scholarly journals Spatial Distribution of Active Sites and Bubble Flux Density

1978 ◽  
Vol 100 (1) ◽  
pp. 56-62 ◽  
Author(s):  
M. Sultan ◽  
R. L. Judd
Keyword(s):  
Heat Flux ◽  
Spatial Distribution ◽  
Flux Density ◽  
Vapor Bubble ◽  
Active Sites ◽  
Heating Surface ◽  
Copper Surface ◽  
Emission Frequency ◽  
Nucleation Sites ◽  
Different Levels

An experimental investigation is presented concerning the boiling of water at atmospheric pressure on a single copper surface in which the spatial distribution of active nucleation sites was investigated at different levels of heat flux and subcooling. The results obtained indicated that the active sites were located randomly on the heating surface, since the distribution of active sites followed the Poisson relationship. Changes in heat flux and sub-cooling did not affect the distribution of active nucleation sites although additional active sites formed among the sites which had already been activated when heat flux was increased. The frequency of vapor bubble emission and the bubble flux density have been studied as well. The results obtained showed that the heat flux had a great effect on the vapor bubble emission frequency although the influence of subcooling was of lesser significance. However, the bubble flux density was found to be non-uniformly distributed over the heating surface contrary to what had been expected.

scholarly journals Interaction of the Nucleation Phenomena at Adjacent Sites in Nucleate Boiling

1983 ◽  
Vol 105 (1) ◽  
pp. 3-9 ◽  
Author(s):  
M. Sultan ◽  
R. L. Judd
Keyword(s):  
Active Sites ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Nucleation Sites ◽  
Theoretical Predictions ◽  
Experimental Findings ◽  
Relationship Of ◽  
The Relationship ◽  
Different Levels

The present investigation is an original study in nucleate pool boiling heat transfer combining theory and experiment in which water boiling at atmospheric pressure on a single copper surface at two different levels of heat and different levels of subcooling was studied. Cross spectral analysis of the signals generated by the emission of bubbles at adjacent nucleation sites was used to determine the relationship of the time elapsed between the start of bubble growth at the two neighbouring active sites with the distance separating them. The experimental results obtained indicated that for the lower level of heat flux at three different levels of subcooling, the elapsed time and distance were directly related. Theoretical predictions of a temperature disturbance propagating through the heating surface in the radial direction gave good agreement with the experimental findings, suggesting that this is the mechanism responsible for the activation of the surrounding nucleation sites.

An Experimental Investigation of the Effect of Subcooling on Bubble Growth and Waiting Time in Nucleate Boiling

1985 ◽  
Vol 107 (1) ◽  
pp. 168-174 ◽  
Author(s):  
E. A. Ibrahim ◽  
R. L. Judd
Keyword(s):  
Heat Flux ◽  
Waiting Time ◽  
Bubble Growth ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Growth Theory ◽  
Growth Time ◽  
Time Data ◽  
Relationship Of ◽  
Different Levels

The effect of subcooling on bubble waiting time and growth time for water boiling on a copper surface was examined in conjunction with measurements obtained over a range of subcooling from 0 to 15°C and three different levels of heat flux 166, 228, and 291 kW/m2. The growth-time data was successfully correlated with a model that combined the bubble growth theory of Mikic, Rohsenow, and Griffith with the bubble departure diameter relationship of Staniszewski, thereby establishing confidence in the measuring procedure. The waiting time data agreed with the predictions of the Han and Griffith waiting time theory at lower levels of subcooling but then showed a behavior contrary to that predicted for higher levels of subcooling.

scholarly journals Analysis of thermal properties, water vapor resistance and radiant heat transmission through different combinations of firefighter protective clothing

2019 ◽  
Vol 69 (06) ◽  
pp. 458-465
Author(s):  
NAEEM JAWAD ◽  
ADNAN MAZARI ◽  
AKCAGUN ENGIN ◽  
HAVELKA ANTONIN ◽  
KUS ZDENEK
Keyword(s):  
Heat Flux ◽  
Water Vapor ◽  
Flux Density ◽  
Heat Flux Density ◽  
Protective Clothing ◽  
Radiant Heat ◽  
Thermal Protective Performance ◽  
Firefighter Protective Clothing ◽  
Protective Performance ◽  
Different Levels

This experimental work is an effort to seek the possibility of improvement in thermal protective performance of firefighter protective clothing at different levels of heat flux density. Improvement in thermal protective performance means enhancement in the time of exposure against the heat flux, which will provide extra time to firefighters to perform their duties without suffering from severe injuries. Four different multilayer combinations of firefighter protective clothing were investigated. Each combination consists of outer shell, moisture barrier and thermal liner. Aerogel sheet was also employed as a substitute to thermal barrier. Initially, properties like thermal resistance, thermal conductivity, and water vapor resistance of multilayer fabric assemblies were investigated. Later on these combinations were exposed to different levels of radiant heat flux density i.e. at 10, 20 and 30 kW/m2 as per ISO 6942 standard. It was noted that those combinations in which aerogel blanket was used as thermal barrier acquire greater thermal resistance, water vapor resistance and have less transmitted heat flux density values.

