Three Dimensional Transient Coupled Radiative—Conductive Heat Transfer in Cylinders Filled with Semi—Transparent Media with Complicated Surface Characteristics

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Some aspects concerning modelling the flow and heat transfer in horizontal mantle heat exchangers in solar water heaters

An experimental and numerical investigation has been undertaken to study the heat transfer process in horizontal mantle heat exchangers used in solar water heaters. A rectangular cavity has been used as a simplified geometry. With the aid of particle image velocimetry (PIV) the flow field in the centre‐plane of the rectangular cavity has been visualized. Three‐dimensional flow simulations were performed using a commercial CFD package. The impinging jet formed by the inlet flow directed towards the opposite wall was found to produce localised turbulence in the cavity, with an inlet Reynolds number as low as 360. This turbulence was found to effect the flow field and heat transfer in the cavity when the inlet Reynolds number was above 1200. It is shown that, with the boundary conditions used in this study, most of the heat transferred was in the bottom half of the cavity. This is not the ideal situation for optimization of solar water heating systems. Copyright © 1999 John Wiley & Sons, Ltd.     

Recent developments of heat and mass transfer in hydromagnetic flows

This review article presents an account of several investigations of heat and mass transfer in the field of magnetohydrodynamics which have been carried out during the last few years by several authors. If an article has been omitted, it is entirely by oversight and not by intention.     

Pool boiling heat transfer experiments in silica-water nano-fluids

Heat transfer measurements taken at atmospheric pressure in silica nano-solutions are compared to similar measurements taken in pure water and silica micro-solutions. The data include heat flux vs. superheat of a 0.4 mm diameter NiCr wire submerged in each solution. The data show a marked increase in critical heat flux (CHF) for both nano- and micro-solutions compared to water, but no appreciable differences in heat transfer for powers less than CHF. The data also show that stable film boiling at temperatures close to the wire melting point are achievable with the nano-solutions but not with the micro-solutions.     

Numerical modelling of boiling heat transfer in microchannels

In this paper we report the results of our modelling studies on two-phase forced convection in microchannels using water as the fluid medium. The study incorporates the effects of fluid flow rate, power input and channel geometry on the flow resistance and heat transfer from these microchannels. Two separate numerical models have been developed assuming homogeneous and annular flow boiling. Traditional assumptions like negligible single-phase pressure drop or fixed inlet pressure have been relaxed in the models making analysis more complex. The governing equations have been solved from the grass-root level to predict the boiling front, pressure drop and thermal resistance as functions of exit pressure and heat input. The results of both the models are compared to each other and with available experimental data. It is seen that the annular flow model typically predicts higher pressure drop compared to the homogeneous model. Finally, the model has also been extended to study the effects of non-uniform heat input along the flow direction. The results show that the non-uniform power map can have a very strong effect on the overall fluid dynamics and heat transfer.     

Numerical modelling of heat and mass transfer in finned dehumidifier

In several heat exchange devices, phase transition occurs in a small region adjacent to the wall, and the secondary phase is present only in a thin layer running along the wall, allowing for decoupling between the fluid dynamic computation of the core flow and the numerical analysis of the secondary phase. This happens in finned dehumidifier, but also in spray cooling or defogging problems. In a finned dehumidifier, or in air conditioning evaporators, the secondary phase is provided by moist air condensation, and may consist of discrete droplets, continuous film or a collection of rivulets. Several levels of approximation may be adopted, depending on the specific problem: perfect drain assumption requires only the addition of a heat source in the energy equation, otherwise the water layer behaviour has to be taken into account. Furthermore, a heat and mass transfer analogy may or may not be appropriate; in the latter case, the solution of the diffusion equation of humidity is required.Here, different levels of approximation are compared with literature experimental data for condensation over a vertical fin. Results show that thermal resistance and gravity effects, in the considered geometry, are negligible, and the condensate takes the form of a collection of still droplets, rather than a flowing film. This has an effect on the actual heat transfer and water layer build-up, and the variation of temperature along the fin induces some discrepancy with respect to the straightforward application of the heat and mass transfer analogy.     

