In this study laminar forced film condensation of vapor flowing in cross-flow direction through the annular space between two concentric cylinders was investigated numerically. To achieve this, governing equations of the vapor and the condensate flow in cross-flow direction between two concentric cylinders were developed. After obtaining the equations in integral forms by using the finite difference technique the vapor boundary layer thicknesses on the inner and outer cylinders and the condensate layer thickness was obtained as a function of the angular position on the cylinders. It was assumed that the condensation took place on the outer surface of the inner cylinder only and the outer cylinder was assumed to be insulated. The computer program developed is capable to calculate the condensate film thickness, vapor boundary layer thickness, the heat flux and the heat transfer coefficient and the interface velocity between the condensate and the vapor layer as a function of the angular position on the cylinders. Effects of changing the free stream velocity flowing in the channel, the radius of the inner cylinder, the temperature difference between the saturated vapor and the wall and the annular space between the concentric cylinders were investigated numerically by using the computer program and the results were presented graphically. Results showed that by increasing the free stream velocity of the vapor in the core, the film thickness decreased and by increasing the radius of the inner cylinder, the temperature difference between the saturated vapor and the wall and the annular space, the film thickness increased.

This thesis presents energy, exergy and economic analyses of the heating system of an existing building, the Konya Central Informatics Technical High School. The heat requirement for each room of the building is found by calculating heat losses. Radiator lengths that can provide the heat requirements are selected. For the exergy analysis, the system is divided into three parts: Heat generator, radiators and rooms. Comparisons are made according to minimum outdoor temperature, insulation quality of the structural elements, fuel type, heating water temperature and heat generator type (boiler, heat pump, cogeneration unit with heat pump) to see their effects on energy usage, exergy consumption, capital costs and annual operating cost of the system. Results show that the largest heat loss is due to infiltration but it should not be reduced because of the fresh air requirement. Minimum energy usage, exergy consumptions and annual operating cost is achieved by using the cogeneration unit with the heat pump. However, due to high capital cost it has a long payback period (45.3 years). The shortest payback period (3.2 years) is calculated for upgrading the windows to 4 mm double glass panes and 12 mm stagnant air gap.

In this study simultaneously developing single phase, laminar and incompressible flow in a micro gap between parallel plates is numerically analyzed by including the effect of variation in thermal conductivity and viscosity with temperature. Variable property solutions for continuity, momentum and energy equations are performed in a coupled manner, for air as a Newtonian fluid. In these analyses the rarefaction effect, which is important for the slip flow regime, is taken into account by imposing slip velocity and temperature jump boundary conditions to the wall boundaries. Mainly, the influence of viscous dissipation, axial conduction, geometric parameters and rarefaction on the property variation effect is aimed to be discussed in detail. Therefore, the effects of variable thermal conductivity and viscosity are investigated simultaneously with the effects of rarefaction, geometric parameters, viscous dissipation and axial conduction. The difference between constant and variable solutions in terms of heat transfer characteristics is related to the effects of viscous dissipation axial conduction and rarefaction. According to results, property variation is substantially effective in the entrance region where temperature and velocity gradients are high. On the other hand, property variation effects are not significant for fully developed air flows in microchannel.

A novel complementary metal oxide semiconductor (CMOS) compatible microchannel heat sink is designed, fabricated, and tested for electronic cooling applications. The proposed microchannel heat sink requires no design change of the electronic circuitry underneath. Therefore, microchannels can be fabricated on top of the finished CMOS wafers by just adding a few more steps to the fabrication flow. Combining polymer (parylene C) and metal (copper) structures, a high performance microchannel heat sink can be easily manufactured on top of the electronic circuits, forming a monolithic cooling system. In the design stage, computer simulations of the microchannels with several different dimensions have been performed. Microchannels made of only parylene showed poor heat transfer performance as expected since the thermal conductivity of parylene C is very low. Therefore an alternative design comprising structural parylene layer and embedded metal layers has been modeled. Copper is selected as the metal due to its simple fabrication and very good thermal properties. The results showed that the higher the copper surface area the better the thermal performance of the heat sinks. Based on the modeling results, the final test structures are designed with full copper sidewalls with a parylene top wall. Several different microchannel test chips have been fabricated in METU-MEMS Research &amp / Application Center cleanroom facilities. The devices are tested with different flow rates and heat loads. During the tests, it was shown that the test devices can remove about 126 W/cm2 heat flux from the chip surface while keeping the chip temperature at around 90&deg / C with a coolant flow rate of 500 &mu / l/min per channel.