Nucleate Boiling With High Gravity and Large Subcooling

1990 ◽  
Vol 112 (2) ◽  
pp. 451-457 ◽  
Author(s):  
M. E. Ulucakli ◽  
H. Merte
Keyword(s):  
Heat Flux ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
Fluid Temperature ◽  
Heater Surface ◽  
Earth Gravity ◽  
Maximum Value ◽  
Wall Superheat ◽  
Nucleation Sites ◽  
Type Of Behavior

Measurements of the heater surface temperature are presented for pool boiling of distilled water in an accelerating system with various subcoolings and levels of heat flux. The ranges of the experimental variables are: heat flux between 0.19 MW/m2 and 1.5 MW/m2, accelerations normal to the flat heating surface from 1 to 100 times earth gravity, and liquid subcoolings between 0 K and 89 K. Increasing sub-cooling first produces an increase and then a decrease in wall superheat, with the eventual cessation of nucleate boiling for certain combinations of conditions. The increase in wall superheat is particularly enhanced at 10g, reaching a maximum value of 9 K at 1.05 MW/m2 with 60 K subcooling. This type of behavior is attributed to the interactions between the fluid temperature distribution in the immediate vicinity of the heater surface as it is influenced by natural convection, the activation of nucleation sites, and the influence of increased buoyancy on the heat transfer associated with each departing bubble.

High Heat Flux Dissipation Using Symmetric Dual-Taper Manifold in Pool Boiling

2019 ◽  
Author(s):  
Aranya Chauhan ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pool Boiling ◽  
Heat Removal ◽  
Heating Surface ◽  
High Heat ◽  
Copper Surface ◽  
High Heat Flux ◽  
Maximum Heat ◽  
Continuous Increase

Abstract The trend of miniaturization in electronics presents a great challenge in the thermal management of devices. The continuous increase in the number of transistors in the processor leads to high heat flux generation, limiting the performance of the device. Boiling heat transfer offers a great heat removal competency while maintaining the low chip temperatures. The critical heat flux (CHF) dictates the maximum heat removal ability, and heat transfer coefficient (HTC) defines the efficiency of the boiling process. This pool boiling study is focused on using a manifold containing a symmetric dual taper over the heating surface. The heat transfer performance of this configuration is evaluated for different taper angles in the manifold. The macro-convection assisted by vapor columns during boiling enhance the CHF and HTC limit significantly. A CHF of 287 W/cm2 with an HTC of 116 kW/cm2°C was achieved with a plain copper surface, representing greater than a 2-fold increases in each over a plain surface.

Copper Inverse Opal Surfaces for Enhanced Boiling Heat Transfer

2017 ◽  
Author(s):  
Hyoungsoon Lee ◽  
Tanmoy Maitra ◽  
James Palko ◽  
Chi Zhang ◽  
Michael Barako ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Bubble Formation ◽  
Copper Surface ◽  
Inverse Opal ◽  
Porous Structures ◽  
Optical Images ◽  
Nucleation Sites ◽  
Enhanced Boiling

Enhanced boiling is one of the popular cooling schemes in thermal management due to its superior heat transfer characteristics. This study demonstrates the ability of copper inverse opal (CIO) porous structures to enhance pool boiling performance using a thin CIO film with a thickness of ∼ 10 μm and pore diameter of 5 μm. The microfabricated CIO film increases microscale surface roughness that in turn leads to more active nucleation sites thus improved boiling performance parameters such as heat transfer coefficient and critical heat flux compared to those of smooth Si surfaces. The experimental results for CIO film show a maximum critical heat flux of 225 W/cm2 (at 16.2°C superheat) or about 3 times higher than that of smooth Si surface (80 W/cm2 at 21.6°C superheat). Optical images showing bubble formation on the microporous copper surface are captured to provide detailed information of bubble departure diameter and frequency.

