Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：Part Ⅱ－Analytical Modelfor Boiling Heat Transfer of Binary Mixtureson Smooth Tubes and Comparison of Analytical Models for both Pure Liquids and Binary Mixture

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Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：part II—Analytical Model for Boiling Heat Transfer of Binary Mixtures on Smooth Tubes and Comparison of Analytical Models for both Pure Liqu

A combined physical model of bubble growth is proposed along with a corresponding bubble growth model for binary mixtures on smooth tubes. Using the general model of Wang et al.^[1] and the bubble growth model for binary mixtures, an analytical model for nucleate pool boiling heat transfer of binary mixtures on smooth tubes is developed. In addition, nucleate pool boiling heat transfer of pure liquids and binary mixtures on a horizontal smooth tube was studied experimentally. The pure liquids and binary mixtures included water, methanol, ethanol, and their binary mixtures. The analytical models for both pure liquids and binary mixtures are in good agreement with the experimental data.     

Heat and mass transfer influence on potential variation in a PEMFC membrane

Water transport in the membrane of a PEM fuel cell is provided essentially by a convective force, resulting from a pressure gradient, an osmotic force, due to a concentration gradient and an electric force caused by the protons migration from the anode to the cathode. Through these three types of force the two-dimensional behavior of electric potential has been studied in this paper. The adopted model in this work is based on the assumption of single phase and multi-species flow, supposed two-dimensional and transient in a porous medium. The species conservation equation is coupled with the energy equation through the diffusion coefficient of water and the heat convective flux. The set of governing equations in the form of convection–diffusion problem has been solved numerically using the finite volume method. The obtained results show the transient two-dimensional effect of heat and mass transfer on the voltage variation within the membrane.     

Nucleate Pool Boiling Heat Transfer of Pure Refrigerants and Binary Mixtures

Heat transfer coefficients in nucleate boiling on a smooth flat surface were measured for pure fluids of R-134a, propane, isobutane and their binary mixtures at different pressure from 0.1 to 0.6 MPa. Series of experiments with different heat flux and mixture concentrations were carried out. The influences of pressure and heat flux on the heat transfer coefficient for different pure fluids were studied. Isobutane and propane were used to make up binary mixtures. Compared to the pure components, binary mixtures show lower heat transfer coefficients. This reduction was more pronounced as the heat flux increasing. Several heat transfer correlations are obtained for different pure refrigerants and their binary mixtures.     

Experimental study of heat transfer enhancement using water/ethylene glycol based nanofluids as a new coolant for car radiators

Traditionally forced convection heat transfer in a car radiator is performed to cool circulating fluid which consisted of water or a mixture of water and anti-freezing materials like ethylene glycol (EG). In this paper, the heat transfer performance of pure water and pure EG has been compared with their binary mixtures. Furthermore, different amounts of Al₂O₃ nanoparticle have been added into these base fluids and its effects on the heat transfer performance of the car radiator have been determined experimentally. Liquid flow rate has been changed in the range of 2–6 l per minute and the fluid inlet temperature has been changed for all the experiments. The results demonstrate that nanofluids clearly enhance heat transfer compared to their own base fluid. In the best conditions, the heat transfer enhancement of about 40% compared to the base fluids has been recorded.     

Analytical modeling of polarizations in a solid oxide fuel cell using biomass syngas product as fuel

An analytical model is developed to study fuel type effect on polarizations and performance of SOFC. We consider especially two types of fuel: pure hydrogen and syngas (mixture of H₂ and CO) produced by biomass gasification. The proposed model is based on simultaneous direct oxidation of H₂ and CO at the anode side and uses the dusty-gas model with appropriate diffusion coefficient (binary or mixture coefficient in porous material) to evaluate the concentration polarization and the Butler–Volmer equation to calculate the activation polarization when ohmic polarization is expressed by the well known Ohm’s law. Results analysis show that a fraction of CO of about 24% in syngas improves the performance of SOFC by 23% compared to that obtained by pure H₂.     

Boiling Heat Transfer Characteristics for Methane,Ethane and Their Binary Mixtures

Abstract

Methane (R50) and ethane (R170) are the dominated components of natural gas and the important components in mixture refrigerants for the mixture Joule–Thomson refrigeration cycle. In this article, experimental investigations on nucleate pool boiling and flow boiling heat transfer characteristics of R50, R170, and their binary mixtures are presented. The effects of saturation pressure, heat flux, mass flux, concentration, and vapor quality on heat transfer coefficients are analyzed and discussed. Firstly, the pool boiling heat transfer data were compared with six well-known correlations. Labuntsov correlation shows the best agreement with a mean absolute relative deviation (MARD) of 11.3%. Secondly, a new flow boiling heat transfer correlation for pure fluids was proposed based on the asymptotic addition of forced convection and pool boiling. The modified enhancement factor and suppression factor were developed to account for their relative contribution. In addition, in order to consider the mass transfer resistance of mixtures, a new mixture factor was deduced. The new flow boiling heat transfer correlations can well predict the experimental data with the MARD of 9.5% for pure fluids and 8.3% for mixtures.     

