DISPERSED FLOW HEAT TRANSFER IN CIRCULAR BENDS

This paper presents an experimental study of dispersed flow heat transfer in 90-degree circular bends. From extensive measurements, two different heat transfer patterns are identified, i.e. heat transfer without and with rewetting. Their intrinsic mechanisms are analysed, based on the present experimental evidence and our previous theoretical studies. Effects of mass flow rate, wall heat flux, system pressure and curvature ratio on heat transfer are also investigated. An empirical criterion is developed to identify the heat transfer pattern in the bend.     

DISPERSED FLOW HEAT TRANSFER IN CIRCULAR BENDS

This paper presents an experimental study of dispersed flow heat transfer in 90-degree circular bends. From extensive measurements, two different heat transfer patterns are identified, i.e. heat transfer without and with rewetting. Their intrinsic mechanisms are analysed, based on the present experimental evidence and our previous theoretical studies. Effects of mass flow rate, wall heat flux, system pressure and curvature ratio on heat transfer are also investigated. An empirical criterion is developed to identify the heat transfer pattern in the bend.     

CONJUGATE HEAT TRANSFER WITH TURBULENT FLOW IN A CIRCULAR TUBE

A steady heat transfer problem has been analyzed as a conjugate problem with turbulent flow in a circular tube. The three kinds of thermal boundary conditions considered here are specified as constant temperature, constant heat flux and constant heat transfer coefficient at the outer surface of the wall.

From the results of numerical calculation for Prandtl numbers in the range 0.01 ≤ Pr ≤ 10 and for Reynolds numbers in the range 10⁴ ≤ Re ≤ 10⁵, it was confirmed that the dimensionless parameter R_c could have significant effects on the heat transfer and the temperature field in the fluid adjacent to the wall.     

FLOW OF A NON-NEWTONIAN FLUID WITH HEAT TRANSFER ABOVE A ROTATING DISK CONSIDERING THE ION SLIP

The steady flow and heat transfer of a conducting non-Newtonian fluid due to the rotation of an infinite nonconducting disk in the presence of an axial uniform steady magnetic field are studied considering the ion slip. The governing nonlinear equations are solved numerically using finite differences, and the solution shows that the ion slip and the non-Newtonian fluid characteristics give some interesting results.     

FLOW OF A NON-NEWTONIAN FLUID WITH HEAT TRANSFER ABOVE A ROTATING DISK CONSIDERING THE ION SLIP

The steady flow and heat transfer of a conducting non-Newtonian fluid due to the rotation of an infinite nonconducting disk in the presence of an axial uniform steady magnetic field are studied considering the ion slip. The governing nonlinear equations are solved numerically using finite differences, and the solution shows that the ion slip and the non-Newtonian fluid characteristics give some interesting results.     

管壳式换热器壳程流动和传热的三维数值模拟

提出了一种管壳式换热器壳程单相流动和传热的三维模拟方法 .用体积多孔度、表面渗透度、分布阻力和分布热源来考虑壳程复杂几何结构造成的流道缩小和流动阻力、传热效应 ,通过数值求解平均的流体质量、动量、能量守恒方程 ,得到壳程流动和换热的分布 .用该方法对一实验换热器进行了流动和传热的模拟 ,计算结果和实验结果吻合良好 .     

MODELING OF HEAT TRANSFER OF UPWARD ANNULAR FLOW IN VERTICAL TUBES

This article presents the modeling of heat transfer of upward annular flow in a smooth tube and a spirally internally ribbed tube. First, analytical models of two-phase flow dynamics and heat transfer of annular flow in flow boiling were derived from the liquid film momentum and energy equations for smooth tubes. Combined with empirical correlations for liquid droplet entrainment and deposition rates in annular flow, modeling of heat transfer of upward annular flow in the smooth tube was conducted. The predicted heat transfer coefficients of annular flow agree with the experimental results very well for the smooth tube. Based on the heat transfer model for smooth tubes, a simplified annular flow heat transfer model for the spirally internally ribbed tube was proposed by modifying the interfacial friction factor. The predicted heat transfer coefficients by the modified heat transfer model for the spirally internally ribbed tube agree with the experimental results to some extent. It is suggested that the heat transfer model for the spirally ribbed tube be further improved by modifying the correlations for liquid droplet entrainment and deposition rates in annular flow, which should describe the feature of annular flow in the spirally internally ribbed tube. Extensive experimental data are needed for this purpose.     

