Supply Air Flow Rate Research Articles

Control logic for a novel HVAC system providing room-based indoor climate control in residential buildings

One of the major changes regarding HVAC systems in new or renovated residential buildings is the incorporation of heating in the ventilation system, providing an air heating system. In buildings equipped with air heating systems, indoor climate is commonly controlled based on a reference zone, which leads to unsatisfactory thermal condition in the other zones. In addition, this strategy is not energy-efficient for buildings due to overheating that can occur in some zones. This study presents control logic for a novel-designed demand-controlled ventilation and air heating system and develops a variable air volume (VAV) control strategy to ensure an energy-efficient operation of the system, while providing a satisfactory indoor climate conditions. The air heating system can change airflow rate and air temperature in each room quickly and accurately enabling the control of indoor climate on a room level within a dwelling. The performance of the air heating system, in terms of its effect on thermal comfort and indoor air quality in each room as well as the stability of the system has been evaluated through a full-scale laboratory experiment. The experimental result indicated the ability of the system to satisfy different temperature set-points ranging from 20 °C to 25 °C in different zones with ±0.3°C deviation from the set-points, while the system was remained stable when temperature set-points were changed in several zones during the experiment. Individual indoor air quality requirements were also satisfied with providing the required supply airflow rate to each zone, while the supply fan was running based on the critical zone reset control strategy to achieve an energy-efficient operation.

Increasing the rate of capture of pollutants by local suction due to the use of an external swirling jet. Part 1. Research methods

To capture pollutants in many buildings and structures, it is necessary to use local suction systems. One of the criteria for the effectiveness of local suction ― the rate of capture of pollutants ― it is proposed to increase due to the impact of a swirling annular jet and the occurrence of a reverse air flow. The circular suction is placed coaxially in an external cylindrical casing, in which the air flow is twisted due to the tangential supply of two supply jets into the casing. A vortex dome is formed, which helps to increase the efficiency of such a vortex suction. The change in the axial velocity of air at a distance from the vortex suction was experimentally and numerically investigated for 11 structures, in each of which 4 different ratios of intake and supply air flow rates were set. In the investigated applications of the proposed suction systems, the speed increases significantly at a certain range of distance from them. A combination of designmode parameters that contribute to the highest rate of capture by vortex suction is determined. The results obtained are useful in designing effective suction systems for capturing pollutants.

Holistic methodology to reduce energy use and improve indoor air quality for demand-controlled ventilation

Ventilation control logics are usually based on the control indicators of occupancy. However, strategies including control of contaminants not linked to occupancy are requested and more feasible with the introduction in the market of low-cost sensors (LCS).In this work, a methodology for the improvement of demand-controlled ventilation (DCV) using measurements of IAQ parameters with LCS, correlation analysis, and co-simulation EnergyPlus/CONTAM is presented. Its goal was reduced annual energy use and the fraction of time with room air concentration of IAQ parameters outside thresholds.The ventilation control sequences of supply airflow rates and recirculation of return air focused on the significant parameters chosen by cross-correlation functions in the de-trended measurements.The results revealed that the methodology successfully developed control sequences that simultaneously reduced annual energy use and the number of hours outside the recommended IAQ guidelines compared to the baselines. In cold cities with excellent outdoor air quality, recirculation could reduce energy use and increase the RH in winter. Further simulations demonstrated that the use of recirculation had a protective effect on the indoor concentrations of PM2.5, assuming low outdoor air quality. However, when using recirculation, it is essential to control the IAQ to avoid excessive pollutants, RH, and temperatures.

