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In the vicinity of the pseudocritical point, supercritical carbon dioxide (sCO2) undergoes a steep change in properties from “liquid-like” to “gas-like” as it is heated at a constant pressure. At the same time, there is a large spike in specific heat which can yield high heat transfer coefficients and heat capacity rates. These unique properties have made sCO2 an attractive working fluid in next generation power and HVAC&R technologies. Microchannel heat exchangers are being used to safely and efficiently utilize the high pressure fluid in these applications. However, prior investigation of heating of supercritical CO2 has primarily focused on circular, uniformly heated channels at relatively low heat flux for nuclear power applications. Thus, it is unclear if models and correlations developed from large circular tube data can be scaled down to the smaller, non-circular channels, with non-uniform heating. In the present work, a methodology is developed to experimentally characterize heat transfer for multiple parallel microchannels with a hydraulic diameter of 0.75 mm and aspect ratio of 1. Experiments are conducted over a range of heat fluxes (20 ≤ q” ≤ 40 W cm−2), mass fluxes (500 ≤ G ≤ 1000 kg m−2 s−1), reduced pressure (1.03 ≤ P[subscript R] ≤ 1.1), and inlet temperatures (20 ≤ T[subscript in] ≤ 100°C) in a parallel square microchannel test article with a single-wall constant heat flux boundary condition. Local and average heat transfer coefficients are experimentally measured and the results are compared to previously developed correlations. The predictive capabilities for the supercritical models were poor, with the lowest mean absolute percent error (MAPE) of 55.3% for the range of bulk fluid temperatures, heat fluxes, and mass fluxes. Interestingly, subcritical correlations were also investigated and yielded much lower MAPE than 80% of the supercritical correlations even though the effects of variable fluid properties were not taken into account. The subcritical correlations did not incorporate property ratios to account for the variability in fluid properties; in some supercritical correlations it was found to add additional uncertainty for the case of the present study. The effects of buoyancy and flow acceleration were also evaluated. Based on dimensionless criteria, buoyancy was expected to play a role in heat transfer, especially when the bulk fluid temperature is below the pseudocritical temperature. However, the relative importance of flow acceleration was inconclusive. Despite the apparent importance of buoyancy effects, heat transfer was not degraded, as would be expected in larger, circular, uniformly heated tubes. The mixed convection could be inducing a density driven swirling with the stratification of low-density fluid near the top (unheated). This would ultimately improve the heat transfer at the bottom portion of the test section channels. Therefore, the flow geometry and the non-conventional heated boundary could be improving the heat transfer even with buoyancy driven effects under supercritical conditions.
High-temperature supercritical CO2 Brayton cycles are promising candidates for future stationary power generation and hybrid electric propulsion applications. Supercritical CO2 thermal cycles potentially achieve higher energy density and thermal efficiency by operating at elevated temperatures and pressures. Heat exchangers are indispensable components of aerospace systems and improve efficiency of operation by providing necessary heat input, recovery, and dissipation. Tubular heat exchangers with unconventionally small tube sizes (tube diameters less than 5 mm) are promising components for supercritical CO2 cycles and provide excellent structural stability. Accurate and computationally efficient estimation of heat exchanger performance metrics at elevated temperatures and pressures is important for the design and optimization of sCO2 systems and thermal cycles. In this study, new Colburn and friction factor correlations are developed to quantify shell-side heat transfer and friction characteristics of flow within heat exchangers in the shell-and-tube configuration. Using experimental and CFD data sets from existing literature, multivariate regression analysis is conducted to achieve correlations that capture the effects of multiple critical geometric parameters. These correlations offer superior accuracy and versatility as compared to previous studies and predict the thermohydraulic performance of about 90% of the existing experimental and CFD data within ±15%. Supplementary thermohydraulic performance data is acquired from CFD simulations with sCO2 as the working fluid to validate the developed correlations and to demonstrate application to sCO2 heat exchangers. A computationally efficient and accurate numerical model is developed to predict the performance of STHXs. The highly accurate correlations are utilized to improve the accuracy of performance pre- dictions, and the concept of volume averaging is used to abstract the geometry for reduced computation time. The numerical model is validated by comparison with CFD simulations and provides high accuracy and significantly lower computation time compared to exist- ing numerical models. A preliminary optimization study is conducted, and the advantage of using supercritical CO2 as a working fluid for energy systems is demonstrated. A microtube heat exchanger is fabricated, and essential design and fabrication guidelines of a compact shell-and-tube heat exchanger with microtubes (with inner diameters of 1.75 mm) are provided. A heat exchanger test rig is used to evaluate the thermohydraulic performance of this heat exchanger with supercritical CO2 and air as working fluids. Thermohydraulic data are reported for more than forty sets of experiments with varying Reynolds numbers for shell and tube flows. Critical performance metrics are calculated from the data and compared with predictions from the numerical model. The average deviations between the experimental and model results fall within 10% for all critical metrics. This excellent agreement validates the numerical model for supercritical CO2 heat exchanger optimization and scale-up. A generalized costing model is developed to estimate the capital costs incurred to manufacture microtube shell-and-tube heat exchangers. This model is utilized in conjunction with an accurate and efficient 2D numerical shell-and-tube heat exchanger performance prediction model to conduct optimization studies with two key objectives - minimization of cost and maximization of heat exchanger power density - on supercritical CO2 microtube heat exchangers utilizing superalloy Haynes 282 as the solid material. A methodology is then demonstrated to optimize these heat exchangers for aerospace applications, and highly compact and cost-effective optimal designs with power density around 20 kW/kg and cost per conductance less than 5 $ · K/W are obtained.
