Takakage Arai

Criticality for stabilized oblique detonation waves around spherical bodies in acetylene/oxygen/krypton mixtures

We have experimentally studied self-sustained oblique detonation waves around projectiles as part of a fundamental investigation of the application of an oblique detonation wave engine and a high-efficiency detonation wave combustor as a power generator. In previous papers we used optical observation to clarify the fluid-dynamic structure of self-sustained oblique detonations stabilized around cone-nosed projectiles. In this study we investigated the criticality for detonation waves. The first expression of the criticality was a mean-curvature coefficient, a rate between a detonation cell width and a mean-curvature radius in which the normal velocity component was the Chapman-Jouguet (C-J) velocity, of 5.03. The mean-curvature coefficient was constant and did not depend on the type of fuel mixture (H 2 /O 2 /Ar or C 2 H 2 /O 2 /Ar), initial mixture pressure, projectile diameter, projectile velocity, or diluent mole fraction. We obtained a more accurate mean-curvature coefficient for stabilized, oblique detonation around symmetric spherical bodies in highly krypton-diluted acetylene/oxygen mixtures that have extremely low C-J velocities. The meancurvature coefficient of 7.8 was determined to be the most important value for stabilizing the self-sustained oblique detonation waves around multidimensional bodies. Based on, experimental results obtained at high-and low-projectile-velocity ranges, it may be concluded that a lower-velocity projectile can stabilize a self-sustained oblique detonation wave more effectively than can a higher-velocity one. In the high-projectilevelocity region, the experimental critical condition is inconsistent with Lees detonation initiation theory, We propose a semiempirical criticality, equation for the stabilization, which was the secondary expression of the criticality and identical with present and past experimental results.

Evaluation of mass transfer coefficient and hydrogen concentration in supersonic flow by using catalytic reaction

The authors propose a new simple method to evaluate hydrogen concentrations in a hydrogen/air supersonic mixing layer without the need for costly apparatus. Catalytic reaction occurs on an electrically heated platinum wire in the supersonic flow of a hydrogen/air mixture. By adopting the technique of a constant-temperature hot-wire anemometer, the heat transfer coefficient and the catalytic heat release rate are measured. A series of experiments with different platinum wire temperatures shows that the platinum wire temperature does not affect the catalytic heat release rate, implying that the rate of mass transfer from the flow to the platinum wire surface is the controlling factor. This means that the catalytic heat release rate gives the mass transfer coefficient of the controlling species, which is hydrogen/oxygen in lean/rich mixtures. It is found that the effect of hydrogen concentration on the ratio of heat and mass transfer coefficients is very weak, suggesting that the mass transfer coefficient, is obtained with reasonable accuracy from the heat transfer coefficient by assuming the equivalent spatial distributions of heat and mass transfer. Based on this result, a method to translate the catalytic heat release rate into the hydrogen concentration of the flow is proposed. To prove the accuracy of this method, hydrogen concentrations of hydrogen/air premixed supersonic flows were measured successfully. Finally, one example applying this method to an actual supersonic mixing layer is presented.

Internal structure of Pseudo‐shock waves in a square duct

This paper is concerned with the internal structure of pseudo‐shock waves in a straight square duct. The experiments were carried out for M1=1.77 using a dual‐beam Laser Doppler Velocimeter (LDV) and Double Exposure Laser Holographic Interferograms, where M1 was the flow Mach number just ahead of pseudo‐shock wave were measured in detail, and the variations of velocity and the boundary layer displacement thickness were clarified. The displacement thickness increased at the shock wave locations and decreased in the regions between the shock waves. The velocity distributions calculated from the density distributions, which were measured by holographic technique, were compared with the results obtained by LDV. As each shock wave in the pseudo‐shock wave was very weak, the flow in the central part of the duct seemed to be an isentropic flow, while the flow near the wall seemed to be an irreversible adiabatic flow.

Experimental study of impulse and heat transfer on pulse detonation engines

The pulse detonation engine (PDE) has recently become recognized as a possible new aerospace propulsion system. In the PDE system, self-sustained detonation waves propagate in tubes and burned gases are repeatedly exhausted backwards from behind the detonation. The PDE system produces a momentum whose absolute value is equal to the exhausted gas momentum, but whose direction is opposite it. The PDE system has high thermal efficiency on account of its constant-volume combustion and has a simple structure composed of tubes. Although the high efficiency of the PDE system has been confirmed, few experimental reports have been based on a fundamental cyclic theory. In the present paper, we apply Endo-Fujiwara simple cycle theory to the PDE in order to analyze the pressure history at an end wall (thrust wall) of the detonation tubes, the net momentum obtained on the PDE by direct-velocity measurement, and heat transfer from the burned gases to the tubes. As well, we estimated the PDE’s specific impulses. Although the model overestimated the constant-pressure time duration by over 25% and simplified the exhaust phase as linear decay, the specific impulse obtained from the present experiments (187-361 sec) are roughly identical to the theory (-23% to +49%). It would be considered that the pressure overestimation by the model was balanced by the experimentally obtained higher specific impulse due to the small filling rate of the PDE. The net specific impulse based on net impulse measurement (direct-velocity measurement) was 67-147 sec, which was 28-60% of the Endo-Fujiwara theory. The difference between the two specific impulses is due to the momentum transfer on the sidewall of the tubes that was neglected in the pressure measurement. Heat, 11-15 MJ/kg, was transferred from the burned gases to the tubes during one-cycle operation. This value was 8-11% of the gross calorific value of 2H2+O2.

