Search

• Coupled thermal and humidity environment is studied for impinging jet ventilation (IJV). • Correlation coefficient between thermal and humidity distributions in IJV is approximately 0.4. • A specific model for predicting thermal and humidity distributions of IJV is proposed. • Key variables in the model that affected by indoor airflow pattern are determined. • Results predicted by the proposed model agree well with the numerical results. Variations of indoor humidity have significant influence on thermal comfort, calculation accuracy of air-conditioning load and selection of design parameters. As one of the most promising advanced ventilation strategies, impinging jet ventilation (IJV) has received much attention. However, few study concerned its coupled indoor thermal and humidity environment and there is also no complete mathematical model for the IJV to synchronously predict its indoor temperature and humidity distributions. Therefore, this study first simulated the thermal and humidity environment in IJV rooms with considering the coupled thermal and moisture transfer effect. The results demonstrate a significant correlation between temperature and humidity distributions in IJV, with a correlation coefficient of approximately 0.4. Then, a simplified theoretical model for predicting temperature and humidity distributions in the IJV was established based on the theory of nodal model, and the key parameters within the model that are influenced by indoor airflow pattern were also identified. Next, the model results were compared with the simulation ones and showed that the proposed model is with a mean absolute error of 0.4 °C for the prediction of temperature distribution and of 0.21 g/kg for the prediction of humidity distribution. These discrepancies are sufficiently minor to validate the accuracy of the proposed model. Last, the potential application value of the proposed model was discussed. Overall, the current study contributes to extend the application of the existing coupled thermal and humidity models to IJV scenarios, and provides a robust theoretical foundation for the more rational design and practical application of the IJV. [ABSTRACT FROM AUTHOR]

Excessive temperature stratification and draught are two prominent drawbacks for impinging jet ventilation (IJV). Using IJV together with ductless personalized ventilation (DPV) (i.e., IJV + DPV) has been proved to be an effective way to reduce temperature stratification, but the draught rate around occupant would be increased. In the current work, an in-depth study on the characteristics of indoor local thermal comfort for IJV + DPV operating with different scenarios is done. Predictive models for the local thermal comfort indicators are developed to optimize the operating parameters (including supply velocity of IJV (V s) and intake airflow rate of DPV (D f)) with different room cooling loads (Q). The results show that the application of IJV can be extended to spaces with high cooling loads (e.g., larger than 60 W/m2) by using together with DPV, but the risk of thermal discomfort increases. To avoid local thermal discomfort, the combination relationship between V s and D f should be properly matched and this relation becomes more stricter with the increase of Q. It also obtains that the maximum cooling load that IJV + DPV can eliminate is less than 170 W/m2. At last, to ensure the IJV + DPV create a good local thermal comfort environment, design chart about the feasible ranges of V s and D f are established for different values of Q. On the whole, the current study not only can provide a basis for the application of IJV + DPV in spaces with high cooling load, but also can provide theoretical guidance for the operation control and optimization design of the IJV + DPV. • Impinging jet ventilation (IJV) coupled with ductless personalized ventilation (DPV) can be used in high cooling load space. • There exists a risk of thermal discomfort for IJV combined with DPV (i.e., IJV + DPV). • Predictive models for thermal comfort indicators are developed for IJV + DPV. • Feasibility of IJV + DPV operating with different cooling loads is discussed. • Design chart for IJV + DPV is presented for achieving good thermal comfort. [ABSTRACT FROM AUTHOR]