Hydrodynamics and heat transfer of turbulent gas suspension flows in tubes—2. Heat transfer

By the method of averaging over the ensemble of turbulent flow realizations, averaged heat transfer equations for a solid phase and a flow as a whole are derived. Closed expressions for the second single-point moments of the solid and carrier phase velocity and temperature fluctuations in terms of the second moments of the carrier phase velocity and temperature fluctuations in a non-uniform turbulent flow are found. Based on these expressions, a set of equations is written for the second single-point moments of the liquid phase velocity and temperature fluctuations in the presence of particles. Heat transfer calculations are carried out for turbulent flow of gas suspension in circular tubes. The effect of the relationship between the thermal and physical properties of the particle material and gas on the thermal characteristics of a two-phase flow is investigated. The predicted Nusselt numbers for a dusty flow agree satisfactorily with the experimental data.     

围护结构热质耦合传递对室内环境的影响

对新建建筑的综合热质耦合传递进行了模拟,建立了围护结构和室内空气的综合热质耦合传递方程。分析了围护结构的热质耦合传递在不同的通风率条件下对室内相对湿度和呼吸舒适度不满意率的影响。模拟结果表明:新建建筑室内通风率为零时,围护结构的热质耦合传递对室内湿环境的影响较大,会明显降低呼吸舒适度;通风率越高,对室内相对湿度的影响越低,呼吸舒适度越高。从全年的对比中可明显看出冬季的室内空气呼吸舒适度明显高于夏季,人们更易接受低温低相对湿度(低焓)环境。     

A new method for determining coupled heat and mass transfer coefficients between air and liquid desiccant

This paper presented the characteristic of liquid desiccant dehumidification based on NTU–Le model. The results showed that the Lewis number Le had little effect on air outlet humidity ratio during desiccant solution dehumidification process. A new method called h_D–Le separative evaluation method was developed for determining coupled heat and mass transfer coefficients between air and liquid desiccant, through which the heat and mass transfer coefficients between air and liquid desiccant were calculated to obtain from experimental inlet and outlet parameters of air and desiccant solution. The effects of the air volume flow rate, temperature, humidity ratio and the solution concentration, temperature on the Lewis number, heat and mass transfer coefficient were analyzed according to experimental data and the h_D–Le separative evaluation method. Based on the computation results, it was concluded that the Lewis number greatly depended on the operation parameters and conditions of the air and desiccant. In addition, the correlations of the heat and mass transfer coefficients were developed. The additional 74 groups of experiments validated the developed correlations by comparison of air/solution parameters change with the calculation data.     

A new mathematical method to solve highly coupled equations of heat and mass transfer in porous media

Heat and mass conservation equations in porous media are coupled and, in general, solved, iteratively, by using the values of temperature and moisture content from previous iteration to calculate source terms. This is the traditional mathematical method and numerical stability is only ensured for small time steps, depending on the source term's magnitudes. This is specially important, when material properties have strong variations with moisture content. This paper presents an unconditionally stable numerical method, conceived accordingly to a new methodology, which considers: (i) linearization of the term giving the vapor exchanged at the boundaries in terms of temperature and moisture content and (ii) introduction of a new generic algorithm to solve, simultaneously, the governing equations, for each time step. Numerical stability of these two methods are compared and it is shown that, in addition to avoid numerical unstability for arbitrary time steps and material properties, convergence is always quickly reached, in the presently proposed calculation method.     

Improvement of heat transfer through fins: A brief review of recent developments

Fins or extended surfaces are generally used in heat exchangers to enhance heat transfer between the main surface and ambient fluid. Various types of simple‐shaped fins, namely, rectangular, square, annular, cylindrical, and tapered, have been used with different geometrical combinations. To satisfy industrial demand, different trials have also been carried out for designing optimized fins. The optimization of fins can be performed either by enhancing heat dissipation at an exact fin weight or by diminishing the weight of the fin by precise heat dissipation. Recently a notable amount of work on some typical fins, like, porous fins and perforated fins, has also been carried out. This paper presents a brief review on heat transfer enhancement using fins of different types considering variable thermophysical and geometric parameters, which will also be useful for future use of geometrical modifications of extended surfaces, based on the cost and availability of space.     

Cooling of spherical products: Part II—heat transfer parameters

This paper presents a study on the determination of the heat transfer parameters, namely surface heat transfer coefficients, thermal conductivities, thermal diffusivities, specific heats and Biot numbers, for the individual product being cooled with water and with air. An analytical model was developed to determine the surface heat transfer coefficients of the products depending on the thermal properties and cooling process parameters. The results of the present study indicate that surface heat transfer coefficients decrease with increasing batch weight in water cooling and increase with increasing air flow velocity in air cooling. The proposed model can be used to determine easily and accurately surface heat transfer coefficients of different spherically shaped objects subjected to cooling.     

