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The inner flow containing the ignition process of a throatless and diverging (constant diverging angle α = 5 deg) rotating detonation engine (RDE) was experimentally investigated by both optical observation and pressure measurement. An RDE that had an acrylic engine wall with a length of 70 mm and an inlet diameter of 20 mm was tested under a low back pressure condition using gaseous C 2 H 4 − O 2 as the propellants. A high-speed camera captured the ignition process, of which there were two types: deflagration-to-detonation transition and direct initiation. In addition to the visualizations of the inner flow, the pressure values in an RDE with an acrylic wall were compared with those of an RDE having a stainless steel wall with 8 pressure ports in the axial direction. The comparison revealed that the RDE exhaust flow was supersonic. A strong self-luminescence and CH* luminosity area near the injector surface during combustion suggested that the short completion of detonation combustion enabled the thermal choking of the subsonic propellant gases even when the channel was diverging. Moreover, the axial position of the erosion in the acrylic wall indicated that the heat release region was near the injector surface. Finally, a theoretical flow model for the RDE was compared to the experimental data, and the results suggested that the inner flow in the RDE can be reasonably described as a quasi-one-dimensional steady flow. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical simulations are conducted based on the Eulerian-Lagrangian method to model a gaseous detonation laden with monodispersed water droplets. The premixed mixture is a slightly diluted stoichiometric hydrogen oxygen mixture at low pressure. The outcome of the interactions of the droplet breakup with the cellular instabilities and the non-uniform flow behind the leading front is analyzed. The simulation results are also analyzed using instantaneous flow fields and Favre average profiles for water droplets. Breakup occurs mainly near the detonation front. The mean final diameter of the water droplets at the end of the breakup process is the same regardless of the initial strength of the leading shock or whether it is lower or greater than the Chapman-Jouguet value. The polydispersity comes from local phenomena behind the leading shock, such as forward jets coming from triple point collisions, transverse waves and a combination of both. The total breakup time is longer than that estimated from post-shock conditions and the present finding is in line with the previous experimental results on spray detonation. [ABSTRACT FROM AUTHOR]

Detonation combustors are considered promising alternatives to conventional combustors because they offer high thermal efficiency and fast combustion. However, especially for the rotating detonation combustor, the theoretical propulsive performance has not been confirmed in experimental studies because the highly unsteady flow field hinders the measurements process. To understand the involved phenomena in more detail, a reflective shuttling detonation combustor (RSDC) with a rectangular combustion chamber was developed. The interior of the chamber can easily be visualized owing to its two-dimensional quality. Utilizing the RSDC, several combustion tests with gaseous ethylene and oxygen were conducted for different values of mass flow rates and equivalence ratios. Combustion modes from the tests were classified into four types based on the fast Fourier transform (FFT) analysis of the luminous intensity of the CH* self-luminescence images captured by a high-speed camera and a band pass filter. Simultaneously, the theoretical total pressure of a conventional isobaric combustor was compared to the static pressure measured at the bottom of the RSDC chamber. For the detonation modes, the ratio between experimentally measured static pressure and the theoretical pressure varied depending on the location in the chamber owing to the distribution of the time-averaged static pressure. Furthermore, the pressure ratio of the detonation modes was up to 18% lower than that of the deflagration mode potentially owing to the flow velocity induced by the detonation waves. [ABSTRACT FROM AUTHOR]

Recently, Kawasaki and Kasahara (2019) reported that reflection point distance , which is a detonation characteristic length relevant to the diffraction process, is a useful measure; i.e., the critical condition for detonation diffraction can be universally expressed in terms of the diffraction point distance, independent of mixture stability. However, their findings were limited to their experimental conditions only. In this study, we performed high-speed visualization of processes of cylindrical (line-symmetric) detonation diffraction around a 90-degree corner for two series of experiments to obtained reflection point distances, l r , as a novel characteristic length, and examined critical conditions of reinitiation expressed in terms of the reflection point distance. In the first experimental series, stoichiometric C 2 H 2 /O 2 mixtures with 50% Ar dilution were employed, and the channel width l c was varied to 5, 10, 15, and 20 mm to investigate the influences of the boundary condition of the flow field. In the second experimental series, H 2 /O 2 , C 2 H 2 /O 2 , or C 2 H 4 /O 2 mixtures with different equivalence ratios were employed to investigate influences of the reaction systems. Our results confirmed that the channel width does not affect the reflection point distance or the critical condition. The critical condition was also independent of fuel species and equivalence ratio, and can be uniquely expressed as l r / l c = 4.0 ± 0.6 in terms of the reflection point distance. [ABSTRACT FROM AUTHOR]

