Forced Convection Film Boiling Heat Transfer Over a Vertical Cylinder

The knowledge of subcooled film boiling heat transfer is important as the basis of understanding the reflooding phenomenon during emergency cooling in a nuclear reactor under a loss-of-coolant accident. In this study, forced convection film boiling heat transfer from a vertical cylinder in Freon-113 flowing upward along the cylinder was measured for the flow velocities ranging from 0 to 1.3 m/s, and liquid subcoolings ranging from 0 to 20 K at pressures near atmospheric. A platinum heater with a diameter of 3 mm was heated by electric current. The heat transfer coefficients obtained are almost independent of vertical positions on the cylinder. The heat transfer coefficients are almost independent of velocity for the velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients at each velocity are higher for higher liquid subcoolings. Improvement of film boiling heat transfer from the vertical cylinder with the increase in flow velocity is much less than that of horizontal cylinder in cross flow previously reported by the authors. This is mainly due to the difference of heat transfer enhancement mechanism; the former is the drag force on vapor flow acted by a liquid flow, and the latter is the pressure gradient near the front stagnation point caused by external potential flow.     

竖直螺旋槽管壁面液膜在蒸发/冷凝时的传热特性的研究

研究竖直螺旋槽管壁面液膜在传热条件下的液膜形成及流动特性，建立了单组分流体的物理和数学模型并得出解析解，且分析了壁面液膜在蒸发，冷凝及无热传输时的液膜厚度分布及速度分布，结果表明，液膜的形状主要受表面张力影响，在表面内弯处流膜较厚，而在槽道起始部液膜较薄，相对于光滑直管，竖直螺旋槽管壁面液膜具有均匀的厚度分布和更好的传热传质性质，特别在冷凝时壁面液膜更薄且分布更加均匀。     

混合气体管内对流凝结传热研究

研究了混合气体在垂直圆管内的对流凝结传热。利用修正的膜模型与Nusselt凝结理论建立了换热数学模型，预测了壁面温度对膜厚度和界面温度的影响，计算了凝结液膜厚度，并与报相热阻法进行比较，研究结果表明该模型更接近实验果，提出了混合气体对流凝结换热与Nusselt凝结的不同。     

竖直圆管内R113的降膜换热特性数值模拟

为开发适用于低温热源的高效降膜蒸发换热装置,本研究采用FLUENT软件对低沸点有机工质氟利昂(R113)在竖直管内汽液两相逆流降膜蒸发进行模拟研究。汽液界面捕捉选用VOF模型,并通过udf编程模拟汽液两相蒸发传热,研究了喷淋密度、热流密度及入口温度对R113降膜蒸发换热的影响。结果表明:在一定结构参数下,存在降膜换热最佳喷淋密度;在一定喷淋密度下,热流密度对降膜换热影响显著,且热流密度越高换热效果越好;随着入口温度升高,降膜换热效果削弱,且高于某温度后其对降膜换热几乎没有影响。     

Fouling Propensity of Modified Heat Transfer Surfaces

Abstract

As much as attention that has been paid to surface treatment as an efficient, and environmentally friendly approach toward fouling mitigation, the characterization of many innovative modified surfaces has become a matter of much debate. The latter is closely associated with the intermolecular interaction energies which would profoundly influence the adhesion of precursors onto the modified surfaces. In this study, based on the extended Derjaguin, Landau, Verwey and Overbeek (DLVO) theory, a new criterion is proposed to predict the propensity of a surface when prone to crystallization fouling or biofouling. Thereafter, the proposed criterion is examined against the present experimental results as well as those from previous studies where the required information for the determination of new criterion is available. The comparison shows that deposit formation onto heat transfer surfaces decreases with increasing the new proposed fouling propensity indicator criterion. Moreover, nearly 75% of the collated crystallization and biological fouling data points are predictable with this criterion and reasons for those that are not in compliance with the proposed criterion are discussed.     