Three-Dimensional Numerical Simulation of Burnout on Horizontal Surface in Pool Boiling

2003 ◽  
Author(s):  
Zoran V. Stosic ◽  
Vladimir D. Stevanovic
Keyword(s):  
Heat Flux ◽  
Residence Time ◽  
Pool Boiling ◽  
Three Dimensional ◽  
Heating Surface ◽  
Heat Fluxes ◽  
Heating Wall ◽  
Two Phase ◽  
Nucleation Sites ◽  
Phase Mixture

Prediction of nucleate boiling mechanisms and burnout conditions, when heat transfer coefficient sharply drops and the heating surface destruction could occur are one of the crucial topics in thermal design and safety analyses of various thermal equipment. Although these phenomena have been intensively investigated for decades, various influencing factors and complexity of coupled thermal and fluid dynamic processes have not yet been fully understood. The integral approach towards prediction of nucleate boiling and burnout conditions requires modelling and numerical simulation of micro level phenomena of bubble rise and departure at a numbers of nucleation sites, as well as macroscopic two-phase mixture behaviour on the heating surface. In this paper multidimensional numerical simulation of the atmospheric saturated pool boiling is performed under high heat fluxes, near to and at the occurrence of burnout conditions. Micro level phenomena on the heating surface are modelled with the key parameters of vapour generation on the heating wall, such as bubble nucleation site density, bubble residence time on the heating wall and certain level of randomness in the location of bubble nucleation. Heat flux is non-uniformly distributed on the heating surface with peaks at the nucleation sites. The nucleation sites are determined by a random function, while the mean number of nucleation sites is prescribed according to the material characteristics and roughness of the heating surface. The applied numerical grids are able to represent the nucleation sites on the heating wall for both fresh (polished) and aged (rough) heaters at the atmospheric pool boiling conditions. The macro level phenomena are modelled with two-fluid model of liquid-vapour flow. The interfacial drag is modelled with appropriate closure laws. The applied modelling and numerical methods enable full representation of the two-phase mixture behaviour on the heating surface with inclusion of the swell level prediction. In this way the integral conditions of nucleate pool boiling with the possibility of burnout are simulated and the critical heat flux conditions are predicted. The result of the three-dimensional numerical simulations and analyses are presented as the extension of the previously published two-dimensional numerical results. Here presented three-dimensional investigation is performed in order to take into account more realistically spatial effects of vapour generation and two-phase flow, such as phase dispersion within the two-phase mixture, than it was able with previously performed two-dimensional investigation. Results are presented for short time period after the initiation of heat supply and vapour generation on the heating surface, as well as for quasi steady-state conditions after several seconds from pool boiling initiation. A replenishment of the heating surface with water and partial surface wetting for lower heat fluxes is shown, while heating surface dry-out is observed for high heat fluxes. The influence of the density of nucleation sites and the bubble residence time on the wall on the pool boiling dynamics is investigated. Also, the influence of the heat flux intensity on the pool boiling dynamics is analysed. Numerical simulations show that decrease of the density of nucleation sites and increase of bubble residence time on the heating surface (characteristics pertinent to fresh-polished heaters) lead to the reduction of critical heat flux values. Obtained results are in excellent agreement with the recent experimental investigations of the upward facing burnout conditions on the horizontal heated plate. Details of the developed numerical procedure are presented. The introduced method of random spatial and temporal generation of the vapour at the heated wall is a new approach. It enables the macroscopic representation of the population of microscopic vapour bubbles, which are generated at nucleation sites on the heater wall, and which burst through liquid micro-layer in thermal-hydraulic conditions close to the burnout. The applied numerical and modelling method has shown robustness by allowing stable calculations for wide ranges of applied modelling boiling parameters (density of nucleation sites and bubble residence time).

Multi-Dimensional Numerical Simulation of Burnout on Horizontal Surface in Pool Boiling

2002 ◽  
Author(s):  
Zoran V. Stosic ◽  
Vladimir D. Stevanovic
Keyword(s):  
Numerical Simulation ◽  
Heat Flux ◽  
Residence Time ◽  
Pool Boiling ◽  
Heating Surface ◽  
High Heat ◽  
Heat Fluxes ◽  
Heating Wall ◽  
Nucleation Sites ◽  
Vapour Generation