The internal flow structures of detonation wave were experimentally analyzed in an optically accessible hollow rotating detonation combustor with multiple chamber lengths. The cylindrical RDC has a glass chamber wall, 20 mm in diameter, which allowed us to capture the combustion self-luminescence. A chamber 70 mm in length was first tested using C 2 H 4 O 2 and H 2 –O 2 as propellants. Images with a strong self-luminescence region near the bottom were obtained, confirming the small extent of the region where most of the heat release occurs as found in our previous research. Based on the visualization experiments, we tested RDCs with shorter chamber walls of 40 and 20 mm. The detonation wave was also observed in the shorter chambers, and its velocity was not affected by the difference in chamber length. Thrust performance was also maintained compared to the longer chamber, and the short cylindrical RDC had the same specific impulse tendency as the cylindrical (hollow) or annular 70-mm chamber RDC. Finally, we calculated the pressure distributions of various chamber lengths, and found they were also consistent with the measured pressure at the bottom and exit. We concluded that the short-chamber cylindrical RDC with equal length and diameter maintained thrust performance similar to the longer annular RDC, further expanding the potential of compact RDCs. [ABSTRACT FROM AUTHOR]

Abstract A two-dimensional (2D) numerical simulation is conducted to clarify the propagation behavior of gaseous detonation in a water spray and its structure. The computational target refers to the experiment conducted by G. Jarsalé et al. , and C 2 H 4 –air gaseous detonation propagates where the water droplets (WDs) are sprayed. The parameters used are the C 2 H 4 –air equivalence ratio and WD mass fraction. The flow field, Favre-averaged one-dimensional profile, and cellular structure are revealed in 2D simulations. Stable propagation of gaseous detonation is observed in the water spray, and the decrease in velocity relative to the Chapman–Jouguet velocity without WDs is as much as 3.2%. Adding WDs changes the cellular pattern, especially for leaner mixtures. The weak triple point decays, and the cell width increases because of the longer induction length due to decreased velocity. The WD presence changes the detonation flow field substantially, and evaporation occurs primarily at 10 mm behind the shock wave. The high-evaporation region propagates at the detonation speed, and the compression wave formed when the detonation reflects from the two-phase medium propagates backward. Furthermore, WD evaporation suppresses the velocity, vorticity, and temperature fluctuations. Rapid evaporation with WDs leads to lower hydrodynamic thickness than that without WDs or in the Zel'dovich–von Neumann–Döring model. [ABSTRACT FROM AUTHOR]

Abstract We describe the critical condition necessary for the inner cylinder radius of a rotating detonation engine (RDE) used for in-space rocket propulsion to sustain adequate thruster performance. Using gaseous C 2 H 4 and O 2 as the propellant, we measured thrust and impulse of the RDE experimentally, varying in the inner cylinder radius r i from 31 mm (typical annular configuration) to 0 (no-inner-cylinder configuration), while keeping the outer cylinder radius (r o = 39 mm) and propellant injector position (r inj = 35 mm) constant. In the experiments, we also performed high-speed imaging of self-luminescence in the combustion chamber and engine plume. In the case of relatively large inner cylinder radii (r i = 23 and 31 mm), rotating detonation waves in the combustion chamber attached to the inner cylinder surface, whereas for relatively small inner cylinder radii (r i = 0, 9, and 15 mm), rotating detonation waves were observed to detach from the inner cylinder surface. In these small inner radii cases, strong chemical luminescence was observed in the plume, probably due to the existence of soot. On the other hand, for cases where r i = 15, 23, and 31 mm, the specific impulses were greater than 80% of the ideal value at correct expansion. Meanwhile, for cases r i = 0 and 9 mm, the specific impulses were below 80% of the ideal expansion value. This was considered to be due to the imperfect detonation combustion (deflagration combustion) observed in small inner cylinder radius cases. Our results suggest that in our experimental conditions, r i = 15 mm was close to the critical condition for sustaining rotating detonation in a suitable state for efficient thrust generation. This condition in the inner cylinder radius corresponds to a condition in the reduced unburned layer height of 4.5–6.5. [ABSTRACT FROM AUTHOR]

