Thermal Radiation in Microchannels

The dip in the temperature at the reactor end is observed because the wall loses energy by radiation to the surroundings. In this case, a thermal reservoir for radiative heat transport at the reactor end was assumed to be at ambient (low) temperature. On the other hand, it is common to use a thermal reservoir temperature 

Boehman [15] made similar observations in his work comparing channels with different aspect ratios and wall emissivities. As the aspect ratio decreases, the effect of radiation increases. For microreactors with smaller aspect ratio, the radiation flux can be computed vhth reasonable accuracy employing a blackbody assumption for the reactor wall. This assumption reduces the computational complexity in obtaining the radiation flux the net radiation equation for gray diffuse emitters involves solving an integro-differential equation, which can be avoided with the blackbody radiation flux assumption [15]. Fu et al. [16] reported that the radiation effect becomes significant 

An alternative to the above modeling approach is to simulate thermal radiation exchange using a conservative variant of the discrete ordinates (DO) radiation model, called the finite-volume (FV) scheme, implemented in the Fluent software package. 

The DO-FV model [ 17] solves the radiative transfer equation (RTF) as a field equation for a finite number of discrete solid angles each associated with a vector direction s fixed in the global Cartesian system. The procedure involves the solution of as many transport equations as there are solid angles. 

In conclusion, radiation effects can be neglected for large aspect ratios typical of microreactors. Blackbody radiation may be used to approximate radiation effects with reduced computational complexity of surface-to-surface radiation. However, for an aspect ratio of 10 or lower, blackbody radiation gives the upper limit of the radiation effect and the net radiation equation needs to be solved to obtain the radiation heat transfer. Heat transfer along the wall is the most critical heat transfer mechanism and needs to be accounted for. 

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