This experimental work reports on detonation behaviors in a curved chamber without inner wall after diffraction of a Chapman–Jouguet (CJ) detonation from a straight channel tangent to the chamber ...outer wall. The upper and lower faces of the chamber receive either soot foils for recording the history of the transmission dynamics, or optical windows for schlieren high-speed visualizations. Tests were carried out with the stoichiometric propane-oxygen mixture at initial temperature T0 = 288 K and initial pressures ranging between 8 kPa and 15 kPa. The primary observation is the existence of an initial pressure range (8–12 kPa) for which, after diffraction transients, a Mach detonation can rotate normal to the outer wall with a constant angular velocity such that the normal front velocity at the wall is larger than the CJ value. The height of the Mach front decreases and its tangential velocity increases with increasing initial pressure. This overdriven detonation results from the irregular reflection of the initially-oblique front as the outer wall tilts with respect to the front, similarly to shock propagation in a continuously-converging channel. The Mach triple-point moves away from the wall and stabilizes its trajectory parallel to the wall. This Mach front shows detonation cells parallel to the wall with a constant mean width very small compared to that on the initial CJ front. A detonation can thus smoothly propagate in a curved chamber without center body, though the Mach front does not sweep the entire equivalent cross-section of the chamber. These observations are consistent with experimental results for RDE chambers without center body, or with linearly-increasing cross sections, that show improved properties of rotating detonation, compared to chambers with constant cross sections. This supports the interest of further studies on the hollow-chamber configuration of RDEs.
The dynamics of detonation transmission from a straight channel into a curved chamber was investigated numerically and experimentally as a function of initial pressure (10 kPa ≤ p0 ≤ 26 kPa) in an ...argon diluted stoichiometric H2–O2 mixture. Numerical simulations considered the two-dimensional reactive Euler equations with detailed chemistry; hi-speed schlieren and OH* chemiluminescense were used for flow visualization. Results show a rotating Mach detonation along the outer wall of the chamber and the highly transient sequence of events (i.e. detonation diffraction, re-initiation attempts and wave reflections) that precedes its formation. An increase in pressure, from 15 kPa to 26 kPa, expectedly resulted in detonations that are less sensitive to diffraction. The decoupling location of the reaction zone and the leading shock along the inner wall determined where transition from regular reflection to a rather complex wave structure occurred along the outer wall. This complex wave structure includes a rotating Mach detonation (stem), an incident decoupled shock-reaction zone region, and a transverse detonation that propagates in pre-shocked mixture. For lower pressures, i.e. ≤ 10 kPa, the detonation fails shortly after ignition. However, the interaction of the decoupled leading shock with the curved section of the chamber results in detonation initiation behind the inert Mach stem. Thereafter, the evolution was similar to the 15 kPa case. Simulations and experiments qualitatively and quantitatively agree indicating that the global dynamics in this configuration is mostly driven by the geometry and initial pressure, and not by the cellular structure in highly compressed regions.
This experimental and numerical work reports on the dynamical behaviour of a shock in an inert gas at the concave wall of a hollow circular chamber. The gas in the chamber was air or He +
O
2
+ 2 Ar ...at initial pressures
p
c
0
ranging from 2 to 12 kPa and initial temperature
T
0
=288 K. The shock was generated using a detonation driven shock tube. The shock dynamics were characterized through high-speed shadowgraph recordings and high-resolution numerical simulations. For each gas and
p
c0
, the experiments evidenced the formation of a Mach reflection along the wall and identified a range of initial pressures for which this configuration rotates with constant stem heights and constant velocities larger than those at the chamber entry. The numerical simulations were capable of capturing the dynamics quantitatively. These results extend to inert gases our previous work with a reactive gas for which we reported on the possibility of a steadily rotating overdriven Mach detonation. The steadiness range is narrower with the inert gases, likely because of the smaller initial pressure ratios at the chamber entry and lower support from the subsonic flow behind the shock. The initial support in the reactive case was more efficient because the discontinuities at the chamber entry were self-sustained Chapman–Jouguet detonations. Further investigations of these Mach rotating regimes should rely only on specific experiments and numerical simulations, for example, on the effect of the chamber dimensions, because of the complex non-dimensional formulation of the problem.
The detonation regime is an alternative to the conventional constant-pressure combustion mode typically used for propulsive systems because of its higher thermal efficiency and temperature and ...pressure of products, and shorter characteristic combustion time and length. The classic implementation is the rotating detonation engine, with the combustion chamber consisting of the annular space between a center-body and an outer cylindrical wall. This experimental study focuses on the effects of the chamber inner geometry, the total mass flow rate, and the detonation cell width on the conditions for detonation rotation. Cylindrical and conical center-bodies with several lengths and half-apex angles are considered to approach the hollow configuration of the RDE chamber. The cell width is varied by testing with mixtures of ethylene and enriched air, with several equivalence ratios and nitrogen dilutions. The combustion modes and the detonation velocities and pressures are characterized by analyzing pressure signals and high-speed camera visualizations. Three detonation regimes are identified, characterized by one or two fronts propagating in the same or opposite directions. Decreasing the center-body length and increasing the half-apex angle increases the measured detonation velocity and pressure. Velocities range between 53 and 89% of the Chapman–Jouguet value, and the pressure reaches about 11 bar. For the conditions tested, higher detonation velocity and pressure are obtained for the conical center-body configuration. Our interpretation is that center-bodies that are too long, or channels that are too narrow, hinder the exhaust of the burned gas. As a result, the proportion of products in the unburned gas mixture ahead of the detonation wave (consisting of fresh and burned gas) increases, resulting in a decrease in the magnitude of the detonation properties.
