Co-flow Jet Research Articles

Co-flow jets application for occupant targeted ventilation: Focus on thermal comfort and air quality

Co-flow air jets can be used in occupant-targeted ventilation, such as personalized ventilation, stratum ventilation, aiming at improving air quality in the breathing zone of occupants. However, these have not been studied sufficiently. The velocity field and cleanness of co-flow jets comprising inner jet of clean air encircled by outer jet of room air were studied under isothermal conditions by physical measurements and CFD simulations at numerous combinations of initial diameter and velocity of the inner and outer jets appropriate for practical application. The co-flow jet had a lower axial velocity decay along the jet direction than that of the single jet. The magnitude (turbulence intensity) and frequency of the velocity fluctuations in co-flow jets were lower compared to those in a single jet. The results revealed that with regard to thermal comfort co-flow jets may provide more cooling compared to single jets, especially when supplied toward to the user. The level of cleanness depended on the initial velocity of the inner and outer jets as well as their diameters. The air on the jet axis of the co-flow was up to 40 % cleaner compared to the single jet. The increased area ratio of outer nozzle to inner nozzle from 1.0 to 3.0 improved the cleanness of the air by at least 10 % to the distance from 8 to 14 initial diameters compared to a single jet. The co-flow jet with velocity ratio of 1.5 had longest concentration core length, Lu, (up to 6 initial diameters to the inner jet) with clean air. The results can be used for design of occupant targeted ventilation.

Study of NO and CO Formation Pathways in Jet Flames with CH4/H2 Fuel Blends

The existing natural gas transportation pipelines can withstand a hydrogen content of 0 to 50%, but further research is still needed on the pathways of NO and CO production under moderate or intense low oxygen dilution (MILD) combustion within this range of hydrogen blending. In this paper, we present a computational fluid dynamics (CFD) simulation of hydrogen-doped jet flame combustion in a jet in a hot coflow (JHC) burner. We conducted an in-depth study of the mechanisms by which NO and CO are produced at different locations within hydrogen-doped flames. Additionally, we established a chemical reaction network (CRN) model specifically for the JHC burner and calculated the detailed influence of hydrogen content on the mechanisms of NO and CO formation. The findings indicate that an increase in hydrogen content leads to an expansion of the main NO production region and a contraction of the main NO consumption region within the jet flame. This phenomenon is accompanied by a decline in the sub-reaction rates associated with both the prompt route and NO-reburning pathway via CHi=0–3 radicals, alongside an increase in N2O and thermal NO production rates. Consequently, this results in an overall enhancement of NO production and a reduction in NO consumption. In the context of MILD combustion, CO production primarily arises from the reduction of CO2 through the reaction CH2(S) + CO2 ⇔ CO + CH2O, the introduction of hydrogen into the system exerts an inhibitory effect on this reduction reaction while simultaneously enhancing the CO oxidation reaction, OH + CO ⇔ H + CO2, this dual influence ultimately results in a reduction of CO production.

Investigation of hydrogen/air co-flow jet flame propagation mechanism in supersonic crossflow

The design of fuel injection schemes is crucial for improving the combustion performance of high-Mach number scramjet. To clarify the feasibility of the coaxial jet injection scheme, high fidelity Large Eddy Simulation of the supersonic coaxial jet flame is conducted. The simulations are in good agreement with the experimental results in terms of time-averaged velocity, temperature, and species distribution. Auto-ignition phenomenon and the characteristics of partially premixed flame are well captured. The introduction of co-flow air increases the vorticity magnitude close to the injection port and downstream near-wall region, which results in a 400 K rise in the time-averaged temperature on the downstream near-wall region and a 4% increase in the proportion of premixed combustion near the injection port. Moreover, the instantaneous distribution of hydroxy radical indicates that the spanwise width of the windward reaction shear layer is reduced utilizing the coaxial jet scheme. Chemical kinetic analysis is applied to reveal the propagation mechanism of partially premixed flames. Thermal explosion is the chemical explosion mode for all coaxial jet flame front, which are dominated by a high-temperature reaction path. The low-temperature reaction path mainly exists in the transverse jet injection port, downstream near-wall region of the single transverse jet and co-flow lifted flame base. These significant findings provide valuable insights for the potential engineering application of the coaxial jet injection scheme to a high Mach number scramjet.

