HYDROSTATIC CORRECTION AND PRESSURE-DROP MEASUREMENT IN MIXED CONVECTION HEAT-TRANSFER IN A VERTICAL TUBE

In vertical, internal, mixed convection flows at high Grashof numbers and low Reynolds numbers, large radial and axial temperature profiles result in property variations which can lead to significant error in the hydrostatic correction applied to pressure drop measurements. The improvement developed here consists of integrating an equation for the mixed mean temperature as a function of axial distance in order to obtain a better estimate for the volume-average temperature, from which a more accurate density for use in the hydrostatic correction can be determined. The pressure drop calculated in constant wall temperature (CWT) situations using the improved correction shows, relative to purely forced flow, much higher flow resistance for aiding flow and a negative region for opposing flow. The technique is useful for the mixed convection region, unnecessary in the turbulent flow region, and inapplicable in the asymptote region of vertical, internal flows with mixed convection heat transfer.