Evolution of liquid film in a crossflow tunnel: Liquid film thickness measurement and effect of droplet impingement on film breakup

Abstract Liquid jet in crossflow tunnel has widespread applications in various industrial devices, with the measurements of liquid film on the bottom surface pivotal in exploring relevant mechanisms such as heat transfer and film breakup. This work reports the measurements of liquid film on the bottom surface of a crossflow tunnel using the brightness-based laser induced fluorescence method under different flow conditions at ambient pressure and temperature. Film breakup phenomena are observed downstream within the tunnel. Employing the shadowgraph method, two distinct patterns of film breakup associated with the droplet impingement positions on the film wave are identified, i.e. bag breakup and membrane breakup. The film thickness is subsequently calculated, and jet impingement and spray impingement of injected liquid on tunnel bottom surface are classified based on the centerline film thickness. A critical jet-to-crossflow momentum flux ratio ( q ) is determined to be proportional to the square of tunnel height. The averaged film thickness across the entire cross-section downstream at a distance of 50 mm from the nozzle is found to increase with the logarithm of q . Besides, the film boundaries are also identified under different flow conditions, which can be well predicted by a quadratic fit with the fitting parameters also correlated to the logarithm of q .