1T Mode Research Articles

Numerical study of the linear and non-linear damping in an acoustically forced cold-flow test rig with coupled cavities

Quantifying the oscillation amplitude of thermoacoustic instabilities remains a critical and challenging issue, as it is a complex balance between driving and damping processes. The New Pressurized Coupled Cavities (NPCCs) setup designed for the study of acoustic damping is analyzed in this work. It is a cold-flow test rig mimicking the geometry of a liquid rocket engine and equipped with an acoustic forcing device. The chamber 1T mode triggers a strong non-linear harmonic response, while the 1T1L and 1T2L exhibit weak non-linearities. Disturbance energy budgets are used in large-eddy simulations to characterize the damping phenomena with the 1T2L and 1T1L forcing. The correct global damping of the system is retrieved, and local damping contributions are extracted. Then, a non-linear term representing the energy transfer to the harmonics is derived from non-linear acoustics theory. Combined with a linear model, this model correctly retrieves the limit-cycle of the 1T mode.

Impact of shear-coaxial injector hydrodynamics on high-frequency combustion instabilities in a representative cryogenic rocket engine

The excitation mechanism of a thermoacoustic instability in a 42-element research rocket thrust chamber with representative operating conditions with respect to European cryogenic rocket engines is investigated in detail. From previous research it was known that the chamber 1T mode can be excited by persistent heat release rate oscillations which are modulated by the resonant modes of the liquid oxygen injectors. The excitation source of the longitudinal injector eigenmodes is investigated in this study. Fibre-optical probes measuring the OH* dynamics from the recess volume of two injectors showed additional frequency content which could neither be explained by the chamber acoustics, nor the acoustics of the injection system. Instead, the temporal evolution of these frequencies correlate with the oxidizer flow velocity. In this work we show that the additional flame modulation originates from a hydrodynamic effect in the injection system. Even though the exact process cannot be precisely identified, an effect designated orifice whistling at the injector inlet orifice seems to be a likely candidate. Combining the new results with previous publications about this combustor, it is now possible to explain past and present observations in terms of the hydrodynamic and thermoacoustic conditions which are necessary for the combustion instability to appear. The conditions, which lead to an injection-driven excitation of the 1T mode are matching frequencies of the 2L mode of the injectors and the chamber 1T mode as well as a Strouhal number between 0.2 and 0.4 based on the length and flow velocity of the injector inlet orifice.

Flame-acoustic response measurements in a high-pressure, 42-injector, cryogenic rocket thrust chamber

This work presents measurements of acoustically driven flame dynamics in a 42-element, cryogenic oxygen-hydrogen rocket thrust chamber under supercritical injection conditions. The experiment shows self-excited combustion instabilities for certain operating conditions, and this work describes the nature of the flame dynamics driving the acoustic field, as far as it can be ascertained from state-of-the-art optical measurements. Optical access has been realized in the combustion chamber with both fibre-optical probes and a viewing window. The probes collect point-like measurements of filtered OH* radiation. Their signals were used to calculate the gain and phase of intensity oscillations with respect to acoustic pressure for both stable and unstable operating conditions. Through the window, synchronized high-speed imaging of the flame in filtered OH* and blue radiation wavelengths was collected. The 2D flame response was related to the local acoustic pressure to investigate the distributed intensity and phase relationships. The flame response from OH* measurements is in agreement with the theory of Rayleigh. For stable conditions the oscillations of combustion and pressure were out of phase, whereas for an excited chamber 1T mode the oscillations were closely in phase. The integrated Rayleigh index from blue imaging was not consistent with the OH* results. The reason lies in the depth of field captured by this type of imaging, and must be used in a complementary fashion together with OH* imaging. The flame response values and 2D visualization presented in this work are expected to be of value for the validation of numerical modelling of combustion instabilities.

A Flow Dynamic Characteristic Analysis of A Single Radial Swirler Combustor

Abstract A diffusion combustor with a single radial swirler in non-reacting condition is investigated via a large eddy simulation (LES). Three dynamic analysis methods – the fast Fourier transform (FFT), proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) – are implemented to investigate the flow dynamic characteristics of the combustor. The kerosene-air combustor analyzed in the study was designed by the German Aerospace Center (DLR). It has a square cross-section and uses kerosene as fuel, which is modeled as a pre-vaporized and surrogated fuel consisting of 242 species. The first tangential(1T) mode in combustor caused by the swirler emerges dominantly in the combustor. This 1T mode exhibits the largest amount energy in the combustor dynamics, as verified by POD, and the DMD analysis determines the frequency of 1876.8 Hz. The fuel injector dynamics is associated with Helmholtz resonator frequency of 816.5 Hz. To analyze the instability, the DMD method is employed to investigate the growth rate of the most dominant dynamic structure.

