Majority of industrial processes such as air conditioning systems and fuel cells involves steam condensation for effective operations. This is due to the large latent heat of water involved in such processes. Presently, reducing the size of condensers and enhancing the heat transfer performance have attracted the interest of researchers thus understanding the steam-side condensation is highly desirable. In a recently published research work, the transition from the filmwise condensation to dropwise condensation has shown a great improvement in the heat transfer. This is a potential consideration in the analysis of flow condensation in hydrophobic mini-channels.
Dropwise condensation can be affected by various factors: droplet sizes, nucleation mechanisms, nucleation density among others. Considering the different effects of individual parameters, development of various strategies to ensure high heat transfer by mitigating their respective effects is highly desirable. For instance, appropriately altering the nanostructure dimensions and spacing has led to the achievement of desirable nucleation density. On the other hand, gravitational forces can use used to induce the coalescence and departure of droplet nucleation in dropwise condensation. Unfortunately, the effects of various modifications and factors affecting the dropwise condensation as well as the development of correlations to predict the dropwise condensation are missing.
To this note, Kansas State University researchers: Dr. Xi Chen and Professor Melanie Derby developed a model for predicting the dropwise condensation. In particular, the model comprised of a droplet departure, and droplet distribution in which heat transferred occurred through a single droplet for different flow conditions. They purposed to determine the heat transfer rates of single droplets and condensation surfaces and also develop various correlations to predict the dropwise condensation. Their research work is currently published in the journal, International Journal of Heat and Mass Transfer.
Briefly, the authors started their work by exploring the heat transfer correlation for dropwise condensation in hydrophobic channels and comparing it with other types of condensation such as filmwise condensation. Secondly, both of the filmwise and dropwise condensation areas were weighed to model the heat transfer coefficients on hydrophobic surfaces. Additionally, resistor analogy and power-law function methods were used to analyze heat transfer in single droplets and development of droplet size distribution respectively. This also enables accurate estimation of the dropwise condensation transfer coefficients. For determination of the droplet departure size, both the drag and droplet-solid adhesive forces were taken into account. Lastly, the results obtained from the developed model were compared to the available data for experimental flow condensation heat transfer.
The study by Xi Chen and Melanie Derby showed successful and excellent agreement between the experimental and model data. For instance, 0.95mm and 1.8 mm mini-gaps produced a heat transfer correlation with mean absolute errors of 9.6% and 8.8% respectively. Additionally, it was noted that the droplet departure size significantly influenced the heat transfer coefficient. Furthermore, the model required no curve fitting, therefore, resulting in mean absolute errors within the desired range. In summary, the study will advance the use of dropwise flow condensation in various industrial processes.
Chen, X., & Derby, M. (2018). Droplet departure modeling and a heat transfer correlation for dropwise flow condensation in hydrophobic mini-channels. International Journal of Heat and Mass Transfer, 125, 1096-1104.Go To International Journal of Heat and Mass Transfer