Heat Release Fluctuations Research Articles

Influence of the co- and counter-swirl on combustion instability of the centrally staged combustor

The centrally staged combustor is an effective way to reduce NOx emissions from combustors. However, combustion instability caused by the mutual coupling between flames and acoustics during the combustion process is almost unavoidable. To better understand this problem, the effect of the swirl rotational direction is investigated in this paper using two different schemes with co-swirl and counter-swirl configurations. Pressure fluctuations and flame dynamics are investigated under self-excited combustion oscillation conditions. The CH* chemiluminescence distribution captured by a high-speed camera is utilized to characterize the flame macrostructure and heat release fluctuations. Furthermore, non-oscillating reaction velocity fields are acquired using particle image velocimetry (PIV) technology. The results indicate that the amplitude and frequency of the counter-swirl scheme are higher than those of the co-swirl scheme at varying main stage equivalence ratios. Combining the results from dynamic mode decomposition and the local Rayleigh index, it is found that the heat release regions of the counter-swirl scheme are mainly concentrated in the shear layer. Higher velocity gradients, vorticities, and strain rates in the inner shear layer (ISL) and outer shear layer (OSL) for the counter-swirl scheme are verified using PIV technology. The driving sources of thermoacoustic oscillations are located in the regions of the ISL, OSL, and the area where the flame impinges on the sidewall of the liner. Additionally, the counter-swirl scheme exhibits larger vorticities and strain rates in the ISL and OSL, facilitating the development of thermoacoustic oscillations.

Acoustic modal analysis with heat release fluctuations using nonlinear eigensolvers

Closed combustion devices like gas turbines and rockets are prone to thermoacoustic instabilities. Design engineers in the industry need tools to accurately identify and remove instabilities early in the design cycle. Many different approaches have been developed by the researchers over the years. In this work we focus on the Helmholtz wave equation based solver which is found to be relatively fast and accurate for most applications. This solver has been a subject of study in many previous works. The Helmholtz wave equation in frequency space reduces to a nonlinear eigenvalue problem which needs to be solved to compute the acoustic modes. Most previous implementations of this solver have relied on linearized solvers and iterative methods which as shown in this work are not very efficient and sometimes inaccurate. In this work we make use of specialized algorithms implemented in SLEPc that are accurate and efficient for computing eigenvalues of nonlinear eigenvalue problems. We make use of the n-tau model to compute the reacting source terms in the Helmholtz equation and describe the steps involved in deriving the Helmholtz eigenvalue equation and obtaining its solution using the SLEPc library.

Effects of pulse power on evaporation characteristics of an n-heptane droplet in a microtube

When combustion instability arises in liquid rocket engines, heat release fluctuations cause local temperature oscillations and affect the evaporation of fuel droplets. In this study, to investigate the responses of droplet evaporation characteristics to temperature oscillations, an n-heptane droplet is formed by continuous fuel injection, and a pulse power is applied to generate temperature oscillations. The pulse power outputs a square wave with a frequency of 0.1–10 Hz and a power of 16.6–30.0 W. A transient heat transfer and droplet evaporation model is established, and measured droplet diameters serve to validate the model. The effects of heating frequency, heating power, and waveforms on droplet vaporization are studied. The results show that pulse power induces oscillations in air temperature, droplet diameter, and evaporation rate, which correspond to the applied heating frequency. Due to heat transfer within the tube, a phase lag is observed in the air temperature relative to the applied power. The dynamic equilibrium between the evaporation rate and fuel flow rate causes the evaporation rate phase to lag behind the droplet diameter phase by a constant of 0.5π, and to lead slightly ahead of the air temperature phase. Increases in heating frequency reduce the amplitudes of air temperature, droplet diameter, and evaporation rate while maintaining their averages constant. Decreases in heating power lower the air temperature, consequently increasing the evaporation rate amplitude and average droplet diameter. At 16.6 W, the droplet descends due to exceeding the critical diameter of 2.02 mm. Replacing the square wave of pulse power with a sinusoidal or triangular wave only results in a decline in air temperature amplitude, consequently reducing the evaporation rate amplitude by an equivalent degree. The amplitude percentage ratio between them remains unchanged at 4.77 but is subject to interference from their phase difference. As frequency increases from 0.1 to 1 Hz, the phase difference rises from −0.08π and converges to 0, while the amplitude percentage ratio increases from 4.77, converging to 4.87.