Solar radiation plays a crucial role in the formation of sea-land breeze (SLB) circulation in coastal areas, and the SLB circulation in turn influences the atmospheric diffusion of pollutants in coastal cities. Therefore, the characteristics of the SLB and its impacts on the pollutant distributions in Shanghai, China, are numerically investigated for three (low, medium, and high) daily solar radiation intensities (DSRIs). The results show that the pollutant dispersion is affected by both the horizontal flow of SLB circulation and the vertical thermal plume (TP). The SLB only transports pollutants within the near-ground layer, while TP removes pollutants from the boundary layer and then improves near-ground air quality. Both the intensities of the SLB and TP increase with the increase of DSRI, however, the relative strength of the two are different at different DSRIs. When the DSRI is high (28.35 MJ/m2/day) or low (9.45 MJ/m2/day) during the daytime, the sea breeze (SB) is much stronger than the TP, so the prevailing flow direction on the land is horizontal, causing large amounts of pollutants to be blown inland. For the medium DSRI (18.90 MJ/m2/day), the TP is more powerful in the early stage of the SB period, so the airflow across the land is dominated by the rising TP. Pollutants are mainly transported to the upper atmosphere and the best air quality can be achieved in the residential region. At night, the worst air quality occurs at low DSRI, because when the DSRI is low, the speed of the land breeze is also low and this decelerates the diffusion of pollutants to the sea by the wind. [Display omitted] • Effect of daily solar radiation intensity (DSRI) on the sea-land breeze (SLB) is studied. • Pollutant dispersion patterns for different SLB intensities are compared. • The height of the low-speed region (i.e., pollutant accumulation) depends on DSRI. • Thermal plume facilitates the pollutant vertical transport at medium DSRI. • Pollutants accumulated on the sea surface are re-blown to land only in case of low DSRI. [ABSTRACT FROM AUTHOR]

An appropriate thermal stratification environment can be formed in impinging jet ventilation (IJV) cooling rooms. In this scenario, the well-mixing hypothesis provided by the conventional mixing ventilation is not applicable to calculate the cooling load of IJV systems. In the present study, a simple method using the cooling load ratio (CLR, α) is proposed for calculating the cooling load in the thermal stratification environment of IJV. Numerical simulations are applied to investigate the effect of several influencing factors on the CLR. Univariate analysis results indicate that the key influencing factors of α are the thermal length scale (L m), supply airflow rate (L) and total heat gain (q *). A prediction model of α (CLR model) is developed related to the three influencing factors of L m , L and q *. The CLR model has a high prediction accuracy in low q * situations (q * ≤ 2.5), with a slightly reduced accuracy in high q * situations. Overall, the maximum relative error of the CLR model is lower than 12%. Moreover, contour plots of α illustrate that α achieves a minimum level of 0.55, and the values of α vary between 0.55 and 1.0 for all the values of L m , L and q *. The CLR model can provide a theoretical basis for the determination of cooling load and energy saving potential of IJV systems during the design stage. [Display omitted] • A simple method for calculating the cooling load is proposed for IJV system. • The simple calculation method is applied using the cooling load ratio (α). • Three influencing factors are determined to develop the prediction model of α. • The values of α range from 0.55 to 1.0 according to the obtained prediction model. [ABSTRACT FROM AUTHOR]

Urban neighborhoods, which contains numerous buildings and gaps between the buildings, are usually simplified as a single air-impermeable block in numerical simulations of urban air environments. Such a simplification reduces the computational costs but underestimates the pollutant removal capacity of the street, because the high-speed wind flow through the building gaps causes a flushing effect on pollutant distribution in the street. However, the influences of the gap flow on traffic pollutant dispersion remain unclear. Therefore, a four-way street network surrounded by four neighborhoods is assumed as a representative urban area, and the characteristics of the gap flow and its correlation with building height (H/W = 0.5–2, W = 30 m) are investigated numerically. The results show that the flow pattern within the street depends on the relative intensity of the flow across the gaps and the parallel bulk flow along the street. For lower (H/W = 0.5) or higher (H/W = 2) building heights, the gap flow is strong enough to break through the barrier of the bulk flow, and then enters the street to dilute the pollutants, causing the concentrations in the leeward region of the street to be lower than those in the windward region. Opposite results are found for medium building heights (H/W = 0.75–1.5), where the flow in the street is dominated by the parallel flow itself. The results also indicate that there exists a critical height, H/W = 1.5, below which the air environment is good in both the streets and neighborhoods. [Display omitted] • Effect of building height (H/W) on the flow across gaps between buildings is studied. • Blocks are set as porous media except first building row by the street in simulation. • Gap flow (GF) affects flow in the street and this depends on flow strength in street. • GF affects pollutants distribution and dispersion in blocks when H/W = 0.5 and 2. • Good air environment can be achieved in both street and neighborhood when H/W < 1.5. [ABSTRACT FROM AUTHOR]

