Flow And Heat Transfer Of Liquid Nitrogen In Micro-Tubes

For the applications in micro cryogenic surgical apparatus (MCSA), the cooling of high temperature super-conductivity (HTS) and other fields, the flow and heat transfer of liquid nitrogen in micro-tubes are investigated systematically.Experiments have been conducted to study the single-phase pressure drop and heat transfer characteristics of liquid nitrogen in four micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The friction factors are compared with the conventional correlations over a Reynolds numbers of 10,000-90,000. The effect of the variable thermal properties of liquid nitrogen, i.e., viscosity and thermal conductivity, on the flow and local heat transfer in the micro-tubes is clarified. The average Nusselt numbers are determined and compared with the correlations for the conventional and micro channels.The flow boiling of liquid nitrogen in micro-tubes are also investigated. The contents mainly include the onset of nucleate boiling (ONB), two-phase flow pressure drop, heat transfer characteristics and critical heat flux (CHF). At ONB, mass flux drops suddenly while pressure drop increases, and apparent wall temperature hysteresis of 1.0 to 5.0 K occurs. Modified Thom model can predict the wall superheat and heat flux at ONB. The adiabatic and non-adiabatic two-phase flow pressure drop characteristics in micro-tubes are examined and compared with four models. Contrary to the conventional channels, homogeneous model enables to predict the experimental data well, whereas the prediction divergences of three separated flow models are rather large. It can be explained by the fact that the density ratio of liquid to vapor for nitrogen is comparatively small, and the mixing of liquid and vapor phase in micro-tube is sufficient at high mass flux. The influences of heat flux, mass flux, pressure and tube diameter on the flow boiling heat transfer coefficients are investigated systematically. Two regions with different heat transfer mechanism can be classified: the nucleate boiling dominated region for low mass quality and the convection evaporation dominated region for high mass quality. For none of the existed correlations can predict the experimental data, a new correlation expressed by Co, Bo, We, Kp, and X is proposed. For 1.931 mm tube, the flow boiling heat transfer characteristics are similar to those of macro channels, and the heat transfer coefficient can be estimated by Chen correlation. Both the CHF and the critical mass quality (CMQ) are higher than those for the conventional channels. According to the relationship that CMQ decreases with the mass flux, the mechanism of CHF in micro-tubes is postulated to be the dryout or tear of the thin liquid film near the inner wall. It is found that CHF increases gradually with the decrease of tube diameter. Moreover, the flow boiling of liquid nitrogen in the narrow annular channels are studied. It is found the pressure drop and heat transfer characteristics are similar to those in micro-tubes.Especially, a kind of stable long-period (50-60 s) and large-amplitude oscillations of mass flux, pressure drop and wall temperatures are observed at ONB for the 1.042 and 1.931 mm micro-tubes. Block phenomenon at ONB is also observed in the cases of high mass flux. The regions for the oscillations, block and stable flow boiling are divided. A physical model of vapor patch coalesced at the outlet is proposed to explain the ONB oscillations and block, and then a mathematic model are proposed to investigate the two-phase flow instability in micro-tubes. The influences of heat flux, micro-tube diameter, the inlet subcooling and system pressure on the inner characteristic curves are obtained. To improve the micro-system stability, three strategies including the decrease of the inlet subcooling, adoption of constant mass flux pump and separated discharge of vapor and liquid phase are proposed.The vapor-liquid two-phase flow patterns of nitrogen are visualized with a high-speed camera in micro-tubes. Four typical flow patterns, namely, bubbly, slug, churn and annular flow are described. Amount of mist and wisps liquid entrainments in the central vapor core are observed not only in the annular flow, but also in the churn flow. This point provides the evidence that the mixing of liquid and vapor phase in micro-tube is sufficient, and can explain why the homogeneous model can predict the pressure drop well. The two-phase flow pattern maps are drawn and compared with those for macro-channels and other fluids. It is found that the annular flow is the main flow pattern, and the surface tension dominated region (slug and churn flow) reduces significantly. Correspondingly, the transition from the bubble flow to slug flow shifts to the higher gas superficial velocity, and the transition lines of slug/churn and churn/annular flow move to the lower gas superficial velocity. Three kinds of long-period and large-amplitude flow pattern transition oscillations are found, and the instantaneous images are used to depict the whole oscillation process.