Confluent Jets Research Articles

An investigation concerning optimal design of confluent jets ventilation with variable air volume

This parametric study aims to predict the performance of confluent jets ventilation (CJV) with variable air volume (VAV) from four CJV design parameters. A combination of computational fluid dynamics (CFD), and response surface method (RSM) has been used to predict the energy efficiency, thermal comfort and IAQ for the four expected vital design variables, i.e., heat load (XH), number of nozzles (XN), airflow rate (XQ) and supply temperature (XTS). The RSM was used to generate a quadratic equation for the response variables exhaust temperature (TE), supply temperature (TP), PMV, DR, εT and ACE. The RSM shows that the TE, TP and PMV were independent of the number of nozzles. The proposed equations were used to generate setpoints optimized for thermal comfort (PMV) for summer, spring and winter cases with different CLO factors and different TS under a scenario where the heat load varied between 10-30 W/m2. TE was used as setpoint to regulate the airflow rate to keep the PMV values close to zero. The results show that by adapting the TS to the CLO factor both thermal comfort and the energy efficiency can be improved. Further energy reduction can be gained by downregulating the airflow rate to keep the TP at a fixed setpoint when the heat load is decreased. This means that a CJV can effectively be combined with VAV to improve environmental performance with good thermal comfort (-0.5 < PMV < 0.5, DR < 20%), above average IAQ (ACE ≈ 106%) and with a higher heat removal efficiency (ε T ≈110%) than conventional mixing ventilation.

Experimental investigations of flow and thermal behavior of wall confluent jets as a heating device for large-space enclosures

The study aimed to explore the effects of inlet air temperature, outdoor air temperature, inlet bulk velocity, and the number of nozzles on wall confluent jets (WCJ) propagating along an external cold wall in a large space enclosure such as a greenhouse. A combination of experimental study and Response surface methodology has been used to predict the flow and thermal behavior of the WCJ for the studied cases. Box-Behnken design was used to determine the case matrix for four of the above-mentioned vital variables for non-isothermal cases. The experimental study employed constant current anemometers to measure the velocity and temperature of the WCJ. Results showed that the WCJ attached to the wall under both isothermal and non-isothermal conditions. This flow behavior suggests that the WCJ can be used to heat the external facades of large-space enclosures. All the stated variables were critical to the decay factor and decay rate of maximum velocity, albeit at varying levels. The velocity decayed faster with an increase in the inlet bulk velocity and outdoor air temperature. It also decayed faster as the number of nozzles and inlet air temperature decreased. The external wall surface temperature and the wall-heating effect increased as the momentum of the jet increased. The surface temperature on the external wall was most influenced by the inlet air temperature and least by the number of nozzles. Correlations of the second-order polynomial for the Response surface models that estimate the rate of velocity decay and temperature on the external wall were obtained.

A Numerical and Experimental Investigation of a Confluent Jets Ventilation Supply Device in a Conference Room

In this study, confluent jets ventilation (CJV) supply devices with three different nozzle arrays (1 × 19, 2 × 19, 3 × 19) were investigated both numerically and experimentally at two different airflow and supply air temperature set-ups. The performance of the CJV supply devices was investigated concerning thermal comfort, indoor air quality (IAQ), and heat removal effectiveness in a conference room environment. A comparison between the experimental and numerical results showed that the ϑ2¯−f model had the best agreement out of the investigated turbulence models. The numerical results showed that the size of the array had a great impact both on near-field development and on the conditions in the occupied zone. A larger array with multiple rows and a lower momentum conserved the inlet temperature and the mean age of the air better than a single-row array with a higher momentum. A larger array with multiple rows had a higher IAQ and a greater heat removal effectiveness in the occupied zone because the larger array conserved the mean age of air better and the buoyancy driven flow was slightly better at removing the heat. Because of the lower inlet velocities, they also had lower velocities at ankle level, which decreased the risk of draft and thermal discomfort.

