scholarly journals Boiling-condensation heat transfer and flow characteristics in ultrathin limited enclosed space based on numerical simulation and visualization experiment

2021 ◽  
pp. 348-348
Author(s):  
Li Cong ◽  
Huang Ying ◽  
Tan Jianping
Keyword(s):  
Heat Transfer ◽  
Flow Characteristics ◽  
Condensation Heat Transfer ◽  
Anhydrous Ethanol ◽  
Upward Movement ◽  
Heating Section ◽  
Working Medium ◽  
Evaporation Heat ◽  
Visualization Experiment ◽  
Enclosed Space

Boiling-condensation heat transfer in ultrathin flat heat pipes are complicated and difficult to observe. In this study, a visualization experiment and simulation analysis in an ultrathin limited enclosed space were carried out. Width of the ultrathin enclosed space was 1 mm, with anhydrous ethanol as the working medium. The enclosed space was oriented vertically with the heating section on the bottom and the cooling section on the top. Flow characteristics of the anhydrous ethanol were photographed using a high-speed camera through the quartz cover. The boiling-condensation heat transfer and fluid flow in the limited enclosed space were simulated. Effective heat transfer coefficient calculated based on the experimental data varied from 1.0 to 1.1 W/?C, while that of the inner wall obtained by the simulation varied within the range of 1.068-1.076 W/?C. The maximum error was 2.9%, which verified the reliability of the simulation results. By analyzing the pressure change in condensation section, it was found that the boiling-condensation heat released in the enclosed space changed periodically, because of the growth and bursting of bubbles and falling of the working medium due to gravity. Restricted by the thickness, the bubbles produced by boiling of the working medium grew in flat and irregular shapes, promoting the upward movement of the rest of the liquid working medium, and a liquid film was formed at the heated inner surface for evaporation heat transfer, which enhanced the heat transfer capacity of the heating section.

Predicting Heat Transfer in Long R-134a Filled Thermosyphons

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
M. H. M. Grooten ◽  
C. W. M. van der Geld
Keyword(s):  
Heat Transfer ◽  
Saturation Temperature ◽  
Good Alternative ◽  
Condensation Heat Transfer ◽  
Air Cooling ◽  
Transfer Coefficients ◽  
Reduced Pressure ◽  
Evaporation Heat ◽  
Evaporation Heat Transfer ◽  
R 134A

When traditional air-to-air cooling is too voluminous, heat exchangers with long thermosyphons offer a good alternative. Experiments with a single thermosyphon with a large length-to-diameter ratio (188) and filled with R-134a are presented and analyzed. Saturation temperatures, filling ratios, and angles of inclination have been varied in wide ranges. A higher sensitivity of evaporation heat transfer coefficients on reduced pressure than in previous work has been found. Measurements revealed the effect of pressure or the saturation temperature on condensation heat transfer. The condensate film Reynolds number that marks a transition from one condensation heat transfer regime to another is found to depend on pressure. This effect was not accounted for by correlations from the literature. New correlations are presented to predict condensation and evaporation heat transfer rates.

Numerical Analysis on Flow Characteristics of Anti-Gravity Flow

2011 ◽  
Author(s):  
Yan Li ◽  
Shuchao Zhang ◽  
Ning Mei
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Fluid Flow ◽  
Flow Characteristics ◽  
Gravity Flow ◽  
Radius Of Curvature ◽  
Micro Channel ◽  
Subcooled Boiling ◽  
Evaporation Heat ◽  
Micro Channels

Fluid flow phenomena in micro channels received wide attention due to its high heat transfer coefficient. As a new technique in the field of micro channel phase-change heat transfer, anti-gravity flow can drive fluid flow by capillary force and create enhanced evaporation heat transfer conditions by promoting the formation of an extended meniscus in the three-phase contact-line region. Resulting from the circumferential discrepancy of degree of superheat, the radius of curvature of intrinsic meniscus decreases rapidly as liquid rising up, leading to the formation of capillary pressure gradient. With the increase of heat flux, subcooled boiling occurs and micro-bubble appears at the bottom of the fluted tube. Under the action of buoyancy and drag force, the bubble rises along the channel and at the same time grows continually for the presence of superheat until its break. This paper focuses on the numerical study of flow characteristics of anti-gravity flow in the micro channel and the influence of bubble under the subcooled boiling circumstance. The results shows that bubble plays a positive role in the formation of anti-gravity flow and the analytical expressions are presented for the rising velocity of liquid, the contact angle and the curvature of the intrinsic meniscus, which are all influenced by heat flux, superheat temperature and the geometric parameters of the channel.

