Ventilation Jet Research Articles

Geometric analysis of turbulent macrostructure in jets laid on flat surfaces for turbulence intensity calculation

European Norms for ventilation contain turbulence intensity requirements in rooms. One of the determining factors is turbulence intensity in ventilation jets. We found an approach for geometric analysis of turbulent macrostructure for subsonic flows with large-scale vorticity i.e. ventilation jets and boundary layers between flows. This approach requires building of simplified turbulent macrostructure chart and performing geometrical analysis of it. In previous works, using the approach we analytically found averaged characteristic of free jets, jets in flows and jets laid on different shape surfaces without requirements of any experimental data. The results of geometrical analysis of heat transfer between flows are used in Ukrainian norms. In this work we found turbulence intensity of wall jets on flat surfaces such as room walls or ceiling without any experimental values. The results coincide with known experimental data and may be used in flow calculation in rooms.

Computational methods design longitudinal jet ventilation systems in parking lots of closed type

Computational methods design longitudinal jet ventilation systems in parking lots of closed type

The accuracy of mapping the airstream of jet fan ventilators by fire dynamics simulator

Jet fan ventilators are often applied for smoke control and support for pollutant dispersal in large enclosures, especially in tunnels and car parks. In the design phase, computational fluid dynamics techniques are most often used to verify the fire safety level afforded by the application of jet fans. However, reliable design based on computational fluid dynamics requires a validation of jet fan airstream modeling. To predict these flows, this investigation evaluated the performance of four turbulence models using large-eddy simulations by comparing the simulation results with the experimental data of both horizontal and sloped jet fan guide vanes. The axial velocity was examined and simulations were prepared with fire dynamics simulator software, from the National Institute of Standards and Technology, which is the most popular program for fire simulations, although it is often criticized for inaccurate mapping of ventilation jet fan airstreams. In addition, this investigation analyzed the computing costs of the simulations, according to the fire dynamics simulator version and grid size, showing that the old version of software (fire dynamics simulator 4) was much faster than new one (fire dynamics simulator 6), and that changing grid size from 30 to 15 cm does not significantly change results.

Heating energy consumption of impinging jet ventilation and mixing ventilation in large-height spaces: A comparison study

Heating energy consumptions are compared when impinging jet ventilation (IJV) and mixing ventilation (MV) systems are used in heating modes in large-height spaces. The indoor thermal environments with different outdoor temperatures are simulated by employing CFD techniques for both the constant volume (CAV) and variable volume (VAV) systems. The results show that IJV can distributes the warm supply air into the occupied zone more easily than MV with a Top–Top configuration. The energy consumptions in IJV for heating fresh air and re-circulated air are much less than those in MV because IJV can overcome the disadvantage of thermal stratification that exists in MV rooms, but the power consumed by the fans for IJV is slightly higher than that for MV due to the lower temperature difference between supply and indoor air in the former, resulting in higher total heating energy consumption for MV when compared with IJV. Moreover, the results also show that with MV being replaced by IJV, the heating load index can be reduced and the lower the outdoor air temperature or the higher the required fresh air flow rate is, the more obvious reducing effect for IJV on heating load index.

Numerical and experimental investigation of turbulent three separated jets

In this work three-dimensional turbulent and separated jets are numerically studied and compared with experimental data. The basic idea of the separated jet burner consists in separating fuel and oxidizer injection to dilute reactants with combustion products before the reactants mix. The computation is achieved using the finite-volume numerical method. The turbulence is modeled using a second order Reynolds Stress Model (RSM) by Launder–Reece–Rodi. A non uniform grid is applied and particularly tightened close to the exit jets in order to capture the near field mechanisms. The validation of the numerical tools is done by comparison with experimental data of the non ventilated jets. Ventilated jets are investigated only numerically. The obtained results show that mixing is improved using three jets compared to the single jet. A comparison between computed ventilated and non ventilated jets is presented. It yields a larger velocity mean value, but a decreasing of the corresponding root mean square of the turbulent velocity fluctuations (rms).

