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... moderate cycle pressure ratios [2]. As a result, there has been a widespread experimental and computational effort to study the integration of detonating PGC into existing systems and harness this potential performance benefit. Historically, most detonating PGC research was conducted using pulse detonation engines (PDEs) due to the simplicity, and ease of control provided by these devices on the laboratory level. A pulse detonation engine generally consists of a tube, closed on one end, with reactants administered at the closed end, and exhaust products exiting at the open end, with the PDE cycle depicted in Fig. 1. Detonation is achieved in the reactants by sufficient ignition energy input, or by use of deflagration-to-detonation transition (DDT) obstacles. After the detonation products expand through the tube exit, the PDE is generally purged or cooled to prevent undesirable auto-ignition of the freshly injected reactants for the following cycle. Depending on the tube geometry, reactant injection scheme, purging or cooling scheme, and the boundary conditions, the PDE cycle is generally limited to maximum operating frequencies of 10-100 Hz. Unfortunately, there are many practical challenges that impede efficient integration of PDEs into existing cycles. Robust valving of fresh reactants generally requires moving parts and adds weight and complexity, while timing and control of the valving and ignition process must be precise for efficient operation. The unsteady admission of fresh reactants and unsteady expulsion of exhaust products generates considerable unsteadiness at the compressor exit and turbine inlet, which can severely degrade performance of those components. Direct detonation initiation is difficult to achieve for less-detonable, commercially-viable hydrocarbon fuels, requiring the use of loss-inducing DDT obstacles. The length of tube required to achieve DDT increases the combustor size, and the relatively slow transition process limits maximum operating frequency. For each reactant mixture, there exist minimum combustor size requirements to support detonation [3], such that compact designs are problematic for less-detonable fuels. While significant progress has been made in overcoming these technical challenges, a new type of detonating PGC, the rotating detonation engine (RDE) has emerged as a popular alternative. An RDE is generally comprised of an annular channel, supporting a continuous detonation wave, which rotates tangentially through the channel at supersonic velocities (Fig. 2). The channel is continuously fed with fresh reactants, which are consumed with each successive pass of the rotating detonation wave, and products are ejected axially downstream. While RDEs were first developed and tested half a century ago [5], RDE research has broadened significantly in the last decade, partially due to the potential benefits over PDEs. The continuous, valveless injection of reactants in RDEs is mechanically and aerodynamically superior to valved PDE designs, as it results in reduced weight and complexity, and reduces the unsteadiness imposed on upstream components. Furthermore, precise timing and control is generally limited to a single RDE initiation event, compared to the repeated cyclic initiation required in PDE operation. As RDEs generally operate in the kilohertz regime, exhaust pulsations are more easily mixed out, reducing the unsteadiness on downstream components. While initiation is still difficult to achieve for less-detonable mixtures, transition to detonation in an RDE is only required 2 American Institute of Aeronautics and Astronautics upon startup, as a stable detonation can be continuously maintained. Geometrically, the RDE is more compact than the PDE, and its annular shape improves its ease of integration with existing components. These considerable advantages make the RDE the leading PGC candidate for combined cycle applications. Despite these advantages, there are significant technical challenges for RDEs that impede widespread adoption. Achieving adequate mixing of reactants remains a challenge for non-premixed designs, while flashback remains a considerable problem for premixed designs. Initiation and stable operation of less-detonable fuels remains difficult, especially when using air as an oxidizer. Excessive heat loading can lead to failure or reduced lifespan of the combustor and downstream components without the addition of active cooling or secondary flow. These challenges highlight the critical need for RDE research to unlock the capabilities of PGC combined cycles. The present RDE facility was constructed as part of the University of Cincinnati Detonation Engine Test Facility, part of the UC Gas Dynamics and Propulsion Laboratory (GDPL). As a part of the existing test facility, it is housed underground within the detonation test bunker and fully isolated from the GDPL. The air-breathing RDE (Fig. 3) is based on the Shank et al. 2012 design [6], and many of its baseline components were manufactured by AFRL. The components in direct contact with the combustion process are machined from stainless steel, while aluminum is used for upstream components. The combustor consists of a center body and outer body which comprise the walls of the annulus, with reactant injection occurring on the upstream end at the base of the annulus. As in the Shank baseline design, air is injected radially-inward from an air plenum through an air injection slot (Fig. 4). Fuel is injected axially through an injection plate from a fuel plenum, perpendicular to the air stream to generate crossflow mixing. The center body is mounted onto the fuel injection plate, while the outer body bolts into the outer wall of the air plenum. The air plenum is fed radially by five, equal-length runners from an upstream manifold, while the fuel plenum is fed axially by a single, central inlet. Initiation is achieved with a predetonator, which enters the annulus tangentially, just above the fuel injection plate. To increase the chamber pressure above ambient conditions, a converging nozzle was developed, which mounts directly to the center body. The entire RDE assembly is mounted horizontally on a modular test stand, with the potential to upgrade this stand to support thrust ...

