Radiation Convection Heat Transfer Research Articles

Entropy generation analysis of MHD nanofluid flow in a porous vertical microchannel with nonlinear thermal radiation, slip flow and convective-radiative boundary conditions

The heat transfer and entropy generation in a magnetohydrodynamic flow of Al2O3–water nanofluid through a porous vertical microchannel with nonlinear radiative heat flux were investigated numerically. Then, combined effects of nanoparticle volume fraction, hydrodynamic slip, magnetic field, suction/injection and thermal radiation on heat transfer and entropy generation were studied. The dimensionless governing equations were solved numerically by applying Runge–Kutta integration method together with shooting technique. In this study, the accuracy of the numerical results was verified by comparing its predictions with exact solutions of model without both radiation effects and buoyancy force. Here, different from previous literature, heat transfer subject to nonlinear thermal radiation, Joule heating and viscous dissipation was solved and analyzed using conjugate convective-radiative heat transfer on the boundary surfaces. Moreover, influences of pertinent parameters on nanofluid velocity, temperature, local and global entropy generation and Nusselt number were discussed in detail and illustrated graphically. Based on the numerical results, it was proved that the global entropy generation decreased with both nanoparticle volume fraction and suction/injection Reynolds number while it increased with Grashof number (Buoyancy force intensity), radiation parameter and conduction-radiation parameter. In addition, it was possible to determine optimum values of slip flow with minimum values of global entropy generation rate. The Nusselt number was also calculated and explored for different conditions. In this way, optimum values of Grashof number with maximum heat transfer on the heated left plate were derived.

Immersed Boundary Method for Radiative Heat Transfer Problems in Nongray Media With Complex Internal and External Boundaries

The immersed boundary method (IBM) is gaining attention in the computational fluid dynamics but its applications in the field of a conjugated radiative–conductive or radiative–convective heat transfer seem limited. Therefore, the paper presents extension of this method to heat transfer problems dominated by thermal radiation in a nongray medium. The present model enables simulation of heat and fluid flows in a domain with complex stationary or moving internal and external boundaries on a fixed Cartesian grid (FCG) by applying the finite volume method. The special attention is paid to modeling thermal radiation and optical phenomena at highly curved, opaque, or transparent boundaries which confine the computational domain or separate zones of different thermophysical and optical properties, e.g., different values of a refractive index. The model is limited to a 2D planar or axisymmetric spaces. Detailed verification procedure proves accuracy and correctness of the developed model and shows its potential application field. The model may be used for simulations of a conjugated radiative–conductive or radiative–convective heat transfer in a nongray medium in a complex domain with opaque or transparent curved internal or external boundaries without unstructured or body fitted mesh generation.

Critical and optimal thicknesses of thermal insulation in radiative-convective heat transfer

Heat transfer through a thermal insulation layer in radiative-convective heat transfer on its nonconcave surface is considered. Combinations of determining parameters at which the heat flux passing through this layer reaches an extremal absolute value (a maximum corresponds to a critical thickness of thermal insulation, and, a minimum, to an optimal). The qualitative analysis of the dependence of the heat flux on the determining parameters makes it possible to choose the thermal characteristics of the insulation and its thickness, providing the desired reduction in the intensity of heat transfer through the insulation with a nonconcave surface of double curvature in conditions of radiative-convective heat transfer.

Convection–radiation heat transfer in solar heat exchangers filled with a porous medium: Exact and shooting homotopy analysis solution

The case of combined conduction–convection–radiation heat transfer usually occurred in solar thermal usages is the aim of the present study. In this work, the type of combined heat transfer in heat exchangers filled with a fluid saturated cellular porous medium is revisited. It is shown the governing strongly nonlinear differential equation, which has been formulated by performing Darcy–Brinkman equation is exactly solvable. A full discussion and exact analytical solution (Dimensionless temperature distribution) in the implicit form is given for further physical interpretation and it is proved the solution is mathematically unique. The uniqueness of solution is also confirmed by a so-called shooting homotopy analysis method. Furthermore, Effects of porous medium shape parameter (s) and radiation parameters (Tr and λ) on the thermal performance are exactly analyzed.

