Numerical investigation of local entropy production rate of a finned oval tube with vortex generators
Résumé
The present paper analyzes the entropy production rate induced by longitudinal vortex generators punched on the fins of a compact heat exchanger. The flow is assumed to be three-dimensional, unsteady and laminar. The entropy production rate due to heat fluxes and drag forces
is studied by means of finite volume methodology. The heat conduction in the fin is also taken into account and hence the induced thermal entropy production is also evaluated in the fin. The effects of the fin efficiency (factor Fi) and of the angle β of the vortex generator are evaluated for the volumetric entropy production rate in the fin and in the flow field. The integration over the whole domain enables to define the entropy production number NS1. The relevance of this exchanger global criterion is illustrated for a specific technical requirement. The results show that the volumetric entropy production rate is predominant in the strong flow gradient zones. These zones correspond to the outer surface of the longitudinal vortex and to the thinning zone of the boundary layer (down-wash zone). This entropy production increases with Fi and β. In the fin the entropy production decreases with Fi. It is shown that the Bejan number (Be) is close to unity, which means that the global entropy production rate due to heat transfer is predominant in the flow field. The effectiveness of the heat exchanger increases with Fi and β, which corresponds to an increase of the Number of Transfer Unit NTU, but a decrease of NS1.