An experimental study of pressure drop and friction factor for laminar and transition opposing mixed convection in entrance region of rectangular duct

The laminar and transition opposing mixed convection was experimentally investigated when the air flow was in the entrance region through a rectangular duct. The pressure drops were measured under different air flowrates, convective heat fluxes and inclination angles, with the maximum value less than 6.0 Pa. The friction factors were calculated and analyzed under different Reynolds numbers, Grashof numbers and inclination angles. The results indicated that the pressure drops increased with the mean air velocity, and the friction factors showed different characteristics when the Reynolds numbers were larger or smaller than 1500. Finally, the correlations of friction factors were fitted considering the influences of Reynolds number, Grashof number, inclination angle and length to diameter ratio, which fitted the experimental data with the deviations less than ±10.0%.

Research highlights
► Laminar and transition opposing mixed convection in duct was investigated.
► Pressure drops in entrance region of rectangular duct were less than 6.0 Pa.
► Influences of air flowrate, heat flux and inclination angle were considered.
► Friction factors showed different characteristics when Re > 1500 or Re < 1500.
► Two correlations of friction factor were fitted with deviations less than 10%.