Mode shape-dependent thermoacoustic interactions between a lean-premixed primary flame and an axially-staged transverse reacting jet

Axial fuel staging enables elevation of the turbine inlet temperature in advanced heavy-duty gas turbine combustors to ∼2000 K, ultimately to achieve more than 65% combined cycle efficiency, while limiting excessive nitrogen oxide production. While previous investigations have identified transverse jet-flow-dependent nitrogen oxides emission trends and the topological properties of reacting shear layers in vitiated crossflow environments, there exists a critical gap in the understanding of axial-fuel-staging-induced combustion instabilities. The present study therefore investigates the link between first- and second-stage flame dynamics in a lean-premixed combustor. We demonstrate that the second-stage jet-in-vitiated-crossflow flame is preferentially coupled to higher order acoustic modes than the upstream primary flame with the same characteristic nozzle dimension. The first- and second-stage-driven acoustic modes (two different timescales) are mutually incompatible, and the ensuing selective excitation is critically dependent on the position of the secondary injector relative to the mode shape. When the second-stage flame is located near a pressure node, or a velocity antinode, the transverse jet-stabilized flame is influenced by the primary flame-driven crossflow velocity oscillations, and exhibits conspicuous flapping motions in the crossflow direction accompanied by periodic partial extinction at the windward and leeward-side flame elements. If the second-stage flame is situated near a pressure antinode, on the other hand, the secondary jet flame's dynamics are governed by a combination of jet merging-induced flame surface destruction and the growth of non-axisymmetric coherent structures. In this case, high-intensity pressure oscillations are sustained by the local heat release fluctuations generated solely from the transverse reacting jets, whereas the upstream primary flame remains nearly unperturbed, or decoupled from the underlying feedback processes. This experimental observation is remarkable, and reflects previously unknown properties originating from the complex interactions between the two axially-staged lean-premixed flames.

The response mechanism of pressure fluctuation to the unsteady heat release in a strong rotating environment

The response mechanism of the pressure fluctuation to the unsteady heat release rate at the ‘unsteady’ combustion state in a strong rotating environment was investigated using a stratified vortex-tube combustor via a large eddy simulation method. Results show that the peak amplitude of heat release fluctuation at the ‘unsteady’ state is just 2.8 × 106 W/m3 and the peak amplitude of pressure fluctuation is always within 3000 Pa based on the vortex-tube configuration, indicating a weak fluctuation degree. The unsteady heat release does not bring about a higher momentum flux fluctuation due to quick momentum damping in the rotating reactive environment. The pressure fluctuation does not relate closely to the momentum flux and its fluctuation; this is due to the effect of the centrifugal force caused by rotating behavior. After splitting the momentum flux into the different components, we found that the tangential component acts in reverse to that of the radial and axial directions in the exterior region. Viz., the pressure increases when the tangential momentum flux increases in the exterior region but decreases when the radial and axial components increase. The laminarization of the fluid suppresses the fluctuations in the radial and axial directions only, therefore the uneven attenuation of the momentum flux fluctuation in different directions is the dominant cause of the enlarged pressure fluctuation in the ‘unsteady’ burning state. Moreover, we find that the pressure fluctuation degree is also closely related to the centrifugal weight coefficient. When this coefficient is far away from zero, the centrifugal effect will become obvious, and vice versa.