The present work seeks to investigate the thermal hydraulic (heat transfer and fluid dynamics) behavior of supercritical (Sc) fluids at both the fundamental and applied levels. The thermal hydraulics of these fluids is not very well known although they have been used in various applications. There are drastic changes in the thermal and hydraulic properties of fluids at supercritical conditions. There has been a lot of focus to effectively utilize these properties changes in many applications such as heat exchangers. This work focuses on studying the forced convective heat transfer of Sc-CO2 in a series of mini semi-circular horizontal tubes and a zig-zag shaped horizontal channel. The problems were investigated numerically by second-order finite volume method using a commercial software FLUENT. Three dimensional Computational Fluid Dynamics (CFD) models were developed to simulate the flow and heat transfer for three different geometries -- a single semi-circular channel, a series of nine parallel semi-circular channels and a zig-zag channel. Grid and accuracy refinement studies were carried out to assess numerical errors. All the computational meshes developed for this study incorporated the first node cell within the viscous sub-layer i.e. y[superscript +]
Fundamentals and Applications of Supercritical Carbon Dioxide (SCO2) Based Power Cycles aims to provide engineers and researchers with an authoritative overview of research and technology in this area. Part One introduces the technology and reviews the properties of SCO2 relevant to power cycles. Other sections of the book address components for SCO2 power cycles, such as turbomachinery expanders, compressors, recuperators, and design challenges, such as the need for high-temperature materials. Chapters on key applications, including waste heat, nuclear power, fossil energy, geothermal and concentrated solar power are also included. The final section addresses major international research programs. Readers will learn about the attractive features of SC02 power cycles, which include a lower capital cost potential than the traditional cycle, and the compounding performance benefits from a more efficient thermodynamic cycle on balance of plant requirements, fuel use, and emissions. Represents the first book to focus exclusively on SC02 power cycles Contains detailed coverage of cycle fundamentals, key components, and design challenges Addresses the wide range of applications of SC02 power cycles, from more efficient electricity generation, to ship propulsion
This Special Issue is a compilation of the recent advances in thermal fluid engineering related to supercritical CO2 power cycle development. The supercritical CO2 power cycle is considered to be one of the most promising power cycles for distributed power generation, waste heat recovery, and a topping cycle of coal, nuclear, and solar thermal heat sources. While the cycle benefits from dramatic changes in CO2 thermodynamic properties near the critical point, design, and analysis of the power cycle and its major components also face certain challenges due to the strong real gas effect and extreme operating conditions. This Special Issue will present a series of recent research results in heat transfer and fluid flow analyses and experimentation so that the accumulated knowledge can accelerate the development of this exciting future power cycle technology.
Near-critical-point supercritical fluid convection is a promising alternative for emerging high-flux thermal management needs because of the high fluid thermal conductivities and specific heats. However, limited information is available on transport processes to guide engineering of high-flux compact supercritical heat transfer equipment, which often have non-uniform heating distributions. To address this need, large eddy simulations (LES) are employed in this dissertation to study supercritical CO2 convection in microchannels and micro-pin-fin enhanced geometries. Following mesh independence studies, the simulation approach is validated with published experimental data as well as relevant empirical correlations. Numerical results are used to assess the applicability of published supercritical convection correlations for microchannel heat exchangers. Parametric studies are conduced to characterize the onset of mixed convection and non-uniform heating effects in microscale test sections. Furthermore, a representative case is evaluated to assess the impact of conjugate heat transfer at microchannel walls on microscale supercritical convection performance. In addition, a new 2D map was suggested to predict zones of heat transfer deterioration and/or considerable mixed convection effects in the microchannel. Finally, thermal-hydraulics performance of parallel-plates and aligned square micro-pin-fins enhanced heat exchangers is investigated and compared against available single-phase flow correlations.
This book introduces two of the most exciting heat pumping technologies, the coabsorbent and the thermal recovery (mechanical vapor) compression, characterized by a high potential in primary energy savings and environmental protection. New cycles with potential applications of nontruncated, truncated, hybrid truncated, and multi-effect coabsorbent types are introduced in this work. Thermal-to-work recovery compression (TWRC) is the first of two particular methods explored here, including how superheat is converted into work, which diminishes the compressor work input. In the second method, thermal-to-thermal recovery compression (TTRC), the superheat is converted into useful cooling and/or heating, and added to the cycle output effect via the coabsorbent technology. These and other methods of discharge gas superheat recovery are analyzed for single-, two-, three-, and multi-stage compression cooling and heating, ammonia and ammonia-water cycles, and the effectiveness results are given. The author presents absorption-related topics, including the divided-device method for mass and heat transfer analysis, and truncation as a unique method for a better source-task match. Along with advanced gax recovery, the first and second principles of COP and exergy calculation, the ideal point approaching (i.p.a.) effect and the two-point theory of mass and heat transfer, the book also addresses the new wording of the Laplace equation, the Marangoni effect true explanation, and the new mass and heat exchangers based on this effect. The work goes on to explore coabsorbent separate and combined cooling, heating, and power (CHP) production and advanced water-lithium bromide cycle air-conditioning, as well as analyzing high-efficiency ammonia-water heat-driven heating and industrial low-temperature cooling, in detail. Readers will learn how coabsorbent technology is based on classic absorption, but is more general. It is capable of offering effective solutions for all cooling and heating applications (industry, agriculture, district, household, etc.), provided that two supplying heat-sink sources with temperatures outdistanced by a minimum of 12-15oC are available. This book has clear and concise presentation and illustrates the theory and applications with diagrams, tables, and flowcharts.