K. C. Schadow

Large-scale coherent structures as drivers of combustion instability

Abstract The role of flow coherent structures as drivers of combustion instabilities in a dump combustor was studied. Results of nonreacting tests in air and water flows as well as combustion experiments in a diffusion flame and dump combustor are discussed to provide insight into the generation process of large-scale structures in the combustor flow and their interaction with the combustion process. It is shown that the flow structures, or vortices, are formed by interaction between the flow instabilities and the chamber acoustic resonance. When these vortices dominate the reacting flow, the combustion is confined initially to the circumference of their cores and further downstream proceeds into their core, leading to periodic heat release, which may result in the driving of high amplitude pressure oscillations. These oscillations are typical to the occurrence of combustion instabilities for certain operating conditions. The basic understanding of the interaction between flow dynamics and the combustion ...

Effect of Flame-Holding Cavities on Supersonic-Combustion Performance

An experimental study was performed to evaluate the e ame-holding and mixing enhancement characteristics of supersonic reacting e ow over acoustically open cavities. Several cone gurations of acoustically open cavities were placed inside a supersonic-combustion duct just downstream of the fuel injection ports. The resulting changes in e ame behavior and combustion characteristics were assessed using schlieren visualization of the uncone ned e ow and wall pressure measurements of the cone ned e ow along the duct. The results were then compared with the baselinecase, which used no cavity. Although thecavitiesimproved thecombustion performancefrom thebaseline, the amount of enhancement was dependent on the particular shape of the cavity as well as the e ow conditions. Certain cavity cone gurations that were strategically placed inside the combustion duct led to a faster increase in the axial pressure force. The data showed that the recovery temperature was higher and the total pressure proe le was more uniform at the exit plane, suggesting enhanced volumetric heat release and faster mixing associated with the cavity- ine uenced e owe eld.

Combustion-related shear-flow dynamics in elliptic supersonic jets

Abstract : An elliptic jet having an aspect ratio of 3:1 was studied and compared to a circular jet at three Mach numbers: M = 0.15, 1 and 1.3. Hot-wire measurements and Schlieren photography were employed in this study. The superior mixing characteristics of an elliptic jet relative to a circular jet, which were found in previous works in subsonic jets, prevail in the sonic jet and are further augmented by the shock structures of the supersonic underexpanded jet. The major and minor axes switch at a distance of 3 diameters from the nozzle, and the spreading rate of the minor axis side is twice that of a subsonic jet. The experimental data are supported by results of the linear instability analysis of the supersonic elliptic jet which show that the initial vortices are bending at the major axis side in a similar way to the process which occurs in a subsonic elliptic jet.

Noncircular jet dynamics in supersonic combustion

The mixing characteristics of circular, small-aspect-ratio elliptic and rectangular jets were studied in subsonic, sonic, and supersonic flows. The experiments were carried out in both cold and hot flows using hot-wire anemometry, thermocouples, and photography. The elliptic and rectangular jets had similar features, with a slightly better mixing performance of the elliptic jet in the subsonic and supersonic flows. The elliptic and rectangular jets had a higher spreading rate relative to the circular jet, epecially at the minor axis plane. In the subsonic jet, the spreading rate was limited to the first five equivalent diameters (De) from the nozzle. In the supersonic underexpanded jet, the spreading rate was three times higher in the entire range (30£>e) measured. The minor axis plane was also characterized by high intensity of near-field pressure fluctuations. The two phenomena can be related to each other when an acoustic feedback occurs. Nomenclature & = aspect ratio D = circular nozzle diameter De = equivalent diameter E = energy of the fluctuating pressure components in the power spectrum / = frequency M = Mach number r = radial coordinate Re = Reynolds number, U0De/v R05 = half-width of the jet; r at with U= U^ /2 Rw 5 = half-width of the reacting jet; r at which

Near acoustic field and shock structure of rectangular supersonic jets

An underexpanded supersonic rectangular jet is studied experimentally at a pressure ratio range of 1-15. The shock-cell and shear-layer structure variation with the pressure ratio is shown to be related to the near-field pressure fluctuations. Near the sonic, fully adapted velocity, the jet is fully symmetric. An abrupt change to a flapping mode occurs at a low Mach number, causing a large increase in the spreading rate, which is also related to the appearance of an upstream propagating screech component

