An experimental investigation on thermal striping Mixing phenomena of a vertical non-buoyant jet with two adjacent buoyant jets as measured by ultrasound Doppler velocimetry

An experimental investigation on the thermal mixing phenomena of three quasi-planar vertical jets, with the central jet at a lower relative temperature than the two adjacent jets, was conducted. The central jet was unheated (ʹcoldʹ), while the two adjacent jets were heated (ʹhotʹ)- The temperature difference and velocity ratio between the heated (h) and unheated (c) jets were, (delta)T(hc)=5°C, 10°C and r = V cold,exit/V hot,exit = 1.0 (isovelocity), 0.7, 0.5 (non-isovelocity) respectively. The typical Reynolds number was -ReD= 1.8 x 10^ 4, where D is the hydraulic diameter of the exit nozzle. Velocity measurement of a reference single-jet and triple-jet arrangement were taken by ultrasound Doppler velocimetry (UDV) while temperature data were taken by a vertically traversed thermocouple array. Our UDV data revealed that, beyond the exit region, our single-jet data behaved in the classic manner. In contrast, the triple-jet exhibited, for example, up to 20 times the root-mean-square velocity values of the single-jet, especially in the regions in-between the cold and hot jets. In particular, for the isovelocity case (V exit=0.5 m/s) with (delta)T(hc)=5°C, we found that the convective mixing predominantly takes place at axial distances, z/D)=2.0 - 4.5, over a spanwise width, x/ D ~|2.25|, centered about the cold jet. An estimate of the turbulent heat flux distribution semi-quantitatively substantiated our results. As for the non-isovelocity case, temperature data showed a localized asymmetry that subsequently delayed the onset of mixing. Convective mixing however, did occur and yielded higher post-mixing temperatures in comparison to the isovelocity case. © 1999 Elsevier Science S.A. All rights reserved.