EXPERIMENTAL AND NUMERICAL HEAT TRANSFER ON A CYLINDER COOLED BY TWO RECTANGULAR JETS OF DIFFERENT HEIGHTS

The present paper presents experimental measurements of heat transfer on a cylinder, cooled by two rectangular jets of the same width but different height, H, in order to investigate the influence of the jet height on the local and the average cooling rates, because one of the problems in the heat transfer with jet flow is the convenience to choose the height of the jet in comparison to the height of the impinged object. The cylinder, heated by electric current, is set at several distances from the jet exit, x, on the symmetrical plane of symmetry, i.e. in a two-dimensional geometry, in order to find the optimal position which realizes the maximum heat transfer. The experimental heat transfer on the impingement shows that the local Nusselt number, defined with the cylinder diameter, D, is greater for the smaller slot because velocity is slightly higher but the average Nusselt numbers, defined with the cylinder diameter, D, are quite similar because the higher slot has a greater surface of impingement. Local and average Nusselt numbers are in qualitative agreement only if are compared with the same "dimensionless distance, x/H, which can be interpreted as the ratio Re-x/Re-H, which is per unit of mass flow rate or is independent of the mass flow rate. Numerical simulations are carried out with a two-equations turbulent model using the RNG k-epsilon approach, on a cylinder with the same thickness of the experiments or without thickness. The numerical simulations of the cylinder without thickness are in acceptable agreement for what concern the average Nusselt numbers. The local Nusselt numbers are in fair agreement only if the cylinder has the thickness of the experiments, independently of the heat flow conditions, i.e. uniformly throughout the thickness or from the inner surface.