Droplet dynamics and heat transfer for dropwise condensation at lower and ultra-lower pressure

• Transients of initial droplet size distribution were experimentally studied.
• Surface coverage of steady condensation was strongly dependent on steam pressure.
• Effective heat transfer area reduced and resistance increased at low pressure.
• Heat transfer resistance distribution was sensitive to the steam pressure.

To investigate the transient characteristics of initial droplet size distribution, steady droplet size distribution and thermal resistance distribution at lower and ultra-lower steam pressure, dropwise condensation at the pressure range from atmospheric to 1.5 kPa has been studied. During the transient process, the initial nucleated droplets satisfied lognormal distribution, and then a bimodal distribution formed, finally revealed an exponential distribution. The peak value was smaller and the evolution was slower with the reduction of steam pressure. The corresponding surface coverage increased to 0.7–0.8 at the steady condensation which was strongly dependent on the pressure. Introducing a dimensionless time, the surface coverage evolution indicated that the time consumed by direct growth increased as the pressure decreased. The effect of steam pressure on droplet size distribution revealed a more scattered distribution, larger departure size, and denser large droplets at low pressure, resulting in the reduction of the effective heat transfer area. By comparing the thermal resistance distribution at various pressures, it showed that large droplets induced a greater proportion of resistance at low pressure. The findings help clarifying the limitations of droplet growth mechanism and offer guidelines for the optimization of surface morphology to enhance the steam condensation at low and ultra-low pressure.