Factors influencing the warm jet spreading distance of an impinging jet ventilation (IJV) system, which is operated in heating mode, are studied and multiple regression analysis is used to develop multivariate regression models for the estimation of the spreading distance. The factors considered are jet discharge height, nozzle area (i.e., nozzle configuration), supply airflow rate and temperature difference between the supply and indoor air (i.e., supply condition). CFD techniques are employed to conduct a set of required numerical experiments, and each simulation condition is determined by the Box-Behnken design (BBD) method. The developed equation suggests that the warm air spreading distance in IJV has a maxima and will not exceed 12 m for any nozzle configuration and supply condition. Moreover, the supply airflow rate should be greater than a threshold and the height and area of the nozzle should also be within particular ranges in order to meet the special requirements of the spreading distance. The larger the required spreading distance there is, the more restrictions there are on the studied variables. With the regression models established for the warm air spreading distance, the optimized combination of these variables for satisfying the design requirement is determined. [ABSTRACT FROM AUTHOR]

The matching relationship of operating parameters between the air supply and radiant systems has significant impact on the indoor thermal comfort and energy efficiency for combined air-conditioning system. The impinging jet ventilation (IJV) and chilled ceiling (CC) combined system (IJV/CC) overcomes the drawback of IJV which cannot be used in spaces with high cooling load. However, the matching relationship between the operating parameters of the IJV and CC are not clear due to limited research on IJV/CC at present. To ensure good thermal comfort and indoor air quality of the IJV/CC in an energy efficient way, this study mainly focuses on exploring the optimal combination of the operating parameters between the IJV and CC (including the supply temperature and supply velocity of IJV and internal surface temperature of CC (T c)). First, the indoor thermal environment for IJV/CC operating with different combinations of design parameters is simulated by CFD. Second, response surface methodology (RSM) is utilized to establish predictive models for various ventilation performance indicators by collecting the simulation results. Then, the optimal combinations of operating parameters between the IJV and CC are determined by using the technique of order preference by similarity to ideal solution (TOPSIS) coupled with the developed RSM models. Finally, control strategies for the IJV and CC are discussed and it suggests setting the value of T c in the range of 20–21 °C and a variable air volume system is preferred for the control of the air supply system to achieve efficient operation of IJV/CC. • Distributions of thermal environments for IJV/CC with high cooling loads are investigated. • Predictive models for various operation performance indicators of IJV/CC are established. • Matching relation between the IJV and CC are presented for efficient operation of IJV/CC. • The internal surface temperature of CC is suggested to set in the range of 20–21 °C. • A variable air volume system is preferred for the control of IJV. [ABSTRACT FROM AUTHOR]

Street networks serve as the main open corridors for wind flow within urban areas, so understanding their ventilation performance and capacity of removing pollutants helps improve urban air quality. However, conventional modelling methods using an exact building model (EBM) are hard-pressed to simulate urban-scale areas, due to their extremely high computing load. To reduce time and cost of numerical simulations, and accurately predict the flow field and ventilation within the street network, a simplified modelling approach (i.e., EBM&PMM) is proposed, in which only the first row of buildings flanking the street is modelled by EBM and the rest of the urban buildings are modelled by a porous media model (PMM). The performance of EBM&PMM is evaluated by three common modelling approaches, i.e., EBM, PMM, and sealed concrete model (SCM) in which the building complexes with gaps are treated as an enclosed entity. A typical crossing street network flanked by four communities (building arrays) is used for this study. The age of air (τ P) and ventilation flux (Q) obtained from the four modelling methods are compared. The results show that EBM&PMM predicts τ P and Q well, and the flow pattern by this model agrees well with EBM. PMM significantly underestimates τ P but exaggerates Q in street networks, and it fails to give the detailed flow patterns. SCM over-predicts τ P , and Q between the community and street is zero. Furthermore, the first row of buildings facing the street plays a decisive role in flow pattern and ventilation effectiveness of the street network. [Display omitted] • Airflow in streets is simulated by exact building model and porous media model (EBM&PMM). • EBM&PMM predicts the flow pattern and ventilation processes well in street networks. • PMM and sealed concrete models fail to simulate flows and pollution in the network. • First building rows beside a street play decisive roles in simulations by the EBM. [ABSTRACT FROM AUTHOR]