Abstract As a novel detonation combustor that differs from a pulse and a rotating detonation engine, a reflective shuttling detonation combustor (RSDC), in which detonation waves shuttle repeatedly, was proposed. In a fan-shaped two-dimensional combustor, detonation waves propagate, repeating attenuation and re-ignition by a shock reflection at the side wall. In the demonstration experiment, chemiluminescence visualization and pressure measurement with ethylene–oxygen mixture were conducted at the same time. As the result, a single shuttling wave coupled with pressure rise was observed in the combustor. The tangential velocity of the wave was 1526 ± 12 m/s and approximately 60% of the estimated Chapman–Jouguet velocity of 2513 m/s. The ratio of pressure in front of the wave to one behind the primary wave or the reflected wave was in good agreement with one-dimensional shock theory, and it was suggested that the rapid reaction behind the reflected shock wave sustained the continuous propagation of the shock wave. [ABSTRACT FROM AUTHOR]

The motion of a fluid in a valveless pulse detonation combustor (PDC) at an operating frequency of 500 Hz was investigated using a novel PDC, in which the inner diameter of the oxidizer feed line was equal to that of the combustor (inner diameter = 10 mm). Gaseous oxygen was supplied to the PDC in a valveless mode. In contrast, supercritical ethylene was injected into the PDC in a direction perpendicular to its axis. The flame propagation speed and backflow of the burned gas were measured using 12 ion probes installed on the side wall. The measured flame propagation speeds in all the combustion tests were 107 ± 4% of the Chapman–Jouguet detonation speed. In addition, the experimental results were compared with those of a one-dimensional numerical calculation. Under the condition that the stop time of the fuel injection was equal to the spark time, the trajectory of the backflowing burned gas was identical to the experimental results, within 7%. Shortening the burned-gas backflow process was found to be essential to achieve a higher operating frequency. [ABSTRACT FROM AUTHOR]

The structure of detonation waves in rotating detonation combustors (RDCs) and their combustion chamber pressure characteristics have not yet been fully clarified due to the complexity and shape of the RDC combustion chamber. Therefore, a disk-shaped RDC was used in this study to visualize the inside of the combustion chamber while simultaneously measuring its pressure. Forward-tilting rotating detonation waves were observed, and a schematic was proposed for them. The initial velocity of the forward-tilting rotating detonation wave was 1200 ± 160 m/s, and it subsequently increased to 1600 ± 160 m/s; meanwhile, the Chapman–Jouguet (CJ) velocity was 2376 m/s. There are several reasons why the velocity may have differed so widely from the CJ value, including the presence of burned gas in front of the detonation wave, the complicated wave structure due to non-uniformity of the mixture in the RDC, insufficient propellant mixing, and the difference between the true and actual wave propagation direction. The velocity and amplitude of the combustion chamber static pressure appeared to be correlated. Averaged combustion chamber static pressure reached 0.432 MPa, which was 89.0% and 92.8% of the fuel and oxidizer plenum pressure, respectively. Dynamic pressure was also estimated using an equilibrium calculation. The resulting dynamic pressure was 0.008 MPa, and estimated total pressure was 0.440 MPa; these values were 90.1% and 94.6% of the fuel and oxidizer plenum total pressure, respectively, even though pressure was lost through the small diameter injector holes. [ABSTRACT FROM AUTHOR]