Cooling effect of air movement on heating performances of advanced air distribution

The cooling effect of air movement is widely recognized as an energy-efficient measure for thermal environment management under the cooling mode, but it is generally neglected under the heating mode. This study proposes that the cooling effect of air movement negatively impacts the energy efficiency of advanced air distribution and should be considered when modulating advanced air distribution under the heating mode. Advanced air distribution like stratum ventilation and wall attachment ventilation supplies warm and clean air into the occupied zone by employing supply air momentum to suppress thermal buoyancy. However, the supply air momentum elevates the air movement in the occupied zone and the cooling effect of air movement negatively impacts the energy efficiency of advanced air distribution under the heating mode. Based on numerical simulations validated by the experiments, the results show the cooling effect of air movement significantly impacts the optimal operation, particularly the optimal supply airflow rate of stratum ventilation and the optimal supply air temperature of wall attachment ventilation. Accounting for the cooling effect of air movement can improve the optimal overall performance of advanced air distribution, particularly when the preference for energy efficiency increases. Compared with the optimization ignoring the cooling effect of air movement, the optimization considering the cooling effect of air movement improves the overall performance by up to 38.7% for stratum ventilation and up to 67.6% for wall attachment ventilation. Therefore, the cooling effect of air movement should be considered for modulating advanced air distribution under the heating mode.

Optimization of supply air flow and temperature for VAV terminal unit by artificial neural network

The purpose of this paper is to optimize the supply air flow rate and temperature of the variable air volume (VAV) terminal unit by artificial neural network (ANN). In general, the setpoint of the VAV terminal unit are supply air flow rate and supply air temperature. These setpoints are determined by the design value, and are calculated by the maximum indoor heating/cooling load and the ventilation requirements. The setpoints calculated based on the design value does not cause any problems in maintaining the indoor environment. However, energy consumption cannot be optimized because it is based on the maximum design value. In order to improve the existing control method, it is important to apply the setpoints according to the real time indoor conditions. The real time indoor condition can be utilized for control through prediction model. In this study, the indoor load, indoor air quality and energy consumption were predicted and used for VAV system control. In addition, the predictive model was developed with the ANN algorithm, and the process of selecting input data and optimizing the predictive model was performed. The performance of the ANN based optimal control algorithm (suggested CASE) for the VAV terminal unit in the target building was compared with those of the dual maximum control algorithm (existing CASE), one of the VAV terminal unit control algorithms. The comparison of heating season showed that the ANN based control algorithm of VAV terminal unit reduced 16.7% of supply fan energy consumption and 19.5% of reheat coil energy consumption compared to the existing CASE using the fixed setpoint.

Numerical Evaluation of the Flow Field of an Isothermal Dual-Corner Impinging Jet for Building Ventilation

The corner impinging jet ventilation is a new air distribution system for use in office environments. This study reports the mean flow field behavior of dual isothermal corner-placed inlets based on an impinging jet in a square-shaped room with the size of 7.2 m × 7.2 m. A detailed numerical study is carried out to evaluate the influence the different configuration parameters, such as the inlet placement, same side or opposite side, and supply airflow rate, have on the flow field. The results show that the highest velocity peak for all cases is obtained at x = 0.5 m and the lowest at x = 3.5 m. The velocity profiles development remains similar when increasing the flow rate. For the zone evaluation, the results show that Case 1 and 2 (V = 20 L/s) meet the requirement of not exceeding 0.15 m/s during the heating season in the occupied zone according the BBR standard both for same-side and opposite-side configurations. For Case 4, the optimal placement of the inlets is opposite to each other when V = 30 L/s for the BBR requirements. Case 1, 2, 3, 4, 5, and 7 all meet the requirement of not exceeding 0.25 m/s during the cooling season both for the same-side and opposite-side configurations. For Case 8, the optimal placement of the inlets is opposite to each other when V = 50 L/s.

Quantification of how mechanical ventilation influences the airborne infection risk of COVID-19 and HVAC energy consumption in office buildings.

