Channel height effect on heat transfer and friction in a dimpled passage

The heat transfer enhancement in cooling passages with dimpled (concavity imprinted) surface can be effective for use in heat exchangers and various hot section components (nozzle, blade, combustor liner, etc.), as it provides comparable heat transfer coefficients with considerably less pressure loss relative to protruding ribs. Heat transfer coefficients and friction factors were experimentally investigated in rectangular channels which had concavities (dimples) on one wall. The heat transfer coefficients were measured using a transient thermochromic liquid crystal technique. Relative channel heights (H/d) of 0.37, 0.74. 1.11, and 1.49 were investigated in a Reynolds number range from 12,000 to 60,000. The heat transfer enhancement (Nu(HD)) on the dimpled wall was approximately constant at a value of 2.1 times that (Nu(sm)) of a smooth channel over 0.37 less than or equal toH/d/less than or equal to1.49 in the thermally developed region. The heat transfer enhancement ratio ((Nu) over bar (HD)/Nu(sm)) was invariant with Reynolds number. The friction factors (f) in the aerodynamically fully developed region were consistently measured to be around 0.0412 (only 1.6 to 2.0 times that of a smooth channel). The aerodynamic entry length was comparable to that of a typical turbulent pou (x(o)/D-h=20), unlike thermal entry length on dimpled surface which was much shorter (x(o)/D-h<9.8). The thermal performance ((<(Nu)over bar>(HD)/Nu(sm))/(f/f(sm))(1/3)similar or equal to1.75) of dimpled surface was superior to that (1.16 <(Nu(HD)/Nu(sm))/(f/f(sm))(1/3)<1.60) of continuous ribs, demonstrating that the hear transfer enhancement with concavities can be achieved with a relatively low-pressure penalty. Neither the heat transfer coefficient distribution nor the friction factor exhibited a detectable effect of the channel height within the studied relative height range (0.37H/d less than or equal to1.49).