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

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.