Jet Mixing Enhancement Research Articles

Influence of thermodynamic conditions on the mixing characteristics of a supercritical nitrogen jet under mode excitation

Fuel injection mixing is one of the critical factors affecting the energy conversion efficiency and emission level of thermal engines. The effect of thermodynamic conditions on mixing characteristics of supercritical cryogenic nitrogen jet under varicose mode excitation is investigated using the unsteady Reynolds-averaged Navier-Stokes method combined with real-gas thermodynamics and transport properties of nitrogen in NIST database. The ambient-to-jet density ratio is defined to represent different thermodynamic conditions and is found to affect the jet mixing enhancement directly. As the density ratio increases from 0.098 to 0.432, the density gradient reduces in the jet shear layer, mitigating flow stratification. Jet mixing is enhanced with a reduction of density potential core length by 24 % and a tripled density spreading angle. Under varicose mode excitation, large vortices are formatted close to the jet orifice, especially for higher density ratio cases. The peak of the jet turbulent kinetic energy is higher and shifts upstream as density ratio increasing. The jet mixing enhancement is further achieved, with the spreading angle increment growing from 1° to 4° and the velocity decay rate increasing from 0.03 to 0.15. As the excitation amplitude rising from 5 % to 15 %, the growth of jet momentum instability in the jet potential core breaks the velocity gradients inhibition on jet mixing. The supercritical jet enters self-similar region earlier, with density potential core length shortened by 51 % and spreading angle increased by 28 % at a middle density ratio of 0.275.

Jet mixing enhancement with Bayesian optimization, deep learning and persistent data topology

We optimize jet mixing using large eddy simulations (LES) at a Reynolds number of $3000$ . Key methodological enablers consist of Bayesian optimization, a surrogate model enhanced by deep learning and persistent data topology for physical interpretation. The mixing performance is characterized by an equivalent jet radius ( $R_{eq}$ ) derived from the streamwise velocity in a plane located $8$ diameters downstream. The optimization is performed in a 22-dimensional actuation space that comprises most known excitations. This search space parameterizes the distributed actuation imposed on the bulk flow and at the periphery of the nozzle in the streamwise and radial directions. The momentum flux measures the energy input of the actuation. The optimization quadruples the jet radius $R_{eq}$ with a $7$ -armed blooming jet after around $570$ evaluations. The control input requires $2\,\%$ momentum flux of the main flow, which is one order of magnitude lower than an ad hoc dual-mode excitation. Intriguingly, a pronounced suboptimum in the search space is associated with a double-helix jet, a new flow pattern. This jet pattern results in a mixing improvement comparable to the blooming jet. A state-of-the-art Bayesian optimization converges towards this double-helix solution. The learning is accelerated and converges to another better optimum by including a deep-learning-enhanced surrogate model trained along the optimization. Persistent data topology extracts the global and many local minima in the actuation space. These minima can be identified with flow patterns beneficial to the mixing.

Numerical analysis on the effect of passive control geometry in supersonic jet mixing enhancement

Abstract This paper presents the numerical analysis of a convergent-divergent circular nozzle with the exit Mach number of 1.69 with and without passive control at the exit. The passive control method opted for this analysis was inward and outward ascending triangular protrusion. This paper explores the influence of the passive control geometry and its blockage area concerning the nozzle exit. The nozzle pressure ratio (NPR) used for carrying out the flow analysis were 3, 4.932, and 6. Two different inward and outward protrusions were used with a height of 1.5 mm and 3 mm. From the results, the potential core length of the protrusion 1.5 mm height was not much changed in the both outward and inward cases. But when the height of the protrusion was increased to 3 mm, there was a noticeable core length reduction at all NPR but with different cases. At the NPR of 6, the potential core length of the inward protrusions 3 mm was reduced by 44 % compared to the plain CD nozzle.

Experimental Investigation of Innovative Vortex Generators in the Mixing Enhancement of Subsonic Jets

Jet mixing becomes necessary for its wide range of applications from household appliances to modern high technology rockets. Various researchers have studied the enhancement of jet mixing and concluded that the most effective jet mixing is due to the engagement of a vortex generator at the exit plane of the nozzle, thereby creating vortices of different sizes to enhance the mixing. To intensify the jet mixing, two similar innovative vortex generators with a total blockage ratio of 3.5% are placed diametrically opposite locations of the convergent nozzle. The Aspect Ratio of the convergent nozzle is 1. A numerical investigation is carried out to assess the effectiveness of the vortex generator for Mach numbers of 0.4, 0.5, 0.6, 0.7 and 0.8. The centerline Pitot pressure decay was calculated and found to exhibit the core length reduction due to the introduction of the Vortex generator. To measure the effectiveness of the jet mixing using vortex generators, the results are compared with the uncontrolled jet.

