Heat transfer and flow characteristics of air-assisted impinging water jets

Abstract

The heat transfer performance of circular, air-assisted liquid water jets is experimentally observed over a wide range of physical parameters, including nozzle tube diameter and surface tension. Dimensional analysis is used to identify the parameters of importance. A new correlation of the local Nusselt number is suggested along with a separate correlation of the average Nusselt number for several values of r/D. The new correlations allow for prediction of heat transfer performance over a much broader range of conditions than previously available. Trends in both local and averaged heat transfer are discussed, including the effect of the liquid Weber number which has not been included in any previous analysis. The maximum stagnation point Nusselt number enhancement was 2.6 times the liquid only value. The maximum enhancement of the averaged Nusselt number (at a radius of five tube diameters) was 1.8 times the liquid only value. A clear trend of decreasing averaged heat transfer with increasing liquid Weber number is observed. Flow visualization of the two-phase flow patterns and water splatter are used to explain the observed heat transfer trends. The two-phase flow pattern affects the heat transfer in the region near the stagnation point, but does not affect the radial flow region. An experiment to quantitatively evaluate water splatter was also conducted. Water splatter is shown to correlate well with both gas Reynolds number and liquid Weber number, and has a significant effect on heat transfer outside the impingement region. Finally, optimal operating points for each tube size in the present study, in terms of the dimensionless pumping power requirement, are observed for several values of r/D . It is shown that small air injection rates into large diameter jets can cause reduced heat transfer.