Fluid dynamics and heat transfer analyses for droplet and spray impingement

Many industries need to cool objects at high-heat removal rates and high temperatures. Electronic components must keep operating temperatures below 85°C at long periods and high-heat fluxes to avoid performance reduction or damage. The metal production industry must rapidly cool pieces at relatively high temperatures using the quenching heat treatment to achieve a specific material quality. Spray cooling is a potential active heat dissipation technique for handling high-heat flux and high temperatures. Passive techniques such as surface and fluid modifications can enhance spray cooling performance and reduce practical problems like thick liquid film and surface fouling. Additional active technique such as magnetic fields is also a prospect for enhancing droplet-surface interaction for cooling application. This research studies single, multiple and spray impingement cooling employing modified surfaces and fluids. A literature review is included in this thesis with several gaps in the droplet and spray cooling state-of-the-art. Four droplet impingement and two spray experimental studies are covered in this thesis. Successive water droplet cooling onto wettable and liquid repellent surfaces is investigated, and results show that subsequent droplets reduce the heat transfer rate due to liquid accumulation for the wettable surface, which in contrast shows the best heat transfer performance. However, the liquid repellent surface transfers heat at a similar droplet heat transfer rate with no liquid film formation. Non-simultaneous water droplet cooling study shows that adjacent droplet interactions reduce the droplet cooling efficiency while impinging onto a wettable surface due to a relatively smaller heat transfer contact area and damped spreading velocity. Also, a heat transfer reduction is observed for increasing time separation between droplets. A study for ferrofluid droplet impinging onto silicon, titanium thin film and liquid repellent surfaces under a magnetic field shows higher heat transfer rate in comparison to pure water and surfactant-water solution. Ferrofluid droplets perform better due to increasing droplet spreading area and magnetic forces in this study. An experimental investigation for a single water droplet impinging onto flat silicon, flat titanium thin film, and titanium thin film micro-structures is carried out. Micro-structured surfaces showed relatively smaller heat transfer rate than that of flat silicon and titanium surfaces due to increasing friction and reduced spreading diameter contact area. High-velocity water spraying for 36 hours onto heat exchanger materials is studied for smooth, rough, and liquid repellent coating and chemically treated surfaces. Although not showing significant mass losses, surface roughness increases more than two times for smooth surfaces after 36-hour spray impact for both aluminium and copper materials, and liquid repellent surface wettability changes from superhydrophobic to hydrophobic after 12 hours of spray. Spray cooling experimental investigation shows that the 200 µm structured surface performed better than all other surfaces in both the single and two-phase regimes, achieving a heat transfer coefficient of 8.13 W cm−2 K−1 with surface temperature of 47◦C at heat flux of 209 W cm−2 and flow rate of 0.93 L min−1 in the steady-state single-phase regime. Flow rate also increases the cooling performance of all surfaces during spray impingement. In transient analysis for quenching, smooth surface performed better than relatively rougher surfaces in the early stage of quenching, being surpassed by rough surfaces for time evolution and increasing spray flow rate. The analysis for spray cooling are assisted by the findings of droplet impingement cooling experiments included in this thesis. As a final remark for the spray cooling study, the results show that the structure size, roughness and spray flow rate should be balanced for optimum spray cooling performance. The droplet and spray impingement results provide in this thesis can be used for spray cooling system design, optimisation, and guidance.