Experimental study of two-phase pressure drop in adiabatic tubes at subatmospheric system pressures

This study addresses the critical prediction of frictional pressure drop in two-phase flow at subatmospheric system pressures, essential for improving heat transfer methodologies in various industrial applications to enhance thermal equipment design efficiency. The pressure drop in the heat transfer equipment is affected by the system pressure, system geometry and working fluid. There is a dearth of literature on two-phase pressure drop in the conventional adiabatic tubes at subatmospheric system pressures. Experimental investigations are conducted using adiabatic tubes of 8, 13.7, and 18 mm diameters and 1500 mm length at 0.25, 0.5, 0.75, and 1 bar system pressures to get two-phase frictional pressure drop. The steam at 0 - 0.98 vapor qualities is used at 32 - 660 kg/m2s mass flux in the adiabatic tubes. The effects of tube diameter, vapor quality, mass flux and system pressure on two-phase frictional pressure drop are investigated. In the liquid-vapor flow, the pressure drop experiences a non-linear increase with changes in vapor quality. The frictional pressure drop in two-phase flow is elevated with higher vapor quality, reduced subatmospheric system pressure, increased mass flux, and a smaller tube diameter. Reducing system pressure from atmospheric to 0.25 bar doubles the frictional pressure drop in an 18 mm tube and increases it by 12 kPa in a 13.7 mm tube at high mass fluxes (>= 56 kg/m2s) and vapor qualities (>= 0.2). A correlation is suggested to predict the two-phase pressure drop with reasonable accuracy in the adiabatic tubes at subatmospheric system pressures.