The Thickness of the Liquid Microlayer Between a Sliding Bubble and a Heated Wall: Comparison of Models to Experimental Data

2008 ◽  
Vol 130 (11) ◽  
Author(s):  
D. Keith Hollingsworth ◽  
Xin Li ◽  
Larry C. Witte
Keyword(s):  
Mechanistic Model ◽  
High Heat ◽  
Heated Wall ◽  
Vapor Bubbles ◽  
Transfer Coefficients ◽  
Data Set ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
And Performance ◽  
First Time

Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend, in part, on the evolution of the liquid microlayer that forms between the bubble and the surface, as the bubble grows by evaporation. A mechanistic model of the microlayer thickness verified by direct observation of the microlayer thickness is needed. This paper describes a comparison of measurements from a recent set of experiments to the results of microlayer models from literature and to the predictions of a new model presented here for the first time. The measurements were produced by a laser-based method developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Microlayer thicknesses of 22–55 μm were obtained for saturated FC-87 and a uniform-temperature surface inclined at 2–15 deg from the horizontal. The basis of each model, input requirements, limitations, and performance relative to this data set is discussed. A correlation is developed based on the structure of the lubrication theory. It collects the measured microlayer thickness presented as a microlayer Reynolds number to within ±10%. This correlation depends only on bubble volume, inclination, and a bubble shape factor, all of which can be determined experimentally to within reasonable accuracy.

The Thickness of the Liquid Microlayer Between a Sliding Bubble and a Heated Wall: Comparison of Models to Experimental Data

2007 ◽  
Author(s):  
Xin Li ◽  
D. Keith Hollingsworth ◽  
Larry C. Witte
Keyword(s):  
Mechanistic Model ◽  
High Heat ◽  
Heated Wall ◽  
Vapor Bubbles ◽  
Transfer Coefficients ◽  
Data Set ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
And Performance ◽  
First Time

Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend, in part, on the evolution of the liquid microlayer that forms between the bubble and the surface. A mechanistic model of the micro-layer thickness verified by direct observation of the microlayer thickness is needed. This paper describes a comparison of measurements from a recent set of experiments to the results of microlayer models from the literature and to the predictions of a new model presented here for the first time. The measurements were produced by a laser-based method developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Microlayer thicknesses of 22 to 55 microns were obtained for saturated FC-87 and a uniform-temperature surface inclined at 2° to 15° from the horizontal. The basis of each model, input requirements, limitations, and performance relative to this data set are discussed.

The Thickness of the Liquid Microlayer Between a Cap-Shaped Sliding Bubble and a Heated Wall: Experimental Measurements

2006 ◽  
Vol 128 (9) ◽  
pp. 934-944 ◽  
Author(s):  
Xin Li ◽  
D. Keith Hollingsworth ◽  
Larry C. Witte
Keyword(s):  
Heat Transfer ◽  
Reynolds Numbers ◽  
High Heat ◽  
Heated Wall ◽  
Transfer Coefficients ◽  
Data Set ◽  
Laser Probe ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Spherical Bubbles

A laser-based method has been developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend on the evolution of the microlayer that forms between the bubble and the surface. Past experiments have used heat transfer measurements on uniform-heat-generation surfaces to infer the microlayer thickness through an energy balance. These studies have produced measurements of 20–100 μm for refrigerants and for water, but they have yet to be confirmed by a direct measurement that does not depend on a first-law closure. The results presented here are direct measurements of the microlayer thickness made from a reflectance-based fiber-optic laser probe. Details of the construction and calibration of the probe are presented. Data for saturated FC-87 and a uniform-temperature surface inclined at 2 deg to 15 deg from the horizontal are reported. Millimeter-sized spherical bubbles of FC-87 vapor were injected near the lower end of a uniformly heated aluminum plate. The laser probe yielded microlayer thicknesses of 22–55 μm for cap-shaped bubbles. Bubble Reynolds numbers range from 600 to 4800, Froude numbers from 0.9 to 1.7, and Weber numbers from 2.6 to 47. The microlayer thickness above cap-shaped bubbles was correlated to a function of inclination angle and a bubble shape factor. The successful correlation suggests that this data set can be used to validate the results of detailed models of the microlayer dynamics.

