Supply Jet Research Articles

CFD of the diffusion movement and concentration distribution of culinary particles in the respiratory zone of restaurant diners

Released aerosol particles during restaurant culinary activity affect diners' health. The air conditioning system is crucial for regulating indoor air quality. However, its improper air distribution increases the individuals' exposure to particle pollution. This study investigates restaurants employing side-up airflow during summer with numerous heat sources and examines the culinary particle diffusion in the diners’ respiratory zone under the combined influence of air conditioning cold jet air supply and culinary heat source heat plume. It elucidates the change rule of the concentration distribution of culinary particles under the combined action of these two heterogeneous airflows.This study investigated the movement and concentration distribution of indoor particle by numerical simulation under various air supply velocities, culinary heat source strengths and positions and tuyere opening modes. In restaurants with culinary sources, the thermal buoyancy by the heat plume causes particles to rise. However, the drag force exerted by the cold air supply jet impedes the particles’ upward motion. The particle concentration distribution is significantly influenced by both the air supply velocity and the relative positioning of the heat source and the tuyere. Particle concentration increases by 27.13 % in the respiratory zone when the air supply jet trajectory is above the pollution emission source than below. Therefore, lowered air supply velocity is ideal with increased horizontal distance between the emission source and the tuyere under the condition of comfort satisfaction. This scenario mitigates the downward movement exerted by the jet on the particles. The drag force is increased with the air supply velocity increasing from 2.5 m/s to 4.0 m/s. Particle concentration is raised to 41.38 % in the respiratory zone. The drag force by the cold jet on the particles is also heightened with the bilateral tuyere than its single-side counterpart which increases particle concentration maximum by 40.30 % in the respiratory zone.

Analysis of the influence of large-scale room circulation on the human microenvironment

In this study, the effects of indoor large-scale circulation on the human microenvironment under natural convection and air-conditioning conditions were investigated by experiments and numerical simulations in a climate chamber. The effect of human thermal plumes on airflow was studied by large-scale circulation period characteristics. The main influencing factors of the vortex structure distribution are analysed under two conditions. The results show that the thermal plume under natural convection condition is the main factor affecting the large-scale circulation, and the indoor large-scale circulation has a significant effect on the microenvironment around the human body. The supply air jet can significantly reduce the large-scale circulation period. Moreover, the air supply jet and the human thermal plume are coupled to form multiple small-scale circulations.

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

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

Use of recirculated air curtains inside ventilated rooms for the isolation of transient contaminant

An air curtain is an aerodynamic barrier for regional isolation and protection; however, research on its application in ventilated rooms is insufficient. This study investigated the effect of a recirculated air curtain on transient contaminant transport when used inside a room with diluted ventilation. The effects of the discharge air velocity, air intake position, and air curtain position were studied using a numerical method. The results were compared with those of diluted ventilation only. The results showed that even when the air curtain recirculated the polluted indoor air, adding a recirculated air curtain based on traditional ventilation significantly reduced the pollution in the protected zone. The containment effect on pollution initially increased and then decreased as the discharge air velocity of the air curtain increased from 0.5 to 4 m/s. A moderate velocity of 1 m/s for a jet width of 0.06 m acted as an effective aerodynamic barrier, with the average concentration in the protected zone reduced by up to 66 % at 600 s. The air intake of the air curtain strengthened the deflection of the air supply jet and vortex in the region. The air curtain exhibited better performance when the air was recirculated from the floor below it. Reducing the protected area did not significantly improve the regional protection capacity. This study guides the joint design of air curtains and air distribution patterns in healthy indoor environments.

Flow characteristics and formation optimization of vortex ring air supply.

The vortex rings ventilation (VRV) is a new type of air supply system comprising vortex rings. Compared with an air supply jet, a vortex ring reduces the loss of fresh air during transportation because of its stable structure. However, during the formation of the vortex ring, it entrains the ambient air and reduces the fresh air in the vortex ring. In this study, a vortex ring generator with a fresh air cavity is proposed to form confined vortex rings. This improved the fresh air ratio of the VRV. Based on previous experiments, a piston-orifice axisymmetric model with a dynamic grid was developed to form an air vortex ring. The flow characteristics of free and confined vortex rings during the generation stage were studied. First, the evolution of free vortex rings with different stroke lengths was studied, and the optimal piston stroke length and radial constraint size were determined. Subsequently, the mixing ratios of the free and confined vortex rings were compared and analyzed. The results showed that the mixing ratio of the confined vortex ring formed by the fresh air cavity in the formation stage was zero. Moving to the location of 4.9 times the orifice diameter ( ), reduced the mixing ratio of the confined vortex ring by 77.78% compared with that of the free vortex ring. In addition, the influence of three inner diameters and four outlet diameters of fresh air cavities on the vortex rings was studied to optimize the size of the vortex ring generator. The results showed that the inner diameter of the fresh air cavity was greater than and that an outlet diameter greater than had little influence on the vortex rings.

