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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.
A timely and comprehensive introduction to CO2 heat pump theory and usage A comprehensive introduction of CO2 application in heat pump, authored by leading scientists in the field CO2 is a hot topic due to concerns over global warming and the 'greenhouse effect'. Its disposal and application has attracted considerable research and governmental interest Explores the basic theories, devices, systems and cycles and real application designs for varying applications, ensuring comprehensive coverage of a current topic CO2 heat transfer has everyday applications including water heaters, air-conditioning systems, residential and commercial heating systems, and cooling systems
Handbook of Generation IV Nuclear Reactors, Second Edition is a fully revised and updated comprehensive resource on the latest research and advances in generation IV nuclear reactor concepts. Editor Igor Pioro and his team of expert contributors have updated every chapter to reflect advances in the field since the first edition published in 2016. The book teaches the reader about available technologies, future prospects and the feasibility of each concept presented, equipping them users with a strong skillset which they can apply to their own work and research. Provides a fully updated, revised and comprehensive handbook dedicated entirely to generation IV nuclear reactors Includes new trends and developments since the first publication, as well as brand new case studies and appendices Covers the latest research, developments and design information surrounding generation IV nuclear reactors
Supercritical CO2 (sCO2) power cycle is an up-and-coming technology to produce electricity from various heat sources. Apart from power cycles, sCO2 can also be used as coolant in centralized cooling system and stand-alone cooling device. However, lack of accurate predication tools such as heat transfer coefficient correlations and insufficient knowledge behind fundamental heat transfer processes can hinder its practical realization in key energy and cooling systems. The overall objective of the study is to extend fundamental knowledge about heat transfer and fluid flow processes in conduits pertinent to sCO2 power cycle. The emphasis here is investigation of heat transfer effects of three testing parameters: heat flux, inlet mass flux and inlet temperature. Experimental setup for this heat transfer study is designed considering limitations due to high pressure rating requirements and thus follows unconventional approach to calculate heat transfer coefficient. Test section chosen is a horizontal stainless steel tubing of inner diameter of 9.4 mm and heated length of 1.23 m with uniform volumetric heat generation within tubing walls. The designed test apparatus and data reduction process are validated with high pressure air experiments. Nusselt numbers are calculated at top, bottom and sidewall locations to demonstrate effects of buoyancy. Enhancement of heat transfer at bottom wall surfaces and deterioration at top wall surfaces is observed as the main effect of buoyancy. It was observed that effects of buoyancy increase with heat flux and decrease with mass flux. Buoyancy effects are also decreased for fluid temperatures higher than pseudocritical temperature. Nusselt numbers calculated from experimental results are compared with Nusselt number from available correlations in literature. It is hinted that near critical region where property variations are significant, one correlation alone may not accurately predict heat transfer for different regimes of geometry, mass flux and heat flux.
Supercritical fluids are increasingly being used in energy conversion and fluid dynamics studies for energy-related systems and applications. These new applications are contributing to both the increase of energy efficiency as well as greenhouse gas reduction. Such research is critical for scientific advancement and industrial innovations that can support environmentally friendly strategies for sustainable energy systems. The Handbook of Research on Advancements in Supercritical Fluids Applications for Sustainable Energy Systems is a comprehensive two-volume reference that covers the most recent and challenging issues and outlooks for the applications and innovations of supercritical fluids. The book first converts basic thermo-dynamic behaviors and “abnormal” properties from a thermophysical aspect, then basic heat transfer and flow properties, recent new findings of its physical aspect and indications, chemical engineering properties, micro-nano-scale phenomena, and transient behaviors in fast and critical environments. It is ideal for engineers, energy companies, environmentalists, researchers, academicians, and students studying supercritical fluids and their applications for creating sustainable energy systems.
Supercritical carbon dioxide (sCO2) can be utilized as a working or heat transfer fluid in various thermal systems with applications in large-scale power cycles; portable power production units, coolant systems and devices. However, there are no sufficient methods and equations of heat transfer coefficient correlations, and in addition insufficient research studies about the mechanisms controlling heat transfer processes for sCO2. This study is motivated by the need to understand the intricate properties of sCO2 heat transfer and fluid dynamics with an emphasis on flow direction and inclination effects. This paper presents the study on effects of gravity, buoyancy on sCO2 flow at temperatures near and away from the pseudocritical temperature. The experimental setup consists of a high temperature and pressure sCO2 heat transfer loop and flow testing facility. Recently researched sCO2 heat exchangers can have tubes oriented at different angles such as 45° or 90° to horizontal. For the optimized design of efficient and cost-effective turbomachinery components utilizing sCO2 as the heat transfer fluid, an understanding of convective heat transfer inside a tube/pipe is equally as important as external heat transfer. A study on sCO2 heat transfer at various inclinations with angles ranging from 0°(horizontal) to 90°(vertical) along with upward and downward flow directions with different inlet temperatures is conducted. Thermocouple-based temperature measurement is utilized at multiple locations within the tube test section axially and circumferentially to study the temperature distributions on the tube surface. Volumetric heat generation is utilized to heat the external wall of the tube test section, Nusselt and Richardson numbers are calculated at circumferential wall location to show the effects of buoyancy and gravity. These Non-dimensional parameters are plotted from experimental data to show the effect of the varying parameters on heat transfer and fluid dynamics properties of the flow. it can be seen that for inlet bulk temperatures near the pseudocritical temperature, buoyant force are stronger but reduce as the inlet temperature and inclination angle is increased the buoyant forces become negligible.