Spray cooling is an effective cooling process with numerous potential applications in rapid cooling from high temperatures, such as quenching of aluminum and steel, and steady removal of concentrated heat loads, such as the cooling of lasers and electronic devices. However, due to the complexity of droplet dynamics, transient evaporation and boiling heat transfer even under simpler conditions, spray cooling process is not well understood. Experiments were conducted to investigate the droplet dynamics and heat transfer of both a monosized droplet stream and a water spray on a hot substrate. Visualization method was used to study the droplet deformation, boiling and rebounding at the substrate temperatures from 393 to 473K. To study the effect of droplet impact rate and Weber number, the...[ Read more ]

Spray cooling is an effective cooling process with numerous potential applications in rapid cooling from high temperatures, such as quenching of aluminum and steel, and steady removal of concentrated heat loads, such as the cooling of lasers and electronic devices. However, due to the complexity of droplet dynamics, transient evaporation and boiling heat transfer even under simpler conditions, spray cooling process is not well understood. Experiments were conducted to investigate the droplet dynamics and heat transfer of both a monosized droplet stream and a water spray on a hot substrate. Visualization method was used to study the droplet deformation, boiling and rebounding at the substrate temperatures from 393 to 473K. To study the effect of droplet impact rate and Weber number, the frequency of impinging droplets was adjusted to 1.5kHz and 11kHz, which give the Weber numbers of 22 and 143, respectively. Droplet coalescing and rebounding were investigated under various impacting angles and surface temperatures. It was found that the coalescing effect could greatly reduce the ratio of the heat transfer area to the droplet volume and, therefore, decrease the heat transfer coefficient. The results were also measured quantitatively by using a laser Phase-Doppler Anemometry system (PDA). Furthermore, using the experimental data obtained by previous studies a correlation model between the heat flux and the spray characteristics was developed, in which the heats removed by forced convection of water film and by droplet evaporation were taken into consideration. By using this correlation model, the heat transfer of spray cooling on a rectangular fin surface was evaluated. The fin temperatures were evaluated numerically by using a two-dimensional steady-state heat equation coupled with the newly developed correlation model for the boundary conditions. An experiment was performed to verify the validity of the model and the results of calculations agree well with the experiment.