| Condensation in microchannels widely exists in pulsating heat pipes,micro heat exchangers and micro fuel cells etc.,which attracts more and more researchers' attention.The heat and mass transfer coefficients in microchannels are several orders higher than those in traditional tubes for their large specific surface-area in limited space.Surface tension behaves as the dominate force in microchannel,which is totally different from traditional tubes where the gravity force plays an important role.The mechanisms of condensation heat transfer enhancement are also different in the two kinds of tubes.During the ethanol-water vapor mixture condensation on a plate,even if on a hydrophilic surface,the form of the condensate film changes and drops are generated for the equivalent surface free energy difference between condensate and liquid film.This non-film condensation is called pseudo-dropwise or Marangoni condensation.The special condensation mode provides a new approach for the control of the microchannel condensation and it is worth investigating the binary vapor condensation in microchannels.A new experimental platform has been constructed in this study.Ethanol-water vapor mixture condensation in microchannel has been systematically studied including the two-phase flow pattern,frictional pressure drop and the heat transfer performance.The surface free energy difference effect on the condensation flow pattern,the frictional pressure drop and the heat transfer performance have been revealed.The results and conclusions are summarized as follows:Visualization study on two-phase flow patterns of ethanol water vapor mixture condensation in trapezoidal microchannels is conducted with high speed camera system and microscope.Annular flow,streak flow,churn flow,droplet flow,injection flow and bubble flow are observed in microchannels under different inlet ethanol concentrations.Variations of surface free energy difference at different microchannel positions and input ethanol mass concentrations are calculated in the paper.The influence of the surface free energy on evolution of the flow patterns is studied.It is found that with the increase of the surface free energy difference,droplets appear on the condensation surface.Droplets could be generated to form the streak flow by the driven force of the vapor side,and the streak flow could merge into the side wall and form a stronger fluctuant liquid film in microchannel.The criterion on streak flow pattern transition is also proposed in the paper.The frequency and flow pattern maps of injection flow for ethanol water vapor mixture condensation in trapezoidal microchannels are also developed according to the surface free energy difference,which could predict the flow pattern transition and lay the foundation for the flow pattern control.The silicon microchannels of triangular cross section are fabricated to provide a condensation surface with temperature gradient,which could control the mixture vapor condensation flow pattern in microchannel.Visualization study on two-phase flow patterns of ethanol water vapor mixture condensation in triangular microchannels is performed with high speed camera system and microscope.Two-phase flow patterns are observed in microchannels under different inlet ethanol concentrations.The same as study in trapezoidal microchannel,variation of surface free energy differences at different microchannel positions and input ethanol mass concentrations are calculated in triangular microchannels.Different from the flow patterns in trapezoidal microchannel,the streak flow and the droplets exhibit spontaneous movement on the condensation surface in triangular microchannel.The surface free energy differences on the condensation surface of triangular microchannels are calculated under different temperature gradients,which reveals the internal mechanisms of droplets and streak self-migration under the condensation on a temperature gradient condensation surface.The frequency and flow pattern maps of injection flow are proposed for better understanding of the ethanol water mixture vapor condensation.The pressure drop characteristics and the heat transfer performances for the ethanol water mixture vapor condensation in different microchannels are both studied.The total pressure drop,frictional pressure drop,decelerated pressure drop,inlet pressure drop and outlet pressure drop are calculated with considering the local pressure drop under different inlet mixture vapor mass fluxes and inlet ethanol mass concentrations.It is found that the total pressure drop and accelerated pressure drop decrease with the increase of the ethanol concentration,while the frictional pressure drop doesn't show a clear change.The experimental results are compared with other existing models and it is found that the existing models could not accurately predict the ethanol water mixture vapor condensation flow in microchannels due to the special flow patterns under higher surface free energy difference.New transition criterion on the emergence of streak flow pattern is proposed based on Kim's universal pressure drop model in microchannels and the predicted data are in good agreement with the experimental values.The heat transfer coefficients of different inlet ethanol concentrations and mass fluxes are calculated with the modified pressure drop model in microchannels.It shows that the heat transfer coefficients increase with the increasing mass flux.The heat transfer coefficients keep constant and then decrease with the increase of inlet ethanol concentration in triangular microchannel,while they rise up at the beginning and then move down with the increase of the inlet ethanol concentration in trapezoidal microchannel.It is found that the heat transfer coefficients in triangular microchannel are higher than those in trapezoidal microchannels under the same inlet ethanol mass concentration.The heat transfer model of ethanol and water vapor mixture condensation is modified by considering the effect of surface free energy difference,and the results are in a good accordance with experimental values. |
PDF Full Text Request