Sub-cavity liquid volume beneath spray droplet impacts into static liquid layers, and initial estimates of the heat flux required to dry out this volume

2015 ◽  
Vol 66 ◽  
pp. 106-116 ◽  
Author(s):  
Nicholas L. Hillen ◽  
John M. Kuhlman
Keyword(s):  
Heat Flux ◽  
Liquid Volume ◽  
Spray Droplet ◽  
Dry Out ◽  
Droplet Impacts

Effect of Fluid Loading on the Performance of Wicked Heat Pipes

Volume 3 ◽  
2004 ◽  
Author(s):  
R. Kempers ◽  
A. Robinson ◽  
C. Ching ◽  
D. Ewing
Keyword(s):  
Heat Flux ◽  
Heat Pipe ◽  
Heat Transport ◽  
Heat Pipes ◽  
Working Fluid ◽  
Fluid Loading ◽  
Maximum Heat ◽  
Horizontal Orientation ◽  
Maximum Heat Flux ◽  
Dry Out

A study was performed to experimentally characterize the effect of fluid loading on the heat transport performance of wicked heat pipes. In particular, experiments were performed to characterize the performance of heat pipes with insufficient fluid to saturate the wick and excess fluid for a variety of orientations. It was found that excess working fluid in the heat pipe increased the thermal resistance of the heat pipe, but increased maximum heat flux through the pipe in a horizontal orientation. The thermal performance of the heat pipe was reduced when the amount of working fluid was less than required to saturate the wick, but the maximum heat flux through the heat pipe was significantly reduced at all orientations. It was also found in this case the performance of this heat pipe deteriorated once dry-out occurred.

Analysis of the effect of liquid volume fraction of liquid helium chamber with wall heat flux for K 500 SC

2018 ◽  
Vol 43 (1) ◽  
pp. 119
Author(s):  
Pranab Bhattachryya ◽  
Anjan Dutta Gupta ◽  
S. Dhar ◽  
Paramita Mukherjee
Keyword(s):  
Heat Flux ◽  
Liquid Helium ◽  
Volume Fraction ◽  
Wall Heat Flux ◽  
Liquid Volume ◽  
Liquid Volume Fraction

Modeling of Dry-Out Incipience for Flow Boiling in a Circular Microchannel at a Uniform Heat Flux

2015 ◽  
Vol 137 (2) ◽  
Author(s):  
Amen Younes ◽  
Ibrahim Hassan
Keyword(s):  
Heat Flux ◽  
Liquid Film ◽  
Flow Regime ◽  
Annular Flow ◽  
Flow Boiling ◽  
Operating Conditions ◽  
Inertia Force ◽  
Saturated Flow ◽  
Working Fluids ◽  
Dry Out

Dry-out is an essential phenomenon that has been observed experimentally in both slug and annular flow regimes for flow boiling in mini and microchannels. The dry-out leads to a drastic drop in heat transfer coefficient, reversible flow and may cause a serious damage to the microchannel. Consequently, the study and prediction of this phenomenon is an essential objective for flow boiling in microchannels. The aim of this work is to develop an analytical model to predict the critical heat flux (CHF) based on the prediction of liquid film variation in annular flow regime for flow boiling in a horizontal uniformly heated circular microtube. The model is developed by applying one-dimensional (1D) separated flow model for a control volume in annular flow regime for steady, and sable saturated flow boiling. The influence of interfacial shear and inertia force on the liquid film thickness is taken into account. The effects of operating conditions, channel sizes, and working fluids on the CHF have been investigated. The model was compared with 110 CHF data points for flow boiling of various working fluids, (water, LN2, FC-72, and R134a) in single and multiple micro/minichannels with diameter ranges of (0.38≤Dh≤3.04 mm) and heated-length to diameter ratios in the range of (117.7 (117.7≤Lh/D≤470)470). Additionally, three CHF correlations developed for saturated flow boiling in a single microtube have been employed for the model validation. The model showed a good agreement with the experimental CHF data with mean absolute error (MAE) = 19.81%.

Comprehensive Comparisons Between Evaporation and Pool Boiling on Thin Micro Porous Coated Surfaces

2006 ◽  
Author(s):  
Chen Li ◽  
G. P. Peterson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Porous Media ◽  
Heated Surface ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Evaporation Heat ◽  
Evaporation Heat Transfer ◽  
Coated Surfaces ◽  
Dry Out

