A new heat-transfer model for dropwise condensation with non-condensable gas (NCG) is developed, which can be used in the design and calculations of a dehumidifier in humidification–dehumidification technology. A single-droplet heat-transfer model is established via the rounded analysis of all the contributing thermal resistances based on the diffusion-layer theory. Combined with the drop size distribution, the heat flux of dropwise condensation with NCG is determined, and the error between the predicted results and previous experimental data is found to be within ±20%. The effects of different parameters on the heat transfer of dropwise condensation with NCG are investigated, and the results indicate that there is an optimal contact angle to maximize the heat flux, which depends on the NCG content. The thickness of the coating layer has little effect on the heat flux at a given thermal conductivity of the coating layer, indicating that the dropwise condensation effect can be improved by increasing the thickness of the coating layer, neglecting the heat-transfer deterioration due to the additional thermal resistance during dropwise condensation with NCG.
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