Effect of radiation and chemical reaction on natural convective MHD flow through a porous medium with double diffusion

The influence of radiation and chemical reaction on a natural convective MHD flow through a porous medium bounded by a vertical infinite surface in the presence of transverse magnetic field is studied. The basic equations governing the flow, heat and mass transfer are reduced to a set of ordinary differential equations by appropriate transformations. Governing equations are solved by perturbation technique for velocity, temperature and concentration, and that has been presented graphically for different values of involved parameters. It is observed that effects of magnetic parameter and radiation parameter in the flow field affect the flow significantly.     

Heat transfer in MHD flow of dusty viscoelastic (Walters’ liquid model-B) stratified fluid in porous medium under variable viscosity

The paper investigates the effects of heat transfer in MHD flow of viscoelastic stratified fluid in porous medium on a parallel plate channel inclined at an angle θ. A laminar convection flow for incompressible conducting fluid is considered. It is assumed that the plates are kept at different temperatures which decay with time. The partial differential equations governing the flow are solved by perturbation technique. Expressions for the velocity of fluid and particle phases, temperature field, Nusselt number, skin friction and flow flux are obtained within the channel. The effects of various parameters like stratification factor, magnetic field parameter, Prandtl number on temperature field, heat transfer, skin friction, flow flux, velocity for both the fluid and particle phases are displayed through graphs and discussed numerically.     

Upshot of Chemical Species and Nonlinear Thermal Radiation on Oldroyd-B Nanofluid Flow Past a Bi-directional Stretched Surface with Heat Generation/Absorption in a Porous Media

A three-dimensional mathematical model is developed to examine the flow of nonlinear thermal radiation Oldroyd-B nanofluid past a bidirectional linearly stretched surface in a porous medium. The flow is induced by temperature dependent thermal conductivity, chemical reaction and convective heat and mass conditions. Novel characteristics of Brownian motion and thermophoresis are accompanied by magnetohydrodynamic and heat generation/absorption.Self-similar transformations are employed to convert the system of nonlinear partial differential equations to a system of ordinary differential equations with high nonlinearity and are solved by strong analytic technique named as Homotopy Analysis method(HAM). Effects of varied arising parameters on involved distributions are reflected through graphical illustrations. From this study, it is perceived that strong magnetic field hinders the fluid's motion and leads to rise in temperature that eventually lowers heat transfer rate from the surface. Further, decrease in heat transfer rate is also observed for enhanced values of thermal radiation parameter. To validate our results, a comparison with already published paper in limiting case is also given and results are found in excellent oncurrence; hence reliable results are being presented.     

Upshot of Chemical Species and Nonlinear Thermal Radiation on Oldroyd-B Nanofluid Flow Past a Bi-directional Stretched Surface with Heat Generation/Absorption ina Porous Media

A three-dimensional mathematical model is developed to examine the flow of nonlinear thermal radiation Oldroyd-B nanofluid past a bidirectional linearly stretched surface in a porous medium. The flow is induced by temperature dependent thermal conductivity, chemical reaction and convective heat and mass conditions. Novel characteristics of Brownian motion and thermophoresis are accompanied by magnetohydrodynamic and heat generation/absorption. Self-similar transformations are employed to convert the system of nonlinear partial differential equations to a system of ordinary differential equations with high nonlinearity and are solved by strong analytic technique named as Homotopy Analysis method (HAM). Effects of varied arising parameters on involved distributions are reflected through graphical illustrations. From this study, it is perceived that strong magnetic field hinders the fluid's motion and leads to rise in temperature that eventually lowers heat transfer rate from the surface. Further, decrease in heat transfer rate is also observed for enhanced values of thermal radiation parameter. To validate our results, a comparison with already published paper in limiting case is also given and results are found in excellent oncurrence; hence reliable results are being presented.     

Hybrid Isothermal Model for the Ferrohydrodynamic Chemically Reactive Species

A hybrid isothermal model for the homogeneous-heterogeneous reactions in ferrohydrodynamic boundary layer flow is established. The characteristics of Newtonian heating and magnetic dipole in a ferrofluid due to a stretchable surface is analyzed for three chemical species. It is presumed that the isothermal cubic autocatalator kinetic gives the homogeneous reaction and the first order kinetics gives the heterogeneous (surface) reaction. The analysis is carried out for equal diffusion coefficients of all autocatalyst and reactions. Heat flux is examined by incorporating Fourier's law of heat conduction. Characteristics of materialized parameters on the magneto-thermomechanical coupling in the flow of a chemically reactive species are investigated. Further, the heat transfer rate and friction drag are depicted for the ferrohydrodynamic chemically reactive species. It is evident that the Schmidt number has increasing behavior on the rate of heat transfer in the boundary layer. Comparison with available results for specific cases is found an excellent agreement.     

