Inner Flow And Heat Transfer Characteristics Of Condensing Droplets In Vapor Flow

Dropwise condensation is an effective phase change heat transfer phenomenonwhich is widely used in air conditioners and power plants. High heat transfer coefficientis required in order to meet the requirements of energy saving. Many researches havebeen made to investigate aspects affecting the heat transfer coefficient of dropwisecondensation, such as the curvature, the interphase heat transfer and thermalconductivity. However, few of these researches care about the effect of the externalsteam flow on heat transfer.Based on micro-flow and heat transfer, this article mainly studies the steam flowrate effects on the droplet inner flow during condensation. The concerned dropletdiameter is about several millimeters, which is close to departure diameter of dropwisecondensation. This article provides a theoretical basis for a comprehensiveunderstanding of dropwise condensation heat transfer mechanism and applications. Avisual experimental facility is set up for single droplet condensation, the condensationheat flux through a single droplet is found to increase with the steam flow rate. Affectedby the droplet interface surface tension, the steam velocity increase has little effect onthe geometry of the droplet condensate, but can cause convection inside the droplet,thus enhancing heat transfer. In order to observe the droplet inner flow field, tracingparticles are added inside the droplet. Particle Velocity Imaging(PIV) method is alsoused in this article. The experimental results show that a flow vortex exists inside thedroplet in the shearing action of the steam. As the steam flow velocity increases, theinner flow rate linearly increase. The droplet inner flow velocity is about2%-5%of thesteam flow velocity within the range of experimental parameters.This article proposes a droplet interface shear stress model and a heat transferthermal model for a droplet in steam flow, and conducts a CFD calculation. The resultsshowed that the steam flow rate and the contact angle are both related with the heattransfer coefficient of the condensing droplet. Only when the droplet contact angleexceeds90°, steam flow significantly enhances condensation heat transfer, and the heattransfer coefficient is100%-250%higher than that without flow. Compared with purethermal conduction model through the droplet, the steam flow model will get a larger temperature difference of interphase heat transfer and thus enhance the condensationheat transfer. In addition, this article finds that the effects of gravity can be ignored inthe study of the condensing droplet inner flow within a steam flow. Therefore, for acertain flow rate of the steam condensation process, the shear stress caused by steamflow at the vapor-liquid interface is the main force driving the droplet inner flow. Theinner flow reduces the condensation heat transfer resistance to some extent, and the heattransfer can be highly enhanced.