Control of Impingement Heat Transfer Using Mist

Abstract

Impingement heat transfer from a circular orifice jet by using latent heat of water mists was studied experimentally. The amounts of mists of about Zauter's mean diameter 14 μm were from 60 to 200 g/h within a range where liquid films were not formed on the target plate and mists were added near the orifice edge. Experiments covered Reynolds numbers from 12,500 to 50,000 and a heat flux is 1,400 W/m 2 . The experimental results indicate that adding mists had little influence on free jet mean velocity profiles and target plate pressure coefficients. On the other hand, mists had a strong influence on temperature and humidity profiles of a free jet and they also influenced Nusselt number distributions on the target plate. Increases of mists and Reynolds number caused increases in Nusselt number on the developed region. In addition, we investigated influence of the way mists were added and these results showed that Nusselt number was influenced not only by the amounts of mists but also by the adding method. Local Nusselt number profiles with mists were closely related to temperature distributions of the free jet at the location corresponding to the target plate.

Second-moment closure simulation of flow and heat transfer in a gas-droplets turbulent impinging jet

“…The main increase in heat transfer rate occurs in the stagnation zone. This finding is observed both in experiments of [15,19] and in this study.…”

“…Reduction of heat transfer is typical for large droplets because they have smaller interfacial surface and their heating and evaporation are much slower then for the smaller droplets. Quantitative agreement between the computation results and measurements of [15,19] is obtained. The main increase in heat transfer rate occurs in the stagnation zone.…”

“…We still do not know the exact reason for this phenomenon. One of the possible reasons for this relatively small difference between SMC or ke 3 model is a very low value of droplets concentration (M L1 < 0.02%) in measurements of [19]. This finding requires the additional investigation and is the potential basis for our future study.-Computations by the SMC and ke 3 model give qualitatively the same results for single-and two-phase jets impingement for the surface of target.…”

“…The use of gas-droplets mist round impinging flow as a coolant causes a significant increase in heat transfer intensity (more than twice in comparison with the single-phase round impinging jet) (curves 2 and 3) because of latent vaporization heat at droplet evaporation. It should be noted that in experiments of [19] and in author's simulation main intensification of heat transfer occurs in the stagnation zone. Only one maximum in distribution of local heat transfer coefficient is explained by a large distance between pipe exit and cooled surface [1,2].…”

“…Perhaps, the only one experimental study of the gas-droplet impinging jets under conditions of impinging surface cooling without boiling is [19]. Heat transfer increases significantly (several times) with addition of droplets in comparison with the singlephase impinging jet of air.…”