Combustion dynamics of high Mach number scramjet under different inflow thermal nonequilibrium conditions

To characterize the impact of thermal nonequilibrium conditions in shock-induced combustion in high Mach number scramjet engines, a comparative numerical investigation has been performed. The combustor configuration was based on the HyShot II scramjet while the total enthalpy is 3.3 MJ/kg which resembles Mach 8 flight condition. The improved delay detached eddy simulation with Park two-temperature approach in combination with the vibration-chemical coupling model was employed in the nonequilibrium simulations. It is shown that under thermal nonequilibrium inflow conditions, noticeable stratification in the thermal state exists in the circumferential direction both in the combustor section and exhaust nozzle. In the inner core where the fuel mixes with the mainstream, flow deceleration caused by the heat release leads to a sufficient transformation of energy between the vibrational and translational-rotational modes approaching the thermal equilibrium state. However, the τv,mix is quite large in the supersonic mainstream, whereby the thermal nonequilibrium dominates. For the case with higher Tv in the inflow, the fuel jet's penetration height is lower while the local viscosity is higher which results in poor mixing between the fuel and air stream. Nevertheless, a 400 K increase in inflow Tv gives a higher effective temperature, which promotes the dissociation reactions, shortens the ignition distance and yields an 8% increase in the overall combustion efficiency. With better combustion performance, an additional 6% deceleration in the supersonic mainstream and the energy exchange rate between vibrational and translational-rotational modes becomes almost four times faster at the combustor outlet, which promotes the restoration of the thermal equilibrium state. Furthermore, under higher inflow vibrational temperature, the shock-induced combustion in supersonic crossflow becomes less stable with noticeable fluctuations in combustion heat release and local static pressure.

Characterisations of self-excited nonlinear oscillations at varied system parameters in a Rijke tube with a premixed laminar flame

Thermoacoustic instability has been one of the major concerns in various industrial combustion systems, such as Rich burn, Quick mix, Lean burn engine and aerospace engines. The complex interactions between heat release and pressure fluctuations could generate thermoacoustic oscillations with high amplitudes and frequencies in a combustion chamber, which causes many combustion problems even severe structural damage. The studies about the effects of various system parameters on thermoacoustic instability would be important due to their crucial roles in characterising the system oscillations. In this study, the effects of different system parameters in a Rijke tube, including equivalence ratio, methane flowrate, burner position and tube inner diameter, have been investigated. The time-domain analysis methods, including both phase space analysis constructed from the time-delayed method and recurrence analysis, have been conducted to explore the system characteristics under different system parameters. Furthermore, the Recurrence Quantitative Analysis (RQA) and Wayland translation error have been processed to quantify the system nonlinearity. The study has shown the effects of system parameters on system frequency response, deterministic nature and nonlinear dynamics. The system oscillation tends to be more complex and unstable when the burner is moved to lower positions or in a large diameter tube. The phase space and recurrence analysis are found to be efficient for characterising the system nonlinearity with useful insights of self-excited thermoacoustic oscillations.

Performance enhancement of swirl-assisted distributed combustion with hydrogen-enriched methane

Effect of hydrogen addition to methane fuel on the performance of swirl-assisted distributed combustion was studied at 5.72 MW/m3-atm thermal intensity using different hydrogen-enrichment (at 0, 10, 20, and 40%) to methane fuel and CO2 and N2 as the flowfield diluents. High-speed chemiluminescence imaging performed (without spectral filtering) exhibited a gradual increase of chemiluminescence intensity along with gradually narrowing of the flame shape for both swirl and distributed combustion with increase in hydrogen-enrichment from increased reactivity in flame brush. Fluctuations of pressure (p′) and heat release (q′) were signals showed the existence of a peak in swirl combustion indicating the possibility of thermo-acoustic coupling. This peak mostly diminished in distributed combustion. Investigation of fluctuation with the proper orthogonal decomposition exhibited no prominent structures in distributed combustion. Measurement of lean blowoff equivalence ratios (ϕLBO) at different combustion conditions showed extended ϕLBO in distributed combustion indicating wider operational limits in distributed combustion. The NO emission increased in conventional swirl combustion while it consistently decreased in distributed combustion even at increased H2 enrichment to methane fuel. The exhaust CO2 gradually decreased with increase in H2 enrichment for both swirl and distributed reaction zones at constant thermal intensity. Reduced pollutants emission with hydrogen enrichment was also observed with air preheats in the range of 373–573 K to support distributed combustion for gas turbine conditions.

