Counterflow Jet Research Articles

Ignition of CH4 intensely diluted with CO2 versus hot O2/CO2 with high oxygen concentration in a counterflow jets

Lean combustion has wide range of applications due to very low emissions and very high efficiency. The study of the ignition of the fuel under dilution conditions is crucial for the satisfactory operation and performance of dilution fuel combustion processes. In present study, the ignition temperatures of CH4 diluted with CO2 were measured in a counterflow ignition facility for the mixtures of 9–25% of CH4 in CO2 and 30%, 35%, and 40% oxygen concentration in the O2/CO2 atmospheres, respectively. The experimental results show that for a fixed fuel concentration, the higher the oxygen concentration, the lower the ignition temperature. The differences between ignition temperatures of CH4 for various oxygen concentration cases decrease notably with the decrease of the CH4 concentration, and are not obvious when the fuel is intensively diluted at 9%. A mechanism has been updated based on our previous research work and named as IDIM (Intensive dilution ignition mechanism). The performance of the USC Mech 2.0, Aramco Mech 2.0, FFCM-1 and IDIM has been evaluated. IDIM can predict well the ignition temperatures of three different oxygen concentrations, while FFCM-1 markedly over-predicts the experimental results for all the cases. The effects of the oxidation pathways of CH3, CH3→CH3O, CH3→CO, CH3→CH2O and CH3→CH2(S), on the ignition temperature have been analyzed in detailed. The pathway of CH3→CH3O→CH2O plays an important role on the ignition when the oxygen concentration is high in the oxidant and the fuel is intensely diluted. Weakening the path of CH3→CH3O→CH2O is the main reason for the over-prediction of FFCM-1. Moreover, the effect of the fuel dilution on the ignition temperature is stronger than that of the oxygen concentration at the fuel concentration of 9%, resulting in that the conversion ratios of CH3 to CH3O, CH2O, CH2(S), and CO are almost same for 30% and 40% oxygen concentrations. Therefore, the ignition temperature is almost insensitive of the oxygen concentration within the range of our experiment.

Drag and heat flux reduction mechanism induced by the combinational forward-facing cavity and pulsed counterflowing jet configuration in supersonic flows

The drag and heat flux reduction characteristics plays a very important role in the conceptual design phase and engineering realization of the aerospace vehicle. In the current study, a novel thermal protection system combining the forward-facing cavity and the pulsed counterflowing jet has been proposed, and three groups of the combinations with the same pulsed counterflowing jet but different forward-facing cavities are established. The waveform and period of the pulsed counterflowing jet are triangular waveform and T = 2 ms respectively, and the diameter of the jet nozzle is set to be d = 4 mm. The forward-facing cavities with L/D = 1.25, 1.33 and 1.75 has been taken into consideration. The cavity is placed at the nose of the blunt body, and the jet nozzle is placed at the center of the cavity surface. In the numerical investigation, an axisymmetric numerical simulation model of the counterflowing jet on the supersonic vehicle nose-tip is established, and a two-dimensional axisymmetric Reynolds-averaged Navier-Stokes (RANS) equations coupled with the two equation k-ω shear stress transport (SST) turbulence model are employed. The wall Stanton number distributions, as well as the surface static pressures, have been extracted from the flow field structures in order to evaluate the drag and heat flux reduction characteristics. Further, the influence of the length-to-diameter ratio of the cavity on the drag and heat flux reduction has been analyzed based on the wall Stanton number and surface pressure distributions. The obtained results show that The combinational forward-facing cavity and pulsed counterflowing jet configuration with L/D = 1.33 owns the best drag and heat flux reduction performance in the range considered in this paper, and the smaller cavity volume is beneficial for the stability of the flow field. There is an optimal value for the length-to-diameter ratio of the forward-facing cavity, and the optimal configuration for the thermal protection may be not the same as that for the drag reduction. This should be dealt with the multi-objective design optimization approaches.

