Numerical investigation of flow and heat transfer in plane channels of variable section


Аuthors

Lushchik V. G.1, Makarova M. S.1*, Medvetskaya N. V.2, Reshmin A. I.1

1. Institute of Mechanics Lomonosov Moscow State University, 1, Michurinsky prospect, Moscow, 119192, Russia
2. Joint Institute for High Temperatures of the Russian Academy of Sciences, 13, Izhorskaya str., Moscow, 125412, Russia

*e-mail: mariia.makarova@gmail.com

Abstract

Numerical simulation of the flow and heat transfer in plane axisymmetric confusor and diffuser with an inclination angle of —5° ÷ —1° and 1° ÷ 5° has been performed using a threeparameter differential model of turbulence, supplemented by the transport equation for turbulent heat flux. A flat channel of variable cross section was selected from the conditions of constant Reynolds number in order to exclude it from the number of variable parameters. It is shown that the restructuring of the flow and the positive pressure gradient in the diffuser lead to turbulization of the flow, and the negative pressure gradient in the confuser leads to laminarization. Confirmation of this is a change in the turbulence energy, which increases significantly in the dif- fuser and leads to intensification of heat transfer, and decreases in the confuser and leads to a decline of heat transfer. In the diffuser and confuser, the flow and heat transfer characteristics differ significantly from the corresponding values in the channels of constant cross section for the same Reynolds number. The study of the influence of the inclination angle in the diffuser and confuser showed that for the same (modulo) angle in the confuser, the average length drag coefficient is significantly higher than in the diffuser, and this excess increases with increasing of the angle. In this case, the average Nusselt number along the length, in contrast to the hydraulic losses, is significantly lower in the confuser than in the diffuser. A numerical study of the influence of the Reynolds number on the flow and heat transfer characteristics in the diffuser showed that with an increase in the Reynolds number, hydraulic losses increase and heat transfer decreases, while in the confuser, on the contrary, hydraulic losses decrease and heat transfer increases. However, this change in the studied range of Reynolds numbers Re = (10—100)×103 is not very significant and does not exceed ±14% for hydraulic losses and ±5% for heat transfer. The study showed that even a slight taper (with angles less than ±4°), which can take place in real channels, significantly affects the characteristics of the flow and heat transfer. It is established that in heat exchangers diffuser-type channels are more preferable in comparison with confuse type channels, since they have lower hydraulic losses and greater heat transfer.

Keywords:

diffuser, confusor, angle of inclination, flow, heat transfer, Reynolds number, differential model of turbulence.

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