Effect of a recirculated air curtain with incomplete coverage of room width on the protection zone in ventilated room

An air curtain is an aerodynamic barrier for contaminants and is frequently installed in the doorway to separate indoor and outdoor spaces. There are few studies on the isolation of areas within a room. In this study, the effect of a recirculated air curtain, which did not completely cover the width of the room, i.e., an incomplete air curtain, was investigated when used inside a ventilated room. The steady contaminant distributions were analyzed numerically. The impacts of air curtain length, air extract position in the air curtain, and position of the contaminant source were studied. The results for the incomplete air curtain were compared with those for complete air curtain and traditional ventilation dilution. The results showed that installing an additional incomplete and recirculated air curtain could reduce the pollution in the protection zone compared with traditional ventilation. The concentrations of contaminant could be reduced by 30.8 and 43.9% in the subzone covered by the air curtain and that not completely covered by the air curtain, respectively. When the curtain length exceeded the source position, the direct invasion of high pollution could be prevented. A relative concentration deviation of up to 26.1% was caused by different air extract positions. Under the coupled airflow field resulting from air supply jets, recirculated curtain airflow, and airflow surrounding exhaust air outlets, a complete air curtain was not necessarily a better choice. This study provides guidance for the flexible use of incomplete and recirculated air curtains for the maintenance of protection zones.

Investigation of the quasi-periodic airflow oscillation and ventilation performance of an enclosed environment with a jet air supply

Unsteady airflows lead to better mixing in ventilated enclosures, facilitating indoor pollutant dispersion and exhausting. In this study, an air jet ventilated enclosure and its flow characteristics were investigated using experimental measurements and computational fluid dynamics simulations, focusing on the quasi-periodic fluctuation of the airflow field and the advantages of such an oscillation pattern for the ventilation efficiency. Starting from a non-converged Reynolds-averaged Navier–Stokes (RANS) simulation, we then used the unsteady RANS method (URANS). As validated by sampling the high-frequency velocity data of representative positions in the jet-ventilated space, the URANS results reflected the unsteady flow characteristics more precisely than the RANS results, further confirming that the supply jet air swung up and down quasi-periodically. The ventilation performance of such an actual quasi-periodic airflow was evaluated by purging a gaseous pollutant from the enclosure, and the results were compared with those in the average steady-state airflow field to showcase the different performance of the unsteady ventilation system.

PIV measurements of opposing-jet ventilation flow in a reduced-scale simplified empty airplane cabin

Mixing ventilation in airplane cabins is driven by overhead supply jets, which may be directed towards each other, interact and merge before entering the passenger area. Although many studies already investigated airplane cabin mixing ventilation, detailed knowledge on the fundamental flow components (i.e. supply jets, interaction zone, merged jet and secondary flows) is rather scarce, but can be useful to obtain new insights into mixing ventilation design optimisation. This paper presents time-averaged 2D particle image velocimetry (PIV) measurements of the flow driven by isothermal opposing plane jets in a reduced-scale water-filled generic airplane cabin. Two configurations of opposing jets, i.e. ceiling jets and lateral jets, are examined with inlet Reynolds numbers ranging from 3,450 to 4,650. The analysis focuses on the mean velocity and Reynolds stresses of the whole flow field and the aforementioned fundamental flow components. Decay rates of the supply jets and decay and growth rates of the merged jet are analysed and the size of the interaction zone is assessed. The supply jet velocity decays very fast due to the interaction with the opposing jet. Within the interaction zone, the turbulence levels are high and turbulence is highly anisotropic. A comparison between the merged jet and a self-similar turbulent plane free jet is presented as well. The measurement data is also valuable for (sub-configuration) computational fluid dynamics (CFD) validation purposes.