This thesis investigates the refrigerant (R-134a) flow in a minitube experimentally. The small scale heat transfer is a relatively new research area and has been in favor since the end of 1970&rsquo / s. Refrigerant flow in mini- and microscale media is a potential enhancement factor for refrigeration technology in the future. For the forthcoming developments and progresses, experimental studies are invaluable in terms of having an insight and contributing to the establishment of infrastructure in the field in addition to leading the numerical and theoretical approaches. The studies in the literature show that low mass flow rate and constant wall temperature approach in minitubes and minichannels were not among the main areas of interest. Therefore, an experimental set-up was prepared in order to perform experiments of two-phase refrigerant flow in a 1.65 mm diameter copper minitube with the constant wall temperature approach. The design, preparation, and modifications of the experimental set-up are explained in this thesis. Two-phase flow and quality arrangements were done by pre-heating the refrigerant at saturation pressure and the constant wall temperature was achieved by a secondary cycle with water and ethylene glycol mixture as the working fluid. The heat transfer coefficient and the pressure drop for the two-phase flow with varying quality values and saturation temperatures of the refrigerant were calculated and compared with the results available in literature.

A computer program, called &ldquo / Cogeneration Design&quot / is developed using Visual Basic 6.0, for conceptually designing cogeneration power plants. Design is focused on power plants to be built in university campuses, where there is mainly heating, hot water, electricity and sometimes cooling demands. Middle East Technical University campus is considered as the primary working area. Before the conceptual design study, detailed information regarding description of the campus, infrastructure, annual electric, water and heat demand covering last 10 years, properties of existing heat plant including natural gas expenses and specifications of the steam distribution pipes and electricity grid are collected and examined in detail. Throughout the thesis, eight different natural gas fired cogeneration power plant designs are developed regarding different gas turbine and steam turbine configurations, for METU Campus, considering the Campus&#039 / properties described above, by using the &quot / Cogeneration Design&quot / program. Then, by means of a thermoeconomic optimization process, cost summary reports are prepared and the feasibility of the designed cogeneration power plants are discussed.

ABSTRACT HEAT REMOVAL FROM A LARGE SCALE WARM WATER STORAGE Kayserilioglu, Yavuz Selim M.S., Department of Mechanical Engineering Supervisor: Prof. Dr. R&uuml / knettin Oskay August 2004, 88 Pages A preliminary experimental study was performed in order to investigate the charging and heat removal characteristics of a sensible heat storage. Two sets of experiments were performed at two aspect ratios. Heat removal processes of these two sets were different while the charging processes were similar. In the first set of experiments, after the charging of the storage unit with relatively warm water was complete, heat removal process was started with simple heat exchangers from different elevations within the storage while the charging of the storage unit was continued. In the second set of experiments, after the charging of the storage unit was complete, heat removal from the storage unit was started without further charging of the storage unit. Charging water was fed into the storage from the top of one side and relatively colder water was drained from the bottom of the opposite side. Internal heat exchangers were used for the heat removal. Vertical temperature profile developments during the charging and heat removal periods were investigated. Thermal stratification was observed in all experiments. Heat exchangers extracted heat from different elevations in different experiments and the trend was that more heat can be extracted in upper elevations. Comparable heat can be extracted from the same elevation of lower and higher aspect ratio. Keywords: Sensible Heat Storage, Heat Removal, Thermal Stratification, Warm Water Storage

In this thesis, several configurations of combined cycle cogeneration systems proposed by the author and an existing system, the Bilkent Combined Cycle Cogeneration Plant, are investigated by energy, exergy and thermoeconomic analyses. In each of these configurations, varying steam demand is considered rather than fixed steam demand. Basic thermodynamic properties of the systems are determined by energy analysis utilizing main operation conditions. Exergy destructions within the system and exergy losses to environment are investigated to determine thermodynamic inefficiencies in the system and to assist in guiding future improvements in the plant. Among the different approaches for thermoeconomic analysis in literature, SPECO method is applied. Since the systems have more than one product (process steam and electrical power), systems are divided into several subsystems and cost balances are applied together with the auxiliary equations. Hence, cost of each product is calculated. Comparison of the configurations in terms of performance assessment parameters and costs per unit of exergy are also given in this thesis.

Finite element solution of solidification process in 2-D Cartesian and axisymmetric geometries is investigated. The use of finite element may result in spurious increase of temperature in the field and the selection of the mushy zone range when used as a numerical tool along with the selection of the mesh size results in large errors in the predicted solidification time. The approach works best for problems where the mushy zone range is finite and the thermal conductivities of both phases are high.

A Theory of Dropwise Condensation