Air Curtain Optimization

The term “impinging jet” refers to a high-velocity fluid stream that is ejected from a nozzle, a narrow opening or an orifice, and which impinges on a surface. As applied to the built environment, impinging jets are used in air curtains to separate two environments subjected to different environmental conditions with the purpose of improving thermal comfort, air quality, energy efficiency and fire protection in buildings. The design and application of state-of-the-art air curtains requires detailed knowledge of the relationship between the separation efficiency of air curtains—their main performance criterion—and a wide range of jet and environmental parameters involving air curtain design. In order to address the current knowledge gaps in the field, this project encompasses an investigation into the impact of different jet and environmental parameters on the performance of air curtains while giving special attention to the study of innovative jet excitation techniques by means of optimizing the separation efficiency of air curtains.  This project is being carried out in close collaboration with the air curtain manufacturer ‘Biddle B.V.’. &nbsp

Air Curtain Optimization

The term “impinging jet” refers to a high-velocity fluid stream that is ejected from a nozzle, a narrow opening or an orifice, and which impinges on a surface. As applied to the built environment, impinging jets are used in air curtains to separate two environments subjected to different environmental conditions with the purpose of improving thermal comfort, air quality, energy efficiency and fire protection in buildings. The design and application of state-of-the-art air curtains requires detailed knowledge of the relationship between the separation efficiency of air curtains—their main performance criterion—and a wide range of jet and environmental parameters involving air curtain design. In order to address the current knowledge gaps in the field, this project encompasses an investigation into the impact of different jet and environmental parameters on the performance of air curtains while giving special attention to the study of innovative jet excitation techniques by means of optimizing the separation efficiency of air curtains.  This project is being carried out in close collaboration with the air curtain manufacturer ‘Biddle B.V.’. &nbsp

CFD simulation of air distributions in a small multi-layer vertical farm:Impact of computational and physical parameters

Computational fluid dynamics (CFD) simulations have been extensively used in designing air distribution systems for controlled environment agriculture (CEA). In recent years, more application studies using CFD simulations can be found for vertical farms due to the increasing interest in indoor vertical farming systems. However, it is well-known that CFD simulations are sensitive to many computational parameters and settings. The requirement of a crop response model in the CFD simulation for a vertical farm makes it even more complicated. Despite increased interest, guidelines for CFD simulations in vertical farms are scarce based on a literature study. Therefore, a systematic sensitivity analysis is conducted for a small generic multi-layer vertical farm with sole source lighting, which was the object of study in the literature before. The impact of a wide range of computational and physical parameters is investigated, including grid resolution, turbulence model, turbulence intensity, discretisation scheme, drag coefficient of the crops and computational time. The analysis shows that in this case (inlet Re = 46,923, Ar = 0.078, cultivated with lettuce), the RNG k-ε turbulence model outperforms other commonly used two-equation turbulence models. Compared to the experimental results from the literature, the simulation results from the first-order upwind scheme show large discrepancies, especially on the coarse grid. Although the influence of drag coefficient on the airflow inside the crop canopy is pronounced, little difference is observed in the air distributions in the vertical farm away from the crops.</p

Experimental and CFD analysis of particle distribution in a controlled test facility:Impact of exhalation flow velocity and direction

The dispersion of exhaled particles in indoor environments is significantly influenced by the airflow conditions. Constant human respiration conditions are widely employed in experiments and CFD simulations for indoor particle transmission studies. However, it remains challenging to reach a definitive consensus on how the exhalation flow velocity/rate and direction interact with the indoor airflow pattern and influence the indoor particle distributions. The goal of this study is to systematically investigate the impact of constant source conditions, specifically the exhalation flow velocity (EFV) and direction (EFD) at the source, on the particle distribution in a controlled test facility by experiments and CFD simulations. First, the velocity, temperature and concentration results obtained from CFD simulations, employing the RNG k-ε turbulence model, are validated with experimental results. Next, the validated computational setup is used for CFD simulations of the same test facility, incorporating two mixing ventilation configurations. Under each ventilation configuration, three different EFV (i.e. 2.33, 4.65, and 9.30 m/s), each with three EFD (i.e. positive y, negative y, and positive x), and a reference case with no EFV/EFD at the source are considered. The effect of the variation of EFV and EFD on the surface-averaged concentration (Cavg) on the breathing height plane (z = 1.2 m) generally remains below 5.8 %, compared to the reference cases with no EFV/EFD. However, when considering the Cavg over a central area near the source, the Cavg deviation can reach up to 29.9 %. This study enhances the understanding of indoor particle transmission and its dependency on the EFV and EFD.</p