Flame Transfer Functions of the Lean Burnout Zone of an RQL Combustion Chamber - Dynamic Response to Primary Zone and Mixing Port Velocity Fluctuations

This article describes an experimental investigation of thermoacoustic flame transfer functions of the lean burnout zone of an rich–quench–lean combustion chamber. With spatial separation of the rich primary from the lean secondary dilution zone, the latter is independently examined. The multi-microphone-method was employed to characterize the combustor acoustic velocity response to acoustic forcing coming from the primary zone and the mixing ports. The lean secondary zone is then treated as a pure acoustic 3-port network element connected to a 2-port Rankine-Hugoniot flame element. Focusing only on heat release fluctuations due to velocity fluctuations, the former are described by two linear superimposed flame transfer functions as a function of the velocity fluctuations coming from the primary zone and the mixing ports, respectively. Based on a non-reacting and a reacting measurement the two flame transfer functions could be extracted from the experimental data. Within this research, flame transfer functions from the new acoustical approach are presented and compared with ones measured using chemiluminescence and a photomultiplier tube. It is found that the inverse diffusion flame in the burnout zone reacts to velocity fluctuations from the primary zone in the low frequency range and a clear low pass behavior is observed. The mixing port velocity fluctuations create a more broadband response. In the presented cases, the flame transfer functions calculated from chemiluminescence match those from the acoustic method very well.

Analysis of Combustion Noise Sources Using Doak's Momentum Potential Theory

Noise emissions of modern lean combustors are related to different sources. Direct combustion noise is generated by heat release fluctuations, while indirect noise sources include the acceleration of entropy and vorticity inhomogeneities through the nozzle-guide-vane or at the combustor exit. The latter noise source is characterized by the coupling of fluctuations convected in non-uniform mean flow, which can e.g. cause vortical or entropic fluctuations to be partly transferred into acoustics. Due to the complexity of the sources, a clear and quantitative separation of the different phenomena in terms of primitive variables presents a significant challenge. This study therefore proposes an alternative framework for the description of combustion noise based on Doak's Momentum Potential Theory (MPT). The MPT defines a Generalized Acoustic Field (GAF) and describes the sound production in terms of mean energy fluxes carried by the respective acoustic, thermal and turbulent fluctuating momentum components. To confirm the ability to identify the different combustion noise sources, the method was applied to Large-Eddy Simulation data of a non-reacting swirl-combustor simulator. Finally, the coherent character and spectral behavior of the GAF were investigated using a Spectral Proper Orthogonal Decomposition (SPOD) analysis and correlated to the source distributions.

Sustainable Combustion Technologies Need Acoustics Research

This paper outlines the challenges of developing hydrogen-fueled combustion systems, which are safe from combustion instabilities. Combustion instabilities can occur in any configuration where a continuously burning flame is housed in an acoustic resonator, such as gas turbine engines, boilers and furnaces. They manifest themselves by large-amplitude oscillations of one or more resonator modes and can cause major damage to the combustion system. Their underlying cause is the following feedback loop: the flow carrying the combustible mixture is perturbed → the flame becomes unsteady and its heat release fluctuates → the fluctuating heat release generates sound → the sound perturbs the flow of the mixture, and the feedback cycle is closed. Various strategies exist to prevent instabilities in conventional combustion systems, which burn hydrocarbon fuels. However, if hydrogen or a hydrogen-blend is used as fuel, the flame's dynamic behaviour changes, and as a consequence, combustion instabilities occur under new and unexpected conditions. In order to give some explicit physical insight, a generic combustion system will be presented and described by an analytical model. Predictions will be made about the impact of hydrogen fuel and other parameters.