Case study of a multiple confluents jets system in a virtual chamber

This article presents a case study of a multiple confluents jets system in a virtual chamber. In this study, it is obtained the behavior of the airflow in an office, simulated by a virtual camera, whose ventilation system is of the horizontal confluent jets type. The ventilation system has an inlet system consisting of four vertical ducts mounted in the corners of the compartment. Each duct has four groups of 32 nozzles, aligned and equally spaced, positioned from the base of the duct close to the floor. Each two groups of nozzles are directed towards each of the walls adjacent to the duct The exhaust system consists of a quadrangular vertical duct mounted close to the ceiling and positioned in the center of the compartment. The study uses a numerical differential model in order to simulate the Computational Fluids Dynamics. Ventilation by horizontal confluent jets creates a horizontal air velocity close to the wall surface as well as the floor surface. With the increase in the exit velocity of the jets, the interactive effect of the confluence of the jets, coming from opposite ducts, in the central area of the wall becomes more evident. The airflow resulting from the interactions of the jets has an upward characteristic towards the exhaust system.

Thermo-fluid-dynamic characteristics of confluent jets for distribution of treated air in small environment

Thermo-fluid-dynamic characteristics of confluent jets for distribution of treated air in small environment

On the Ventilation Performance of Low Momentum Confluent Jets Supply Device in a Classroom

The performance of three different confluent jets ventilation (CJV) supply devices was evaluated in a classroom environment concerning thermal comfort, indoor air quality (IAQ) and energy efficiency. The CJV supply devices have the acronyms: high-momentum confluent jets (HMCJ), low-momentum confluent jets (LMCJ) and low-momentum confluent jets modified by varying airflow direction (LMCJ-M). A mixing ventilation (MV) slot jet (SJ) supply device was used as a benchmark. Comparisons were made with identical set-up conditions in five cases with different supply temperatures (TS) (16–18 °C), airflow rates (2.2–6.3 ACH) and heat loads (17–47 W/m2). Performances were evaluated based on DR (draft rating), PMV (predicted mean vote), ACE (air change effectiveness) and heat removal effectiveness (HRE). The results show that CJV had higher HRE and IAQ than MV and LMCJ/LMCJ-M had higher ACE than HMCJ. The main effects of lower Ts were higher velocities, DR (HMCJ particularly) and HRE in the occupied zone as well as lower temperatures and PMV-values. HMCJ and LMCJ produce MV conditions at lower airflow rates (&lt;4.2 ACH) and non-uniform conditions at higher airflow rates. LMCJ-M had 7% higher HRE than the other CJV supply devices and produced non-uniform conditions at lower airflow rates (&lt;3.3 ACH). The non-uniform conditions resulted in LMCJ-M having the highest energy efficiency of all devices.

Air diffusion system design in large assembly halls. Case study of the Congress of Deputies parliament building, Madrid, Spain

The paper describes and analyses the air distribution solution adopted in the refurbishment of the Spanish Congress of Deputies assembly hall. A new approach is proposed for the design of displacement ventilation systems. The system performance is methodically validated both with laboratory tests, computational fluid dynamics analysis and experimental field measurements related to the environmental performance of the built environment. In assembly halls, microclimate and displacement ventilation systems, which supply air by means of floor mounted outlets, have been shown to perform adequately. In this case study floor air supply is not feasible. The solution adapted uses parapet mounted diffusers, using a confluent jet flow parallel to the floor plane above the occupied area. All the tools used to analyse the performance of this system showed that the admixing overhead supply, with ceiling return, and flowing parallel to and above the occupied zone is drawn back into the occupied zone, creating a plume effect similar to that of floor mounted displacement outlets. Thus, this confirms that the displacement effect can be accomplished using a high induction turbulent overhead supply with ceiling return. Therefore supplying air directly into the occupied zone, at or near to floor level, is not a prerequisite to achieve the desired displacement effect. This report concludes that, using the correct design parameters, as set out in this paper, the displacement ventilation effect is independent of the plane of air supply. The conclusions suggest that accepted criteria for the definition and design of displacement systems should be redefined.

