The application of an electric field between a capillary and heated surface results in the ability to control the free boundary flow from discreet drops to jets to sprays. Through an experimental study, the impingement heat transfer was evaluated over a range of operating and geometrical parameters using subcooled ethanol as the working fluid. The ability to change the mode of impinging mass did change the surface heat transfer. The characteristics of the impinging mass on heat transfer was dependent on capillary flow rate, applied voltage, capillary to heated surface spacing, capillary geometry, and heat flux. Enhancement occurred primarily at low heat fluxes (below 30 W/cm2) under ramified spray conditions where the droplet momentum promoted thin films on the heated surface. Higher heat fluxes resulted in greater vapor momentum from the surface minimizing the effect of different modes. However, under ramified spray conditions less mass was impacting the heated surface showing that heat transfer rates at higher heat fluxes were achievable with less mass, resulting in greater evaporation efficiency.

Proposed Correlation for Forced Convection Boiling Heat Transfer in Mini and Micro Channels With CO2 as a Fluid

2014 ◽  
Author(s):  
Mayank I. Vyas ◽  
Salim A. Channiwala ◽  
Mitesh N. Prajapati
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Boiling Heat Transfer ◽  
Micro Channels

After reviewing the available literature on flow boiling heat transfer in mini/micro tubes and channels, it is felt that there is need for predictive correlations which is applicable over wide range of parameters. In present work a new correlation for two-phase flow boiling heat transfer coefficient is developed, which has considered nucleate boiling and convective boiling heat transfer effect. To develop this correlation we have considered total 651 data points, which have been collected from the open available literature covering different operational conditions and different dimensions of channels. We have selected CO2 as a working fluid because it does not contain chlorine, hence an efficient and environmentally safe refrigerant and would be potential replacement for R-22. CO2 has unusual heat transfer and two-phase flow characteristics, and is very different from those of conventional refrigerant. Also a comparison of present correlation with the best published correlation for CO2 is done. The results of this comparison indicate that the new developed correlation is superior to published best correlation for CO2. Present correlation is also compared with best published correlation for all fluids and with the correlation developed by using CO2 data. The results of these both case, indicate that the present correlation is superior.

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