Manufacturing process of liposomal Formation: A coarse-grained molecular dynamics simulation

A method of producing liposomes has been previously developed using a continuous manufacturing technology that involves a co-axial turbulent jet in co-flow. In this study, coarse-grained molecular dynamics (CG-MD) simulations were used to gain a deeper understanding of how the self-assembly process of liposomes is affected by the material attributes (such as the concentration of ethanol) and the process parameters (such as temperature), while also providing detailed information on a nano-scale molecular level. Specifically, the CG-MD simulations yield a comprehensive internal view of the structure and formation mechanisms of liposomes containing DPPC, DPPG, and cholesterol molecules. The importance of this work is that structural details on the molecular level are proposed, and such detail is not possible to obtain through experimental studies alone. The assessment of structural properties, including the area per lipid, diffusion coefficient, and order parameters, indicated that a thicker bilayer was observed at higher ethanol concentrations, while a thinner bilayer was present at higher temperatures. These conditions led to more water penetrating the interior of the bilayer and an unstable structure, as indicated by a larger contact area between lipids and water, and a higher coefficient of lipid lateral diffusion. However, stable liposomes were found through these evaluations at lower ethanol concentrations and/or lower process temperatures. Furthermore, the CG-MD model was further compared and validated with experimental and computational data including liposomal bilayer thickness and area per lipid measurements.

Effect of increasing bypass ratio on supersonic co-flowing jet with finite lip thickness

The article presents the investigation on the effect of increasing bypass ratio on a supersonic co-flowing jet with finite lip thickness for a jet exit Mach number of 2.0. Co-flowing jets of differing bypass ratios (BR = ms/mp, where ms and mp are the mass flow rate of secondary and primary jets, respectively) 3.69, 9.0, and 16.0 were employed to explore the impact of high bypass secondary jets on the primary jet. A single-jet BR 0 is used for comparison. The supersonic core lengths of the single jet and the co-flow jet for BR 3.69, BR 9.0, and BR 16.0 were metrics for quantifying the mixing. The surrounding jet enhances the wake dominance in the nozzle lip, and the supersonic core length shortens. High bypass co-flowing jets are extremely efficient at shortening the supersonic core length compared to the low bypass counterpart, and hence, the jet mixing increases. The radial Mach number profiles quantitatively represent different zones in the co-flowing jet. The variation of these zones with bypass ratios highlights the dynamic interaction between the primary and secondary jets. The physical reason for apparent jet mixing and flow categories has been discussed based on considerations about changes in the flow field and shock structure.

Comparative Analysis of Aerodynamic Lift and Drag of Commercially Used Air-foil Incorporated with Co-flow Jet

In this research, a baseline airfoil is modified by incorporating a Co-flow Jet into the airfoil. Co-flow jet is also called zero net mass flux as the same amount of highly pressurized air is injected towards the streamlines of an airfoil from the leading edge and sucks this air without changing the amount in the trailing edge using an air pump. A performance boost is expected for the use of the Co-flow Jet. An airplane’s performance is based on a few parameters such as the coefficient of lift, coefficient of drag, boundary layer separation, and many others. In this research, a comparative analysis is run between a baseline airfoil and a CFJ airfoil. NACA-4412 is chosen as the target airfoil. The research shows how the incorporation of CFJ in an airfoil increases the coefficient of lift (CL) and decreases the coefficient of drag (CD). Thus, it is possible to maintain a better CL/CL ratio by using CFJ. The research also mentions the increase in stall angle due to the use of CFJ. It is a computational analysis performed in the software Ansys. For simplicity, the analysis is limited to two-dimensional analysis. This research may bring a better improvement in the aircraft and aerodynamic sectors. Though the results obtained from the research may not fully be similar to the hypothesis, it has been tried to maintain that the results of the simulation hardly differ from the theoretical analysis.