The effects of room length on jet momentum flux

As to mixing ventilation in indoor environments, the turbulent jet plays a major role in driving the air movement, contaminant transport, and heat transfer. The main characteristic of a turbulent jet is its momentum flux. By entrainment of air, the flow of a jet increases and may enhance the flooding of contaminant. In investing the jet’s momentum flux, it is generally regarded that the supply jet collides with the opposing wall and the jet is transformed into a wall jet. However, this is not always true if a jet is not sufficiently strong, or the length of a room is large. Therefore, this study adopted computational fluid dynamics (CFD) to investigate the supply jet development and its momentum flux by varying the room length. Initially, the width of the air supply inlet was the same with that of the room. By defining n as the ratio of room length and height, when n = 3, there is a horizontal a vortex which is the normal behaviour. When the room length increased further, the supply jet was unable to collide with the opposing wall. This investigation got two vertical vortices at the room end which is new. The two new vertical vortices were most pronounced for n = 5. It is possible that increasing the length of the room introduces a gradual transition towards a flow in a rectangular duct. This flow is probably very much governed by the side walls. Therefore, this study reduced the width of the air supply inlet by half and maintained the same flow rate. However, a single vertical vortex was identified at the room end for n = 5. In both scenarios, the supply jet may create new vortices that would enhance the flooding of contaminants.

СОВЕРШЕНСТВОВАНИЕ МЕСТНОГО ОТСОСА ОТ ПЛАЗМЕННОЙ РЕЗКИ МЕЛКОСЕРИЙНЫХ ДЕТАЛЕЙ

One of the main reasons for the deterioration of the environmental situation in the Russian Federation is air pollution by emissions of harmful substances into the environment, the intensification of which is caused by the decentralization of the sources of their formation. Theoretically, the interaction of a heat jet arising in a room with a supply jet has been studied, and, based on the results obtained, a method for localization and removal of hazards has been developed. The work also gives an ecological and economic assessment of the developed means of protecting air from pollution

Numerical investigation of the dynamic erosion behavior in fluidic oscillators with a periodic oscillating jet

Fluidic oscillators (FOs) containing no moving parts have received extensive attention in recent decades. Applications with FOs in some complex conditions are generally characterized by a high-speed jet of particle-laden flow that periodically switches between two parallel channels. This oscillating flow makes FOs prone to erosion. Using the CFD method, this study investigated the dynamic erosion behavior coupled with periodic oscillating jets. Three primary locations of erosion were observed, namely, in the vicinities of the nozzle, splitter, and vents. The effects of flow rate, particle size, and solid content on erosion were evaluated. It was found that the location of maximum erosion did not remain same. The amount of erosion in the splitter and vents fluctuated cyclically due to the periodic oscillation of the supply jet. Additional erosion occurred during the start-up process of the vertical feedback oscillator, whereas the curved feedback oscillator showed better performance in this respect.

Transport characteristics of dense gaseous contaminants in a large space in the presence of obstacles

In industrial buildings, large cross-sections and obstacles lead to relatively low velocity in the large space except in the vicinity of air supply jets. The dimensionless number that represents the dense gas transport characteristics is revised to effectively represent the indoor gaseous contaminant transport mode in the large space. Based on the computational fluid dynamics (CFD) model validated by experimental data, a group of orthogonal design simulations have been conducted to examine the ranges of the dimensionless number for each transport mode. The indoor gaseous transport modes in the vicinity of the source are divided into passive, transitional, and active transport modes, featured by the magnitude of the revised dimensionless number θ. When θ exceeds 29.38, the transport is in active mode, and when θ is less than 13.61, the transport is in passive mode. The critical value of θ from the passive to the transitional stage is in the range of 13.61–19.94. The critical value of θ from the transitional to the active stage is in the range of 20.68–29.38. When the gaseous contaminant is in active transport mode, the contaminants tend to move downward in the vicinity of the source, the dimensionless concentration distribution shows vertical stratification in the overall space, and the overall ventilation efficiency is notably higher than the non-active transport modes.

