Influence of Spanwise Pitch on Local Heat Transfer for Multiple Jets with Crossflow

Abstract

The influence of spanwise jet-to-jet spacing on local heat transfer distribution due to multiple impinging circular air jets from an in-line rectangular array on a surface parallel to the jet plate is studied experimentally. The length-todiameter ratio of the nozzles of the jet plate is 1.0. The flow, after impingement, is constrained to exit in one direction from the confined passage formed between the jet plate and the target plate. Mean jet Reynolds numbers based on the nozzle-exit diameter d covered are 3000, 5000, 7500, and 10,000, and jet-to-plate spacings studied are d, 2d, and 3d. Spanwise pitches considered are 2d, 4d, and 6d, keeping the streamwise pitch at 5d. For all configurations, the jet plates have ten spanwise rows in the streamwise direction and six jets in each spanwise row. The flat heat transfer surface is made of thin stainless steel metal foil. Local temperature distribution on the target plate is measured using thermal infrared camera. Wall static pressure on the target plate is measured in the streamwise direction to estimate crossflow velocities and individual jet velocities. Heat transfer characteristics are explained on the basis of flow distribution. A simple correlation is developed to predict the streamwise distribution of the Nusselt number averaged over each spanwise strip resolved to one jet hole as a function of jet-flow and crossflow distributions.diameter of the jet, m G act = actual jet mass velocity based on jet-hole area, kg=s m 2 G c = channel crossflow mass velocity based on channel cross-sectional area, kg=s m 2 G ideal = theoretical jet mass velocity based on jet-hole area, kg=s m 2 G j = individual jet mass velocity based on jet-hole area, kg=s m 2 G j = mean jet mass velocity based on jet-hole area for the array, kg=s m 2 h = heat transfer coefficient, W=m 2 K k = thermal conductivity of the jet fluid, W=m K l=d = aspect ratio of the nozzle _ m j = mean mass flow rate from the nozzle, kg=s N c = number of streamwise rows of jets Nu = local Nusselt number (hd=k) Nu o;multijet = stagnation Nusselt number due to a jet in each spanwise row Nu o;single jet = stagnation Nusselt number due to a single jet estimated from the correlation corresponding to the jet-row Reynolds number Nu segment = Nusselt number averaged over each segment Nu strip = Nusselt number averaged over each spanwise strip resolved to one jet hole P = absolute static pressure at the nozzle exit, Pa P a = atmospheric pressure, Pa P o = absolute static pressure in the plenum or at the nozzle inlet, Pa Pr = Prandtl number Q = heater input power, W q 00 = net heat flux imposed on the target plate, W=m 2 R = characteristic gas constant of air, J=kg K Re = mean jet Reynolds number of flow exiting the nozzle (4 _ m j =d) Re j = individual spanwise-row jet Reynolds number T amb = ambient air temperature, K T j = reference jet fluid temperature on the target plate, K T o= plenum air temperature, K T w = target-plate surface temperature, K V j = mean jet velocity at the nozzle exit, m=s x = distance in the streamwi...

A Numerical Study Of Micro-Jet Impingement Cooling Inside A High Pressure Turbine Vane

“…The analysis revealed an optimal jet to target distance for maximum heat transfer between 2D and Sd, with 3D being considered a good initial design value. Likewise to Katti and Prabhu [34], an optimal spanwise spacing of 4D was also found. Van Treuren [28.29] studied three different jet to target ratios, 1D, 2D, and 4D.…”

“…The experimental work of Katti and Prabhu [34] investigates the effect of spanwise spacing on the heat transfer distribution. Their analysis reveals that the non-uniformity in jet flow and non-linearity of the channel flow increases with decreasing jet to target spacing and spanwise pitch.…”

“…Likewise, the experimental work of Brevet et al [35], Huang et al [37], and Katti and Prabhu [34], all evaluate the local and average heat transfer values using large diameter orifices, D>3.1mm. In the experimental work mentioned, the effect of jet diameter on the hydrodynamics and heat transfer has not been studied within constant array geometry.…”

“…ReAvg=SK & 15K, will be compared to established correlations yielding the streamwise resolved Nusselt number. This includes the experimental work of Florschuetz [31], Bunker [33], and Katti and Prabhu [34].…”

Experimental and numerical investigation of heat transfer in an array of impingement jets on a concave surface

“…The jet fountain is an important factor affecting the heat transfer characteristics when multi-jet impingement is considered. Katti and Prabhu [15] experimentally investigated the effect of different span-wise pitches on the local heat transfer coefficient distribution on the plate by an array of multi-jet impingement. Their study showed that a span-wise pitch of 4 times diameter of hole performs better than that of 2 or 6 times, according to the analysis of the Nu and the pressure loss coefficient.…”