Laryngeal clefts

Laryngeal clefts

Entransy analysis and application of a novel indoor cooling system in a large space building

Researchers are giving increased attention to large space buildings such as airports and railway stations because of the high energy consumption of their indoor cooling systems. The indoor cooling system of a large space building is essentially a heat transfer process among several heat sources and heat sinks. Entransy analysis and T–Q diagrams are clear and direct methods for describing the heat transfer process in large space buildings. Based on entransy dissipation, the equivalent thermal resistance can be defined in order to evaluate the indoor cooling systems in large space buildings. Minimizing thermal resistance is the best way to achieve high-temperature cooling. Methods for reducing the equivalent thermal resistance from the heat sources to the heat sinks in large space buildings include simplifying the heat transfer process, reducing mixture loss among heat sources, minimizing unnecessary mixing of hot and cold fluids, and handling solar radiation directly. Based on this analysis, a radiant floor and displacement ventilation system is proposed. Compared with a conventional jet ventilation system, the radiant floor system has lower equivalent thermal resistance, especially for direct solar radiation. The new system was applied in Xi’an Xianyang International Airport, and it demonstrated approximately 34% better energy performance compared to the conventional system.

Experimental investigation of the flow behavior of an isothermal impinging jet in a closed cabin

The impinging jet concept has been proposed as a new ventilation strategy for use in office and industrial buildings. To improve the performance of impinging jet ventilation system to create a better thermal comfort environment, quantifying the detailed information of the flow behavior of impinging jet should be an essential prerequisite. This study reports an experimental study of an isothermal turbulent jet impinging normally on a flat surface in a closed cabin. The air jet issued from a round pipe with an inner diameter D. The distance between the pipe exit and the flat impingement plate was 9D. The Reynolds number, based on the jet centerline velocity at pipe exit and the pipe inner diameter, was 10,338. Measurements were performed in the free- and wall-jets using cross hot-wire anemometry, mean velocity, turbulence intensity, and power spectrum results being presented. In addition, a multi-scale analysis technique based on empirical mode decomposition was used to analyze and gain deeper insight into the multi-scale characteristics of coherent structures of the jet impingement on flat surface.

On-site measured performance of a radiant floor cooling/heating system in Xi’an Xianyang International Airport

In large space buildings such as airports, radiant floor systems are an attractive alternative to conventional all-air jet ventilation systems due to their advantages in thermal comfort and energy efficiency. This paper focuses on the operating performance of the radiant floor system in Terminal 3 of Xi’an Xianyang International Airport, the first airport terminal in China to adopt radiant floors for cooling. In summer, the typical cooling capacity of the radiant floor is 30–40W/m2 with a mean chilled water temperature of 18°C; this range increases to 110–140W/m2 with solar radiation of 120–170W/m2. The incremental effect of the solar radiation on the cooling capacity of radiant floor is analyzed with simple calculation methods. In winter, the radiant floor provides a heating capacity of 30–70W/m2 with a hot water temperature of 35–40°C, which is sufficient to satisfy the heating requirement. Compared with the jet ventilation system adopted in Terminal 2 of the same airport, indoor thermal comfort is significantly improved with the radiant floor system in Terminal 3 in both cooling and heating modes. Moreover, the novel system’s indoor air temperature distribution throughout the whole space and its utilization of high-temperature chilled water and low-temperature hot water reveal its potential for energy conservation.

Distribution of Indoor Air Quality under Air Conditioning Running

People are constantly committed to seeking new ways that energy saving ventilation can improve indoor air quality. In this paper, methods of theoretical analysis and numerical simulation combining established indoor ventilation jet crash fluid dynamics model to simulate the indoor air, thermal comfort and indoor air quality issues.

Application of multi-objective optimization techniques to improve the aerodynamic performance of a tunnel ventilation jet fan

This paper describes the design optimization of a tunnel ventilation jet fan through multi-objective optimization techniques. Four design variables were selected for design optimization. To analyze the performance of the fan, numerical analyses were conducted, and three-dimensional Reynolds-averaged Navier–Stokes equations with a shear stress transport turbulence model were solved. Two objective functions, the total efficiency of the forward direction and the ratio of the reverse direction outlet velocity to the forward direction outlet velocity, were employed, and multi-objective optimization was carried out to improve the aerodynamic performance. A response surface approximation surrogate model was constructed for each objective function based on numerical solutions obtained at specified design points. The non-dominated sorting genetic algorithm with a local search procedure was used for multi-objective optimization. The tradeoff between the two objectives was determined and described with respect to the Pareto-optimal solutions. Based on the analysis of the optimization results, we propose an optimization model to satisfy the objective function. Finally, to verify the performance, experiments with the base model and the optimization model were carried out.