The evaporation and pool boiling on micro porous coated surfaces have been shown to provide among the highest heat transfer rates achievable from any type of surfaces. The heat transfer modes in these surfaces, present a number of interesting similarities and also, some fundamental differences, which are the result of the liquid supply methods to the heated surface. For the evaporation from porous coated surfaces, the liquid return to the heated surface is assisted by the capillary pressure at the liquid-vapor interface; while for pool boiling, gravity is the principal driving force that rewets the surface. In order to better understand the physical phenomena that governs the flow behavior of both the liquid and vapor phases, and the heat transfer process inside the porous media, comprehensive comparisons between these return mechanisms and their respective characteristics, and the performance and the critical heat flux (CHF) for each have been made, based on similar physical situations. These systematic comparisons illustrate that at a lower heat flux, the evaporation and pool boiling curves are almost identical due to the similar heat transfer modes, i.e., convection and nucleate boiling. While with further increases in heat flux, the heat transfer performance of the evaporation on micro porous media is generally superior to pool boiling on an identical surface. This shift is believed to be due to the fact that for evaporation on micro porous media, the heat transfer mode is dominated by the film evaporation, while in pool boiling, it is principally the result of fully developed nucleate boiling. It was also observed that the impact of the effective thermal conductivity of the porous coating on pool boiling performance is larger than for evaporation heat transfer on the identical micro porous coated surfaces. In general, the experimental data indicated that the CHF for evaporation heat transfer is much higher than for pool boiling on the same surfaces. The mechanism of CHF for evaporation on porous coated surfaces is believed to be the capillary limit; while for pool boiling the limit is the result of the hydrodynamic instabilities. This difference in mechanisms is clearly demonstrated by the experimental observations, where initially, the dry out process of the porous coated surfaces during evaporation is gradual, while for pool boiling; the entire surface reaches dry out in a very short time. In addition, the sensitivity of the CHF to the thickness of the porous coatings at a constant volumetric porosity and pore size, as well as the various optimal volumetric porosity of the CHF at a given thickness, are clearly the results of the differences induced by the various CHF mechanisms.

Experimental and Numerical Characterization of Droplet-Induced Spreading-Splashing Transition in Surface Cooling

2016 ◽  
Author(s):  
Taolue Zhang ◽  
J. P. Muthusamy ◽  
Jorge Alvarado ◽  
Anoop Kanjirakat ◽  
Reza Sadr
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Weber Number ◽  
High Speed ◽  
Single Phase ◽  
Surface Heat ◽  
Droplet Impingement ◽  
Surface Heat Transfer ◽  
Dry Out

The effects of droplet train impingement on spreading-splashing transition and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of HFE-7100 droplet train was generated using a piezo-electric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet impingement velocity. A thin layer of Indium Tin Oxide (ITO) was coated on a translucent sapphire substrate, which was used as heating element. High-speed and infrared imaging techniques were employed to characterize the hydrodynamics and heat transfer of droplet train impingement. Numerically, the high frequency droplet train impingement process was simulated using ANSYS-Fluent with the Volume of Fluid (VOF) method [1]. The heat transfer process was simulated by applying constant heat flux conditions on the droplet receiving surface. Droplet-induced spreading-splashing transition behavior was investigated by increasing the droplet Weber number while holding flow rate constant. High speed crown propagation images showed that at low-Weber number (We < 400), droplet impingements resulted in smooth spreading of the droplet-induced crown. However, within the transitional droplet Weber number range (We = 400–500), fingering and splashing (i.e. emergence of secondary droplets) could be observed at the crown’s rim. At high droplet Weber number (We > 800), breakup of the crown was observed during the crown propagation process in which the liquid film behaved chaotically. Droplet-induced spreading-splashing transition phenomena were also investigated numerically. Reasonable agreement was reached between the experimental and numerical results in terms of crown morphology at different droplet Weber number values. The effects of spreading-splashing transition on surface heat transfer were also investigated at fixed flow rate conditions. Time-averaged Infrared (IR) temperature measurements indicate that heat flux-surface temperature curves are linear at low surface temperatures and before the onset of dry-out, which indicate that single phase forced convection is the primary heat transfer mechanism under those conditions. Numerical heat transfer simulations were performed within the single phase forced convection regime only. Instantaneous numerical results reveal that droplet-induced crown propagation effectively convect heat radially outward within the droplet impingement zone. Under high heat flux conditions, a sharp increase in surface temperature was observed experimentally when dry-out appeared on the heater surface. It was also found that strong splashing (We > 800) is unfavorable for heat transfer at high surface temperature due to the onset of instabilities seen in the liquid film, which leads to dry-out conditions. In summary, the results indicate that droplet Weber number is a significant factor in the spreading-splashing transition and surface heat transfer.