This paper presents an EnergyPlus-based parametric analysis to investigate the infection risk of Coronavirus Disease 2019 (COVID-19) under different mechanical ventilation scenarios for a typical medium-sized office building in various climate zones. A Wells-Riley (WR) based Gammaitoni-Nucci (GN) model was employed to quantitatively calculate the airborne infection risk. The selected parameters for the parametric analysis include the climate zone, outdoor air fraction, fraction of infectors, quanta generation rate, and exposure time. The loss and deposition of particles are not considered. The results suggest that the COVID-19 infection risk varies significantly with climate and season under different outdoor air fraction scenarios since the building heating and cooling load fundamentally impacts the supply airflow rate and thus directly influences the amount of mechanical ventilation, which determines the dilution ratio of contaminants. This risk assessment identified the climate zones that benefit the most and the least from increasing the outdoor air fraction. The climate zones such as 1A (Honolulu, HI), 2B (Tucson, AZ), 3A (Atlanta, GA), and 7 (International Falls, MN) are the most energy-efficient locations when it comes to increasing the outdoor air fraction to reduce the COVID-19 infection risk. In contrast, the climate zones such as 6A (Rochester, MN) and 6B (Great Falls, MT) are the least energy-efficient ones. This paper facilitates understanding a widely recommended COVID-19 risk mitigation strategy (i.e., increase the outdoor airflow rate) from the perspective of energy consumption.Electronic Supplementary MaterialSupplementary material is available for this article at 10.1007/s12273-022-0937-5 and is accessible for authorized users.

Effect of blind angles on thermal decay in the UFAD system in summer

• Thermal decay depending on blind angle examined two heat transfer effects. • The effect of change in conduction at the bottom surface of the concrete slab on the rate of convective heat transfer. • The effect of change in solar radiation at the top surface of the raised floor on convective heat transfer at its bottom surface. • Optimization of the UFAD system based on analysis of thermal decay and chiller energy consumption, and their dependence on Phi. The objective of this study was to explain and quantify the effect of blinds on the heat transfer paths causing thermal decay and the resulting effects on diffuser discharge temperatures, temperature profiles, supply airflow rate, and energy consumption. To analyze the direct effects of convective heat transfer on thermal decay as blind angle changes, two processes must be examined. The first is the effect of the return temperature on convective heat transfer during operating hours, and the resulting change in conduction rate at the bottom surface of the concrete slab. The second is the effect of change in solar radiation at the top surface of the raised floor on convective heat transfer from its bottom surface, which affects thermal decay. This paper performed an analysis of thermal decay and cooling energy consumption according to the blind angle through a comparison of experimental data and simulation results. Phi means stratification in the room; lower values mean more stratification, which should be considered as a UFAD systems design. The most notable result of this study is the return temperature. It can be seen that during better temperature stratifications, the return temperature was higher due to the plume phenomenon. Although blinds decrease the total indoor cooling load with their shielding effect on the solar radiation load, the rise in return temperature increased the cooling coil load of the air conditioner and the heat storage in the concrete slab. This study will help to understand the relationship between the blind angle, the thermal stratification, and cooling energy usage in the UFAD systems.

Effect of UV LED Intensity on The Growth of Red Lettuce in Indoor Hydroponics

Microclimate control is increasingly widespread in limited agricultural environments. This is especially important for the cultivation of plants that tolerate significantly different thermal and hygrometric conditions. Nevertheless, there is much to be done in automation and control technology in this area to achieve the best results in both quantitative and qualitative terms of the product. This applies especially to horticultural crops that are sensitive to the cultivation environment and microclimate. This work aims to characterize the microclimate parameters in a confined agricultural environment with perforated ducts for air conditioning supply. For this work, a microclimate control unit was used instead of a lettuce crop. It was placed into a confined agricultural environment at different locations in the space to obtain the main microclimate parameters. After setting the input of the microclimate environment, the instrument measured a series of physical quantities (temperature, radiant temperature, humidity, and air velocity). Tests were carried out by taking the optimum day temperature constant for growing lettuce and by varying the supply airflow rate by setting the fan speed at 30%, 50%, and 80%. The results of these tests are essential for performing real-time control of the microclimate environment and for managing parameters for optimization of the entire system. In addition, the air velocity test showed adequate velocity reduction and good air mixing. The values obtained are generally acceptable for indoor cultivation and the conditions created are suitable for growing plants in such an environment. Light is an essential need for plants so that plants can carry out the photosynthesis process properly. In indoor DWC hydroponics system, the source of UV light is LED lights for plants. Some of the advantages of using LED light include a small light spectrum, less heat production, low power consumption, and wavelengths of 660 m and 450 m that are needed by plants. This research project aims to create a DWC hydroponic system for growing red lettuce in an indoor hydroponics and see the effect of LED grow light on the growth of red lettuce. DWC hydroponics uses AB-Mix nutrients that are channeled through inch PVC pipes using pump power. The hydroponic rack used has a height of 1.7 m and a width of 40 cm and has 3 shelves, where each shelf has 9 nutrient containers. The red lettuce plants in the DWC system were provided with different light treatments by installing shading nets with different percentages of light penetration, namely, 75%, 50%, and 0%. From the results of these treatments, the average yield of red lettuce was 300 grams on the top shelf, 400 grams on the middle shelf, and 600 grams on the bottom shelf.