A two-dimensional investigation is conducted to clarify the velocity decrease mechanism of a detonation wave propagating within a rotating detonation engine (RDE) chamber, focusing on the effect of injection conditions of the fuel and oxidizer. Two numerical studies are conducted; the first imitates the RDE chamber with injection ports in a rectangular computational domain, and the second simplifies the flow field in the RDE chamber based on observations of the first simulation. The injection is parameterized by the geometrical jet and the gas conditions (i.e., premixed or non-premixed). In the first simulation of the RDE chamber, the total length of the injection nozzles constitutes 20% of the length of the chamber's lower end. The number of nozzles, N , ranges from 10 to 500 under premixed and 25 and 125 under the non-premixed condition. As the nozzle interval increases, more burned gas exists in front of the detonation wave. Under the non-premixed condition for N = 25, the propagation velocity decreases to 84% of the CJ velocity, and 82.4% of the injected C 2 H 4 is burned by the detonation. The CJ velocity is theoretically calculated, based on the amount of burned C 2 H 4 , to be 2283 m/s, which is not close to the propagation velocity of the simulation, 1999 m/s. In the simplified model in the second study, C 2 H 4 , O 2 , and burned gas are quiescently placed rather than injected, and the role of the upper wall is also examined. Significant velocity decrease, down to 86% of the CJ velocity, is observed when C 2 H 4 and O 2 are placed separately with a large width of burned gas between them. The amount of incompletely combusted C 2 H 4 is inversely proportional to the propagation velocity except in the case with an upper wall. Therefore, burned gas has a larger effect on propagation velocity in the non-premixed condition. [ABSTRACT FROM AUTHOR]

Abstract: Direct initiations and stabilizations of three-dimensional conical detonation waves were attained by launching spheres with 1.06–1.31times the C–J velocities into detonable mixtures. We conducted high time-resolution Schlieren visualizations of the whole processes over unsteady initiations to stable propagations of the stabilized Oblique Detonation Waves (ODWs) using a high-speed camera. The detonable mixtures were stoichiometric oxygen mixtures with acetylene, ethylene or hydrogen. They were diluted with argon in a 50% volumetric fraction, and a 75% diluted mixture was also tested for the acetylene/oxygen. The direct initiation of detonation by the projectile and the DDT process like the re-initiation appeared in the initiation process of stabilized ODW. This process eventually led to the stabilized ODW supported by the projectile velocity and the ringed shape detonation wave originating in the re-initiation. We modeled the spatial evolution of stabilized ODW after the re-initiation based on its C–J velocity and angle. The model qualitatively reproduced the measured development rate of stabilized ODW. We also discussed about the detonation stability for the curvature effect arising from the three-dimensional nature of stabilized ODW around the projectile. The curvature effect attenuated the detonation wave below its C–J velocity at the vicinity of projectile. The propagation limits of curvature effect will be responsible for the criticality to attain the stabilized ODWs. By accessing the detailed distributions of propagation velocities and curvature radiuses, the critical curvature radiuses normalized by the cell sizes experimentally revealed to be 8–10 or 15–18 for mixtures diluted with each 50% argon or 75% argon/krypton. [Copyright &y& Elsevier]

Abstract: The geometry and characteristic length of diffraction and re-initiation during a two-dimensional detonation propagation were revealed by visualization. C2H4 +3O2 (unstable), 2C2H2 +5O2 +7Ar (stable) and 2C2H2 +5O2 +21Ar (stable) were used as the test mixtures. Experiments were performed over the deviation angle range from 30° to 150° and the initial pressure range from 15.8 to 102.3kPa. By self-emitting photography, we confirmed that the geometry and the characteristic length of diffraction are not different among test gases, with the exception of the fan-like structure of re-initiation that occurred regardless of whether the mixture was unstable or stable. We conducted a compensative experiment by changing the deviation angle and initial pressure, and summarized the detonation diffraction by shadowgraph. At deviation angles larger than 60°, we measured the distances from the vertex of the channel corner to the point where the transverse detonation wave reflected on the under wall (=wall reflection distance) and confirmed that wall reflection distances are approximately in the range of 10–15 times the cell width, whether the mixture is unstable or stable. [Copyright &y& Elsevier]

