太阳能热水器传热传质和强化传热研究

在低温太阳能光热光伏联合应用试验台的基础上,结合GB/T 17049—2005,利用Gambit、Ansys Fluent和Tecplot软件,对全玻璃真空管太阳能热水器进行传热传质和强化传热分析。结果表明:所建立的二维数值计算模型,能准确反映同一条件下,全玻璃真空管太阳能热水器的变化趋势;在数值模拟基础上,确定了单面受热时的最佳安装角度为51°,加装反光板类似双面受热的最佳安装角度为38°;在粗略估算和细化分析的基础上,确定了不同真空管结构的最佳导流板长度及安装位置;通过实验和数值模拟,确定了58mm×1 800mm为优化的全玻璃真空管结构。     

Heat and Mass Transfer in Open-Cycle OTEC Systems

The temperature difference between surface and deep water in the oceans represents a vast resource of thermal energy. A promising method of harnessing this resource is the open-cycle ocean thermal energy conversion (OC-OTEC) system, which utilizes steam evaporated from the surface water to power the turbine. In this paper the state of the art of heat and mass transfer related to evaporation and condensation of steam at low pressures in OC-OTEC is summarized and relevant research issues are discussed.     

Heat and Mass Transfer in Multicomponent Condensation and Boiling

This paper reviews the state of the art in design methods for multicomponent condensation and boiling. In multicomponent condensation, a range of methods of varying complexity are available to calculate the local heat and mass transfer rates; the complexity depends on the detail with which the mass transfer resistances are calculated. These methods are briefly reviewed and their interrelationships along with those of their expertmental validation are discussed. Mixture boiling research has thus far followed different lines and very little effort has gone into understanding the liquid multicomponent diffusion process in boiling. Research work has been mainly concentrated on understanding the dynamics of bubble growth; heat transfer calculations are done using empirical corrections to coefficients calculated for pure fluids. The extensive research in these areas is reviewed and also the few previous studies on convective two-phase heat transfer to mixtures are described. In conclusion, possible design recommendations are offered. It is suggested that, although condensation and boiling heat transfer in mixtures have hitherto been studied separately, a combined research approach may produce further advances.     

涡轮薄层污泥干化的热质传递过程与关键参数研究

涡轮薄层污泥干化是涉及导热、对流传热与传质、高速旋流相耦合的复杂过程,掌握污泥的耦合干化机理与规律、确定干化设备核心参数,是该技术成功应用的关键。本文用机理分析方法构建涡轮薄层干化过程传热、传质的数学模型,基于该模型对污泥干化过程开展数值模拟,揭示涡轮薄层干化过程单一气相和气固两相流的速度、温度和含水率的分布规律,探究涡轮薄层干化的关键技术及参数。设计开发涡轮薄层污泥干化系统并开展实验研究。结果表明：干化机内部的桨叶阵列设计能够实现物料的顺利运输,桨叶末端是干化机内混合传热效果最佳位置,最优桨叶安装角度为45°,出口污泥含水率可降至20%。     

湿化器的传热传质机理和性能分析

从传热传质基本原理出发,建立了ＨＡＴ循环的重要部件－湿化器的数学模型,研究了湿化器的工作性能,并指出了目前某些处理方法的不合理之处。     

板式石蜡储热器传热的数值模拟

在相变储热器中采用强化传热技术，克服相变材料的低导热性能，是目前国内外研究的热点。应用FLUENT软件数值模拟了翅片强化板式石蜡储热器的凝固传热过程，得到随时间变化的相界面位置、总凝固时间、壁面热流、翅片温度分布等，并进一步分析了翅片对不同长宽比叫的储热器的强化传热效果。模拟结果表明，只有当ω≥1时，翅片才能对储热器起到明显的强化传热作用，研究结果可为相变储热器的优化设计提供可靠的依据。     

Approximate Solution to the Spray Heat Transfer Problem at High Surface Temperatures and Liquid Mass Fluxes

ABSTRACT

A basic energy balance that includes phase change has been used to describe the boiling heat transfer process. By using the differential form of this energy balance, the relative change in the heat transfer coefficient can be determined when the surface and coolant temperature change. This represents a general solution to the boiling heat transfer problem under high flux conditions where fully mixed thermal boundary layer exists, although the solution procedure is approximate. The results agree quite well with experimental data. Further work remains to prescribe the heat transfer process near the critical heat flux and Leidenfrost point. This approach vastly reduces the empiricism and data required for boiling heat transfer processes, and also existing data can be used to generalize to a wide range of conditions.     