Numerical investigation of the flow behavior of an isothermal corner impinging jet for building ventilation

The corner impinging jet concept has been proposed as a new air distribution system for use in office environments. The present paper reports the mean flow field behavior of an isothermal corner-based turbulent impinging jet in a room. A detailed experimental study is carried out to validate the numerical simulations, and the predictions are performed using three turbulence models. RNG k − ε model was chosen for this study. This study investigates the influence different configuration parameters such as jet discharge height, diffuser geometry (shape and size) and supply airflow rate have on the flow field. The results show that the diffuser geometries used in this study had in general a minor effect on the velocity developments along the centerline of the floor, maximum velocity decay and jet spreading rate except for some specific cases. When evaluating the triangle geometry cases, the results show that all the cases with volume flow <20 L/s are able to meet Boverket's building regulations velocity requirement both for summer and winter. The applicability evaluation show that the results can be considered for room sizes between ≈25 and 100 m 2 . In addition, the wall confinement effect (90° vs. 180°) is having a significant impact on the maximum velocity decay for corner impinging jet ventilation. In the regression analysis the results shows that the distance along the diagonal centerline of the room has the most impact on the evaluation of maximum velocity decay and jet spreading rate. • Corner impinging jet shows higher max. velocity compared to center wall placed. • The most important parameter affecting jet velocity decay and spreading rate is x A • The results are applicable for room sizes between 25 and 100 m 2 .

A determination method on the supply airflow rate with the impact of leakage and bypass airflow of computer room air conditioning

Raised-floor air supply (RFAS) system is widely used in data centers (DCs) for its superior thermal environment and air distribution. However, the occurrence of leakage and bypass airflow (LBA) results in a poor thermal environment and ineffective air cooling in a computer room. Most existing DCs either failed to consider the above airflow effects incurring hot spots, or considered these effects using over-cooling which resulted in high energy consumption. In this paper, we propose a determination method of supply airflow rate for computer room air conditioning (CRAC). With the two-dimensional network diagram of airflow and heat transfer in a DC, the relationship of airflow rates between server demand and CRAC supply is deduced based on the conservation of energy. Then a CFD model was built according to a practical DC and its accuracy was verified by field tests, and the room thermal environment is simulated under three different supply airflow rates obtained by traditional method, proposed method and empirical method. The thermal environment performance is evaluated and compared using SHI. Besides, the energy consumption of these three cases is also investigated. Compared with traditional method, the proportion of racks with SHI>0.2 is reduced from 39.5% to 24.2% and 17.2% for proposed method and empirical method, respectively. The corresponding incremental energy consumption of the empirical method is 4.8 times higher than that of the proposed method. The comparison results show that the proposed determination method of supply airflow rate is a proper option that can reduce hot spots with an energy saving of 406.24 kW of the cooling system.

