• Energy Research

Abstract In this work, the dynamic response of a propane-burnt (C3H8) jet diffusion flame in a longitudinal tube to acoustic waves produced from a loudspeaker are studied. For this, 2-D numerical simulations are conducted by using FLUENT to investigate the interaction of acoustics-flow-flame. Unsteady RANS simulations with one-step Eddy-Dissipation (ED) combustion model are used in the model. And acoustic fluctuations are generated by using User Defined Functions (UDF). The numerical model is validated first by comparing the numerical results with the experimental measurements in the absence of a flame. Further validation is performed by comparing with flame-involved experimental results available in the literature. It is shown that the jet and the flame characteristics are highly sensitive to its axial location, especially when standing waves are present in the tube. The jet experiences large velocity fluctuations. Flow reversal is observed, when the jet is placed at acoustic velocity antinodes. However, the jet and flame are much stable in the velocity node region. Due to the large-amplitude acoustic disturbances, an interesting unsteady mushroom-shaped flame is observed. The numerical model is then used to determine the flame transfer function (FTF), which is important to characterize the flame-acoustic coupling behaviors. It is shown that the flame transfer function is nonlinear. Furthermore, it depends strongly on not only the amplitude of the acoustic disturbances but also the frequency. Such strong responses of the flame to acoustic waves are due to the ‘resonance’ effect of the tube. The present work opens up new applicable way to model and characterize the flame-acoustics-flow interaction via flame transfer function.

Abstract Large-amplitude thermoacoustic oscillations are unwanted in gas turbines due to the detrimental damage to combustors. Thus predicting the onset of such oscillations and a better understanding of the unstable behaviors are important. In this work, the stochastic properties of thermoacoustic oscillations in the subthreshold region of thermoacoustic systems are investigated theoretically and numerically. The energy conversion from the unsteady heat release to sound is mainly achieved via two ways: one is caused by inherent turbulent fluctuations, and the other is due to the flame response to the acoustic waves. The turbulence-induced non-coherent heat release fluctuation is characterized by colored noise, and the coupling between the unsteady heat release and acoustic pressure is characterized by a 3rd order polynomial. Both stochastic averaging and stochastic normal form are utilized to approximate the responses of the system as a Markov process. The result shows that the correlation time τc and intensity D of the colored noise have significant but opposite effects on the dynamics of thermoacoustic oscillations, including the most probable amplitude, autocorrelation of the system as well as the correlation time. In addition, the resonance-like behaviors in signal-to-noise ratio (SNR) are observed, which denotes the emergence of coherence resonance (CR) in this annular combustion system. The optimal noise intensity, at which SNR is maximized, becomes sensitive to the variation of τc, as τc is larger than certain value. Lastly, the extent of the system coherent motions relies significantly on the proximity to the supercritical Hopf bifurcation point. Being closer to the stability boundary is found to increase the strength of system association. Meanwhile, SNR of the colored noise-induced motion becomes more distinguished, and the optimal noise intensity is shifted to a smaller value. This variation can serve as a precursor to predict the onset of thermoacoustic instability, as the correlation time remains unchanged.

Ammonia is an alternative renewable green fuel with significant potential for addressing climate change concerns. Blending ammonia with methane has emerged as a viable strategy to improve the laminar burning velocity of ammonia. In this study, we propose a mechanism for methane/ammonia combustion, comprising 31 species and 131 chemical reaction steps, and investigate the emissions of CO and NO, along with electrical power output and energy efficiency of a micro-thermal photovoltaic (MTPV) system fueled with premixed ammonia/methane/oxygen. Three key parameters are identified as: 1) the inlet mixture flow velocity, 2) the CH₄ mole fraction blended ratio (ξCH₄), and 3) the material of the micro-combustor. The MTPV system achieves its highest energy efficiency (5.8 %) at an inlet velocity (vin) of 2.3 m/s, and reaches its maximum electrical power output (6.9 W) at vin = 7.2 m/s. Further, increasing ξCH4 can enhance electrical power output (ξCH₄ = 0.9 yields 1.37 W more than that at ξCH₄ = 0.1). Finally, altering the micro-combustor material is shown to have little effects on electrical power output, NO emissions, and energy efficiency. However, the MTPV system made of quartz is found to reduce CO emissions by 15 % and 12 % in comparison with those systems made of Sic and steel, respectively.

Abstract Three-dimensional numerical simulations of methane/air premixed combustion in micro-planar quartz combustor are performed with detailed chemical reaction mechanism. Three types of flame propagation modes including planar, U-shaped and inclined flames are observed with increasing inlet velocity. Numerical results show reasonable agreement with experiments. It is found that flame structure and location are greatly affected by inlet velocity. Meanwhile, the variation of blowout limit with respect to equivalence ratio is non-monotonous, i.e., it increases first and then decreases when the equivalence ratio ranges from 0.9 to 1.1. Further analysis on inclined flame shows that the shorter the flame length, the wider the blowout limit. Moreover, the effect of combustor material on flame structure, flame location and temperature distribution as well as radiant efficiency is studied and compared. Results indicate that thermal conductivity can not only affect the flame structure and flame location, but it can also determine the blowout limit, which is due to the heat recirculation along the streamwise direction. Among the investigated combustor materials, a broadest blowout limit, most uniform temperature distribution and highest radiant efficiency can be achieved in nickel combustor.