Unsteady actuation of counter-flowing wall jets for gust load attenuation

Counter-flowing wall jets actuated on the upper surface of an airfoil are investigated for the purpose of controlling gust encounters. For periodic and transient actuations, phase-averaged lift force and particle image velocimetry measurements are presented for a NACA 0012 airfoil, at a Reynolds number of 660,000, for a range of reduced frequencies and three jet locations, xJ/c=0.08, 0.60 and 0.95. For periodic actuation, amplitude of lift oscillations decrease and phase delay increase with increasing reduced frequency. The effect of reduced frequency on the amplitude and phase is more significant for blowing locations near the leading-edge and with increasing angle of attack. Transient actuation reveals the slow response of the separated flow, and therefore lift, with the delay becoming more pronounced for blowing near the leading-edge. Estimated time constants are similar to previous observations for forced separation and reattachment.

Drag and heat flux reduction induced by the pulsed counterflowing jet with different waveforms on a blunt body in supersonic flows

The drag and heat flux reduction characteristics plays a very important role in the conceptual design phase and engineering realization of the aerospace vehicle. In the current study, the flow field properties around a blunt body with three different pulsed counterflowing jets in the supersonic flow with the freestream Mach number being 3.98 are investigated numerically. In this paper, there are three different kinds of pulsed jets with the sinusoidal, triangular and rectangular waveforms are established, and the periods of the pulsed jets are all set to be T = 1.0 ms. The jet nozzle is placed at the nose of the blunt body. In the numerical investigation, an axisymmetric numerical simulation model of the counterflowing jet on the supersonic vehicle nose-tip is established, and the two-dimensional axisymmetric Reynolds-averaged Navier-Stokes (RANS) equations coupled with the two equation k-ω shear stress transport (SST) turbulence model are employed. The wall Stanton number distributions, as well as the surface static pressures, are extracted from the flow field structures in order to evaluate the drag and heat flux reduction characteristics. Further, the influence of the pulsed jet waveform on the drag and heat flux reduction is analyzed based on the wall Stanton number and surface pressure distributions. The obtained results show that the variations of the wall Stanton number and surface pressure distributions induced by the pulsed jet with the same period but different waveforms, all have an obvious periodicity and hysteresis phenomenon. At the same time, it is found that the drag and heat flux reduction under the triangular wave has the best effect, and the pulsed jet with the triangular wave has a better comprehensive performance than the other two waveforms.

Turbulence structure of a counter-flowing wall jet

Although several studies have been carried out to analyze the characteristics of counter-flowing wall jets (CFWJs), their internal turbulence structure is yet to be fully understood. In this paper, a CFWJ issuing into a main flow is numerically examined using a three-dimensional, unsteady, improved delayed detached-eddy simulation. The results of the simulation are validated with the experimental results and are presented with pertinent discussions. The interaction of the jet with the wall and the main flow leads to the oscillation of the stagnation point (SP), further enhancing the turbulence. The analysis of the instantaneous flow field reveals the presence of a feedback mechanism between the stagnation region and the shear layer of the counter-flowing wall, resulting in the excitation of the jet. This feedback mechanism is responsible for the oscillation of the SP. To further evaluate the internal structure of turbulence, the coherent structures within the flow are identified using a vortex identification criterion. These structures are also quantitatively evaluated using proper orthogonal decomposition. The analysis of the coherent structures revealed the presence of large-scale structures in the vicinity of the SP. The complex dynamics of these structures and their interactions is presented with pertinent discussions.

Numerical investigation of an active TPS for a wing leading edge exposed to high temperature air behind a strong bow shock wave

An array of subsonic counter-flow jets is studied as an active thermal protection system (TPS) for wing leading edges of hypersonic vehicles. The performance is numerically estimated in the model case of a circular cylinder on the basis of the 2D compressible Navier-Stokes equations. In contrast to a single subsonic jet, an array of jets is robust against variation of the angle of attack; high cooling effectiveness is confirmed up to 5° variation. The coolant gas (air) discharged from channels embedded in the cylinder covers over a wide range of the front surface of the cylinder. The feasibility of the active TPS is also discussed.

