Vertical length effect on dropwise condensation heat transfer at low heat flux - Part II: Theoretical model suitable for non-coated metal surfaces

Rivulet flows can form and thereby affect heat transfer during dropwise condensation on long vertical surfaces. Under noncondensable gas conditions, including in heat exchangers in small modular reactors, droplet/rivulet heat transfer becomes more complicated. A new dropwise condensation heat transfer model is proposed herein to incorporate vertical length effects in the presence of noncondensable gas. Heat transfer through rivulets and the sweeping effects of rivulet flows were modeled to obtain heat flux through node-based calculations. Droplet/rivulet parameters, which are crucial for calculating heat transfer in the model (departure droplet radius, rivulet velocity, and sweeping period), were compared to experimental results. The model's prediction of heat flux is in good agreement with the experimental data and exhibits better accuracy than existing models, with an error range of -40 % for 76 % of the data.