Efficient and controlled mixing of fuel with fast-moving air is a challenging physical problem relevant to hypersonic systems. Although mixing happens at the molecular level through diffusion, the macroscopic phenomena such as entrainment and vorticity dynamics resulting from the shear layer instabilities of the mixing fluids play a significant role in the overall efficiency of the process. With a focus on improving mixing at extreme flow conditions, this paper presents a fundamental study of a novel, high-speed, pulsed co-flow system integrated with ultra-high frequency actuators that operates at 11-20 kHz. This injection system consists of a supersonic actuation air jet at the inner core that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle, with pulsing occurring at a designated frequency. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a steady co-axial configuration operating at the same input pressure. Experiments also indicate a strong connection between the frequency and unsteady amplitude of the actuation jet to the supersonic flow mixing phenomena. This paper reports the design details of the injector assembly and flow mixing characteristics captured using phase-locked microschlieren and planar laser-induced fluorescence (PLIF) techniques.
more »
« less
This content will become publicly available on June 12, 2024
Experimental Investigation of a High-Frequency Pulsed Supersonic Co-Axial Injector using Optical Diagnostics
With an increased national emphasis on the development of high-speed systems, especially
those utilizing air-breathing propulsion, there is a strong demand for efficient means of enhancing
the mixing of fuel injection and an air stream that may be moving at supersonic speeds. In recent
years, small scale fluidic-based actuators have shown promise in their ability promote efficient
mixing in these scenarios due to their small size and having no moving parts. High-frequency
pulsed co-axial actuators developed at Tuskegee University are one such device that only rely
on a compressed gas source and particular geometric designs to enable rapid pulsed injection.
These actuators are able to efficiently atomize injected fluids, such as fuels or oxidizers, and
they can be designed to pulse at frequencies in excess of 15 kHz. This paper presents the
results of an experimental investigation of one such device using advanced optical and laser
diagnostics to measure both the spectral content and the unsteady velocity fields of the pulsed jet
output. The flowfield is visualized with high-speed schlieren imaging, and the near-field acoustic
emission is measured by a microphone. Focused laser differential interferometry (FLDI) and
planar particle image velocimetry (PIV) are conducted within the actuator’s pulsed jet output.
The spectral content measured from FLDI closely matches the acoustic spectra. While the
phase-locked velocity fields demonstrate the pulsing behavior of the actuator’s core flow jet, the
calculated velocities appear to be lower than what was anticipated based on inspection of the
high-speed schlieren images. Velocity fields are presented for the actuator operating in both
steady and pulsed modes of operation, and the results indicate that the pulsed actuation jet has
the potential to significantly enhance mixing of the annular stream when it is introduced.
more »
« less
- Award ID(s):
- 1900177
- NSF-PAR ID:
- 10433927
- Date Published:
- Journal Name:
- AIAA Aviation 2023
- Issue:
- 4240
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
This paper presents the velocity and vorticity fields of a supersonic, pulsed co-flow injection system, measured using particle image velocimetry (PIV). The active injection system consists of an actuation air jet from a nozzle (1 mm ID, 1.5 mm OD) that provides large mean and fluctuating velocity profiles to the shear layers of a fluid stream injected through a 460 μm diameter annular space surrounding the nozzle. The injector assembly is designed to operate in three modes in the present study. In the first case, annular fluid was injected at 70 m/s without actuation and treated as baseline flow. In the second case, a steady under-expanded supersonic jet with an exit velocity of 350 m/s interacts with the injected annular fluid. In the last case, the actuation jet pulsing at 15.5 kHz introduces highfrequency streamwise vortices and shockwaves to the annular stream. PIV data indicate that steady and pulsed actuation significantly modifies the velocity and vorticity fields of annular flow. While steady actuation caused intensive shear and high vorticity streaks in the flow, the high-frequency vortex generated in pulsed actuation resulted in highly fluctuating velocity and vorticity fields that favor enhanced mixing between the annular stream and supersonic actuation jet. Quantitative data from this study confirm the potential of this injector system for flow mixing and control under extreme conditions.more » « less
-
