Length Of Combustion Chamber Research Articles

Analysis of the effect of combustion chamber length on the gas oscillations characteristics

In this paper, the case of the influence of a change in the length of the vortex combustion chamber on the amplitude-frequency characteristics of gas oscillations is considered. Experimental data were obtained, on the basis of which the graphs of the dependence of the amplitude and frequency of the combustion chamber on the excess air ratio were received.

Experimental investigations of equivalence ratio effect on nonlinear dynamics features in premixed swirl-stabilized combustor

In this work, the dynamics of the pressure oscillations and flame behaviors in a premixed swirl-stabilized combustor were experimentally investigated. As equivalence ratio Φ varied between 0.55 and 1.35, five combustion states were distinguished based on the characteristics of the acoustic pressures and flame dynamics. The permutation entropy (PE) and proper orthogonal decomposition (POD) were used to analyze the fluctuating pressure signal complexities and spatial CH⁎ chemiluminescence images, respectively. The results showed that the magnitude of the PE was at a differentiable level. In addition, the POD modes exhibited distinguishable features when the system was in different combustion states. Further analyses of the state transitions indicated that the system underwent a supercritical Hopf bifurcation and a pair of fold bifurcations when Φ was in the lean condition. The state transition from quasi-periodic oscillation to limit-cycle oscillation was attributed to the movement of the flame location, which enhanced the coupling of the acoustics and combustion by reducing the phase difference between the pressure oscillations and heat release fluctuations. In addition, a hysteresis phenomenon was observed for the trajectory of the fluctuating pressure root-mean-square (RMS) value in the forward and backward processes of the equivalence ratio under the lean condition. A low-order model was used to capture the bifurcations; the theoretical results agreed well with the experimental data. As for the rich condition, a supercritical Hopf bifurcation was captured when the system switched between the dual-frequency oscillation mode and the rich stable state. Furthermore, the influence of combustion chamber length Lc on the combustion stability in relation to Φ was investigated. Parameter Lc was divided into steady and unsteady regions depending on whether the combustion system could reach limit-cycle oscillation. The PE was demonstrated to be effective in distinguishing the stable state and limit-cycle oscillation even for combustors of different lengths. Understanding the nonlinear dynamics by analyzing experimental data could be critical to developing models for the prediction and suppression of thermoacoustic instabilities.

On the impact of fuel injection angle in Euler–Lagrange large eddy simulations of swirling spray flames exhibiting thermoacoustic instabilities

This study deals with the fundamental problem of combustion dynamics in gas turbine combustors operating with liquid fuel. In this framework the present work proposes the study of an academic liquid fueled combustor sensitive to thermoacoustic instabilities, simulated via high-fidelity Large Eddy Simulations. The experimental setup addressed is SICCA-spray from EM2C laboratory featuring both stable and unstable flames depending on the combustion chamber length. The proposed analysis, based on the Euler-Lagrange modeling approach, studies the impact of the spray injection angle θ on both the stable flame and the triggering of the longitudinal combustor acoustic mode when using a longer quartz tube. For the liquid injection modeling, the FIM-UR semi-empirical model is adopted with three different θ values: θ=35∘,45° and 60°. In stable conditions, the spray angle is proven to have a negligible impact on the flame anchoring point, however, the mean flame length and fuel distribution are found to be slightly modified by the velocity at which droplets enter the combustion chamber. For the thermoacoustically unstable conditions, two well-established stable limit cycles with the same frequency and similar amplitudes are obtained when fuel is injected at θ=45∘ and 60°. Contrarily, the system stabilizes when θ=35∘ pointing to the importance of the dynamics of the liquid film layer formed inside the injector for this setup. Likewise, this liquid film layer dynamics and its modeling appear critical as already suggested by previous studies on the same configuration. The detailed analysis of the thermoacoustically unstable different predictions is then performed through the investigation of the spatial fields of the local Rayleigh index obtained following the novel extension in the frequency domain of the Rayleigh criterion complemented by the application of Dynamic Mode Decomposition. It confirms that the injection angle of the liquid spray has a significant effect on the thermoacoustic response of the system. Indeed the influence of θ on the dynamics of the liquid fuel when entering the combustion chamber is proven to have an impact on the synchronization mechanism governing the liquid phase with respect to acoustics sustaining the observed limit cycles. More specifically, couplings at the liquid phase level are evidenced by introducing two novel indices correlating the fluctuations of liquid fuel volume fraction and evaporation rate with pressure.

