Numerical model of the gas-liquid ejection is established based on the Eulerian-Eulerian method of multiphase flow model, coupled with population balance equation to describe the size variation of droplet population during the preparation of slush nitrogen using the ejector-based atomization cooling method. According to four-stage droplet solidification process-liquid supercooling stage, nucleation recalescence stage, freezing stage and solid tempering stage, the corresponding heat transfer equations are established for each stage to analyze the interaction between gas flow and droplet population. The evolution of key parameters (including pressure, velocity, and volume fraction) in the gas-liquid two-phase flow field of ejector is determined by analyzing the flow behavior during the gas-liquid ejection. As the gas inlet pressure gradually increases from 0.11 MPa to 0.13 MPa, the ejection coefficient rapidly increases from 7.689 to 9.161. Although the pressure continues to increases thereafter, the ejection coefficient gradually stabilizes, indicating that the atomization effect first improves with increasing inlet pressure and then stabilizes. As for the heat transfer characteristics during the solidification process of droplet population, the effects of gas velocity and temperature on the solidification rate of droplets and the effective utilization of cold energy are analyzed for the varying droplet diameters. Besides, the total solidification rate of the droplets and the energy efficiency ratio of the system are evaluated for different ejection coefficients. As the droplet size increases, the displacement during nucleation first increases and then decreases, while the displacement at complete solidification exhibits an approximately exponential growth.

An innovative air distribution, synthetic fountain jet ventilation (SFV), was proposed in this study to reduce cross-infection risk in hospital wards in winter. A computational fluid dynamics (CFD) method validated by experimental data was employed to investigate airflow pattern and exhaled particle dispersion in a four-bed ward under SFV and two configurations of mixing ventilation (MV). Zoning effect on particles under SFV was quantified using the concentration gradient between sub-zones (eta). The significance and correlation of supply air velocity (V-s), supply air temperature (T-s), and supply inlet height (H-s) with the zoning effect were statistically evaluated. An improved Wells-Riley model was employed to quantify the potential of SFV in reducing the cross-infection risk. Results showed that upward fountain jet under SFV followed the direction of positive buoyancy, thereby restricting lateral and longitudinal dispersion of exhaled particles in winter. The zoning effect was significantly affected by V-s and the average air velocity in the central zone (V-ave). A highly significant positive correlation was found between eta and V-ave (p < 0.01), as V-ave significantly affected particle removal and suspension. A "vertical air channel" formed between the ceiling-level outlet and the floor under a high V-ave, enhancing particle removal whilst reducing particle suspension. Calculations of infection probability demonstrated that SFV effectively reduced the risk of cross-infection among patients in winter. Compared with the two configurations of MV, the average infection probability in Sub-zones 2, 3, and 4 located outside the source zone under SFV was reduced by 55.04% and 76.52%, respectively.

Understanding the inactivation kinetics of airborne influenza viruses is essential for accurately assessing infection risk and developing effective ambient control strategies. However, most existing studies focus on virus stability during the aerosol suspension phase, with limited quantitative data on the critical evaporation phase immediately after droplet release. Here, we developed a novel controllable relative humidity (RH), temperature, and particle size aerosol quantification experimental system, using H1N1 and H3N2 influenza viruses to quantify the inactivation rate. The mixing tube outlet achieved droplet evaporation equilibrium at specific temperature and RH (±0.5°C, ±1.5% RH); the inertial impactor enabled particle size selection; the aerosol remained suspended within the rotating drum maintaining ±0.3°C and ±2% RH stability. Results indicated that at 25°C, influenza virus survival during the evaporation phase exhibited a U-shaped relationship, with H3N2 showing higher sensitivity than H1N1; however, at 35°C, the viability of influenza viruses increased with rising RH. Notably, the inactivation rate constants of H1N1 influenza viruses during evaporation exceeded those in the suspension phase by two orders of magnitude, providing the first kinetic quantification of this rapid inactivation process. These findings demonstrate that virus inactivation during the evaporation phase is a dominant yet previously underestimated pathway and establish a new bioaerosol experimental system for quantifying the stability of airborne pathogens under controlled ambient conditions.

