EFFECT OF RIDGE SHAPES ON TURBULENT HEAT-TRANSFER AND FRICTION IN A RECTANGULAR CHANNEL

Turbulent heat transfer and friction in a channel with various-shaped ridges mounted on two opposite walls has been investigated experimentally. Experiments were conducted for three ridge shapes, namely triangular, semicircular, and square cross sections. The ridges are oriented transversely to the main stream in a periodic arrangement. The technique of real-time holographic interferometry is used to measure local and average heat transfer. The ridge pitch-to-height(radius) ratio varies from 8.0 to 20; the Reynolds number varies from 7800 to 50000; and the ridge height(radius)-to-channel hydraulic diameter ratio is 0.08. The results show that the three types of ridged channels have comparable thermal performances. It is also noted that the semicircular and triangular ridge geometries are less likely to yield the hot-spots behind the ridges than the square one. Moreover, a comparison of the average heat transfer coefficient of the wall with thermally active ridges and that with thermally nonactive ridges is made. Compact correlations in terms of roughness parameters are developed for both the fully developed heat transfer coefficients and friction factors. The correlations can be used in the design of turbine blade cooling passages.