동축 와류형 분사기의 연소안정성에 대한 분사기 리세스 및 연소실 길이의 영향

동축 와류형 분사기의 연소안정성에 대한 분사기 리세스 및 연소실 길이의 영향

Analysis of the maximum flight Mach number of hydrocarbon-fueled scramjet engines under the flight cruising constraint and the combustor cooling requirement

Scramjet is the optimal propulsion system for hypersonic vehicles, and it is important to study its maximum flight Mach number. Upper limit and its influence factors of the flight Mach number of dual-mode scramjet used in hypersonic vehicles are investigated in this paper. Scramjet performance analysis model with the constant-area heating process, cooling requirement analysis model of the combustor and cruise model of the scramjet-powered hypersonic vehicle are established. Specific impulse of dual-mode scramjet engine with hydrocarbon fuel is similar to that of liquid oxygen kerosene rocket engine when the freestream Mach number is above 10. The ratio of hypersonic vehicle reference area to scramjet inlet capture area is introduced to establish the size relationship between the scramjet engine and the hypersonic vehicle. Considering the balance of thrust and drag, smaller ratio of hypersonic vehicle reference area to scramjet inlet capture area leads to higher flight Mach number that hypersonic vehicle can cruise. When the angle of attack is 10 degrees, the ratio of hypersonic vehicle reference area to scramjet capture area should be less than 5.5 to ensure that the vehicle can work at Mach 8. The maximum length to diameter ratio of the combustor is used to indicate active cooling requirements, and it is less than 5 under the cooling requirement when flight Mach number is above 9. Reducing the heat flux at the combustor wall and increasing the fuel heat sink is helpful to increase the allowable combustion chamber length, so as to increase the maximum flight Mach number of the scramjet engine on the premise of meeting thermal protection requirement.

Combustion instability characteristics in a dump combustor using different hydrocarbon fuels

ABSTRACTThe combustion instability characteristics in a model dump combustor with an exhaust nozzle were experimentally investigated. The first objective was to understand the effects of operating conditions and geometric conditions on combustion instability. The second objective was to examine more generalised parameters that affect the onset of combustion instability. Three different premixed gases consisting of air and hydrocarbon fuels (C2H4, C2H6, C3H8) were burnt in the dump combustor at various inlet velocity, equivalence ratio and combustion chamber length. Dynamic pressure transducer and photomultiplier tube with a bandpass filter were used to measure pressure fluctuation and CH* chemiluminescence data. Peak frequencies and their maximum power spectral densities of pressure fluctuations at same equivalence ratios showed different trends for each fuel. However, the dynamic combustion characteristics of pressure fluctuations displayed consistent results under similar characteristics chemistry times regardless of the used hydrocarbon fuels. The results showed that characteristic chemistry time and characteristic convection time influenced combustion instabilities. It was found that the convective-acoustic combustion instability could be prevented by increasing the characteristic chemistry time and characteristic convection time.

Investigation of the effect of geometry of combustor on combustion characteristics of solid-fuel ramjet with swirl flow

The investigation on the effect of geometry of combustor on combustion characteristics of solid fuel ramjet (SFRJ) with swirl flow by numerical and experimental approaches is presented. Experiments were conducted on connected pipe facility for ignition of a solid fuel ramjet using high-density Polyethylene (HDPE) as solid fuel. The length of combustion chamber and port diameter of solid fuel are varied independently. An in-house code has been developed to explore the flow features with chemical reactions in the combustion chamber. The obtained results show that both the temperature in recirculation zone and the distance of flame surface to fuel surface in redevelopment zone could be reduced significantly with the decreasing of the chamber length. On the other hand, decreasing the port diameter can increase the temperature of flame surface, enhance the local heat transfer and regression rate in the redevelopment zone. Besides, the average regression rate is increased with decreasing the length of combustion chamber and port diameter.

