Radiation heat transfer in space based sensors: a thermal analysis of spectrometer

Abstract

Optical spectrometers have been of interest in remote sensing because of their ability to decipher an image scene based on its spectrum. Spectrometer generally consists of fore optics, glass window, cold shield, order sorting filters (OSF), detector arrays and cry cooler. Since weight constraints are of prime importance in payload design, optimum size of cryo-cooler must be known. The cooling load depends on the ability of assembly’s parts to absorb, transmit and radiate energy emanating from distant scene and incident on spectrometer’s aperture. The experimentation with small scale components is particularly expensive and requires sophisticated measuring and calibrating instruments. So there is a need to develop a computational study which can closely speculate the thermal parameters of assembly and their interdependency. A computational study has been carried out for the Dewar cooling assembly using Ray optics and Heat transfer modules of COMSOL Multiphysics in the spectral range of 800 to 5000 nm. It is essential to maintain the detector array at low temperatures (~ 90 K) to achieve desired signal to noise ratio. This study analyses the effect of OSF design on temperature and total heat flux on detector array. Ray tracing coupled with heat conduction and surface to surface radiation have been modelled separately to calculate resultant incident flux on detector array. The propagation of rays inside Dewar assembly is modelled using Snell’s law, Fresnel equations and eikonal equations while Radiative Transfer Equation (RTE) is used for solving surface to surface radiation heat transfer. Semi-transparent media such as glass window and OSF having avelength dependent refractive indices different than surrounding medium also take part in radiation. This phenomenon has been modelled using radiation in participating media interface by Discrete Ordinate Method (DOM). Rays undergo attenuation in semi-transparent media and heats up the domain which acts as a heat source. It was found that the temperature of OSF and OSF mounting plays an important role in determining the temperature of detector array. A coating as a dielectric thin film for order sorting of wavelength has been designed which acts as a band-pass filter in the wavelength range of 800 to 5000 nm. I expect that these simulations, once validated with experiments, lead to a better understanding of the trade-offs involved in the operation of optical spectrometer for remote sensing application.