Convection–radiation heat transfer study of moving fin with temperature-dependent thermal conductivity, heat transfer coefficient and heat generation

In this article, the simultaneous convection–radiation heat transfer through a moving fin with heat generation is studied. The heat transfer coefficient, thermal conductivity and heat generation are variable and supposed to be temperature dependent. The heat transfer in fin with longitudinal rectangular profile was carried out by using the Differential Transformation Method (DTM). The results indicated that the fin tip temperature increases with an increase in the heat generation gradient and a decrease in the Peclet number and the radiation–conduction parameter. Also, the optimal temperature distribution for a longitudinal rectangular fin is accomplished at stationary fin with heat generation-without radiation case. Furthermore, the limiting cases are obtained and found to be in good agreement with the numerical results.

Study of coupled transient radiation-natural convection heat transfer across rectangular cavities in the vicinity of low emissivity thin films for innovative building envelope applications

The coupled natural convection-radiation heat transfer across cavities is an extremely important research issue commonly encountered in many innovative building envelope applications. Extensive both experimental and numerical studies were conducted to analyse the natural convection flow coupled with radiation heat transfer across vertical and inclined rectangular cavities, proposing accurate empirical or regression correlations according to the particularities of the studied cavity problem. Nevertheless, there is a lack of correlations accurate and adapted enough to model coupled natural convection-radiation across cavities in the vicinity of low emissivity surfaces. The present work aims to cover this subject, through an exhaustive numerical analysis validated and evaluated enough with the use of experimental data. A numerical model is firstly updated to simulate infrared radiation and convective heat transfer across a sophisticated innovative building envelope prototype, while 8 different approximations have been implemented in the model in order to evaluate a variety of available correlations on the subject. Formerly the numerical approach presented here used available experimental data for the same prototype. The air’s temperature experimental data served as input to the model boundaries in order to exactly impose the same initial conditions for obtaining the results. Furthermore, experimental data regarding the temperature on the sample’s interfaces were used for validating the proposed theoretical model. Satisfactory enough agreements are observed between the theoretically simulated and experimentally measured temperatures and the model is afterwards evaluated to determine the most accurate approximation. Finally, the numerical study presented in this paper concluded in the proposition of 2 new updated correlations to model natural convection across vertical cavities in the vicinity of low emissivity surfaces with the perspective to further support their implementation into Computational Fluid Dynamic (CFD) codes.

Inverse Boundary Design Problem of Combined Radiation-convection Heat Transfer in Laminar Recess Flow

In the present work, an inverse analysis of combined radiation and laminar forced convection heat transfer in a two-dimensional channel with variable cross sections is performed. The conjugate gradient method is used to find the temperature distribution over the heater surface to satisfy the prescribed temperature and heat flux distributions over the design surface. The fluid is considered to be a gray participating medium with absorption, emission and isotropic scattering. The discrete ordinate method is used to solve the radiative transfer equation. The effect of radiation-conduction parameter is studied on the amount of heat transfer from the heater surface.

Heat transfer in concentrated solar air-heaters filled with a porous medium with radiation effects: A perturbation solution

In this study, convection-radiation heat transfer in solar air-heater filled with a porous medium is investigated. The effects of porous media shape parameter (s), Forchheimer number (F) and radiation parameter (ζ) on the collector efficiency are discussed. Approximate analytical solutions for the dimensionless velocity, temperature, and Nusselt number are obtained using perturbation method. The radiation heat transfer has been modeled using the Rosseland approximation method. A discussion on the accuracy and limiting cases of the solution is represented and a good agreement has been found between the analytical solutions and previous works. The results showed that by increasing the radiation parameter, the efficiency of the collector was improved. Also, the collector heat removal factor enhances when the porous media shape parameter increases.

