Droplet train impingement is a fundamental approach to mimic the complicated interactions between the fluid and the substrate in advanced thermal engineering applications in industry. Differently from previous studies, the main original contribution of this study is to perform an inclined droplet train impingement on a non-uniformly heated surface. Ethanol was used as the liquid for droplet train impingement applications, while glass substrate was selected as the target surface. The inclined flow angle was 63 degrees. Both optical and thermographic observations were performed on the target surface by focusing on the droplet impact area. Three experimental sets were created with the Weber numbers 667.57, 841.90, and 998.01. A surface temperature range was selected between 85.00 °C and 200.00 °C, which was above the boiling point of the ethanol. The maximum spreading length was measured at 0.97 mm at the surface temperature of 82.00 °C for the experiment with the Weber number of 998.01, whilst the minimum spreading length was found at 0.18 mm at the highest surface temperature for the experiment with the Weber number of 667.57. A uniform splashing direction was observed above 170.00 °C for all experiments, which meant that the sign of the transition regime appeared.

The present study considers the impingement of a train of ethanol droplets on heated aluminum and glass surfaces. The surface temperature is allowed to vary in the interval 140 C-240 C. Impingement is considered with an inclination of 63 degrees. The droplet diameter is 0.2 mm in both aluminum and glass surface experiments. Thermal gradients are observed with a thermographic camera. It is found that in comparison to glass, the aluminum surface displays very small liquid accumulations and better evaporation performance due to its higher thermal conductivity. The relatively low thermal conductivity of glass results in higher thermal gradients on the surface. The droplet impact area on the aluminum surface is smaller than the corresponding area for the glass surface. Interestingly, the liquid accumulation area is not symmetrical. Moreover, the extension of the droplet train impact region decreases on increasing the surface temperature because higher temperature values allow greater surface energy levels that enhance significantly the evaporation rate.

Steady-state hydrodynamic patterns of ethanol droplet train impingement on the heated aluminum surface is investigated in the surface temperature range of 80°C-260°C using two different Weber numbers (We) of 618 and 792. Instead of a vertical train impingement, the droplet train is sent to the aluminum surface with an incline of 63 degrees. Changes in the spreading length are observed at different surface temperatures for two different We values, which are obtained by using two different pinholes with 100 and 150 μm diameters. The greatest spreading length is seen at the lowest surface temperature (80°C) and it continuously decreases until the surface temperature of 200°C. Above 200°C, the spreading length remains stable which is most probably because of the Leidenfrost effect. The spreading lengths of the experiments with 100 μm are 46.4% smaller than the experiments with 150 μm. Also, splashing angles are observed for both We values. The ranges of splashing angle observations are 140°C-200°C and 170°C-185°C for We values of 792 and 618, respectively.