Airflow pattern and performance of wall confluent jets ventilation for heating in a typical office space

To investigate an air distribution system with the potential to provide a comfortable thermal environment and efficient ventilation performance in heating mode, this study compared the numerical results of the RNG k-[Formula: see text] model, realizable k-[Formula: see text] model and SST k-ω model with experimental data obtained from a typical office with wall confluent jets ventilation (WCJV) for heating. The SST k-ω model was chosen for numerical study of WCJV. The findings showed that the WCJV performed better than mixing ventilation (MV) in the heating mode because WCJV could entrain less ambient air and maintain the jets’ momentum and energy to reach the occupied zone. The effects of supply air temperature difference (in the range of 2.00°C to 7.79°C) and supply air velocity (in the range of 2.20 m/s to 8.50 m/s) on the jet velocity profiles and ventilation performance of WCJV were also quantified. The results showed that the jet velocity profile was more sensitive to the supply air velocity than to the supply air temperature difference. Accordingly, reducing the supply air velocity might result in poorer thermal environment and air quality in the occupied zone because the warm air jets cannot spread out over the floor area. The results can be helpful in understanding WCJV used for ventilation under heating mode.

Experimental and numerical investigations of a new ventilation supply device based on confluent jets

In developed countries, heating, ventilation, air conditioning (HVAC) systems account for more than 10% of national energy use. The primary function of a HVAC system is to create proper indoor environment. A number of ventilation strategies have been developed to minimize HVAC systems' energy use whilst still maintaining a good indoor environment. Among these strategies are confluent jet ventilation and variable air volume. In this study, an air supply device with a novel nozzle design that uses both of the above-mentioned strategies was investigated both experimentally and numerically at three different airflow rates. The results from the numerical investigation using the SST k - ω turbulence model regarding velocities and flow patterns are validated by experimental data carried out by Laser Doppler Anemometry. The results from both studies show that the flow pattern and velocity in each nozzle is directly dependent on the total airflow rate. However, the flow pattern does not vary between the three different airflow rates. The numerical investigation shows that velocity profiles for each nozzle have the same pattern regardless of the airflow rate, but the magnitude of the velocity profile increases as the airflow increases. Thus, a supply device of this kind could be used for variable air volume and produce confluent jets for the airflow rates investigated.

Comparison of ventilation performance of three different air supply devices: a measurement study

ABSTRACTPeople today spend a significant part of their time in an indoor environment, whether it be home, school, vehicle or workplace. This has put greater demands on indoor environment, in terms of both air quality and thermal comfort. The main objective of building ventilation is to take care of pollutants and lower their concentration, but it is also used to cool or heat indoor air. The aim of this paper was to study the behavior of three different ventilation supply devices, i.e. mixing supply device, displacement supply device and confluent jet supply device, in an office room. Different cases have been studied experimentally with different airflow rates, supply air temperature and supply devices. The results shows that mostly that we can expect, but the results show a very small difference in ventilation efficiency between the different systems and in theory there should be a larger difference.

On the Influence of Array Size and Jet Spacing on Jet Interactions and Confluence in Round Jet Arrays

This work uses computational models to study the effects of confluence and jet-to-jet interactions for inline matrices of confluent round jets. In total, 12 different confluent jet arrangements, having various jet array sizes and dimensionless jet spacing, S/d0, have been investigated. The array size varies from 6 × 6 to 10 × 10 jets, while S/d0 varies between 1.75≤S/d0≤4.0. The Reynolds number, based on the nozzle exit diameter, is between 2200 and 6600. The results show that both jet spacing and jet array size largely influence the jet-to-jet interactions and flow field development in confluent jet arrays. The jet interactions in the investigated setups result in regions of negative static pressure between jets, jet deformation, high spanwise velocity, and jet displacement. Generally, smaller jet spacing and larger array size result in stronger influence of jet interactions. After the jets have combined, the confluent jets form a zone with constant maximum streamwise velocity and decay of turbulence intensity, called a confluent core zone (CCZ). During the CCZ, the combined jet will have asymmetric spreading rates leading to axis-switching. The entrainment rate of the CCZ is constant, but the volumetric flow rate of the combined jet is substantially affected by the degree of entrainment before the jets have combined.

