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The mixing ventilation (MV) system could spread airborne infectious diseases in airplane cabins, such as the flu and Corona-virus infection (COVID-19). As a result, it is critical to improve the current design of airline cabin ventilation systems. Six innovative customized ventilation (PV) systems were proposed and investigated through numerical simulations in this study to reduce pollutant transport. At first, two environmental chambers' experimental data on airflow, air temperature, and pollutant concentration were utilized to validate the Computational Fluid Dynamics (CFD) model used for the present numerical study. The six PVs were then studied to analyze the distributions of air velocity, air temperature, pollutant concentration, and carbon dioxide (CO2) concentration in a part of the Boeing 767 cabin using the validated CFD model. It is found that the passenger's personal outlet set up on both sides of his or her head (PV-1) has the best distribution of carbon dioxide concentration, velocity, temperature, and contaminant concentration when compared to the six personalized air distribution systems. It can reduce pollutant concentrations more than the other proposed systems. The present study may shed new light on the design of PVs with the potential for reducing the spreading of airborne infectious diseases. [ABSTRACT FROM AUTHOR]

In this study, a comprehensive literature survey of all the experimental data available in literature regarding optimal heat rejection pressure on transcritical carbon dioxide systems is carried out. Based on the phenomenological analysis and quantitative assessment of the data, a Poisson regression analysis is performed and a new dimensionless correlation to predict the optimal heat rejection pressure is developed. The influences of relevant parameters on heat rejection pressure, such as ambient temperature and temperature at the gas cooler are taken into account in the correlation development. The proposed correlation is tested against experimental measurements and is further validated by comparison to all existing experimental data collected from literature. Additionally, the predictive ability of the newly proposed correlation is discussed, tested against experimental data found in literature and compared with existing correlations. Upon comparison with existing correlations, the new generalized dimensionless correlation proposed in this study predicts the optimal heat rejection pressure with an average deviation of 1.31% and a standard deviation of 4.26 bar. Thus, becoming the most accurate correlation up to date, yet providing a much wider range of temperature applicability than the correlations currently available in literature. [ABSTRACT FROM AUTHOR]

The aim of the present study was to extend the knowledge on the suitability and performance of different ventilation retrofit solutions for school buildings located in a temperate climate. A unique approach was used, where four similar and adjacent classrooms in the same school unit located north of Copenhagen, Denmark, were retrofitted either with a decentralized, balanced supply and exhaust mechanical ventilation unit with heat recovery; automatically operable windows with an exhaust fan; automatically operable windows with alternating counter-flow heat recovery through slots in the outside wall; or a visual feedback display unit showing the current classroom carbon dioxide concentration, thus advising when the windows should be opened. For comparison, one classroom retained the original approach for achieving ventilation by manual opening of windows. One year after retrofitting the classrooms carbon dioxide concentrations, temperatures, energy use, and window and door opening behavior were recorded during a four week period in the heating season in January. The measured carbon dioxide concentrations were significantly lower in the classrooms with the mechanical ventilation system and the system with automatic window opening and an exhaust fan as compared with the classrooms with automatic window opening and heat recovery, with visual carbon dioxide feedback and where windows were opened manually. The automatically controlled windows were open for 71% of the occupied time including breaks with an exhaust fan and for 49% with heat recovery. The façade windows were open up to 17% of the occupied time including breaks in the classrooms with manual window opening (with or without visual feedback). The classroom temperature was generally within the recommended thermal comfort range. The present results indicate that in temperate climates the mechanical ventilation system and both systems with automatic window opening are the recommended systems for classrooms in temperate climates. Providing simply visual feedback on the current carbon dioxide concentration, as a motivation for window opening, did not do so. [ABSTRACT FROM AUTHOR]

Temperature and pressure effects on quenching distance have been studied for difluoromethane (R32) and ammonia (R717), two mildly flammable refrigerant gases. By studying ignition and quenching behavior in conditions beyond standard test conditions, the authors aimed at understanding whether combustion takes place in practical situations. First, the quenching distance of R32 and R717 in the temperature range of 298 to 333 K and pressure range of 0.79 to 1.12 atm was measured. The quenching distance decreased slowly with increasing temperature and decreased steeply with increasing pressure. Second, the authors examined the relationship between quenching distance and burning velocity at various temperature and pressure. It was found that quenching distance is essentially related to (unburned gas density × burning velocity)−1, and changes in quenching distance observed in the present experiment can be explained by the temperature and pressure dependences of burning velocity and unburned gas density. It was also found that the quenching distance and burning velocity for R32 were not affected much by the humidity of air in the practical condition. [ABSTRACT FROM PUBLISHER]

This article proposes an ejector model based on the real properties of CO2, which includes the critical mode and sub-critical mode (the critical mode means the primary and suction flows are both choked, and the sub-critical mode refers to only the primary flow choking). Moreover, a dynamic model of the transcritical CO2ejector expansion refrigeration cycle is developed to simulate system responses at different ejector operational modes. The prediction results by the ejector model and system model are compared with available experimental data, respectively. Furthermore, the dynamic responses of the ejector expansion refrigeration cycle based on the entire ejector model are compared with those predicted upon the ejector model only with critical mode. The present results show the entrainment ratio provided by the ejector model coincides well with the experimental data, and most data lie within ±10% error. The system model predicts the gas cooler pressure and evaporator pressure with errors of 1.8% and 4.2% for the measured results, respectively. Moreover, the pressure and mass flow rates of the system based on the ejector model only with critical mode are higher than that by the entire ejector model. The proposed model is useful to predict performances accurately and conduct dynamic analysis reasonably. [ABSTRACT FROM PUBLISHER]

Two-phase ejector studies in recent years have shown that the performance of an ejector cycle is strongly dependent on the performance of the ejector. However, measuring the performance of a two-phase ejector is not always straightforward. In the present study, several different metrics that have been proposed for measuring the performance of ejectors are presented and analyzed. Performance metrics originally proposed for single-phase ejectors as well as those proposed specifically for two-phase ejectors are both considered. A simple numerical ejector model is used to simulate ejector operation and calculate the performance of the ejector based on the various performance metrics. The various ejector performance metrics are then compared based on the numerical results. Experimental data from previous studies for R134a and CO2 two-phase ejectors is also presented, and the ejector performance metrics are compared based on the available experimental data as well. It is seen that for a given ejector operating point, the different performance metrics can yield very different values. Based on the data from available ejector studies, it seems that CO2 ejectors can achieve somewhat better performance than R134a ejectors. It is also seen that better ejector performance does not necessarily correlate to better ejector cycle performance. [ABSTRACT FROM AUTHOR]