Mechanism of convection heat transfer on a heated crystal surface

This paper considers the mechanism of convection heat transfer on a heated crystal surface. An attempt was made to divide the heat transfer process into three links. Phonon theory is used to study and define the "convective power" of a crystal. The results indicate that the convection heat transfer process is discrete. The order of convective power of the crystal is extremely high and is approximately proportional to T ∼︁ T. It can be regarded that the energy exchange is finished in the course of one collision between the heated solid and media particles that is demonstrated by thermal conduction in the gas. The value of the heat transfer coefficient depends on the properties, states, and motion of the media. © 2000 Scripta Technica, Heat Trans Asian Res, 29(7): 573—580, 2000     

Interfacial heat transfer during subcooled flow boiling

In order to develop a mechanistic model for the subcooled flow boiling process, the key issues which must be addressed are wall heat flux partitioning and interfacial (condensation) heat transfer. The sink term in the two-fluid models for void fraction prediction is provided by the condensation rate at the vapor-liquid interface. Low pressure subcooled flow boiling experiments, using water, were performed using a vertical flat plate heater to investigate the bubble collapse process. A high-speed CCD camera was used to record the bubble collapse in the bulk subcooled liquid. Based on the analyses of these digitized images, bubble collapse rates and the associated heat transfer rate were determined. The experimental data were in turn used to correlate the bubble collapse rate and the interfacial heat transfer rate. These correlations are functions of bubble Reynolds number, liquid Prandtl number, Jacob number, and Fourier number. The correlations account for both the effect of forced convection heat transfer and thickening of the thermal boundary layer as the vapor bubble condenses which in turn makes the condensation heat transfer time dependent. Comparison of the measured experimental data with those predicted from the correlations show that predictions are well within ±25% of the experimentally measured values. These correlations have also been compared with those available in the literature.     

Contact angles and interface behavior during rapid evaporation of liquid on a heated surface

Photographic observations of the boiling phenomena have played an important role in gaining insight into the boiling mechanism. This paper presents a brief historical review of the available literature on the photographic studies in pool and flow boiling. This is followed by the results of the photographic studies conducted in the authors' laboratory on liquid droplets impinging on a heated surface. Liquid-vapor interface and contact line movements are observed through a high speed camera at high resolution. The effect of surface roughness and surface temperature on dynamic advancing and receding contact angles has been studied. In addition, the effects of rapid evaporation on advancing and receding contact angles, liquid-vapor interface motion, and the dryout front propagation have been investigated.     

Convection heat and mass transfer along a vertical heated plate with film evaporation in a non-Darcian porous medium

A numerical analysis has been carried about to study the heat and mass transfer of forced convection flow with liquid film evaporation in a saturated non-Darcian porous medium. Parametric analyses were conducted concerning the effects of the porosity ε, inlet liquid Reynolds number Re_l, inlet air Reynolds number Re_a on the heat and mass transfer performance. The results conclude that better heat and mass transfer performances are noticed for the system having a higher Re_a, a lower Re_l, and a higher ε. Re_l plays a more important role on the heat and mass transfer performance than Re_a and ε. For the case of ε = 0.4 and Re_a = 10,000, the increases of Nu and Sh for Re_l = 50 are about by 33.9% and 35.3% relative to the values for Re_l = 250.     

Forced convection heat transfer from a heated circular cylinder with arbitrary surface temperature distributions

A Fredholm-type boundary integral expression for evaluation of the forced convection heat transfer from an object with arbitrary surface temperature distributions is proposed. The Fredholm kernel function for a heated circular cylinder was calculated by numerical simulation of the forced convection fields, and then generalized heat transfer coefficients for arbitrary surface temperature distributions were defined. By use of the generalized heat transfer coefficients, it is shown that the difference in local heat transfer characteristics between the case of an isothermal cylinder and that of a uniform heat flux one can be interpreted only as the difference of the surface temperature distributions. Moreover, the mechanism of the effect of the surface temperature distribution on the characteristics of forced convection heat transfer from a cylinder is clarified in detail through the generalized heat transfer coefficients. © 1999 Scripta Technica, Heat Trans Asian Res, 28(6): 484–499, 1999     

