In this study, effect of surface roughness on convective heat transfer and fluid flow in two dimensional parallel plate microchannels is analyzed by numerically. For this purpose, single-phase, developing, laminar fluid flow at steady state and in the slip flow regime is considered. The continuity, momentum, and energy equations for Newtonian fluids are solved numerically for constant wall temperature boundary condition. Slip velocity and temperature jump at wall boundaries are imposed to observe the rarefaction effect. Effect of axial conduction inside the fluid and viscous dissipation also considered separately. Roughness elements on the surfaces are simulated by triangular geometrical obstructions. Then, the effect of these roughness elements on the velocity field and Nusselt number are compared to the results obtained from the analyses of flows in microchannels with smooth surfaces. It is found that increasing surface roughness reduces the heat transfer at continuum conditions. However in slip flow regime, increase in Nusselt number with increasing roughness height is observed. Moreover, this increase is found to be more obvious at low rarefied flows. It is also found that presence of axial conduction and viscous dissipation has increasing effect on heat transfer in smooth and rough channels.

Performance of a refrigerator can be characterized with its ability to maintain a preset low temperature by spending the least amount of electricity. It is important to understand natural convection inside a refrigerator for optimizing its design for performance. Computational Fluid Dynamics (CFD) together with experiments is a very powerful tool for visualizing flow and temperature fields that are essential for understanding a phenomenon that involves both fluid and heat flow. In this aspect, simulations are performed for compartment and total refrigerator models using the package program Fluent which is based on finite volume method. An experimental study is performed to determine the constant wall temperature boundary conditions for the numerical models. Effect of radiation is also investigated by comparing the numerical study of a different full refrigerator model with a similar one in literature. While evaluating the radiation effect, convection boundary condition is selected by defining overall heat transfer coefficient between the ambient room air at a constant temperature and the inner surfaces of the walls. Based on assumptions, related heat transfer analyses are done using compartment and total refrigerator model analyses. Performing CFD simulations of a refrigerator cabinet for visualizing the flow and temperature fields which is the aim of the study is achieved and some observations that can be useful in design optimization are made.

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.

Estimating the temperature distribution in transformer components in the design stage and during the operation is crucial since temperatures above the thermal limits of these components might seriously damage them. Thermal models are used to predict this vital information prior to actual operations. In this study, a two-dimensional, steady-state model based on the Finite Difference Method (FDM) is proposed to estimate the temperature distribution in the three-phase, SF6 gas insulatedcooled power transformer. The model can predict the temperature distribution at the specific discredited locations in the transformer successfully. This study also compares predicted temperatures of the model proposed in this study with the results of the previous study which is based on Finite Element Method (FEM) and the results of the research performed by the designers of the transformer. The results show that FDM model proposed in this study can be used to estimate the temperature distribution in the transformer with an acceptable accuracy and can be an alternative of the previous study which is based on FEM.

A nanofluid is the suspension of nanoparticles in a base fluid. Nanofluids are promising for heat transfer enhancement due to their high thermal conductivity. Presently, discrepancy exists in nanofluid thermal conductivity data in the literature, and enhancement mechanisms have not been fully understood yet. In the first part of this study, a literature review of nanofluid thermal conductivity is performed. Experimental studies are discussed through the effects of some parameters such as particle volume fraction, particle size, and temperature on conductivity. Enhancement mechanisms of conductivity are summarized, theoretical models are explained, model predictions are compared with experimental data, and discrepancies are indicated. Nanofluid forced convection research is important for practical application of nanofluids. Recent experiments showed that nanofluid heat transfer enhancement exceeds the associated thermal conductivity enhancement, which might be explained by thermal dispersion, which occurs due to random motion of nanoparticles. In the second part of the study, to examine the validity of a thermal dispersion model, hydrodynamically developed, thermally developing laminar Al2O3/water nanofluid flow inside a circular tube under constant wall temperature and heat flux boundary conditions is analyzed by using finite difference method with Alternating Direction Implicit Scheme. Numerical results are compared with experimental and numerical data in the literature and good agreement is observed especially with experimental data, which indicates the validity of the thermal dispersion model for explaining nanofluid heat transfer. Additionally, a theoretical analysis is performed, which shows that usage of classical correlations for heat transfer analysis of nanofluids is not valid.

