Mixed Condensation Heat Transfer Model On Heterogeneous Surface With Hydrophilic/hydrophobic Stripes

Condensation heat transfer generally exists in nature and human life,and is an indispensable energy transmission path for industrial production.Dependent on the surface wettability,condensation is divided into two modes:filmwise condensation and dropwise condensation.Usually,hydrophilic surface behaves filmwise condensation but hydrophobic surface behaves dropwise condensation.The film-like condensation heat transfer coefficient is low,and the drop-like condensation heat transfer coefficient is higher than that of the film-like condensation.Due to the surface processing technology,it is difficult to maintain the high heat transfer and strong durability of the completely hydrophobic surface at the same time.Film-like droplet-like mixed condensation occurs on the heterogeneous surface with hydrophilic/hydrophobic stripes,introducing a new drop-off mode,which has a larger room for heat transfer.However,The research of condensation heat transfer on heterogeneous surface mainly focuses on the experiment.There still lack model to guide the design of heterogeneous surface to enhance condensation heat transfer.This thesis paper establishes a mixed condensation heat transfer model on heterogeneous surface with hydrophilic/hydrophobic stripes.This thesis paper establishes a mixed condensation heat transfer model on heterogeneous surface with hydrophilic/hydrophobic stripes.Found the critical condition that the heat transfer effect of condensation on heterogeneous surface is better than that of completely hydrophobic surface.Provides theoretical guidance for optimizing the heterogeneous surface structure and improving the heat transfer performance of condensation.On the heterogeneous surface,dropwise condensation occurred on the hydrophobic stripes,while filmwise condensation occurred on the hydrophilic stripes.In the condensation heat transfer model,condensate droplet detachment modes and size on hydrophobic stripe are focused on.Energy-mass transfer occurs at the boundary of the hydrophilic-hydrophobic stripes on the heterogeneous surface,which affects the droplet shedding pattern and size on the hydrophobic stripes.(1)When the condensate droplet on the hydrophobic stripe touches the hydrophilic-hydrophobic boundary line,it is sucked away by the neighboring hydrophilic stripe.Because the size of the sucked-off liquid droplets is small,the dropwise condensation heat transfer on hydrophobic stripe is enhanced.(2)It is noted that the radius of the sucked droplet is related to the distance from the droplet to the hydrophilic-hydrophobic boundary line.(3)When the width of the hydrophobic stripe(WDWC)exceeds a critical value,the droplet detachment mode in the center of the hydrophobic stripe switches to sliding.The hydrophilic stripe absorbs droplets from the hydrophobic stripe(excluding the droplets that slide off the center of the hydrophobic stripe),causing the liquid film on the hydrophilic stripe to become thicker,and the thickness of the liquid film is the key to affecting the film-like condensation.Therefore,the hydrophilic stripe sacrifices its own heat transfer performance and improves the heat transfer performance of the hydrophobic stripe.As result,the model prediction matches well with the measurement data in literature for mixed condensation heat transfer model on heterogeneous surface.It is found that both heat transfer coefficients on hydrophobic stripe and hydrophilic stripe decrease with increased WDWC.However,when the WDWC is increased,the area ratio of dropwise condensation to filmwise condensation is increased.Therefore,there is an optimal hydrophobic stripe width(WDWCo)to obtain maximum heat transfer coefficient on the heterogeneous surface.Base on the model analysis,this paper identifies the key factors that affect WDWCo from six parameters:the thickness of the hydrophobic coating(δp)and the width of the hydrophilic stripe(WFWC).With the combination of small δp and large WFWC(δp≤1μm and WFWC≥0.7 mm),the condensation heat transfer coefficient of heterogeneous surfaces is always smaller than that of completely hydrophobic surfaces.In the case of other combined parameters(δp>1 μm or WFWC<0.7 mm),when WDWC is optimized according to the model in this paper,condensation heat transfer can be enhanced.WDWCo increases with decreasing δp or increasing WFWC.