Linear Acoustic Analysis of the Preburner of an Oxidizer-Rich Staged Combustion Engine

The linear acoustic behavior of an oxidizer-rich staged combustion engine preburner assembly that was similar to that of the RD-170 has been investigated by using real fluid speeds of sound to accommodate supercritical pressures. The speeds of sound, and hence the resulting frequencies, were approximately 20% higher than would be expected from ideal gas assumptions, which is a critical point of difference between the acoustic behavior of the preburner versus the main chamber. The results show that the mode shapes in the preburner–turbine inlet assembly are largely invariant with respect to the boundary condition used for the turbine blade row, but the frequencies are noticeably affected. The 2L mode is totally suppressed for both the acoustically open case and the case with the blade row impedance condition. The remaining longitudinal modes are damped. Both the frequency and the mode shape of the 1T mode remain totally invariant because the oscillations are confined to the injector regions and do not exhibit acoustic velocity with components normal to the impedance surface. The lack of damping of the 1T mode by the blade rows, combined with the relatively high magnitudes of acoustic velocity near the combustion zone, suggests that the 1T mode is a likely candidate to experience instability if it is suitably energized by the heat release.

Application of dynamic mode decomposition to acoustic-modes identification and damping in a 3-dimensional chamber with baffled injectors

The dynamic mode decomposition (DMD) is applied to a 3-dimensional chamber in order to identify acoustic resonant modes and quantify their damping. The DMD is one of post-processing methods, which can be applicable with acoustic field information in a full or a partial domain from numerical and experimental data. For application of the DMD method, the 3-dimensional chamber with baffled injectors is selected, in which gaps in baffled injectors enhanced acoustic damping and it has been shown that there exists an optimum gap for maximum damping in the previous works. Damping coefficients are evaluated in order to investigate whether the DMD method is able to predict the influence of gaps in baffled injectors on the damping of the first tangential (1T) mode or not. In addition to the 1T mode, the first longitudinal (1L) mode, the 1T1L mode, and higher modes are extracted and damping coefficients for them are estimated using the DMD method on 2-dimensional planes and they reveal that the baffle gap affects significantly only specific modes and show a good agreement with the damping factors from the previous work in predicting the optimum gap for maximum acoustic damping.

Intense Tangential Pressure Oscillations Inside a Cylindrical Chamber

Intense tangential pressure oscillations due to oscillatory combustion are generated experimentally inside a cylindrical chamber and are analyzed to understand their characteristic features. To generate the tangential mode, a coaxial injector is installed offcenter on the closed side of the chamber. Hydrogen and nitrogen-diluted oxygen are used as working gases under atmospheric conditions. The features of the side-wall pressure in the first tangential mode (1T mode) at high amplitude, whose amplitude is over 30% of the chamber pressure, differ from those of the side-wall pressure in the 1Tmode at low amplitude in the followingmanner: 1) The positive half-wave of the pressure oscillation has twin sharp peaks. 2) The amplitude of the positive half-wave (zero to positive peak) of the pressure oscillations is larger than that of the negative half-wave (negative peak to zero). The acoustical features of the signals are reproduced and investigated by conducting an analysis and anumerical simulation under similar configurations. It is found that the characteristic feature of the intense signal is originated in the nonlinearity due to the large amplitude of the pressure oscillation.

Raman spectra of single micrometer-sized tubular ZnO

Micrometer-sized tubular ZnO crystal was fabricated by a hydrothermal method, and was characterized by scanning electron microscopy (SEM). Back-scattering micro-Raman spectrum was applied to investigate the vibrational modes of the microtubes with the comparison to that of ZnO powder. Raman scattering in a single micrometer-sized tubular ZnO was also recorded with different polarization configurations consequently. Most of the observed vibrational modes were assigned on the basis of group theoretical analysis. Besides the typical Raman-active modes assigned to ZnO, a new mode with Raman shift of 860 cm −1 was observed which has not been reported in previous relative researches yet. In addition to this, the Raman signal of E 1T mode at 410 cm −1 increased dramatically and become asymmetric in some cases.