Thermal modeling of short pulse collimated radiation in a participating medium”

Abstract

In most traditional engineering applications, such as in the thermal analysis of boilers,furnaces, internal combustion engines, etc., as temporal variations in thermal quantities of interest are much slower than the time scale associated with the propagation of radiation,the transient term from the radiative transfer equation is usually neglected.The rapid progress in the field of short pulse laser in a participating medium has lead to some interesting applications such as materials processing, optical tomography, remote sensing,laser micro-surgery etc.The temporal radiative signals from a medium irradiated by short pulse lasers offer more useful information which reflects the internal structure and properties of the medium than that by the continuous light sources.The time scales of such applications are usually in the order of 10-12 to 10-15 seconds. Therefore, the consideration of the transient term in the radiation transport equation is necessary.To understand the physical phenomena involved in this complicated transport process, two cases are considered such as 1) one dimensional model and 2) a two dimensional model.The models assume a participating medium bounded by diffusely emitting and reflecting boundaries, one of the boundaries is irradiated with a short pulse laser beam.The gray wall assumption resembles more to the practical application as compared to the simplified assumption of black wall. Due to reflective characteristics of the gray boundary surface, the temporal spread changes significantly by the multiple reflections and partial transmissions at the surfaces, this draws attention to the present problem considered.The finite volume method is applied to solve the transient radiative transfer equation governing the above said physical phenomena.The fully implicit scheme is used to iii discretize the transient term.In the proposed approach, one does not have to split the intensity into diffused and collimated part unlike the case with other existing methods
like DTM, DOM, and REM etc.Intensity of radiation can directly be evaluated by solving the governing transient radiative transfer equation.The proposed methodology is compared with other existing methods.The effects of optical
thickness, scattering albedo, emissivity, and linear anisotropic scattering on the transmitted and reflected signals are studied.The performance of two different spatial schemes: STEP and CLAM also have been tested. It is seen that the CLAM scheme yields results to a greater accuracy in case of two dimensional problems and, hence, correctly predicts the speed of photon whereas STEP scheme overpredicts the same.