Simulation of Convective Heat Transfer of Gas–Liquid Bubble Train Flow in Wet Microtube

In this paper, a direct numerical simulation of a two‐phase incompressible gas–liquid flow for simulation of bubble motion and convective heat transfer in a microtube is presented. The microtube radius is 10 μm. The interface between the two phases is tracked by the volume of fluid method with the continuous surface force model. Newtonian flows are solved using a finite volume scheme based on the PISO algorithm. Numerical simulation is done on an axisymmetric domain with a periodic boundary condition for different values of pressure gradient, void fraction, and bubble period. Mean pressure gradient is fixed for each simulation. The superficial Reynolds numbers of gas and liquid phases studied are 0.3 to 7 and 5 to 210, respectively. Numerical results are coincident with the Serizawa regime map, and there is a linear relation between the void fraction and gas flow ratio. Simulation shows local Nusselt number increases in the presence of a gas bubble.     

Investigation of Electrohydrodynamically-Enhanced Convective Heat and Mass Transfer from Water Surface

Enhancement of forced flow evaporation rate by applying electric field (corona wind) has been experimentally evaluated in this study. Corona wind produced by a fine wire electrode which was charged with positive high DC voltage impinges to water surface and leads to evaporation enhancement by disturbing the saturated air layer over the water surface. The study was focused on the effects of corona wind velocity, electrode spacing and air flow velocity on the level of evaporation enhancement. Two sets of experiments, i.e., with and without electric field, have been conducted. Data obtained from the first experiment were used as reference for evaluation of evaporation enhancement at the presence of electric field. Applied voltages ranged from corona threshold voltage to spark over voltage at 1 kV increments. The results showed that corona wind has great enhancement effect on the water evaporation rate, but its effectiveness gradually diminishes by increasing air flow velocity. Maximum enhancement ratios were 7.3 and 3.6 for air velocities of 0.125 and 1.75 m/s, respectively. Finally two empirical correlations were obtained for prediction of electrohydrodynamic evaporation enhancement and its coefficient of thermal performance.     

Forced Convective Heat Transfer Experiment of Supercritical Water in Different Diameter of Tubes

Forced convective experiment of supercritical water was performed in Inconel-625 tubes of 4.62 mm, 7.98 mm and 10.89 mm in diameter. The water flowed upward, covering the ranges of pressure of 23.4 MPa to 25.8 MPa, mass flux of 90 kg/m^2s to 3,281 kg/m^2s, local bulk temperature of 102-384 ℃, inner wall temperature of 167-669℃ and heat flux of up to 2.41 MW/m^2. The results exhibited severe deteriorated and enhancement heat transfer. The experimental results can be calculated by the Jackson＇s correlation and the Bishop＇s correlation mostly. But some data with strong effects of the buoyancy force and the variations of flow regimes can not be predicted properly.     

Unsteady Convective Heat Transfer of Power-Law Fluid with Variable Fluid Properties in a Concentric Annulus Originating from a Polymer Flooding Process

We study the unsteady convective heat transfer of power-law fluid with variable fluid properties in a concentric annulus with isothermal surface. The problem is originated from the polymer flooding process between a sucker rod and oil well. A new power-law rheological model is proposed, which takes the effects of temperature on fluid viscosity and thermal conductivity into account. Numerical solutions are presented for velocity and temperature fields using the Chebyshev spectral method coupled with the strong stability-preserving Runge–Kutta time discretization. The exponential convergence is verified by accuracy testing between a smooth exact solution of the Partial Differential Equations (PDEs) with source terms and the numerical approximation of manufactured solutions. It is found that heat transfer is enhanced in the variable power-law index model, and a decrease in power-law index of pseudoplastic fluids promotes heat transfer due to the increased Nusselt number. Moreover, the influences of other parameters on convective heat transfer behaviors are discussed in detail.     

Convective Heat Transfer Coefficients in Concentric Annuli

Abstract

The geometric shape of a passage's cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, the diameter ratio of the annular space plays an important role. The purpose of this study was to determine to what extent research has been done on convective heat transfer in smooth concentric annuli and, if possible, to improve on or contribute to existing theories. It was found that although various correlations exist, they are not in good agreement. For this study, experiments were conducted with a wide range of annular diameter ratios. The Wilson plot method was used to develop a convective heat transfer correlation for annular diameter ratios of 1.7 to 3.2. For Reynolds numbers (based on the hydraulic diameter), in the range of 4000 to 30000, the deduced correlation predicted Nusselt numbers accurately within 3% of experimental values.     

