Numerical study of heat and mass transfer of binary mixtures condensation in mini-channels

This work presents a model to study condensation heat and mass transfer characteristics of binary mixtures inside mini-channels. By considering the mass and heat transfer resistances in the vapor phase, the conservation equations of mass, species, momentum and energy are solved using higher order finite element method. The model is validated by comparing the predicted results with experimental data from the literature, in particular for the case of methane/ethane mixtures at different compositions and working conditions in a tube of 1.0 mm in diameter. The results show a reasonably good prediction of the heat transfer coefficient and pressure drop by comparing with these experimental data. The model is then used to study the effect of mass flux, wall heat flux and system inlet pressure on the heat and mass transfer resistances during condensation of binary mixtures.     

Effect of Operating Parameters on the Heat Transfer and Liquid Film Thickness of Revolving Heat Pipe

This paper presents a study on the effects of operating parameters on the liquid film thickness and heat transfer of revolving heat pipe. The effects of speed, radius of rotation, evaporator and condenser temperatures, and mass of the working fluid are considered. Also, the effects of these parameters on the maximum heat transfer and minimum mass of the working fluid supplied to the heat pipe are considered. A simplified theoretical model is presented to estimate the heat transfer and the liquid film thickness. The theoretical model is used to determine the driven forces on the control volume. The system of equations associated with the heat pipe model is solved using the fourth-order Runge–Kutta method through a numerical code written in MATLAB. The results show that the heat transfer increases by decreasing the mass of the working fluid and increasing the temperature difference through the heat pipe. They also show that the liquid film thickness increases with the decrease in temperature difference and with increase in the mass of fluid. The maximum heat transfer increases with the increase in the rotation speed. The minimum mass of the working fluid supplied to the heat pipe increases with the increase in temperature difference and with the decrease in the rotation speed.     

Mathematical modelling of moisture desorption in a porous medium

This paper discusses heat and mass transfer in desorption drying. A basic equation system is derived to describe coupled heat and mass transfer in a porous medium with moisture desorption under temperature gradients and a vacuum environment. The desorption mushy zone model is used to obtain an exact solution for coupled heat and mass transfer with a moving desorption mushy zone in a porous half-space. The results are analysed numerically to demonstrate the effects of various parameters on desorption.     

Direct numerical simulation of interphase heat and mass transfer in multicomponent vapour–liquid flows

A Volume-of-Fluid methodology for direct numerical simulation of interface dynamics and simultaneous interphase heat and mass transfer in systems with multiple chemical species is presented. This approach is broadly applicable to many industrially important applications, where coupled interphase heat and mass transfer occurs, including distillation. Volume-of-Fluid interface tracking allows investigation of systems with arbitrarily complex interface dynamics. Further, the present method incorporates the full interface species and energy jump conditions for vapour–liquid interphase heat and mass transfer, thus, making it applicable to systems with multiple phase changing species. The model was validated using the ethanol–water system for the cases of wetted-wall vapour–liquid contacting and vapour flow over a smooth, stationary liquid. Good agreement was observed between empirical correlations, experimental data and numerical predictions for vapour and liquid phase mass transfer coefficients. Direct numerical simulation of interphase heat and mass transfer offers the clear advantage of providing detailed information about local heat and mass transfer rates. This local information can be used to develop accurate heat and mass transfer models that may be integrated into large scale process simulation tools and used for equipment design and optimization.     

Heat and mass transfer in a non-isothermal fixed bed solid adsorbent reactor: a uniform pressure-non-uniform temperature case

A uniform pressure model is presented to describe the heat and mass transfer in a fixed bed of solid adsorbent in a finned reactor. This model neglects the resistance to mass diffusion but takes into account the resistances to heat diffusion through two coefficients: the heat conductivity of the adsorbent bed and the heat transfer coefficient between the adsorbent bed and the fins. An experiment has been conducted to validate this model and the two heat transfer coefficients are obtained by an identification technique. When the temperature of the closed reactor is modified on one side of the reactor, large temperature inhomogeneities inside the reactor are observed and mass transfer occurs through a heat pipe effect: the model explains that effect which is observed experimentally. That uniform pressure model is more adapted to describe the history of solid adsorbent reactors used in thermal processes than uniform temperature models proposed by other authors.     

