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Within this project, the heat transfer and pressure drop characteristics of CO2 (R-744) were investigated at supercritical pressures (cooling) and subcritical pressures (evaporation). The measurements were carried out with a microchannel type (MPE) tube with 25 parallel ports and a port diameter of 0.787 mm. The experimental data collected confirm that CO2 offers high heat transfer coefficients at supercritical pressures. The comparison of these data with common correlations shows good correspondence. The comparison of the measured pressure drop data with calculation models is satisfactory as well. At evaporation the situation is not as clear. The experiments at high mass fluxes have shown a strongly decreasing heat transfer coefficient from a certain vapour fraction upwards. A comparison with a calculation model for dry-out has shown that this drop has to be expected. On the contrary, at low mass fluxes no influence of the velocity was detected. None of the heat transfer calculation models investigated takes this phenomenon into account. The two-phase pressure drop correlations yield too low values in general, but for a first estimation models from the literature can be taken.
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.
We live in interesting times in which life as we know it is being threatened by manmade changes to the atmosphere in which we live. On the global scale, concern is focused on climate change due to greenhouse gas emissions, and on a national scale, atmospheric pollution produced by combustion processes is of concern. A possible approach is through the development of new ideas and innovative processes to the current practices. Among the available options, multi-generation processes such as the trigeneration cycle, battery storage system, solar power plants and heat pumps have been widely studied, as they potentially allow for greater efficiency, lower costs, and reduced emissions. On the other hand, some researchers had been working to increase the potential of energy generation process through heat recovery under the steam generator, organic Rankine cycle, and absorption chillers. In this Special Issue on "Thermal Systems” of fundamental or applied and numerical or experimental investigation, many new concepts in thermal systems and energy utilization were explored and published as original research papers in this “Special Issue”.
"Multiphase flow and heat transfer have found a wide range of applications in several engineering and science fields such as mechanical engineering, chemical and petrochemical engineering, nuclear engineering, energy engineering, material engineering, ocea"