The effect of cross-ventilation in buildings on air quality within street canyons is evaluated by employing computational fluid dynamic (CFD) models. The impacts of the airflow entering the canyons through the upstream building windows on flow field and pollutant distribution were investigated in both regular and staggered canyons, and the influences of window-opening-percentage (WOP) for different street width to building height aspect ratios (AR) was studied. The numerical results show that the airflow through the windows of the upstream building can destroy the primary vortex and leads to a new flow pattern within the canyon. When the WOP is increased from 0 to 10%, the ability of the flow for pollutant dilution and dispersion can be improved. This causes the pollutant concentrations decreased by 23% to 27% in the street. And these reductions are greater for staggered arrangements than that for regular arrangements. The influences of the WOP on air flow inside the canyon decrease with increasing the AR. A limiting aspect ratio exists, beyond which the flow structure and the pollutant concentration in the canyon will not be changed no matter what the WOP is. [ABSTRACT FROM AUTHOR]

Conventional numerical simulations are hard-pressed to give the details of air flow in the whole urban area because of the limitation of computing resources, and this challenge can be effectively resolved when urban building complexes are modelled by the porous media model (PMM). However, the resistance coefficient (C f) needed to run PMM has not been determined so far. Therefore, an anisotropic prediction model of C f is developed for simulating wind flow through building complexes, and a wind direction index (a θ) is defined to characterize the anisotropy of C f. Significance tests are conducted to analyze the influence of building configuration, approaching wind velocity and a θ on C f. It is found that C f depends mainly on building area density (λ p) and a θ , so a model related to λ p and a θ for predicting C f is obtained by regression analysis. The feasibility of C f within PMM is validated by results from the fully resolved building model (FRBM). The results show that PMM predicts the macroscopic airflow through the building array generally well except for streamwise velocity near the ground and at mid-height of the array, especially the macroscopic variations of vertical velocity and pressure simulated by PMM agree well with FRBM. PMM can also predict the macroscopic turbulence energy well for most of the regions inside the building array. Moreover, the relative differences between PMM and FRBM are about 2.8% for blocking effect on airflow and 1.5% for the ventilation capacity within the array, which are measured by pressure loss and in-canopy velocity, respectively. [Display omitted] • Porous media model (PMM) is used to simulate block effects by buildings on wind flow. • The momentum source term of PMM is reasonably simplified in simulations. • Anisotropy of resistance coefficient (C f) is analyzed by using wind direction index. • An anisotropic prediction model of C f is developed for the buildings when using PMM. • PMM with C f can be employed to simulate macroscopic flow well in a building complex. [ABSTRACT FROM AUTHOR]

Impinging jet ventilation (IJV) is a new ventilation strategy for using in public and industrial buildings with large spaces. It combines advantages of both displacement ventilation (DV) and mixing ventilation (MV). Compared with DV and MV, however, there is a potential risk associated with the IJV system, i.e., particle resuspension from the floor due to the supply air, by which the high momentum jet is directed towards the floor. In the present study, particle concentrations under different supply airflow conditions were measured in an IJV room with indoor pollutant sources. The effect of supply airflow on the resuspension of the deposited particles and the critical supply velocities causing the resuspension for different supply heights and particle sizes were analyzed by using the measured data in a model IJV room. The results show that the critical supply velocity is increased with the increase of supply height and the decrease of particle diameter. Meanwhile, in practical applications, there is little risk of the significantly degrading IAQ induced by particle resuspension via supply airflow in an IJV room. Comparing the resuspension amount caused by the supply airflow of an IJV system with that caused by an occupant walking past, a sharp rise of supply velocity in a short time span in an IJV room will not result in significantly worse IAQ. [ABSTRACT FROM AUTHOR]

To explore the effect of traffic pollution on indoor air quality of naturally ventilated buildings in the vicinity of a street canyon, the wind flow and pollutant distributions in and around buildings with different window opening percentages (i.e. WOP, the percentage of the total window opening area to the total facade area) were investigated by three-dimensional numerical simulations. The numerical results show that the WOP changes the pressure distribution around the downstream building, which is due to the infiltration of air into the street canyon through the opening windows of both the upstream and downstream buildings. When the indoor air of the downstream building is supplied by the outdoor air from the street canyon, the ventilation flux will be increased with increasing WOP. If the indoor air is taken in from the leeward side of the downstream building, however, the trend of the ventilation flux is found reverse. The results also indicate that the effective source intensity, which is introduced to quantify the amount of traffic pollutant entering into buildings through unit ventilation area, decreases as the WOP increases. When the WOP reaches 10%, the averaged effective intensity is reduced by 30% compared to the reference case when all windows are closed. It means that if a naturally ventilated room in the downstream building has a fixed ventilated area over different seasons, the room will take in more pollutants from outdoors in winter than in other seasons. [ABSTRACT FROM AUTHOR]

