Fluid Flow And Heat Transfer Of Steam And Steam-noncondensable Gas Mixtures Condensation In Microchannels

As an important component part of the modern miniaturization technology, microchannels play predominant roles in heat and mass transfer fields. The study on flow and heat transfer characteristics of condensation with microchannel as carrier is also becoming an important subject, however, the mechanisms on two-phase flow and heat transfer of phase change in microchannels need to be further studied. A new steam condensation experimental platform has been constructed in this study, and a unique experimental technology was designed to measure the local heat transfer rate and steam quality and dimensionless parameters by testing the temperature profile of copper cooler measured by thermocouples. On this basis, the flow patterns and their transition mechanism, the characteristics of heat transfer and pressure drop for steam and mixture gas condensation in microchannels were investigated systematically. This can offer dependable references for micro-sized condenser in practical engineering application.The visualization sudy on two-phase flow patterns of steam condensation was conducted using high speed video system and microscope, and the flow pattern maps for steam condensation in silicon microchannels were also developed using steam mass flux and steam quality as the coordinates. This study further investigated the effect of steam mass flux and microchannel geometries on the flow patterns and their transition, analyzed the importance of acting force at the flow patterns transitional position, and offered a criterion on two-phase flow patterns transition based on the critical quality correlation. The visualization study indicated that four main flow regimes, annular, droplet, injection and intermittent (bubble) flow appeared in the silicon microchannels; two modes of injection flow were observed in the microchannel, single bubble injection mode and multiple bubble injection mode; the pulsation of injection flow was mainly influenced by aspect ratio; the deformation of bubbles was induced by surface tension. It can also be found that the region occupied by annular/droplet flow in the microchannel increased with the increasing steam mass flux, steam quality or microchannel scale. The dimensionless analysis domenstrated that surface tension and inertia played major roles in determining the flow patterns.The characteristics of two-phase flow frictional pressure drop for steam condensation in silicon microchannel with different geometries were studied. The effect of steam mass flux, steam quality, cooling water flow rate, hydraulic diameter and microchannel aspect ratio on two-phase frictional pressure drop were investigated. After compared our experimental results with other existing models, a new frictional pressure drop predictive model was established based on experimental data. The calculated and experimental results domenstrated that the effect of deceleration pressure drop, contraction pressure drop between inlet plenum and microchannel outlet on total pressure were more significant than other pressure loss; the local two-phase frictional pressure drop increased with the increasing of steam mass flux or steam quality, and the cooling water volume flow rate also affected the local frictional pressure drop. It can also be concluded that the local two-phase flow frictional pressure drop was not only influenced by microchannel hydraulic diameter but also by aspect ratio. When the hydraulic diameter was smaller or aspect ratio was larger, the higher frictional pressure drop would be obtained. The existing pressure drop predictive model can not predict the frictional pressure drop under this experimental condition accurately. New pressure drop predictive models was thus developed for steam condensation in microchannels, taking into account fluid properties, forces acting on the fluid and microchannel geometrical parameters, which showed good agreement with experimental data.The characteristics of heat transfer for steam condensation in microchannels were studied. The steam temperature can be determined according to the saturation pressure, which can be calculated via the new pressure drop predictive model. The local heat transfer rate can be measured by testing the temperature profile of copper cooler. The thermal resistance of silicone grease was also measured, and then the temperature of microchannel inner surface and the local heat transfer coefficient can be determined. The experimental results showed that the condensation heat transfer coefficient increased with the increasing steam mass flux, steam quality and the ratio of effective heat transfer area and volume. The heat transfer model established by Moser was modified, and the simulation results were in a good accordance with experimental values.The study on steam condensation with non-condensable gas was carried out. The effect of noncondensable gas on condensation two-phase flow patterns and the characterastics of heat transfer and frictional pressure drop were investigated. The visualization study results showed that the special intermittent annular flow was found in the microchannel under the condition of larger mole fraction of noncondensable gas; the apical area of injection was much larger and the neck of injection was longer for mixture gas with lower mole fraction of noncondensable gas in comparison with pure steam condensation; meanwhile, the noncondensable gas resulted in the decrease of flow patterns transitional steam mass flux and qulity. The experimental results also indicated that the friction pressure drop increased with the increasing mole fraction of noncondensable gas when the steam mass flux was fixed. Unlike nature convective condensation heat transfer, the mole fraction of noncondensable gas had little effect on Nusselt number. Based on experimental data, the predictive correlation of Nusselt number for mixture gas condensation in microchannels was established, which can make an accurate prediction.