On the effect of the slot height in the cooling of a circular cylinder with a rectangular jet

The two main problems in the cooling of a cylinder with a rectangular jet of the same width are the choice of the jet height and the distance between the jet exit and the cylinder. The present paper presents experimental measurements of the heat transfer on a cylinder, with diameter D = 10 mm, cooled by rectangular jets with low turbulence, the same width of the cylinder but different height, H, 2.5 and 5 mm, respectively, in order to study their influence on the local and the mean cooling rates. The cylinder to be cooled is heated by electric current and is set on the symmetrical plane of symmetry, i.e. the geometry is two-dimensional, at several distances from the jet exit, x, in order to find the position which realizes the maximum heat transfer. The experimental measurements of the local heat transfer are performed along the circumference of the cylinder at several angles from the impingement for Reynolds number, defined with the cylinder diameter, D, and the average velocity, in the range ReD = 5000–22,000. The comparison between the two slots is performed at the same Reynolds number, ReD, which means that the smaller slot has a slightly greater maximum velocity. The experiments show that on the impinging point the local Nusselt number is greater for the slot with H = 2.5 mm, while at greater angles from the impingement, i.e. 45°, 90°, 135° and 180°, the local Nusselt numbers have different behaviors. The mean Nusselt numbers are similar for the two slots, at the same Reynolds number, ReD. Local and mean Nusselt numbers, obtained with the two jets, are in qualitative agreement if they are compared with the dimensionless distance, x/H, which can be interpreted as the ratio of the Reynolds numbers, Rex/ReH, independent of the mass flow rate or per unit of mass flow rate. The maximum of the local Nusselt number on the impinging point is obtained at greater distances for smaller Reynolds numbers, e.g. in the range from x/H = 15 at Re = 5000 to x/H = 9 at Re = 22,000, while the distance is shorter along the cylinder at greater angles from the impingement. The maximum of the mean Nusselt number is similar to that on the impingement for the two jets. The experimental heat transfer is also examined from the point of view of the slot efficiency, i.e. taking into account the mechanical power necessary to move the fluid in the slot jet. The conclusion is that the smaller slot, H = 2.5 mm or D/H = 4, has a greater efficiency as cooling system because it removes more heat at the same mechanical power