Study of Mixing Enhancement in Helium Jet/Shock Interaction by Laser Pulses

The mechanism of low-density jet mixing enhancement by a pulsed laser plasma is investigated through a combined numerical and experimental method. A helium jet is injected in parallel to a Mach 2.4 supersonic airstream, and interacts with an oblique shock generated by a compression ramp of 20 deg. The helium jet Mach numbers are set to be 1.01 and 1.80 for the sonic and supersonic fuel jets in the scramjet combustor, respectively. For jet mixing enhancement control, the laser pulses at frequencies ranging from 5 to 25 kHz are introduced inside the jet. The experimental results show that laser pulses significantly improve the jet mixing with jet width increased by 30% seen from the averaged schlieren images. Three-dimensional unsteady Reynolds-averaged Navier–Stokes equations are solved, and the numerical results further reveal that the jet mixing enhancement is strongly associated with large-scale vortex rings stimulated by the laser pulses interacting with shock waves. These large-scale vortices prompt the jet cross-sectional area and mixing efficiency two times larger. However, the total pressure recovery coefficient is reduced by 0.16% downstream. Meanwhile, the turbulent kinetic energy of jet flow is significantly increased and favorable for the jet mixing.

Flow visualization experiment of jet mixing enhancement using pulsed jets

Flow visualization experiment of jet mixing enhancement using pulsed jets

Jet mixing enhancement of a supersonic twin jet nozzle using rectangular tabs

Computational study on the development of mean flow and mixing capability of a rectangular tab placed at the exit of a Mach 1.6 single and twin jet nozzles has been presented. Placing two identical conical nozzles side by side separated by a distance of 1.5 times exit diameters makes the twin jet configuration. The mitigation technique for twin jet cross coupling effects by rectangular tabs for the enhancement of aerodynamic and acoustic characteristics is validating. The results are relevant to situations wherever shock cell structures, potential core length, jet mixing related noise are of concern. Centre line pressure decay characteristics shows that there is an abrupt reduction in the core length and suppression of shock cell structure at off design conditions. The results are in good agreement with the experimental results.

Effect of tab parameters on the near-field mixing characteristics of a Mach 1.5 elliptic jet

The effect of delta-tabs with its apex leaning downstream on the jet mixing characteristics of a Mach 1.5 elliptic jet of aspect ratio (AR) 3.37 is reported in this paper. The delta-tabs were placed along the minor and major axes of the elliptic nozzle, referred to as tab configuration C1 and C2, respectively. For each tab configuration, four different orientation angles varying from θ= 90∘ to 150∘ were studied, where θ= 90∘ is the tab positioned normal to the nozzle exit and θ= 150∘ is the tab oriented in the downstream direction at the nozzle exit. The tab effects on jet manipulation were investigated via shadowgraph visualization, and the mean Pitot pressure was measured along the jet centerline and along the orthogonal planes of the jet. The non-dimensonalized supersonic core length Lc*, average shock-cell spacing Ls¯*, and the axis-switching location xas* were examined in detail to understand the jet characteristics of the natural and manipulated jets. The mixing capability of the manipulated jets quantified based on the reduction in supersonic core length ΔLc* depends strongly on the ideally expanded jet Mach number Mj, tab configuration (C1 and C2), and θ. As such, the ΔLc* for the manipulated jet at θ= 135∘ is the largest, revealing the optimal tab orientation for jet mixing enhancement, as also observed for the circular jet [Zaman et al., “Control of an axisymmetric jet using vortex generators,” Phys. Fluids 6(2), 778–793 (1994)]. The relationship between ΔLc* and ΔLs* is presented to understand the correlation between the jet mixing and noise characteristics of the manipulated jet. Discussion is conducted based on the dependence of ΔLc* and ΔLs* with θ, which provides useful insight into the jet flow physics. With suitable placement of tabs C1 and C2, the axis switching could be either stopped or augmented. The superior mixing characteristics of an elliptic jet relative to a circular jet, which were found in previous works on the supersonic over-expanded jet [Kumar and Rathakrishnan, “Characteristics of a supersonic elliptic jet,” Aeronaut. J. 120(1225), 495 (2016)] prevail in the supersonic jet under-expanded jet as well.