Impinging jet ventilation (IJV) is believed to create good indoor thermal comfort and air quality (IAQ) in an energy efficient way. However, since there is little discussion about the relationship between its heating performance and airflow patterns, operation control of ventilation parameters (e.g., supply temperature, airflow rate, discharge height and nozzle shape) is not clear. To achieve a good heating performance for IJV, operation conditions are optimized by using data-driven model coupled with technique of order preference by similarity to ideal solution (TOPSIS). Draught discomfort (PD), air change efficiency (ACE) and energy utilization coefficient (EUC) are modeled as performance indicators. First, response surface methodology (RSM) and artificial neural network (ANN) are employed to establish predictive models for the performance indicators, a comparison between the two methods are then performed from prediction error analysis. It shows that both RSM and ANN can predict the ACE and EUC well, but compared to ANN, RSM is less reliable in estimating PD and may fall into the risk of misjudging comfort level. Then, the ANN is used together with the TOPSIS to optimize the IJV for heating and a square nozzle is better preferred among the studied nozzle shapes. Moreover, optimal conditions of supply parameters are determined and verified for different nozzle characteristics. Overall, the current results not only can provide reference for the design and operation control of ventilation parameters of IJV for heating, but also contribute to the practical applications of ANN in different ventilation scenarios. • RSM and ANN methods are used for modeling the thermal environment of IJV for heating. • Prediction accuracy between RSM and ANN for the same performance indicator is compared. • A multiple-criteria optimization study for IJV is conducted by ANN-based TOPSIS method. • Optimal conditions of ventilation parameters are obtained for the best performance. [ABSTRACT FROM AUTHOR]

Abstract: Full-scale experiments were carried out to explore the effects of the initial temperature difference between the interior and exterior and the vent characteristics on the transient development of natural ventilation driven by thermal buoyancy. Air temperature and tracer gas concentration in a test chamber were measured and the experimental results were compared with the theoretical predictions. It was found that the initial temperature difference has a large influence on the thermal stratification, the concentration distribution of tracer gas, the flow rate and the removal rate of tracer gas. The time taken to reach steady-state ventilation is shorter for a larger vent if the initial indoor temperature is greater than or equal to the outdoor temperature. For a pre-cooled room, the time taken for the ventilation to transform the airflow direction is shorter for a larger vent. Increasing the vent area would yield a greater flow rate and thus improve the efficiency of gas removal. Experimental results also show that the vent shape has little impact on the flow rate. [Copyright &y& Elsevier]

Abstract: The time series of indoor and outdoor CO concentrations in natural ventilating rooms were monitored continuously and simultaneously in summer, and the relationships between indoor and outdoor CO concentrations at two sampling locations in the central area of Shanghai were examined and discussed. Linear regression analysis was employed to correlate indoor concentration levels with outdoor concentrations, and the diurnal cycles of the indoor and outdoor CO concentrations show positive correlations at both sampling sites. The monitoring data also indicate that the indoor to outdoor ratios of CO concentrations in the test natural ventilating rooms are less than but close to 1. In addition, a predicting model based on a mass balance assumption of indoor pollutants was applied for estimating the indoor CO concentrations by using the outdoor test data as inputs, resulting in good agreement with the test curves for the two sampling sites. [Copyright &y& Elsevier]

Abstract: Indoor air quality and pollutant dispersion in natural ventilation rooms have received considerable attention in recent years. In order to evaluate the pollutant concentration levels in the occupant zone in a displacement natural ventilation (DNV) room, the mathematical model in previous studies are employed and extended to estimate pollutant transportation in a DNV room with the consideration of both the incoming air concentration and indoor pollutant source, resulting in exponential type expressions in time for the concentration in the lower zone and the mean concentration in DNV rooms. The results show that for reducing the concentration in the lower zone to a certain value, the concentration of inflow air and indoor emission rate should be less than their critical values for a given effective opening area of the window. The effect of thermal radiation of the inner surface on the reduction of pollutant concentration in the upper zone is greater than that in the lower zone of a DNV room. In addition, the effective opening area of the window primarily influences the final concentration level in the DNV room, and only affects the decay rate of indoor pollutant concentration when the pollutant concentration of incoming air or the strength of the indoor source approach their critical values. [Copyright &y& Elsevier]