Experimental investigation on the effect of swirling flow on combustion characteristics and performance of solid fuel ramjet

Solid-fuel ramjet converts thermal energy of combustion products to a forward thrust without using any moving parts. Normally, it uses air intake system to compress the incoming air without swirler. A new design of swirler has been proposed and used in the current work. In this paper, a series of firing tests have been carried out to investigate the impact of using swirl flow on regression rate, combustion characteristics, and performance of solid-fuel ramjet engines. The influences of swirl intensity, solid fuel port diameter, and combustor length were studied and varied independently. A new technique for determining the time and space averaged regression rate of high-density polyethylene solid fuel surface after experiments has been proposed based on the laser scan technique. A code has been developed to reconstruct the data from the scanner and then used to obtain the three-dimensional distribution of the regression rate. It is shown that increasing swirl number increases regression rate, thrust, and characteristic velocity, and, decreases air-fuel ratio, corner recirculation zone length, and specific impulse. Using swirl flow enhances the flame stability meanwhile negatively affected on ignition process and specific impulse. Although a significant reduction of combustion chamber length can be achieved when swirl flow is used. Power fitting correlation for average regression rate was developed taking into account the influence of swirl number. Furthermore, varying port diameter and combustor length were found to have influences on regression rate, combustion characteristics and performance of solid-fuel ramjet.

The impacts of three flamelet burning regimes in nonlinear combustion dynamics

Axisymmetric simulations of a liquid rocket engine are performed using a delayed detached-eddy-simulation (DDES) turbulence model with the Compressible Flamelet Progress Variable (CFPV) combustion model. Three different pressure instability domains are simulated: completely unstable, semi-stable, and fully stable. The different instability domains are found by varying the combustion chamber and oxidizer post length. Laminar flamelet solutions with a detailed chemical mechanism are examined. The β probability density function (PDF) for the mixture fraction and Dirac δ PDF for both the pressure and the progress variable are used. A coupling mechanism between the volumetric Heat Release Rate (HRR) and the pressure in an unstable cycle is demonstrated. Local extinction and reignition are investigated for all the instability domains using the full S-curve approach. A monotonic decrease in the amount of local extinctions and reignitions occurs when pressure oscillation amplitude becomes smaller. The flame index is used to distinguish between the premixed and non-premixed burning mode in different stability domains. An additional simulation of the unstable pressure oscillation case using only the stable flamelet burning branch of the S-curve is performed. Better agreement with experiments in terms of pressure oscillation amplitude is found when the full S-curve is used.

Solid fuel ramjet fuel optimization for maximum thrust to drag ratio and impulse density subject to geometric restraints on missile outer mold line

Tactical solid fuel ramjet propulsion system designs are often restricted by a fixed geometric envelope that the vehicle's outer mold line must be contained within. These constraints fix the combustion chamber diameter and length as well as the maximum intake area profile. With the intake area restrained to some maximum value, the maximum air flowrate into the combustor is likewise limited to a specific value for any given design freestream Mach number and altitude. Optimization of fuels for the maximum thrust to drag ratio or specific impulse must consider the linkage of geometric restraints on the selection of fuel materials. In this paper, equations are developed for thrust to drag, net specific impulse, impulse density, and a newly defined performance parameter, the momentum density. These equations are cast in terms of the ratio of the intake area to the aerodynamic reference area. Thermochemical analyses are conducted for a range of common polymers and solid particulate additives. The optimization equations show that as the intake to reference area ratio decreases, maximum performance shifts from materials having the highest heat of combustion per unit mass to materials that maximize the ratio of heat of combustion to the stoichiometric fuel to air ratio, and to operating air to fuel ratios that approach the upper flammability limits. As the intake area to reference area increases, the optimization tends towards fuel that maximize the heating value alone and to operating air to fuel ratios that approach the lower flammability limits. The conclusion of the analysis is that for solid fuel ramjet designs subject to restrictions on inlet area, maximum performance occurs with high density materials that maximizes heat release and minimizes the stoichiometric air to fuel ratio.

The contribution of intrinsic thermoacoustic feedback to combustion noise and resonances of a confined turbulent premixed flame

