Solar radiation offers phenomenal potential for energy conversion with energy densities on the order of 1000W/m2 in locations with regularly clear skies. As always, the difficulty lies in finding a solar-electric conversion technology capable of producing electricity at a competitive cost. The SolarCAT (Solar Compressed Air Turbine) system produces electricity by releasing stored compressed air through a series of turbines with solar dish concentrators providing the required heat for efficient conversion to electricity. To minimize impact on capital cost, high recuperator effectiveness targets are sought but unlike typical fuel-fired micro-turbines, raising the recuperator effectiveness of the solar power system yields a benefit in overall system capital cost. Improving efficiency lowers the size and cost of the largest element of the system, namely the dish. In this study potential techniques for achieving a highly compact heat-transfer media were reviewed. Folded fin, packed beds, micro-tubes, lattice frame structures, metal foams, woven textile, and micro-machining techniques were assessed. Textile structures were selected as an appropriate medium to replace the internal folded fin of the SolarCAT recuperator. The relatively long flow (>150mm) path through the proposed screen wafers requires a model for fully-developed forced convective flow between parallel plates. A mathematical model was developed by integrating the results from the work of several authors in the field of textiles and porous media. #100 mesh sintered screen wafers were brazed between two 0.25mm stainless steel sheets and destructively tested to assess their tensile strength. Although iii optimization of the braze parameters was not completed, it was found that many samples survived exposure to internal pressures in excess of 50MPa. This study found that the use of sintered screen wafers to replace the internal folded fin of the SolarCAT recuperator would have advantages over the current design with respect to both overall recuperator effectiveness, size, and cost. Textile structures can be tailored to have wide range of fluid and heat-transfer properties depending on the application. The manufacturing process is relatively simple and could be cost-effective for high-volume production.

Solar radiation offers phenomenal potential for energy conversion with energy densities on the order of 1000W/m2 in locations with regularly clear skies. As always, the difficulty lies in finding a solar-electric conversion technology capable of producing electricity at a competitive cost. The SolarCAT (Solar Compressed Air Turbine) system produces electricity by releasing stored compressed air through a series of turbines with solar dish concentrators providing the required heat for efficient conversion to electricity. To minimize impact on capital cost, high recuperator effectiveness targets are sought but unlike typical fuel-fired micro-turbines, raising the recuperator effectiveness of the solar power system yields a benefit in overall system capital cost. Improving efficiency lowers the size and cost of the largest element of the system, namely the dish. In this study potential techniques for achieving a highly compact heat-transfer media were reviewed. Folded fin, packed beds, micro-tubes, lattice frame structures, metal foams, woven textile, and micro-machining techniques were assessed. Textile structures were selected as an appropriate medium to replace the internal folded fin of the SolarCAT recuperator. The relatively long flow (>150mm) path through the proposed screen wafers requires a model for fully-developed forced convective flow between parallel plates. A mathematical model was developed by integrating the results from the work of several authors in the field of textiles and porous media. #100 mesh sintered screen wafers were brazed between two 0.25mm stainless steel sheets and destructively tested to assess their tensile strength. Although iii optimization of the braze parameters was not completed, it was found that many samples survived exposure to internal pressures in excess of 50MPa. This study found that the use of sintered screen wafers to replace the internal folded fin of the SolarCAT recuperator would have advantages over the current design with respect to both overall recuperator effectiveness, size, and cost. Textile structures can be tailored to have wide range of fluid and heat-transfer properties depending on the application. The manufacturing process is relatively simple and could be cost-effective for high-volume production.

