Two Phase Spray Cooling Heat Transfer Mechanisms On Smooth And Micro-, Nano-, Hybrid-structured Surfaces

Numerous industrial applications require the removal of high heat fluxes, such aselectronic systems, high-power lasers, energy weapons and aerospace satellite. Spraycooling is one of the best solutions due to its high heat dissipating capability with lowcoolant mass fluxes at low wall superheats, precise temperature control, low cost andreliable long-term stability. Spray cooling has been used in hybrid vehicle electronics,power inverter modules, laser surgery and so on.Continual and intermittent spray cooling heat transfer experiments on a flat surfacewere performed and the spray droplet parameters were similar. The results show that ifthe heat flux is quite high, the continual spray cooling is better than the intermittentspray cooling since it has a higher heat transfer capability and the effective heat transfercoefficient is equivalent to the intermittent spray cooling in the boiling region. If theheat flux is not high, however, intermittent spray cooling should be used since it is moreeffective than continual spray cooling in the non-boiling region. The effects of the spraycycle, duty ratio and the spray or non-spray time were studied to make more efficientuse of the coolant in intermittent spray cooling.The spray characteristics and the differences between spray cooling on smooth andenhanced silicon surfaces with micro-, nano-and hybrid structures was investigated.The shadowgraph technique was used to measure the spray droplet parameters andpictures were got by a camera during the heat transfer process. The two-dimensionalheat conduction analytical equations were derived by local thermal non-equilibriummodel. The spray cooling experiments show that the heat transfer for the enhancedsurfaces was much greater in the thin film and partial dryout regions than for the smoothsurface. There is an optimal groove depth corresponding to a given droplet parameter,groove width and stud size for the liquid to completely take advantage of the areaenlarged by the micro-structures and enhance the heat transfer. Adding carbon nanotubes on the top surfaces of the micro-structured surfaces was more effective as themicro-structure sizes were smaller than the droplet diameter. The wall temperaturedifference and the temperature fluctuations were both small for a wide range of powers.The spray cooling heat transfer is mainly influenced by the droplet parameters. A numerical simulation on the micrometer-sized droplet impinging on a flat, unheatedsurface has been investigated based on the spray cooling technology, using the VOFmodel with static contact angle, experimental dynamic contact angle, Kistler model andBlake model for dynamic contact angles. The results show that the simulation resultswith the Blake model agree well with the experimental results and the contact anglecalculated by the spreading velocity and set as the boundary condition in simulationmight not be equal to the apparent contact angle. The development of the spreadvelocity and the the contact angle was described and the droplet parameter effects wereanalyzed by changing the droplet diameters and the velocities.A micrometer-sized droplet impinging on enhanced surfaces with micro-grooves ormicro-studs whose size were the same order with the droplet was simulated bythree-dimensional model using dynamic contact angle. The surface area wetted by fluidat various positions was calculated and the results show that the surfaces withmicro-studs were superior for the bottom surface wetted by the droplet.