EFFECTIVENESS OF MICRO-DROPLET TRAIN AND MICRO-JET IMPINGEMENT IN SURFACE COOLING

In this study, the heat transfer effectiveness of a single micro-droplet train and a micro-jet impingement has been investigated experimentally and numerically. A piezo-electric droplet generator was used to produce micro-scale droplets with the ability to control droplet properties such as droplet impingement frequency, droplet velocity and droplet Weber number. The same droplet generator was used to produce a single micro-jet by switching off the function generator. An infrared imaging system in combination with a translucent sapphire substrate with a thin layer of ITO (Indium Tin Oxide) was used in all the heat transfer experiments. Numerically, ANSYS-Fluent was used to simulate the droplet and jet impingement processes using the Volume-of-Fluid approach coupled with the Level Set method (CLS-VOF). Structured 2D axi-symmetric meshes were used for simulating mono dispersed droplet impingement cases. Dynamic mesh adaption technique was also used to capture the propagation of the droplet-induced crown with time-dependent spatial and temporal resolutions. A good agreement was reached between experimental and numerical data in terms of droplet-induced crown diameter, time-averaged temperature distribution, and Nusselt number. Results for fixed flow rate condition show that droplet train impingement exhibits better heat transfer performance than circular jet impingement. The better thermal performance of droplet train impingement was attributed to the periodic droplet-induced crown propagation nature, which improves the heat transfer process and liquid usage efficiency. From the numerical simulations, it is evident that the average radial momentum within the impinged liquid film plays an important role in the overall heat transfer process.