Solar radiation offers phenomenal potential for energy conversion with energy densities on the order of 1000W/m2 in locations with regularly clear skies. As always, the difficulty lies in finding a solar-electric conversion technology capable of producing electricity at a competitive cost. The SolarCAT (Solar Compressed Air Turbine) system produces electricity by releasing stored compressed air through a series of turbines with solar dish concentrators providing the required heat for efficient conversion to electricity. To minimize impact on capital cost, high recuperator effectiveness targets are sought but unlike typical fuel-fired micro-turbines, raising the recuperator effectiveness of the solar power system yields a benefit in overall system capital cost. Improving efficiency lowers the size and cost of the largest element of the system, namely the dish. In this study potential techniques for achieving a highly compact heat-transfer media were reviewed. Folded fin, packed beds, micro-tubes, lattice frame structures, metal foams, woven textile, and micro-machining techniques were assessed. Textile structures were selected as an appropriate medium to replace the internal folded fin of the SolarCAT recuperator. The relatively long flow (>150mm) path through the proposed screen wafers requires a model for fully-developed forced convective flow between parallel plates. A mathematical model was developed by integrating the results from the work of several authors in the field of textiles and porous media. #100 mesh sintered screen wafers were brazed between two 0.25mm stainless steel sheets and destructively tested to assess their tensile strength. Although iii optimization of the braze parameters was not completed, it was found that many samples survived exposure to internal pressures in excess of 50MPa. This study found that the use of sintered screen wafers to replace the internal folded fin of the SolarCAT recuperator would have advantages over the current design with respect to both overall recuperator effectiveness, size, and cost. Textile structures can be tailored to have wide range of fluid and heat-transfer properties depending on the application. The manufacturing process is relatively simple and could be cost-effective for high-volume production.

Understanding the mechanisms that serve to increase heat transfer provides valuable knowledge to minimize the size and maximize the performance of compact heat exchangers. This document presents a detailed experimental heat transfer study of six scaled up louvered fin geometries that are typical of those found in modern louvered fin compact heat exchangers. Heat transfer measurements were performed over a range of Reynolds numbers and with two different boundary conditions. A fully heated boundary condition allowed the effects of the thermal field to be observed while an adiabatic boundary condition allowed the effects of the flow field to be observed. The results indicated that the complex thermal and flow field patterns that developed within the louvered fin geometries strongly affected the heat transfer of individual louvers. In the entrance region of the louvered array, the effects of the flow field were dominant while in the fully developed region of the louvered arrays, the effects of the thermal field were dominant. A companion two-dimensional CFD study indicated that the heat transfer trends of the louvers resulting from both the thermal and flow fields were well predicted. Based on heat transfer performance, it was determined that the theta = 27°, Fp/Lp = 1.52 geometry performed the best at Re = 230 and Re = 370, while the theta = 39°, Fp/Lp = 0.91 geometry performed best at Re = 1016. / Master of Science

Solar radiation offers phenomenal potential for energy conversion with energy densities on the order of 1000W/m2 in locations with regularly clear skies. As always, the difficulty lies in finding a solar-electric conversion technology capable of producing electricity at a competitive cost. The SolarCAT (Solar Compressed Air Turbine) system produces electricity by releasing stored compressed air through a series of turbines with solar dish concentrators providing the required heat for efficient conversion to electricity. To minimize impact on capital cost, high recuperator effectiveness targets are sought but unlike typical fuel-fired micro-turbines, raising the recuperator effectiveness of the solar power system yields a benefit in overall system capital cost. Improving efficiency lowers the size and cost of the largest element of the system, namely the dish. In this study potential techniques for achieving a highly compact heat-transfer media were reviewed. Folded fin, packed beds, micro-tubes, lattice frame structures, metal foams, woven textile, and micro-machining techniques were assessed. Textile structures were selected as an appropriate medium to replace the internal folded fin of the SolarCAT recuperator. The relatively long flow (>150mm) path through the proposed screen wafers requires a model for fully-developed forced convective flow between parallel plates. A mathematical model was developed by integrating the results from the work of several authors in the field of textiles and porous media. #100 mesh sintered screen wafers were brazed between two 0.25mm stainless steel sheets and destructively tested to assess their tensile strength. Although iii optimization of the braze parameters was not completed, it was found that many samples survived exposure to internal pressures in excess of 50MPa. This study found that the use of sintered screen wafers to replace the internal folded fin of the SolarCAT recuperator would have advantages over the current design with respect to both overall recuperator effectiveness, size, and cost. Textile structures can be tailored to have wide range of fluid and heat-transfer properties depending on the application. The manufacturing process is relatively simple and could be cost-effective for high-volume production.