Wing design optimization and stall analysis with Co-flow Jet active control

Coupled with Co-flow Jet (CFJ) technology, the Non-dominated Sorting Genetic Algorithm II was utilized for the multi-objective combination optimization of an optimized Co-flow Jet wing, based on National Advisory Committee for Aeronautics (NACA) 6421. A high-precision numerical simulation using the delayed detached eddy simulation model was performed on the optimized wing to investigate the three-dimensional flow separation characteristics after static stall. The stall improvement was investigated by adjusting the momentum coefficient of the injection. The results show that the optimized wing exhibits significant improvements in aerodynamic performance and corrected aerodynamic efficiency. At an angle of attack of 10°, the average lift increased by 16.25% and the drag decreased by 27.23% compared to the CFJ6421 wing, while effectively addressing the problem of low modified aerodynamic efficiency of the CFJ wing at lower angles of attack. By utilizing higher momentum and improving the boundary layer control capability, flow separation is effectively suppressed, thus achieving the goal of stall recovery of the CFJ wing.

Closed-loop Flow Control Method Based on Deep Reinforcement Learning using a Co-flow Jet

Closed-loop Flow Control Method Based on Deep Reinforcement Learning using a Co-flow Jet

Large eddy simulation of hydrogen/air co-flow jet flame in the strut-based supersonic combustor

The exploration of efficient combustion organization schemes is crucial as the expansion of scramjet to higher Mach number becomes a cutting-edge trend. This paper evaluates the combination of strut flameholder and co-flow jet injection scheme through Large Eddy Simulation. The study primarily investigates the combustion stabilization and flame blowout characteristics in various internal circular airflow rates and coaxial/eccentric design scenarios. The simulation results show that the partially premixed flame dominates the combustion process. The findings reveal that the proposed scheme can quickly increase the mixing and combustion efficiency to 20 % and 80 % within a 30 mm range post-strut. The mass flow rate setting of the inner air jet plays a pivotal role in determining the combustion process. Optimal combustion performance is attained with the use of an internal air jet with a diameter of 1 mm in the current configuration. Although an eccentric design is employed to enhance mixing, a 1.5 mm diameter air jet fails to maintain flame stabilization due to the influence of cold high-speed air on the velocity and energy of the recirculation zone. These conclusions provide valuable insights for the design of high-Mach scramjet.

Thermodynamic analysis of nano-electrospray induced gas jets

Nano-electrospray (nES) produces a plume of charged liquid droplets which have drag interactions with the surrounding gas after they are emitted from a source capillary. The resulting induced gas flow has often been considered unimportant, but we have demonstrated that a gas microjet of significant velocity up to 10s of m/s can be produced. In this work we present a thermodynamic framework that enables analysis of gas jet generation from electrosprays and introduce the important metrics for such analysis, effectiveness (a measure of momentum transfer from the electrosprayed aerosol to gas) and efficiency (a measure of energy conversion from electrical energy generating the electrospray to gas kinetic energy). This analytical framework is applicable to any two-phase flows consisting of discrete conservative-force-driven particles which exchange momentum with an inert, otherwise quiescent fluid medium to yield a co-flowing two-phase jet. We apply this framework to sprays of water from nano-electrospray emitters to demonstrate that increasing the applied electrical potential difference, increasing liquid mass flowrate, and decreasing droplet size all can increase electrospray induced gas jet strength, but only the latter two do so while also increasing the momentum transfer effectiveness and energy conversion efficiency.

An elementary model for an advancing autoignition front in laminar reactive co-flow jets injected into supercritical water

In this paper we formulate and analyze an elementary model for the propagation of advancing autoignition fronts in reactive co-flow fuel/oxidizer jets injected into an aqueous environment at pressures exceeding the critical point of water. This work is motivated by the experimental studies of autoignition of hydrothermal flames performed at the high pressure laboratory of the NASA Glenn Research Center. Guided by experimental observations, we use several simplifying assumptions that allow the derivation of a simple, still experimentally feasible, mathematical model for the propagation of advancing ignition fronts. We analyze the model by means of asymptotic and numerical techniques and discuss feasible regimes of propagation of advancing ignition fronts. We hope that the present study will be helpful for the interpretation of existing experimental data and guiding of future experiments.

