Microscopic Mechanism Of Steam Dropwise Condensation At Low Pressure And Heat Transfer Enhancement

Vapor condensation is a ubiquitous phenomenon in nature and daily life. In industry, condensation plays an essential role in various applications. Condensation heat transfer at low steam pressure has drawn much attention due to its important applications in the field of waste heat recovery, such as low temperature multi-effect evaporation desalination, low temperature heat pump, vacuum boiler and heat pipe. The heat transfer performance greatly reduces with the decrease of steam pressure due to the increased interfacial thermal resistance. Compared with conventional filmwise condensation, dropwise condensation becomes an ideal enhancement technology, which can effectively reduce the thermal resistance of condensate and improve the transfer process at liquid-vapor interface. In this dissertation, droplet dynamic characteristic, droplet size distribution and temperature evolution of droplet surface during steam condensation were investigated by experimental test, numerical simulation and theoretical analysis to clarify the microscopic mechanism of dropwise condensation at low pressue. And the interface effect superhydrophobic surface was used to promote the coalescence-induced jumping behavior to accelerate the renewal frequency of condensed droplets for condensation heat transfer enhancement at low steam pressure.The experimental system of condensation heat transfer at low pressure was designed and set up to investigate the effect of steam pressure on the droplet dynamic behavior and heat transfer performance of dropwise condensation. The results indicated the heat transfer coefficient decreased slowly first and then reduced rapidly with the decrease of steam pressure. Combined with the droplet heat transfer model, the mechanism of condensation heat transfer at low pressure was discussed and it indicated that the effect of transfer process at liquid-vapor interface on droplet growth increased. The retention effect of droplet departure was found in dropwise condensation at low pressue, which revealed the microscopic mechanism between the droplet dynamic and heat transfer performance. By introducing the droplet dynamics, the classical dropwise condensation heat transfer model was modified to predict the heat transfer performance of dropwise condensation at different pressures.The transient characteristics of initial droplet size distribution and surface coverage at different steam pressures were investigated experimentally to clarify the effect of steam pressure on droplet growth. During the transient process, the initial nucleated droplets satisfied lognormal distribution, and then a bimodal distribution formed, finally revealed an exponential distribution. The peak values were smaller and the evolution of peaks was slower at lower pressure. The effect of steam pressure on droplet size distribution revealed a more scattered distribution, larger departure size, sparser small droplets and denser large droplets at lower pressure. The dynamic contact angle hysteresis model was introduced to describe the droplet departure retention at low pressure and explain the rapid shedding of pulsating droplet at atmospheric pressure. From the perspective of the local energy barrier, the non-continuous droplet departure at low pressure was explained.Infrared thermography was used to map the droplet interfacial temperature, intuitively illustrating the microscopic heat transfer process caused by droplet dynamic behaviors. The measurement evidenced non-uniform temperature and gradient between the apex of the droplet and the contact line, which was closely related to surface subcooling. Steam condensation took place discontinuously and unstably, which was influenced by droplet behaviors and interface evolutions. The non-continuous condensation on large droplet surfaces occurred by the refreshment of liquid-vapor interface due to droplet dynamic behaviors, which was different from the heat transfer way of small droplets. Compared with superhydrophobic surface, the hydrophobic surface was more conducive to the droplet nucleation and growth. In the initial stage of condensation, the surface temperature distribution was more non-uniform and the temperature fluctuation caused by droplet dynamics was more severe.Starting from the initial nucleation, the effect of surface structures and condensing conditions on the wetting mode of nucleated droplets was investigated by model analysis. Surface Evolver simulation and experimental observation. According to the structure characteristic of superhydrophobic surface and multiscale growth of condensed droplets, the spatial confinement effect on droplet growth was proposed to explain the wetting transition of condensed droplets on the nanostructured superhydrophobic surfaces. A tapered nanostructured copper oxide surface was fabricated to spatially control the initial nucleation state and the wetting mode of condensed droplets. The invasive factor was introduced to describe the infiltration degree of condensed droplets. And the variation of the apparent contact angle and contact angle hysteresis with surface subcooling was accurately predicted. Due to the energy barrier of cluster growth, droplet nucleation tended to occur in the upper part of tapered nanostructures at the small surface subcooling, which promoted the formation of condensed droplets in partially wetting state and the coalescence-induced jumping. However, with the increase of surface subcooling, the nucleation size greatly reduced and droplet nucleation randomly occurred in nanostructures. As a result, the superhydrophobicity broke down due to the pinning effect of contact line and the adhesion work.By controlling oxidative etching time, four superhydrophobic surfaces with different nanostructures were prepared to investigate the influence of surface structures and condensing conditions on the coalescence-induced droplet jumping. The results indicated that with the increase of length of nanostructures and spacing between nanostructures, the jumping size increased while the jumping frequency decreased. With the increase of surface subcooling, the initial jumping velocity decreased and the optimal jumping size increased due to the increase of infiltration degree of condensed droplets. Condensed droplets formed the completely wetting state with the further increase of surface subcooling. The wetting characteristic was introduced to analyze the effect of surface property and condensing condition on the initial jumping velocity. The results showed that the droplet jumping behavior was decided by the surface structure, condensing condition and droplet size.The wetting mode transition of condensed droplets on superhydrophobic surface was experimentally observed. It showed that the transition from partially wetting to completely wetting with the increase of surface subcooling and the constant wetting mode with the decrease of surface subcooling. Steam condensation heat transfer performances of superhydrophobic surfaces were significantly affected by the wetting state and dynamic behavior of condensed droplets. The heat transfer performance of dropwise condensation at low steam pressure was improved by nanostructured superhydrophobic surface. Compared with filmwise condensation, the heat transfer coefficient of superhydrophobic surface was higher which exceeded the heat transfer coefficient of dropwise condensation on the smooth hydrophobic surface at low surface subcooling. By comparing the heat transfer performance of the three superhydrophobic surfaces, it indicated that the enhancement heat transfer range of condensation on superhydrophobic surface could be widened by the rational design and optimization of surface structures, which provided the experimental foundation and guiding principle for the condensation heat transfer enhancement at low steam pressure.