超临界R134a水平圆管内冷却换热模拟研究

采用增强壁面函数的标准k-ε模型对超临界R134a水平圆管内冷却换热进行了模拟研究.分析了管内不同截面上流体温度、速度和湍动能的分布情况及对应关系。研究了质量流量和浮升力对换热系数的影响。结果表明,流体速度随着温度的降低而减小,并且最大速度处对应着最高温度和最小湍动能.换热系数随着质量流量的增加而增大,其峰值出现在准临界温度附近。浮升力在似液体区的影响较大,对流体换热起到增强的效果。     

METHOD AND SOME RESULTS ON EXPERIMENTAL STUDY OF UNSTEADY TURBULENT HEAT TRANSFER

Abstract

An experimental setup is described for studying unsteady heat transfer where both the heat generation rate in the channel wall and the fluid flow rate vary in time. For measuring the surface temperature and the wall heat flux, the temperature field in the thermal sublayer is used. The results of the first experiments carried out for a sudden change in channel wall heat generation are discussed. Unsteadiness rates much greater than those in previous work known to the authors were achieved     

Effects of chemical reaction on combined heat and mass transfer by laminar mixed convection flow from vertical surface with induced magnetic field and radiation

The elevated temperature electromagnetic materials production system in chemical engineering requires increasingly more refined theoretical and computational models for describing multiple, simultaneous thermophysical effects. Motivated by this application, the present paper addresses heat and mass transfer in a chemically reacting laminar mixed convection flow from a vertical sheet with inducedmagnetic field. The governing equations of the flow are solved analytically using a perturbation technique. The influences of various established parameters on the flow, induced magnetic field, and heat and mass transfer are studied graphically in the present analysis. Finally, we also obtained expressions for shear stress, current density and Nusselt number, and discussed the results through tables.     

Perturbation technique for unsteady MHD mixed convection periodic flow, heat and mass transfer in micropolar fluid with chemical reaction in the presence of thermal radiation

An analytical study is presented for the problem of unsteady hydromagnetic heat and mass transfer for a micropolar fluid bounded by semi-infinite vertical permeable plate in the presence of first-order chemical reaction, thermal radiation and heat absorption. A uniform magnetic field acts perpendicularly to the porous surface which absorbs the micropolar fluid with a time-dependent suction velocity. The basic partial differential equations are reduced to a system of nonlinear ordinary differential equations which are solved analytically using perturbation technique. Numerical calculations for the analytical expressions are carried out and the results are shown graphically. The effects of the various dimensionless parameters related to the problem on the velocity, angular velocity, temperature and concentration fields are discussed in detail.     

Temperature prediction for CO2 laser heating of moving glass rods

This paper presents a heat transfer model to calculate the temperature field in moving glass rods heated by a CO₂ laser. Conduction and radiation heat transfer in radial and axial directions are taken into account in the current model. The Rosseland diffusion approximation is incorporated to analyze the radiation heat transfer in the glass rod. A two-band model is used to simulate the spectral property of the glass. Results of the simulation show that glass rods of sufficiently large optical thickness should be treated as a semitransparent medium for radiative transfer, and it is reasonably accurate to assume it to be opaque to CO₂ laser irradiation. It has been shown that the resulting temperature profile is strongly dependent on the laser parameters, i.e., the size of laser beam and the power of the laser. The diameter and speed of the moving glass rod are also important in determining the temperature field although the convective heat transfer coefficient between the glass rod and the environment has little effect.     