Experimental evaluation of an intermittent air supply system – Part 1: Thermal comfort and ventilation efficiency measurements

Spaces with high occupancy density e.g.; classrooms, auditoriums and restaurants, provide challenges to ventilate at a lower energy use due to elevated temperatures. To meet occupants' thermal comfort requirements traditional systems use a lot of energy. Alternative ventilation strategies that optimize high air movements in the occupied zone allow human activities at elevated temperatures while attaining improve occupants' perception and acceptance of the indoor climate at a low energy use. This paper presents an experimental evaluation of a novel ventilation strategy for high occupancy spaces that provides fresh air and thermal comfort in the sitting zone through a controlled intermittent air jet system. The strategy uses ceiling mounted high momentum air jet diffusers (AJD) made from ventilation duct fitted with nozzles that generate confluent jets. The jets coalesce into a single two-dimensional jet which is directed downwards in the sitting zone. This paper presents an experimental evaluation/analysis of the proposed system with regard to ventilation efficiency and thermal comfort measurements in a classroom mockup. Results show that the system qualifies to be used as a primary ventilation system and has local air change index >1 inside the jet, and a ventilation efficiency >50%. The system also provides better thermal climate than mixing and displacement ventilation at elevated temperatures.

Investigation in the Near-Field of a Row of Interacting Jets

Multiple interacting jets (confluent jets) are employed in many engineering applications, and the significant design factors must be investigated. Computational fluid dynamics (CFD) is used to numerically predict the flow field in the proximal region of a single row of round jets. The numerical results that are obtained when using the low Reynolds k-ε are validated with the experimental data that are acquired by particle image velocimetry (PIV). PIV was used to measure mean velocity and turbulence properties in the proximal region of a row of six parallel coplanar round air jets with equidistant spacing at low Reynolds number (Re = 3290). The low Reynolds k-ε underpredicts the streamwise velocity in the onset of the jets' decay. The characteristic points are determined for various regions between two neighboring jets. The comparison of the merging point (MP) and the combined point (CP) computed from measurements and simulations shows good agreement in the different regions between the jets. In this study, a computational parametric study is also conducted to determine the main effects of three design factors and the interactions between them on the flow field development using response surface method (RSM). The influences of the inlet velocity, the spacing between the nozzles, and the diameter of the nozzles on the locations of the characteristic points are presented in the form of correlations (regression equations). CFD is used to numerically predict the characteristic points for a set of required studies, for which the design values of the simulation cases are determined by the Box–Behnken method. The results indicate that the spacing between the nozzles has a major impact on the flow characteristics in the near-field region of multiple interacting jets. The RSM shows that the inlet velocity has a marginal effect on the merging and CPs. All of the square terms are removed from the response equations of MP, and only one two-way interaction term between inlet velocity and spacing remains in the regression model with a marginal effect. The square of the nozzle diameter contributes in the regression equations of CP in some regions between the jets.

An experimental investigation of the flow and comfort parameters for under-floor, confluent jets and mixing ventilation systems in an open-plan office

There is a new trend to convert the workplaces from individual office rooms to open offices for motivating money saving and better communication. With such a shift the ability of existing ventilation systems in meeting the new requirements is a challenging question for researchers. The available options could have an impact on workers' health in terms of providing acceptable levels of thermal comfort and indoor air quality. Thus, this experimental investigation focuses on the performances of three different air distribution systems in an open-plan office space. The investigated systems were: mixing ventilation with ceiling-mounted inlets, confluent jets ventilation and underfloor air distribution with straight and curved vanes. Although this represents a small part of our more extensive experimental investigation, the results show that all the purposed stratified ventilation systems (CJV and UFAD) were more or less behaving as mixing systems with some tendency for displacement effects. Nevertheless, it is known that the mixing systems have a stable flow pattern but has the disadvantage of mixing contaminated air with the fresh supplied air which may produce lower performance and in worst cases occupants' illness. For the open-plan office we studied here, it will be shown that the new systems are capable of performing better than the conventional mixing systems. As expected, the higher air exchange efficiency in combination with lower local mean age of air for corner-mounted CJV and floor-mounted UFAD grills systems indicates that these systems are suitable for open-plan offices and are to be favored over conventional mixing systems.

