Investigations on Minichannel Gas Cooling
Axial pressure profiles and temperature profiles numbers have been measured with a heatable annular minichannel with a hydraulic diameter of 1.23mm and a channel length of 94mm. The measured data has been reduced to local friction factors and Nusselt numbers, covering 1500 < Re < 14000 and 0.0005 < q+ < 0.004 . It was found, that the friction factors for the adiabatic experiments matched very well the correlation of Chen, taking in account the measured channel surface roughness. For the heated cases, a pronounced effect of the heating intensity q+ (a nondimensional quantity proportional to the temperature increment of the gas) was proven, where higher heating rates effected a deterioration of the heat transfer, especially in the transition range 6000 < Re < 10000 of this configuration. It could be shown, that this is due to the increased gas viscosity near the wall, while acceleration effects do not yet play a significant role. Influence of the pressure (0.1-0.4MPa) and the gas species (N2, He, Ar) were not evident.
|Plots of the local Fanning friction factor and local Nusselt number at two Reynolds numbers, measured in the ITHEX annular duct minichannel experiment. A clear effect of laminarization is visible for high heating rates q+