Etude de l'intensification des échanges de chaleur par convection dans des canaux de section triangulaire et rectangulaire

Abstract

Most of the research work applied to the intensification of heat transfer through convection in turbulent flow, has been devoted to local heat flow and local friction losses. Intensification is obtained by means of turbulators inside smooth channels (fins, steps). In the re-sticking zone of the boundary layer, the heat transfer is maximum and the friction factor is minimum, the latter being maximum in the free flow zone. Maximum values are more than ten times higher than those observed in an undisturbed analogous stream. As early as 1958, those results have been applied to the construction of heat exchangers by which the heat transfer coefficient rises faster than the hydraulic resistance (discovery No 242). Such heat exchangers (water-air) are built from ribbed fin-plates and cross-tubes. The ribs act as turbulators inside the channels which are allowed between plates and tubes. The present paper deals with the influence of geometrical shape and distribution of the turbulator on thermal and hydraulic efficiencies of heat transfer surfaces. The results are presented as graphs ▪ where: Nu, Nu R = Nusselt number for channels with and respectively, without, turbulators; ε, ε R= friction loss factor for channels with, and respectively, without, turbulators; l'/d = pitch of trottling=ratio between the length of smooth piece l' and its hydraulic diameter d; d∗/d=rate of throttling=ratio between hydraulic diameter of the throttled section d∗, and that of the smooth piece d. Experimentations have been performed with 16 different combinations of l'/d and d∗/d, in channels of the same triangular section and Reynolds number varying between 400 and 5500; in channels with different triangular sections; in channels with rectangular sections. Main results are: the heat transfer intensification depends on the parameters l'/d and d∗/d; the maximum intensification depends on the shape of the channel section, the highest values being obtained with triangular shaped channels; the most efficient turbulators ▪ consist of two dimensioned surfaces generated along the channel radius. New radiators for farm tractor-engines have been developed. Their volume and weight are half those of conventional radiators. Their construction does not require any significant modification of the traditional manufacturing technique of such type of equipment and their utilization in dusty environments does not raise any particular difficulty.