Abstract: A computer model for predicting aerosol dispersion in indoor spaces was validated with experimental data found in the literature. The validated model was used to explore the effect of the area or point source locations on aerosol particle transportation in ventilation rooms with rough surfaces. Two different ventilation strategies were studied: mixing ventilation (MV) and underfloor air distribution (UFAD) system. The simulation results show that in MV, the particle concentration and removal effectiveness are little dependent on the position of the pollutant sources. In UFAD, the source location has a strong impact on the distribution and removing of the contaminants. The particle removal performance strongly depends on the ventilation efficiency and the particle deposition loss in the room with rough surfaces. The important consideration in both the relative ventilation efficiency and the deposition rate is the relative position of the sources to the main airflow and the occupied zone in an UFAD room. [Copyright &y& Elsevier]

Local interference flow induced by the movement of human bodies or objects indoors can directly affect breathing air quality. However, very few studies have been carried out on the flow induced by moving objects due to high experimental costs. As an alternative, computational fluid dynamic (CFD) method has been employed in predicting the flow induced by moving objects indoors. During the numerical computations, the mesh needs to be updated continuously for the moving objects behave as part of the boundary. Although the dynamic mesh update (DMU) methods have been used in the CFD method, the performances of these DMU methods vary widely, and it is still lack of analysis on the selection of the DMU methods. In this study, the DMU methods provided by ANSYS FLUENT are used for comparing the numerical performances between computational and experimental results. Five aspects of numerical performance, accuracy, mesh quality, mesh generation complexity, continuity, and computational efficiency, are discussed integrally. The results show the dynamic layering (DL) method is preferred for the case of linear movement due to the high computational efficiency and accuracy though it is difficult and time-consuming in mesh generation. Spring-based smoothing & remeshing (SS&R) method can be used when the complicated-shaped or nonlinearly moving objects are part of the boundary. Both computational efficiency and accuracy of overset mesh (OM) method are low when the moving objects are clinging to the boundary, so this method is not recommended for simulating the flow fields. [Display omitted] • Numerical analysis is conducted on local flow induced by moving objects in a room. • Computational performances of three dynamic mesh update methods are analyzed. • Dynamic layering method performs best among the three methods for linear movement. • Low mesh quality is found in spring-based smoothing & remeshing method. • Overset mesh method is not suitable for moving object clinging to the boundary. [ABSTRACT FROM AUTHOR]

Convective transport of particles in a narrow horizontal channel with porous walls is analyzed by considering the gravitational effect. Particle concentration, penetration and deposition velocity in three different channels are obtained by using the theoretical solutions. The results show that the differences between the concentrations in the three channels become evident with the increase of the permeability coefficient and slip factor of porous materials, especially for fine particles. The wall structure of the channel has significant influences on particle deposition velocity. When both the upper and lower walls are porous, the deposition in the channel is weakened. In addition, particle deposition velocities on the porous walls are consistent with each other, and the velocities are higher than those on the solid walls. [Display omitted] • Particle transport in porous channels is studied considering diffusion and gravity. • Permeability and slip effect of porous media strongly affect particle deposition. • Deposition in the channel is weakened when both upper and lower walls are porous. • Deposition velocity on the porous wall keeps consistent regardless of channel type. • Effects of permeability on deposition are the same when only one wall is porous. [ABSTRACT FROM AUTHOR]