Numerical study of hydrodynamic perturbations caused by filiform spark discharge near wall

A numerical simulation of the high-speed jet caused by the decay of the thermal cavern after cylindrical 10 mm long spark discharge located on the wall in ambient air is performed. It is shown that after the shock wave retreats, two counter-flowing jets are formed in a low-density hot gas, moving inside the thermal cavern from the electrodes to the center of the area. A strong distortion of the hot gas region is observed and a cold gas suction into the heat cavern is appeared. At the next phase, the formed hot gas jets collide, and the flow is directed radially from the axis of the initial discharge channel. Finally, the hot low-density gas takes the form of half the toroid, which size increases in time. The central part of the flowfield is occupied with the cold gas. The jet velocity and momentum are simulated at variation of the discharge energy (5-500mJ). The simulation results are considered to be consistent with the experimental data previously published by the authors.

Experimental study of the swirling flow effect on the efficiency of the local ventilation system

The paper presents the results of an experimental study of the local ventilation system. It is shown that the use of radially swirled counter-flow jet leads to significant increase in the efficiency of gas removing. This fact is confirmed by the flow visualization for different operating regimes of the device (with and without swirling). The velocity distribution along the central axis of the device was also measured. It is also shown that the use of swirling flow leads to significant increase in the velocity of removal flow. Study of the system operating parameters for different direct and counter flow ratio was carried out.

Numerical Study of the Effects of a Counterflow Jet on the Drag Reduction of a Blunt Body in a Hypersonic Flow

A numerical investigation has been conducted for the drag reduction of a counterflow jet with a blunt body at Mach 6. The computational study was carried out by solving two-dimensional axisymmetric Reynolds-averaged Navier–Stokes (RANS) equations. The Spalart–Allmaras one-equation turbulence model was used in this study. The jet flow of exit Mach number 2.86 interacts with the free stream and changes the single shock structure to a multiple shock structure. The purpose of this study was to investigate the influence of the stagnation pressure based on a parameter, the momentum parameter ratio (MPR), which can characterize the jet. The result shows that the flow field can be categorized as being in a long penetration mode (LPM) or short penetration mode (SPM) depending on the penetration length of the free-stream flow. In the LPM, under overexpanded jet conditions, the shock structure fluctuates continuously, so the flow field is unstable. On the other hand, in the SPM, the shock structure is nearly fixed, and the entire flow field is stable. For this reason, even if the penetration length of the flow field in the SPM is less than that in the LPM, the aerodynamic drag can be reduced by up to 40%. Therefore, it becomes clear that jet pressure conditions can significantly improve aerodynamic performance by reducing the drag applied to the blunt body.

Ignition of CH4 intensely diluted with N2 and CO2 versus hot air in a counterflow jets

The ignition temperatures of CH4 intensely diluted with N2 and CO2 have been measured in a counterflow ignition facility with mixtures of 6–25% CH4 in N2 and 9–25% CH4 in CO2. The simulation by GRI Mech 1.2, GRI Mech 3.0, USC Mech 2.0, and Aramco Mech 2.0 cannot predict well the present experimental results. A modified mechanism has been proposed by substitution of the news channels of CH3 + O and CH2 + O2 for the corresponding channels in USC Mech 2.0 and updating some reaction accordingly. The modified mechanism has been verified by present experiments and other experiments under the intensely diluted CH4 condition. The analysis by the modified mechanism shows that when CH4 is intensely diluted with N2 or CO2, the path of CO → CO2 becomes important to produce ignition, because the reaction CO + OH = CO2 + H can provide sufficient radical H for the rapid growth of the radical pool, thus, the main oxidation chain of CH3 is extended furtherly to CO2, that is, CH3 → CH2O → HCO → CO → CO2. The channels of CH3 + O becomes increasing important for the ignition with the decease of the CH4 concentration, because the reaction CH3 + O = CO + H + H2 significantly promotes the ignition of the intensively diluted CH4 due to facilitating the rapid growth of the radical pool. Consequently, the secondary CH3 oxidation path, CH3 → CO → CO2, makes additional but critical contribution to produce ignition. The channels of CH2 + O2 are CH2 + O2 = CO + H2O, CH2 + O2 = CO2 + H + H, CH2 + O2 = CH2O + O, CH2 + O2 = CO2 + H2, and CH2 + O2 = CO + OH + H. Among the channels of CH2 + O2, the variation of the effects of the reaction CH2 + O2 = CO + OH + H on the ignition is the most significant when the concentration of CH4 changes from 20% to 6%. Therefore, the third CH3 oxidation path, CH3 → CH2(s) → CH2 → CO → CO2, makes evident contribution to produce ignition under intensely diluted fuel condition due to CH2 + O2 = CO + OH + H. When diluent gas N2 is replaced by CO2 in intensely diluted CH4, the ignition temperature in the CH4/CO2 is higher than that in the CH4/N2, which is mainly attributed to the higher heat capacity of CO2 compared to N2.