Rheology and Convective Heat Transfer Properties of Borate Cross-Linked Nitrogen Foam Fracturing Fluid

This article considers the rheology and convective heat transfer characteristics of borate cross-linked guar and borate cross-linked foam fracturing fluid by conducting experiments on a large-scale test loop at 30 MPa. The results show that severe chemical degradation of borate cross-linked guar occurred as temperature increased. When temperature was higher than the threshold value, the cross-linker was almost disabled and the guar was no longer cross-linked. It was also found that the viscosity of borate-cross-linked foam fracturing fluid was proportional to the increment of foam quality, which was inversely proportional to the increase of temperature. The influence of fluid behavior index on the velocity gradient of non-Newtonian fluid at the wall is tremendous, resulting in a negative temperature-dependent convective heat transfer coefficient. Moreover, to calculate the convective heat transfer coefficients of these two fluids and also to match the experimental data, a multiplier should be utilized to account for the shear-dependent thermal conductivity and heat transfer enhancement. The contribution of shear-induced bubble-scale microconvection was significant for the heat transfer enhancement of foam fluid. The correlations between the viscosity and the convective heat transfer coefficient of borate cross-linked guar and borate cross-linked foam fracturing fluid deduced from the experimental data were also obtained.     

Comparative Study of Convective Heat Transfer Performance of Steam and Air Flow in Rib Roughened Channels

A comparative experimental study of heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted by using an infrared camera. Effects of Reynolds numbers and rib angles on the steam and air convective heat transfer have been obtained and compared with each other for the Reynolds number from about 4,000 to 15,000. For all the ribbed channels the rib pitch to height ratio(p/e) is 10, and the rib height to the channel hydraulic diameter ratio is 0.078, while the rib angles are varied from 90° to 45°.Based on experimental results, it can be found that, even though the heat transfer distributions of steam and air flow in the ribbed channels are similar to each other, the steam flow can obtain higher convective heat transfer enhancement capability, and the heat transfer enhancement of both the steam and air becomes greater with the rib angle deceasing from 90° to 45°. At Reynolds number of about 12,000, the area-averaged Nusselt numbers of the steam flow is about 13.9%, 14.2%, 19.9% and 23.9% higher than those of the air flow for the rib angles of 90°,75°, 60° and 45° respectively. With the experimental results the correlations for Nusselt number in terms of Reynolds number and rib angle for the steam and air flow in the ribbed channels were developed respectively.     

Effects of High-Order Slip/Jump,Thermal Creep,and Variable Thermophysical Properties on Natural Convection in Microchannels With Constant Wall Heat Fluxes

Natural convection gaseous slip flows in open-ended vertical parallel-plate microchannels with symmetric wall heat fluxes are numerically investigated. A second-order model, including thermal creep effects, is considered for velocity slip and temperature jump boundary conditions with variable thermophysical properties. Simulations are performed for wide range of Rayleigh numbers from 5 × 10^{? 6} to 5 × 10^{? 3} in the continuum to slip flow regime. The developing and fully developed solutions are examined by solving the Navier–Stokes and energy equations using a control volume technique. It is found that the second-order effects reduce the temperature jump and the slip velocity, whereas thermal creep strongly increases the slip velocity in both developing and fully developed regions. Moreover, the rarefaction effects increase the flow and heat transfer rates considerably, while decreasing the maximum gas temperature and friction coefficient as compared to the continuum limit. It was also shown that the axial temperature variations of the gas layer adjacent to the wall in the modeling of the thermal creep are of paramount importance and neglecting these variations, which is common in literature, leads to unphysical velocity and temperature distributions.     