On the Energy Decay for a Thermoelastic Contact Problem Involving Heat Transfer

A dynamic unilateral contact problem between two thermoelastic beams is considered. Under thermal boundary conditions involving heat transfer, the evolution problem is shown to possess an energy decaying exponentially to zero, as time goes to infinity.     

Heat and Mass Transfer in Adsorption-chemical Cooling with Phase Transitions in a Sorbent

The complex model of heat and mass transfer during adsorption and chemical heat conversion is presented. It includes combined effect of physical and chemical adsorption and accompanying ammonia condensation/evaporation directly inside the sorber taking into account the conjugate heat transfer between the sorbent and the heat-transfer fluid. It was found that the specific cooling power and average cooling temperature are one-valued function of a ratio of the sorber length to the mass flow of heat-transfer fluid. The use of condensation/evaporation in a sorbent can increase the temperature effect more than twice.     

Soret Effects on Non-Fourier Heat and Non-Fickian Mass Diffusion Transfer in a Slab

In this article, non-Fickian and non-Fourier models were used for the rapid heat and mass transfer problem in a slab. The boundaries are convective and insulated sides, and the solution takes the Soret effect into consideration. The Laplace transform method is used to obtain a local-analytic solution in the transformed domain and then inverted to obtain the results in physical quantities. The results show that the temperature and concentration curves possess discontinuities when the relaxation-time in the non-Fourier heat conduction and non-Fickian mass diffusion problem is considered. These discontinuities are caused by wave propagation. Propagated waves are reflected at the insulated side and grow weaker as time passes. When considering the Soret effect for mass transfer, the intensity of concentration rise in the discontinuities will increase.     

Heat and Mass Transfer with Condensation in Non-Saturated Porous Media

Heat and mass transfer with condensation in porous media was numerically studied coupling with distributions of temperature, vapor concentration, pressure and water saturation. The transport processes in porous media displayed two distinct stages, or early and late stages. Different characteristics during these stages were described with the consideration of physical nature. A systematical investigation was conducted to explore the influence of different parameters on the processes, including the temperature, the particle diameter, the porosity, and the thermal conductivity of the particle. A modeling experiment was conducted to partially validate the calculation.     

Numerical Study of Heat Transfer and Analysis of Optimal Fin Pitch in a Wavy Fin-and-Tube Heat Exchanger

In this paper an analysis of laminar heat transfer and fluid flow in a wavy fin-and-tube heat exchanger has been carried out. Three-dimensional (3D) numerical simulation results of a circular tube heat exchanger were compared with published numerical and experimental results. The computational fluid dynamics (CFD) procedure was validated by comparing average Nusselt numbers, and good agreement between published and calculated results has been accomplished. The influence of inlet air velocity, varying from 0.5 to 5 m s^?1, as well as fin pitch, varying from 0.4 to 4 mm, on heat transfer and pressure drop conditions has been studied. The results have shown that there is an optimal fin pitch for each air velocity, which gives the best heat exchanger performance from the heat transfer point of view.     