This experimental work investigates the possibility to non-dimensionalize the limits and the distances of the deflagration-to-detonation transition process (DDT). The deflagration was ignited using ...jets of hot gases generated by the impact of a Chapman–Jouguet detonation on a multi-perforated plate. The tube was 1 m long with a square cross section
40
×
40
mm
2
. The reactive mixtures were the stoichiometric compositions of hydrogen, methane, and oxygen (
1
-
x
)
H
2
+
x
CH
4
+
1
/
2
(
1
+
3
x
)
O
2
with the composition parameter
x
ranging from 0 to 1. The initial pressure
p
0
was varied from 12 to 35 kPa, and the initial temperature was 294 K. The widths of the detonation cells and the conditions and distances for DDT were obtained as functions of
x
,
p
0
, the thickness of the plate, and the number and diameter of its perforations. The cell width was used as the reference length. The non-dimensional DDT distances correlate well with the non-dimensional number representing the surface re-ignition effect in the form of a concave increasing function. The non-dimensional DDT limits appear to be independent of the surface dissipation phenomena in the perforations. These trends are found to be independent of the regularity of the detonation cells. DDT processes are very dependent on the system configuration and the ignition conditions, but our analysis suggests that the proper selection of non-dimensional numbers based on the system characteristics can predict the DDT limits and distances to a reasonable approximation.
The transition between different combustion regimes is investigated experimentally for a stoichiometric argon-diluted n-decane/O
2
mixture. The focus is put on the influence of initial temperature (
...T
= 420–470 K) and pressure (
P
= 1.5–3 bar) on the regime transitions. Fast schlieren visualization (≥ 120 kHz) and high-speed pressure and temperature measurements are used to monitor the evolution of the reactive processes inside a combustion chamber of square cross-section (40 mm × 40 mm × 172 mm). Results for
P
≥ 2.5 bar and the entire temperature range and for
P
= 2 bar and
T
≥ 440 K show three distinct stages following the adiabatic compression of fresh gases induced by the propagation of a flame into the chamber/test section, namely a cool flame, a main heat release stage, and detonation onset. For
P
= 1.5 bar, however, only the first two stages of the process are observed in the temperature range studied. A two-stage autoignition phenomenon, typical of large hydrocarbons, occurs systematically in the end gas and generates consecutive reactive fronts. The transition to detonation appears to result from the acceleration of the aforementioned fronts toward the speed of sound in fresh gases. Notably, the compression history plays a key role in setting conditions for detonation onset. Our results are in agreement with classical transition maps available in the literature.
The aim of this study is to experimentally investigate the combustion regime transitions of a single-component kerosene surrogate (n-decane). For this purpose, a deflagration is generated by a spark ...in a constant-volume vessel with a length-to-width aspect ratio of 4.3. By consuming the unburnt gas, the flame behaves as a piston that compresses the end gas. The pressure and temperature of the end gas increase with the flame propagation until autoignition conditions are reached. Ultrafast schlieren visualizations are set up to monitor the dynamics of the reactive processes, whereas the time-pressure evolution associated with non-dimensional (0D) numerical models is used to characterize the unburnt gas thermodynamic conditions. Once the thermodynamic conditions needed to initiate the autoignition reactions are reached in the end gas, a transition between deflagration (flame speed of approximately ten m/s) and autoignition fronts is observed (propagation velocity ˜200 m/s). This transition occurs for various fuel equivalence ratio values (0.75 – 1). For the strongest thermodynamic conditions, once the velocity of this autoignition front reaches the speed of sound, a new reactive front propagating up to 1800 m/s is observed, thus indicating the transition to detonation combustion mode. Parametric studies indicated that the occurrence of both transitions was a function of the pressure, temperature and equivalence ratio for n-decane fuel. A small initial temperature variation (approximately 40 K) could change the phenomenology of the constant-volume combustion from deflagration to detonation through the autoignition process, even for lean mixtures. The results obtained using our experimental setup show that the transition between autoignition and detonation is due to the acceleration of the autoignition front towards the speed of sound in the unburnt gas.
The propulsive potential of a reactive mixture that uses detonation as a combustion process is studied. An experimental set up is built up to determine the thrust and the impulse developed in single ...and multi-operating cycles by the detonation products of a reactive mixture contained in a cylindrical combustion chamber (CC). One end of the CC, called the thrust wall (TW), is closed and supports the thrust. The other end is open into atmosphere for the exhaust of the detonation products. The detailed flow features are experimentally investigated by means of pressure gauges inside the CC and in the immediate vicinity of open-ended of CC. The specific impulse I
sp
reached in our device, with C
2
H
4
= 3 O
2
as a detonative mixture, is about 200 s. The overpressure profiles recorded on the TW show clearly that the flow inside the CC is self-similar. Consequently, a relationship between I
sp
and the Chapman Jouguet's characteristics of the detonative mixture is established. The maximum operating frequency can be linked to the scale of the reactive mixture contained in CC and the maximum averaged thrust is deduced. The atmospheric shock wave produced is found to be equivalent to that of a strong point source of explosion of energy E
O
liberated by the reactive mixture. For the pulsed detonation results, the same thrust and impulse profiles as in single shot was obtained, but it was found a deficit about 30% in the level of thrust and consequently on the specific impulse. This deficit is mainly due to the quality of the filling of the CC and of the renewing the reactive mixture because of low value of length to diameter ratio of CC.
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