Citations

... The presence of a combustion front was detected with an ion probe mounted in the same plane of the dynamic pressure transducer. Details of the ion probe implemen-tation can be found in a study by St. George et al. [34] as the described apparatus follows their methodology. All signals were recorded with a computer via an NI PICe-6363 high-speed data acquisition device. ...
Article
This work is an experimental demonstration of a novel resonant pulse combustor technology for which sustained pressure gain can be achieved at thermal equilibrium. The resonant pulse combustor employs an actively driven air inlet valve with passively modulated fuel injection such that the device operating frequency is specified by the valve set-point frequency. This configuration enables more control over the combustor operation compared to passive valve and valveless resonant pulse combustors and enables the durability necessary for consideration in practical applications. Combustor performance is characterized with measured stagnation pressure gain as a function of stagnation temperature and fuel mass flux. Steady-state stagnation pressure ratios between 1.005 and 1.025 are demonstrated for gaseous (ethylene) and liquid (gasoline) fuels at temperature ratios from 2.8 to 4.5. Baseline performance measurements for a well-studied passive valve resonant pulse combustor are presented for comparison. The proposed active valve technology provides a direct path for translation from research-type devices to practical gas turbine applications by addressing key challenges inherent of existing resonant pulse combustor technologies.
... This pressure gain allows for additional work to be extracted by turbomachinery, thus increasing the cycle thermal efficiency. Although the operation of an RDC is conceptually simple, practical implementations [1][2][3][4][5][6][7][8][9] of the Communicated by E. Gutmark. B M. Gamba mirkog@umich.edu 1 University of Michigan, 1320 Beal Avenue, Ann Arbor, MI 48109, USA concept introduce additional uncontrolled processes. ...
Article
Full-text available
In this work, we experimentally investigate secondary waves in rotating detonation combustor (RDC) operation. Secondary waves are finite-strength periodic perturbations of the field, which manifest as reacting fronts and/or pressure rise rotating around the annulus superimposed to one or more detonation waves. Secondary waves interact with the detonation wave, affecting the operation of the RDC, as well as the potential of realizing pressure gain. Through analysis of high-speed end-view chemiluminescence imaging in the detonation channel, the characteristics (speed, multiplicity, and strength) of different systems of waves are identified. The analysis indicates that in addition to the main detonation wave, two secondary wave systems are present: (1) a pair of co-rotating waves moving counter to the detonation wave at a speed near the acoustic speed of the products of combustion and (2) a wave moving counter to the detonation wave at a speed near that of the detonation wave. These two types of secondary waves are consistently observed in three canonical injection schemes. We further investigated the impact of the wave pair on the structure of the main detonation wave in one inlet configuration. By constructing conditional phase-averaged distributions of the pressure and OH* emission over the interaction between the detonation and secondary waves, we reconstruct the structure of the detonation wave during the interaction. The results show that there is a nonlinear interaction between the detonation and secondary waves, which results in an augmentation of the pressure rise (increase by as much as 60%) across the detonation wave as well as partial suppression of the heat release as the two waves interact (variation up to an order of magnitude). This nonlinear interaction is supported by differences in the temporal response of the air and fuel streams subject to the propagation of secondary waves, generating fill region stratification leading to the partial suppression of heat release when the secondary wave collides with the main detonation wave.