Active Combustion Control in a Coaxial Dump Combustor

Abstract Suppression of combustion instability in a dump combustor was demonstrated with open-loop and closed-loop active control systems. Pressure oscillations and CH-emission sensors were used to monitor the combustion process. Acoustic drivers were used to modulate the fuel and modify the shear layer at the dump. The open-loop active-control system applied high frequency acoustic forcing to break down the coherence of the large-scale vortical structures, thus reducing the periodic heat release which excites the instability. The closed-loop controller used the CH-emission signal or the pressure fluctuations, after filtration, to lock the acoustic excitation at various relative phase angles. The pressure locking was more effective in suppressing the oscillations at a relative phase angle range of 250-330°. The reduced oscillations were observed by both CH and pressure sensors. The CH locking was significantly less effective relative to the pressure locking, but had a similar range of phase angles in whic...

Flow characteristics of orifice and tapered jets

Two types of small‐aspect‐ratio slot jets were investigated. An orifice slot jet with a 3:1 aspect ratio was shown to have features similar to an elliptic jet with a well‐designed contraction. A larger spreading rate and enhanced turbulent mixing, relative to a circular jet, can be obtained using this orifice nozzle, which is advantageous for many practical applications. By adding a conical contraction to the slot jet (tapered slot jet) the characteristics were totally altered. The axes switching, typical to an elliptic jet, did not occur and the spreading rate at the major axis plane was larger than at the minor axis plane. The turbulence amplification in the initial core region was higher than both in the circular and elliptic jets. The transition from the conical contraction of the nozzle to the slot shape of the outlet section produced three‐dimensional vortical structures. These structures interacted with the asymmetric vortices shed from this noncircular configuration to enhance the small scale turb...

Subsonic and supersonic combustion using noncircular injectors

Nonreacting and combustion tests were performed for subsonic, sonic, and supersonic conditions using noncircular injectors in a gas generator combustor. The noncircular injectors, including square, equilateral-, and isosceles-triangular nozzles, were compared to a circular injector. The flowfields of the jets were mapped with hot-wire anemometry and visualized using spark schlieren photography. The combustion characteristics were visualized by high-speed photography and thermal imaging, and the temperature distribution was measured by a rake of thermocouples . The present tests conducted at high Reynolds and Mach numbers confirmed earlier results obtained for the low range of these numbers, i.e., the combination of large-scale mixing at the flat sides with the fine-scale mixing at the vertices is beneficial for combustion. Large-scale structures provide bulk mixing between the fuel and air, whereas fine-scale mixing contributes to the reaction rate and to better flameholding characteristics.

Initial Development of Noncircular Jets Leading to Axis Switching

This paper discusses the underlying mechanisms for the deformation of coherent structures which occurs in the initial stage of the axis switching of noncircular jets. The generalized shooting method is applied to jets with elliptic-core and equilateral-triangular-core regions of constant flow. The analysis reveals that in order to have the deformation, three requirements must be present in the behavior of the eigenmodes of noncircular jets: 1) the eigenfunctions are localized without excessive overlapping; 2) the amplification rates of the corresponding eigenmodes are comparable; and 3) sufficient phase speed difference exists between the eigenmodes. The qualitative behavior of the noncircular jets found through the numerical analysis is compared with experimental results and are in good correlation with them.

On the role of large and small-scale structures in combustion control

Abstract Experiments were performed to actively control combustion between coaxial air and fuel jets. The objective was to initiate the combustion as close as possible to the flameholder surface and to maintain uniform combustion in the entire mixing region. Forcing was applied to both the air and fuel streams. at different frequencies and amplitudes. The air jet was excited at its preferred mode frequency while the fuel was forced at higher harmonics in order to trigger and amplify the initial instabilities of the coaxial jet. Nonreacting tests showed that the combined forcing was promoting an earlier transition to small-scale turbulence at the nozzles exit. Consequently. the combustion was enhanced and became uniformly distributed along the flame. contrary to the reference unforced flame where intense combustion started only at a distance from the flameholder and was limited to the region where vortical structures developed. The optimal combination of forcing parameters are presented and discussed.