Influence of velocity on the transient auto-ignition of ethylene jet in a hot coflow

The difficulty of ignition is one of the main problems in the combustion chamber of the air-breathing combined engine. Based on this, the transient auto-ignition in the turbulent non-premixed flames of ethylene in a high-speed jet in hot coflow was studied using a newly developed burner. High-speed hydroxyl (OH) planar laser-induced fluorescence (up to 10 kHz) and large eddy simulations (LES) were performed to investigate the auto-ignition process. The experimental conditions included injection velocities of 334–491 m/s, and the effect of jet velocity on the mixing process, ignition process, and flame stabilization mechanisms were investigated. The results showed that the numerical simulation results were consistent with the experimental data. The velocity could affect the auto-ignition position, whereas it had no effect on the auto-ignition time. The increase in fuel jet velocity is beneficial to the mixing of fuel and high-temperature gas and helps to increase the premixed combustion zone during the formation of fire kernels. Moreover, as the velocity increased, the flame status changed from a transitional flame to a lifted flame, the growth rates of the flame volume and total heat release increased, and the flame stabilization mechanism changed from flame propagation to auto-ignition.

Distortion elimination for serpentine duct at various Mach numbers using co-flow jet active flow control

This paper numerically investigates Co-flow Jet (CFJ) active flow control (AFC) for eliminating the AGARD M2129 serpentine inlet distortion with throat Mach number (Mth) varying from 0.42 to 0.79. The configuration was optimized at design Mth of 0.79 in a prior study. The current objective is to investigate the CFJ S-duct performance at off-design Mth with the fixed geometry. The high-order FASIP in-house code is used to conduct 3D Reynolds Averaged Navier-Stokes (RANS) simulation with the one-equation Spalart-Allmaras (S-A) turbulence model. The CFD simulation is well validated with the experiment of AGARD test cases in the studied Mach numbers. Results show that CFJ virtually eliminates the flow distortion for all the studied Mth. However, the off design Mth have lower energy efficiency. At the design Mth, CFJ requires the lowest power coefficient (Pc) to eliminate the flow separation and distortion. The exergy analysis indicates that CFJ S-duct at the design Mth has the highest system energy efficiency with the EIPR (ratio of exergy increase to the power required) exceeding 1. At the off-design Mth, EIPR is lower than 1 because the shifts of separation-onset spot and adverse-pressure-gradient region make CFJ injection and suction no longer at the most efficient location. This leads to the rising of the CFJ power coefficient even though the actual power is less. Nevertheless, the overall system is still very efficient at off-design Mth for the CFJ S-duct with the CFJ energy expenditure recovered by 72%, 90%, 96% for Mth of 0.42, 0.55, and 0.69, respectively.

Experimental design for a novel co-flow jet airfoil

The Co-flow Jet (CFJ) technology holds significant promise for enhancing aerodynamic efficiency and furthering decarbonization in the evolving landscape of air transportation. The aim of this study is to empirically validate an optimized CFJ airfoil through low-speed wind tunnel experiments. The CFJ airfoil is structured in a tri-sectional design, consisting of one experimental segment and two stationary segments. A support rod penetrates the airfoil, fulfilling dual roles: it not only maintains the structural integrity of the overall model but also enables the direct measurement of aerodynamic forces on the test section of the CFJ airfoil within a two-dimensional wind tunnel. In parallel, the stationary segments are designed to effectively minimize the interference from the lateral tunnel walls. The experimental results are compared with numerical simulations, specifically focusing on aerodynamic parameters and flow field distribution. The findings reveal that the experimental framework employed is highly effective in characterizing the aerodynamic behavior of the CFJ airfoil, showing strong agreement with the simulation data.

