Mixed convective heat transfer of water in a pipe under supercritical pressure

Because of the rapid properties variation of fluid under supercritical pressure, there is a violent secondary flow in a heated pipe, which will certainly complicate the heat transfer of fluid in a pipe under supercritical pressure. In this paper, a numerical study is conducted for the laminar developing mixed convective heat transfer of water under supercritical pressure. The velocity field and temperature field are given, and the influence of different parameters on flow and heat transfer is investigated in detail. The results show that secondary flow has a great influence on velocity and temperature distributions and thus affects the friction factor and the Nusselt number remarkably. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 608–619, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20079     

Evolution and heat transfer after droplet impact on heated liquid film with vapor bubbles inside

Abstract

To better understand the droplet impact on the liquid film with vapor bubbles in spray cooling, a two-dimensional symmetric numerical model is set up using the Coupled Level Set and Volume of Fluid method (CLSVOF). Three simulative cases are taken, considering the effects of film thickness and the presence of vapor bubbles or not. The main purposes of this paper are to investigate the evolution of vapor bubbles during droplet impact and to identify the effect of vapor bubbles on convection heat transfer. The results indicate that vapor bubbles will detach from the wall and break up at the surface of the liquid film during droplet impact, for a thinner film, later a “sawtooth” liquid film appears at the non-impact region. However, for a thicker film, no bubbles rupture and the detached bubbles will flow inside the liquid film and then some will merge into larger bubbles. In the presence of vapor bubbles, the crater radius is larger for a thicker liquid film. The presence of vapor bubbles will facilitate the subcooled droplet to spread to the heated wall, leading to a substantial increase in surface heat flux.     

Evaporative heat transfer characteristics of a water spray on micro-structured silicon surfaces

Experiments were performed to evaluate the evaporative heat transfer characteristics of spray cooling of water on plain and micro-structured silicon surfaces at very low spray mass fluxes. The textured surface is made of an array of square micro-studs. It was found that the Bond number of the microstructures is the primary factor responsible for the heat transfer enhancement of evaporative spray cooling on micro-structured silicon surface in the present study. A qualitative study of evaporation of a single water droplet on plain and textured silicon surface shows that the capillary force within the microstructures is effective in spreading the deposited liquid film, thus increasing the evaporation rates. Four distinct heat transfer regimes, which are the flooded, thin film, partial dryout, and dryout regimes, were identified for evaporative spray cooling on micro-structured silicon surfaces. The microstructures provided better cooling performance in the thin film and partial dryout regime and higher liquid film breakup heat flux, because more water was retained on the heat transfer surface due to the capillary force. Heat transfer coefficient and temperature stability deteriorated greatly once the liquid film breakup occurred. The liquid film breakup heat flux increases with the Bond number. Effects of surface material, system orientation and spray mass flux were also addressed in this study.     

Unsteady conjugate laminar heat transfer of a rotating non-uniformly heated disk: Application to the transient experimental technique

Presented in the paper are the results of an investigation of 2D heat conduction effects on the transient heat transfer of a rotating disk heated up to a non-uniform initial temperature and suddenly subjected to unsteady cooling by still air. A self-similar solution of the transient laminar convective heat transfer confirmed that the heat transfer coefficient rapidly becomes time-independent and equal to its value at steady-state conditions. An analytical solution of the unsteady two-dimensional heat conduction inside a disk made of Plexiglas^® confirmed that the known infinite-slab approach can still be used as a transient technique for determining heat transfer coefficients. Use of the regular heat transfer regime theory for the same purpose can be recommended only for the cases with the moderate initial temperature non-uniformity.     

Heat transfer for flow boiling of water and critical heat flux in a half‐heated round tube under low‐pressure conditions

Heat transfer for flow boiling of water and critical heat flux (CHF) experiments in a half‐circumferentially heated round tube under low‐pressure conditions were carried out. To clarify the flow patterns in the heated section, experiments in the round tube under the same conditions were also carried out, and their results were compared. The experiments were conducted with atmospheric‐pressure water in test sections with inner diameter D = 6 mm, heated length L = 360 mm, inlet water subcooling ΔT_in = 80 K, and mass velocity G from 0 to 2000 kg/(m²·s) for the half‐circumferentially heated round tube and from 0 to 7000 kg/(m²·s) for the full‐circumferentially heated tube. The experimental data demonstrated that the wall temperature near the outlet of the half‐circumferentially heated tube remained almost the same until CHF. It was found that burnout occurred when the flow regime changed from churn flow to annular flow, and the liquid film on the heated wall dried out although liquid film on the unheated wall remained. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 149–164, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10022     

Enhancement of natural convective heat transfer from tall,vertical heated plate to water

An enhancement technique was developed for natural convection heat transfer from a tall, vertical heated plate to water. Rectangular grid fins attached to the base plate were utilized as a heat transfer promoter. These grid fins redirect the high‐temperature fluid ascending along the base plate toward the outside of the boundary layer and introduce the low‐temperature ambient fluid toward the base plate instead. The heat transfer coefficients of thus‐treated surfaces were measured and compared with a nontreated surface and a surface with conventional vertical plate‐fins. The highest performance was achieved for the experimental surfaces. In particular, the experimental surfaces with 5‐mm‐high, nonconducting grids and with 10‐mm‐high, conducting grid fins show 27% and 80% higher heat transfer coefficients compared to the turbulent heat transfer coefficients of the nontreated surface, respectively. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 178–190, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10083     