Inappropriate thermal stratification characteristics can degrade the ventilation performance of impinging jet ventilation (IJV). In the present study, an analysis method using dimensionless parameters is proposed for predicting the thermal stratification, and the Box-Behnken design method is employed to determine the simulation cases of influencing factors for thermal stratification. The thermal performance in IJV cooling room is investigated by computational fluid dynamics (ANSYS Fluent), and the effects of supply air parameters, inlet area (S *) and height (h *) on thermal stratification are discussed, respectively. A further supply air parameter is represented by the thermal length scale (L m), which is applied to evaluate the combined effect of inertial force and thermal buoyancy on supply airflow. The results show that the driving force on supply airflow changes from buoyancy dominated to inertial dominated with increasing L m , leading to the thermal stratification profiles changing from thermal stratification to uniform distribution. For small L m , the thermal stratification profiles are similar for different inlet areas, while the dimensionless air temperature decreases with increasing inlet area. However, the larger inlet area promotes a more uniform temperature distribution for large L m. In addition, the thermal stratification profiles are similar regardless of the inlet height for both small and large L m conditions, indicating that the dimensionless air temperature is a weak function of inlet height. Finally, multiple regression models related to L m , S * and h * are developed for predicting the thermal stratification, which can provide a reference basis for the determination of temperature distribution in IJV cooling rooms. ● Thermal stratification (TS) of impinging jet ventilation (IJV) is studied. ● L m is used to evaluate the combined effect of inertial and buoyancy forces on TS. ● Prediction models of TS are developed for IJV cooling rooms (θ ank , Δ θ oz and Δ θ fc). ● Effect of L m on Δ θ fc is less than that on θ ank and Δ θ oz. [ABSTRACT FROM AUTHOR]

Thermal stratification is the key to improving energy efficiency and indoor air quality for impinging jet ventilation (IJV) system in cooling mode. However, unclear thermal stratification characteristics of IJV system can degrade the ventilation performance of the IJV. In the present study, the thermal stratification characteristics of an IJV is investigated by the validated simulation model for different supply air parameters and internal heat sources in interior zone of office buildings. The critical value of supply air parameters for the formation of thermal stratification is determined by thermal length scale, which considers the combined effect of inertial force and thermal buoyancy on the supply airflow. In addition, the cooling load in an IJV cooling room caused by the heat source intensity can be calculated directly and the heat source distribution is negligible. The results also indicate that the thermal stratification height (H) only depends on supply air parameters, while the temperature difference between head and ankle level (Δ T) is affected by both supply air parameters and cooling load. The predictive models of H and Δ T are obtained based on the simulation results, which could provide the theoretical basis for optimizing the design scheme of IJV system in interior zone of office buildings. • Thermal stratification (TS) of impinging jet ventilation for cooling is studied. • Critical value of supply air parameter (L m) is determined for the formation of TS. • Thermal stratification height H is mainly affected by L m but not cooling load (q). • Temperature difference between head and ankle (Δ T) is affected by both L m and q. • Predictive models of H and Δ T are obtained for design of impinging jet ventilation. [ABSTRACT FROM AUTHOR]

Impinging jet ventilation (IJV) system is believed to combines the advantages of good indoor air quality (IAQ) and high energy efficiency. However, there is few study about the optimization of multiple performance characteristics and the significance of design variables on the ventilation performance is not clear for IJV. This study addresses an approach based on Taguchi method with grey relational analysis for optimizing an IJV system with multiple performance characteristics (including draught discomfort (PD), temperature difference between the head and ankle level (Δ T), air change efficiency (ACE) and energy utilization coefficient (EUC)). The supply inlet height, supply velocity, supply temperature and return outlet height are selected as factors that affecting the ventilation performance of IJV and L 9 (43) orthogonal array is employed to design the experiments. First, the optimal combinations for PD, Δ T , ACE and EUC are determined separately by Taguchi method coupled with numerical simulations and the significance of the studied factors in promoting each evaluation index is revealed and ranked through main effect analysis and ANOVA. Second, the optimal combination of design variables corresponding to the best overall performance for IJV is determined by converting multi-objective into a single-objective according to Taguchi-based grey relational analysis. Then the important factors influencing the overall performance is ranked and the heights of supply inlet and return device are found to be significant factors in improving the overall performance. Finally, a confirmatory test is conducted to verify the improvement of performance characteristics and satisfactory results are obtained. • A multi-objective optimization study for impinging jet ventilation (IJV) is conducted. • Thermal comfort, indoor air quality and energy efficiency are considered together. • Significance of factors influencing the overall performance of IJV is revealed. • An optimal combination of design variables is obtained for the best performance. [ABSTRACT FROM AUTHOR]