不同振动参数下脉动气流横掠圆柱体的传热研究

在脉动气流强化传热实验台上进行了层流（雷诺数RP为171）中不同振动参数（频率厂分别为15Hz、30Hz、45Hz、59Hz,振幅Pmax为45～286Pa）下脉动气流横掠高温共烧陶瓷发热管的传热实验研究．结果表明：层流中低频大振幅的脉动气流能显著强化圆柱体的传热过程,相对努塞尔数随着压力振幅的增大而增大,随着脉动频率的增大而减小,最大相对努塞尔数N“,可超过2．55;在Re=171时,脉动气流强化传热的经验公式为Nux=0．2684＋0．5867pmax^0.388.3/f^0.3170,该经验公式的相关系数为R=0．9941,揭示了脉动气流强化圆柱体传热过程对振动参数的非线性依赖性．     

Modeling of Heat Transfer Across the Interface in Two-Fluid Flows

A model is presented in this article to deal with heat transfer across the interface separating two immiscible fluids. It is suitable to be incorporated into interface-tracking methods, such as volume-of-fluid (VOF) methods, because a sharp interface is available in these approaches. The temperature at the interface and the heat flux through it are calculated in such a way that the continuity of the two properties at the contact surface is satisfied explicitly. With use of these values, the temperature either at the centroid or on a face of the interface cell can be estimated, which serves as Dirichlet boundary condition for the energy equation. The temperature field is then calculated by solving the energy equations for the two fluids simultaneously in an implicit way. This method is first assessed via testing on two heat conduction problems in which two solids are in contact. Good agreement between numerical solutions and theory is obtained. To demonstrate its capability, it is applied to two kinds of heat transfer problems, one being the collapse of a heated water column in a cavity, and the other the falling of a molten tin droplet in an oil tank. The effect of fluid flow on the heat transfer is clearly illustrated.     

Local Heat Transfer Coefficients of a High-Frequency Synthetic Jet during Impingement Cooling over Flat Surfaces

It has been shown that synthetic jets can enhance heat transfer in air-cooling during natural convection heat transfer. Those meso scale devices are expected to be one of the methods of choice for cooling confined space, low heat-generating electronics. The present study focuses on the local and global heat transfer coefficients of a high-frequency meso scale synthetic jet. The experiments have been completed with synthetic jets, which are 12.5 mm in diameter and 2 mm thick with a square orifice of 1 mm. A synthetic jet has been driven at the resonance frequency of 4500 Hz, and voltage was between 30 V and 50 V. Earlier studies have focused on understanding the effect of voltage and driving frequency on the average heat transfer effect, while the current study aims for determining local heat transfer. A microscopic infrared thermal imaging technique was used to acquire local temperature distributions, and the data were analyzed for local convective and radiative heat transfer coefficients. Four square heaters (each with a different size) have been studied in the current study to determine the effect of the characteristic length as well. Heat transfer enhancements over the specific heater sizes are presented, and it is found to be between 4 and 10 times of natural convection.     

Rewetting of Hot Vertical Surfaces by a Falling Liquid Film

In this work, a transient heat conduction model is developed for rewetting a hot wall surface by a falling liquid film. In the model, the heat conduction in the rewetted wall is assumed to be two‐dimensional. Convection heat transfer from the hot surface to rewetting fluid is considered negligible in the dry surface region ahead of the wet front. The numerical solution indicates that the rewetting process is mainly controlled by two‐dimensional heat conduction in the rewetted wall, even for the walls of low Biot number, especially at low initial temperatures. The effects of Biot number and initial wall temperature on the rewetting velocity are investigated. Comparison of the results with previous studies is presented.     

Frictional and Heat Transfer Characteristics of Single-Phase Microchannel Liquid Flows

In this paper, we review the literature on flow frictional and heat transfer characteristics of single-phase liquid flows through microchannels. The work accentuates the existing discord between experimental observations of microscale transport process characteristics and the corresponding theoretical predictions on the basis of the classical paradigms. The role of microscale effects in inducing such disparity between experimental and theoretical frameworks, as indicated by various researchers, is critically discussed. Theoretical models and empirical correlations, proposed in the literature, for comprehending microscale flow and thermal characteristics are also highlighted for ready reference. In closure, aspects of microscale liquid flow and heat transfer requiring further scrutiny are identified, and possible future research directions are prescribed.     

