Heat transfer characteristics of double, triple and hexagonally-arranged droplet train impingement arrays

In this study, hydrodynamics and heat transfer of multiple droplet trains impinging a pre-wetted solid surface have been investigated experimentally. A piezo-electric droplet generator has been designed and constructed, which is capable of producing double, triple and hexagonally-arranged droplet trains. A translucent sapphire substrate coated with a thin layer of indium tin oxide (ITO) was used as flat heating element. The effects of droplet Weber number, impact spacing and impingement pattern on liquid film hydrodynamics and heat transfer have been evaluated using high speed optical imaging and IR thermal imaging techniques. High speed images show that a hump was formed between two impact craters for double droplet train impingement. Surface jet flows were observed among impact craters for triple and hexagonally-arranged droplet train impingement arrays. Heat transfer results reveal that horizontal impact spacing and impingement pattern play significant roles in cooling performance. For double droplet train impingement, it was found that higher impact spacing leads to better cooling performance both locally (i.e. within the impingement zone) and globally (i.e. outside the droplet impingement zone). For triple droplet train` impingement, there is an optimum impact spacing for heat transfer. For hexagonally arranged droplet train impingement arrays, lower impact spacing leads to better cooling performance locally. However, higher impact spacing leads to better cooling performance globally. Comparisons have been made between droplet train impingement and circular jet impingement for various impingement patterns. Heat transfer measurements show that droplet train impingement leads to better cooling performance for various impingement patterns, In summary, results reveal that the combined effects of the droplet Weber number and impingement pattern are significant factors in the study of droplet-induced surface heat transfer phenomena. (C) 2017 Elsevier Ltd. All rights reserved.