چكيده به لاتين
The phase change from vapor to liquid at constant pressure is called condensation. Condensation on the surface is done in two ways, layer and dropwise. In layered condensation, a layer of condensed liquid covers the entire surface, while in dropwise condensation, individual droplets of very different sizes are dispersed on the surface. dropwise condensation occurs on hydrophobic and superhydrophobic surfaces. The heat transfer rate of droplet condensation is highly dependent on the preparation of the hydrophobic surface. The two main characteristics of hydrophobic surfaces are the presence of micro-nano structures and their low surface energy. In fact, if the condensing surface has a low surface energy, such as hydrophobic or superhydrophobic surfaces, the vapor condenses as tiny droplets in the nucleation sites, but if the condensing surface has a high surface energy (which is usually the case with metal surfaces), the vapor It condenses as a layer of liquid on the surface. Due to the low thermal conductivity of the condensed liquid in layered condensation, the heat transfer capacity of this regime is much lower than dropwise condensation.
One of the methods of creating dropwise condensation is changing the contact angle of the surface by creating a microstructure on the surface. The morphology of droplet placement on these structures is Wenzel, Cassie and Partial Wenzel, the droplet can have a constant contact angle with the surface and variable radius, or it can grow with a constant contact surface with variable contact angle and surface and change the radius. until the contact angle reaches a maximum value and after reaching a certain radius, it is removed from the surface by the weight force prevailing over the surface tension force. The approach of this thesis is to numerically model the growth of single droplets, coalescing and condensation of droplets on surfaces with different wettability of microstructures and taking into account the presence or absence of contact angle changes during condensation. In each time step, if the contact surface of the droplet with the surface is less than the width of the microstructure, the droplet grows with a constant contact angle and the radius of the contact surface increases, and if the droplet completely covers the surface of the microstructure, the growth changes with the contact angle and the radius remains constant. surface contact occurs. It can be seen that the amount of growth and heat transfer rate of small droplets with a low contact angle is higher than that of large droplets with a high contact angle.
