Shoji Sakaue

Effect of Expansion Ramp Angle on Supersonic Mixing using Streamwise Vortices

For supersonic mixing enhancement, the use of large scale streamwise vortices is the most advantageous. The present study investigates flow charactorestics of mixing region and breakdown process of streamwise vortices, which is produced by expansion ramp and backward facing step, by using schlieren visualization and 3D -PIV. The expansion ramp angle is changed to vary the circulation of streamwise vortices. The results show that circulation of streamwise vortex, distance between streamwise vortices, and velocity difference between free-stream and inside of streamwise vortices have an effect on the expansion ratio of mixing region and breakdown process of streamwise vortices. It is found that there is the suitable expansion ramp angle to expand the mixing region widely. And, it is clarified that the lateral vortex ring surroundi ng streamwise vortices play an important role on the breakdown process of the streamwise vortices.

Fluctuation of Mass Flux and Concentration on Supersonic Mixing using Streamwise Vortices

To clarify supersonic mixing process, the mixing evaluation method was proposed. Our method can measure the instantaneous fluctuations of mass flux and concentration simultaneously by using double-hot-wire probe and CVA (Constant Voltage Anemometer) circuit with 500 kHz bandwidth. In the present study, we first demonstrate the usefulness and accuracy of our method to measure the air/helium mixing layer, and the results of the mean distributions and the instantaneous fluctuations of mass flux and concentration show that our method can capture the mixing process of supersonic mixing. We also carried out the mixing evaluation on supersonic mixing field using streamwise vortices. Nomenclature aw = over heat ratio A, B = calibration constants c = helium concentration Cw = heat capacity of the hot wire Iw = current through the hot wire M = time constant of the hot wire PDR = power dissipation ratio of the hot wire Ra = resistance of the unheated wire at ambient temperature Ta Rref = resistance of the unheated wire at reference temperature Tref Rw = resistance of the hot wire at operating temperature Ta = ambient temperature Tref = reference temperature Vw = voltage across the hot wire α = temperature coefficient of resistance

Streamwise Vortices Introduced by 'Hyper-Mixer' on Supersonic Mixing

For supersonic mixing enhancement, large scale streamwise vortices seem to be the most advantageous. In SCRAM jet (supersonic combustion ramjet) engine, the device which introduces both the large scale streamwise vortices and fuel into supersonic flow needs to be installed on the chamber wall to alleviate the heavy aerodynamic and heat load, so that, it must have the high ability to mix the fluids in near-wall and freestream, efficiently. In the present study, we made effort to clarify the mixing performance of such wall-mounted ramp injectors. And , we clarified that Hyper-mixer type wall-mounted ramp injector is effective to expand the mixing field in high Mach number flow.

MIXING TRANSITION IN SUPERSONIC STREAMWISE VORTICES

The present paper describes our recent experimental studies on the supersonic mixing and combustion enhancement using streamwise vortices in spanwise- row configuration. Firing tests verify streamwise vortices of Γ /ν =3 × 10 4 to be powerful for supersonic combustion enhancement. Cold-flow hot-wire measurements show that Kolmogorovs −5/3 power law region appears in the spectrum of ρu-fluctuations in the vortices for Γ /ν above about 10 4 . This is not inconsistent with the minimum Reynolds number for the mixing transition proposed by Dimotakis (2000).

Supersonic Streamwise Vortices Breakdown in Scramjet Combustor

†‡ ‡ † § For supersonic mixing enhancement, experiments suggested that large scale streamwise vortices seemed to be the most advantageous, for example, FFT spectrum analysis of mass flux fluctuations showed that the power spectrum partly follows Kolmogorov’s -5/3 power law, indicating that the breakdown of streamwise vortices occurred to generate small scale turbulent eddies of high mixing power. The present study examined the breakdown process of streamwise vortices by using PIV, schlieren visualization, hot-wire anemometer and CFD. The experimental results showed that the breakdown of streamwise vortices into small scale turbulence occured at the vortex Reynolds number ReΓ = Γ/ν = 1.3 × 10 4 or more, and that the lateral structure on streamewise vortices took a part in its breakdown process. From the computational analysis, the oscillation of shear layer from ramp could make the streamwise vortices unstable and cause its breakdown.

On the Receptivity Process of Supersonic Laminar Boundary Layer

The receptivity of supersonic boundary layer is examined numerically at a freestream Mach number M 1 = 2.2. The external disturbance is introduced by forcing weak sinusoidal u-fluctuation on a narrow strip of the wall. The result shows that the intensity of excited T-S wave is proportional to that of T-S wave number spectrum contained in the external disturbance. The receptivity to time periodic Mach waves incident onto leading-edge is also examined. When the Mach wave is coming from below the plate, the excited flow is found to be governed by the T-S wave in the upper side boundary layer, and by the forced wave in the lower side boundary layer.

Mixing Enhancement on the Afterburner with Fuel Injection Struts for Hypersonic Vehicle

JAXA’s recent model of the Pre-Cooled Turbojet engine, which is called S-engine, is developed for the flight test at Mach 2. To improve the combustion efficiency, the cold flow field, which simulate the afterburner’s one, is investigated by Schlieren Photographing, Particle Image Velocimetory, Hot Wire Anemometry and Instability analysis. The results showed that Karman vortex sheet was generated behind the strut and it still remained at the exit of the afterburner. It indicated that the mixing of air and fuel was not enough. Then, to enhance mixing, zigzag shaped struts were proposed. As the result, the zigzag struts succeeded to weaken the Karman vortex sheet.

Propagation characteristics of tunnel compression waves with multiple peaks in the waveform of the pressure gradient: Part 1: Field measurements and mathematical model

A high-speed train entering a tunnel generates a compression wave. When the compression wave reaches the exit portal of the tunnel, a micro-pressure wave radiates outward. The magnitude of the micro-pressure wave is approximately proportional to the pressure gradient of the compression wave arriving at the exit portal. As the micro-pressure wave can cause environmental problems, tunnel entrance hoods have been installed at many portals of long slab track tunnels on the Japanese high-speed railway, the Shinkansen to reduce the magnitude of the micro-pressure wave. In this study, field measurements were taken in a Shinkansen long slab track tunnel with a hood at its entrance. The compression wave distorts during its propagation through a long slab track tunnel. The dependence of the propagation characteristics on the initial compression waveform was clarified on the basis of field measurements on different trains and hood window configurations. It was shown that compression waves with a waveform of the pressure gradient that has shallow valleys tend to steepen more easily and that the optimum window pattern of the hood depends on the length of the tunnel. Furthermore, a mathematical model corresponding to the results of the field measurements was proposed to describe the distortion of the compression waves.

Measurement Method of Mass Flux and Concentration in Supersonic Mixing by Hot-Wire Anemometry

In the present study, we made efforts to realize a measurement method of instantaneous fluctuations of mass flux and concentration with 500 kHz bandwidth to explain supersonic mixing process. In our method, by using double-hot-wire probe which has two kinds of wires with different responses to the variations of mass flux and concentration, we were able to explain the mixing flow field with the help of calibration maps of each wire. Experiments were carried out in a 2D supersonic air/helium mixing layer at the Mach number of 2.4. The results show that the proposed method is very effective for the analysis of supersonic mixing. However, in selection of each hot wire, its diameter and thermal conductivity, both should be considered.