Abstrakt

Accurate control of cooling parameters is required in ever wider range of technical applications. It is known that reducing the dimensions of the size of nozzle leads to an increase in the economy of cooling and improves its quality. Present study describes research related to the design and construction of the nozzles and microjet study, which may be applied in many technical applications such as in metallurgy, electronics, etc. Using liquids such as water, boiling is likely to occur when the surface temperature exceeds the coolant saturation temperature. Boiling is associated with large rates of heat transfer because of the latent heat of evaporation and because of the enhancement of the level of turbulence between the liquid and the solid surface, Garimella and Rice (1995). This enhancement is due to the mixing action associated with the cyclic nucleation, growth, and departure or collapse of vapour bubbles on the surface. In the case of flow boiling, such as boiling under impinging jets, the interaction between the bubble dynamics and the jet hydrodynamics has significant effect on the rate of heat transfer. The common approach used to determine the rate of boiling heat transfer is by using a set of empirical equations that correlate the value of the surface heat flux or the heat transfer coefficient with the fluid properties, surface conditions, and flow conditions The basis of microjet technology is to produce laminar jets which when impinging the surface have a very high kinetic energy at the stagnation point, Mikielewicz and Muszynski (2009), Mikielewicz et al (2011). Boundary layer is not formed in those conditions, while the area of film cooling has a very high turbulence resulting from a very high heat transfer coefficient. Applied technology of jet production can result with the size of jets ranging from 20 to 500μm in breadth and 20 to 100μm in width. The paper presents the investigation of a single water microjet cooling forming an evaporating liquid film on the impingement surface. Examined is the influence of liquid subcooling on the extent of the critical heat flux. Tests were conducted under steady state conditions. Developed also has been a theoretical model of surface cooling by evaporating microjet impinging in the stagnation point, where the highest heat transfer coefficient occurs.

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