Downward piston ventilation assisted with a localized - Chair exhaust for mitigation of pathogen transport in educational spaces

This study proposes the integration of a localized - chair exhaust (LCE) device and a downward piston ventilation (DPV) in an educational space. The main aim is to assess the proposed system ability in providing a safe educational space similar to the case implementing the 2 m distancing between students at lower energy consumption. This allows the decrease of the DPV high supply flowrate without jeopardizing the protection level compared to the standalone DPV. A three-dimensional computational fluid dynamics (CFD) model of the educational space with DPV + LCE combination was developed as simulator software. The CFD model was validated experimentally and a parametric study was conducted to determine the best DPV supply airflow rate when assisted with LCE. The DPV flowrate should be capable of (i) preserving high protection levels, (ii) maintaining acceptable thermal comfort levels, while (iii) reducing the energy bill burden. Hence, an infected student was assumed sitting in the middle of the space and the exposure levels of the surrounding students were evaluated using the intake fraction (iF) index. It was found that for a practical use in educational spaces, the DPV system should be operated at 60 l/s per person with LCE at 20 l/s to maintain the high protection potential of the DPV system at 51% energy savings in the overall system energy consumption. Furthermore, the energy performance of the proposed system was compared to that of other ventilation systems implemented in educational spaces while offering same or better protection levels.

Ventilation indices for evaluation of airborne infection risk control performance of air distribution

Air distribution is an effective engineering measure to fight against respiratory infectious diseases like COVID-19. Ventilation indices are widely used to indicate the airborne infection risk of respiratory infectious diseases due to the practical convenience. This study investigates the relationships between the ventilation indices and airborne infection risk to suggest the proper ventilation indices for the evaluation of airborne infection risk control performance of air distribution. Besides the commonly used ventilation indices of the age of air (AoA), air change effectiveness (ACE), and contaminant removal effectiveness (CRE), this study introduces two ventilation indices, i.e., the air utilization effectiveness (AUE) and contaminant dispersion index (CDI). CFD simulations of a hospital ward and a classroom served by different air distributions, including mixing ventilation, displacement ventilation, stratum ventilation and downward ventilation, are validated to calculate the ventilation indices and airborne infection risk. A three-step correlation analysis based on Spearman's rank correlation coefficient, Pearson correlation coefficient, and goodness of fit and a min-max normalization-based error analysis are developed to qualitatively and quantitatively test the validity of ventilation indices respectively. The results recommend the integrated index of AUE and CDI to indicate the overall airborne infection risk, and CDI to indicate the local airborne infection risk respectively regardless of the effects of air distribution, supply airflow rate, infectivity intensity, room configuration and occupant distribution. This study contributes to airborne transmission control of infectious respiratory diseases with air distribution.

Stratum-air-distributed radiant-convective room air conditioner for heating

The radiant-convective room air conditioner has been demonstrated to improve the coefficient of performance and the heating capacity effectively. However, the applicability of the radiant-convective room air conditioner in modulating the indoor thermal environment is unclear. This study proposes a stratum-air-distributed radiant-convective room air conditioner and evaluates its effectiveness for providing thermal comfort under heating mode with experimentally validated CFD simulations. Results show that the heat output of the radiant panel of the room air conditioner accounts for 20.7 %- 34.5 % of the total heat output. The supply airflow rate should be properly determined to balance the highly coupled heat output and air distribution. Compared with the conventional room air conditioner, the proposed room air conditioner under proper operation more effectively warms the occupied zone because of the enlarged heat output and improved air distribution, thereby increasing the predicted mean vote from −0.15 to −0.02, reducing the vertical air temperature difference from 6.7℃ to 0.8℃, and enhancing the thermal utilization effectiveness from −0.09 to 0.25. In addition, the draft rating and radiant temperature asymmetry of the proposed room air conditioner are thermally acceptable. Therefore, the proposed room air conditioner can modulate the indoor thermal environment energy-efficiently from the aspects of both overall and local thermal comfort.

