Local instantaneous convective heat transfer characteristics from mechanical and supply pulsed radial reattaching nozzles
Furlow, John Scott
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The heat transfer characteristics of combined mechanically and supply pulsated radial reattaching, CPRJR, nozzles were documented as a function of nozzle exit angle (0° and 20°), non-dimensional gap height (0.05 and 0.13), non-dimensional flow guide height (0.8 and 1.16), mechanical pulsation rate (5 and 10 Hz), supply pulsation rate (10, 20, 30, and 40 Hz), phase angle (0° and 180°), and Reynolds number (1683 and 2366). Air was forced through a supply pulsation mechanism and then through a pulsated nozzle diverter apparatus. The air impinged on a heated steel plate whereupon instantaneous heat flux and surface temperature measurements were collected and analyzed on instantaneous, time-averaged, and integrated area bases. Frequency analysis was made, via an FFT algorithm, of local instantaneous surface temperature and heat flux data. Nozzle exit velocity data was taken and a frequency analysis was performed. CPRJR nozzle heat transfer can be characterized as a frequency interference of mechanical and supply pulsation effects. CPRJR heat transfer values were similar to their mechanically pulsed equivalent excepting areas of low heat transfer that corresponded to the "flow off' portion of the supply pulsation cycle. The flow was blocked in the "flow off" portion of the pulsation cycle in an attempt to scour the boundary layer; however, air was entrained thus preventing any scouring of the boundary layers. For measured CPRJR cases, flow rate measurements were based upon the average flow rate over one mechanical pulsation cycle where the "on slot" supply pulsed flow rate was twice the mechanical flow rate. Variation in nozzle exit angle decreased the reattachment radius and caused a wide range of Nu number variation. Increasing of non-dimensional gap height and non-dimensional flow guide height induced a 50% increase in reattachment radius and a 50% drop in Nu number values. Increase in mechanical pulsation from 5 Hz to 10 Hz has little influence on reattachment radius and induces slightly higher heat transfer. Variation of phase angle and supply pulsation frequency produced a wide range of increased and degraded heat transfer values. Heat transfer from CPRJR nozzles was found to be consistently higher, up to 68% than the corresponding mechanically pulsated nozzle, for a supply-to-mechanical pulsation ratio of 1:1 with a phase angle, ct;=180°. A brief investigation was made to evaluate the basing of the "on slot" flow rate equal to the mechanical flow rate. Heat transfer results from this flow rate measurement scheme produced heat transfer variations generally less, 3% to 53%, than the corresponding PRJR cases. From the these results, it is concluded that supply pulsation does not enhance convective heat transfer.