... The presence of a combustion front was detected with an ion probe mounted in the same plane of the pressure transducer. Details of the ion probe implementation can be found in a study by George et al. [17] as the described apparatus follows their methodology. The valve position was recorded by an absolute positioner connected to the ball valve through a shaft. ...
... The presence of a combustion front was detected with an ion probe mounted in the same plane of the pressure transducer. Details of the ion probe implementation can be found in a study by George et al. [17] as the described apparatus follows their methodology. The valve position was recorded by an absolute positioner connected to the ball valve through a shaft. ...
... The cross section can be described as a confined JIC geometry with a 90 • turn, where one or multiple dye containing jets are issuing into a water crossflow. The generic RDC geometry was developed by Shank [12], and extensively investigated in combustion experiments by others [3,[35][36][37][38][39]. This investigation focuses on the spatial mixing in the detonation channel with the goal of understanding and improving the mixing characteristics of the RDC. ...
Article
Full-text available
Reactant mixing has been identified as one of the driving factors for successful and stable operation of Rotating Detonation Combustors (RDC). This work investigates the stationary mixing in a scaled model RDC cross section in a water tunnel. Two configurations with one and five dyed water jets injecting into a confined water crossflow modeled the mixing scheme used in the radially inward injecting RDC at TU Berlin. The influence of several parameters on the mechanisms driving the mixing quality was investigated, with the objective of improving future injection strategies. The parameters studied were: the position of the fuel injectors relative to the RDC outer wall, the shape of the corner between the oxidizer injection slot and the detonation annulus, and the ratio of fuel to oxidizer momentum flux. High-speed PLIF imaging of the longitudinal plane centered at the middle jet injection hole, as well as of several planes perpendicular to the longitudinal plane, confirmed the existence of a strong shear layer and recirculation zone at the RDC outer wall corner that significantly influenced the mixing. Depending on the jet location and the jet-to-crossflow velocity ratio, different mechanisms impacted the reactant mixing.
... The expected outlet-flow direction of detonation products with respect to the chamber axis is qualitatively indicated by arrows, and the details of the actual flow remain a subject of active investigation. The axial geometry (Fig. 1a) has been studied most extensively, where the RDWs propagate between two coaxial cylinders, with fresh gas injection at the chamber bottom and burned gas ejection at the top [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. In the radial geometries, the combustion chamber is defined by the space between two disks bounded by an outer cylindrical wall for the centripetal version (Fig. 1b), or an inner cylindrical wall for the centrifu-Axial (a) Radial centripetal (b) Radial centrifugal (c) Fig. 1 Primary geometries of RDE: chamber walls ( ). ...
Article
Full-text available
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.
... Stoddard et al [25] used PCBs to sense the temporal fluctuations due to the oblique shock wave in the RDC. George et al. [26] used PCBs to find the direction of the waves. While pressure evolution is captured by PCB sensors, wave passage is not always correlated to the presence of active combustion, and may indicate a decaying blast wave or shock reflection. ...
... While pressure evolution is captured by PCB sensors, wave passage is not always correlated to the presence of active combustion, and may indicate a decaying blast wave or shock reflection. George et al. [26] therefore used fast response IPs to track the detonation wave. ...
