Numerical study of swirl cooling enhancement by adding mist to air: Effects of droplet diameter and mist concentration

Swirl cooling with air/mist mixture coolant is a promising heat transfer method, for which the combined influence of swirl and mist should be clearly understood. In this work, numerical simulations on the swirl cooling performance of the leading edge of a turbine vane using mist/air mixture and air are implemented for comparative study. The Eulerian-Lagrangian particle tracking method is adopted to investigate the two phase cooling. The coolant air impacts tangentially the inner surface of the circular swirl chamber through two rectangular nozzles, and the mists with different concentrations (5%, 10%, and 20%) and droplet diameters (1 mu m, 2.5 mu m, 5 mu m, 7.5 mu m, and 10 mu m) are included to explore their effects on the cooling performance. The results demonstrate that the circumferentially-averaged Nusselt number, Nu(cir), increases with the increase of mist concentration, and this effect becomes prominent when the droplet diameter increases. For droplet diameter of 10 mu m and mist concentration of 20%, Nu(cir) can increase by 12.6% to 309.7% as compared to pure air. In addition, the empirical correlations of the global-averaged Nusselt number, and friction coefficient to the mist diameter and mist concentration are given for the first time. It is concluded that the thermal performance factor, which can be used to evaluate comprehensively the cooling characteristics, increases with the increase of mist concentration and droplet diameter. The empirical correlations and thermal performance evaluation provide the basis for further theoretical analysis of the cooling performance using air/mist mixture.