导热油管内强制对流换热系数的计算

以一种常见导热油为例对有机热载体锅炉的对流受热面传热系数进行计算分析。     

Free Convective Heat Transfer in Tilted Longitudinal Open Cavity

Heat transfer experiments were performed to investigate the effects of inclination and channel height-to-gap ratio on free convection in a simulated fin-passage with a strategic aim of devising a criterion for selecting the optimal fin length that could provide the maximum free convective capability. The ranges of parameters investigated include the Grashof number, up to 500,000; channel height-to-gap ratios of 1, 2, and 3; and tilt angles of 0°, 30°, 60°, 90°, 120°, 150°, and 180°. Selections of local and spatially averaged Nusselt number results demonstrate the manner by which the Grashof number, tilt angle, and channel height-to-gap ratio interactively affect the heat transfer. In conformity with the experimentally revealed heat transfer physics, the correlation of a spatially-averaged Nusselt number over two parallel walls and the bottom surface of an open-ended channel is derived that permits the individual and interactive effects of the Grashof number, tilt angle, and channel height-to-gap ratio on heat transfers to be evaluated. A criterion for selecting the optimal height-to-gap ratio of the fin channel is subsequently formulated as a design tool for maximizing the convective capability of a free convective fin assembly.     

Heatline Analysis on Heat Transfer and Convective Flow of Nanofluids in an Inclined Enclosure

This article presents a heatline method to analyse the transport mechanism of heat transfer and convective flow of nanofluids in an inclined square enclosure, where a heated thin plate located in the middle of the enclosure. The fluid flow, heat transfer, and heat transport characteristics are illustrated using streamlines, isotherms, Nusselt number and heatlines. Results show that fluid flow and temperature fields strongly depend on Rayleigh number, inclination angle, solid volume fraction, types of nanoparticles and the plate length, and the maximum strength of heatfunction increases as the inclination angle and Rayleigh number increase.     

填充床内对流换热的数值研究

为研究空气流入高温填充床时小球直径和空气流速变化对填充床内对流换热和压力损失等的影响,利用孔隙尺度介观方法对顺序排列多孔介质小球的三维填充床进行数值计算,数值计算与实验结果吻合较好。结果表明:填充床内固相和气相间存在热的非平衡性;当小球直径从2.8增大到5.6 mm时,在最高温度上游对流换热强度减小,在最高温度下游对流换热强度增大,同时,压力损失和最大无量纲速度减小;气体流速增大时,填充床内产生湍流运动。     

Seed Bubble Guided Heat Transfer in a Single Microchannel

We use the seed bubble concept for manipulating the evaporative heat transfer in a heated microchannel with smooth surfaces. Using this concept, separation of bubble nucleation and growth is obtained to simplify the heat transfer system. Not only is the temperature excursion at the boiling incipience eliminated, but also the heat transfer system displays well-ordered and repeated flow characteristics. The heat transfer rates and wall temperatures can be controlled through adjusting the seed bubble frequency. The method provides a thermal management solution for microsystems and a tool for the study of the intricate flow and heat transfer.     

Numerical Study of Nanofluid Convective Heat Transfer in Sinusoidal Tubes

ABSTRACT

The waviness of tube wall and adding nanoparticles to fluid as two passive enhanced heat-transfer techniques are dully accepted; however, the combined effect of their simultaneous usage has not been dealt with, yet. Therefore in the present study, the convective heat transfer of nanofluid laminar flow inside straight tube and sinusoidal tubes under constant heat flux boundary condition was documented. The nanofluid used in this study was Al₂O₃/water with volume fractions from 0 to 4%. The effects of Reynolds number, volume fractions of nanoparticles, and the geometry of sinusoidal tubes upon the heat-transfer coefficient were investigated. The results showed that using sinusoidal tubes enhances heat-transfer coefficients. Also, it was observed that increasing Reynolds number leads to higher heat-transfer coefficients in the convergent section. Moreover, it was observed that increasing the sinusoidal wave amplitude augments the convective heat-transfer coefficients; however, the increase in Nusselt number was slight. Furthermore, adding nanoparticles enhances heat transfer especially in large wave amplitude sinusoidal tubes.     