Conference Paper
Recently, pressure gain combustion (PGC) has been a subject of intense study because of its potential to increase the thermodynamic efficiency of power generating gas turbines by several percentage points. The rotating detonation combustion/combustor (RDC) can provide large pressure gain within a small volume through rapid heat release by detonation wave(s) that propagate continuously in the circumferential direction. The RDC has been investigated mainly for propulsion applications using hydrogen fuel. In contrast, we present experimental results from an RDC operated on methane and oxygen-enriched air mixtures to represent the reactants in advanced power generating gas turbines. The propagation of detonation and oblique shock waves in the RDC is investigated through High Speed Video (HSV) imaging and Ion Probe (IP) data. HSV imaging requires optical access to the RDC, which can be difficult especially when the RDC is integrated with the gas turbine inlet hardware. Additionally, HSV systems are quite expensive. In contrast, IPs are inexpensive and have the advantages of small size and flexibility in the placement location and can be flush mounted causing minimal interference with the propagating wave. In this study, the detonation wave is tracked by high-resolution HSV imaging at framing rate of 200 kHz. At the same time, IPs are used to detect the rotating oblique shock wave inside the RDC, and different analysis techniques are explored to quantify the wave speed. IP voltage data are analyzed by differentiation, correlation and fast-Fourier transform methods to compute the wave speed (or rotation frequency), and the results are compared with those from the HSV image analysis. The uncertainty of different methods is discussed, and finally, the analysis techniques are applied to investigate the wave characteristics during an experiment.
... This process has been used to track combustion fronts rather effectively over the years in different setups. The working principle of an ion probe is discussed in detail in [14]. The threaded instrumentation ports (10 mm in diameter) in the pulsejet are machined in such a way that at a given axial location, two sensors (pressure and ion probe) are diametrically opposite to each other. ...
Article
Full-text available
An experimental analysis of valved pulsejets based on the Curtis-Dyna design and the concomitant results are discussed in the current paper. By altering the combustor length, the tail pipe length and by adding a flare at the aft-end, twelve different pulsejet configurations are tested. An axially-distributed array of piezoelectric pressure sensors and ion probes reveal the pressure and combustion dynamics inside these devices. Evidence is attained to support the claim that valved Curtis-Dyna pulsejets of the tested configurations behave like a Helmholtz resonator. Each cycle of a pulsejet is composed of temporally and spatially restrained combustion events. Altering the geometry induces an amplitude modulated low frequency instability inside the pulsejet that is characterized by sinusoidallyvarying peak cycle pressures. The operating frequency, peak pressures and combustion activity of the pulsejets are characterized to reveal that reliable pulsejet operation requires proper amount of coupling — defined by low time lags — between the pressure peaks and combustion events.
... There are also three ion probes (blue tabs/circles) in the second row of stations +60°, −180°and −60°. The ionization probe circuit used here [29] gives a negative voltage that correlates with the strength of ionization present. We shall henceforth use the color scheme of red, green and blue for presenting pressure traces from the stations −60°, +30°a nd +180°, respectively. ...
Article
Recent investigations into the rotating detonation phenomenon have involved its inception and sustenance in hollow combustors, in contrast to the traditional annular rotating detonation combustor (RDC) designs. Despite this proof-of-concept, the mechanism of propagation of detonation waves in hollow combustors is unclear. On the other hand, the decades-old issue of high frequency combustion instabilities, especially in rocket engines, has been known to produce distinct shock waves that are in-sync with regions of intense combustion, the reason for which is widely attributed to the Rayleigh criterion. In this paper, we argue that there is a considerable overlap in the physics behind the reported rotating detonations in hollow RDCs and the high frequency tangential combustion instabilities that are known to wreak havoc on engines. To support this notion, an atmospheric hollow combustor is experimentally tested to attain the baseline performance. It is then ‘transformed’ into a hollow RDC by the use of a flow-turning obstacle that diverts the combustible ethylene-air mixture towards the outer wall. Two distinct mechanisms are found to cause rotating detonations in a hollow combustor, and subsequently predicate its stability. The observed modes are analogous to the behavior exhibited by planar detonations at the near-limit. This explains not only the widely observed velocity and pressure deficits in rotating detonations, but also the “steep-fronted” “detonation-like” behavior noted in high frequency combustion instabilities.
... There are also three ion probes (blue tabs/circles) in the second row of stations +60°, −180°and −60°. The ionization probe circuit used here [29] gives a negative voltage that correlates with the strength of ionization present. We shall henceforth use the color scheme of red, green and blue for presenting pressure traces from the stations −60°, +30°a nd +180°, respectively. ...