Drag and heat flux reduction induced by the pulsed counterflowing jet with different periods on a blunt body in supersonic flows

In the current study, three different kinds of pulsed jets with the period being 0.5 ms, 1.0 ms and 2.0 ms are established, and sinusoidal waveforms are used in all three pulsed jets. The pulsed counterflowing jets with the same amplitude but different periods on the nose of a blunt body in supersonic flows are investigated numerically, and an axisymmetric numerical simulation model of the counterflowing jet on the supersonic vehicle nose-tip is established. The obtained results show that the wall Stanton number and surface pressure change periodically with the pulsed jet, and the variations of the parameters show a strong hysteresis phenomenon. The hysteresis phenomenon becomes less significant as the period increases. At the same time, a better drag and heat flux reduction is obtained under larger period pulsed jet conditions. With the increase of the period of the pulsed jet, there is a significant decline in the maximum peak values of the wall Stanton number and surface pressure. Compared with the steady jet, the pulsed jet is more effective in heat flux reduction, but the drag reduction of the pulsed jet is less effective than the steady jet.

Numerical experiment on the flow field properties of a blunted body with a counterflowing jet in supersonic flows

The prediction of the drag and heat flux reduction characteristics is a very important issue in the conceptual design phase of the hypersonic vehicle. In this paper, the flow field properties around a blunted body with a counterflowing jet in the supersonic flow with the freestream Mach number being 3.98 were investigated numerically, and they are obtained by means of the two-dimensional axisymmetric Reynolds-averaged Navier-Stokes (RANS) equations coupled with the two equation standard k-ε turbulence model. The surface Stanton number distributions, as well as the surface static pressures, were extracted from the flow field structures in order to evaluate the drag and heat flux reduction characteristics. Further, the influences of the jet pressure ratio and the jet Mach number on the drag and heat flux reduction were analyzed based on the detailed code validation and grid independency analysis process. The obtained results show that the flow cell Reynolds number has a great impact on the heat flux prediction, and its best value is 5.0 for the case studied in the current study. However, the flow cell Reynolds number and the grid scale both have only a slight impact on the prediction of the surface static pressure distribution, as well as the turbulence model. The larger jet pressure ratio is beneficial for the drag and heat flux reduction, and the smaller jet Mach number is beneficial for the heat flux reduction. Further, the long penetration mode is beneficial for the drag reduction, but it is not beneficial for the heat flux reduction.

Numerical investigation of drag and heat flux reduction mechanism of the pulsed counterflowing jet on a blunt body in supersonic flows

To design a kind of aerospace vehicle, the drag and heat flux reduction are the most important factors. In the current study, the counterflowing jet, one of the effective drag and heat flux reduction concepts, is investigated numerically by the two-dimensional axisymmetric Reynolds-averaged Navier-Stokes equations coupled with the SST k-ω turbulence model. An axisymmetric numerical simulation mode of the counterflowing jet on the supersonic vehicle nose-tip is established, and the numerical method employed is validated by the experimental schlieren images and experimental data in the open literature. A pulsed counterflowing jet scheme is proposed, and it uses a sinusoidal function to control the total and static pressures of the counterflowing jet. The obtained results show that the long penetration mode does not exist in the whole turnaround, even in a relatively small range of the jet total and static pressures, and this is different from the phenomenon obtained under the steady condition in the open literature. At the same time, it is observed that the variation of the physical parameters, such as the Stanton number induced by the pulsed jet, has an obvious periodicity and hysteresis phenomenon.