Decoupling transient effects of factors affected by air recirculation devices on contaminant distribution in ventilated spaces

The use of indoor air recirculation devices, such as air cleaners and air curtains, results in an airflow interaction between air supply jets and recirculated airflows. It is essential to quantify the independent effects of supply air, contaminant source, and initial condition in a ventilated room with air recirculation. In this paper, the transport process of a contaminant with air recirculation is mathematically derived based on a fixed flow field. A linear expression is established to describe the transient contributions of different influencing factors. The indices of revised transient accessibility of supply air (RTASA), contaminant source (RTACS), and initial condition (RTAIC), are defined to quantify the effect of each factor by incorporating the effect of air recirculation. The prediction accuracy is verified through a comparison with simulation results. The proposed indices are applied to evaluate a ventilated room with an air cleaner and recirculated air curtain. The results indicate that the recirculated airflows from both the air cleaner and air curtain increase the RTASA of each air supply inlet to distant areas. The purification effect (30%) occurs only after a certain period for RTASA and RTACS, while it operates from the initial stage for RTAIC owing to the initial high contamination. The RTACS in the protected zone is reduced significantly owing to the air curtain, with a decrease from 1.33 to 0.77 at 600 s at a height of 1.2 m in the curtain jet. The proposed method can be used to evaluate indoor environments with various air recirculation devices.

Study of Radial Wall Jets from Ceiling Diffusers at Variable Air Volume

The knowledge of the air velocity distribution in the supply jets is essential when designing ventilation and air conditioning systems. In this study, we tested and analyzed the velocity distributions in the radial wall jets—these jets are commonly used in ventilated rooms. Tests included jets from two ceiling diffusers of different constructions, at three airflow rates. The mean air speed distributions were measured with a 16-channel hot-sphere anemometer both in the self-similarity zone and in the terminal zone. A specially developed method of converting the mean speed to mean velocity was used. The measurement results show that the spread coefficients of the jets from both diffusers were the same, but the positions of the virtual origin were different. Due to the friction of the jet with the ceiling and the transfer of momentum to the recirculating flows, the momentum flux in the self-similarity zone decreased by up to 50%. An improved method for calculating velocity distributions in radial wall jets was developed and validated. This method takes into account the decrease of momentum, non-zero position of the jet origin, and faster velocity decrease in the terminal zone. A reliable method of predicting air velocity distribution in radial wall jets (RWJs) from ceiling diffusers may allow to properly select the diffuser size, its location, and the range of flow rate changes. The design process for variable air volume systems can be facilitated.

Analysis of the factors influencing the airflow behavior in an impinging jet ventilation room

A validated CFD model is employed to predict the airflow behavior in an impinging jet ventilation (IJV) room with cool, isothermal or warm jets. By using the numerical results, the influences of jet discharge height, supply grille shape and room height on the jet flow behavior as well as the draught discomfort are analyzed for IJV operating in heating scenarios. The results indicate that the warm supply jet of IJV rises upward to the ceiling after spreading along the floor for a certain distance due to thermal buoyancy, resulting in a limited dispersion area, while the cool and isothermal jets of IJV always spread along the whole floor. When IJV is used for space heating, the lower the jet discharge height, the farther the supply air spreads along the floor, and the supply grille shape and room height almost have no effect on the air dispersion area. The results also show that the energy-efficient advantage of IJV in its heating mode compared to the mixing ventilation (MV) system is more remarkable in higher rooms. Moreover, there is a risk of draught discomfort in IJV heating rooms and it is recommended to wear socks to avoid this discomfort.

Experimental study on the flow field and economic characteristics of parallel push-pull ventilation system

Abstract Push-pull ventilation systems provide excellent control of contaminants and harmful gases. However, since both a push inlet and a pull outlet are used in the push-pull ventilation system, the flow rate required by the system is large. In that case, the energy consumption of the system is large. The purpose of this paper is to study the flow field and economic characteristics of a parallel push-pull ventilation system by reducing the flow rate of the exhaust outlet, which will be achieved by reducing the size of the exhaust hood. The three commonly used push-pull ventilation systems were analyzed: a high velocity push-pull system with high air supply velocity, a low velocity push-pull system with wide airflow and small velocity, and a parallel push-pull system with wide airflow and uniform air supply velocity. Results showed that the parallel push-pull ventilation system was the only one in which the flow rate of the exhaust outlet could be reduced, reducing the overall energy consumption. Under conditions of the parallel air supply jet, the diffusion range of contaminants in the push-pull flow field was the smallest and reducing the exhaust air flow rate did not affect the capture efficiency of pollutants. These results may be useful in guiding the design of push-pull ventilation system and optimize economic constraints.

A modified turbulence model for simulating airflow aircraft cabin environment with mixed convection.