Multidimensional numerical simulation of the atmospheric saturated pool boiling is performed under high heat fluxes, near to and at the occurrence of burnout conditions. Heat flux through the vessel bottom wall is varied and its influence on the pool boiling dynamics is analysed. Dynamics of vapour generation on the heating wall is modelled through the density of nucleation sites and the bubble residence time on the wall. The nucleation sites are determined by a random function. The applied numerical grid is able to represent the nucleation sites on the heating wall for both fresh (polished) and aged (rough) heaters at the atmospheric pool boiling conditions. Results are presented for short time period after the initiation of heat supply and vapour generation on the heating surface, as well as for quasi steady-state conditions after two seconds from pool boiling initiation. The results show a replenishment of the heating surface with water and partial surface wetting for lower heat fluxes, while heating surface dry-out is predicted for high heat fluxes. The influence of the density of nucleation sites and the bubble residence time on the wall on the pool boiling dynamics is investigated. Numerical simulations show that decrease of the density of nucleation sites and increase of bubble residence time on the heating surface (characteristics pertinent to fresh-polished heaters) lead to the reduction of critical heat flux values. Obtained results are in excellent agreement with the recent experimental investigations of the upward facing burnout conditions on the horizontal heated plate. Details of the developed numerical procedure are presented. The introduced method of random spatial and temporal generation of the vapour at the heated wall is a new approach. It enables the macroscopic representation of the population of microscopic vapour bubbles, which are generated at nucleation sites on the heater wall, and which burst through liquid micro-layer in thermal-hydraulic conditions close to the burnout. The applied numerical and modelling method has shown robustness by allowing stable calculations for wide ranges of applied modelling boiling parameters (density of nucleation sites and bubble residence time).

Photographic Study of Nucleate Pool Boiling on a Horizontal Surface

1965 ◽  
Vol 87 (1) ◽  
pp. 17-27 ◽  
Author(s):  
R. F. Gaertner
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Active Sites ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Horizontal Surface ◽  
Transfer Model ◽  
Nucleate Pool Boiling ◽  
Platinum Surface

A photographic study was made of saturated nucleate pool boiling at a pressure of one atmosphere. Over 1000 still photographs and 12 high-speed motion pictures were taken of water boiling from a 2-in-dia flat horizontal surface facing upward. Two surfaces were studied, a 2/0 polished platinum surface and a 4/0 polished copper surface. The platinum surface was studied in the heat flux range of 14,700 to 176,000 Btu/hr, sq ft, and the copper surface from the incipient boiling heat flux of 10,500 Btu/hr, sq ft to the maximum flux of 493,000 Btu/hr, sq ft. Data were obtained for the breakoff diameters of discrete bubbles, and for the populations of active sites at heat fluxes up to 58,600 Btu/hr, sq ft. At least three, and possibly four, heat-transfer regions were found to exist in nucleate boiling, depending upon the mode of vapor generation. The vapor structures on the surface progressed through a sequence of first discrete bubbles, then vapor columns and vapor mushrooms, and finally vapor patches, as the surface temperature was increased. These individual vapor structures, or combinations of them, determine the mechanism of heat transfer in the four nucleate boiling regions. It was concluded that any heat-transfer model or design equation which is based on the dynamics of individual bubbles, or on any other single mechanism, must be in serious error.

THE MECHANISM OF RAISING AND QUANTIFICATION OF SPECIFIC HEAT FLUX AT BOILING OF NANOFLUIDS IN FREE CONVECTION CONDITIONS

2017 ◽  
pp. 25-34
Author(s):  
V.N. Moraru
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Capacity ◽  
Specific Heat ◽  
Chemical Properties ◽  
Heating Surface ◽  
Physical Models ◽  
Superheated Liquid ◽  
Two Phase ◽  
Boiling Process

The results of our work and a number of foreign studies indicate that the sharp increase in the heat transfer parameters (specific heat flux q and heat transfer coefficient _) at the boiling of nanofluids as compared to the base liquid (water) is due not only and not so much to the increase of the thermal conductivity of the nanofluids, but an intensification of the boiling process caused by a change in the state of the heating surface, its topological and chemical properties (porosity, roughness, wettability). The latter leads to a change in the internal characteristics of the boiling process and the average temperature of the superheated liquid layer. This circumstance makes it possible, on the basis of physical models of the liquids boiling and taking into account the parameters of the surface state (temperature, pressure) and properties of the coolant (the density and heat capacity of the liquid, the specific heat of vaporization and the heat capacity of the vapor), and also the internal characteristics of the boiling of liquids, to calculate the value of specific heat flux q. In this paper, the difference in the mechanisms of heat transfer during the boiling of single-phase (water) and two-phase nanofluids has been studied and a quantitative estimate of the q values for the boiling of the nanofluid is carried out based on the internal characteristics of the boiling process. The satisfactory agreement of the calculated values with the experimental data is a confirmation that the key factor in the growth of the heat transfer intensity at the boiling of nanofluids is indeed a change in the nature and microrelief of the heating surface. Bibl. 20, Fig. 9, Tab. 2.

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