Lubricants and liquid cooling are used in many industrial applications to ensure reliability and longevity of machinery. Oil cooling of both electrical and mechanical applications is of interest since oil is better suited for electrical applications than water and already available in the system as a lubricant. Brazed plate heat exchangers (BPHEs) have many advantages compared to other heat exchanger types commonly used in oil cooling applications. Flow maldistribution inside BPHEs can arise with highly viscous fluids like oil. Since flow is hard to measure when plate heat exchangers are brazed together, Computational Fluid Dynamics (CFD) can be used instead. This study investigates parameters that could affect flow distribution inside BPHEs with the CFD-tool COMSOL Multiphysics. The study is made on three different geometries at different detail levels. The purpose of the study is to expand the knowledge about fluid behavior in BPHEs and how it affects efficiency. It was proved from the Bernoulli equation that flow velocity, gravity and Reynolds number were some parameters that could affect flow distribution inside BPHEs. Two simplified models were built for evaluation of viscosity, gravity and Reynolds number. A more detailed model was provided by SWEP representing the fluid domain of a full-size distribution zone model. Model validation and mesh independence study were made with expressions due to the lack of experimental data. Investigations of viscosity, gravity and Reynolds number were made through isolation and alteration of the respective parameter. The validation and mesh independence study proved the models trustworthy and detailed enough to capture the physical behavior. Small deviations from expected validation results can be explained with the assumptions and simplifications made in the process. Results show that flow maldistribution increases with viscosity differences between channels. Viscosity maldistribution is greater for oil than for water. It is important to consider how the fluid viscosity changes with temperature under the respective working conditions. Gravity has no effect on flow distribution as long as it acts along or opposite the main flow direction. As plate heat exchangers are generally placed vertically, gravity will not affect flow distribution. Gravity has a significant effect on flow distribution if plate packages are places horizontally. High Reynolds numbers have a positive effect on flow distribution and reduce the difference between highest and lowest velocities across the outlet. Very low flow velocities should therefore be avoided since it increases flow maldistribution.

A microwave heat exchanger (MHE) is a device which converts microwave (MW) energy into usable form of heat energy. The working principle of the MHE is based on a collective effect of electromagnetic wave propagation, heat transfer and fluid flow, so the development of an efficient device requires complicated experimentation with processes of different physical nature. A peculiar phenomenon making the design of MHE even more challenging is extit{thermal runaway}, a nonlinear phenomenon in which a small increase in the input power gives rise to a large increase in temperature. Such high temperature may result in material damage through excessive thermal expansion, cracking, or melting. In this Thesis, we report on an initial phase in the development of a computational model which may help clarify complicated interaction between nonlinear phenomena that might be difficult to comprehend and control experimentally. We present a 2D multiphysics model mimicking operation of a layered MHE that simulates the nonlinear interaction between MW, thermal, and fluid flow phenomena involved in the operation of the MHE. The model is built for a triple layered (fluid-ceramic-fluid) MHE and is capable of capturing the S- and SS-profiles of power response curve which determines steady-state temperature solution as a function of incident power. The model is implemented on the platform of the COMSOL Multiphysics modeling software. We show that a MHE with particular thickness and dielectric properties of the layers can operate efficiently by keeping temperatures during thermal runaway under control. Overall temperatures increase rapidly as soon as the local maximum temperature reaches a critical value. This condition is held true both in absence and in presence of fluid flow. It is demonstrated that the efficiency of the MHE dramatically increases when thermal runaway is achieved. As the amount of heat energy, which is being transferred to the fluid from the heated dielectric, increases, incident power required to achieve thermal runaway also increases. It is also shown that, with appropriate length of the layered MHE, thermal runaway can be achieved at a lower power level. While the model developed in this Thesis studies the basic operation of a three layered MHE, it can further be developed to investigate optimum design parameters of the MHE of other structures so that maximum thermal efficiency is achieved.