Drag reduction investigation for hypersonic lifting-body vehicles with aerospike and long penetration mode counterflowing jet

This study describes numerical computations of aerospike and counterflowing jet as the drag reduction approach of a hypersonic lifting-body model for Mach 8 flow at 40 km altitude. The three-dimensional Reynolds-average Navier–Stokes equations and the laminar condition have been utilized to obtain the flow field properties. Both steady-state and time-accurate computations are performed for the models in order to investigate the drag reduction effect and the periodic oscillation characteristics of long penetration mode (LPM) jet. The obtained results show that both methods can significantly modify the external flowfields and strongly weaken or disperse the shock-waves of the vehicle, then achieve an obvious drag reduction effect in the range of small angles of attack. This value is 7.25% for the model with aerospike and 8.80% for the model with counterflowing jet at angle of attack of 6°. Compared with the aerospike, the counterflowing jet shows a better drag reduction effect in the gliding angle of attack, and this leads to a larger lift-to-drag ratio of 3.58. Meanwhile, the displacement of center of pressure of the whole vehicle is smaller in the flight phase. The oscillation frequency of the counterflowing jet at angle of attack of 6° is 444 Hz, and the oscillation characteristics of the drag force is due to the variation of the pressure distribution induced by the oscillation of the shock wave structure. The results may be of high practical significance and show the possibility of developing a feasible system using the counterflowing jet as an active flow control to reduce the drag force during the gliding phase with maximum lift-to-drag ratio.

초음속 환경에서 역분사 공기 제트의 압력 변화에 따른 유동 특성 및 항력 감소

초음속 환경에서 역분사 공기 제트의 압력 변화에 따른 유동 특성 및 항력 감소

Flow Separation Control with a Plasma Actuator Over a Metallic NACA 4418

The flow control over an airfoil was experimentally investigated with a Dielectric Barrier Discharge (DBD) plasma actuator. A NACA 4418 airfoil was used and the fact that it is metallic is the main difference with the numerous articles about plasma actuators over airfoils. Metallic airfoils are normally avoided to reduce risk of arcing and electromagnetic interferences. The plasma actuator was located at x/c = 0.1 measured from the leading edge. Through the Schlieren visualization, the characteristics of the actuator in a quiescent ambient was observed, showing a double wall jet due to the interaction of the plasma actuator and the metallic airfoil used as substrate. One of them is a co-flow wall jet and the other one is a counter-flow wall jet. Time-averaged Particle Image Velocimetry (PIV) images were used to examine the flow for different angles of attack at Rec = 40.000 and Rec = 200.000 when plasma is on and off. Velocity maps show that the plasma actuation delays the separation for both low and moderate Reynolds numbers. The turbulent kinetic energy (TKE) and Reynolds stress values are reduced when plasma can control the flow, however plasma actuator increases these values when it does not avoid the separation. The velocity magnitude profiles show the influence of the co-flow wall jet near exposed-electrode but the counter-flow is not observed. A reduction of the wake with plasma actuation is also seen. The results confirm the authority of plasma actuators to control the flow separation over a metallic airfoil. The effectiveness and the global performance in applications such as wind turbines or UAVs at moderate Reynolds number, where metallic airfoils are used, is expected to be improved avoiding the separation with plasma actuators.