Application of radiant floor cooling in a large open space building with high-intensity solar radiation

Radiant floor cooling is suitable for environmental control in large space buildings with high-intensity solar radiation caused by a large transparent envelope. A simple method for predicting the performance of radiant floor cooling systems with solar radiation in steady state is proposed in this paper, and the key factors that influence performance are obtained. Cooling capacity of the radiant floor is significantly increased with high-intensity solar radiation and high-temperature building envelope surfaces in large space buildings. The effect of shelter of indoor furniture (such as chairs) on the performance of radiant floors is analyzed. The lowest floor surface temperature is located at the sheltered floor, and the cooling capacity of the sheltered radiant floor is also lower than that of the unsheltered radiant floor. A case study applying radiant floor cooling in a large space building is then analyzed, and the results are compared with those of a conventional all-air jet ventilation system. At the same indoor air temperature, the radiant cooling system can provide better thermal comfort at a lower operative temperature, and it presents a significant improvement in energy efficiency, with total energy demand 20–30% lower than the conventional all-air system.

Experimental study of a non-isothermal wall jet issued by a displacement ventilation system

This paper reports a new set of experimental data and presents an in-depth analysis of the flow physics of a non-isothermal jet stream produced by a large quarter-round corner-mounted displacement diffuser. The air velocity, temperature and turbulence intensity inside the displacement ventilation (DV) jet have been thoroughly analyzed and compared with the reported findings of previous studies and model predictions. Through the experiment, it is observed that the DV jet development is significantly altered by buoyant forces and can be divided into four distinctive zones. The results of the present study refine the physical understanding of the coupled thermal-fluid fields characteristic of a DV jet stream, and are useful for improving the design of the DV system tested.

Investigation on the flow and thermal behavior of impinging jet ventilation systems in an office with different heat loads

This paper presents the flow and temperature field within an office using impinging jet ventilation (IJV) under different heat loads ranging from 17 to 65 W per square meter floor area. The measurement was carried out in a full-scale test room to verify the reliability of three turbulence models, i.e., the RNG k–ɛ, SST k–ω and v2¯−f models. It is found that all the tested models show good agreements with measurements, while the v2¯−f model shows the best performance, especially on the overall temperature prediction.The v2¯−f model is used further to investigate a number of important factors influencing the performance of the IJV. The considered parameters are: cooling effect of chilled ceiling, external heat load as well as its position, number of occupants and supplied air conditions. The interaction effect of chilled ceiling and heat sources results in a complex flow phenomenon but with a notable feature of air circulation. The appearance and strength of the air circulation mainly depends on the external heat load on window and number of occupants. It is found that with higher external heat load on window (384 W and 526 W), the air circulation has a strong tendency towards the side wall in the opposite direction to occupant, while with lower power on window (200 W) the air circulation has a strong tendency in the center of the room and extends to a larger area. When two occupants are present, two swirling zones are formed in the upper region. The effects of air circulation consequently alter the temperature field and the level of local thermal comfort.

High-efficiency design of a tunnel ventilation jet fan through numerical optimization techniques

This paper describes a numerical optimization procedure for performance improvement of a tunnel ventilation jet fan. We employ optimization techniques based on a PRESS (predicted error sum of squares)-based-averaging (PBA) model to improve the aerodynamic performance of a tunnel ventilation jet fan. For numerical analyses, three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations with a shear stress transport turbulence model are discretized using finite volume approximations and solved on hexahedral grids to evaluate the total efficiency at operating conditions as the objective function. Numerical results are verified by using experimental data for the total efficiency and the effective outlet velocity at operating conditions. Further, four geometric variables are used defining the meridional length and the thickness profile at the hub and shroud in the jet fan rotor as the design variables for numerical optimization. The results of numerical optimization show a significant improvement in the total efficiency of the optimum model in comparison with the base model.

Study on the Flow Numerical Simulation of Jet Ventilation in Multi Tunnel Excavation

Pressed ventilation in heading face is actually turbulent jet ventilation. In the blind drift, the air ducts are usually laid on one side of the drifting tunnel forming the wall-attached jet along the side of the tunnel and in order to dilute and pour out harmful gas and dust, and create an backflow at the same time, the airflow of the ventilation jet must reach at the head of the tunnel. Therefore, the forced ventilation in heading face is actually restrained wall attached jet ventilation in confined space.