Convection Condensation Heat Transfer of Steam-Air Mixture With Heat Pipe Heat Exchanger

A heat pipe heat exchanger (HPHE) was used to investigate the heat transfer performance of steam-air mixture condensation, analogous to the dew condensation of flue gas, when the steam volume fraction ranged from 0 to 20%, and the inlet temperature of steam-air mixture varied from 70 to 120°C. Self-assembled monolayers (SAMs) treatment with n-octadecyl mercaptan was adopted to modify the condensation surface of the heat pipe. The comparisons were conducted to examine the influence of SAMs on condensation heat transfer of steam-air mixture vapor. The results indicate that the convection condensation heat transfer coefficient increases with the increase of steam volume fraction and Re number of the steam-air mixture. As the inlet temperature increases, the heat transfer coefficient decreases accordingly. The heat transfer performance can be improved by the SAMs surface, with a heat transfer enhancement ratio up to 1.6 at a condition of 20% of the steam volume fraction and 1500 Re number.     

Heat Transfer Characteristics of Gaseous Flows in Micro-Channel with Negative Heat Flux

Two-dimensional compressible momentum and energy equations are solved to obtain the heat transfer characteristics of gaseous flows in micro-channels with constant heat flux for which the value is negative for no-slip flow. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian method. The computations are performed for channels with constant heat flux ranging from ?10⁴ to ?10² W/m². The channel height ranges from 10 to 100 μ m and the aspect ratio of the channel height and length is 200. The stagnation pressure is chosen such that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure is fixed at the atmosphere. The wall and bulk temperatures in micro-channels with negative heat flux are compared with those of positive heat flux cases obtained in our previous work and also those of the incompressible flow in a conventional sized channel. In the case of fast flow, temperatures normalized by heat flux have different trends whether heat flux value is positive or negative. A correlation for the prediction of the wall temperature of the gaseous flow in the micro-channel is proposed. The rarefaction effect is investigated for the cases of channel height of 10 μ m with slip boundary conditions. The magnitudes of viscous dissipation term and compressibility term are also investigated. The effect of each term on heat transfer characteristics is discussed.     

Unsteady Phenomena in the Double-Diffusive Convection Flows at High Rayleigh Number

A two-dimensional dual-mesh hybrid numerical method is used to investigate some small scale phenomena in double-diffusive convection flows at high Rayleigh number. With a horizontal temperature gradient, the interface of the two-layer stratified system becomes tilted and wavy, resulting in two typical flow patterns. The quasi-steady flow pattern is scattered with abundant salt fingers and hook-like plumes, which agree well with experimental data available in the literature qualitatively. It is found that three-dimensional modeling should be carried out in order to have better agreement with experimental data. In addition, the stratified system is found to be sensitive to the buoyancy ratio.     

Laminar Flow and Heat Transfer Phenomena Across a Confined Semicircular Bluff Body at Low Reynolds Numbers

The present study is concerned with the simulation of incompressible Newtonian fluid flow and heat transfer over a long semicircular bluff body in a channel at low Reynolds numbers. In particular, wall effects on the forced convection from a (heated) semicircular cylinder confined in a horizontal channel are investigated for Reynolds number = 1–40 and blockage ratio = 16.67–50% for air as the working fluid. Flow and thermal fields are found steady for the preceding range of settings. The onset of flow separation increases as the wall confinement increases. The size of the recirculation zone downstream of a semicircular cylinder is seen to increase almost linearly with Reynolds number for a fixed blockage ratio, but it decreases with increasing blockage ratio for a fixed Reynolds number. As expected, total drag coefficient and its components decrease with increasing value of Reynolds number. However, with increasing blockage ratio, the values of these drag coefficients increase. On the basis of equal projected area, the total drag coefficient for the present flow system is found to be greater than the corresponding drag in the case of the unconfined semicircular cylinder. Similarly, the overall drag in the case of a confined semicircular cylinder is found to be greater than that of a confined circular cylinder for the appropriate range of dimensionless control parameters. The maximum augmentation in heat transfer for blockage ratios of 25% and 50% is found to be approximately 16% and 51% with respect to the corresponding value at the blockage ratio of 16.67% at Reynolds number = 1. Finally, the correlations of wake length, drag coefficient, and average Nusselt number are obtained.     

