Hidrofobik mikro-talaşlanmış yüzeylerde buhar yoğuşmasında oluşan ısı transferi ve yüzey ıslanma olgusunun incelenmesi

Efficiency and performance of a system that involves condensation process have been one of the primary considerations over the last decades for a number of applications including thermal management systems such as vapor compression (refrigeration) cycle, electronic cooling etc. In fact, predicting the performance of a system is essential to assess and enhance system components such as condensers and evaporators. Thus, the primary objective of this study is to investigate the condensation phenomena using various characteristics of surface topographies and surface wetting at different saturation pressures ranging from 1.02 Bar to 1.25 Bar. For this purpose, a number of tasks were completed to provide an in depth understanding. The first task focused on analyzing the performance of a heat pipe at various orientations of a condenser (-90º to +90º). In addition, the condenser part of a heat pipe was considered as a vertical plate with different surface topographies such as unstructured, square groove and V-groove. Then, heat flux and heat transfer coefficients were calculated as a second task based on the experiments carried out over untreated surfaces. In addition, some specifications of surface topographies including surface energy and droplet contact angle were examined while two treated cases before and after coating by Ultra-ever-dry chemical were studied. Another task was accomplished by comparing experimental results of a set of studies by other authors with analytical results of the current study for each sample before coating at a constant vapor pressure of 1.02 Bar. In the final task, IPTES silane coating was applied at various vapor pressures to observe the condensation process on a vertical surface with different topographies and wettings (hydrophobic surface). Today's conventional thermal management systems working by means of conduction and convection which are not sufficient when a high amount of waste heat is generated. Tightly packaged double-sided PCBs and high power LEDs can be considered as examples of such thermal problems as they produce undesired local heat that may lead to device failures. To ensure the system reliability, more effective new cooling techniques need to be developed. Thus, a special study with a heat-pipe-embedded PCB was performed. Moreover, a series of numerical models were developed to determine the local temperature distribution and validate analytical and experimental results. The results show that spreading resistance is significantly lowered due to the effective heat spreading when heat pipe is flattened and embedded inside the plates with known conductivities. Thermal resistances at each case were also compared with the conventional cooling of a PCB made up of polymer or aluminum. The proposed cooling method shows a reduction in the overall resistance by approximately 50%. Furthermore, novel surface characteristics were studied as another task during the condensation experiments. Hydrophobic surfaces are some of those novel surfaces that are widely used to increase liquid repellence typically by controlling liquid-solid contact angle. These micro-nano-textured surfaces are fabricated by using advanced manufacturing technologies and methods including etching, electrospinning, and chemical vapor deposition. While these fabricated surfaces can be used for a variety of purposes, such as protecting a material from external effects and avoiding deposition, they are also considered to improve heat transfer mechanisms. Thus, a set of experimental and analytical studies was conducted to investigate the influence of micro-nano-textured surfaces on liquid wetting phenomena. Since the liquid-wall interaction is a key parameter for droplet formation, the effect of liquid wetting phenomena on heat transfer performance of a heat pipe condenser was also investigated as another interest of this study. For this purpose, copper samples with different surface topographies at micro-scale were subjected to a chemical treatment process by using dip-coating and spray-coating techniques. Before and after coating with Ultra-ever-dry, measurements were performed with deionized water in order to determine the difference in droplet contact angle at the surfaces to be used. In the analytical study, capillary Laplace equation with available analytical correlations from the literature was used to predict the contact angles and surface energies. Results clearly show that the coated square-grooved surface has a larger average contact angle than the v-grooved surface by 4.6 %. The influence of super-hydrophobic surface on the condensation heat transfer rate compared to the untreated surfaces was also investigated and explained in this task. Thus, experimental investigations on liquid transport and heat transfer during water vapor condensation on subcooled, nano-coated microstructured copper substrates were performed in a generic setup. Two types of surfaces were used, an unstructured sample as a reference surface and a sample with longitudinally oriented micro-structures of square geometry. The first set of experiments was performed with micro-structured surfaces without coating of nano particles while other measurements were carried out after spray-coating the micro-structures with nano particles using a commercially available mixture (Ultra-ever-dry). Finally, the effect of a different surface topography was studied by comparing surface temperature, heat flux and HTC. In general, it is found that HTC drops for all surfaces as temperature difference (Tv,ch – Ts) increases. It is also realized that micro-grooves on the surfaces do not result in a significant enhancement of HTC and heat flux which show a similar behavior with the unstructured surface. Specifically, the coated surfaces show lower HTC values compared to the uncoated samples. Another task focused on understanding the fundamentals of condensation phenomenon during dropwise and filmwise modes on micro-structured surfaces and validating the experimental results in the condensation process at 1.02 Bar. Analytical calculations and experiments for condensation were carried out at vertically aligned copper substrates. An experimental setup was used to observe the impact of three different surface topographies on the condensation process under various operation parameters such as saturation pressure of vapor and inlet temperature of cooling liquid. Experimental heat transfer coefficients measured at an unstructured surface was found to be higher than those of the surface embossed with V-grooves and square-grooves by 30 % and 7.2 % respectively. Moreover, the impact of vapor pressure inside a vapor chamber on condensation at various surface topographies and wetting (hydrophobicity) conditions was studied. The hydrophobic surface was prepared in a dip coating process by applying perfluoro-silane coating since it causes a strong promotion of DWC as the droplet contact angle is increased. The effects of the vapor pressure on DFWC on both untreated and treated surfaces that include the unstructured and micro-structured surfaces were performed at various Re numbers. Experimental results show that an increase in the droplet contact angle with coating is achieved for all surfaces. As a result, after the coating is applied, the largest droplet contact angle is obtained (130.9º ±2.0º) for the microstructure square groove surface when compared to other samples. In conclusion, thermal resistance of the embedded heat pipe is found to be lower than the conventional heat pipe's thermal resistance and the change in the effective thermal conductivity of the embedded heat pipe with respect to the angle of the heat pipe inclination was investigated. As the angle of inclination is changed from -90° to 90°, the effective thermal conductivity increases by 68.7% at the maximum heat flux of 76 W/cm2. Furthermore, applicability of ultra-ever dry coating in condensation tests was questioned. It was determined that Ultra-ever dry coating is not a suitable coating technique for the condensation tests because this type of coating cannot be applied in high temperature gradients although it provides a large contact angle more than 165° over super-hydrophobic surface. On the other hand, unstructured surface gives a good enhancement in condensation heat transfer than micro-structures. The condensation heat rate is elevated with the rise in vapor pressure for all surfaces.