Numerical investigation of ventilation performance of different air supply devices in an office environment

The aim of this study was to compare ventilation performance of four different air supply devices in an office environment with respect to thermal comfort, ventilation efficiency and energy-saving potential, by performing numerical simulations. The devices have the acronyms: Mixing supply device (MSD), Wall confluent jets supply device (WCJSD), Impinging jet supply device (IJSD) and Displacement supply device (DSD). Comparisons were made under identical set-up conditions, as well as at the same occupied zone temperature of about 24.2 °C achieved by adding different heat loads and using different air-flow rates. Energy-saving potential was addressed based on the air-flow rate and the related fan power required for obtaining a similar occupied zone temperature for each device.Results showed that the WCJSD and IJSD could provide an acceptable thermal environment while removing excess heat more efficiently than the MSD, as it combined the positive effects of both mixing and stratification principles. This benefit also meant that this devices required less fan power than the MSD for obtaining equivalent occupant zone temperature. The DSD showed a superior performance on heat removal, air exchange efficiency and energy saving to all other devices, but it had difficulties in providing acceptable vertical temperature gradient between the ankle and neck levels for a standing person.

Numerical study of a ventilation system based on wall confluent jets

This study presents numerical investigation of an air supply device based on wall confluent jets in a ventilated room. Confluent jets can be described as multiple round jets issuing from supply device apertures. The jets converge, merge, and combine at a certain distance downstream from the supply device and behave as a united jet, or so-called confluent jet. The numerical predictions of the velocity flow field of isothermal confluent jets with three Reynolds-averaged Navier–Stokes turbulence models (renormalization group k-ϵ, realizable k-ϵ, and shear stress transport k-ω) are reported in the present study. The results of the numerical predictions are verified with detailed experimental measurements by a hot wire anemometer and constant temperature anemometers for two airflow rates. The box method is used to provide the inlet boundary conditions. The study of the airflow distribution shows that a primary wall jet (wall confluent jet) exists close to the supply device along the wetted wall, and a secondary wall jet is created after the stagnation region along the floor. It is presented that the flow field of the primary and secondary wall jet predicted by turbulence models is in good agreement with the experimental data. The current study is also compared with the literature in terms of velocity decay and the spreading rate of the primary and secondary wall jet, the results of which are consistent with each other. Velocity decay and the spreading rate of the secondary wall jet in vertical and lateral directions were studied for different inlet airflow rates and inlet discharge heights. The comparative results demonstrate that the flow behavior is nearly independent of the inlet flow rate. Inlet discharge height is found to have impact close to the inlet, where the velocity decays faster when the jet discharges at higher level. The decay tendency is similar as the jet enters into the room for all discharge heights.

Experimental investigation of a ventilation system based on wall confluent jets

The flow behavior of isothermal and non-isothermal wall confluent jets (WCJ) ventilation system was investigated experimentally in a mock-up office environment. Two flow regions were identified: first, a primary region is developed below the supply device, with axis along the inlet wall, and a secondary wall-jet forms along the floor. The velocity and temperature fields were recorded by traversing a hot wire and thermistor anemometers for both primary and secondary regions. The results show self-similarity characteristic of the velocity and temperature profiles for both isothermal and non-isothermal WCJ. Maximum velocity decay and its spreading rate are linear and independent of the inlet airflow rate. Minimum temperature difference and its spread also show linear decay. The spreading rates for maximum velocity are similar in both the primary and secondary regions normal to the inlet wall and floor, respectively, which is consistent with previous studies on wall jets. The variation of draught rating is evaluated via mean velocity, turbulence intensity and temperature. Although the WCJ has slow velocity decay, which enables it to cover almost the entire floor of the test room, the draught is acceptable in the occupied zone according to ISO 7730. PMV (predicted mean vote) and PPD (predicted percentage dissatisfied) are presented for the occupied zone of the room.

