No abstract

Thermal radiation greatly affects the transient thermal response of translucent materials in many practical applications, such as radiative heat shields and ignition and flame spreads for translucent plastics. However, because of the complexities that transients impose, less work has been done on the transient analysis of combined radiation-conduction heat transfer than on steady-state analysis. In this study, the transient heat transfer analysis of a polycarbonate (PC) layer was done with the use of the two-flux method and implicit finite difference formulations. The radiative and conductive properties of PC available in the literature, together with computer implementation prepared on the basis of the two-flux method and implicit finite difference formulations, were used to obtain the transient thermal response of a PC layer. On the basis of these results, we show that, compared to the conduction-alone case, the PC layer responded faster when radiation effects were considered. It is also shown that the internal reflectivity of boundaries had a great effect on the thermal response of the layer, whereas the thermal conductivity had a minor influence.

No abstract

The applicability and usefulness of combustion in porous media is of much interest due to its competitive combustion efficiency and lower pollutants formation. In the previous works, the focus has been on the effects of combustion and heat transfer parameters such as excess air ratio, thermal power, solid conductivity, convective heat transfer coefficient, and radiation properties on centerline temperature and pollutant formations. A premixed combustion scheme and a fixed porous medium with constant geometrical parameters have been used in these works; therefore, the effects of porous material parameters have been less considered. In this research, the effects of geometrical parameters of porous medium, namely porosity and permeability, on centerline temperature distributions, peak flame temperature, flame structure, and gas mixture preheating have been investigated by numerical methods. To this, a two-dimensional axis-symmetric physical model of porous burner is considered. As the most typical porous burners, a two stage one which has preheating porous zone (PPZ) and combustion porous zone (CPZ) is studied. The continuity, momentum, energy, turbulence, and species transport equations are solved employing a one-step chemical reaction mechanism with an eddy-dissipation model for rate of reactions. The turbulence is modeled with two transport equations which are not considered in similar works. The combustion regime is assumed to be diffusion and combustion parameters are fixed in all cases. Porosity effects on the structure and temperature characteristic of the flame are probed in a wide range for PPZ and CPZ. Critical permeability is defined and permeability effects on flame characters in both of the preheating and combustion regions are studied thoroughly.

Polymer films possess excellent optical properties, such as high transparency, and thermal characteristics, like low heat conductivity, as well as further polymer specific advantages. Consequently, polymer films have an outstanding potential for many solar applications. They are already used for encapsulation of photovoltaic (PV) cells, as convection barrier in solar collectors and as substrate or adhesive layers for glazing. In translucent polymers, energy can be transferred internally by radiation in addition to conduction. Since radiant propagation is very rapid, it can provide energy within the layer more quickly than diffusion by heat conduction. Thus, the transient thermal response of a layer for combined radiative and conduction may be extremely different from that of conduction alone. In this paper, the behavior of a heat conducting, absorbing, and emitting layer of Polypropylene is investigated during the transient interval when both conductive and radiative heat transfer are considered. The governing differential equations include the equation of radiative heat transfer within the material coupled to the transient energy equation, which contains both radiative and conductive terms. The solution procedure is based on nodal analysis and Hottel’s zonal method extended by the ray tracing method. The transient energy equation including the radiative internal energy source is solved using a time marching finite difference procedure with variable space and time increments. In addition, effects of variable parameters including optical thickness and thermal conductivity of the layer are investigated carefully.