Airflow characteristics by air curtain jets in full-scale room

A new air distribution pattern, air curtain jet ventilation was presented. The ventilation or airflow patterns and the air velocity produced by air curtain jet were investigated in detail. To identify the airflow characteristics of this novel air curtain jet ventilation system, a full-scale room was used to measure the jet velocity with a slot-ventilated supply device, with regards to the airflow fields along the vertical wall as well as on the horizontal floor zones. The airflow fields under three supply air velocities, 1.0, 1.5 and 2.0 m/s, were carried out in the full-scale room. The experimental results show the velocity profiles of air distribution, the airflow fields along the attached vertical wall and the air lake zones on the floor, respectively. The current experimental research is helpful for heating, ventilation and air conditioning (HVAC) engineers to design better air distribution in rooms.

터널 환기용 제트홴의 성능 향상을 위한 수치최적화

This paper presents an optimization procedure for performance improvement of a tunnel ventilation jet fan. Optimization techniques based on response surface approximation (RSA) are employed to improve the aerodynamic performance of a tunnel ventilation jet fan. For numerical analysis, three-dimensional Renolds- averaged Navier-Stokes (RANS) equations with shear stress transport turbulence model are discretized by using finite volume approximations and solved on hexahedral grids to evaluate the total efficiency at the operating condition as the objective function. Four geometric variables defining the meridional length and the thickness profile at the hub and shroud in the jet fan rotor are selected as design variables for the numerical optimization. The results of the numerical optimization show that the total efficiency of the optimized model is significantly improved in comparison with the base model.

Experimental assessment of the velocity and temperature distribution in an indoor displacement ventilation jet

Displacement Ventilation (DV) offers a great opportunity to improve indoor air quality while reducing the energy consumption compared to mixing ventilation. A frequent problem with displacement ventilation is however the draft discomfort caused by the cold air movement at foot level and excessive temperature difference between head and ankle. Despite numerous past studies, there is still a lack of detailed data regarding the velocity and temperature distribution inside DV jets. In this experimental study, measurements of the air velocity and temperature are performed in the DV jet produced by a flat wall-mounted DV diffuser, using a fine mesh focused on the 0.00–0.20 m height range. Two supply conditions are studied. Three planes of measurements are covered: the vertical longitudinal plane, the vertical transversal plane, and the horizontal plane. Measurements at the outlet of the wall-mounted DV diffuser are also performed. The drop of the DV jet at the diffuser exit, the variation of jet thickness, and the lateral spreading of the flow are discussed. The non-uniformities of velocity and temperature in each plane of measurements are also highlighted and documented. Finally, the measured results are discussed in regard to thermal comfort. Although the measurements presented in this paper are obtained with one specific DV diffuser, the results allow for better understanding of this type of air jet.

Variety of Expiratory Resistance Between Different Continuous Positive Airway Pressure Devices for Preterm Infants

Continuous positive airway pressure (CPAP) systems for preterm infants work with conventional ventilators or use a jet ventilation system. It is assumed that the most important advantage of jet-CPAP systems is a lower expiratory resistance (R(E) ). We investigated the R(E) of seven different CPAP systems. We studied two primary-care CPAP systems, three jet-CPAP generators, and two conventional CPAP devices. All devices were adjusted at 6 mbar and connected with a test lung simulating a standardized expiration volume. Maximum pressure increase during expiration was measured and maximum R(E) was calculated. In primary-care CPAP devices, the maximum R(E) of the Benveniste valve was 9.7 mbar/L/s (SD 1.2) while that of the Neopuff was 102.8 mbar/L/s (SD 7.9) (P < 0.01). In jet-CPAP devices, the R(E) of the Infant Flow was 6.8 mbar/L/s (SD 1.7), the one of the Medijet REF 1000 was 43.5 mbar/L/s (SD 1.5), and that of the Medijet REF 1010 was 36.7 mbar/L/s (SD 0.3) (P < 0.01). In conventional CPAP systems, the R(E) of the Baby Flow was 29.7 mbar/L/s (SD 1.1) and that of the Bubble CPAP was 37.1 mbar/L/s (SD 4.3) (P < 0.01). All CPAP devices created an R(E). Jet-CPAP devices did not produce lower R(E) than conventional CPAP devices.