Heat transfer characteristics of a cylindrical porous radiant air heater under the influence of a two-dimensional axisymmetric radiative field

Abstract

This work presents the heat transfer characteristics of a new type of porous radiant air heater (PRAH) that operates on the basis of an effective energy conversion method between flowing gas enthalpy and thermal radiation. In this system, four porous layers consisting of porous radiant burner (PRB), high-temperature (HT) section, first heat recovery section, and second heat recovery section are considered. These layers are separated from each other by three quartz glass walls. The PRB generates a large amount of thermal radiative energy, which is converted into gas enthalpy in heat recovery layers. In the HT section, the gas enthalpy of the exhaust flow from PRB converts into thermal radiation. At each layer, the gas and solid phases are considered in non-local thermal equilibrium and combustion in the PRB is modelled by considering a non-uniform heat generation zone. The homogeneous porous media, in addition to its convective heat exchange with the gas, may absorb, emit, and scatter thermal radiation. In order to determine the thermal characteristics of the proposed PRAH, a two-dimensional model is used to solve the governing equations for porous medium and gas flow and the discrete ordinates method is used to obtain the distribution of radiative heat flux in the porous media. Computational results show that this type of air heater has much higher efficiency than the conventional ones. In addition, it is clarified that the high optical thickness and low scattering coefficient for porous layers improve the heater efficiency. In order to validate the applied numerical method at the present analysis, the results are compared with some reported theoretical and experimental results by other investigators and good agreement is found.for this purpose. This is due to the extremely high heat transfer coefficient and large heat transfer surface area for the gas flow through porous matrix. For the first time, Echigo [1] studied the problem of recovering energy from hot exhaust gases by placing a porous layer normal to the gas flow in exhaust ducts. In that study, the solid matrix was assumed not to scatter radiant energy. His analysis revealed that a substantial amount of gas enthalpy might be recovered to radiative energy travelling upstream from the porous segment. Wang and Tien [2] extended Echigo's analysis using the two-flux radiation model and the scattering effect of the porous layer was considered in their computations. Another theoretical study on heat recovery (HR)

Investigation of transient conduction–radiation heat transfer in a square cavity using combination of LBM and FVM

“…He used DOM to solve the radiative transport equation in a 2D square cavity with low temperature boundary condition containing isothermal absorbing-emitting medium. Finite-volume method (FVM) is widely employed for calculating the radiative quantities in radiative heat transfer problems [4,[13][14][15][16][17]. The computational grids in FVM for the radiative heat transfer are correspondent with the FVM grids that are utilized in the solution of the momentum and energy equations [18].…”

Numerical simulation for the determination of the radiative properties of spherical packed bed porous media: A COMSOL Multiphysics Study

“…12,[25][26][27] With the significant progress in computational tools over the last decades, numerical methods have been developed for the prediction of the radiative properties of porous media. Many numerical simulations have been performed for spherical packed bed porous media by using different methods, alone or combined, 28 such as Monte Carlo ray-tracing (MCRT), 29,30 the radiative distribution function identification (RDFI), 31 discrete ordinate, 32 two-flux methods, 24 Finite Volume Method (FVM), 33,34 Finite Element Method (FEM), 35 the Mie theory and the discrete dipole approximation (DDA) 36 etc. Coquard and Baillis suggested a Monte Carlo method for the determination of the radiative characteristics of opaque and spherical, 28 and later, absorbing and scattering particles beds.…”

Discrete vs. continuum-scale simulation of radiative transfer in semitransparent two-phase media

“…Examples include high-temperature chemical reactions in packed beds [1][2][3], porous radiant burners [4][5][6], combustion furnaces for solid feedstock [7], high-flux solar receivers [8][9][10], and porous heat exchangers [11]. Traditionally, radiative transfer in such media has been modeled using the equation of radiative transfer with appropriate average radiative properties [12,13].…”