Study On Enhanced Condensation Process And Heat Transfer Under Corona Discharge

The gas-liquid phase change process plays an irreplaceable role in thermal power generation,seawater desalination,artificial environment control and other fields.Exploring a method to efficiently control the heat and mass transfer on the surface of the heat exchanger has become an important issue to promote the sustainable development of world energy.Many studies have been conducted on passive or active enhancements of condensation heat transfer,such as improving surface wetting characteristics or utilizing external fields.Aiming at the problems of high condensation nucleation energy barrier on hydrophobic surface and noncondensable gas layer hindering condensation,this paper proposes a technical route for corona discharge to enhance dropwise condensation heat transfer on hydrophobic surface.Through the visualization and identification of the microscopic behavior characteristics of droplet nucleation,growth,coalescence and shedding on different wettability surfaces,combined with the heat transfer experiments to verify the condensation strengthening effect of corona discharge and system energy efficiency,revealing electrical Mechanism of corona discharge on condensationenhancing processes on surfaces with different wettability.Based on the mechanism analysis and experimental results,a new method to enhance the dropwise condensation heat transfer by combining active and passive technologies is explored,which provides a research basis for the design and application of hydrophobic surface heat exchangers.Firstly,the mechanism of surface properties,electric charge and electrostatic field on the condensation process is discussed from the perspective of kinetics and thermodynamics.The variation law of the Gibbs free energy barrier required for the homogeneous nucleation of water vapor with the size of the cluster is analyzed,and it is concluded that the nucleation rate of water vapor is mainly related to the Gibbs free energy barrier and supersaturation.The work required for the heterogeneous nucleation of vapor increases with the increase of the contact angle,indicating that the energy barrier to be overcome to condense on the hydrophobic surface is higher than that on the hydrophilic surface.The effects of surface charge and dielectrophoresis on the saturated vapor pressure of the condensation surface were analyzed.Secondly,the microscopic behavior characteristics of the nucleation,growth,coalescence and shedding of droplets on hydrophilic,hydrophobic and superhydrophilic surfaces under different negative corona discharge conditions were obtained by high-definition camera observation.An adaptive threshold binarization algorithm was used to identify and analyze the droplet morphology and particle size distribution.The visual analysis results show that negative corona discharge can effectively accelerate the condensation and detachment of droplets on hydrophilic and hydrophobic surfaces,and the closer the distance to the negative corona needle,the better the promoting effect.In addition,negative corona discharge could also promote the formation of more nucleation sites on the hydrophobic surface,which to a certain extent makes up for the defect of low nucleation density on the hydrophobic surface due to high contact angle.The nucleation density and shedding speed of droplets on hydrophobic surfaces also increased with the increase of negative corona voltage.Then,the parametric analysis of the corona discharge enhanced condensation heat transfer system is carried out,and the parameters include surface wettability characteristics(hydrophilic,hydrophobic,superhydrophilic)and negative corona electric field strength(10kV,15 kV,20 kV),in order to further optimize the proposed A combination of active and passive technologies.The advantages of the superhydrophilic surface is that the liquid film has good ductility,low thickness,and low thermal resistance,which is conducive to heat conduction.The disadvantage is that the thickness of the liquid film increases with the increase of the condensation speed,which is not suitable for scenarios with fast condensation speed.The advantage of the hydrophobic surface is that the droplets fall off quickly,which is beneficial to clean the surface and promote condensation.The disadvantage is that the nucleation energy barrier is high and the nucleation density is low.The disadvantage of hydrophilic surfaces is that the droplets are difficult to fall off the surface,and the high thermal resistance of the droplets themselves hinders the surface condensation heat transfer process.Negative corona discharge can promote the condensation heat transfer process on surfaces with different wettability to a certain extent.Under the condition of 15 kV negative corona,the average heat flux density of hydrophilic,hydrophobic and metasurface increased by 38.0%,30.0% and 32.4% compared with the case without corona.Within a certain range,the promotion effect of corona discharge on condensation heat transfer on hydrophobic surfaces increases with the increase of negative corona voltage.The energy efficiency analysis of the corona discharge enhanced condensation heat transfer system is carried out and the results show that the negative corona discharge enhanced condensation heat transfer system has extremely low power consumption,and the power consumption under the conditions of 10 kV,15 kV,and 20 kV negative corona is about0.1W,0.48 W,and 1.4W,respectively.Combined with theoretical and experimental research results,the mechanism of corona discharge enhancement of condensation heat transfer on hydrophobic surfaces is summarized.Finally,the energy efficiency analysis of the corona discharge enhanced condensation heat transfer system is carried out.The results show that the negative corona discharge enhanced condensation heat transfer system has extremely low power consumption cost,and the power consumption under the conditions of 10 kV,15 kV and 20 kV negative corona is about 0.1 W,0.48 W,1.4W.Combined with theoretical and experimental research results,the mechanism of corona discharge enhancing condensation heat transfer on hydrophobic surface is summarized:(1)The directional migration of hydrated ion clusters increases the vapor pressure of water vapor near the condensation surface;(2)The surface charge of the liquid core reduces the water vapor.The energy barrier condensed on its surface;(3)the electric field force promotes the droplets to fall off the surface;(4)the corona wind can effectively destroy the non-condensable gas layer on the surface.