Natural convection heat transfer of water in a horizontal gap with downward-facing circular heated surface

An experimental study of natural convection heat transfer from a downward-facing horizontal circular heated surface in a water gap has been carried out. The results were correlated in different forms of Nusselt number vs Rayleigh number according to different independent variables. The effects of different characteristic length and temperature were discussed and the gap size is the preferred characteristic length, the average fluid temperature between bulk temperature and the saturated temperature is the preferred film temperature. For the estimation of the natural convection heat transfer under the present conditions, empirical correlations in which Nusselt number is expressed as a function of Rayleigh number, or Rayleigh and Prandtl numbers both, may be used. However, the best accuracy is provided by an empirical correlation which expresses the Nusselt number as a function of the Rayleigh and Prandtl numbers, as well as the gap width-to-heated surface diameter ratio, the dimensionless temperature. Artificial neural networks have been trained successfully for analyzing the influences of the gap width-to-heated surface diameter ratio and the wall temperature difference between the temperature of wall and ambient fluid on natural convection heat transfer based on the experimental data in the present study. The results show that the Nusselt number will increase by increasing the gap ratio and decrease by increasing the wall temperature difference.     

A numerical investigation on the influence of liquid properties and interfacial heat transfer during microdroplet deposition onto a glass substrate

This work investigates the impingement of a liquid microdroplet onto a glass substrate at different temperatures. A finite-element model is applied to simulate the transient fluid dynamics and heat transfer during the process. Results for impingement under both isothermal and non-isothermal conditions are presented for four liquids: isopropanol, water, dielectric fluid (FC-72) and eutectic tin–lead solder (63Sn–37Pb). The objective of the work is to select liquids for a combined numerical and experimental study involving a high resolution, laser-based interfacial temperature measurement to measure interfacial heat transfer during microdroplet deposition. Applications include spray cooling, micro-manufacturing and coating processes, and electronics packaging. The initial droplet diameter and impact velocity are 80 μm and 5 m/s, respectively. For isothermal impact, our simulations with water and isopropanol show very good agreement with experiments. The magnitude and rates of spreading for all four liquids are shown and compared. For non-isothermal impacts, the transient drop and substrate temperatures are expressed in a non-dimensional way. The influence of imperfect thermal contact at the interface between the drop and the substrate is assessed for a realistic range of interfacial Biot numbers. We discuss the coupled influence of interfacial Biot numbers and hydrodynamics on the initiation of phase change.     

The effect of gap distance on the heat transfer between a heated finned surface and a saturated porous plate

Experiments were performed to investigate the effect that the presence of a gap has on the heat transfer between a heated finned surface and a saturated porous plate with an average pore radius of 200 μm. There was evidence that the vapour generated beneath the heated surface can escape to the vapour grooves more easily when a gap distance is introduced. This seemed to decrease the vapour penetration into the porous plate. The heat transfer performance of the heated finned surface initially increased as the gap distance was increased from 0 to 500 μm, but remained relatively unchanged for gap distances of 500–900 μm.     

Heat transfer in a forced wall jet on a heated rough surface

Many stUdies of wall Jets on smooth, flat and curveds~es haVe been examined over the past fifty years.Glauertll] realized the first comprehensive study of thewall jet. A survey by Launder and Rodiln summarizedmost of the known results on tulbulent jet flows on flatsauce. The stUdy of the effect of significant rouglmesscaused by the deposition of pericles on a heated wallsubmited tO acoustic vibrations, can be interesting foraPPlications in problems of cooling in engine brineblades, for exa…     

The heat transfer and fluid flow of a partially heated microchannel heat sink

Numerical modeling of the conjugate heat transfer in microchannel heat sink is presented. As the most of the cooling applications deals with the partial heated sections, the influence of the heating position on the thermal and hydrodynamic behavior is analyzed. The laminar fluid flow regime and the water as a working fluid are considered. It is observed that partial heating together with variable viscosity has a strong influence on thermal and hydrodynamic characteristics of the micro-heat sink.     

Effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of horizontal tubes

An experimental study has been conducted to investigate the effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of various kinds of copper tubes. Plain, spiral, corrugated, and low-finned tubes were selected as test tubes. In this work, to increase the wettability of distilled water on copper tubes, a novel hydrophilic surface treatment method using plasma was employed. The experiments show that every kind of hydrophilic surface treated tube tested in the work exhibits superior evaporation heat transfer performance as compared with that of the same kind of untreated tube. It is found out that during the evaporation process, the high wettability of the surface obtained through hydrophilic treatment induces film flow on the tubes while sessile drops are formed on untreated tubes. The film has a smaller thickness as well as a greater heat transfer area than the sessile drops, and this yields higher heat transfer rate for hydrophilic surface treated tubes than that for untreated tubes.     

Convective heat transfer over a heated square porous cylinder in a channel

A numerical study is made of the unsteady flow and convection heat transfer for a heated square porous cylinder in a channel. The general Darcy–Brinkman–Forchheimer model is adopted for the porous region. The parameters studies including porosity, Darcy number, and Reynolds number on heat transfer performance have been explored in detail. The results indicate that the average local Nusselt number is augmented as the Darcy number increases. The average local Nusselt number increases as Reynolds number increases; in particular, the increase is more obvious at a higher Darcy number. In contrast, the porosity has slight influence on heat transfer.     

Effects of ridged walls on the heat transfer in a heated square duct

Turbulent flows in rectangular cooling ducts of rocket engine thrust chambers are characterized by secondary motions of Prandtl’s first and second kinds. These secondary currents play a prominent part in heat transfer between the thrust chamber and the cooling gas conveyed in the duct. Previous numerical and experimental works reveal that attaching ridges on the walls of the duct causes the formation of new secondary flows of Prandtl’s second kind. These new structures are likely to increase the heat transfer. The present study has investigated numerically, through large eddy simulations, the effect of different forms of ridges on heat transfer in straight square duct flows.     

Natural convective heat transfer of heated packed beds

Natural convection heat transfer of heated packed bed was investigated. Experiments were performed for a single heated sphere buried in unheated packed beds varying its locations and for packed beds with all heated spheres varying the heights of packed beds from 0.02 m to 0.26 m. Mass transfer experiments using a copper electroplating system were performed based upon the analogy between heat and mass transfer. The diameter of sphere was 0.006 m, which corresponds to Ra_d of 1.8 × 10⁷. For the single heated sphere cases, the measured results agreed well with the existing natural convection heat transfer correlations for packed beds and even with those for a single sphere in an open channel. For all heated sphere cases, the average heat transfers decrease with increasing packed bed heights.     

Effects of adiabatic extensions on heat transfer through a differentially heated square cavity

This study is concerned with transient and steady state heat transfer by natural convection in a differentially heated cavity. The purpose is to evaluate a passive approach for enhancing heat transfer through the cavity. In this study, the effects of three different corner geometries (including sharp, round and straight corners) and adiabatic extensions of various dimensions on natural convection heat transfer are investigated numerically. The numerical results show marginal variations of the heat transfer rates among the three different corner shapes and a strong dependence of heat transfer enhancement on the Rayleigh number. For a given Rayleigh number, the enhancement of heat transfer by adiabatic extensions is limited.     

Analysis of heat transfer coefficients and evaporation in a solar still

The dependence of the glass cover temperature, the individual heat transfer coefficients, the overall upward heat flow factor, the fraction of upward heat flow utilized for evaporation, and the rate of water evaporation on the basic parameters has been studied. A semi-empirical equation for estimation of the glass cover temperature has recently been proposed by the authors. An analysis has been made of the capability of the new method to compute accurately the glass cover temperature, the overall upward heat flow factor, the rate of water evaporation, the fraction of upward heat flow utilized for evaporation over an extensive number of combinations of the basic parameters.     

Interfacial suction effects of a porous layer on filmwise condensation heat transfer enhancement

Considering the liquid transverse suction effect at the porous layer interface, a mathematical model was presented to investigate the influence of the porous layer characteristic parameters on condensation heat transfer. The results revealed that the enhancement ratio increased with the increase of the porous layer thickness and permeability. The effective thermal conductivity of the porous layer was, however, of little significance for condensation heat transfer enhancement. Also, the enhancement mechanism was analyzed by comparing the thermal resistances within the external condensate film and the porous layer. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 568–577, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10058