Measurement of the Thickness of the Liquid Microlayer Between a Sliding Bubble and a Heated Wall

2005 ◽  
Author(s):  
Xin Li ◽  
D. Keith Hollingsworth ◽  
Larry C. Witte
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
High Heat ◽  
Bubble Surface ◽  
Heated Wall ◽  
Transfer Coefficients ◽  
Laser Probe ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Spherical Bubbles

A laser-based method has been developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend on the evolution of the microlayer that forms between the bubble and the surface. Past experiments have used heat transfer measurements on uniform-heat-generation surfaces to infer the microlayer thickness through an energy balance. These studies have produced measurements of 20 to 100 μm for refrigerants and for water, but they have yet to be confirmed by a direct measurement that does not depend on a first-law closure. The results presented here are direct measurements of the microlayer thickness made from a reflectance-based fiber-optic laser probe. Details of the construction and calibration of the probe are presented. Data for saturated FC-87 and a uniform-temperature surface inclined at 2° to 15° from the horizontal are reported. Millimeter-sized spherical bubbles of FC-87 vapor are injected near the lower end of a uniformly heated aluminum plate. The bubbles grow rapidly and change from a spherical to an elliptical shape and finally to a cap-shape with a large section of the bubble surface sliding along the microlayer adjacent to the wall. This evolution is captured by high-speed (1000 frames/sec) images in plan and side views. These image sequences allow measurements of bubble speed, acceleration, and size, but they do not attempt to resolve the microlayer itself. The laser probe yielded microlayer thicknesses of 22 to 55 microns for the cap-shaped bubbles. Bubble Reynolds numbers range from 600 to 4800, Froude numbers are from 0.9 to 1.7, and Weber numbers are from 2.6 to 47.

An Analytical Study of the Optimized Performance of an Impingement Heat Sink

2004 ◽  
Vol 126 (4) ◽  
pp. 528-534 ◽  
Author(s):  
S. B. Sathe ◽  
B. G. Sammakia
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Heat Sink ◽  
High Performance ◽  
Cross Flow ◽  
High Heat ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Flow Through

The results of a study of a new and unique high-performance air-cooled impingement heat sink are presented. An extensive numerical investigation of the heat sink performance is conducted and is verified by experimental data. The study is relevant to cooling of high-power chips and modules in air-cooled environments and applies to workstations or mainframes. In the study, a rectangular jet impinges on a set of parallel fins and then turns into cross flow. The effects of the fin thickness, gap nozzle width and fin shape on the heat transfer and pressure drop are investigated. It is found that pressure drop is reduced by cutting the fins in the central impingement zone without sacrificing the heat transfer due to a reduction in the extent of the stagnant zone. A combination of fin thicknesses of the order of 0.5 mm and channel gaps of 0.8 mm with appropriate central cutout yielded heat transfer coefficients over 1500 W/m2 K at a pressure drop of less than 100 N/m2, as is typically available in high-end workstations. A detailed study of flow-through heat sinks subject to the same constraints as the impingement heat sink showed that the flow-through heat sink could not achieve the high heat transfer coefficients at a low pressure drop.

Spatial and Temporal Wall Temperature Fluctuations in Two-Phase Flow in Microgap Coolers

2010 ◽  
Author(s):  
Jessica Sheehan ◽  
Avram Bar-Cohen
Keyword(s):  
Heat Transfer ◽  
High Heat ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Volumetric Cooling ◽  
Very High

Heat transfer to an evaporating refrigerant and/or dielectric liquid in a microgap channel can provide very high heat transfer coefficients and volumetric cooling rates. Recent studies at Maryland have established the dominance of the annular flow regime in such microgap channels and related the observed high-quality peak of an M-shaped heat transfer coefficient curve to the onset of local dryout. The present study utilizes infrared thermography to locate such nascent dryout regions and operating conditions. Data obtained with a 210 micron microgap channel, operated with a mass flux of 195.2 kg/m2-s and heat fluxes of 10.3 to 26 W/cm2 are presented and discussed.

Numerical Investigation on Forced Convection in Circular Tubes With Septa

2008 ◽  
Author(s):  
D. Corrente ◽  
O. Manca ◽  
S. Nardini ◽  
D. Ricci ◽  
G. Masullo
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Forced Convection ◽  
High Heat ◽  
Ideal Gas ◽  
Transfer Coefficients ◽  
Pressure Drops ◽  
Heat Transfer Coefficients ◽  
Heating And Cooling ◽  
High Heat Transfer

Heat transfer in fluids is very important in many industrial heating and cooling equipments. Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions or by increasing thermal conductivity of the fluid. Another possibility to increase heat transfer with gas is to employ extended surfaces. When gas flows in a tube, septa with one or more openings can be used as fins. Furthermore, if the openings are arranged to give a spiral motion around the cylinder axis wall-fluid contact area increases. As a consequence the presence of the septa can significantly augment pressure drops. In this paper a numerical investigation is carried out on forced convection in circular isothermal tubes. The fluid is air and ideal gas model is employed. Septa are introduced and several shapes and arrangements are analyzed. The investigation is accomplished by means of the commercial code Fluent. A turbulence model is used. Results are presented in terms of temperature and velocity fields, local and average heat transfer coefficients and pressure drops. The aim of this study is to find the shape and arrangement of septa such to give high heat transfer coefficients and low pressure drops.

scholarly journals Properties of Kerosene-Aluminium Nanofluid used to Estimate the Overall Heat Transfer Rates during Regenerative/Film Cooling of Thrust Chambers