The influence of Intrinsic Thermoacoustic (ITA) feedback on the combustion noise spectrum produced by a confined, turbulent, premixed, swirl flame is investigated. The analysis is based on the understanding that sound is generated by unsteady heat release resulting from turbulent fluctuations on the one hand, and from the response of the flame to incoming acoustic perturbations on the other. The former effect is described by a source term for combustion noise, i.e. a spectral distribution of unsteady heat release rate, the latter by the flame transfer function. Both quantities are identified from time series data for fluctuating velocity and heat release rate, generated with large eddy simulation of premix swirl burner. The combustion noise source term and the flame transfer function are then introduced in an acoustic network model of the test rig in order to compute the spectral distribution of the sound pressure level at a certain location in the combustion chamber. Results for the noise spectrum are in good agreement with experiment, showing a broadband component and well-defined peaks. The frequencies of the peaks correspond to either acoustic cavity or ITA resonances. The acoustic network model is used for parametric studies, where the acoustic reflection coefficient at the combustor exit is varied. Remarkably, it is found that the magnitude of the ITA peak increases with decreasing values of the acoustic reflection coefficient, and vice versa. Furthermore, the influence of combustion chamber length on resonance frequencies is explored. It is observed that the frequency of the ITA resonance is insensitive to combustor length. This behaviour is observed qualitatively also in experiments.

Numerical investigation on hydrogen/air non-premixed combustion in a three-dimensional micro combustor

In order to obtain a higher combustion efficiency and a lager heat transfer from outer wall, a new type of micro cylindrical combustor with suction pipe, mixing pipe, diffuser pipe and shrinkage pipe and the combination of cavity and backward-facing step is designed and a numerical investigation on non-premixed hydrogen/air reacting flow inside three micro combustors has been carried out at atmospheric pressure. Moreover, the combustion characteristics and working performance of micro combustors are also investigated. Results show that the cavity and the backward-facing step in micro combustor is useful for the heat recirculation and the flame stability. Moreover, the combustion efficiency and the heat transfer from outer wall can be improved by increasing the length of combustion chamber within limited range. A high and uniform outer wall temperature distribution is desirable for the L2 micro combustor, therefore, the L2 micro combustor is more suitable for the micro thermo photovoltaic (MTPV) system when hydrogen volume flow rate is equal to 150mL/min and equivalence ratio φ is equal to 1.0.

FLAME FRONT PROPAGATION IN COMBUSTION SYSTEM WITH SEMI-OPEN COMBUSTION CHAMBER WITH DIFFERENT COMPRESSION RATIO

Researches of the combustion system with semi-open combustion chamber for spark ignition engines. This system was elaborated in Aircraft Engine Department of Heat Engineering Institute of Warsaw University of Technology. The researches concern the determination of influence CR on a flame front propagation into combustion chamber when CR and ignition advance are varied. The model combustion chamber make up with the constant volumes ratio of the prechamber volume to the prechamber plus main combustion chamber volumes, of 28%, but the CR were varied: 6:1, 8:1, 10:1 and 12:1, by changing of combustion chamber length. The researches were performed using rapid compression machine (RCM). The combustion sequences were recorded using high-speed digital camera with speed of 5000 frames per second. Simultaneously with combustion sequences recording, the high-speed pressure measurements were performed. The results of the visualization researches, with the pressure measurement results are presented in the paper. The research results show that the growth of compression ratio caused an improvement of combustion system performances; an increase of maximum cycle pressure and useful work field. The improvement of the performances is caused mainly by shortening of the combustion chamber length and approaching of supposed combustion course.

Geometrical parameter evaluation of a 0.5 MWth bench-scale oxy-combustion burner

Carbon capture and storage (CCS) technologies are one of the most relevant energy pathways to mitigate medium-term climate change effects. They will contribute up to 20% of the total CO2 emission reduction by 2035. Oxy-combustion is considered a promising CCS technology applicable in fossil-fuel power plants. Much effort has been made to develop oxy-combustion at large scale. However, there are still fundamental issues and technological challenges that must be addressed before this technology can be considered for commercialization. Recent research on coal oxy-combustion experimental facilities indicates stabilization problems and ignition delays when combustion occurs in CO2-rich environments. Advanced burner designs are required to ensure technological and economic feasibility of the process. Computational Fluid-Dynamics tools were used to evaluate the behaviour of a 0.5MWth bench-scale oxy-burner. Key geometrical parameters were analysed, such as combustion chamber diameter, chamber length and angle of the quarl. Simulations showed that a length of 6m ensures complete coal combustion. The results indicated that the combustion chamber diameter and the quarl angle have an influence on coal ignition, stability, flame shape and CO concentration. Higher values of those parameters produced greater hot recycled flow, which means higher temperatures and greater flame stability. Coal ignition was also improved. A 1500-mm chamber diameter and a 50° quarl angle were selected as optimal values for both parameters. CFD modelling makes it possible to analyse the behaviour of different oxy-burner configurations and to optimize the combustion process under oxy-combustion conditions.

