FRICTION CHARACTERISTICS OF WATER, R-134a AND AIR IN SMALL TUBES

This manuscript provides a systematic test of friction characteristic for air, water and liquid refrigerant R-134a in ten tubes with inside diameters from 0.173 to 4.01 mm. The test results show that the conventional minor losses correlations for large tubes may be adequately used to estimate the related inlet, exit and acceleration components for water, refrigerant and low speed air flow in micro tubes. However, for air flow at high Reynolds number condition, the minor losses were over estimated in comparing to the test present results. The laminar-turbulent transition Reynolds number varies from 1,200 to 3,800, and increases with decreasing tube diameters. The test friction factors for water and refrigerant R-134a agree very well with the conventional Blasius (f = 0.079Re_d^-0.25) and Poiseuille (f = 16/Re_d) equations in turbulent and laminar flow regime. However, for air flow, the measured friction factors are significantly lower than those predicted by Blasius for smaller tubes. The discrepancy between the measured and predicted values increased with decreasing tube diameters. This implies that the postulation by Kakac, et al. [2] for flow characteristics in small tubes can be applied for air flow but not for water and refrigerant flow in the Reynolds number region tested.