Wind-tunnel experiments on cross-ventilative cooling in a generic isolated building with one heated wall:Impact of opening size

This paper presents wind-tunnel experiments of cross-ventilative cooling in a generic isolated building with an interior heated side wall. Two different sizes of openings are considered: large and small openings. Particle image velocimetry (PIV) is used to determine velocities in the vertical centerplane. Air temperatures in the vertical centerplane are measured using negative temperature coefficient (NTC) sensors. Surface temperatures on the heated wall are measured using an infrared camera. Surface heat fluxes are obtained using heat flux sensors. In both cases the indoor airflow is dominated by the jet through the openings, with higher velocities in the building with large openings. The air temperatures measured with small openings are up to 7.5 % larger than those with large openings. The surface heat fluxes are up to 20 % higher in the building with large openings. The interior convective heat transfer coefficients vary considerably across the heated wall for both opening sizes and can be very different (up to 5 times higher) from those obtained by existing internal convective heat transfer coefficient correlations. The measurement results give insight into the complexity of ventilative cooling and can be used to validate computational fluid dynamics (CFD) simulations of cross-ventilative cooling.</p

Experimental and Numerical Analysis of Mixing Ventilation at Laminar, Transitional and Turbulent Slot Reynolds Numbers (Experimentele en numerieke analyse van mengventilatie bij laminaire, transitionele en turbulente slot-Reynoldsgetallen)