Film models for transport phenomena with fog formation: the fog film model

In a preceding paper (Brouwers and Chesters, Int. J. Heat Mass Transfer35, 1–11 (1992)) possible supersaturation in a film and in the bulk of a binary mixture has been discussed. In the present analysis the exact conditions for fog formation and the magnitude of the fogging and superheated regions in the film are first determined. Next, the governing equation of diffusion and energy (coupled with the saturation condition) of the fog layer is solved numerically. An evaluation of various mixtures of water vapour and air illustrates the substantial effect of fog formation on heat and mass transfer rates. Subsequently, a thorough asymptotic analysis of the fog layer's governing equation yields an excellently matching approximation solution. Furthermore, this solution leads to analytical film model correction factors for the combined effects of fog formation and injection/suction on transfer rates. Finally, the fog film model is applied to channel flow of a binary mixture. This approach provides new procedures for the computation of condensers and evaporators, allowing both fog formation in the film (affecting transfer rates) and/or in the bulk (affecting the incremental balances of mass and energy).     

Predicting Methods for Flow Boiling Heat Transfer of a Non-Azeotropic Mixture Inside a Single Microchannel

At this time, a widely accepted model that can predict flow boiling heat transfer in microchannels with different fluids, geometries, and operative conditions is still missing. Depending on the working fluid, a predicting correlation can lead to accurate estimation or give rise to errors up to 50% and higher. The situation is further complicated when the working fluid is a zeotropic mixture of two components, due to the additional mass transfer resistance that must be estimated. In the recent years much attention has been paid to the possible use of fluorinated propene isomers in substitution for high-global-warming-potential refrigerants. The available hydrofluoroolefins cannot cover all the air-conditioning, heat pump, and refrigeration applications when used as pure fluids because their thermodynamic properties are not suitable for all the operating conditions, and therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. The adoption of new mixtures poses the problem of how to extend the correlations developed for pure fluids to the case of flow boiling of mixtures in microchannels. In this work, a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) has been considered: The local heat transfer coefficient during flow boiling of this mixture in a single microchannel with 0.96 mm diameter has been measured at a pressure of 14 bar, which corresponds to a bubble temperature of around 26°C. This flow boiling database, encompassing more than 300 experimental points at different values of mass velocity, heat flux, and vapor quality, is compared with available correlations in the literature. The introduction of a correction to account for the additional mass transfer resistance is discussed, and such correction is found to be necessary and proper to provide a correct sizing of the evaporator.     

Effect of sweep gas on hydrogen permeation of supported Pd membranes: Experimental and modeling

Membrane reactor processes are being increasingly proposed as an attractive solution for pure hydrogen production due to the possibility to integrate production and separation inside a single reactor vessel. High hydrogen purity can be obtained through dense metallic membranes, especially palladium and its alloys, which are highly selective to hydrogen. The use of thin membranes seems to be a good industrial solution in order to increase the hydrogen flux while reducing the cost of materials. Typically, the diffusion through the membrane layer is the rate-limiting step and the hydrogen permeation through the membrane can be described by the Sieverts’ law but, when the membrane becomes thinner, the diffusion through the membrane bulk becomes less determinant and other mass transfer limitations might limit the permeation rate. Another way to increase the hydrogen flux at a given feed pressure, is to increase the driving force of the process by feeding a sweep gas in the permeate side. This effect can however be significantly reduced if mass transfer limitations in the permeate side exist. The aim of this work is to study the mass transfer limitation that occurs in the permeate side in presence of sweep gas. A complete model for the hydrogen permeation through PdAg membranes has been developed, adding the effects of concentration polarization in retentate and permeate side and the presence of the porous support using the dusty gas model equation, which combines Knudsen diffusion, viscous flow and binary diffusion. By studying the influence of the sweep gas it has been observed that the reduction of the driving force is due to the stagnant sweep gas in the support pores while the concentration polarization in the permeate is negligible.     

采用混合工质的热声驱动脉管制冷实验研究

在计算了惰性气体二元混合工质的热力学参数之后,采用氦-氩混合气体作为工质来提高热声驱动脉管制冷的性能。实验结果表明,热声驱动脉管制冷机采用适当配比的氦-氩工质可获得比纯氦更加优越的制冷性能。     

A new method for reducing the numerical dispersion in mathematical modeling of miscible injection process

Molecular diffusion and mechanical dispersion are major factors affecting the concentration profiles during the miscible injection process. Solving the diffusivity equation always has some source of numerical error, which is called numerical dispersion. Reducing this error is the key parameter to studying the effect of diffusion and dispersion. In this article, a two-dimensional model has been generated and, according to diffusivity equation, the grid shapes have been defined depending on the front velocity of a grid’s position to reduce the numerical dispersion of the system. The predicted concentration profile by this method is in good agreement with the real concentration.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：PartI－Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

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Nucleate Pool Boiling of Pure Liquids and Binary Mixtures ：Part I—Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.     