MODELING OF HEAT TRANSFER OF UPWARD ANNULAR FLOW IN VERTICAL TUBES

This article presents the modeling of heat transfer of upward annular flow in a smooth tube and a spirally internally ribbed tube. First, analytical models of two-phase flow dynamics and heat transfer of annular flow in flow boiling were derived from the liquid film momentum and energy equations for smooth tubes. Combined with empirical correlations for liquid droplet entrainment and deposition rates in annular flow, modeling of heat transfer of upward annular flow in the smooth tube was conducted. The predicted heat transfer coefficients of annular flow agree with the experimental results very well for the smooth tube. Based on the heat transfer model for smooth tubes, a simplified annular flow heat transfer model for the spirally internally ribbed tube was proposed by modifying the interfacial friction factor. The predicted heat transfer coefficients by the modified heat transfer model for the spirally internally ribbed tube agree with the experimental results to some extent. It is suggested that the heat transfer model for the spirally ribbed tube be further improved by modifying the correlations for liquid droplet entrainment and deposition rates in annular flow, which should describe the feature of annular flow in the spirally internally ribbed tube. Extensive experimental data are needed for this purpose.     

MHD FLOW AND HEAT TRANSFER IN A VISCOUS FLUID OVER A NON-ISOTHERMAL STRETCHING SURFACE WITH THERMAL RADIATION IN SLIP-FLOW REGIME

The present work is concerned with the effects of surface slip conditions and thermal radiation on an electrically conducting fluid over a non-isothermal stretching surface in the presence of a uniform transverse magnetic field. Similarity transformation is used to transform the partial differential equations describing the problem into a system of nonlinear ordinary differential equations, which is solved analytically. The effects of various parameters on the velocity and temperature profiles as well as on the local skin-friction and the local Nusselt number are discussed in detail and displayed through graphs.     

ANALYTICAL STUDY OF HEAT TRANSFER TO LAMINAR FLOW OF NON-NEWTONIAN FLUIDS IN ANNULUS

An analytical study is made of the problem of laminar flow heat transfer to pseudoplastic fluids in a concentric circular tube annulus. The solution is obtained for simultaneously developing velocity and temperature profiles and constant wall heat flux. Constant property results are presented for different values of flow behavior index, n, and several inner to outer tube radius ratios and Prandtl numbers. Variable property solutions, with strongly temperature-dependent consistency index are obtained. The effect of viscous dissipation on the results of heat transfer is also presented.     

FINITE ELEMENT ANALYSIS FOR LAMINAR FLOW AND HEAT TRANSFER IN FINITE ROD BUNDLES

This investigation deals with the application of finite element method to solve the thermohydraulic problem of laminar fully developed flow in the interior and wall sub-channels of finite fuel rod bundles. A variational principle has been used for the solution of the momentum and energy equations. Wall shear stress and temperature distributions, ?Re and Nusselt numbers are obtained for the sub-channels of different configurations. The results are compared with solutions generated by collocation and finite difference methods.     

FINITE ELEMENT ANALYSIS FOR LAMINAR FLOW AND HEAT TRANSFER IN FINITE ROD BUNDLES

This investigation deals with the application of finite element method to solve the thermohydraulic problem of laminar fully developed flow in the interior and wall sub-channels of finite fuel rod bundles. A variational principle has been used for the solution of the momentum and energy equations. Wall shear stress and temperature distributions, ƒRe and Nusselt numbers are obtained for the sub-channels of different configurations. The results are compared with solutions generated by collocation and finite difference methods.     

UNSTEADY MHD HEAT AND MASS TRANSFER BY MIXED CONVECTION FLOW IN THE FORWARD STAGNATION REGION OF A ROTATING SPHERE AT DIFFERENT WALL CONDITIONS

This study is focused on the problem of MHD heat and mass transfer by mixed convection flow in the forward stagnation region of a rotating sphere in the presence of heat generation and chemical reaction effects. The surface of the sphere is maintained at constant fluid temperature and species concentration. The governing equations of the problem are converted into ordinary differential equations by using suitable similarity transformations. Two cases are considered, namely, constant wall temperature and mass (CWTM) and constant heat and mass fluxes (CHMF). The obtained self-similar equations for both cases are solved numerically using an efficient iterative implicit finite-difference method. The numerical results are compared with previously published results on special cases of the problem and found to be in excellent agreement. The obtained results are displayed graphically to illustrate the influence of the different physical parameters on the velocity components in x- and y-directions, temperature, and concentration profiles as well as the local surface shear stresses and local heat and mass transfer coefficients.     