A three-dimensional non-equilibrium model for an intermittent adsorption cooling system

Intermittent adsorption cycles, driven by low temperature heat, like solar heat, instead of electricity or natural gas, can achieve substantial fossil energy savings. In this paper, the mathematical model for the coupled heat and mass transfer in the adsorber of an intermittent adsorption cooling system is set up. The model includes four submodels: heat transfer in heating/cooling fluids, heat transfer in the metal tube, heat transfer in the fins, and heat and mass transfer in the adsorbent. The model for the heat and mass transfer in the adsorbent is a three-dimensional non-equilibrium model which takes into account both the internal and the external mass transfer resistance in the adsorbent. An experiment has been done to validate the model. With some modifications, the model can be used in system optimization and design of adsorption cycles driven by solar energy or waste heat.     

Heat and mass transfer effectiveness and correlations for counter-flow absorbers

A simplified linear coupled heat and mass transfer model for counter-flow absorbers is validated by comparing its predictions with those of a numerical turbulent flow model. The simplified model lends itself to the formulation of a mass transfer effectiveness and a heat transfer effectiveness for counter-flow absorbers. The effectiveness is relatively insensitive to variations in the operating conditions of the absorber and depends mainly on the number of transfer units (NTU) and the capacity ratio. Available experimental data on a vertical tube absorber are analyzed using the simplified model to obtain heat and mass transfer correlations.     

Mathematical simulation of lithium bromide solution laminar falling film evaporation in vertical tube

For utilization of the residual heat of flue gas to drive the absorption chillers,a lithium-bromide
falling film in vertical tube type generator is presented.A mathematical model was developed to
simulate the heat and mass coupled problem of laminar falling film evaporation in vertical tube.In the
model,the factor of mass transfer was taken into account in heat transfer performance calculation.The
temperature and concentration fields were calculated.Some tests were conducted for the factors
such as Re number,heating flux,the inlet concentration and operating pressure which can affect the
heat and mass transfer performance in laminar falling film evaporation.The heat transfer performance is
enhanced with the increasing of heat flux.An increasing inlet concentration can weaken the heat
transfer performance.The operating pressure hardly affects on heat and mass transfer.The bigger inlet
Re number means weaker heat transfer effects and stronger mass transfer.The mass transfer
obviously restrains the heat transfer in the falling film solution.The relation between dimensionless
heat transfer coefficient and the inlet Re number is obtained.     

Heat and mass transfer in a flat plate solar solid adsorption refrigeration ice maker

A uniform pressure model is presented to describe the heat and mass transfer in an adsorbent bed for a flat plate solar ice maker. This model accounts for heat and mass transfer in a porous bed in a two-dimensional transient process. An experiment has been conducted to validate this model and the calculated results are in good agreement with experiments. With the help of this model, the transient analysis and performance prediction of an intermittent solar powered solid refrigerator can be presented.     

Development of a microchannel evaporator model for a CO2 air-conditioning system

This paper presents the development and verification of a heat exchanger model for evaluating the thermal performance of an evaporator for a CO₂ mobile air-conditioning system. The model has been developed, on the basis of the finite volume method, with emphasis placed on the air-side heat and mass transfer processes. The governing equations are derived from mass and energy balances using the newly developed air-side heat transfer and friction loss correlations for microchannel heat exchangers under both dry and wet conditions. The calculated air-side heat transfer and pressure drop data are in good agreement with measured data. However, the refrigerant-side pressure drop estimation for microchannel tubes usually underestimates the measured value. The simulation results and importance of selecting appropriate heat transfer and pressure drop correlations for the microchannel heat exchanger are addressed.     

利用热管强化吸附床内的传热传质

为了强化吸附式制冷吸附床内的传热传质，设计了利用高效传热元件热管作为内翅片的吸附床。在能量守恒关系和吸附平衡方程的基础上建立了吸附床的数学模型，并对此模型用数值方法进行了求解。求解结果表明利用热管元件可以显著的改善吸附床内的传热传质过程，缩短了吸附式制冷的循环时间，提高了系统的效率，该数学模型为吸附床的设计参数的选择和优化等提供了依据。     

FINITE ELEMENT ANALYSIS OF COUPLED HEAT. MASS,AND PRESSURE TRANSFER IN POROUS BIOMATERIALS

Abstract

The finite element method was used to solve Luikov's system of partial differential equations for neat, mass, and pressure transfer in capillary porous bodies. The finite element predictions were validated by comparing with exact solutions and the analytical results given by Mikhailov and Shishedjiev [1]. An application of the finite element method to the drying of wood (spruce) and a comparison based on an eigenvalue solution for simultaneous heat and mass transfer [2] are also provided. This technique was applied to study the coupled transport process in a silicon gel. The simulation indicated that the results obtained from the heat, mass, and pressure transfer model showed a marked difference from the results obtained by the heat and mass transfer model.     