液膜流动传热稳定性综述

本文综述了液膜传热流动性的背景及意义,回顾了沿平板液膜、表面波、多相液膜、加热液膜及波动液膜的国内外研究进展,总结了液膜已有的理论结果,并展望了液膜技术的发展前景。     

Experimental and Numerical Analysis of Heat Transfer in a Tall Vertical Concentric Annular Thermo-siphon at Constant Heat Flux Condition

The present work deals with the numerical and experimental analyses to study the detailed behavior of the thermally induced flow of water in an open vertical annulus, circulating through a cold leg forming a closed loop thermo-siphon. Spatio-temporal behavior of fluid flow is also studied for variety of heat fluxes. The annuli in the present study have a radius ratio of 1.184 and aspect ratio (length to annular gap) equal to 352. The objective of the present work is to quantify the effect of heating on design parameters such as liquid and wall temperatures, mass flow rate, and heat transfer coefficient. Experiments have also been conducted on a similar system with water at constant heat flux of 1 kW/m², 2.5 kW/m², 5 kW/m², 7.5 kW/m², 10 kW/m², 12.5 kW/m² and 15 kW/m². For numerical purpose, a two-dimentional solver has been developed for direct numerical simulation of the essential thermally induced flow dynamics The numerical solution was thus performed for Rayleigh numbers ranging between, 4.4 × 10³ and 6.61 × 10⁴ which correspond to the given heat flux, respectively.     

Heat Transfer in Separated Flows on the Pressure Side of Turbine Blades

Heat transfer in separated flows on the pressure side of a typical high lift turbine profile is numerically investigated by means of an in-house CFD code. The numerical code was first validated on attached flows in turbine blades. To obtain flow separation cases, the profile is subject to large negative incidences so that a separation bubble is obtained at the pressure side. The numerical results are compared to available experimental data for code validation. It is shown how local minima and maxima values of the heat transfer coefficient are related to the separation and reattachment points, where the velocity component perpendicular to the wall is shown to have a significant effect on the heat transfer.     

Mechanistic Modeling of the Convective Heat Transfer Coefficient in Gas-Liquid Intermittent Flows

The development of a mechanistic procedure to estimate the convection heat transfer in horizontal gas-liquid intermittent—or slug—flow is presented. In broad terms, the mean convective heat transfer coefficient is calculated following an averaging procedure based on the unit cell model of the slug flow pattern. The flow parameters (i.e., unit cell frequency, liquid slug and elongated bubble length and velocity, and liquid hold-up) were obtained from empirical data for air/water flows in a 15 m-long, 25.4 mm ID copper pipe and for natural gas (mostly methane and ethane) and oil or water flows in an actual size, 200 m-long, 150 mm ID steel pipe. A time-averaging procedure based on the unit cell parameters was then used to calculate the mean convective heat transfer coefficient. The slug flow parameters taken on the small scale air/water loop and the actual size pipeline were used for comparisons. Heat transfer data from the small scale air/water loop were used to validate the results calculated using the averaging procedure. Finally, the approach herein proposed also showed good agreement with previously published data and well-known correlations.     

Heat Transfer and Pressure Drop Characteristics of Nanofluid Flows Inside Corrugated Tubes

An experimental investigation is carried out to study the heat transfer and pressure drop characteristics of multiwalled carbon nanotubes (MWCNTs)/heat transfer oil nanofluid flows inside horizontal corrugated tubes under uniform wall temperature condition. To provide the applied nanafluids, MWCNTs are dispersed in heat transfer oil with mass concentrations of 0.05, 0.1, and 0.2 wt%. The Reynolds number varies between 100 and 4,000. Three tubes with hydraulic diameters of 11.9, 13.2, and 15.5 mm are applied as the test section in the experimental setup. Tubes are corrugated four times on the cross section; that is, there are four different helices around the tube. Depths of the corrugations are chosen as 0.9, 1.1, and 1.3 mm, and pitch of corrugation is 14 mm. The acquired data confirm the increase of heat transfer rate as a result of utilizing nanofluids in comparison with the base fluid flow. However, corrugating the tubes decreases the heat transfer rate at low Reynolds numbers. The highest increase in heat transfer rate is observed for the Reynolds numbers for which the smooth tube is in the transition regime and the corrugated tube reaches the turbulent flow, that is, Reynolds number in the range of 1,000 to 3,000. Rough correlations are proposed to predict the Nusselt number and friction factor.     

立式燃气锅炉强化传热技术进展及趋势

概要介绍了立式燃气锅炉的结构型式与特点,综述了现有燃气锅炉烟管强化传热技术,展示了几种锅炉烟管的新型强化结构,预测了立式燃气锅炉烟管强化传热技术的发展方向,为立式燃气锅炉的高效设计及提高其热效率奠定了基础。