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.
Impingement jet is widely used in industrial fields because it provides a high heat transfer coefficient near the stagnation region. However, few methods exist to control the impingement heat transfer. Recently, peculiar diffusion process "axis-switching'' in three dimensional free jet has been reported, and the possibility of axis-switching controlling the diffusion and mixture process has begun to attract attention. In this report, we have studied experimentally the effect of the orifice configuration on the impingement heat transfer using non-circular orifices. In addition, "axis-switching phenomenon'' was showed clearly by the flow visualization using the hydrogen bubble method. Orifice configurations are the regular polygons with 3 to 6 sides. Heat transfer experiments covered the distance between the orifice-to-target plate is 4 to 8 and Reynolds number is 5 × 10 4 and the constant heat flux is 600 W/m 2. The flow was visualized in Reynolds number 1,500. In the free jet from the regular polygon orifice, when the number of sides on the orifice increased, the occurring location of "axis-switching phenomenon'' moved towards the orifice exit and the tendency of the iso-Nusselt number profile became the concentric profile upstream. With the decrease in the number of sides on the orifice, the iso-Nusselt number profile after "axis-switching phenomenon'' was kept at the downstream.
Impingement jets are widely used in industries because they provide a high heat transfer coefficient near the stagnation region. However, few methods exist for controlling impingement heat transfer. Recently, a peculiar diffusion process called "axis switching" for three-dimensional free jet has begun to attract attention, and there is a novel possibility of control diffusion and mixture process using this phenomenon. In this paper, we report on the effect of non-circular polygonal orifice shapes on impingement heat transfer. In addition, we demonstrate axis-switching phenomenon by using flow visualization with hydrogen bubbles. Orifice configurations are the regular polygons with 3 to 6 sides. Heat transfer experiments covered the distance between the orifice-to-target plate is 4 to 8 and Reynolds number is 5 × 10 4 and the heat flux is 600 W/m 2 . The flow was visualized in Reynolds number 1,500. For a free jet emerging from a regular polygonal orifice, the location of axis-switching phenomenon shifts toward the orifice exit as the number of sides on the orifice is increased. The iso-Nusselt number profile tends to take the shape of a concentric circle farther upstream. However, with a decrease in the number of sides of the orifice, the iso-Nusselt number profile after axis switching remains downstream.
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