Leakage of cryogenic helium into a vacuum chamber can induce supersonic jet impingement, posing risks of local overpressure and overheating that threaten stability and safety. However, the localized effects resulting from jet impingement remain insufficiently explored. A two-dimensional axisymmetric model with the SST k-omega turbulence model and the real gas model is established to investigate flow and heat transfer characteristics of supersonic impinging jets from thermal shield helium leakage. Two key factors are evaluated, including dimensionless impingement distance (Lp/D = 5-120) and surface shape (flat, convex, and concave). Results indicate that increasing Lp/D from 5 to 120 reduces the secondary peaks in velocity, temperature, pressure, and heat flux profiles. The dimensionless impingement distance strongly affects the Mach disk sizes in the nearfield region (Lp/D = 5 and 10). At Lp/D = 100 and a central angle of 90 degrees, the convex surface reduces the average Nusselt number by 8.68% compared with the flat surface, while the concave surface increases it by 12.78%, indicating that the convex surface is superior in overheating risk mitigation. Moreover, the average heat flux on the target surface consistently exceeds the reference value (10 kW/m2) for the liquid helium film boiling threshold, while the wall pressure remains below 100 kPa, indicating that overheating risk arises prior to overpressure risk during loss of coolant accidents. These findings can provide references for the risk assessment and thermal protection design of superconducting magnets, as well as the optimization of component layouts in cryostats.

Viral loads of different-sized droplets are fundamental for accurately assessing transmission risks of respiratory infectious diseases in indoor environments. However, current sampling techniques lack experimental data on exhaled droplets larger than 10 mu m, posing a significant challenge in identifying the dominant transmission routes and preparing for the potential emergence of 'Disease X'. This study developed a novel sampler based on the aerodynamic characteristics of respiratory droplets, including gravitational deposition and inertial impaction, to characterize viral loads in full-size-range respiratory droplets. The sampler components were designed and optimized through the computational fluid dynamics (CFD) simulations, and their performance was evaluated using inert particle aerosols, demonstrating effective collection of respiratory droplets across five size ranges: 1-2.5 mu m, 2.5-5 mu m, 5-10 mu m, 10-50 mu m, and >50 mu m. The sampler achieved over 84% collection efficiency for droplets larger than 50 mu m, with minimal loss (<15%) for droplets smaller than 10 mu m, and consistent performance (fluctuations <15%) across various respiratory conditions. In clinical validation, SARS-CoV-2 RNA was detected in respiratory droplets from 4 out of 5 COVID-19 patients, ranging from nondetectable to 9.11 (>50 mu m), 8.17 (10-50 mu m), 4.95 (5-10 mu m) and 5.91 (1-5 mu m) log(10) RNA copies per 15-min sampling, respectively. These findings offer a systematic quantification of SARS-CoV-2 viral distribution at the source, across the full-size-range of respiratory droplets, providing previously lacking data. This novel sampler enables comprehensive source characterization and supports effective non-pharmaceutical intervention strategies for infection control.

Intensive care units (ICUs) are high-risk areas for hospital-acquired infections. Although impinging jet ventilation (IJV) shows great potential in cooling conditions, its ventilation efficiency in heating conditions remains questionable. This study numerically investigates the performance of IJV in a four-bed ICU, examining key factors such as air inlet position, outdoor temperature and air change rate. The interaction between the warm supply air current and cold air currents from exterior walls and windows determines airflow characteristics and respiratory droplet dispersion. Positioning the air inlet near the window promotes early mixing of warm and cold air currents, resulting in a more uniform temperature distribution (vertical gradients: < 0.2 degrees C/m vs. 0.83 degrees C/m). Lower outdoor thermal loads (e.g. 20 degrees C vs. -10 degrees C) enhance air mixing and droplet removal efficiency. Increasing the air change rate (ACH) from 4 to 12 ACH reduces the intake fraction of fine droplets (<5 mu m) by adjacent susceptible patients from 0.48% to <0.05%. Temperature differences between patient-level air and outlet air remain within 1.0 degrees C if ACH >= 8, ensuring thermal comfort. This study has validated IJV's potential for ICU applications and has provided theoretical guidance for its design.

