EFFECT OF DEFLECTED INFLOW ON FLOWS IN A STRONGLY-CURVED 90 DEGREE ELBOW

摘要

Wall pressure measurements and flow visualization were conducted for a 90 degree elbow with an axis curvature radius the same as its inner diameter (125 mm, R_c/D = 1). Reynolds numbers 320,000 and 500,000, based on the inner diameter and bulk velocity, were examined. A deflected inflow, having an almost constant velocity slope, was introduced in the present study. The velocity at the inside was 20% faster than the bulk velocity in the plane one diameter upstream of the elbow inlet. Ensemble averaged pressure distributions showed that no difference of normalized pressure could be found in cases of Reynolds numbers of 320,000 and 500,000. Comparisons with a uniform inlet flow case proved that a low-pressure region at the intrados of the elbow was weakened and that a high-pressure region outside strengthened in the deflected inflow case. The present case had a characteristic pressure distribution that the pressure downstream of the elbow increased at the inside until two diameters downstream from the elbow exit. Flow visualization concluded that the corresponding pressure increase was caused by a collision of a strengthened secondary flow convected from the extrados. The unsteady pressure distribution in the present case showed that a circumferential extent of a strongly fluctuating region in the inside and downstream of the elbow decreased, comparing with the uniform inlet flow case. Power spectral density functions of pressures exhibited that the fluctuation having the Strouhal number (based on the inner diameter and bulk velocity) of 0.6 existed in the downstream region of the elbow, which is 0.1 larger than that of the uniform inflow case.