A validated analytical model is proposed to quickly calculate the transient indoor air temperature and wall temperature by using Laplace transformation and the method of constant variation. The analytical model is further combined with predicted mean vote (PMV) model to evaluate the indoor thermal environment. The quantitative influence of occupants' ventilation behavior (OVB) during the off-periods on indoor thermal comfort is studied for an intermittent heating (IH) room in the whole heating season. The results indicate that the OVB during the off-periods of IH system can significantly reduce the indoor thermal comfort level during the heating periods. The predictive model of minimum PMV during the heating periods is thus determined to provide a theoretical basis for balancing the OVB during the non-heating periods and indoor thermal comfort during the heating periods. In addition, considering the occupants who have the need for both ventilation and indoor thermal comfort, preheating may be a solution while the heating energy consumption will be increased. The increasing rate of heating energy consumption is approximately 32.5%–74.9%. To reduce the extra heating energy consumption caused by the preheating, occupants need to limit the OVB during the non-heating periods of IH system. • Indoor thermal environment (ITE) is studied in intermittent heating buildings. • ITE is studied for occupant ventilation behavior during non-heating period (OVBNH). • OVBNH can significantly affect indoor thermal comfort during heating period (ITCH). • Preheating may be an effective solution to meet the need for both OVBNH and ITCH. • The extra heating energy consumption caused by preheating is around 32.5%–74.9%. [ABSTRACT FROM AUTHOR]

For impinging jet ventilation (IJV) system, the energy efficiency is believed to be further improved by splitting the return vent from the exhaust device. However, how to properly set the return devices of IJV is unclear when it comes to a comprehensive consideration of the thermal comfort, indoor air quality (IAQ) and energy efficiency. In the present study, numerical simulations are performed to determine the thermal comfort indices (draught comfort (PD) and temperature gradient between the head and ankle levels (Δ T)), IAQ indices (air change efficiency (ACE) and contaminant removal efficiency (ε)) and energy efficiency index (η) for an IJV system with return outlet height (H) ranged from the ceiling to the floor level. The technique for order preferences by similarity to an ideal solution (TOPSIS) method is employed to determine the optimal return vent height by considering these performance evaluation indices simultaneously. The results indicate that when the H increases, the values of Δ T and η decrease, while the values of PD, ACE and ε increase. An optimal range of H (1.2–1.5 m above the floor) is recommended for the best overall performance of IJV. Moreover, a comprehensive graph of H is displayed in this paper, which could provide directions for optimizing the positions of exhaust and return devices. Image 1 • Impinging jet ventilation (IJV) with separate exhaust and return devices is studied. • Thermal comfort, indoor air quality and energy efficiency are considered together. • Short-circuit flow phenomenon of supply cold air might be occurred in the IJV. • An optimal return vent height (H) range for IJV is achieved by using TOPSIS method. [ABSTRACT FROM AUTHOR]

Numerical simulations are performed to predict the spatial distributions of indoor gaseous contaminant and aerosol particles (represented by the CO 2 produced by human respiration and the resuspended 2.5 μm particles caused by occupants' walking, respectively) concentration in a large-height space equipped with impinging jet ventilation (IJV) and mixing ventilation (MV) systems in heating modes. The local age of air, contaminant concentration of the breathing zone and ventilation efficiency for the removal of contaminants are compared between the IJV and MV rooms for both the constant volume (CAV) and variable volume (VAV) systems with different outdoor temperatures. The numerical results indicate that the mean age of air in the breathing zone of IJV is about 37–47% less than that of MV due to the different airflow patterns. Both the CO 2 and 2.5 μm particles distribute uniformly in the IJV room for the CAV and VAV systems, but the CO 2 is mainly distributed in the middle region of the room height and the 2.5 μm particles concentrate at the floor level away from the exterior wall in the studied MV room, which results in the contaminant concentrations of the breathing zone in most studied cases of IJV being lower than those in MV. The results also show that the ventilation efficiency in the IJV is higher than that in the MV with the same supply and exhaust configurations of this study, suggesting that the former is more advantageous in providing good indoor air quality (IAQ) than the latter. Image 1 • Indoor air quality in the impinging jet ventilation (IJV) heating room is studied. • IJV is compared with the mixing ventilation (MV) system under the same conditions. • Dispersions of both gaseous contaminant and particle are simulated for IJV and MV. • Mean age of air in the breathing zone in IJV is 37–47% less than that in MV. • Ventilation efficiency for the removal of contaminants is higher in IJV than in MV. [ABSTRACT FROM AUTHOR]