天然气催化反应壁面传热传质的数值研究

应用二维数学模型研究天然气催化燃烧壁面反应及载体气固界面上热质传递与流动耦合的现象。通过改变载体通道入口混合气体(天然气与空气)的温度、质量分数及壁面上反应的条件来评价努塞尔数和舍伍德数。模拟结果表明,努塞尔数和舍伍德数并不单独取决于燃料质量分数及壁面条件,而取决于壁面的反应速率。模拟同时表明,进口的温度对通道壁面传质影响最大,其它影响较小。     

Heat transfer by laminar flow of an elastico-viscous liquid along a plane wall with periodic suction

The problem of heat transfer by the laminar flow of an elastico-viscous liquid along a plane wall with periodic suction has been considered. A perturbation technique has been used to obtain an approximate solution of the differential equations. The flow phenomenon has been characterized by the non-dimensional parameters like the elastic number (S), the Reynolds number (R), the Prandtl number (P) and the Eckert number (E). The effects of these parameters on the temperature distributions and the rate of heat transfer at the wall have been studied.     

MHD stagnation point flow towards a stretching sheet

The steady two-dimensional MHD stagnation point flow towards a stretching sheet with variable surface temperature is investigated. The governing system of partial differential equations are transformed into ordinary differential equations, which are then solved numerically using a finite-difference scheme known as the Keller-box method. The effects of the governing parameters on the flow field and heat transfer characteristics are obtained and discussed. It is found that the heat transfer rate at the surface increases with the magnetic parameter when the free stream velocity exceeds the stretching velocity, i.e. ε>1, and the opposite is observed when ε<1.     

Hybrid nanofluid flow over two different geometries with Cattaneo–Christov heat flux model and heat generation: A model with correlation coefficient and probable error

The authors scrutinize the steady, MHD flow of SiO₂−MoS₂/water hybrid nanofluid towards two different geometries i.e. a wedge and a cone. The Tiwari and Das model is implemented with a generalized–Fourier's model, popularized as Cattaneo-Christov heat flux model. Analysis of heat transfer also incorporates the effects of suction, heat generation and thermal radiation. To showcase the relationship between engineering quantities and pertinent parameters involved in the study, the correlation coefficient for heat transfer coefficient and the skin friction coefficient is computed followed by the computation of probable error and statistical declaration. Similarity transformations are utilized to remodel the constitutive laws of flow in non-dimensional form. Numerical computation of non-linear, coupled O.D.E.’s is performed with the support of the Runge-Kutta-Fehlberg scheme and shooting method. Graphical and tabular illustrations of computed results are provided to report the variation in flow properties with the fluctuation in physical parameters. In both cases, i.e. flow close to a wedge and a cone, the temperature of hybrid nanofluid enhances on intensifying the thermal radiation and experiences a decrement with thermal relaxation parameter and magnetic field. Rising values of the suction parameter, thermal relaxation parameter, and thermal radiation cause increment in heat transfer coefficient. Interestingly, it was spotted that the heat generation parameter has contrary effects on temperature distribution over the two geometries.     

流体低速绕流振动圆柱对流换热数值研究

运用Fluent的动态网格技术,对空气低速绕流振动圆柱的对流换热进行了研究,分析了流动和振动参数对换热的影响。数值计算表明,在本文计算范围内,壁面振动可使换热强化,最大可强化9倍,换热的强化随振幅和频率的增大而增大。场协同分析表明,圆柱振动强化换热的原因在于速度场和温度梯度场之间的协同程度得到了改善。     

Numerical investigation on properties of attack angle for opposing jet thermal protection system

The three-dimensional Navier-Stokes equation and the k-ε viscous model are used to simulate the attack angle characteristics of a hemisphere nose-tip with an opposing jet thermal protection system in supersonic flow condition. The numerical method is validated by the relevant experiment. The flow field parameters, aerodynamic forces, and surface heat flux distributions for attack angles of 0°, 2°, 5°, 7°, and 10° are obtained. The detailed numerical results show that the cruise attack angle has a great influence on the flow field parameters, aerodynamic force, and surface heat flux distribution of the supersonic vehicle nose-tip with opposing jet thermal protection system. When the attack angle reaches 10°, the heat flux on the windward generatrix is close to the maximal heat flux on the wall surface of the nose-tip without thermal protection system, thus the thermal protection is failure.     