对流换热实验中忽略壁面轴向导热的误差分析

以水和空气在钢管中的热充分发展的层流过程为对象,通过数值计算分析了轴向导热给圆管内对流换热的一维壁面导热带来的误差。在二维导热模型的基础上,添加等热流、绝热边界条件,用Fortran语言编写数值计算程序进行计算,定义了相对误差。结果表明,相对误差近似和壁厚与内径的比值成正比,而不同的流体介质数值大小不同。     

Simplified models for coupled heat and mass transfer in falling-film absorbers

A linearized coupled model is developed for the heat and mass transfer in falling-film absorbers. Its accuracy is established by comparing the predictions with those of a non-linear model and a numerical simulation. Under certain conditions, the linearized model reduces to the log-mean-difference formulation. The linearized model yields analytical expressions that are used to determine heat and mass transfer coefficients from the experimental data for a horizontal tubular absorber and a vertical tube absorber. The overall Nusselt number and Sherwood number for the tubular absorber increase with increasing film Reynolds number and inlet cooling water temperature. The cooling water temperature distribution predicted by the linearized model agrees well with measurements.     

Non-Fourier effect in the presence of coupled heat and moisture transfer

The objective of this paper is to study heat and mass transfer coupling under non-Fourier effect. The non-Fourier model is applied on Luikov’s equations. The governed equation is called Luikov’s Corrected (LC) equations and is solved by hybrid analytical–numerical method. Heat and mass propagation in LC field is shown to be wavelike; so, wavefront speed is obtained. Applying non-Fourier model instead of Fourier’s law sensitizes Luikov’s equations to respond in fast transient process. Further, the effect of heat and mass transfer coupling on interpreting non-Fourier affects is investigated. It is seen that the presence of heat and mass transfer coupling reduces wavefront speed with respect to uncoupled non-Fourier heat transfer.     

Diffusivity measurement of semi-transparent media: model of the coupled transient heat transfer and experiments on glass, silica glass and zinc selenide

The aim of this theoretical and experimental study is to provide a complete methodology to estimate the intrinsic diffusivity of semi-transparent media from flash method experiments. A semi-analytical model describes the coupled conductive-radiative transient heat transfer in a slab. The relevance of the model used for the inversion is then investigated. Experimental results are presented for several semi-transparent samples: float glass, SiO₂ and ZnSe. Measurements of rear face temperature are obtained by optical way (infrared detection) for a wide temperature range (293-700 K), for several radiation boundary conditions (black or gold coatings). The results are compared with those obtained through an other set-up and an other experimental procedure.     

Characteristics of combined heat transfer of superheated steam drying: Numerical study on coupled convection and gas radiation heat transfer

A numerical study on a combined radiation and forced convection heat transfer of superheated steam, which is a radiation participating real gas, in thermally developing laminar flow through a parallel‐plate channel has been conducted to investigate characteristics of superheated steam drying. The integrodifferential energy equation was solved using an implicit finite‐difference technique with a marching solution procedure and an exponential wide‐band model for the treatment of the radiative transfer part. Comparison of results with and without gas radiation in various conditions shows that fluid radiation decreases the temperature of the main stream, but increases the total heat flux at a heat transfer surface. Furthermore, the results show that the fluid radiation decreases the inversion point temperature approximately to 150 to 240 °C with the increase of optical thickness. This numerical result agrees in an order of magnitude with the previous experimental studies, but is about 100 K lower than that of former theoretical predictions without considering fluid radiation. © 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 385–399, 2000     

Effect of corrugated plates in an in-phase arrangement on the heat transfer and flow developments

In the present study, the numerical results of the heat transfer and flow developments in the corrugated channel under constant heat flux conditions are presented. The test section is the channel with two opposite corrugated plates which all configuration peaks lie in an in-phase arrangement. The corrugated plates with three different corrugated tile angles of 20°, 40°, and 60° are tested with the height of the channel of 12.5 mm. The model was simulated for the Reynolds number and heat flux in the ranges of 400–1600 and 0.5–1.2 kW/m², respectively. The flow and heat transfer developments are simulated by using the k-ε standard turbulent model. A finite volume method with the structured uniform grid system is employed for solving the model. The predicted results are validated by comparing with the measured data. There is reasonable agreement from the comparison between the numerical data and experimental data. Effects of relevant parameters on the heat transfer and flow developments are discussed. Due to the breaking and destabilizing in the thermal boundary zone, the corrugated surface has significant effect on the enhancement of heat transfer and pressure drop.