Flow And Heat Transfer Of Jet Impingement Of LN₂ In Mini-micro Tube

Cryogenic jet impingement cooling is employed in numerous fields such as cooling of high-heat flux electronics, micro cryogenic surgical apparatus, and so forth. Experiments were conducted to study the flow and heat transfer of jet impingement of LN2 in mini-micro tube.The effects of jet velocity, heat transfer surface and jet mouth-surface distance, in the confined annular space, in the tube with inner diameter of 1.931mm, have been investigated systematically. The axial temperature distribution shows excellent linearity. The heat transfer order of contaminated surface is equivalent to pool boiling. Convective and boiling impingement cooling of up to 85W/cm2 from hemispherical surface has been measured in a jet velocity of 1.1m/s. Measured critical heat transfer (CHF) is 30-50% higher than the value of semi-theoretical prediction models. The flat surface with a needle shows the best heat transfer for the remarkable forced convection effect in small superheat, but the hemispherical surface shows the best cooling performance in a relatively high superheat for the largest heat transfer surface. The local heat transfer coefficient decreases monotonously with the increase of jet mouth-surface distance different from the free surface jet impingement.A heat transfer dimensionless equation was obtained from the data of the flat with a needle and hemispherical surface, with the Reynolds number ranged from 3500 to 9600. Compared to published correlations for stagnation zone of free surface jet impingement with regular cooling substances, it is found that the experiment Nusselt number is markedly higher than the calculated value from the correlations. The cooling performance of the flat with a needle and hemispherical surface shows the tendency of close with the increase of Reynolds number; meanwhile, the influence of jet mouth-surface distance is weakened.The theory of laminar boundary layer was applied to analyze the characteristics of heat transfer with confined annular jet impingement. On the hypothesis that treating the total heat transfer surface as stagnation zone, heat transfer criterion was obtained used the method of integral. The results of analysis are lower than the experiment data, with an error within 15%, but show the same heat transfer tendency. The comparisons show that the jet impingement in confined annular space are influenced by both the jet velocity and turbulivity.At low jet velocities and large jet mouth-surface distance, compared to pool boiling and forced convective boiling in mini-micro tubes, jet impingement of LN2 in confined annular space show the similar boiling heat transfer characteristics and flow patterns. Periodical oscillations are observed for three heat transfer surface at high jet velocity. Moreover, at high heat flux, the returning flow appears frequently along with the periodical oscillations, which make the oscillation amplitudes increased sharply. At low jet velocities, the oscillations over hemispherical don't show the specific amplitude, and the jet mouth-surface distance makes no remarkably influence to the oscillations. For the remarkable forced convection effect, the oscillation amplitudes of steady-state over the flat with a needle surface are the smallest.