Analysis of Combined Radiation and Forced Convection Heat Transfer in 3D Laminar Flow over an Inclined Forward Facing Step

In the current study, a numerical investigation of three-dimensional combined convection-radiation heat transfer over an inclined forward facing step (FFS) in a horizontal rectangular duct is presented. The fluid is treated as a gray, absorbing, emitting and scattering medium. To simulate the incline surface of FFS, the blocked-off method is employed in this study. The set of governing equations for gas flow are solved numerically using the CFD technique to obtain the temperature and velocity fields. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation heat transfer mechanisms are presented in the energy equation. For computation of radiative term in energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinates method (DOM) to find the divergence of radiative heat flux distribution inside the radiating medium. The effects of optical thickness, radiation-conduction parameter and albedo coefficient on heat transfer behavior of the system are carried out.

Method of calculating heat transfer in furnaces of small power

This publication presents the experiences and results of generalization criterion equation of importance in the analysis of the processes of heat transfer and thermal calculations of low-power heat generators cooled combustion chambers. With generalizing depending estimated contribution of radiation and convective heat transfer component in the complex for the combustion chambers of small capacity boilers. Determined qualitative and quantitative dependence of the integrated radiative-convective heat transfer from the main factors working combustion chambers of small volume.

Heat transfer enhancement and pressure drop penalty in porous solar heaters: Numerical simulations

A comprehensive numerical study is performed on the fluid flow and heat transfer within a porous solar heater. The effects of porous material on the heat transfer enhancement and pressure drop are presented in details. Also, the attention is focused on the effects of several parameters on the combined convection–radiation heat transfer and flow structures. Volume averaged equations are applied to simulate the transport phenomena within the porous substrate. Furthermore, the regular continuity, momentum, and energy equations are used in the clear fluid region. These equations are discretized using the control volume technique. It is found that the Nusselt number increases by inserting the porous substrate to the heater. These augmentations are up to 3, 4.4 and 5.9 times for δ=1/3, 2/3 and 1, respectively at Da=10-2. Also, the pressure drop increases with an increase in the porous layer thickness and decrease in the Darcy number.

Thermal Analysis of Irradiated Fuel Subassemblies and Fuel Pins During Storage in Concrete Pits of Head-End Facility

Irradiated fuel subassembly (SA)/fuel pins, with significant decay heat are transported from reactor and stored in hot cells (HCs) before reprocessing. During transportation they are heavily shielded and no forced cooling is provided. The HCs are made of concrete structures, the outer surfaces of which are force cooled. During these processes, the fuel pin clad temperature and concrete temperatures are to be limited within specific safety limits. These temperatures are function of the decay power and geometric details of surrounding structures. To predict these temperatures, three-dimensional conjugate conduction–convection–radiation heat transfer analysis has been carried out. For this purpose, the computational fluid dynamics (CFD) code STAR-CD has been utilized, wherein individual fuel pins, steel cans, hexagonal wrapper, lead shielding blocks, and concrete structures have been considered in detail. Based on parametric studies pertaining to fuel pin transportation, it is established that for a decay power of 150 W, natural convection is adequate with maximum clad temperature of 686 K. From the studies related to storage in HCs, it is seen that nine fast breeder test reactor (FBTR) SA can stored in hot cell-1 (HC-1), with a decay power of 31.3 W per SA, to respect the temperature limits. For 3 prototype fast breeder reactor (PFBR) cans and 2 FBTR cans stored in hot cell-3 (HC-3), a decay power of 12.5 W per FBTR can and 44 W per PFBR can, can be handled without exceeding temperature limits.

Perturbation analysis of magnetohydrodynamics oscillatory flow on convective-radiative heat and mass transfer of micropolar fluid in a porous medium with chemical reaction

This paper deals with the perturbation analysis of mixed convection heat and mass transfer of an oscillatory viscous electrically conducting micropolar fluid over an infinite moving permeable plate embedded in a saturated porous medium in the presence of transverse magnetic field. Analytical solutions are obtained for the governing basic equations. The effects of permeability, chemical reaction, viscous dissipation, magnetic field parameter and thermal radiation on the velocity distribution, micro-rotation, skin friction and wall couple stress coefficients are analyzed in detail. The results indicate that the effect of increasing the chemical reaction has a tendency to decrease the skin friction coefficient at the wall, while opposite trend is seen by increasing the permeability parameter of the porous medium. Also micro-rotational velocity distribution increases with an increase in the magnetic field parameter.