Numerical investigation of azimuthal thermoacoustic instability in a gas turbine model combustor

Self-excited spinning mode azimuthal instability in an annular combustor with non-swirling flow is investigated using large eddy simulation (LES). Compressible Navier–Stokes equations are solved with a flamelet combustion model to describe the subgrid chemistry-turbulence interactions. Two flamelet models, with and without heat loss effects, are compared to elucidate the non-adiabatic wall effects on the thermoacoustic instability. The azimuthal modes are captured well by both models with only marginal differences in the computed frequencies and amplitudes. By comparing with the experimental measurements, the frequencies given by the LES are approximately 10% higher and the amplitudes are well predicted. Further analysis of the experimental and LES data shows a similar dominant anti-clockwise spinning mode, under which a good agreement is observed for the phase-averaged heat release rate fluctuations. Dynamic mode decomposition (DMD) is applied to shed more light on this spinning mode. The LES and experimental DMD modes reconstructed for their azimuthal mode frequencies agree very well for the heat release fluctuations. The DMD mode structure for the acoustic pressure from the LES shows a considerable non-zero profile at the combustor outlet, which could be essential for azimuthal modes to establish in this annular combustor. Finally, a low-order modelling study was conducted using an acoustic network combined with the flame transfer function extracted from LES. The results show that the dominant mode is associated with the plenum showing a first longitudinal and azimuthal mixed mode structure. By tuning the plenum length to match the effective volume, the predicted frequency becomes very close to the measured value.

Pollution formation and the response of premixed hydrogen blending flame to the external excitation using large eddy simulation: The application of an artificial neural network and genetic algorithm

BackgroundThe regulations for combustion pollution are getting more stringent every day. To decrease the level of pollution, serious attention to the unstable operating conditions of the flame is essential, as any flame may suffer from a level of instability. Further, the application of hydrogen-hydrocarbon blending fuels is developing in the industries. MethodsTherefore, the current study investigates the pollution produced by unstable combustion. An artificial neural network applying 4-15-1 neurons is also used by the input parameters of excitation frequency, amplitude, equivalence ratio, and hydrogen content. It is shown that the prediction is well for emissions. Using a genetic algorithm, the operating conditions corresponding to a minimum pollution emission are determined. Additionally, the importance of the governing parameters is evaluated in the pollution formation mechanism, and the excitation frequency is set in the first rank. Significant findingsHydrogen addition makes the NOx to be increased by a factor of 2.5 and CO to be mitigated by a rate of 2. This factor for increasing the excitation amplitude from 0.05 to 0.45 is around 5. Furthermore, the flame response to the inlet excitation is completely assessed using the heat release fluctuation, transfer function magnitude, and phase. Adding more hydrogen was shown to shift the cut-off frequency.

Equivalence ratio independence and dependence ranges of system responses for a nonlinear thermoacoustic oscillation in a Rijke tube

Thermoacoustic instabilities in a combustion system remains a challengeable problem in industry over decades. It usually involves with the nonlinear interaction between unsteady heat release and pressure fluctuations. The nonlinear oscillation excited by this interaction can be intense enough to result in combustion problems even severe damage of the engine. Therefore, it is important to understand the system response to various parameters. In this study, the system nonlinearity to equivalence ratio change was mainly focused on. A Rijke tube system with a premixed methane-air laminar flame was utilised to trigger the self-excited oscillations with varied equivalence ratios (Φ). The time-domain analysis methods including phase difference, Recurrence Plot (RP) and Recurrence quantification analysis (RQA) were applied to obtain the characteristics of system nonlinearities. Both Φ-independence and Φ-dependence ranges of system frequency responses have been discovered. The results of phase difference and recurrence analysis have also validated the finding of both ranges. The corresponding characteristics of system nonlinearity in both ranges have been investigated. The similarity among the systems in the Φ-independence range under the same burner position and methane flowrate can be highlighted from the system frequency response, phase difference and system dynamics. It is found that the phase difference analysis, RP and RQA utilised in this study are capable of investigating the nonlinearities of the self-excited thermoacoustic oscillation.