Heat transfer in forced convection boiling of oil–non-azeotropic binary refrigerant mixtures

In this paper, the results of the heat transfer, forced convection, boiling characteristics of non-azeotropic refrigerant mixtures and oil are presented. This includes heat transfer coefficients for pure and binary mixtures under boiling conditions outside enhanced surface tubing. Local convective heat transfer coefficients have been determined using a modified Wilson-plot technique. Heat transfer correlations were established as a function of the binary mixture mass flow rate, and oil concentration, as well as key flow parameters.     

Formulation for predicting deposition velocity of particles in turbulence

Relationships for the particle concentration and convection velocity profile has been obtained by the adaptation of the random surface renewal model [1], [2], [3], [4], [5] to the particle continuity and momentum equations of the nonisothermal turbulence boundary-layer flows; particle transport mechanisms of Brownian and turbulent diffusion, eddy impaction, particle inertia, and thermophoresis are included. This proposed model provides a useful framework for coupling these modeling parameters with analytical equation of the particle deposition velocity. The predictions obtained on the basis of this equation have been found to be in good agreement with experimental values of deposition velocity for fully-developed turbulent pipe flow.     

Energy analysis of non-Newtonian nanofluid flow over parabola of revolution on the horizontal surface with catalytic chemical reaction

Nanofluids are special functional fluids, which are designed to reduce the loss of energy and maximize the transport of heat. The thermophoresis and Brownian motion of the particle are important factors in the transport of heat in these fluids. The rise in heat transport shows encouraging effects in control of dissipation of energy and reduces entropy generation. In the current study, two-dimensional non-Newtonian Casson nanofluid flow on an upper horizontal surface of a parabola is investigated. The impact of catalytic surface chemical reactions has been account also due to its industrial importance. For this flow problem, the governing equations are modeled using the law of conservation of mass, momentum, heat, and concentration equation. The fitting transformations are taken to change governing couple partial differential equations and domain into local similar ordinary differential equation and domain of [0,∞). Using the "RK4" approach with Newton's shooting schemes via MATLAB tools, the numerical solution of dimensionless governing equations is sorted. It is observed that the Casson fluid parameter caused a drop in temperature profile, and the chemical reaction parameter is the source of the rise in the temperature field.     

Non-Darcy effects on conjugate double-diffusive natural convection in a variable porous layer sandwiched by finite thickness walls

Steady conjugate double-diffusive natural convective heat and mass transfer in a two-dimensional variable porosity layer sandwiched between two walls has been studied numerically. The Forchheimer–Brinkman–extended Darcy model has been used to solve the governing equations in the saturated porous region. The flow is driven by a combined buoyancy effect due to both temperature and concentration variations. An exponential variation of the porosity near the hot wall is considered. The vertical walls are impermeable and subjected to a horizontal gradient of both temperature and concentration while the horizontal walls are adiabatic. A finite volume approach has been used to solve the dimensionless governing equations and the pressure velocity coupling is treated with the SIMPLE algorithm. The model has been validated with available experimental, analytical/computational studies.     

Phase equilibria of benzene–cyclohexane binary mixtures using a solid–liquid–vapor equation-of-state

Based on our previously developed solid–liquid–vapor equation of state (EOS), we have calculated phase equilibria of benzene, cyclohexane, and their mixtures. The model predictions for phase behaviors of pure compounds and vapor–liquid phase equilibria of the binary system have been straightforward and agreed well with the reported data. However, solid–liquid phase equilibria of the binary system are not well correlated to the experimental data with the present unified EOS model. Then, from this fact, we have found that the coexisting solid phases in the present binary system exist in two different solid structures (or two different Gibbs free-energy curves). We have developed a model to overcome such a problem within the present unified EOS model and successfully correlated the experimental data for wide ranges of temperatures and pressures.     

Effect of finite normal interfacial velocity on free convection heat and mass transfer rates from an inclined plate

An analytical study has been carried to examine the effects of finite normal interfacial velocity on simultaneous heat and mass transfer rates in a laminar, free convection binary boundary layer along an inclined plate. The plate is maintained at uniform temperature and/or concentration. Analyses cover the range of Schmidt numbers, .22–5.0, which represent the diffusion of commonly occuring and technologically important gases and vapors in air. Numerical results are presented in tabular forms for better comparision. The study have revealed that in case of simultaneous transfer the increase in interfacial velocity substantially decreases the rate of heat transfer, specially in case of diffusion of species of low Schmidt numbers. Mass transfer rate both incase of pure and combined transfer also decreases with the increase of the normal interfacial velocities.