Large amount of cold energy stored in liquefied natural gas (LNG) exhibits huge application potentials and also the issues of significant cold load fluctuations and low utilization efficiency, in view of which this study proposes an integrated LNG cold energy-based system combining liquid air energy storage (LAES) and air separation unit (ASU), designated as LNG-LAES-ASU. Based on the variations in LNG temperature and electricity pricing, three operating modes (i.e., energy storage, power generation, and conventional modes) have been proposed for the system. The thermodynamic performance of the system is evaluated through process simulation, sensitivity analysis of key parameters and exergy analysis. The results show that the integrated system increases the exergy efficiency by 10 %, raises the ASU product output by 15 %, reduces the operating costs by 38 %, and enhances the overall revenue by 49 % compared with the standalone LNG cold energy ASU. Additionally, the system achieves a round trip efficiency of 72.9 % and an exergy efficiency of 75 % with an air expansion temperature of 150 degrees C. Economically, the system exhibits a dynamic payback period of 5.55 years within a 20-year lifespan, which could be further reduced with higher external heat source temperatures. This system can enhance the spatiotemporal coordination of LNG cold energy supply and demand by integrating its utilization with the power grid peak-shaving operations, thereby providing an efficient solution for large-scale LNG cold energy utilization.

Data center cooling systems are substantial energy consumers, and managing the heat generated by electronic devices is becoming more complex as chip power levels continue to rise. The single-phase immersion cooling (SPIC) server with oil coolant is numerically investigated using the validated Re-Normalization Group (RNG) k-epsilon model. For the investigated scenarios where coolant velocity at the tank inlet is 0.004 m/s and the total power is 740 W, the heat transfer between the heat sinks and the coolant is dominated by natural convection, although forced convection mediates the overall heat transfer inside the tank. The maximum velocity of coolant through the heat sink is 0.035 m/s and the average heat transfer coefficient is up to 75.8 W/(m2K). The geometry of the heat sink is important for the cooling performance. Increasing both the fin thickness and number enhances the natural convection effect of the heat sink, but also increases the flow resistance. The heat sink with a fin thickness of 3 mm performs the best, reducing the average graphics processing unit (GPU) temperature from 71.3 degrees C to 68.6 degrees C. A heat sink with an optimal fin number of 16 reduces the average GPU temperature to 67.7 degrees C. As for the effect of fin height, increasing it from 15 to 30 mm results in increases in the heat transfer area and flow rate by about 72% and 32%, respectively, which reduces the average GPU temperature to 65.2 degrees C. Therefore, the importance of fin parameters ranks in the following order: fin height, number, and thickness. This study highlights the potential application of oil coolants in SPIC systems and offers theoretical guidance for the efficient design of natural convection cooling solutions.

Hydrogen has been extensively utilized in the chemical industry and is now increasingly adopted as an energy carrier, leading to a wide array of applications in energy storage, transportation, metallurgy, and aerospace. Despite numerous studies on hydrogen leakage risk assessment, the focus has predominantly been on equipment failure-induced scenarios, neglecting evaluations under normal operational conditions. This limitation results in overly specific or conservative conclusions that are not directly applicable to hydrogen energy equipment in typical service settings. This paper investigates key parameters in hydrogen leakage models, including space dimensions and ventilation conditions. Focusing on typical hydrogen storage and transportation equipment-such as hydrogen pipelines, onboard cylinders, stationary hydrogen storage containers, and mobile hydrogen storage units-we analyze common installation and operational spaces and ventilation conditions. Based on existing standards and relevant research, we examine possible failure points and hydrogen leakage rates. Our research emphasizes that hydrogen safety is a product of both equipment design and environmental factors, advocating against solely relying on equipment-specific hydrogen leakage rates for assessing overall hydrogen safety. By examining specific parameters in hydrogen leakage models for various application scenarios, we scrutinize hydrogen concentration distributions under stringent conditions, providing a foundational reference model for hydrogen leakage management in diverse energy applications. Additionally, our findings inform the selection and positioning of hydrogen concentration sensors to bolster safety protocols in hydrogen energy environments.