2019 ◽  
Vol 9 (1S3) ◽  
pp. 165-169
Keyword(s):  
Heat Transfer ◽  
Film Cooling ◽  
Space Station ◽  
High Heat ◽  
Nano Particles ◽  
Transfer Coefficients ◽  
Transfer Rates ◽  
Rocket Propulsion ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer

Large heat transfer rates are always desired for rocket propulsion applications as high heat loads are associated at the nozzle exit. Different strategies have been employed in order to have high heat transfer coefficients including use of liquid nitrogen, spray cooling etc. ISRO has planned to use aluminium based nano-particles with kerosene in order to cool launching vehicles including GSLV Mk III as it is the heaviest rocket that can carry large payloads. Recently, ISRO has announced to install its own International Space Station (ISS) in future and in such applications larger payloads are to be carried by the rocket. In this work, an analytical study on the thermodynamic properties of the aluminium nano-particles based kerosene nanofluid has been done and an attempt has also been made to develop a temperature and pressure dependent correlation that can be used in computational analysis of thrust chambers while film/regenerative cooling.

Heat Transfer Analysis of Microgrooved Evaporator and Condenser Surfaces Utilized in a High Heat Flux Two-Phase Flow Loop

2007 ◽  
Author(s):  
Edvin Cetegen ◽  
Thomas Baummer ◽  
Serguei Dessiatoun ◽  
Michael Ohadi
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
High Heat ◽  
Phase Flow ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Pressure Drops ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Microgrooved Surface

This paper investigates the heat transfer and pressure drop analysis of micro grooved surfaces utilized in evaporators and condensers of a two-phase flow cooling loop. These devices utilize the vapor-liquid phase change to transfer large amounts of heat, and they offer substantially higher heat flux performance with lower pumping power than most liquid cooling technologies. Microgrooved surfaces, combined with force-fed evaporation and condensation technology discussed in this paper yield high heat transfer coefficients with low pressure drops. Our most recent results, aiming to test the limits of the technology, demonstrated dissipation of almost 1kW/cm2 from silicon electronics using HFE 7100 as the working fluid. In a compact two phase system, the heat generated by the electronic components can be absorbed by microgrooved evaporators and rejected through the microgrooved surface condensers to liquid cooled slots with high heat transfer coefficients and low pressure drops on the refrigerant side. In the case of air-cooling, the same microgrooved surface heat exchanger can reject heat with a heat transfer coefficient of 3847 W/cm2 and a pressure drop of 4156 Pa. These heat transfer processes have the added capability of being combined and used together in a self-contained system cooled either by liquid or air.

scholarly journals Numerical Analysis of Water Forced Convection in Channels with Differently Shaped Transverse Ribs

2011 ◽  
Vol 2011 ◽  
pp. 1-25 ◽  
Author(s):  
Oronzio Manca ◽  
Sergio Nardini ◽  
Daniele Ricci
Keyword(s):  
Heat Transfer ◽  
Forced Convection ◽  
High Heat ◽  
Constant Heat Flux ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Nusselt Numbers ◽  
High Heat Transfer ◽  
Friction Factors ◽  
Turbulent Water

Heat transfer enhancement technology has the aim of developing more efficient systems as demanded in many applications. An available passive method is represented by the employ of rough surfaces. Transversal turbulators enhance the heat transfer rate by reducing the thermal resistance near surfaces, because of the improved local turbulence; on the other hand, higher losses are expected. In this paper, a numerical investigation is carried out on turbulent water forced convection in a ribbed channel. Its external walls are heated by a constant heat flux. Several arrangements of ribs in terms of height, width, and shape are analyzed. The aim is to find the optimal configuration in terms of high heat transfer coefficients and low losses. The maximum average Nusselt numbers are evaluated for dimensionless pitches of 6, 8, and 10 according to the shape while the maximum friction factors are in the range of pitches from 8 to 10.

scholarly journals Heat Transfer Characteristics of Swirling Impinging Jets

2010 ◽  
Author(s):  
Karl J. Brown ◽  
Tim Persoons ◽  
Darina B. Murray
Keyword(s):  
Heat Transfer ◽  
Thermal Imaging ◽  
High Heat ◽  
Impinging Jets ◽  
Transfer Coefficients ◽  
Film Sensor ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer ◽  
Hot Film ◽  
Hot Film Sensor

Impinging jets are well renowned for their abilities to achieve significantly high heat transfer coefficients. It has been found in previous studies that the incorporation of a swirl in the flow can enhance heat transfer but it can also decrease it. The objective of this research is to discover the defining characteristics of swirling impinging jets and to find an optimal swirl geometry for enhancing the heat transfer of the system. Using thermal imaging and hot film sensor techniques the heat transfer distribution of the swirling jet is examined in the context of the flow field; heat flux fluctuations at the impinging surface are investigated also.

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