Numerical study on heat transfer characteristics of multiple air jet impinging at lower jet to plate spacing

Abstract

For multi-nozzle jet impinging, the effects of impinging distance (Z/D), streamwise spacing (X/D) and spanwise spacing (Y/D) on heat transfer characteristics should be considered. The heat transfer characteristics for multi-nozzle jet impinging structure with different nozzle spacings were investigated by numerical method. When the impinging distance is constant (Z/D = 0.2), the effects of Reynolds number (Re = 12,000–28,000), streamwise spacing (X/D = 3, 4, 6, 8, 10), spanwise spacing (Y/D = 3, 4, 6, 8, 10) and impinging nozzle arrangement (in-line structure and staggered structure) are considered. The Standard k-ε model was chosen for simulating after comparison to the relevant experimental values. The influences of nozzle spacing and the Reynolds number on the heat transfer characteristics were examined. The Colburn factor j and friction factor f were investigated in relation to Re. The performance criteria η of the Reynolds number ranging from 12,000–28,000 was calculated. The calculations indicated that the jet impinging heat transfer coefficient is highest under the structure of X/D = 3 and Y/D = 3. Compared to the in-line arrangement, the staggered arrangement has a higher average Nusselt number. This is because the turbulent kinetic energy of the staggered structure on the impinging target surface is larger and the heat exchange efficiency is higher. However, the gap between the average Nusselt numbers of the staggered and in-line nozzle arrangements decreases with an increase in Reynolds number. Given the same heat exchange surface, the average Nusselt number is highest under the X/D = 8 and Y/D = 8 arrangements. This is because the larger the impinging nozzle spacing, the fewer the number of nozzles. Therefore, the greater the speed of the gas reaching the nozzle outlet, the greater the average Nu of the target surface.