The proper ventilation of buildings and other enclosures such as airplanes, trains, ships and cars is of primary interest in engineering with respect to human (thermal) comfort, energy efficiency and sustainability.One of the most commonly applied ventilation methods is mixing ventilation, which is based on the injection of an air jet in the upper part of the room. The momentum of the jet should ensure mixing of the fresh supply air with the room air, and the diluted air should subsequently be extracted from the room. Although a lot of research has been conducted on mixing ventilation in the past decades, there are still several issues that are not resolved. The dissertation consists of two parts, both of which address current issues in mixing ventilation studies: (I) experimental and numerical work on isothermal transitional mixing ventilation in anidealized simplified reduced-scale model; (II) experimental and numerical work on mixing ventilation in a full-scale complex enclosure in an urban environment, driven by both wind and buoyancy. Both parts consist ofa combination of unique measurements, either full-scale or reduced-scale, and state-of-the-art Computational Fluid Dynamics (CFD) simulations. Part I A wide range of experimental andnumerical studies have been conducted in the past to analyze the flow patterns associated with ventilation in general and with forced mixing ventilation in particular. However, the vast majority of these ventilationstudies focused on fully turbulent flows (high Reynolds numbers). Low Reynolds (Re) numbers can indicate the presence of a transitional flow regime inside the room, which can be distinguished from turbulent flow by the presence of relatively large coherent structures (vortices). Severalpublications have indicated the fact that transitional flow can be present in different types of room airflow, either in the supply jet region or in other regions of low velocities (e.g. corners of the room, vicinity of buoyant plumes). However, to the best knowledge of the author, onlya limited number of studies has dealt with room airflow at transitionalslot Reynolds numbers so far, either experimentally or numerically. In addition, there is no consensus on the capabilities of CFD to predict transitional room airflow. To be able to come to a conclusion regarding the capability of steady Reynolds-averaged Navier-Stokes (RANS) CFD simulations to predict transitional room airflow, high-quality experimental data sets should be available. The lack of such a data set, and subsequently the lack of consensus on the capabilities of CFD to predict transitional room airflow are the primary reasons for the work performed on this topic for a mixing ventilation case, and which is presented in Chapters 2-5. In Chapter 2, a reduced-scale experimental setup to study ventilation flow at low Reynolds numbers (transitional flow) is presented. The reduced-scale model is used to perform Particle Image Velocimetry (PIV) measurements of mixing ventilation flow at transitional slot Reynolds numbers for a free plane jet. The inlet height for the studied configuration is h/L = 0.0667, with L the characteristic dimension of the cubic test section (L = 0.3 m). Flow visualizations show that the roomairflow is transitional for the range of studied slot Reynolds numbers (800 A second set of PIV measurementsof forced mixing ventilation flow is presented in Chapter 3. The experimental setup is to a large extent similar to the one presented in Chapter 2. However, the experiments presented in Chapter 3 are conducted for an inlet height h/L = 0.1, which corresponds to a plane wall jet issued from a smooth contraction. The PIV measurements focus on both the instantaneous and the time-averaged velocity and vorticity fields, as well as on the turbulence intensity. The vorticity profiles indicate a solid-bodyrotation in the large recirculation cell. The instantaneous vector fields show Kelvin-Helmholtz-type instabilities as a result of the large velocity gradient in the shear layer of the wall jet. The Strouhal number based on the vortex formation frequency is shown to increase with increasing Reynolds number. Application of the Okubo-Weiss function indicates the presence of vortical structures in the wall jet region and the presence of a vortex train in the outer region of the wall jet. Chapter 4 presents steady RANS CFD simulations of forced mixing ventilation at transitional slot Reynolds numbers. The experimental data set presented in Chapter 3 is used to assess the capability of four commonly used RANS turbulence models to predict transitional room airflow. Three popular linear two-equation models are tested (RNG k-ε, low-Re numberk-ε, SST k-ω), as well as one second-order closure model (Reynolds Stress Model (RSM)). Both the dimensionless velocities and the turbulent kinetic energies are compared on three vertical lines in the enclosure. The results show that three out of the four turbulence models provide results that are in close agreement with the measurement results. The results obtained with the RNG k-ε model show the largest deviations with the measurements, which can be attributed to an overprediction of turbulent kinetic energy in the wall jet region. In addition, it is shown that the different turbulence models provide different predictions for the air exchange efficiency, with differences between two models being as high as 44%. In addition to the correct prediction of the time-averaged flow pattern, it is of interest to see whethersteady RANS models can predict the dispersion of pollutants in a room with sufficient accuracy, which is the topic of Chapter 5. CFD simulations with steady RANS models often employ the standard gradient-diffusion hypothesis, in which the turbulent mass fluxes are related to the mean mass gradient using the turbulent (or eddy) mass diffusivity. The relativeinfluence of convective and turbulent mass fluxes in the transport process is analyzed and the role of these fluxes in the prediction accuracy of RANS and Large Eddy Simulations (LES) is clarified for this particular case. It is shown that the standard gradient-diffusion hypothesis is not always valid. However, the turbulent mass fluxes are about one order of magnitude smaller than the convective fluxes. As a result, the invalidity of the standard gradient-diffusion hypothesis does not lead to significant deviations in the predicted mean pollutant concentration field using steady RANS CFD simulations in the case under study. Part II A literature study has shown that well-documented experimental data sets of complex ventilation flow are hardly available. As a result, there is a strong lack of experimental data to validate numerical models for realistic/complex situations. Furthermore, CFD studies of natural mixing ventilation are usually performed for relatively simple building geometries. This part of the dissertation presents full-scale measurements of wind velocity and a range of environmental conditions in and around a complex semi-enclosed stadium situated in an urbanarea. The measurement results are used to validate a CFD model of the stadium and its surroundings, which is subsequently used to assess the natural mixing ventilation of the interior air volume of the stadium. Chapter 6 presents an analysis of full-scale measurements of thermal conditions and natural ventilation in a large semi-enclosed stadium in Amsterdam, the Netherlands. Due to similarity requirements (Reynolds, Grashof, and Richardson numbers) that cannot be fulfilled in the windtunnel, full-scale measurements are the only means to obtain a reliabledata set for a realistic summer situation. The full-scale measurements indicate a certain degree of repeatability on three consecutive evenings; both the wind conditions and the indoor and outdoor thermal conditionsonly show small differences between the three evenings. As a result, the measured CO2 concentration decay curves, and the calculated air exchange rate (ACH) values only show small deviations between the three evenings. Although there might be problems with repeatability and uncontrollable boundary conditions when performing full-scale measurements, in some particular cases, as the one presented here, full-scale measurements canprovide useful experimental data to validate CFD models of natural ventilation. Chapter 7 presents a coupled CFD modeling approach for urban wind flow and indoor natural ventilation of a large semi-enclosed stadium on a high-resolution grid. The computational grid is constructed using a specific procedure to efficiently and simultaneously generate the complex geometry and the high-resolution body-fitted grid for both the outdoor and indoor environment, based on translation and rotation of pre-meshed cross-sections. A grid-sensitivity study indicates that a 5.5 million cell grid provides nearly grid-independent results. The coupled CFD simulations are validated using full-scale (on-site) wind velocity measurements. The natural ventilation of the current configuration,as well as alternative ventilation configurations is analyzed. From theCFD simulations it is concluded that small geometrical modifications can increase the ACH values by up to 43%. A CFD analysis of the influence of wind direction and urban surroundings on the computed air exchange rate is presented in Chapter 8. The computational model of the stadium is the same as the current stadium configuration as studied in Chapter 7. To assess the influence of the wind direction and urban surroundings, simulations are performed for eight wind directions and for acomputational model with and without the surrounding buildings. The simulated differences in ACH between wind directions can be as high as 152%(with surrounding buildings). Furthermore, comparing the simulations with and without taking into account the urban surroundings for each wind direction shows that neglecting the surrounding buildings can lead to overestimations of the ACH with up to 96%. Finally, Chapter 9 presents non-isothermal unsteady RANS CFD simulations of CO2 concentration decay from the abovementioned semi-enclosed stadium. The boundary conditions for the CFD simulations are based on the measured conditions. The CO2 concentration decay curves obtained with the unsteady CFD simulations are compared with measured CO2 concentration decay curves and showa fair to good agreement. The validated model is used to detect regionswith lower ventilation efficiencies, i.e. stagnant regions and recirculation zones inside the stadium, resulting in higher CO2 concentrations. The largest spatial gradients are present in the beginning of the CO2 concentration decay process, and can be as high as 700 ppm (= 37%) betweenthe northern and southern part of the stadium. In addition, a specific piecewise linear technique is applied for the concentration decay methodto determine the ACH values based for smaller time intervals. This is important because the semi-logarithmic decay curve itself is not linear because the value of ACH changes over time as a result of decreasing buoyancy forces. Using this technique, it is shown that the ACH values strongly decrease as a function of time, from about 2 h-1 at the beginning ofthe concentration decay simulations to about 0.3 h-1 at the end (t > 4000 s). Chapter 10 provides a discussion on the research andrecommendations for future work are listed. Finally, Chapter 11 (summary and conclusion) concludes this dissertation.nrpages: 258status: publishe

