FLOW AND HEAT TRANSFER PHENOMENA IN A 90° BEND

Abstract

This paper deals with the overall performance of a two-dimensional, 90°, potential flow bend having a contract ion ratio of 2:1 and using air as the working fluid. It deals as well with the effect on the boundary layer and the heat transfer rate at the inner bend surface due to the cooling and curvature of the inner wall. The overall characteristics of the bend are reported for four inlet velocities from 60 to 175 ft./sec. Static pressure distributions for the inner and outer walls of the bend and velocity and turbulence profiles at the bend inlet, mid-bend, and bond outlet are given. Both the pressure dis tributions and the velocity profiles are compared to those predicted by ideal potential flow and neglecting the velocity decay at the walls due to the boundary layer, agreement is very good. The characteristics of the boundary layer formed at the inner surface of the bend arc reported for the four in- let velocities. Velocity profiles are plotted for isother mal flow conditions in dimensional form, universal velocity profile form, and logarithmic form. The effects on the velocity profiles due to varying inlet velocities and temp erature differences between the wall and the flow’ of up to 60°F are shown to be small. The temperature profiles for all the variations of speed and temperature difference are shown and as the temp erature difference increases the temperature boundary layer thickens slightly. The convective heat transfer coefficient is reported for a point on the inner surface at raid-bend. Evaluations by the accepted heat transfer correlation and from shear stress results agree but actual measurements using a heat flow transducer show that the heat transfer rate is substan tially reduced due to the effect of curvature. These re ductions in heat transfer occur in a region where the accel eration which the flow experiences due to turning is from 730 to 3,3'30 times the acceleration duo to gravity.