Effect of Mixing Section Acoustics on Combustion Instability in a Swirl-Stabilized Combustor

An experimental study was performed to investigate the characteristics of two different combustion instability modes in a swirl-stabilized combustor. The first is the eigenfrequency corresponding to the half-wave of the combustion chamber section, and the second is the quarter-wave eigenmode of the inlet mixing section. The purpose of this study is to understand the effects of the swirl number on each combustion instability mode and analyze their generalized characteristics. Premixed gases composed of hydrocarbon fuels (C2H4 and CH4) and air were burned by independently varying the experimental conditions. Three dynamic pressure transducers and a photomultiplier tube were installed to detect pressure oscillations and heat release fluctuations in the inlet and combustion chamber sections, respectively. A high-speed camera was used to capture the instantaneous flame structures. In the swirl-stabilized combustor, the bands of the dominant frequencies were strongly dependent on the swirl number of the swirler vane. When the swirl number was low, the entire combustion system was often coupled with the quarter-wave eigenmode of the inlet mixing section. However, as the swirl number increased, the combustion instability mode was almost independent of the mixing section acoustics. Analysis of the phase difference and flame structure clearly demonstrated the differences between each eigenmode. The results provide new insights into the effect of the resonance mode in the inlet mixing section on combustion instability, depending on the swirl number in the swirl-stabilized combustor.

Effects of Acetone-Butanol-Ethanol (ABE) addition on HCCI-DI engine performance, combustion and emission

Recent research has shown the potential of long-chain alcohols such as butanol for biofuel. However, the production quantity of butanol is very low and consumes high energy in its purification process. To solve such a problem, one of the effective approaches is to use acetone-butanol-ethanol (ABE) directly from the fermentation process. This study aims to investigate the effects of ABE-diesel blends in an HCCI-DI engine. It was found that although the presence of oxygen in ABE could theoretically promote complete combustion and eventually decrease HC and CO emissions, other significant factors may have played more dominant roles in affecting the HC and CO emissions. The experimental investigation on an HCCI-DI engine fuelled with ABE also did not reduce PM and Soot emissions. Furthermore, the peak in-cylinder combustion pressure decreased with fluctuating heat release rate. However, their p-V areas were larger than pure diesel fuel, which implied a higher indicated work produced per cycle. The addition of ABE also successfully decreased CO2 and NOx emissions. Moreover, improved engine performance in terms of higher BTE, lower BSFC and EGT were observed. All in all, considering its satisfying improved engine performance; ABE has the potential to become a promising alternative biofuel.

Effect of helmholtz resonator shape on suppression of thermo-acoustic instability

In combustion chambers thermo-acoustic instabilities arise due to the coupling between the unsteady heat release and pressure fluctuations. The instabilities created cause serious structural vibration, flame blow out, increase in heat transfer and damage, thereby reducing the life span and performance of jet engines, gas turbines and industrial burners. Therefore, there is need to develop a simple technique for effective control of these instabilities. The present work involves experimental study of thermo-acoustic instabilities occurring in a Rijke tube and their suppression by using Helmholtz resonators of different shape (a passive method of suppression). The combustor model is replaced by Rijke tube which is an experimental setup extensively employed for research purpose. Rijke tube is provided with a co-axial pre-mixed gas burner as the source of heat which could be placed at any desired position. When a heat source with proper air to fuel ratio is placed at about one fourth of the tube length, 3rd mode of instability is obtained and a loud noise of frequency 681 Hz is produced. A comparative study of thermo-acoustic instability suppression using cylindrical, spherical and conical Helmholtz resonators has been performed. Spherical resonator has been found to be the most effective one imparting a maximum damping effect of 20 dB and an average damping effect of 10.5 dB.