CONJUGATED HEAT TRANSFER IN COCURRENT FLOW MULTI-STREAM HEAT EXCHANGERS

Considering the cross-sectional velocity profile to be piecewise-constant in each stream of a multi-stream heat exchanger for cocurrent thermally developing flow, this study analytically solves the related conjugated Graetz problem by using an integral transform method. Further, it obtains an analytical solution in an explicit form to the fluid temperatures that vary two-dimensionally. A numerical example is provided for the case of a triple-tube heat exchanger. The numerical results demonstrate the effects of the thermal conductivity ratios of the fluids and the Péclet number ratios on the temperature distribution in the streams. In addition, in order to show the importance of considering streamwise variations in the overall heat transfer coefficients when designing laminar flow heat exchangers, the amount of exchanged heat calculated by the presented analytical solution is compared with that predicted on the basis of bulk temperatures and constant overall heat transfer coefficients.     

汽-液-液三相垂直管内充分发展流传热性能

研究了垂直管内连续相水和分散相戊烷泡滴之间的流动和传热性能 .在全面考虑了汽体的膨胀、水蒸气的蒸发、界面张力的作用、汽液两相的相互作用和滑移、泡滴的破碎等影响因素的基础上 ,建立了与实验结果吻合较好的一维流体力学模型和传热模型     

HEAT TRANSFER IN THREE-PHASE FLUIDIZED BEDS WITH NON-NEWTONIAN PSEUDOPLASTIC SOLUTIONS

Heat transfer experiments have been conducted in three-phase fluidized beds (TPFB) of 3 and 5 mm glass spheres. To assess the effect of non-Newtonian flow behavior, concentrated pseudoplastic xanthan solutions were employed. The gas and liquid superficial velocities range from 0.81 to 14.4 cm/s and 1.27 to 9.0 cm/s respectively. The impact of these parameters as well as the effect of the effective liquid viscosity, and the solid size on heat transfer in TPFB are discussed. All heat transfer coefficients can be discribed satisfactorily by a new correlation which predicts the Nu-values in TPFB with pseudoplastic solutions with ±9.13% average standard deviation.     

垂直下降沸腾管两相流传热恶化研究

<正>1引言 传热恶化有不同的类型,就工程换热设备设计和运行的实际需要来说,人们最关心的是以下几点：（1）传热恶化发生的位置（壁温飞升起始点）,一般用。cr来表示;（2）传热恶化发生后壁温飞升的最大值,一般用A儿。。来表示;（3）壁温飞升最大值的位置,一般用K。。。来表示;（4）传热恶化发生后的。mi。。本文着重分析讨论后3个问题。     

A THEORETICAL STUDY OF THE LIMITING HEAT TRANSFER COEFFICIENT IN FLUIDIZED BEDS

A new model is presented to describe the heat transfer process occurring in fluidized beds under the limiting condition of very short contact times between the heat transfer surface and the emulsion phase. Unlike other work in this area the proposed model assumes that the particles are in constant motion close to the surface throughout the heat transfer process. The effect of the non-continuum gas film assumption (the Smoluchowski effect) when particles are very close to the surface is found to be of secondary importance when compared with the movement of particles close to the surface. Expressions for both the instantaneous and time averaged heat transfer coefficients for individual and a spatial distribution of parlicles are presented.     

添加剂对流动沸腾传热的影响

研究了添加剂对垂直管内流动沸腾传热的影响,选用W/O型聚丙烯酰胺(简称PAM)和十八烷胺(简称ODA)为添加剂,研究添加剂水溶液的性质、添加剂浓度、流速、热通量、聚合物分子量等因素对流动沸腾传热的影响.采用X光射线光电子能谱仪测试了添加剂分子在管壁上的吸附情况,并对壁面特性进行了分析.研究结果表明,W/O型聚丙烯酰胺和十八烷胺都具有强化流动沸腾传热的性能,而且有最佳浓度范围。PAM还具有减阻性能,因而可提高流速来强化流动沸腾传热。     

HEAT TRANSFER AND FLUID FLOW IN A NONISOTHERMAL CONSTRAINED VAPOR BUBBLE

A Constrained Vapor Bubble (CVB) with a relatively large Bond number formed by partially underfilling liquid in an evacuated cavity is capable of high thermal conductance. Il operates on the principle of closed loop phase-change along with capillarity to circulate the working fluid. Analytical investigations were conducted to compare with existing experimental data. A steady-state fluid flow model combined with a two-dimensional heat transfer model was developed and solved to yield key operating parameters ( i.e., temperature and liquid meniscus curvature) of the CVB. The modeling results of the outside wall temperature in the evaporator were found to agree well with the measured experimental data. An area average heat transfer coefficient was used to characterize the heat transfer on the inside wall of the evaporator. The value of this heat transfer coefficient was found to increase with the heat flow rate. The fluid flow model with the heat transfer model in the evaporator to provide the energy balance was used successfully to fit the experimental curvature data. The mass flow rate in the bottom corners of the CVB was found to be higher than that in the top corners due to the gravitational body force.