Experimental And Theoretical Investigations On Microscale Characteristics Of Vapor Condensation Process

For the excellent heat transfer efficiency, vapor dropwise condensation has been given extensive attention by many investigators, thoroughly studies on the characteristics of this process will help further understand the fundamental mechanism, facilitate the vapor condensation heat and mass transfer enhancement and process energy saving. Microscale characteristics of vapor condensation process have been investigated in the present thesis, including the microscale-space and microscale-time characteristics and the effect of microscale space on condensation pressure drop.In the present work, a physical model in terms of the molecular cluster theory is presented to describe the state of steam molecules in bulk steam phase before condensing on the cooled surface and the resultant forming process of primary droplets. With the existence of surface subcooling, the steam molecules become clusters before condensing on the cooled surface, and a certain cluster size distribution forms in the bulk steam phase close to the surface. After then, the clusters contact the cooled surface and deposit on it randomly, acting as the nucleate embryos. Since the clusters'energies will not dissipate instantly, some of the deposited clusters are able to migrate on the surface in a limited area and incorporate or be incorporated by the other clusters on their ways, accompanied by the condensation of other clusters from vapor phase. At the same time, the evaporation on the surface is on-going as well. After energies have been dissipated completely, the clusters stay on the locations with high free energies, such as the pits and grooves on the surface. Finally, if the seated, post incorporated clusters were not smaller than the minimal size defined in the classic dropwise condensation theory, they become the primary droplets and grow up through the condensation of the other clusters on their surfaces. Molecular dynamics (MD) method was used to simulate the clustering process of steam molecules within a vapor layer near to the condensing surface.To prove the clustering model for steam condensation, the presupposition is whether the vapor molecules become clusters before condensing on the cooled surface. Inferring from the results of the molecular cluster theory, if the size distributions of the primary droplets and the droplets growing up through direct condensation without coalescence on the condensing surface satisfy the Lognormal distribution function, the same size distribution should exist in the bulk gas phase and steam molecules should become clusters before condensing. To verify the present physical model, the initial dew point condensation process of moist air has been investigated experimentally using high speed camera and microscope, the size distribution corresponding to the droplets with size of several microns before coalescence was obtained. The results show that the droplet size distribution obviously satisfies the typical cluster size distribution, Lognormal distribution. Furthermore, the size distribution corresponding to the primary droplets with size of several nanometers was analyzed based on the experimental image reported in literature, the primary droplet size distribution satisfies Lognormal function as well. These results indicate that the size distribution of the droplets from primary to pre-coalescing size satisfy the Lognormal function. In addition, after comparing the initial condensation processes on the surfaces with different wettabilities, it was found that the initial stages of dropwise and filmwise condensation are identical, from nucleating to growing up through direct condensation and then to coalescing, their discrepancy just occurs after coalescing among droplets. The contact lines of droplets on hydrophobic surface can get back to be circular immediately, while those on hydrophilic surface are pinned and remain the shapes right after coalescing. It can be concluded that the liquid-solid interfacial effect influences the condensation mode through affecting the behavior of contact line of condensate.The transient droplet size distributions in the first sub-cycle of initial dropwise condensation were investigated experimentally, and the characteristics on micro-time-scale of vapor condensation have been revealed. The size distribution from primary droplets to those before coalescence (first generation droplets) satisfy Lognormal function, there is only one droplet number peak on the size distribution plots, resulted from the growing and coalescing, the position of the droplets number peak corresponding to the first generation droplets shifts towards the direction with larger droplet size and however with its value decreasing. At the same time, on the bare areas appearing on the condensing surface resulting from coalescences among droplets, nucleation occurs again, then another droplets number peak appears at the droplet radius of about 0.5 to 1μm constantly, the bimodal size distribution forms. Also, the droplet number decreases rapidly due to the coalescence of the first generation droplets, resulting in lower and lower value of first droplet number peak, and it disappears finally. On the other hand, the initially nucleated droplets periodically form on the bare areas always have the same size, so the location of second droplet number peak keeps constant relatively. After long enough time evolution, the droplet size distribution approaches to the steady state as described in classic dropwise condensation. In addition, the investigations on initial dropwise condensation of different pressure steam on polycarbonate (PC) surface indicate that the evolutions of transient condensation stages on low thermal conductivity surface are affected by steam pressure obviously, demonstrating different features from the steady state dropwise condensation process. The abrupt contraction pressure drop at entrance of and the condensation frictional pressure drop in the parallel multi-microchannels were investigated experimentally in the present work to elucidate the effect of micorscale space on condensation pressure drop. Pressure and temperature before and after the entrance of parallel multi-microchannels have been measured using compressed air and steam as working fluids. The results indicate that the existing correlations which were commonly used to predict the abrupt contraction pressure drop coefficient in single channel with commercial and small size cannot reasonably predict the trend for the case concerned in this work. A new empirical formula for the abrupt contraction pressure drop coefficient against Reynolds number (range from 3100 to 19000) in one single micro-channel for air has been correlated, and its availability has been tested by the pressure drop data of saturated steam single phase flow. Microchannel condensation frictional pressure drop gradients predicted by the most accepted models, such as those by Koyama et al, Garimella et al, and Cavallini et al for R134a, NH3, FC-72 and steam have been compared at different mass fluxes of 100,300,500 and 700 kg·m-2·s-1, the obvious discrepancies among the predictions were found. Frictional pressure drop of steam and FC-72 condensation in parallel multi-microchannels (consist of 6 parallel 1 mm×1.5 mm channels with 1.5 mm in space between each other machined in aluminium substance) were measured very accurately and compared with the three models calculations, mass flux ranges of FC-72 and steam were 140～750 kg·m-2·s-1 and 80～200 kg·m-2·s-1, respectively. Koyama and Garimella models can estimate steam condensation frictional pressure drop within the deviation of±20%, Cavallini model overestimates it for about 3 folds; while for FC-72, Cavallini model has a better agreement with our measurements. Koyama model was modified to be able to predict frictional pressure drop of FC-72 and steam condensation in microchannels finally.