Experimental investigations of momentum and heat transfer in pulse detonation engines

The pulse detonation engine (PDE) has recently become recognized as a possible new aerospace propulsionsystem. In the PDE system, self-sustained detonation waves propagate in tubes and burned gases are repeatedly exhausted backward from behind the detonation. The PDE system produces a momentum whose absolute value is equal to the exhausted gas momentum, but whose direction is opposite it. The PDE system has high thermal efficiency on account of its constant-volume combustion and has a simple structure composed of tubes. Although the high efficiency of the PDE system has been confirmed, few experimental reports have been based on a fundamental cyclic theory. In the present paper, we apply the Endo-Fujiwara simple cycle theory to the PDE in order to analyze the pressure history at an end wall (thrust wall) of the detonation tubes, the net momentum obtained on the PDE by direct-velocity measurement, and heat transfer from the burned gases to the tubes. Additionally, we estimated the PDEs specific impulse. Although the model overestimated the constant-pressure time duration by over 25% and simplified the exhaust phase as linear decay, the specific impulse obtained from the present experiments (187–361 s) is roughly identical to the theory (−23% to +49%). The pressure overestimation by the model was balanced by the experimentally obtained higher specific impulse due to the small filling rate of the PDE. The net specific impulse based on net impulse measurement (direct-velocity measurement) was 67–147 s, which was 28%–60% of the Endo-Fujiwara theory. The difference between the two specific impulses is due to the momentum transfer on the sidewall of the tubes that was neglected in the pressure measurement. Heat, 11–15 MJ/kg, was transferred from the burned gases to the tubes during one-cycle operation. This value was 8%–11% of the gross calorific value of 2H 2 +O 2 .

H2 Concentration Profile in Cold Supersonic Hydrogen-Air Mixing Layer* (Evaluation using Catalytic Reaction on Constant Temperature Pt Wire)

Hydrogen was injected normally to a Mach 1.8 cold air stream with a backward-facing step to investigate mixing flow field in a scramjet combustor. Catalytic reaction on constant temperature Pt wire was used to measure the mixing condition of t^ and C>2. It was new technique to investigate H2 mixing condition. The amount of heat release due to the catalytic reaction, which corresponds to the concentration of H2 and/or C>2 on the surface of the catalyst, was measured spatially so that the local mixing condition of H2 and O2 was cleared. The results showed that there were two core regions, which would have good mixing condition for supersonic combustion, in the mixing layer. The development of core regions along flow direction was also clarified.

LDV Investigation of Turbulence Phenomena in Multiple Shock Wave/Turbulent Boundary Layer Interactions

The turbulence phenomena in multiple shock wave/turbulent boundary layer interactions in a square duct have been investigated using a two-component laser Doppler velocimeter (LDV). The free stream Mach number M ∞, flow confinement δ ∞/h and unit Reynolds number Re just upstream of the interaction were of 1.78, 0.3 and 13×106 m−1, respectively. First, the interaction flow fields were visualized by using schlieren photographs and laser holographic interferograms. Second, the time-mean and fluctuating velocity fields were explored in detail using the LDV. Spatial distributions of turbulence intensity, Reynolds shear stress, turbulence kinetic energy and turbulence production are presented.

Hydrogen Concentration Measurements of Supersonic Hydrogen-Air Shear Layer Using Catalytic Reaction

To investigate development of an air-hydrogen supersonic shear layer and distribution of hydrogen concentration, a hydrogen jet was injected into a cold air supersonic freest reem in a paralell direction. The free stream Mach number was about 1.81. Using a catalytic reaction on a thin platinum wire, heat release due to catalytic reaction, a heat transfer coefficient and hydrogen concentration were measured. It was shown that the paralell injection was found to affect on mixing condition. The effect of paralell injection on hydrogen concentration profile was clarified. It seemed that there was the stoichiometric condition at the outer edge of shear layer. It was confirmed that the diffusion of Hydrogen, including turbulent mixing, had an effect of flow configuration.

Three-dimensional structure of pseudo-shock waves in a rectangular duct

The three-dimensional structure of a pseudo-shock wave in a straight square duct was investigated using a 2 color 4 beam LDV and a color schlieren method. The Mach number just upstream of the pseudo-shock wave was 1.83, the Reynolds number was 6.8xl04, and the confinement parameters were about 0.2 for the upper and side walls. It was shown that the structure of the pseudo-shock wave depends on the characteristics of the wall boundary layer and that the formation of the second shock is due to the variation of the displacement thickness of the boundary layer (aerodynamic nozzle) in the pseudo-shock wave.

Mixing Enhancement by Normal Gas Injection in Supersonic Mixing Layer

It is important to understand the supersonic mixing layer in Scramjet engine combustor. In this paper, the helium gas through the tube (outer diameter: 1.0mm, inner one: 0.7mm) was injected normally into the supersonic mixing layer in order to enhance the supersonic mixing. The flows were analyzed by LDV measurements. It was found that the supersonic mixing was enhanced by the helium gas injection and shock-wave induced by the tube. Furthermore, the some terms in turbulence kinetic energy equation were also discussed.