PURE NH3 AND CO2 HEAT TRANSFER AND PRESSURE DROP IN HORIZONTAL SMOOTH TUBES FOR NH3 TWO-STAGE AND NH3/CO2 CASCADE REFRIGERATION SYSTEMS: A REVIEW

2010 ◽  
Vol 18 (02) ◽  
pp. 85-100 ◽  
Author(s):  
C. Y. PARK ◽  
P. S. HRNJAK
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Flow Boiling ◽  
Condensation Heat Transfer ◽  
Transfer Coefficients ◽  
Two Stage ◽  
Heat Transfer Coefficients ◽  
Evaporation Heat ◽  
Predicted Values ◽  
Cascade Refrigeration

This paper presents a review of differences and similarities of in-tube heat transfer and pressure drop between ammonia (NH3) and carbon dioxide (CO2) from the perspective of the design of heat exchangers for NH3 two-stage and CO2/NH3 cascade refrigeration systems. The focus is on differences in thermophysical properties and thus different characteristics of heat transfer and pressure drop. A brief summary of published literatures about CO2/NH3 cascade refrigeration systems is provided and literature review of available correlations and developed correlations are presented for flow boiling and condensation heat transfer and pressure drop. Because of large deviation of calculated values with exiting correlations from measured results, a new correlation to predict flow condensation heat transfer coefficients was developed based on experimental results for CO2 at -15°C. From comparison of measured and predicted values, it is shown that some correlations, previously published in open literature, can be used to calculate flow boiling heat transfer coefficients for NH3 at -20°C, if a flow pattern can be appropriately determined for a flow condition. Also, it is presented that existing correlations can predict well the heat transfer coefficients for CO2 flow boiling at -15 and -30°C. It is shown that some correlations can predict pressure drop relatively well for NH3 and CO2 two-phase flow. The NH3 and CO2 flow evaporation heat transfer and pressure drop characteristics at -40°C are compared with predicted values.

A Study of Condensation Heat Transfer in a Single Mini-Tube and a Review of Korean Micro- and Mini-Channel Studies

2003 ◽  
Author(s):  
M. H. Kim ◽  
J. S. Shin ◽  
C. Huh ◽  
T. J. Kim ◽  
K. W. Seo
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Flow Characteristics ◽  
Saturation Temperature ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Condensation Heat Transfer ◽  
Transfer Coefficients ◽  
Round Tube ◽  
Two Phase

This paper reviews recent Korean studies of flow characteristics, flow boiling, and flow condensation in micro- and mini-channels. The characteristics of local heat transfer and pressure drops were experimentally investigated using condensing R134a two-phase flow, in a single round tube, with an inner diameter of 0.691 mm. New experimental techniques were developed to measure the condensation heat transfer coefficient. Tests were performed for a mass flux of 100 to 600 kg/m2s, a heat flux of 5 to 20 kW/m2, and a saturation temperature of 40°C. The experimental local condensation heat transfer coefficients and two-phase frictional pressure gradients are shown. Comparisons of experimental data with existing models reveal that the correlations failed to predict the present data. This study contains the unique sub-millimeter-diameter, single round tube, condensation data reported in the literature.

scholarly journals NUMERICAL STIMULATION OF RAYLEIGH BERNARD CONVECTION IN WAVY ENCLOSURES

2012 ◽  
pp. 274-277
Author(s):  
S. SUBHA
Keyword(s):  
Heat Transfer ◽  
Rayleigh Number ◽  
Flow Characteristics ◽  
Practical Interest ◽  
Nusselt Numbers ◽  
Numerical Stimulation ◽  
Vertical Walls ◽  
Finite Volume Approach ◽  
Fluid Flow Characteristics ◽  
Enclosed Space

Enclosures are frequently encountered in practice, and heat transfer through them is of practical interest. Heat transfer in enclosed space is complicated by the fact that fluid in the enclosure, in general, does not remain constant. The fluid adjacent to the hotter surface rises and the fluid adjacent to the cooler one falls, setting a rotionary motion within the enclosure that enhances the heat transfer through the enclosure. This paper describes a numerical predication of heat transfer and fluid flow characteristics inside an enclosure bounded by horizontal wavy walls and two periodic straight vertical walls. Governing equation were discretized using an implicit finite difference method, based on finite volume approach. Simulation was carried out for a range of Rayleigh number (104-106) and Aspect ratio (0.35-0.75) for the fluid having Prandtl number 0.71. Results are presented by streamlines, isotherms and local Nusselt numbers. It is observed that flow and thermal field inside the enclosure are affected by the shape of enclosure and heat transfer rate increases as Rayleigh number increase.

Visualization Experiment on the Steady Operating Process of the close Loop Pulsating Heat Pipe

2013 ◽  
Vol 732-733 ◽  
pp. 318-321 ◽  
Author(s):  
Xun Wang ◽  
Yun Zhao Li ◽  
Cheng Si Yang ◽  
Xiao Wen Zhang
Keyword(s):  
Heat Pipe ◽  
Flow Direction ◽  
Flow Characteristics ◽  
Heating Power ◽  
Pulsating Heat Pipe ◽  
Heating Section ◽  
Operating Process ◽  
Visualization Experiment ◽  
Long Time ◽  
Churn Flow

A visualization experimental investigation for the pulsating heat pipe (PHP) was carried out to obtain the flow characteristics in the PHP steady operating process with controlling the input heat flux and condensing temperature. The experimental results show that the flow patterns are mainly slug flow with the intermittent pulsation at low heating power, churn flow, and annular flow with steady unidirectional circulation at high heating power. The flow direction switch phenomenon was also observed and analyzed. When the PHP heating section is occupied by vapor slugs for a long time, the backflow phenomenon provides liquid to the bottom of the heating section and reforms a new liquid slug.

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