Numerical modelling and analysing of conjugate radiation- convective heat transfer of fin-tube radiator of spacecraft

This paper covered the problem of assessing the effectiveness of the section of the fin-tube radiator of space thermal control system. The task of calculating the conjugate radiation-convective heat transfer is presented. The results of numerical simulation are described.

Thermal diffusivity measurements in solids by photothermal infrared radiometry: Influence of convection–radiation heat losses

This work demonstrates that in photothermal experiments performed in frequency domain the heat losses due to convection and radiation should be taken into account at low frequencies for poor heat conductors. From a model-solution of the heat diffusion equation a dimensionless frequency dependent parameter M = ZH, with sample's thermal impedance Z and H the convection-radiation heat transfer coefficient, turns out to adequately quantify the importance of the effect of those heat losses. A straightforward photothermal infrared radiometry setup was designed to demonstrate the above hypothesis. Disc shaped samples of different test materials were heated by square wave modulated illumination at one of their surfaces at different frequencies using an amplitude modulated laser beam, and the temperature at the rear surfaces was monitored as a function of time using an infrared sensor. The frequency dependence of peak-to-peak values of the temperature signals was found to be consistent with the amplitude spectrum obtained by Fourier transforming the data. The frequency dependence of the peak-to-peak amplitude was compared with a theoretical model with and without taking convection and radiation induced heat losses (CRHL) into consideration. It is found that for poor heat conductors at low modulation frequencies the conventional model without CRHL does not fit well the experimental data, while the extended model leads to good agreement, resulting in reliable values for the thermal diffusivity. For the investigated samples, the contribution to the signal of thermal wave reflection at the back side of the sample turn out to have a minor effect on the signal spectrum.

Heat transfer enhancement and pressure drop penalty in porous solar heat exchangers: A sensitivity analysis

Porous materials have a positive effect upon the heat transfer enhancement and a negative impact on pressure drop. Combined convection–radiation heat transfer inside a porous solar heat exchanger with a sensitivity analysis is performed to calculate the effects of porous material on the heat transfer rate and the pressure drop. Reynolds and Darcy numbers and porous substrate thicknesses are selected as the influence parameters. The analysis is carried out by using Response Surface Methodology (RSM). Also, the input parameters for optimization process are obtained by numerical methods. The computational simulations are done for different Reynolds numbers (1⩽Re⩽100), Darcy number (10−6⩽Da⩽10−2) and dimensionless porous substrate thickness (1/3⩽δ⩽1). It is found that the reductions in pressure drop ratio with increasing Darcy number are in the vicinity of 58% and 23% for δ=1/3 and 1, respectively and Da=10−6–10−2. Note that above values are obtained at Re=100. However, the augmentation in Nusselt number with increasing the porous substrate thicknesses is in the vicinity of 96% for δ=1/3–1 and Da=10−2. Also, the maximum errors between the RSM and CFD results are in the vicinity of 3% and 1.37% for the Nusselt number and pressure drop ratio, respectively.