Performance of corrugated actuator-tabs of aspect ratio 2.0 on supersonic jet mixing enhancement

The present experimental investigation explores the ability of the corrugated actuator-tabs (or simply, the corrugated actuators) of aspect ratio 2 in encouraging the mixing of Mach 1.73 circular jet at different levels of expansion conditions. Essentially, rectangular, triangular, and semi-circular corrugated actuators are deployed at the opposite positions across the diameter of the nozzle outlet and compared with the plain or simple actuators located at the same positions. Quantitative and qualitative observations were conducted by pitot pressure measurements and shadowgraph flow visualization. Interestingly, the characteristic decays are higher for the semi-circular and triangular corrugated actuators, exhibiting superior mixing than the rectangular corrugated actuator in the far-field. However, the core length reduction for rectangular corrugation is greater than for triangular and semicircular corrugation geometry. As high as 96.5 % core length reduction was obtained for the rectangular corrugated actuators operating at the overexpansion condition. The pressure profiles along and perpendicular to these actuators confirmed that the deployment of the corrugation geometry significantly reduces the flow asymmetry. The shadowgraphic flow visualizations reveal that the plain and corrugated actuators weaken the waves and shorten the cells when compared to the uncontrolled or free jet.

Mixing Enhancement by Pulsed Energy Deposition in Jet/Shock Wave Interaction

The mechanism of the supersonic round jet mixing enhancement by a pulsed energy deposition is studied numerically by solving the three-dimensional unsteady Reynolds-averaged Navier–Stokes equations with the second-order scheme in space and time. A round jet at different Mach numbers (, 1.76, 3.00) is parallelly injected into a coflowing air stream of Mach number 2.5, interacting with the oblique shock generated by a compression ramp of 20°. The density ratio between the jet and external flow is 0.5. The laser pulse with at 20 kHz is modeled as an ellipsoid for the energy addition, and proposed to put above or inside the jet for jet mixing control. It is found that the energy deposition inside the jet is much more effective for the jet mixing amplification compared with that above the jet. The jet mixing enhancement is achieved through the large-scale vortices, initiated by the energy deposition zone interacting with the oblique shock wave. It is shown that the jet cross-sectional area is doubled, and the turbulent kinetic energy is increased, even reaching 10 times the original level at the downstream location. However, a slight decrease about 0.3% of the total pressure recovery coefficient is observed far downstream.

Effect of the Backward Facing Step on a Transverse Jet in Supersonic Crossflow

A transverse jet in the supersonic crossflow is one of the most promising injection schemes in scramjet, where the control or enhancement of jet mixing is a critical issue. In this paper, the effect of the backward facing step on the characteristics of jet mixing was investigated by three-dimensional large eddy simulation (LES). The simulation in the flat plate configuration (step height of 0) was performed as the baseline case to verify the computation framework. The distribution of the velocity and pressure obtained by the LES agreed well with the experiment, which shows the reliability of the LES code. Then, two steps with a height of 1.0D and 1.58D (D is the injector diameter) were numerically compared to the non-step baseline case. The comparison of the three cases illustrates the effect of the large-scale recirculation region on the variable distribution, and shock and vortex structures in the flow field. In the windward region, the shear layers become thicker, and the convection velocity of the shear vortexes reduces. In the leeward region, the wake vortices almost disappear while the counterrotating vortex pairs (CVPs) expand in the spanwise direction. In the area upstream of the jet, the separation bubble works with the upstream large-scale recirculation zone to entrain the jet into the upstream near-wall zone. At last, a comparison of the overall mixing performance of the three cases revealed that the penetration depth and mixing efficiency increased with the step height increasing.