The effects of injector size on the dynamics and instabilities of lean premixed swirling flame

The external- and self-excited premixed swirling flames are experimentally studied to understand the thermoacoustic instabilities under different injector sizes. We use high-speed CH* chemiluminescence, formaldehyde laser-induced fluorescence and particle image velocimetry to measure the dynamic characteristics of the flame surface and flow field. Reducing the size of the injector from 20 mm to 16 mm changes the thermoacoustic system's stability. The flame transfer function shows that reducing the size of the injector significantly reduces the phase delay, enhances the response amplitude, and annihilates the local gain extrema. The low-order network model analysis is performed, and it is found that the phase variation caused by changing injector size is the main reason for the induced thermoacoustic instability. The pixel-by-pixel decomposition of CH* images is used to extract the weighted gain and phase maps of local heat release oscillation. The swirling flame with a small injector size of 16 mm has a short time delay and a slow phase velocity, which results in strong in-phase interference of the local heat release oscillators and enhanced gains. Reducing the size of the injector strengthens the inner and outer shear layers, thereby suppressing the heat release fluctuation of the flame base. The enhanced vorticity supply of the shear layer increases the size and strength of the outer vortex ring, resulting in strong heat release fluctuations at the flame tip, which is responsible for its large unadulterated fluctuation intensity. These changes in the flow field change the relative heat release intensity and fluctuation amplitude between the flame base and tip. The out-of-phase interference among multiple vortices causes weak flame response at high frequencies.

Planar Laser-Induced Fluorescence and Chemiluminescence Analyses of CO2-Argon-Steam Oxyfuel (CARSOXY) Combustion

Strict regulations and acts have been imposed to limit NOx and carbon emissions. The power generation industry has resorted to innovative techniques to overcome such a low level of tolerance. Amongst those in the literature, CO2-argon-steam oxyfuel (CARSOXY) gas turbines have theoretically been proven to offer an economically sustainable solution while retaining high efficiency. Although theoretical studies have characterized CARSOXY, no experimental evidence has been provided in the literature. Therefore, this paper attempts to experimentally assess CARSOXY in comparison to a CH4/air flame. OH* chemiluminescence integrated with OH Planar Laser-Induced Fluorescence (PLIF) imaging has been utilized to study flame stability and flame geometry (i.e., the area of highest heat intensity (AOH¯Max center of highest heat intensity (COH¯Max)) over a range of working fluid Reynolds’ numbers and oxidizing equivalence ratios. In addition, the standard deviation of heat release fluctuations (σOH*/OH¯) has been utilized as the base-criteria to compare the stability performance of CARSOXY to CH4/air combustion. Moreover, turbulence-chemistry interactions have been described using Damköhler numbers and by plotting Borghi regime diagrams. This paper suggests a modified numerical approach to estimate Damköhler numbers and plot regime diagrams for non-premixed combustion by utilizing the Buckingham π theorem based on experimental observations and results. CARSOXY flames showed lower flame intensity than that of the CH4/air flame throughout the entire Re interval by approximately 16%, indicating higher heat release. The Damköhler numbers of the CARSOXY flame were also greater than those of the CH4/air flame in all conditions, indicating more uniform CARSOXY flames. It was found that the tendency of the CARSOXY flame of approaching the concentrated reaction zone is greater than that of the CH4/air flame.

Response of MCS Low-Frequency Gravity Waves to Vertical Wind Shear and Nocturnal Thermodynamic Environments

AbstractThis study investigates the sensitivities of mesoscale convective system (MCS) low-frequency gravity waves to changes in the vertical wind and thermodynamic profile through idealized cloud model simulations, highlighting how internal MCS processes impact low-frequency gravity wave generation, propagation, and environmental influence. Spectral analysis is performed on the rates of latent heat release, updraft velocity, and deep-tropospheric descent ahead of the convection as a signal for vertical wavenumber wave passage. Results show that perturbations in midlevel descent up to 100 km ahead of the MCS occur at the same frequency as gravity wave generation prompted by fluctuations in latent heat release due to the cellular variations of the MCS updrafts. Within a nocturnal environment, the frequency of the cellularity of the updrafts increases, subsequently increasing the frequency of wave generation. In an environment with low-level unidirectional shear, results indicate that wave generation mechanisms and environmental influence are similar among the simulated daytime and nocturnal MCSs. When deep vertical wind shear is incorporated, many of the low-frequency waves are strong enough to support cloud development ahead of the MCS as well as sustain and support convection.