The Cu-Cl thermochemical cycle is a promising method to generate hydrogen as a clean fuel for human use in the future. The cycle can be coupled to nuclear reactors to supply its heat requirements. The cycle generates hydrogen by splitting water molecules through a series of chemical reactions. Thermal management within the cycle is crucial for improving its thermal efficiency. The cycle has an average theoretical efficiency of around 46% without any heat recovery. The efficiency may increase up to 74%, if all heat associated with the products of the cycle’s steps is recycled internally. The products of the different processes that transfer heat are; oxygen, hydrogen, and molten CuCl. The heat carried by oxygen and hydrogen can be recovered by the use of conventional heat exchangers. However, recovering heat from molten CuCl is very challenging due to the phase transformations that molten CuCl undergoes, as it cools down from liquid to solid states. This thesis presents a new model that predicts the fluid flow and heat transfer in a direct contact heat exchanger, designed to recover the heat from molten CuCl, through the physical interaction between CuCl droplets and air. Numerical results for the variations of temperature, velocity, heat transfer rate, and so forth, are given for two cases of CuCl flow. The predicted dimensions of the heat exchanger were found to be a diameter of 0.13 m, and a height of 0.6 and 0.8 m for 1 and 0.5 mm droplet diameters, respectively. The results obtained provide valuable insights for the equipment design and scale-up of the Cu-Cl cycle. / UOIT

Solar energy is an important form of renewable energy that can be used as an alternative to fossil fuels. It can be used to produce electricity or to provide heat. One particular application is using solar energy for a domestic hot water system. The purpose of this research is to improve the thermal performance of a solar domestic hot water (SDHW) system. Experimental research was conducted to study the thermal performance of a shell-and-3coil heat exchanger and a shell-and-4coil heat exchanger using either water or glycol as working fluids on the tube side. An experimental set-up simulating a SDHW system was designed and constructed. The set-up contained a 270 L storage tank, a shell-and-three coil heat exchanger or a shell-and-four coil heat exchanger, and electrical heaters to simulate the solar collector. At the inlets and outlets of the storage tank and the heat exchanger the temperatures, pressures, and flow rates were measured to determine the thermal performance. The results from the experiment tests were analyzed in terms of the overall heat transfer coefficient product (UA) and the pressure drop (?P) between the inlet and outlet of the heat exchanger. The UA value of the shell-and-4coil heat exchanger was higher than the UA value of the shell-and-3coil heat exchanger. For example, at a heat transfer rate of 2000 W for water, the UA values were 240 W/K and 270 W/K for the shell-and-3coil heat exchanger and the shell-and-4coil heat exchanger, respectively. With respect to glycol, at a heat transfer rate of 2000 W the UA values were 197 W/K and 215 W/K for shell-and-3coil, and shell-and-4coil heat exchanger, respectively. The degradation of the thermal performance of the shell-and-3coil was offset by benefits, such as reduction in mass, volume, labor cost and the final cost. A reasonable agreement between theoretical and experimental results in terms of the UA value was observed. The thermal performance of each coil in both heat exchangers was below that predicted by the relevant heat transfer correlations. A performance factor was calculated for each coil. For both glycol and water, and both heat exchangers, the performance factors for the inner most and outer most coils were 0.70 and 0.53, respectively. However, there is a slight difference in the performance factors of coils between the inner most and the outer most coils for the 3-coil and 4-coil heat exchangers. For these coils the performance factors varied from 0.55 to 0.67.

No description available.

AN ABSTRACT OF THE THESIS OF Daniel Grohmann, for the Master of Science degree in Mechanical Engineering on May 5, 2014, at Southern Illinois University Carbondale DESIGN AND CHARACTERIZATION OF A COMPACT HEAT EXCHANGER FOR USE WITH NANOFLUIDS Major Professor: Dr. Kanchan Mondal This research is aimed to design and characterize an experimental plain channel, plate heat exchanger and further compare the performance of water based nanofluids as with that of water. The thesis discusses the designing and fabrication of the heat exchanger such that several parameters such as temperature, flow rate, nanoparticle concentration, and length of channels can be varied. Three sizes of heat exchangers were fabricated. Experiments were conducted to remove heat from air at different temperatures by the various heat exchanger fluids, namely water, 0.5, 0.75 and 1 vol.% alumina in water. Flow rates of the cold fluid were varied in order to change the Reynolds Number while maintaining a laminar regime. In the experiments with water as the heat exchanger fluid, it was found that the heat exchange did not follow pure counter current flow conditions presumably due to end effects. A correction factor, F, for the log mean temperature difference value was calculated for each case to estimate the mean temperature difference. The effectiveness values were found to be greater than 0.75 for most cases. It was also found that the thermal entrance length was larger than the length of the channels for the shortest heat exchanger. In addition, it was observed that the laminar regime Nusselt number values were similar to values reported in literature for flow through mini and micro channels. Convective heat transfer coefficients were calculated and were found to be of the order of those reported for water. It was also discovered that Prandtl number was the most influential property for this study. As opposed to expectations, the use of nanofluids was found that it did not significantly improve heat transfer than that of the water alone. The only case that the nanofluids had a significant enhancement in the performance was that of the 6in plate at 1 vol.% of 11% increase. The rest of the study showed that it had no increase or negative effects.