Numerical analysis of enhanced mixing due to grooves in convergent – Divergent nozzle

The focus of this study is to improve the mixing at the exit of supersonic jet and estimate the decay of total pressure and Mach number by introducing interior grooves on top and bottom side alongside the divergent length of the convergent divergent nozzle. Geometrical model of the nozzle is created using CATIA software and analysis is performed using ANSYS. The decay of the total pressure and Mach number is studied for over expansion (4 bar), optimum expansion (6 bar) and under expansion (8 bar) instances by fixing Mach number at exit of the nozzle as 1.8 for nozzle configurations with and without inside grooves. The grooves act as passive controls ensuing significant enhancement of jet mixing. The decay of Mach number and total pressure alongside the jet centreline axis is greater for grooved nozzle in contrast to plain nozzle. The centreline axis total pressure decay comparison shows semi-circular grooved nozzle decays 30% faster than the plain circular nozzle, whereas square grooved nozzle decays 62% faster than the plain circular nozzle in under expansion condition. The characteristics of decay increased with increase in groove length.

URANS study of pulsed hydrogen jet characteristics and mixing enhancement in supersonic crossflow

In this paper, three-dimensional pulsed hydrogen jet in supersonic crossflow (PJISC) is investigated by the unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with the k-ω shear stress transport (SST) turbulence model. The numerical validation and mesh resolution have been carried out against experiment firstly. The effects of the pulsed frequency and amplitude on the jet flow field and mixing performance in supersonic cross-flow are all addressed. It significantly changes the distribution of the hydrogen jet flow by comparing with the steady jet in supersonic crossflow. The fuel jet penetration, mixing efficiency, decay rate of the maximum hydrogen mass fraction and total pressure losses are used to quantitatively analyze the mixing performance. The mixing of fuel and incoming air flow is enhanced by the pulsed jet, especially for the case of 50 kHz, which is the optimal pulsed frequency while considering the effects of jet excitation frequency in the present simulations. The decay rate of the maximum mass fraction of hydrogen in the far field downstream is related to the frequency of the pulse jet. Moreover, the pulsed frequency and amplitude have little effects on the total pressure recovery coefficient for the cases studied in the present simulations.

Large-eddy simulation of circular jet mixing: Lip- and inner-ribbed nozzles

Jet mixing of a circular nozzle in two different configurations is extensively investigated using large-eddy simulation (LES), i.e., the lip-ribbed one with an annular rib at the nozzle exit, and the inner-ribbed one with a rib placed 1.0D upstream of the nozzle exit; the latter one is particularly selected to generate unsteady reattachment of the separated shear layer near the nozzle exit. A jet issuing from a hydrodynamically smooth pipe with fully developed turbulent flow into a larger cylindrical domain (a ‘pipe jet’) is used as the baseline configuration for comparison. The Reynolds number based on the mean (bulk) velocity in the pipe and the pipe diameter is 6000. Validation of the LES approach is performed on the baseline pipe jet using particle image velocimetry (PIV) and laser-induced fluorescence (LIF) measurements; perfect agreement is obtained in terms of the mean streamwise velocity and concentration and their fluctuation parts. The LES results show that both ribbed jet configurations exhibit higher levels of mixing than the pipe jet; the most rapid decay of the centreline concentration and the largest entrainment are observed in the inner-ribbed jet. The inner-ribbed nozzle induced high flow fluctuations behind the rib due to frequent reattachment of flow to the pipe wall that are similar to what is known as fluid flapping motion. These flapping motions give rise to the energetic large-scale structures at the pipe exit and serve as an effective control imposed on the jet. The ‘slice’ proper orthogonal decomposition analysis performed on the flow helps to identify the jet's ‘preferred modes’ by selecting the most energetic structures with a specified azimuthal wavenumber. In the lip-ribbed jet, the separated shear layer modified the frequencies of the axisymmetric mode but made a limited contribution to the energy level of the jet preferred mode. In the inner-ribbed jet, however, the jet preferred modes were extensively altered. The single- and double-helical modes (m=1 and 2) were substantially intensified, with increasing energy levels up to 58%, giving rise to the significant enhancement of jet mixing, especially in the upstream regions.

Numerical simulation of jet mixing enhancement using rectangular control jets

The effect of rectangular control jet on the mixing of round jet with ambient air was studied numerically, in the present work. The effect on the length of main jet potential core and the penetration depth of the control jet, due to variations in velocity and width of the control jet, was studied. The mechanism of jet mixing enhancement was also studied. The results indicated that due to the rectangle jets, the main core region was bifurcated. Thus, there were two definitions for the length of main jet core: the length of centerline potential core and the length of actual potential core. Both lengths decreased with increase in control jet velocity. Besides, the length of the actual potential core decreased with increase in control jet width. However, for the length of centerline potential core, there was a specific width, at which the length of centerline potential core reached a minimum value. Finally, the deformation of the main jet was a factor, more critical compared to the streamwise vortices, which had an effect on the control jet.