The forced convection (air supply jet) and the natural convection (thermal plume of passenger) co-exist in an aircraft cabin simultaneously. Due to the notable difference of the Reynolds numbers for the two convection processes, the traditional RANS method can hardly simulate the forced/natural convection flows accurately at the same time. In addition, the large geometric ratio between the main air supply inlet and the whole cabin leads to difficulties in grid generation for the cabin space. An efficient computational model based on the standard k-e model is established to solve these problems. The coefficients in the dissipative equation are modified to compensate the enlarged numerical dissipation caused by coarse grid; meanwhile, the piecewise-defined turbulent viscosity is introduced to combine the forced and natural convection. The modified model is validated by available experimental results in a Boeing 737-200 mock-up. Furthermore, the unsteady characteristic of the aircraft cabin environment is obtained and analyzed. According to the frequency analysis, it turns out that the thermal plume is the main factor of the unsteady fluctuation in cabin.Electronic Supplementary Material (ESM)Supplementary material is available in the online version of this article at 10.1007/s12273-020-0609-2.

Optimization of air flows in Orthodox church by numerical simulation

Frequently modern temples have certain problems with heating and ventilation, despite the strict design rules and the installation of religious buildings. There is a necessity to organize proper air exchange and ensure proper heating of the air throughout the space in a complex building. Moreover, to calculate the geometry of natural flows and supply jets, ensure the removal of polluted air bypassing crowded areas and the most important thing is to protect wall paintings and art objects from sedimentation of soot particles and dirt. Using the method of electronic digital modeling, the effectiveness of the proposed model of temple ventilation was confirmed, which is able to provide maximum protection and preservation of the interior decoration and works of art in the room at any mode of operation. Using the created model, specific design characteristics of ventilation and air heating systems were obtained and evaluated. Based on this, it was proposed to develop a project for their reconstruction.

Ceiling-fan-integrated air conditioning: Airflow and temperature characteristics of a sidewall-supply jet interacting with a ceiling fan

Ceiling-Fan-Integrated Air Conditioning (CFIAC) is a proposed system that can greatly increase buildings' cooling efficiency. In it, terminal supply ducts and diffusers are replaced by vents/nozzles, jetting supply air toward ceiling fans that serve to mix and distribute it within the room. Because of the fans' air movement, the system provides comfort at higher room temperatures than in conventional commercial/institutional/retail HVAC. We have experimentally evaluated CFIAC in a test room. This paper covers the distributions of air-speed, temperature, and calculated comfort level throughout the room. Two subsequent papers report tests of human subject comfort and ventilation effectiveness in the same experimental conditions. The room's supply air emerged from a high-sidewall vent directed toward a ceiling fan on the jet centerline; we also tested this same jet on a fan located off to the side of the jet. Primary variables are: ceiling fan flow volumes in downward and upward directions, supply air volume, and room-vs-supply temperature difference. Velocity, turbulence, and temperature distributions are presented for vertical and horizontal transects of the room. The occupied zone is then evaluated for velocity and temperature non-uniformity, and for comfort as predicted by the ASHRAE Standard 55 elevated air speed method. We show that temperatures are well-mixed and uniform across the room for all of the fan-on configurations, for fans both within or out of the supply jet centerline. The ceiling fan flow dominates the CFIAC airflow, and even though non-uniform is capable of providing comfortable conditions throughout the occupied area of the room.

Mixing ventilation driven by two oppositely located supply jets with a time-periodic supply velocity: A numerical analysis using computational fluid dynamics

Ventilation is of primary importance for the creation of healthy and comfortable indoor environments and it has a significant impact on the building energy heating and cooling demand. The aim of this study is to assess the application of time-periodic supply velocities to enhance mixing in mixing ventilation cases to reduce heating and cooling energy demands. This paper presents computational fluid dynamics (CFD) simulations of a generic mixing ventilation case, in which the time-averaged velocities and pollutant concentrations from a reference case with constant supply velocities were compared with those obtained from a case with time-periodic supply velocities (sine function). The unsteady Reynolds-averaged Navier-Stokes (URANS) CFD simulations indicate that the use of time-periodic supply velocities can reduce high pollutant concentrations in stagnant regions, reduces the overall time-averaged pollutant concentrations and increases contaminant removal effectiveness with about 20%. The influence of the period of the sine function was assessed and the results showed that for the periods tested, the differences are negligible. Finally, the URANS approach was compared with the large eddy simulations (LES) approach, indicating that URANS leads to very similar results (NMSE < 3.2%) as LES and can thus be regarded as a suitable approach for this study.