The heat transfer characteristics of a circular water jet impinging on a moving hot solid were investigated experimentally. In the experiments, distilled water at room temperature was used as the test coolant. The circular jet issued from a 5-mm-diameter pipe nozzle, fell vertically downward, and impinged on a horizontal moving sheet made of 0.3-mm-thick stainless steel. The initial temperature of the sheet, the jet velocity, and the moving sheet velocity were varied systematically. The initial temperature of the moving sheet was set to 100, 150, 200, or 250°C. The mean velocity at the nozzle exit was 0.4, 0.8, or 1.2 m/s, and the moving velocity was 0.5, 1.0, or 1.5 m/s. Observations made using flash photography and thermography showed that the location of the front edge of the liquid film formed upstream of the jet impact point depends on all of these factors. The local heat flux is very small in the dry area, increases steeply near the front edge of the liquid film, and reaches a peak. If the distance between the front edge of the liquid and the jet impact point is relatively large, a second peak appears near the jet impact point. An experimental correlation was developed for predicting peak heat fluxes near the front edge of the liquid, although it has no theoretical background. The correlation agrees moderately well with the experiments.

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The spray cooling of moving hot solids is widely performed in the steel industry. Understanding flow and heat transfer when droplets impinge on moving hot solids is important. By simultaneous visualization with flash photography and temperature measurement using thermography, the flow and heat transfer of a droplet train obliquely impinging on a moving solid at high temperatures was experimentally investigated. A rectangular test piece (SUS303) was heated to 500 °C at a moving velocity of 0.25-1.5 m/s. The test liquid was water at approximately 25 °C. The pre-impact droplet diameter, impact velocity, and inter-spacing between two successive droplets were 0.69 mm, 2.2 m/s, and 2.23 mm, respectively. The tilt and torsional angles were 50° and -30-60°, respectively. No coalescence of droplets was observed; the droplets deformed independently on the moving solid, even though the torsional angle generated a velocity component along the width of the solid. The surface temperature of solid after droplet impingements depended on the experimental conditions. Wavy temperature profile was obtained when the moving distance of solid was large during two successive collisions. The temperature changed continuously for the small distances. In this regard, a simple model considering droplet movement, collisional deformation behavior, and solid migration can explain this phenomenon by the overlap of the cooling regions of the droplets. Furthermore, experimental and numerical analyses show that the heat removal rate of individual droplets is constant at approximately 12.5 MW/m 2 and depends on the total contact time when multiple droplets collide.

Spray cooling on moving hot solids is widely used in metal heat treatment processes. Understanding coolant droplet collision behavior with moving hot solids is of great importance toward improving heat treatment temperature control technology. Via flash photography, we experimentally investigated the hydrodynamics of droplet train obliquely impinging on a hot moving solid. The test piece was a rectangular steel piece (SUS303) heated to 500°C, 550°C, or 600°C with a moving velocity of 0.5 m/s, 1.0 m/ s, or 1.5 m/s. The test liquid was water at approximately 20°C. The pre-impact diameter of droplets, droplet impact velocity, and inter-spacing between every successive two droplets were 0.64 mm, 2.2 m/s, and 1.91 mm, respectively. The tilt angle of the droplet train to the vertical was 50°. No coalescence of droplets was seen-the droplets deformed independently on the moving solid. The measured results of the maximum diameter and the residence time of the droplets agreed well with the empirical formulas that can be used for droplet impact on a stationary solid. It was found that the dynamics of a droplet train impinging on a hot moving solid are the same as the dynamics of a droplet train impinging on a hot stationary solid when the droplets deform independently on a moving solid. Taking advantage of said property such that it is equivalent to the dynamics of a droplet train impinging on a hot stationary solid, we proposed a critical condition for droplet coalescence and experimentally confirmed the validity of the critical condition.