Performance Improvement of The Radiant Cooling Panel Using Pressure-Vacuum Swing Adsorption with A Solid Desiccant Dehumidification

A growing environmental Impact of global warming and rapidly increasing cost of the electrical energy cause researchers in the field of refrigeration and air conditioning to develop the sustainable cooling technologies. The aim of this study was to investigate and design a radiant cooling panel with solid desiccant dehumidifier which was suitable for tropical climate to achieve thermal comfort. The influence of an interior ventilation fan to thermal comfort assessment was also examined. The designed system with CFD (Computation fluid dynamics) simulation used ANSYS software to compare with the measured data. The total area of the ceiling and wall radiant cooling panels were 16.83 m2 and 16.32 m2 with 26.28 L/min of supplied water by the cooling tower. The solid desiccant dehumidifier was the pressure-vacuum swing adsorption technology with containing 20 kg of silica-gel. The flow rate of supply air was approximately 90 kg/h. The results indicated that the PMV (Predicted mean voted) could be improving with the uses of cooling panel with dehumidifier during the day. The lower temperature and humidity ratio can improve thermal comfort. The patterns of temperature, the relative humidity and the PMV from CFD simulation were in the same agreement with the experimental results. In the case of the ventilation fan installed in the radiant cooling panel with dehumidifier, the average value of predicted mean vote was -0.2. Consequently, the PPD (Predicted percentage dissatisfied) was reduced to 6.2%. The daily electric power consumption was 5.94 kWh which was lower than the conventional air conditioning system about 16.27%.

Dynamic prediction of the pre-dehumidification of a radiant floor cooling and displacement ventilation system based on computational fluid dynamics and a back-propagation neural network: A case study of an office room

This study was carried out to solve the problem of condensation in radiant floor cooling systems. Computational fluid dynamics simulation and back-propagation neural network prediction were employed to conduct thorough research to predict the effects of the displacement ventilation dehumidification phase in an office building located in Jinan, China. The effects of the air supply temperature ( T as), air supply flow rate ( V as), air supply humidity ratio ( H as), floor temperature ( T floor), initial indoor temperature ( T ini) and relative humidity ( H ini) on the duration and energy consumption of pre-dehumidification were investigated. The big data show the air dew point temperature ( T ad) produced the most significant effect on the pre-dehumidification duration and energy consumption, while T as would cause the least significant effect. With the decrease of T ad, the pre-dehumidification duration and energy consumption were, respectively, decreased by 59.1% and 44.2%. Furthermore, with the variation of V as, the energy consumption exhibited a fluctuating trend. This study provides a novel and effective method to assess the pre-dehumidification control of radiant floor surfaces by considering different initial indoor conditions and air supply parameters.

Improvement of dynamic foam stability with low-frequency acoustic sound

Exposing the froth phase of a flotation system to low-frequency acoustic sound was reported to improve the process efficiency, which was thought to be due to the improvement in the froth stability. However, no direct evidence of the stabilization effect of sound on froths or foams is available to date. The present work aims to resolve this issue by experimentally studying the effect of low-frequency acoustic sound on the stability of a foam, the two-phase counterpart of flotation froth, across a wide range of experimental conditions. Three different setups consisting of a flotation column or a bottom-driven mechanical flotation cell, with the foam being exposed to sound from above or below, were tested. The equilibrium height of the foam with a given flow rate of air supply was measured at steady state using the Bikerman’s method. The equilibrium foam height was found to increase within certain ranges of sound frequencies and amplitudes, and at various surfactant concentrations (3–50 ppm) and superficial gas velocities (0.3–2.0 cm·s−1). An increase in sound amplitude or superficial gas velocities was also found to broaden the range of sound frequency that led to the stabilization effect. These findings were largely common for the three different setups tested, suggesting that this is a versatile method for improving the stability of foams. A strong correlation was also found between the stabilization of foam demonstrated in the present work and the stabilization of froth reported in a previous study, implying that the stabilization mechanisms could be similar.