Effective mixing in supersonic and hypersonic flow conditions is critical for developing next-generation high-speed air-breathing transport systems. Efficient fuel mixing with fast-moving air leads to a better economy and fewer pollutants. Ultimately the mixing is a molecular diffusion problem. However, the macroscopic phenomena, such as entrainment and vorticity dynamics resulting from the shear layer instabilities of the mixing fluids, play a significant role in the overall efficiency of the process. This paper studies a novel, coaxial injector system integrated with a high-frequency microjet actuator operating at 15.5 kHz for improving mixing in extreme flow conditions. This co-flow system consists of a highfrequency supersonic actuation air jet at the inner core that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a steady co-axial configuration operating at the same input pressure. This paper reports the flow mixing characteristics of the injection system captured using planar laser-induced fluorescence (PLIF).more » « less
-
Experiments were carried out to observe the flow inside counterflow atomizers over a range of operating conditions and fluid properties. Liquids used were water and propylene glycol, while the gas was either air or helium. Liquid flow rates ranged from 10 ml/min to 40 ml/min, with gas liquid ratio (GLR) ranging from 0.1 to 0.6. The primary experiments used the 7-BM line of the Advanced Photon Source in Argonne National Laboratories with a 2.6 mm atomizer produced from (Poly)Ethyl-Ether-Ketone (PEEK). The X-Ray beam was operated in phase contrast mode, leading to interference patterns near the gas-liquid interface and enabling a qualitative understanding of the flow structure. Complementary optical work applied laser shadowgraphy to a 1 mm orifice atomizer constructed with quartz capillary tubing. A diffuse pulsed Nd:YAG laser backlight captured instantaneous gas-liquid interface positions in the internal flow. With both techniques, two distinct flow behaviors are observed corresponding to low and high GLR values. At low GLR, the inertia of the injected gas is insufficient to penetrate the liquid downflow. The gas stream entering the mixing chamber in the upstream direction is immediately deflected by the denser liquid and enters the discharge tube around a central liquid jet, which is sheared and accelerated by the surrounding gas, leading to breakup. A distinct frequency of jet breakup is observed inside the discharge tube, with the liquid jet oscillating and fragmenting against the walls. The situation at high GLR is quite different, however, as the incoming gas stream asymmetrically penetrates upstream into the mixing chamber, taking the form of a high-speed jet confined along one wall, and displaying a flapping instability as it encounters the liquid flowing downstream. This flapping causes violent mixing, resulting in a highly disturbed interface, along with the generation of liquid ligaments and gas bubbles. This two-phase mixture enters the discharge tube with no liquid jet formation evident for this case. The transition between these two regimes is explored by changing the liquid viscosity and gas molar mass, and weak sensitivity to fluid properties is observed. Further, quantitative image analysis techniques applied to the low and high GLR cases allow extraction of the frequencies of the liquid jet in the discharge tube at low GLR, as well as the flapping mode at high GLR.more » « less
-
The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertioelastic instabilities. The nonlinear evolution of these instabilities generates a state of turbulence with significantly different spatiotemporal features compared to Newtonian turbulence, termed elastoinertial turbulence (EIT). We ex- plore EIT by studying the dynamics of a submerged planar jet of a dilute aqueous polymer solution injected into a quiescent tank of water using a combination of schlieren imaging and laser Doppler velocimetry (LDV). We show how fluid elasticity has a nonmonotonic effect on the jet stability depending on its magnitude, creating two distinct regimes in which elastic effects can either destabilize or stabilize the jet. In agreement with linear stability analyses of viscoelastic jets, an inertioelastic shear-layer instability emerges near the edge of the jet for small levels of elasticity, independent of bulk undulations in the fluid column. The growth of this disturbance mode destabilizes the flow, resulting in a turbulence transition at lower Reynolds numbers and closer to the nozzle compared to the conditions required for the transition to turbulence in a Newtonian jet. Increasing the fluid elasticity merges the shear-layer instability into a bulk instability of the jet column. In this regime, elastic tensile stresses generated in the shear layer act as an “elastic membrane” that partially stabilizes the flow, retarding the transition to turbulence to higher levels of inertia and greater distances from the nozzle. In the fully turbulent state far from the nozzle, planar viscoelastic jets exhibit unique spatiotemporal features associated with EIT. The time-averaged angle of jet spreading, an Eulerian measure of the degree of entrainment, and the centerline velocity of the jets both evolve self-similarly with distance from the nozzle. The autocovariance of the schlieren images in the fully turbulent region of the jets shows coherent structures that are elongated in the streamwise direction, consistent with the suppression of streamwise vortices by elastic stresses. These coherent structures give a higher spectral energy to small frequency modes in EIT characterized by LDV measurements of the velocity fluctuations at the jet centerline. Finally, our LDV measurements reveal a frequency spectrum characterized by a −3 power-law exponent, different from the well-known −5/3 power-law exponent characteristic of Newtonian turbulence.more » « less