Combined Conduction–Convection–Radiation Heat Transfer of Slip Flow Inside a Micro-Channel Filled with a Porous Material

Combined conduction–convection–radiation heat transfer is investigated numerically in a micro-channel filled with a saturated cellular porous medium, with the channel walls held at a constant heat flux. Invoking the velocity slip and temperature jump, the thermal behaviour of the porous–fluid system are studied by considering hydrodynamically fully developed flow and applying the Darcy–Brinkman flow model. One energy equation model based on the local thermal equilibrium condition is adopted to evaluate the temperature field within the porous medium. Combined conduction and radiation heat transfer is treated as an effective conduction process with a temperature-dependent effective thermal conductivity. Results are reported in terms of the average Nusselt number and dimensionless temperature distribution, as a function of velocity slip coefficient, temperature jump coefficient, porous medium shape parameter and radiation parameters. Results show that increasing the radiation parameter $$(T_{r})$$ and the temperature jump coefficient flattens the dimensionless temperature profile. The Nusselt numbers are more sensitive to the variation in the temperature jump coefficient rather than to the velocity slip coefficient. Such that for high porous medium shape parameter, the Nusselt number is found to be independent of velocity slip. Furthermore, it is found that as the temperature jump coefficient increases, the Nusselt number decrease. In addition, for high temperature jump coefficients, the Nusselt number is found to be insensitive to the radiation parameters and porous medium shape parameter. It is also concluded that compared with the conventional macro-channels, wherein using a porous material enhances the rate of heat transfer (up to about 40 % compared to the clear channel), insertion of a porous material inside a micro-channel in slip regime does not effectively enhance the rate of heat transfer that is about 2 %.

Numerical analysis of 3D regimes of natural convection and surface radiation in a differentially heated enclosure

Numerical analysis of 3D regimes of convection-radiation heat transfer in cubic enclosure with two isothermal faces and adiabatic walls is performed. The mathematical model is constructed in dimensionless variables “vector potential-vorticity vector-temperature” in the Boussinesq approximation and with regard to diathermal medium filling the enclosure. 3D temperature and velocity fields, medium motion trajectories in a wide range of key parameters are obtained. Correlations for the integral heat exchange coefficient as a function of the key process characteristics are found.

A decoupled procedure for convection-radiation simulation in scramjets

Following an order analysis of key parameters, a decoupled procedure for simulation of convection-radiation heat transfer problems in supersonic combustion ramjet (scramjet) engine was developed. The radiation module of the procedure consisted of Perry 5GG weighted sum gray gases model for spectral property calculation and discrete ordinates method S4 scheme for radiative transfer computation, while the flow field was computed using the Favre average conservative Navier-Stokes (N-S) equations, in conjunction with Menter’s k-ω SST two-equation model. A series of 2D supersonic nonreactive turbulent channel flows of radiative participants with selective parameters were simulated for validation purpose. Radiative characteristics in DLR hydrogen fueled and NASA SCHOLAR ethylene fueled scramjets were numerically studied using the developed procedure. The results indicated that the variations of spatial distributions of the radiative source and total absorption coefficient are highly consistent with those of the temperature and radiative participants, while the spatial distribution of the incident radiation spreads wider. It also demonstrated that the convective heating is significantly affected by the complexity of the flow field, such as the shock wave/boundary layer interactions, while the radiative heating is simply an integral effect of the whole flow field. Although the radiative heating in the combustion chambers reaches a certain level, an order of magnitude of 10 kW/m2, it still contributes little to the total heat transfer (<7%).

Influence of Variable Viscosity on Hydromagnetic Non-Darcy Convective-Radiative Heat Transfer Over a Stretching Sheet with Non-Uniform Heat Source/Sink

In this article, the effects of variable temperature-dependent viscosity and thermal conductivity on non-Darcy mixed convection over a stretching sheet embedded in a porous medium in the presence of thermal radiation, non-uniform heat source/sink, and Ohmic dissipation due to transverse magnetic field and electric field are investigated. Comparison of the present numerical results of Nusselt number are found to be in excellent agreement with the earlier published results under some limiting cases.The effects of non-uniform heat source/sink, porosity, Hartmann number, and viscosity parameter are analyzed on velocity and temperature profiles. We see that the fluid velocity increases due to an increase in the porosity parameter, and the effect of increasing the Hartman number increases the temperature profiles for both PST and PHF cases. Another important finding is that the non-uniform temperature-dependent heat source/sink parameters lead to an increase in the temperature profile for both PST and PHF cases.