Near-field mixing of jets issuing from an array of round nozzles

This article presents results of an experimental study of the confluence of low Reynolds number jets in the near field of a 6×6 in-line array of round nozzles. Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) were employed to measure mean velocities and turbulence statistics. The comparison of the results from PIV and LDA measurements along different cross-sectional profiles and geometrical centerlines showed good agreement. However, LDA enabled more accurate results very close to the nozzle exits. The evolution of all the individual jets in the array into a single jet showed flow regions similar to twin jets (i.e., initial, converging including mixing transition, merging and combined regions). The lateral displacements play an important role for a confluent jet, where all jets to some degree are deflected towards the center of the nozzle plate. The jet development in terms of velocity decay, length of potential core and lateral displacement varies significantly with the position of the jet in the array. A comparison with single jet and twin jets flow showed considerable differences in velocity decay as well as location and velocity in the combined point. The flow field of confluent jets showed asymmetrical distributions of Reynolds stresses around the axis of the jets and highly anisotropic turbulence. Additionally, the lateral displacement as well as the turbulence development in the proximal region of the studied confluent jet was shown to be dependent on Reynolds number.

Numerical and experimental investigation of the near zone flow field in an array of confluent round jets

Numerical simulations, using three different turbulence models (i.e., standard k–ε, RNG k–ε and Reynolds Stress Model [RSM]) is performed in order to predict mean velocity field as well as turbulence characteristics in the near zone of a 6×6 in-line array of unconfined confluent round jets. The numerical results are compared with experimental data acquired by Particle Image Velocimetry (PIV).All the turbulence models used are able to reproduce the mean velocity field and the development of turbulent kinetic energy of the confluent round jets, but in general, the standard k–ε and RSM model show better agreement with experimental data than the RNG model. In terms of mean velocity the second-order closure model (RSM) is not found to be superior to the less advanced standard k–ε model in spite of the mean flow curvature present in the flow field. The RSM model, however, provides information on individual Reynolds stresses. RSM show satisfactory agreement of streamwise normal Reynolds stress and shear stress, but generally underpredicts the normal Reynolds stress in the spanwise direction.In comparison with plane twin jets, confluent round jets show a longer merging region. Within the merging region the maximum velocity of the confluent jets decay linearly. As the jets enter the combined region confluent jets have hardly any velocity decay, which leads to a higher maximum velocity for a combined confluent jet than a single round jet.The jet’s position within the configuration has a substantial impact on the velocity decay, length of the potential core, and the lateral displacement of the confluent jets. Side jets show faster velocity decay, shorter potential core and higher turbulence level compared to central jets. Side jets are also deformed and has a kidney shaped cross-section in the merging region. Corner jets interact less with neighboring jets compared to side jets, thereby extending the potential core and reducing the velocity decay in the merging region compared to side jets.

ASSESSMENT OF CONFLUENT JETS FOR IMPROVED PERFORMANCE OF PERSONALIZED VENTILATION MOUNTED ON THE DESK

A unique method for inhaled air quality improvement by personalized ventilation by control of the free convection flow and its interaction with the clean air based on confluent jets was studied. The confluent upward jets were generated close to the front of the human body by openings at the front edge of a desk board pressed against the abdominal area. The inner jet supplied clean air, while the assisting outer jet supplied polluted room air. Comprehensive experiments were performed under isothermal conditions (26°C) in a full-scale room with a thermal manikin simulating realistically human body shape and the generated free convection flow. Tracer gas measurements were performed to identify the impact of the control method on inhaled air quality. Results showed the use of confluent jet of polluted air improved the inhaled air quality compared to the single jet application.