Heat transfer model for gas–liquid slug flows under constant flux

This paper investigates the mechanisms leading to enhanced heat and/or mass transfer rates in two-phase non-boiling slug flows. The problem is analyzed in a minichannel geometry subjected to a constant heat flux boundary. Local Nusselt numbers, obtained using Infrared thermography are analyzed in both entrance and fully developed flow regions. These novel measurements highlight the physics governing slug-flow heat transfer and results indicate that optimized slug geometries can yield up to an order of magnitude heat transfer enhancement. Finally, based on the physics identified, a heat transfer model is developed which is also applicable to similar mass transfer problems.     

Heat and Mass Transfer in an Indirect Contact Cooling Tower: CFD Simulation and Experiment

The present work is focused on the computational analysis of heat and mass transfer in an indirect contact cooling tower. The main objectives of the study are to contribute to the understanding of heat and mass transfer mechanisms involved in the problem and to check the possibility of making use of a commercial computational fluid dynamics (CFD) code for simulating mass and heat transfer phenomena occurring in indirect cooling towers. The CFD model uses as boundary conditions the temperatures of the tubes obtained by a correlation model developed by Mizushina. The available mass transfer correlations for indirect cooling towers are presented and compared with a correlation obtained from CFD simulations. The assumption of analogy between heat and mass transfer is also discussed.     

CFD方法与间接蒸发冷却换热器的三维数值模拟

采用计算流体力学(CFD)和数值传热学方法,对间接蒸发冷却器内流体流动与热质交换过程进行简化和假设,建立了换热器内三维层流流动与传热的数学物理模型。采用交错网格离散化非线性控制方程组,编制了三维simple算法程序。对间接蒸发冷却器内的流场、温度场及浓度场进行数值模拟研究,得到换热器内的流体流动状态和热流分布,并分析了通道宽度变化对换热器内流体流动与换热的影响。     

Hydrogen storage in metal hydride tanks equipped with metal foam heat exchanger

This paper presents a two-dimensional mathematical model to evaluate transient heat and mass transfer in a metal hydride tank (hereinafter MHT) with metal foam heat exchanger. The model is validated by comparison with experimental data. A good agreement is obtained.     

3D heat transfer analysis in a loop heat pipe evaporator with a fully saturated wick

A practical quasi three-dimensional numerical model is developed to investigate the heat and mass transfer in a square flat evaporator of a loop heat pipe with a fully saturated wicking structure. The conjugate heat transfer problem is coupled with a detailed mass transfer in the wick structure, and incorporated with the phase change occurring at the liquid–vapor interface. The three-dimensional governing equations for the heat and mass transfer (continuity, Darcy and energy) are developed, with specific attention given to the wick region. By comparing the results of the numerical simulations and the experimental tests, the local heat transfer mechanisms are revealed, through the obtained temperature distribution and the further derived evaporation rates along the liquid–vapor interface. The results indicate that the model developed herein can provide an insight in understanding the thermal characteristics of loop heat pipes during steady-state operation, especially at low heat loads.     

基于有限元方法的溴化锂降膜吸收传热传质的数值研究

吸收器是吸收式制冷系统的重要部件.溴化锂溶液的降膜吸收是吸收器中最常见的传质传热形式之一.通过对溴化锂溶液在降膜吸收过程中传质和传热特性的分析,使用基于有限元法的COMSOL Multiphysics软件,建立了溴化锂溶液和水蒸汽降膜吸收的物理数学模型,计算了液膜内部温度和质量分数的分布、界面处传质通量、界面处传热通量...     

Prediction of Heat Transfer Coefficient in Flow Boiling over Tube Bundles Using ANFIS

The applicability of the artificial intelligence technique called ANFIS (for adaptive neuro fuzzy inference system) to model the flow boiling heat transfer over a tube bundle is studied in this paper. The ANFIS model is trained and validated with the experimental data from literature. The heat flux, mass flux, and row height are taken as input and the flow boiling heat transfer coefficient as output. The developed model performance is evaluated in terms of performance parameters such as root mean square error, mean square error, correlation coefficient, variance accounted for, and computational time. The preceding parameters of the model are then determined for different combinations of type and number of membership functions. The model is found to predict experimental heat transfer coefficient within an error of ±5%. The developed model is also compared with the artificial neural network model and is found to be better in predicting the flow boiling heat transfer coefficient. The developed model is further used to observe the variation of heat transfer coefficient of the individual rows and bundle for intermediate value of parameters such as heat flux and mass flux that are not included in the analysis of experimental data. The analysis is able to provide complete information about variation of heat transfer coefficient of individual rows and the bundle with respect to heat flux and mass flux.