On the effect of wind direction and urban surroundings on natural ventilation of a large semi-enclosed stadium

Natural ventilation of buildings refers to the replacement of indoor air with outdoor air due to pressure differences caused by wind and/or buoyancy. It is often expressed in terms of the air change rate per hour (ACH). The pressure differences created by the wind depend - among others - on the wind speed, the wind direction, the configuration of surrounding buildings and the surrounding topography. Computational Fluid Dynamics (CFD) has been used extensively in natural ventilation research. However, most CFD studies were performed for only a limited number of wind directions and/or without considering the urban surroundings. This paper presents isothermal CFD simulations of coupled urban wind flow and indoor natural ventilation to assess the influence of wind direction and urban surroundings on the ACH of a large semi-enclosed stadium. Simulations are performed for eight wind directions and for a computational model with and without the surrounding buildings. CFD solution verification is conducted by performing a grid-sensitivity analysis. CFD validation is performed with on-site wind velocity measurements. The simulated differences in ACH between wind directions can go up to 75% (without surrounding buildings) and 152% (with surrounding buildings). Furthermore, comparing the simulations with and without surrounding buildings showed that neglecting the surroundings can lead to overestimations of the ACH with up to 96%.</p

The influence of wind direction on natural ventilation: Application to a large semi-enclosed stadium

Natural ventilation is a commonly applied way in building engineering to ensure a healthy and comfortable indoor climate. In this paper CFD simulations of the natural ventilation of a large semi-enclosed stadium in the Netherlands are described. Simulations are performed to assess the air exchange rate for a total of eight wind directions. The CFD model consists of both the complex stadium geometry and the urban environment in which the stadium is located. A grid sensitivity analysis is conducted, furthermore, validation of the CFD model is performed using full-scale 3D wind velocity measurements. Comparison of the calculated air exchange rates showed that the wind direction has a significant effect on the air exchange rate; differences up to 100% were found for the air exchange rate, which can be explained by the position and size of the buildings upstream of the stadium

On the effect of wind direction and urban surroundings on natural ventilation of a large semi-enclosed stadium

Natural ventilation of buildings refers to the replacement of indoor air with outdoor air due to pressure differences caused by wind and/or buoyancy. It is often expressed in terms of the air change rate per hour (ACH). The pressure differences created by the wind depend - among others - on the wind speed, the wind direction, the configuration of surrounding buildings and the surrounding topography. Computational Fluid Dynamics (CFD) has been used extensively in natural ventilation research. However, most CFD studies were performed for only a limited number of wind directions and/or without considering the urban surroundings. This paper presents isothermal CFD simulations of coupled urban wind flow and indoor natural ventilation to assess the influence of wind direction and urban surroundings on the ACH of a large semi-enclosed stadium. Simulations are performed for eight wind directions and for a computational model with and without the surrounding buildings. CFD solution verification is conducted by performing a grid-sensitivity analysis. CFD validation is performed with on-site wind velocity measurements. The simulated differences in ACH between wind directions can go up to 75% (without surrounding buildings) and 152% (with surrounding buildings). Furthermore, comparing the simulations with and without surrounding buildings showed that neglecting the surroundings can lead to overestimations of the ACH with up to 96%.</p