A compact heat exchanger is a device used to transfer thermal energy between two or more fluids. The most extensive use of compact heat exchangers occurs in the commercial trucking industry. Most compact heat exchanger designs contain tubes carrying one fluid and external fins through which passes another fluid. To enhance the fin-side heat transfer in a compact heat exchanger, which is typically the air side of the heat exchanger, louvers are manufactured into the fins. Louvered fins initiate the growth of new boundary layers such that the average convective heat transfer coefficient is higher than that which would occur for a continuous fin. Approximately 85% of the total thermal resistance occurs on the air side of the heat exchanger. To design more space and weight efficient heat exchangers, it is imperative to gain a fundamental understanding of the mechanisms that serve to increase the heat transfer on the air side. This thesis presents the heat transfer results of three scaled-up louvered fin geometries and compares these results to six additional models in which the louver angle, fin pitch and Reynolds number were varied. Two experiments were performed to determine the reference temperature used for the calculation of the heat transfer coefficients. The use of two reference temperatures allowed the effects of the flow field and thermal field to be separated. This thesis also presents details of an optimization study performed for a louvered fin array. The results of the experimental study showed that the hot thermal wakes formed at the entrance louver have an adverse effect on the heat transfer of downstream louvers. Measuring the adiabatic wall temperature of the louvers in the array showed the effect of these thermal wakes. The experimental study showed that the optimal louver geometry was Reynolds number dependent. For the lower two Reynolds numbers of ReLp = 230 and 370, the Fp/Lp = 1.52, q = 27° model was found to be the best performer, which does not agree with previous studies. For ReLp = 1016, the Fp/Lp = 0.91, q = 39° model was shown to have optimal heat transfer performance, which is in agreement with a previous study performed by Chang and Wang (1996). / Master of Science

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.

Compact heat exchangers are usually characterized by a large heat transfer surface per unit of volume. These characteristics are useful when thermal energy between two or more fluids must be exchanged without mixing. Most compact heat exchangers are liquid-to-air heat exchangers, with approximately 85% of the total thermal resistance occurring on the air side of the heat exchanger. To reduce the space and weight of a compact heat exchanger, augmentation strategies must be proposed to reduce the air side resistance. However, before any strategies to augment the air side heat transfer can be proposed, a thorough insight of the current mechanisms that govern air side heat transfer is required. The tube wall heat transfer results presented in this paper were obtained both experimentally and computationally for a typical compact heat exchanger design. Both isothermal and constant heat flux tube walls were studied. For the experimental investigation, a scaled-up model of the louvered fin-tube wall was tested in a flow facility. Although computational results for the isothermal tube wall are shown, control of the experimental isothermal tube wall proved to be unrealistic and only heat transfer measurements along the constant heat flux tube wall were made. For the constant heat flux tube wall, reasonable agreement has been achieved between the measurements and the steady, three-dimensional computational predictions. The results of the study showed that high heat transfer coefficients existed at the entrance to the louver array as well as in the louver reversal region. Vortices created at the leading edge of the louvers augmented heat transfer by thinning the tube wall boundary layer. Results indicate that an augmentation ratio of up to 3 times can occur for a tube wall of a louvered fin compact heat exchanger as compared to a flat plate. / Master of Science

This thesis examines the development of a high efficiency heat pump system using thermoelectric (TE) and reticulated metal foam (RMF) technologies to power a vehicle`s battery thermal management system. The focus is split into two areas: first a review of TE???s sourcing or removing heat, second an examination of compact heat exchanger (HX) design. Five TE suppliers were investigated to understand the performance and limitations of their TE modules. Testing showed the Kyrotherm product to be superior so it was used as a design basis. RMF???s are known to be an effective means to improve the performance of compact heat exchangers, thus HX???s were evaluated with RMF foams compressed to varying densities in order to understand their potential in conjunction with thermoelectric devices. Experimental results showed performance was limited due to adequate bonding, yet still on par with the highest efficiency technologies currently on the market.

Approved for public release, distribution is unlimited / This study concentrates on the empirical characterization of a staggered pin-fin array heat exchanger placed in a modular, rectangular wind tunnel. A full analysis of the heat transfer and pressure drop behavior was conducted on various pin-fin shapes, sizes, and configurations. The study was based on airflow over a wide range of Reynolds numbers in the laminar regime. The empirical data gathered can be used to corroborate and develop better numerical models to characterize the performance of such heat exchangers as well as scale down to the micro level for comparison with micro-heat exchangers. / Lieutenant, United States Navy