Low Peclet gaseous flows through circular pipe under two different thermal conditions

Several small-scale flow networks involve low Peclet gaseous flows when subjected to heat transfer. Flow phenomena associated with these conditions are characterized by significant axial conduction. Although the available literature is replete with low Peclet flows for liquid metals, the studies for low Peclet gaseous flows are limited. Since gaseous flows also experience considerable changes in their properties during heat transfer, their separate and complete analysis becomes necessary. This work presents numerical solution of the complete forms of transport equations for mass, momentum and total energy. Peclet number (Pe) in the range [1–100] is considered with property variation retained. The solutions are obtained using the academic version of FLUENT-2020-R2 with SIMPLEC algorithm. Both the conditions of constant wall temperature (CWT) and constant wall heat flux (CHF) are used as the prescribed thermal boundaries. The influence of varying degrees of supplied heat at the wall is also analyzed. It is observed that both the CWT and CHF cases experience increasing backward penetration of heat with reduction in Pe and with an increase in the supplied heat. This represents an increase in Nusselt number for CWT cases where fully developed Nusselt number approaches 4.08 corresponding to Pe = 1 for the minimum wall temperature case. With increase in wall temperature, this value rises further to 4.17 for the highest heating rate, as against 3.65 for high Pe laminar flows without considering property variation. For CHF cases, the fully developed Nusselt number remains close to 4.34 for low Pe cases and it drops to 4.27 for the highest Pe of 100. Notably, Nusselt number is found to drop to a minimum before attaining asymptotic value. The radial advection term of the energy equation manifests itself to be primarily responsible for minimum Nusselt number. Thus, the analysis of low Pe gaseous flows requires consideration of radial advection term and property variation. These unreported results are significant inasmuch as the design of small-scale heat exchangers need to be optimal under space constraints.