Lifting of Single and Coaxial Nonpremixed Jet Flames Under Transverse Acoustic Forcing

The dynamics of gaseous reacting turbulent nonpremixed single and coaxial jet flames in unforced and acoustically forced environments was investigated at atmospheric pressure. Two burner configurations were used: one having a single fuel jet and one having a coaxial jet of fuel and oxidizer streams. The flames were exposed to transverse acoustic waves tuned to produce pressure antinode acoustic forcing at various frequencies and amplitudes. The flame responses were recorded using high-speed Schlieren and OH* chemiluminescence imaging. The center jet was set to be the same between the single and the coaxial jets at two different Reynolds numbers. The annular flow of the coaxial jet was set to have the same bulk velocity as the ambient coflow, a configuration here called a coaxial single jet. The configuration was so named because it was expected that a coaxial jet having the same annular bulk velocity as the ambient coflow would produce results similar to that of the single jet in the same coflow. It was found instead that the flame lifting behavior was considerably different, even qualitatively. The flow visualizations revealed differences in large-scale structures that explain the different lifting regimes.

Wind tunnel experimental study of aerodynamic characteristics of co-flow jet wing with built-in air source

To further investigate the aerodynamic characteristics and control mechanisms of co-flow jet wings, this study presents the design of a co-flow jet wing (CFJ-CLARKY18) with an embedded micro centrifugal compressor. The CFJ-CLARKY18 wing, based on the CLARKY18 airfoil, incorporates a miniature centrifugal compressor positioned horizontally within the wing's cavity, with its rotor axis perpendicular to the wing chord line, thus alleviating geometric constraints and the compressor's exhaust swirl. The integration of the co-flow jet device with the wing is achieved through a split-flow channel design to ensure uniform co-flow jet injection, followed by verification of jet uniformity and calibration of six jet momentum coefficients. Subsequently, static and dynamic control experiments of the co-flow jet wing are conducted in an open-return wind tunnel, utilizing force measurement systems and high-speed particle image velocimetry tools to investigate its aerodynamic characteristics and control mechanisms. Static stall tests indicate that, compared to the original wing, the CFJ-CLARKY18 wing exhibits a significant increase in lift and a noticeable delay in stall angle. At low angles of attack, an increase in co-flow jet momentum leads to a reduction in drag coefficient, with the possibility of achieving negative drag (thrust) even at high co-flow jet momentum coefficients. The results of dynamic stall tests with force measurements indicate that, while keeping the reduced frequency constant, an increase in co-flow jet momentum coefficient results in higher maximum lift and average lift coefficients, accompanied by a significant reduction in average drag coefficient and the hysteresis loop area of the lift coefficient. Despite the increased reduced frequency exacerbating the wing's stall effect, the stall effect weakens significantly once the co-flow jet momentum coefficient is increased.

Automated adaptive chemistry for Large Eddy Simulations of turbulent reacting flows

Large Eddy Simulations (LES) of turbulent reacting flows carried out with detailed kinetic mechanisms have a key role for the discovery of the physical and chemical processes occurring in combustion systems, and are essential for the development of efficient, stable, and non-pollutant technologies. Nevertheless, these simulations require a large amount of computational resources, making their utilization for large-scale systems, such as industrial burners and gas turbines, impractical. In this work, we combine state-of-the-art machine learning algorithms and model reduction methods to deliver a fully automated strategy for performing LES with adaptive chemistry. This strategy is based on the Sample-Partitioning Adaptive Chemistry (SPARC) algorithmic procedure, which consists of four steps: the generation of a training dataset, its partitioning in clusters, the generation of a set of reduced chemical mechanisms specifically tailored to each cluster and, lastly, the numerical simulation of the case of interest with adaptive chemistry enabled by an on-the-fly classification of every grid point.The SPARC approach has already been demonstrated to substantially reduce the computational effort of reactive flows simulations. However a non-negligible level of user interventions is needed, upon which the method’s success critically depend. Therefore, with the goal of boosting the performance of this workflow and minimise the user-specified degrees of freedom, we plug in and exploit the Local Principal Component Analysis augmented with an automated Bayesian-optimised search for optimal clustering solutions, and the Computational Singular Perturbation method with an additional layer of automation based on the Tangential Stretching Rate for minimally-sized reduced mechanisms. We employ a cheap and easy-to-generate 1-dimensional-flames training database and we demonstrate the efficiency, accuracy and robustness of this strategy with an application to LES of the Adelaide Jet in Hot Coflow (AJHC) burner, a turbulent reacting flow exhibiting intense turbulence-chemistry interactions.