The effect of gradient exhaust strategy and blind plate installation on the inhibition of backflow and thermal stratification in data center cabinets is systematically investigated in this study through numerical methods. The validated Re-Normalization Group (RNG) k-epsilon turbulence model was used to analyze airflow patterns within cabinet structures equipped with backplane air conditioning. Key findings reveal that server-generated thermal plumes induce hot air accumulation at the cabinet apex, creating a 0.8 degrees C temperature elevation at the top server's inlet compared to the ideal situation (23 degrees C). Strategic increases in backplane fan exhaust airflow rates reduce server 1's inlet temperature from 26.1 degrees C (0% redundancy case) to 23.1 degrees C (40% redundancy case). Gradient exhaust strategies achieve equivalent server temperature performance to uniform exhaust distributions while requiring 25% less redundant airflow. This approach decreases the recirculation ratio from 1.52% (uniform exhaust at 15% redundancy) to 0.57% (gradient exhaust at equivalent redundancy). Comparative analyses demonstrate divergent thermal behaviors: in bottom-server-absent configurations, gradient exhaust reduces top server inlet temperatures by 1.6 degrees C vs. uniform exhaust, whereas top-server-absent configurations exhibit a 1.8 degrees C temperature increase under gradient conditions. The blind plate implementation achieves a 0.4 degrees C top server temperature reduction compared to 15%-redundancy uniform exhaust systems without requiring additional airflow redundancy. Partially installed server arrangements with blind plates maintain thermal characteristics comparable to fully populated cabinets. This study validates gradient exhaust and blind plate technologies as effective countermeasures against cabinet-scale thermal recirculation, providing actionable insights for optimizing backplane air conditioning systems in mission-critical data center environments.

During a loss of coolant accident (LOCA) caused by a rupture on helium-cooled thermal shields, an underexpanded jet of cryogenic helium forms in the cryostat, increasing pressure in the cryostat and heat load on superconducting magnet walls. The real gas effect of helium at low temperatures has significant impacts on the fluid flow and heat transfer characteristics, which has not yet been fully investigated. In this work, simulations of the helium underexpanded jets during a LOCA with the ideal gas assumption and the NIST real gas model are compared. Besides, the effect of breach size on helium underexpanded jets is also investigated. Results show that the simulation with the ideal gas assumption significantly underestimates the average pressurization rate in the vacuum chamber and the average heat flux on the center solenoid wall (CS-1) compared to the NIST real gas model, with relative errors of 38.59 % and 30 % in 1 s, respectively. As the breach diameter increases from 120 mm to 240 mm, a non-proportional relationship exists between the breach diameter and the pressurization rate. For all the breach sizes discussed, there is no risk of overpressure in the vacuum chamber during the early stage of the LOCA, whereas the risk of liquid helium film boiling occurs in the superconducting magnet cooling system.

In epidemiological prospective modelling, assessing the hypothetical infectious quanta emission rate (Eq) is critical for estimating airborne infection risk. Existing Eq models overlook environmental factors such as indoor relative humidity (RH) and temperature (T), despite their importance to droplet evaporation dynamics. Here we include these environmental factors in a prospective Eq model based on the airborne probability functions with emitted droplet distribution for speaking and coughing activities. Our results show relative humidity and temperature have substantial influence on Eq. Drier environments exhibit a notable increase in suspended droplets (cf. moist environments), with Eq having a 10-fold increase when RH decreases from 90 % to 20 % for coughing and a 2-fold increase for speaking at a representative summer indoor environment (T = 25° C). In warmer environments, Eq values are consistently higher (cf. colder), with increases of up to 22 % for coughing and 9 % for speaking. This indicates temperature has a smaller impact than humidity. We demonstrate that indoor environmental conditions are important when quantifying the quanta emission rate using a prospective method. This is essential for assessing airborne infection risk.

