Adiabatic horizontal and vertical pressure drop of carbon dioxide inside smooth and microfin tubes at low temperatures

This paper presents the pressure drop data and the analysis of adiabatic CO2 flow in horizontal and vertical smooth and microfin tubes at saturation temperatures around -20°C. The test tubes had 3.48 mm inner diameter smooth tube and a 3.51 mm meltdown diameter microfin tube. The test was performed over a mass flux range of 200-800 kg/m2 s and at saturation temperatures of -25°C and -15°C. The effects of various parameters-mass flux, saturated temperature, and tube diameter-on pressure drop were qualitatively analyzed. The analyses showed that the frictional pressure drop characteristics of vertical two-phase flow were much different from that of the horizontal two-phase flow. The microfin tube can be considered as "very rough tube" having the roughness of "fin height." The data were compared with several correlations. The existing frictional pressure drop correlation is sufficient to predict the horizontal pressure drop in smooth tube. For the vertical pressure drop, the simple combination of the frictional pressure drop and void fraction model was in comparatively good agreement. However, the qualitative results showed that there were some limits to cover the different mechanisms related to the interfacial shear stress. The average enhancement factors and penalty factors evidenced that it was not always true that the internally finned geometry guaranteed the superior in-tube condensation performance of microfin tube in refrigeration system and air-conditioning systems.