Optimization of Co-Flow Jet Parameters for Ahmed Body Application

This study evaluates the drag reduction strategy of suction and blowing on idealize automotive vehicle, Ahmed Body. Optimization approach is adapted in order to analyse the effect of slot location, momentum coefficient and slot angle on the vehicle which experiencing drag. Despite all the efforts that have been done to reduce the Ahmed body drag using various active flow control system, most of the drag reduction were only less than 15%. A 25° Ahmed body with build in co-flow jet is modelled using a CAD software. The flow around the Ahmed body is simulated at Reynolds number based on length Re = 4.29 × 106. The governing equation were solve using an open source software package, which has been validated against experimental data. Pressure Implicit with Splitting of Operator (PISO) algorithm is applied to solve the equation. The outcome of the simulation are varies depending on the variables. Some show a decrease in drag while there are also that actually increase the drag of the system. This are caused by the suction and blowing slots that effect the surrounding air flow whether it is reducing or increasing the wake size downstream of the body. The result shows the momentum coefficient and location of both suction and blowing jet played an important role of manipulating the flow around the body and reducing the drag. The velocity contours indicated that the key to drag reduction is by using 40 m/s jet velocity, placement of suction and blowing away from each other.

Computational analysis of influence of CFJ components on aerodynamic performance

This research aims to explore the effect of the co-flow jet (CFJ) airfoil injection jet velocity, injection height, and injection mass flow rate on the aerodynamic coefficient through numerical analysis. Five distinct circumstances are generated by adjusting the jet velocity, injection height, and mass flow rate, and each of the cases underwent numerical investigation. For this computational analysis, Reynolds averaged Navier–Stokes (RANS) solver has been utilized by employing the Spalart–Allmaras turbulence model, and the flow is treated as incompressible. The higher injection slots reduce the coefficient of lift (CL) due to poor aerodynamic shape, which can be overcome by injecting higher jet velocities, whereas lower jet velocities injecting into lower injection slots do not raise the CL even though they have streamlined aerodynamic shape. Hence, even if CFJ airfoils with poor geometry perform less aerodynamically, this problem is not intractable because it can be solved by injecting high jet velocities; meanwhile, injecting low velocities will result in reduced aerodynamic performance for CFJ airfoils with good geometry. The CFJ airfoil with the highest jet velocity and lowest injection slot increases the CL by a maximum of 65% compared to the baseline airfoil, which is higher than all other CFJ airfoils taken into consideration.

Influence of orifice thickness on the flow field of co-flow jets

The present study analyses the numerical simulation results of subsonic flow through co-flow orifices. Analysis is done on the effects of modifying orifice thickness on the co-flow jet characteristics. The combined frontal area of the co-flow jet and central jet is maintained constant (20 mm2). The primary orifice diameter (D) is kept as 3mm and the orifice thickness, i.e the co-flow jet is formed as four segments of a concentric circle around the core jet, and the edge distance between them ranges from 0.16D to 0.5D. The 0.16D co-flow is shown to lengthen the potential core of the main jet at nozzle pressure ratios of 1.2, 1.4, 1.6, and 1.8, but the 0.33D co-flow and the 0.5D co-flow shorten the potential core length, enhancing mixing. The present study analyses the Mach numbers that correspond to the Nozzle Pressure ratios of 1.2, 1.4, 1.6, and 1.8, which are 0.517, 0.71, 0.84, and 0.96, respectively. The 0.5D annular gap is found to be more effective which boosts primary jet mixing.