Conventional buses, as an indispensable part of the urban public transport system, impose cross-infection risks on passengers. To assess differential risks due to dynamic staying durations and locations, this study considered four spatial distributions (i = 1-4) and six temporal scenarios (j = 1-6) of buses. Based on field measurements and a risk assessment approach combining both short-range and room-scale effects, risks are evaluated properly. The results showed that temporal asynchrony between infected and susceptible individuals significantly affects disease transmission rates. The Control Case assumes that infected and susceptible individuals enter and leave synchronously. However, ignoring temporal asynchrony scenarios, i.e., the Control Case, resulted in overestimation (+30.7 % to +99.6 %) or underestimation (-15.2 % to -69.9 %) of the actual risk. Moreover, the relative difference ratios of room-scale risks between the Control Case and five temporal scenarios are impacted by ventilation. Short-range risk exists only if infected and susceptible individuals have temporal overlap on the bus. Considering temporal and spatial asynchrony, a more realistic total reproduction number (R) can be obtained. Subsequently, the total R was assessed under five temporal scenarios. On average, for the Control Case, the total R was estimated to be +27.3 % higher than j = 1, -9.3 % lower than j = 2, +12.8 % higher than j = 3, +33.0 % lower than j = 4, and + 77.6 % higher than j = 5. This implies the need for a combination of active prevention and real-time risk monitoring to enable rigid travel demand and control the spread of the epidemic.

Finned-tube heat exchanger (FTHE) is often used as an evaporator in commercial products of separated heat pipe (SHP). The working conditions of FTHE in gravity-assisted SHP are significantly different from those working in refrigerators and air conditioners. Although FTHE is widely used in commercial products of SHP, previous research on its characteristics is very limited. In this paper, a mathematical model for a SHP with FTHE as the evaporator and plate heat exchanger as the condenser is established and verified with experiments. Parametric analyses are carried out to investigate the influences of evaporator design parameters: air inlet velocity, number of tube rows, tube diameter, and fin pitch. With the increasing of air velocity, number of tube rows and tube diameter, and the decreasing of fin pitch, the heat transfer rate increases, while the energy efficiency ratio (EER) decreases monotonically. Using the total cost of the ten-year life cycle as the performance index, the structure parameters of the evaporator with a given heat transfer rate are optimized by the method of orthogonal experimental design. It is found that the total cost can differ as large as nearly ten times between groups. Among the three factors investigated, the number of tube rows has a significant impact on the total cost of the evaporator. With more tube rows, the total cost will be less. The impacts of fin pitch and tube diameter are insignificant. These results are of practical importance for the engineering design of FTHE in gravity-assisted SHP.

As the requirement for Li-ion battery thermal management system (TMS) in electric vehicles (EVs) rises, an integrated direct cooling thermal management system with high compactness and energy efficiency has been developed. The integrated system is based on a vapor-compression refrigeration cycle, forming a dual-evaporator system, and adopts a direct cooling scheme on the battery side, which brings high coupling effects. A novel decoupling control strategy was proposed and a weight-based fuzzy logic control algorithm was applied to implement the strategy, achieving effective temperature control and reasonable refrigerant distribution between the battery side and cabin side. Experiments conducted under various scenarios demonstrated that the average temperature of both the battery pack and cabin could be stably controlled around different target values. Maximum fluctuations were effectively limited to less than 0.7 °C in different scenarios. The implementation of this decoupling control strategy has opened up new possibilities for the application of direct cooling TMS in multi-target systems.

采用计算流体力学方法模拟高温高压工况下规则球床堆内超临界氮的流动与传热特性,系统分析了入口流量、填充颗粒直径、堆积床高径比对流动和传热性能的影响规律.结果表明:堆积床内相邻颗粒层间存在流动滞止区,该区域流体温度高于其它低阻力区域.随着液氮入口流量和堆积床高径比的增大以及填充颗粒直径的缩小,气体流动阻力逐渐升高,气体与堆芯的换热更加充分,进而出口氮气温度变高.

The cryostat in Chinese Fusion Engineering Testing Reactor (CFETR) provides a vacuum and low temperature environment to limit the convective heat exchange for superconducting magnets. If the thermal shield is broken due to the rupture of components or welds, the helium gas in cooling tube will flow into the cryostat, which can increase the thermal load of superconducting magnets and even cause damage to the system. A two-dimensional numerical model, using ideal gas model and SST k - ω model, is built to simulate the loss of vacuum accident that helium gas flows into the cryostat under a postulated breach. The velocity at monitoring points, the velocity contours and the average heat flux on the walls of superconducting magnets are obtained under different initial pressure conditions. The results show that a shock wave can be observed near the gas inlet. When the initial pressure in the cryostat ranges from 1000 Pa to 10000 Pa, the velocity at the axis in front of the shock wave is almost unaffected, while the velocity after the shock wave decreases. However, the average heat flux on the walls of superconducting magnets increases rapidly as the pressure increases, which needs to be considered in designing the safety system.