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     

The maximum and minimum values of the heat q transmitted from metal to boiling water under atmospheric pressure

The quantity of heat transmitted from a metal surface to boiling water increases as the temperature difference ΔT is increased, but after the ΔT has reached a certain limit, quantity Q decreases with further increase in ΔT. This turning point is the maximum value of heat transmitted. The existence of this point was actually observed in the experiment. Under atmospheric pressure, ΔT corresponding to the maximum value of heat transfer for water at 100°C falls between 20–40°C, and Q is between 1080 000 and 1 800 000 kcal/m² h (i.e. between 2000 and 3000 kg/m² h, if expressed in constant evaporation rate at 100°C); this figure is larger than the maximum value of heat transfer as was previously considered. Also, the minimum value of heat transfer was obtained, and in the Q-ΔT curve for the high temperature region, the burn-out effect is discussed.     

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 of transient natural convection heat transfer of aqueous nanofluids in a differentially heated square cavity

Transient natural convection heat transfer of aqueous nanofluids in a differentially heated square cavity is investigated numerically. The effective thermal conductivity and dynamic viscosity of nanofluids are predicted by using the proposed models that take the contribution of Brownian motion of nanoparticles into account. Three different Rayleigh numbers and five different volume fractions of nanoparticles are considered. The development of natural convection is presented through the evolutions of the average Nusselt number along the cold side wall. The predicted flow development times and time-averaged Nusselt numbers are scaled with Rayleigh number. In addition, the time-averaged Nusselt numbers are presented in terms of volume fraction of nanoparticles. It is shown that at constant Rayleigh numbers, the time-averaged Nusselt number is lowered with increasing volume fraction of nanoparticles.     

Interfacial heat transfer during microdroplet evaporation on a laser heated surface

A comprehensive experimental and numerical investigation on water microdroplet impingement and evaporation is presented from the standpoint of phase-change cooling technologies. The study investigates microdroplet impact and evaporation on a laser heated surface, outlining the experimental and numerical conditions necessary to quantify the interfacial thermal conductance (G) of liquid-metal interfaces during two-phase flow. To do this, continuum-level numerical simulations are conducted in parallel with experimental measurements facilitating high-speed photography and in-situ time-domain thermoreflectance (TDTR). During microdroplet evaporation on laser heated Al thin-films at room temperature, an effective interfacial thermal conductance of G_eff = 6.4 ± 0.4 MW/m² is measured with TDTR. This effective interfacial thermal conductance (G_eff) is interpreted as the high-frequency (ac) interfacial heat transfer coefficient measured at the microdroplet/Al interface. Also on a laser heated surface, fractal-like condensation patterns form on the Al surface surrounding the evaporating microdroplet. This is due to the temperature gradient in the Al surface layer and cyclic vapor/air convection patterns outside the contact line. Laser heating, however, does not significantly increase the evaporation rate beyond that expected for microdroplet evaporation on isothermal Al thin-film surfaces.     

Dehumidification heat transfer on copper surfaces

Heat transfer during dehumidification on a vertical copper surface is studied and a correlation was developed to relate the heat transfer coefficient, relative humidity, and surface-to-ambient temperature difference. The heat transfer coefficient at various humidity levels and surface-to-ambient temperature differences were obtained experimentally. Variations of about ～200% in heat transfer coefficient were observed between low and high humidity levels. Unlike for pure condensation, it was observed that the heat transfer coefficient is higher for filmwise condensation than for dropwise condensation. It is hypothesized that the non-condensable gas insulates regions where condensation does not take place.     

Experimental study of natural convection heat transfer of air in a cube below atmospheric pressure

Experimental study was carried out on the temperature profile of natural convection of air in a 50 mm cube with the temperature difference of 30 K between two opposing vertical walls. The objective is to investigate the essential aspect of heat transfer in the wide range of Rayleigh number (Ra) by lowering the pressure. The pressure was varied from 5.40 kPa (40.5 mmHg) to 99.99 kPa (750.0 mmHg). These correspond to Ra=1.04×10³ to 3.56 × 10⁵. The results show that the temperature distribution and Nusselt number approach those of conduction state as the pressure decreases.     

常压下狭缝流动沸腾传热的研究

以蒸馏水为工质，在常压下，对间隙为1mm的环形狭缝通道中的流动传热进行了实验研究。分别将狭缝通道中的单相强制对流和过冷沸腾的实验数据与传统的Dittus-Boelter型关系式的计算结果进行了比较。通过分析狭缝通道中流动沸腾的传热特性认为，过冷沸腾传热比单相强制对流传热加强；质量流速对狭缝通道中的流动沸腾传热有较大影响。     

Roughness of heat transfer surfaces

A scanning electron microscope was used mainly for order-of-magnitude measurements in the present study. Further, only a few measurements were possible due to the short time the equipment was available. However, these limited measurements showed a significant change in roughness dimensions and shape due to heating of Inconel and, perhaps more importantly, demonstrated a new means to measure natural roughness elements smaller than the radius-of-curvature of the typical industrial profilometer. It seems clear that the scanning electron microscope offers considerable potential for more complete characterization of the texture of the surfaces employed in convective and radiative heat transfer applications. By using electronic signal processing, it is likely that useful measurements of the size spectra could be readily recorded and reduced. The improved characterization of the shape is also valuable.     

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.