Abstract: The propagation of curved detonation waves of gaseous explosives stabilized in rectangular-cross-section curved channels is investigated. Three types of stoichiometric test gases, C2H4 +3O2, 2H2 +O2, and 2C2H2 +5O2 +7Ar, are evaluated. The ratio of the inner radius of the curved channel (r i) to the normal detonation cell width (λ) is an important factor in stabilizing curved detonation waves. The lower boundary of stabilization is around r i/λ =23, regardless of the test gas. The stabilized curved detonation waves eventually attain a specific curved shape as they propagate through the curved channels. The specific curved shapes of stabilized curved detonation waves are approximately formulated, and the normal detonation velocity (D n)−curvature (κ) relations are evaluated. The D n nondimensionalized by the Chapman–Jouguet (CJ) detonation velocity (D CJ) is a function of the κ nondimensionalized by λ. The D n/D CJ−λκ relation does not depend on the type of test gas. The propagation behavior of the stabilized curved detonation waves is controlled by the D n/D CJ−λκ relation. Due to this propagation characteristic, the fully-developed, stabilized curved detonation waves propagate through the curved channels while maintaining a specific curved shape with a constant angular velocity. Self-similarity is seen in the front shock shapes of the stabilized curved detonation waves with the same r i/λ, regardless of the curved channel and test gas. [Copyright &y& Elsevier]

Abstract: We studied experimentally the shock waves and combustion waves generated by a hypersonic spherical projectile in an explosive mixture. An acetylene/oxygen mixture diluted with argon (2C2H2 +5O2 +7Ar) was used with various initial pressures (detonation cell sizes) to observe optically with a shadowgraph imaging system a shock-induced combustion (SIC), a stable oblique detonation wave (ODW), and a wave called a Straw Hat type consisting of a strong SIC and ODW. The criticality of stabilizing an ODW around a projectile is expressed by the ratio of the projectile diameter, d, to the cell size, λ, as d/λ =3.63–4.84. Although the Straw Hat type wave in the vicinity of criticality is an unstable phenomena, it has been mainly observed by a single frame picture to date, so that it is difficult to discuss the time history of its wave structure. In this study, it was remarkable to directly carry out continuous optical observations using a high speed video camera which can continuously film 100 pictures with a 1μs frame speed so as to allow an investigation of the sustaining mechanism of the unstable wave structure. Our results allowed the identification of an increase in unsteadiness in the relative distance between the projectile fronts and the transition points to an ODW as the time increased. They also showed local explosions in the SIC region near transition point transformed the ODW front upstream. [ABSTRACT FROM AUTHOR]

Abstract: Oblique detonation waves, which are generated by a fundamental detonation phenomenon occurring in bent tubes, may be applied to fuel combustion in high-efficiency engines such as a pulse detonation engine (PDE) and a rotating detonation engine (RDE). The present study has experimentally demonstrated that steady-state oblique detonation waves propagated stably through rectangular-cross-section bent tubes by visualizing these waves using a high-speed camera and the shadowgraph method. The oblique detonation waves were stabilized under the conditions of high initial pressure and a large curvature radius of the inside wall of the rectangular-cross-section bent tube. The geometrical shapes of the stabilized oblique detonation waves were calculated, and the results of the calculation were in good agreement with those of our experiment. Moreover, it was experimentally shown that the critical condition under which steady-state oblique detonation waves can stably propagate through the rectangular-cross-section bent tubes was the curvature radius of the inside wall of the rectangular-cross-section bent tube equivalent to 14–40 times the cell width. [ABSTRACT FROM AUTHOR]

Abstract: There are not many studies on DDT with no obstacles and the initiation of DDT near the end of a closed tube. Therefore in the present study we experimentally investigate the mechanism of the combustion wave transition to a detonation wave when there are no obstacles. In particular, we show that a local explosion near the tube wall is necessary for the initiation of a detonation. Parameters that we varied are the wall configuration, distance between the ignition point and the wall, and initial filling pressure. The combustion waves and the compression waves are visualized using the Schlieren optical system. From the results, we found it is necessary for the combustion wave to reach four walls so that the detonation could be initiated by the local explosion. In the conditions of the present experiment, we exhibited that the local explosion did not occur in the vicinity of a single wall and four orthogonal walls; instead, the local explosion occurred in a situation with five orthogonal walls. The time of the local explosion and the detonation initiation is 2.6±1.1 and 2.0±0.1 times the characteristic time for the combustion wave to propagate hemispherically from an ignitor and reach the four walls. [ABSTRACT FROM AUTHOR]