薄层毛细多孔介质湿区干燥过程相变传热传质常压模型

对液相歼余饱和度Sir作了一个定义,把含湿毛细多孔介质干燥区域分为湿区和干区。建立了以液相饱和度S和温度T为参数的湿区干燥过程相变传热传质常压模型。采用全隐式有限差分方法以模型进行了数值计算。数值解表明,模型能很好地预测湿区干燥过程中的液相饱和度S和温度T的变化。     

谐振子系统量子卡诺热泵的最佳特性参数

研究了量子卡诺热泵的最优性能,导出了量子热泵最佳泵热率Ⅱ０与致热系数Ψ的关系,得到了以Ⅱｍ和Ｅ为目标的最佳特性参数。     

Conjugate Heat and Mass Transfer from a Drying Rectangular Cylinder in Confined Air Flow

Heat and mass transfer from a porous body subject to convective drying is investigated numerically based on Luikov's equations. The air flow is assumed incompressible, two-dimensional, laminar, confined in a channel, and parallel to the rectangular-shaped solid. The finite-volume method is used and the computed temporal and spatial variations of moisture content, temperature, concentration, and flow parameters for two different flow rates are analyzed. Two flow configurations are studied: with and without a flow divider upstream of the cylinder in an attempt to eliminate the presence of separation zones and study their effect on drying. It was found that such effects may greatly affect the drying process, along with frontal area stagnation and the thickness of the body.     

Numerical Analysis Of Heat Transfer Parameters Using Ferrofluid

Heat transfer, thermal management, and acoustics are the important parameters that determine the effectiveness of any miniaturized electronic circuit. As the density of electronic components is increasing, the reliability of such systems is of great concern. The proposed work discusses a novel technique of a magnetic pumping fluid system without conventional cooling components. Here the integrated effect of the conjugate heat transfer and magnetic field is taken into account to study flow phenomena. Two different fluids—water and kerosene—as a base fluid for ferrofluid are evaluated for physical quantities such as pressure, temperature, and heat transfer coefficient to measure the performance of a magnetic cooling system. Flow in the cooling setup is optimized by using numerical analysis and number of experiments on the given setup. It was found that flow depends on the magnetic field gradient, temperature gradient, fluid properties, and material combination. In addition, flow characteristics depend on the influence of magnetic field and temperature of the fluid. Analysis results show that the heat transfer and pressure head in the kerosene-based ferrofluid are more stable compared to the water-based ferrofluid. A better magnetic fluid with both high magnetization and a high pyromagnetic coefficient can enhance the flowability of the ferrofluid to a large extent.     

Investigation of Heat and Mass Transfer in a Lid-Driven Cavity Under Nonequilibrium Flow Conditions

Gaseous flow and heat transfer in a lid-driven cavity under nonequilibrium flow conditions is investigated using the direct simulation Monte Carlo method, from the slip to the free-molecular regime. The emphasis is on understanding thermal flow features. The impact of the lid velocity and various degrees of rarefaction on the shear stress and heat flux rates are analyzed. The role of expansion cooling and viscous dissipation on the heat transfer mechanism is investigated. Complex heat flow phenomena, such as counter-gradient heat transfer, are revealed by the simulations which the conventional Navier-Stokes-Fourier equations are not able to capture, even in the slip-flow regime.     

Experimental Study on the Heat and Mass Transfer of Teflon-Coated Tubes for the Latent Heat Recovery

Abstract

The heat and mass transfer on stainless steel and Teflon-coated tube surfaces in a steam-air mixture flow were experimentally studied to obtain design data for a heat exchanger to be used in the latent heat recovery from flue gases. The test section consisted of three horizontal tubes with a pitch of 67 mm. The diameter of the tubes was 25.4 mm, and the tube wall thickness was 1.2 mm. The steam-air mixture flowed vertically from top to bottom. The Teflon coating degraded the overall heat transfer coefficient by 19%, as compared to the bare stainless steel tube. The degradation of the overall heat transfer coefficient of the Teflon-coated tube comes from the additional heat transfer resistance due to Teflon coating. Its magnitude on heat transfer resistance is comparable to the in-tube heat transfer resistance. Nusselt and Sherwood numbers of Teflon-coated surfaces have a different trend compared to bare stainless steel surface in the higher Reynolds number of our test. The higher contact angle of the condensate on the Teflon-coated surface, about 25°, is the reason for the kind of trend observed.