Convective Heat Transfer and Fluid Dynamic Characteristics of SiO2 Ethylene Glycol/Water Nanofluid

Nanofluids comprised of silicon dioxide (SiO₂) nanoparticles suspended in a 60:40 (% by weight) ethylene glycol and water (EG/water) mixture were investigated for their heat transfer and fluid dynamic performance. First, the rheological properties of different volume percents of SiO₂ nanofluids were investigated at varying temperatures. The effect of particle diameter (20 nm, 50 nm, 100 nm) on the viscosity of the fluid was investigated. Subsequent experiments were performed to investigate the convective heat transfer enhancement of nanofluids in the turbulent regime by using the viscosity values measured. The experimental system was first tested with EG/water mixture to establish agreement with the Dittus-Boelter equation for Nusselt number and with Blasius equation for friction factor. The increase in heat transfer coefficient due to nanofluids for various volume concentrations has been presented. Pressure loss was observed to increase with nanoparticle volume concentration. It was observed that an increase in particle diameter increased the heat transfer coefficient. Typical percentage increases of heat transfer coefficient and pressure loss at fixed Reynolds number are presented.     

变物性对有凸出热源通道换热计算的影响

本文基于一侧含有三个均匀分布的凸出热源竖壁、另一侧为绝热竖壁的二维垂直换热通道,研究了物性变化相比于常物性和Boussinesq假设对通道内最大温度和平均Nu数的影响。通过网格无关性验证,证实了计算程序的有效性,并修正了无量纲最大温度的公式为幂函数形式。计算结果验证了,在Gr〉7．5×10。时,变物性对非对称通道换热的必要性。     

缸内对流换热与气体流动的计算分析

摘要本文将内燃机燃烧室简化成轴对称的二维空间,将计算缸内流动的二维模型与边界层模型相结合,分析了缸内气体的迁移特性与对流换热.文中介绍了在内燃机工作过程中缸内气体边界层的分布与变化、边界层对对流换热的影响,给出了对流挟热系数沿燃烧室表面的分布与变化.与实测结果的比较表明,本文的模型具有较高的精度.     

Analysis of Laminar Mixed Convective Heat Transfer in Horizontal Triangular Ducts

Numerical simulations for fluid flow and heat transfer in triangular ducts are carried out. Flow is considered to be laminar, hydrodynamically, and thermally developing. Heat transfer by both forced and natural convection is taken into account. Simulations are carried out for constant wall temperature cases which are at a higher temperature than the inlet temperature of the fluid. The effect of Rayleigh number on bulk mean temperature and Nusselt number is studied. Isotherm and secondary velocity profile formed because of natural convection is shown at different locations with varying Rayleigh number. The effect of the apex angle of the triangular duct on Nusselt number and bulk mean temperature is studied. Results are compared with the cases of mixed convective heat transfer in a square duct.     

Mathematical Modeling of Heat and Mass Transfer during Convective Dehydration of an Anisotropic Cylindrical Foodstuff

Two‐dimensional, unsteady‐state mass transfer was studied for air drying of an anisotropic finite cylindrical body. A mathematical model was developed for predicting the temperature of the drying sample at any time and moisture in any position in the drying sample at any time. The anisotropic nature of the drying material was considered in the mathematical model by taking into account the different moisture diffusivities in the axial and radial directions. A cut fresh green bean was used as an anisotropic material and a pilot‐scaled dryer was set up to investigate the drying behavior of this foodstuff. Several length‐to‐diameter ratios of fresh green beans were considered and the mathematical model was validated by comparison of the predicted values of average moisture content with the experimental data. The model was used to predict the moisture distributions inside the drying samples. Different moisture content distributions in the axial and radial directions during drying confirms the anisotropic nature of cut green bean samples. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21119     

气泡尺寸对气缸盖沸腾换热的影响

在应用欧拉多相流模型仿真计算气液两相流沸腾换热时,离散相的气泡尺寸常常被看作常数,而实际上往往气泡具有不同的形状和尺寸,因此研究气泡尺寸大小对仿真计算结果的影响显得至关重要.以ANSYS Workbench为仿真计算平台,在计算流体动力学模块CFX中,用气液两相流沸腾换热计算模型,对不同气泡尺寸下柴油机气缸盖与冷却水腔所组成的流固耦合传热系统进行了整场离散、整场求解,得到了冷却水腔中气液两相流流场分布特性和气缸盖温度场分布,通过与试验结果的对比分析证明了计算模型的有效性.结果表明,在气泡尺寸大小为1,mm的情况下,仿真结果更接近试验结果,并且考虑气液两相流沸腾换热能够有效地降低气缸盖火力面排气道鼻梁区的最高温度,以此降低此处的热负荷.