Contaminant Crossover in Residential Energy Recovery Ventilators: Mass Spectrometric Analysis and Introducing Remediation Measures

Energy recovery ventilators (ERV) are increasingly present in residential environments to enable energy-efficient provision of controlled outdoor air ventilation. In this work, we investigated pollutant transport through a typical residential ERV as a potential pathway for re-entrainment of indoor air pollutants into the outdoor ventilation air supplied to an indoor space. Specifically, we investigated the transfer of volatile organic compounds (VOCs) through the sandwiched membrane matrix of the ERV core, between two adjacent air streams. Pollutant transfer efficiency is calculated for experiments intentionally injecting two common indoor VOCs (acetone, isopropanol (IPA)) and the behavior of transfer is studied for different ERV exhaust and supply flowrates (supply, exhaust, balanced scenarios). Maximum pollutant transfer efficiency of 17% is recorded for isopropanol at balanced (equal supply and exhaust airflow rates) conditions at intake and exhaust air lines. Maximum pollutant transfer efficiency of 26% and a minimum of 5.3% for unbalanced CFM settings are obtained. For VOCs studies, we observed short response times of &lt;10s from starting injection of VOCs into indoor exhaust air stream until the concentration at the indoor supply air stream reaches to steady state. This results concluded that, ERVs typically have a lower response time to pollutant re-entrainment.

Numerical investigations on the beating behavior of self-excited combustion instability in a hydrogen-fueled Rijke type combustor

Beating, a specific oscillation mode of combustion instability, occurred in a premixed hydrogen fueled Rijke tube was investigated. A two-dimensional transient numerical model for the self-excited combustion instability was developed and well validated. In a beating cycle, the amplitude of pressure oscillations increased to 1400 Pa and then decreased to 70 Pa, exhibiting an obvious feature of low frequency modulation. The joint time-frequency analysis showed that the dominant frequency shifted from 230 Hz to 330 Hz over time at each cycle. To shed lights on the inherent mechanisms of beating instability, the coupling processes between the combustion and flow dynamics were investigated. At each beating cycle, the shapes of flame were relatively stable in growing stage, while strongly fluctuated in transition stage and recovered to stable state in decaying stage. Notably, due to the large amplitude of pressure oscillations, an obvious recirculation zone was formed around the flame and sucked the high temperature exhausted gas to the flame root during the transition stage. Accordingly, the high temperature area was formed around the flame and affected the oscillation mode. By employing the proper orthogonal decomposition analysis, the apparent oscillations at 1 Hz were observed in the modes of heat release rate, velocity vectors and temperature. Later, it was found that with the increase of the amplitude of pressure oscillations, the mass flow rate of air supply at tube inlet dramatically decreased, which reduced the oxygen around the flame and changed the fluctuation mode of heat release rate. The coupling process was also identified by the phase difference and Rayleigh index.

Contaminant removal and contaminant dispersion of air distribution for overall and local airborne infection risk controls

Proper air distribution is crucial for airborne infection risk control of infectious respiratory diseases like COVID-19. Existing studies evaluate and compare the performances of different air distributions for airborne infection risk control, but the mechanisms of air distribution for airborne infection risk control remain unclear. This study investigates the mechanisms of air distribution for both overall and local airborne infection risk controls. The experimentally validated CFD models simulate the contaminant concentration fields in a hospital ward based on which the airborne infection risks of COVID-19 are evaluated with the dilution-based expansion of the Wells-Riley model. Different air distributions, i.e., stratum ventilation, displacement ventilation, and mixing ventilation, with various supply airflow rates are tested. The results show that the variations of the overall and local airborne infection risks under different air distributions and different supply airflow rates are complicated and non-linear. The contaminant removal and the contaminant dispersion are proposed as the mechanisms for the overall and local airborne infection risk controls, respectively, regardless of airflow distributions and supply airflow rates. A large contaminant removal ability benefits the overall airborne infection risk control, with the coefficient of determination of 0.96 between the contaminant removal index and the reciprocal of the overall airborne infection risk. A large contaminant dispersion ability benefits the local airborne infection risk control, with the coefficient of determination of 0.99 between the contaminant dispersion index and the local airborne infection risk.