Abstract Transient growth of acoustic disturbances could trigger thermoacoustic instability in a combustion system with non-orthogonal eigenmodes, even with stable eigenvalues. In this work, feedback control of transient growth of flow perturbations in a Rijke-type combustion system is considered. For this, a generalized thermoacoustic model with distributed monopole-like actuators is developed. The model is formulated in state-space to gain insights on the interaction between various eigenmodes and the dynamic response of the system to the actuators. Three critical parameters are identified: (1) the mode number, (2) the number of actuators, and (3) the locations of the actuators. It is shown that in general the number of the actuators K is related to the mode number N as K = N 2 . For simplicity in illustrating the main results of the paper, two different thermoacoustic systems are considered: system (a) with one mode and system (b) that involves two modes. The actuator location effect is studied in system (a) and it is found that the actuator location plays an important role in determining the control effort. In addition, sensitivity analysis of pressure- and velocity-related control parameters is conducted. In system (b), when the actuators are turned off (i.e., open-loop configuration), it is observed that acoustic energy transfers from the high frequency mode to the lower frequency mode. After some time, the energy is transferred back. Moreover, the high frequency oscillation grows into nonlinear limit cycle with the low frequency oscillation amplified. As a linear-quadratic regulator (LQR) is implemented to tune the actuators, both systems become asymptotically stable. However, the LQR controller fails in eliminating the transient growth, which may potentially trigger thermoacoustic instability. In order to achieve strict dissipativity (i.e., unity maximum transient growth), a transient growth controller is systematically designed and tested in both systems. Comparison is then made between the performance of the LQR controller and that of the transient growth controller. It is found in both systems that the transient growth controller achieves both exponential decay of the flow disturbance energy and unity maximum transient growth.

Abstract Industrial combustion systems such as power generation gas turbines, rocket motors, furnaces and boilers often face the problem of large-amplitude self-excited pressure oscillations that occur due to the onset of thermoacoustic instability. To prevent the onset of such instability, understanding the effects of fuel–air equivalence ratio ϕ, and fuel flow rate Vf on self-excited nonlinear thermoacoustic oscillations is of fundamental and practical importance. Experimental investigation of the roles of these parameters on triggering thermoacoustic instability in a swirl combustor has received very little attention. In this work, we design a swirling thermocoustic combustor and conduct a series of experimental tests. Autocorrelation and recurrence analysis of phase space trajectories reconstructed from the acoustic pressure time trace are performed. These experimental tests allow us to study the effect of the fuel–air equivalence ratio ϕ on the onset of thermoacoustic instability by varying the fuel volume flow rate Vf. We demonstrate that the fuel volume flow rate and the equivalence ratio play different but critical roles on generating thermoacoustic instability at different frequencies and amplitudes. Maximum sound pressure level can be as high as 135 dB. In addition, mode switching, (i.e. frequency swap) is found to occur between approximately ω3 ≈ 510 Hz and ω1 ≈ 170 Hz, depending on the equivalence ratio ϕ. Furthermore, the dominant frequency corresponding to the maximum amplitude is shown to be shifted by approximately 20%, as the fuel flow rate Vf is increased and the combustion condition is changed from lean to rich. These findings are quite useful for designing a feedback control strategy to stabilize an unstable combustor. The present work opens up an applicable means to design a stable swirling combustor.

Abstract In order to achieve a better energy conversion efficiency, a T-shaped micro-combustor with porous medium and ring-shaped ribs is proposed and studied in this work. For this, 3D model of a premixed H2/air-fuelled combustor is developed with k-e turbulence and EDC chemical reaction models applied. The model is first validated with the experimental data available in the literature and then used to evaluate the effects of (1) the rib, (2) inlet flow velocity of the fuel–air mixture, (3) the axial location, (4) the porosity of porous medium, and (5) its size. The temperature difference is observed to be reduced by 60.73 K than that of conventional I-shaped combustor with the same surface area as the inlet velocity is 5 m/s. In addition, the size and location of porous medium are found to affect temperature distribution. To gain insights on the energy losses and thermodynamic efficiency, entropy generation analysis is conducted. It is revealed that the thermodynamic second law efficiencies of the combustors with porous medium are found to exceed 60%. This study proposed a novel design of a micro-combustor with porous medium implemented for micro-thermophotovoltaic system which can achieve a more uniform outer wall temperature and a higher energy conversion efficiency.

Abstract The present work considers a convection-driven T-shaped standing-wave thermoacoustic system. To gain insights on the conversion process of heat to sound and to study the nonlinear coupling between unsteady heat release and acoustic disturbances, thermodynamic analysis, numerical and experimental investigations are conducted. Three parameters are examined: (1) the inlet flow velocity, (2) heater temperature and (3) heat source location. Their effects on triggering limit cycle oscillations are first investigated in 2D numerical model. As each of the parameters is varied, the head-driven acoustic signature is found to change. The main nonlinearity is identified in the heat fluxes. To characterize the transient (growing) behavior of the pressure fluctuation, the thermoacoustic mode growth rate is defined and calculated. It is found that the growth rate decreases first and then ‘saturates’. Similar behavior is observed by examining the slope of Rayleigh index. Furthermore, the overall efficiency of converting the input thermal energy into acoustical energy is defined and calculated. It is found that the energy conversion efficiency can be increased by increasing the inlet flow velocity. To validate our numerical findings, a cylindrical T-shaped duct made of quartz-glass with a metal gauze attaching on top of a Bunsen burner is designed and tested. Supercritical bifurcation is observed. And the experimental measurements show a good agreement with the numerical results in terms of mode frequency, mode shape, sound pressure level and Hopf bifurcation behavior.