Modified Extremum-Seeking Closed-Loop System for Jet Mixing Enhancement

A modified extremum-seeking closed-loop control system is proposed for jet mixing enhancement. The system introduces a variable perturbation component that may act to adjust adaptively the searching range of the optimal control signal. The actuator is a single pulsed minijet injected radially into the main jet before its issue. The excitation frequency for the minjet is controlled by an electromagnetic valve. The open-loop control measurements indicate that, given the duty cycle and the mass flow rate ratio of the minijet to the main jet, the decay rate of jet centerline mean velocity, which provides a measure for jet mixing, displays a maximum at the optimum frequency ratio , where is the preferred-mode frequency of the main jet. Comparison is made between the modified and classical systems. Although both may find automatically the optimal and hence a maximum , the modified system achieves a contraction in the fluctuating , when the control target is reached, by 70%, and subsequently an increase in by 12%. The new system further exhibits a better robustness than the classical.

Mixing enhancement and penetration improvement induced by pulsed gaseous jet and a vortex generator in supersonic flows

A good injection strategy with high penetration and mixing efficiency relates to the overall performance of the airbreathing hypersonic propulsion system. In the current study, the transverse injection flow field with a vortex generator placed in front of jet has been investigated, and the jet mixing enhancement and penetration improvement induced by the pulsed jet has been evaluated as well. The obtained results predicted by the three-dimensional Reynolds-average Navier – Stokes (RANS) equations coupled with the two equation k-ω shear stress transport (SST) turbulence model show that the jet-to-crossflow pressure ratio has an great impact on penetration depth and mixing efficiency in the steady jet flow field. In the case of lower jet-to-crossflow pressure ratio, higher mixing efficiency but lower penetration depth are shown. On the other hand, compared with the steady jet flow field, the pulsed jet flow field with the periodic variation of jet-to-crossflow pressure ratio of the fuel injection has better comprehensive performance, and the penetration depth and mixing efficiency are improved simultaneously.

Passive Control Strategy for Mixing Ventilation in Heating and Cooling Modes Using Lobed Inserts

Mixing ventilation is commonly used to control thermal comfort in a room by means of air jets. The jet diffusers should distribute the fresh air and energy, for heating or cooling, in the entire occupied zone. Therefore, the design of the diffusers must aim, beyond aesthetic aspect, the ability to provide a good mixing between the jet and the ambient air. Enhancement of jet mixing with the ambient air by means of lobed inserts into a diffuser, was recently proposed in an European patent, as a promising and low-cost solution for improving the performance of HVAC systems. In this paper, an experimental investigation on thermal comfort generated by a classical multi-cone ceiling-mounted diffuser is proposed. Its performance is compared with the same diffuser equipped with lobed inserts. A simplified heated manikin simulates the presence of a human body in the test room. Thermal comfort was analyzed based on traditional pointwise measuring probes and on the standard ISO 7730. It is revealed that the thermal comfort was significantly improved using the lobed diffuser compared to the conventional one, without increase of pressure drop and sound pressure levels.

Numerical Simulations of Jet Mixing Enhancement in a Round Jet Exhaust Using SJA

Symmetric synthetic jet is used to stimulate a Mach number 0.9 and high temperature jet for the mixing enhancement. The movement of the piston-type synthetic jet is simulated by dynamic mesh based on URANS numerical methods. Jet flapping, streamwise vortex and geometric axis transformation are the jet mixing mechanism. However, the jet mixing enhancement effect depends on penetration area, which consists of penetration depth and spanwise. And the penetration area is determined by actuator nozzle diameter and blowing jet momentum. In addition, a 54% reduction in potential core length and maximum 45.8K decrease in potential temperature are achieved.

Effect of Inner Nozzle Lip Thickness on Co-flow Jet Characteristics

AbstractThis paper presents the experimental results of subsonic and underexpanded sonic jets delivered from a central nozzle surrounded by a co-flow. The role of central nozzle lip thickness on the co-flow jet characteristics has been studied. The jet-mixing enhancement is achieved in the case of jet from the thick-lip nozzle compared to that of thin-lip nozzle. The extent of supersonic zone for the thick-lip is much shorter than the thin-lip jet.