MHD Maxwell flow modeled by fractional derivatives with chemical reaction and thermal radiation

Heat transfer in a time-dependent flow of incompressible viscoelastic Maxwell fluid induced by a stretching surface has been investigated under the effects of heat radiation and chemical reaction. The magnetic field is applied perpendicular to the direction of flow. Velocity, temperature, and concentration are functions of z and t for the modeled boundary-layer flow problem. To have a hereditary effect, the time-fractional Caputo derivative is incorporated. The pressure gradient is assumed to be zero. The governing equations are non-linear, coupled and Boussinesq approximation is assumed for the formulation of the momentum equation. To solve the derived model numerically, the spatial variables are discretized by employing the finite element method and the Caputo-time derivatives are approximated using finite difference approximations. It reveals that the fractional derivative strengthens the flow field. We also observe that the magnetic field and relaxation time suppress the velocity. The lower Reynolds number enhances the viscosity and thus motion weakens slowly. The velocity initially decreases with increasing unsteadiness parameter δ. Temperature is an increasing function of heat radiation parameter but a decreasing one for the volumetric heat absorption parameter. The increasing value of the chemical reaction parameter decreases concentration. The Prandtl and Schmidt numbers adversely affect the temperature and concentration profiles respectively. The fractional parameter changes completely the velocity profiles. The Maxwell fluids modeled by the fractional differential equations flow faster than the ordinary fluid at small values of the time t but become slower for large values of the time t.     

Numerical Investigation of Heat Transfer Characteristics of scCO2 Flowing in a Vertically-Upward Tube with High Mass Flux

In this work, the heat transfer characteristics of supercritical pressure CO₂ in vertical heating tube with 10 mm inner diameter under high mass flux were investigated by using an SST k-ω turbulent model. The influences of inlet temperature, heat flux, mass flux, buoyancy and flow acceleration on the heat transfer of supercritical pressure CO₂ were discussed. Our results show that the buoyancy and flow acceleration effect based on single phase fluid assumption fail to explain the current simulation results. Here, supercritical pseudo-boiling theory is introduced to deal with heat transfer of scCO₂. scCO₂ is treated to have a heterogeneous structure consisting of vapor-like fluid and liquid-like fluid. A physical model of scCO₂ heat transfer in vertical heating tube was established containing a gas-like layer near the wall and a liquid-like fluid layer. Detailed distribution of thermophysical properties and turbulence in radial direction show that scCO₂ heat transfer is greatly affected by the thickness of gas-like film, thermal properties of gas-like film and turbulent kinetic energy in the near-wall region. Buoyancy parameters Bu < 10⁻⁵, Bu* < 5.6 × 10⁻⁷ and flow acceleration parameter K_v < 3 × 10⁻⁶ in this paper, which indicate that buoyancy effect and flow acceleration effect has no influence on heat transfer of scCO₂ under high mass fluxes. This work successfully explains the heat transfer mechanism of supercritical fluid under high mass flux.     

Prediction of Heat Transfer and Fluid Flow Effects on Entropy Generation in a Monolithic Catalytic Converter Using Large-Eddy Simulation

In the present work, heat transfer and fluid flow and their effects on entropy generation in a realistic catalytic converter of a Lada Niva 21214 vehicle are studied using large eddy simulation. At first, the pressure drop over the catalytic converter is measured for dry air at constant temperature (T=298 K), different volumetric flow rates, and extrapolated to large volumetric flow rates for dry air (T=298 K) and for the exhaust gas under realistic engine conditions (T=900 K) using the Darcy–Forchheimer relation. Then, coupled heat and fluid flow phenomena inside the catalytic converter are analyzed for nonreacting isothermal conditions and nonreacting conditions with conjugate heat transfer by using the large-eddy simulation. The predicted pressure drop agrees well with the measured and extrapolated data. Based on the obtained numerical results, the characteristic flow features are identified, namely: the impinging flow with stagnation, recirculation, flow separation and laminarization within the fine ducts of the monolith, which depends on the heat transfer through temperature-dependent thermophysical properties of exhaust gas. Moreover, due to high-velocity gradients at the wall of the narrow ducts in the monolith, entropy production by viscous dissipation is observed predominantly in the monolith region. In contrast, entropy production due to heat transport is relatively small in the monolith region, while it overwhelms viscous dissipation effects in the pipe regions.     

管内超临界压力水的混和对流换热

本文利用FLUENT6.0软件,数值研究了超临界压力下水在光管内作层流流动和换热特性,着重考察了重力引 起的二次流的影响。研究发现垂直管中由于浮力作用,速度曲线不再是抛物线,而成M状,并在壁面附近出现峰值;重 力作用下,物性的剧烈变化在水平管垂直流动方向引起了很强的二次流,从而影响了阻力特性和换热特性。