Paper 27: Heat Transfer to Steam-Water Mixtures Flowing in Uniformly Heated Tubes in which the Critical Heat Flux has been Exceeded

1967 ◽  
Vol 182 (8) ◽  
pp. 258-267 ◽  
Author(s):  
A. W. Bennett ◽  
G. F. Hewitt ◽  
H. A. Kearsey ◽  
R. K. F. Keeys
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Wall Temperature ◽  
Heat Transfer Process ◽  
High Mass ◽  
Uniform Heat Flux ◽  
Wall Region ◽  
Temperature Profiles ◽  
Axial Temperature ◽  
Dry Out

Experiments are described on evaporative heat transfer to boiling water in upflow in a vertical electrically heated 0·497-in inside diameter tube at 1000 lbf/in2 (abs.). The main objects were to measure the surface temperature profiles in the region beyond the dry-out point in the channel where liquid ceased to flow on the channel wall, and to investigate the behaviour of the dry-out ‘interface’ between the ‘wetted wall’ and the ‘dry wall’ regions. The test section was made from ‘Nimonic’ as this can withstand the highest temperatures in the ‘dry wall’ region and also has a low temperature coefficient of electrical resistivity, thus allowing a uniform heat flux to be maintained with wide axial temperature variation. The temperature in the ‘dry wall’ region first increased rapidly with distance from the dry-out point, after which it either increased at a slower rate or, at high mass velocities, even decreased. The dry-out ‘interface’ moved reversibly down and up the channel as the heat flux was increased and decreased. Local surface temperatures showed no hysteresis with cycling of heat flux, in contrast with the pool boiling situation. A method of predicting the wall temperature profile in the ‘dry wall’ region has been developed. In this method, the heat-transfer process is considered as being in two steps: wall to superheated steam continuum, and steam continuum to water droplets. The first step was calculated from standard single-phase steam heat-transfer correlations, and the second step was calculated on the basis of simultaneous heat transfer to, and steam diffusion from, the droplets. It was important to take account of the slip between the droplets and the steam. Satisfactory agreement was obtained between measured and predicted wall temperature profiles.

scholarly journals Spray droplet impaction models and their use within AGDISP software to predict retention

2012 ◽  
Vol 65 ◽  
pp. 85-92 ◽  
Author(s):  
W.A. Forster ◽  
G.N. Mercer ◽  
W.C. Schou
Keyword(s):  
Leaf Surface ◽  
Simulation Software ◽  
Surface Adhesion ◽  
Physical Processes ◽  
Spray Droplet ◽  
Retention Models ◽  
Crucial Effect ◽  
Model Predictions ◽  
Spray Formulation ◽  
Droplet Impacts

The retention and distribution of spray droplets within the plant canopy have a crucial effect on the biological efficacy of pesticides To maximise spray retention droplets that impact a leaf must remain on the plant Three outcomes are possible when a droplet impacts a leaf surface adhesion bounce or shatter Those droplets that bounce or shatter can continue their journey through the canopy depositing at lower levels in the canopy or on the ground Mathematical models based on the physical processes involved in the bounce/ adhesion and shatter of droplets have been developed improved and described These processbased retention models have recently been implemented within an experimental build of the spray application simulation software AGDISP This has allowed differences in total spray retention to plants due to the spray formulation used or vegetative species studied to be predicted This paper discusses these new tools illustrates the effect different spray formulations and application parameters have on predicted retention and compares model predictions with measured retention

Study on Heat Transfer Mechanism of Isolated Bubble Nucleate Boiling With MEMS Sensors

2010 ◽  
Author(s):  
Tomohide Yabuki ◽  
Osamu Nakabeppu
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Bubble Growth ◽  
Local Heat ◽  
Nucleate Boiling ◽  
Heat Transfer Mechanism ◽  
Mems Sensors ◽  
Local Heat Flux ◽  
Dry Out

This paper describes an experimental investigation of heat transfer mechanism beneath isolated bubble during nucleate boiling with MEMS sensors having high temporal and spatial resolution in temperature measurement. The MEMS sensor fabricated for the boiling research includes eight thin film thermocouples and an electrolysis trigger on the topside of 20 × 20 mm2 silicon substrate and thin film heater on the backside. The electrolysis trigger initiates bubble growth by supplying hydrogen gasses as bubble nuclei with the electrolysis of the water by two electrodes. In the experiment, temperature fluctuation beneath an isolated bubble during saturated nucleate boiling of water was measured with the sensor. The measurement data presented strong evaporation and dry-out of the microlayer in the bubble growth phase and rewetting of the dry-out area in the bubble departure phase. Moreover, heat transfer induced by the boiling bubble was evaluated by computing local heat flux through a transient heat conduction simulation in the sensor substrate using the measured data as boundary condition. The heat transfer analysis shows that the local heat flux in the microlayer evaporation area has high value of the order of MW/m2, and the maximum value of about 2 MW/m2 is indicated near the center in an early phase of the bubble growth. On the other hand, the heat flux is very low of around zero at the dry-out area, where microlayer had disappeared completely, and slight increase was observed at the rewetting area. Total heat transferred from the surface reached to about half of latent heat in the bubble until the bubble departure. Finally, initial thickness of the microlayer under the bubble was estimated by integrating the derived local heat flux. As the result, it was distributed in a few μm within the measurement area.

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