Heat transfer reduction using combination of spike and counterflow jet on blunt body at high Mach number flow

Heat transfer reduction around blunt bodies is one of the important issues in the field of high speed aerodynamics. Using of spikes and counterflow jets each of them separately for reducing of drag force and heat transfer is well known. The present work is description of flow field around a hemispherical nose cylinder with a combination of spike and counterflow jet at free stream of Mach number of 6. The air gas was injected through the nozzle at the nose of the hemispherical model at sonic speed. In this numerical analysis, axisymmetric Reynolds-averaged Navier-Stokes equations was solved by k-ω (SST) turbulence model. The grid study was done and the results are validated with experimental results for spiked body without jet condition. Then the results presented for different lengths of spike and different pressures of counterflow jets. The results show a significant reduction in the peak heat transfer about 60%–78% for different models compared to the spherical cylinder model without any jet and spike. Furthermore, also our results indicate that the heat transfer reduction is increased even more with increasing of the length of the spike.

Experimental and numerical analysis of liquid counterflow jets: Study of resurgent jet generation

Experimental and numerical analysis of liquid counterflow jets: Study of resurgent jet generation

High-frequency counter-flow plasma synthetic jet actuator and its application in suppression of supersonic flow separation

We come up with a control strategy for suppression of supersonic flow separation based on high-frequency Counter-flow Plasma Synthetic Jet Actuator (CPSJA). The main purpose of this investigation is to verify if its control authority can be enhanced by the jet/shock interaction. We use a blunt nose to generate a bow shock, a step on a flat plate to introduce a massive separation in a Mach 2 wind tunnel, and the CPSJA to generate Plasma Synthetic Jet (PSJ). In this study, pulsed capacitive discharge is provided for an array of CPSJAs, which makes the actuation (discharge) frequency f1 = 1 kHz, f2 = 2 kHz and f3 = 3 kHz. We use the high-speed schlieren imaging and fast response pressure transducers as well as a numerical simulation to investigate the quiescent PSJ properties, the interaction between the jet and bow shock, and its disturbance effect on the downstream separated region. The schlieren images show that PSJ is characterized by a succession of vortex rings; the jet strength weakens with the increase of frequency. A 4.5 mN jet thrust is found for all the frequencies. The simulation results show that jet/shock interaction produces vorticity in the vortex ring of the jet, enhancing turbulent mixing in PSJ so that a great deal of momentum is produced into the flow. We found the downstream flow is significantly disturbed by the enhanced actuation. Actuation with frequency of f2, f3 which is close to the natural frequency fn of the separation bubble suppresses the separation with the upstream laminar boundary layer being periodically attenuated, which has a better control effect than f1. The control effect is sensitive to the position where PSJ interacts with the shear layer, but the amount of energy deposited in one pulse is not crucial in a separation reduction in the experiment.

Upstream penetration behavior of the developed counter flow jet resulting from multiple jet impingement in the crossflow of cylindrical duct

Experimental and numerical investigations of ultimate upstream penetration of developed counter flow jet formed as result of impingement of multiple round jets radially injected into a high-temperature confined crossflow have been performed. These investigations aimed to reveal an influence of strength of the developed counter flow jet on mixing intensity of jets in crossflow of cylindrical duct. Based on analysis of experimental data, the analytical dependence between dimensionless parameter of upstream penetration depth of counter flow jet and the square root of jets-to-mainstream momentum-flux ratio J has been established. The dependence proved to consist of linear region as well as nonlinear and asymptotic ones. For a given geometry an ultimate upstream penetration depth of the counter flow jet has been estimated to be approximately 2.1–2.3 diameter of the cylindrical duct. The corresponding value obtained on the base of numerical simulation turned out to be 1.8–2.0 diameter of the cylindrical duct. The dimensionless parameter h/D of radial jet penetration depth proved to be an appropriate to describe adequately an empirical character of upstream penetration of the counter flow jet within the linear region. Based on results of numerical simulation, axial velocity, temperature and pressure profiles as well as centerline turbulent kinetic energy contours have been obtained. It has been has also shown that an increase of the momentum-flux ratio promotes mixing in upstream recirculation flow zone and improves overall mixing performance as well.