No description available.

Master of Science / Department of Mechanical and Nuclear Engineering / Akira T. Tokuhiro / In the STAR-LM (Secure Transportable Autonomous Reactor-Liquid Metal) reactor concept developed at Argonne National Laboratory (ANL), a supercritical CO2 (S-CO2) Brayton cycle is used as the power conversion system because it features advantages such as a higher efficiency due to less compressive work, and competitive cost due to a reduced complexity and size. From the components of the cycle, high performance of both the recuperator and precooler has a large influence on the overall cycle efficiency and plant economy. One attractive option for optimizing the performance of the cycle is to use an high efficiency heat exchanger such as the Printed Circuit Heat Exchanger (PCHE) manufactured by Heatric. The PCHE is a compact heat exchanger with high effectiveness, wide operating range, enhanced safety, and low cost. PCHEs are used in various industrial applications, but are relatively new to the nuclear industry. In this study, performance testing of a PCHE using supercritical CO2 and water as heat transfer media were performed at ANL. The heat transfer characteristics of the PCHE under operating conditions of the STAR_LM precooler were investigated. The S-CO2 , defined the “hot-side”, had its outlet condition near the pseudocritical point at 7.5MPa (~31-32 C). We found that of all the thermophysical properties undergoing rapid change near the critical point, heat transfer for S-CO2 is strongly correlated with the specific heat of CO2. Additional experiments performed with different bulk temperatures and pressures on the hot side also supported this conclusion. We proposed plotting the heat transfer results, (Nu2 + Pr2/3) versus (RePr4/3), based on an order-of-magnitude analysis, to reveal the close proximity of the outlet to pseudocritical conditions. In order to check the experimental results, a nodal model of a segmented PCHE using a traditional log-mean temperature difference method was developed. This approach provided the temperature distribution along the heat exchanger. Additionally a CFD simulation (FLUENT) of a 4-layer, zig-zag channeled PCHE was developed. Comparison of the simulation and LMTD nodal model revealed that indeed specific heat strongly influenced the heat transfer.

No description available.

Extruded Al-Mn alloy are used in heat exchanger applications due to their light weight and good thermal conductivity. Depending on the application, the units may be subjected to external corrosion, which can lead to perforation of the tube. The industrial test most commonly used to assess heat exchanger alloys is the seawater acetic acid test (SWAAT). This is a cyclic fog at 40°C and pH 2.9. In the present study, it was found that pits developing in extruded Al-Mn tubes during the SWAAT test are purely crystallographic. Furthermore, a mechanistic understanding for crystallographic pitting has been developed. The SWAAT test can be of relatively long duration and, typically, does not yield information on the underlying corrosion initiation and propagation mechanisms. In the present study, alternate methods to assess pitting corrosion were elaborated. A drop testing procedure has been successfully implemented to study the mechanism of pit initiation. It was revealed that pits initiated within the aluminium matrix in the vicinity of grain boundaries. A close link between large second-phase particles and pit initiation was established. No preferred grain orientation for pit initiation was evident. Scanning electron microscopy and associated tomography were undertaken for the first time to clarify the mechanism of pit propagation. The pit walls were oriented {100}, while the fast-dissolving planes were {110} and {111}. The findings were in accordance with previous literature. Corrosion penetrated deeper into the alloy when the corrosion front was close to a grain boundary. Pit walls were cathodic to the aluminium matrix, possibly due to enrichment of alloying elements at pit walls. The effect of alloy additions on the corrosion behaviour of extruded aluminium alloys was investigated. Alloys with varying copper, iron and manganese contents were compared. Shot noise analysis and post-mortem analyses were undertaken. The increased amount of manganese in solid solution delayed the transition from micropits to stable pitting. This delay is attributable to second-phase particles that are less cathodic to the aluminium matrix in alloys with increased manganese content. Increasing copper decreased the size of the dissolved polyhedra during stable pitting. Furthermore, pits propagated faster in alloys rich in copper. This could be attributed to an increased level of copper enrichment at the pit walls. Finally, more second-phase particles were present in alloys with increased iron levels. Additionally, pits located in those alloys propagated deeper than pits located in alloys with low levels of iron. A competition between two different types of cathodes, enrichment layer and second-phase particles, is suggested. In conclusion, the effect of microstructure and alloy additions on the corrosion mechanism for crystallographic pitting developed during the project was clarified.