Based on the well-known theoretical studies related to the film condensation inside vertical tubes, a known temperature distribution is prescribed as boundary condition at the inner surface of the tube wall. But, in reality, there is a thermal interaction between the condensate fluid and conduction through the wall where the temperature variation along the inner surface of the tube wall is unknown and this unknown temperature profile should be determined by taking account of this interaction. In other words, the heat conduction equation for the tube wall and the energy equation for the condensate fluid flow should be coupled and solved simultaneously. Therefore, this type of problem is named &ldquo / conjugate condensation heat transfer problem&rdquo / . Subject to the conjugate condensation heat transfer problem in the industrial applications, there are two different fluid flows separated by a tube where the vapor flowing inside the tube condensates whereas the other one is heated and it flows externally in the counter current direction in the annular passages. Because of its fundamental and practical importance, in this doctoral thesis, the studies are focused on the analytical and numerical investigation of conjugate heat transfer due to the steam condensation inside vertical tubes which is cooled externally by a fluid flowing in the counter current direction. The unknown wall temperatures of the condenser tube, condensate liquid layer inside the tube and the turbulent coolant flow outside the tube are coupled. A computer code, named ZEC, containing condensation conjugate heat transfer model is developed in FORTRAN 90 Language. This code and the models it contains are assessed against the various experimental databases. The predictions of the code ZEC are found to reasonably agree with the experimental results over a wide range of conditions. Therefore, this developed code, ZEC, may be used for the preliminary design of in-tube condensers and for the performance evaluation of such condensers in operation.

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.

This thesis presents numerical modelling and analysis of a solar Organic Rankine Cycle (ORC) for electricity generation. A regression based approach is used for the working fluid property calculations. Models of the unit&rsquo / s sub-components (pump, evaporator, expander and condenser) are also established. Steady and transient models are developed and analyzed because the unit is considered to work with stable (i.e. solar + boiler) or variable (i.e. solar only) heat input. The unit&rsquo / s heat exchangers (evaporator and condenser) have been identified as critical for the applicable method of analysis (steady or transient). The considered heat resource into the ORC is in the form of solar heated water, which varies between 80-95 0C at a range of mass flow rates between 2-12 kg/s. Simulation results of steady state operation using the developed model shows a maximum power output of around 40 kW. In the defined operation range / refrigerant mass flow rate, hot water mass flow rate and hot water temperature in the system are identified as critical parameters to optimize the power production and the cycle efficiency. The potential benefit of controlling these critical parameters is demonstrated for reliable ORC operation and optimum power production. It is also seen that simulation of the unit&rsquo / s dynamics using the transient model is imperative when variable heat input is involved, due to the fact that maximum energy recovery is the aim with any given level of heat input.

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Abstract This investigate is designated to the viewpoint that arrangement array of multichip modules are both staggered and in-line. Moreover, here we will discuss and compare the effects and differences of the relevantparameters caused by change Reynolds number (Re) in the experiment.In this experiment, I adopt ¡§transient heat transfer method with thermochromic liquid crystal¡¨ to research multichip modules array change to 3 ¡Ñ 5 and in-line or staggered multichip modules array to probe into the effects of over high density electronic multichip array space to length ratio to heat transfer effects when over high density electronic multichip array space to length ratio are S/L= 4/20,6/20,8/20 in the 8mm¡B12m¡B16mm respectively. The conditions are as following when every row center of chip convection heat transfer coefficient are measured: standard height to length ratio is H/L=10/20 and the height is 20mm. By observing the relationship of the varying parameters, as we can see in the analyze multichip of the experiment Re range form 1394 to 5025, we are able to improve thermal management. The experimentresule: (1) At higher values of Re the heat transfer effects are gain more,atteribute main flow field separation and reattachment is form behind the downstream modules. (2) In over high density multichip array 12S (S/L=6/20) proper are use for Re higher 4135 more than, at 8S (S/L=4/20)the lower Re can be thermocumulate in chip center. (3)When Re is 4135~5025,the heat transfer effects from staggered array is superior to in-line array. If Re range is 1394~3210,the thermal conduction is opposite. Therefore, Re is still the key that decides the efficacy of over high density electronic multichip array heat transfer effects.