Close contact routes, including short-range airborne and large-droplet routes, play an important role in the transmission of SARS-CoV-2 in indoor environments. However, the exposure risk of such routes is difficult to quantify due to the lack of data on the close contact behavior of individuals. In this study, a digital wearable device, based on semi-supervised learning, was developed to automatically record human close contact behavior. We collected 337,056 s of indoor close contact of school and university students from 194.5 h of depth video recordings in 10 types of indoor environments. The correlation between aerosol exposure and close contact behaviors was then evaluated. Individuals in restaurants had the highest close contact ratio (64%), as well as the highest probability of face-to-face pattern (78%) during close contact. Accordingly, university students showed greater exposure potential in dormitories than school students in homes, however, a lower exposure was observed in classrooms and postgraduate student offices in comparison with school students in classrooms. In addition, restaurants had the highest aerosol exposure volume for both short-range inhalation and direct deposition on the facial mucosa. Thus, the classroom was established as the primary indoor environment where school students are exposed to aerosols.

Origin of differently sized respiratory droplets is fundamental for clarifying their viral loads and the sequential transmission mechanism of SARS-CoV-2 in indoor environments. Transient talking activities characterized by low (0.2 L/s), medium (0.9 L/s), and high (1.6 L/s) airflow rates of monosyllabic and successive syllabic vocalizations were investigated by computational fluid dynamics (CFD) simulations based on a real human airway model. SST k−ω model was chosen to predict the airflow field, and the discrete phase model (DPM) was used to calculate the trajectories of droplets within the respiratory tract. The results showed that flow field in the respiratory tract during speech is characterized by a significant laryngeal jet, and bronchi, larynx, and pharynx-larynx junction were main deposition sites for droplets released from the lower respiratory tract or around the vocal cords, and among which, over 90% of droplets over 5 µm released from vocal cords deposited at the larynx and pharynx-larynx junction. Generally, droplets’ deposition fraction increased with their size, and the maximum size of droplets that were able to escape into external environment decreased with the airflow rate. This threshold size for droplets released from the vocal folds was 10–20 µm, while that for droplets released from the bronchi was 5–20 µm under various airflow rates. Besides, successive syllables pronounced at low airflow rates promoted the escape of small droplets, but do not significantly affect the droplet threshold diameter. This study indicates that droplets larger than 20 µm may entirely originate from the oral cavity, where viral loads are lower; it provides a reference for evaluating the relative importance of large-droplet spray and airborne transmission route of COVID-19 and other respiratory infections.

To quantify the risk of the transmission of respiratory infections in indoor environments, we systematically assessed exposure to talking-and breathing-generated respiratory droplets in a generic indoor environment using computational fluid dynamic (CFD) simulations. The flow field in the indoor environment was obtained with SST k-omega model and Lagrangian method was used to predict droplet trajectories, where droplet evaporation was considered. Droplets can be categorized into small droplets (initial size <= 30 mu m or <= 10 mu m as droplet nuclei), medium droplets (30-80 mu m) and large droplets (>100 mu m) according to the exposure characteristics. Droplets up to 100 mu m, particular the small ones, can contribute to both short-range and long-range airborne routes. For the face-to-face talking scenario, the intake fraction and deposition fractions of droplets on the face and facial mucosa of the susceptible were up to 4.96%, 2.14%, and 0.12%, respectively, indicating inhalation is the dominant route. The exposure risk from a talking infector decreases monotonically with the interpersonal dis-tance, while that of nasal-breathing generated droplets maintains a relatively stable level within 1.0 m. Keeping an angle of 15 degrees or above with the expiratory flow is efficient to reduce intake fractions to <0.37% for small droplets. Adjusting the orientation from face-to-face to face-to-back can reduce exposure to small droplets by approximately 88.0% during talking and 66.2% during breathing. A higher ventilation rate can reduce the risk of exposure to small droplets but may increase the risk of transmission via medium droplets by enhancing their evaporation rate. This study would serve as a fundamental research for epidemiologist, healthcare workers and the public in the purpose of infection control.