Integration of a wood pellet burner and a Stirling engine to produce residential heat and power

The integration a Stirling engine with a pellet burner is a promising alternative to produce heat and power for residential use. In this context, this study is focused on the experimental evaluation of the integration of a 20 kWth wood pellet burner and a 1 kWe Stirling engine. The thermal power not absorbed by the engine is used to produce hot water. The evaluation highlights the effects of pellet type, combustion chamber length and cycling operation on the Stirling engine temperatures and thermal power absorbed. The results show that the position of the Stirling engine is highly relevant in order to utilize as much as possible of the radiative heat from the burner. Within this study, only a 5 cm distance change between the Stirling engine and the pellet burner could result in an increase of almost 100 °C in the hot side of the engine. However, at a larger distance, the temperature of the hot side is almost unchanged suggesting dominating convective heat transfer from the hot flue gas. Ash accumulation decreases the temperature of the hot side of the engine after some cycles of operation when a commercial pellet burner is integrated. The temperature ratio, which is the relation between the minimum and maximum temperatures of the engine, decreases when using Ø8 mm wood pellets in comparison to Ø6 mm pellets due to higher measured temperatures on the hot side of the engine. Therefore, the amount of heat supplied to the engine is increased for Ø8 mm wood pellets. The effectiveness of the engine regenerator is increased at higher pressures. The relation between temperature of the hot side end and thermal power absorbed by the Stirling engine is nearly linear between 500 °C and 660 °C. Higher pressure inside the Stirling engine has a positive effect on the thermal power output. Both the chemical and thermal losses increase somewhat when integrating a Stirling engine in comparison to a stand-alone boiler for only heat production. The overall efficiency of the pellets fired Stirling engine system reached 72%.

Effects of the number of tangential passages on spray characteristics of a bipropellant atomizer

The liquid–liquid bipropellant pressure swirl atomizers have been widely used in rocket engines to take advantage of their high mixing efficiency within the short length of the combustion chamber. In this kind of engine, the uniform mass distribution and propellant mixture ratio have great influence on combustion efficiency and instability. The present work investigates the effect of the number of tangential passages on the spray cone angle, mass distribution, and propellant mixture ratio of a bipropellant atomizer. Thus, three atomizers were designed, manufactured, and evaluated experimentally. The results showed that the number of tangential passages has some influence on the circumferential mass distribution; however, spray angle and propellant mixture ratio have not presented significant variations.

Experimental Study of the Combustion Regimes Occurring in a Laboratory Combustor

This article examines the combustion regimes occurring in a small-scale laboratory cylindrical combustor, in which the burner and the exhaust port are mounted at the top end of the combustion chamber. Flue-gas composition data and hydroxyl radical chemiluminescence (OH*) imaging are presented as a function of the air inlet preheat temperature and excess air coefficient, which in the present configuration implies also changes in the air inlet velocity. For three of these combustor operating conditions, detailed in-combustor measurements of temperature and of O2, CO2, CO, unburned hydrocarbons, and NOx concentrations are also reported. The flue-gas data reveal that, for a given air inlet preheat temperature, the air inlet velocity has an important impact on the NOx emissions, which decrease as the air inlet velocity increases. Furthermore, as the air inlet temperature increases, the burner is able to operate with higher excess air coefficients. For a given air inlet temperature, the combustion regime in the present combustor develops from conventional lean combustion at lower air inlet velocities (or lower excess air coefficients) to flameless combustion at higher air inlet velocities (or higher excess air coefficients). The OH* images and both the temperature and the chemistry fields indicate that the reaction zone for the conventional lean combustion regime is well defined and established near the burner with relatively high gradients along the combustion chamber length. In contrast, the flameless combustion regime, which occurs for higher reactants jet momenta, is characterized by a more distributed reaction zone, located far from the burner, as revealed by the chemiluminescence images that show relatively small gradients disperse over a large volume of the combustion chamber. In line with this, the spatial temperature and gas species concentration gradients are much lower in this case than in the case of the conventional lean combustion regime.

Hydrogen Fuelled Micro Gas Turbine Combustion Chamber

An introductory overview of the micro and small-scale devices for the combustion in micro gas turbines is presented at the beginning of this paper. The work, thereafter presents the development of a micro combustor design, where the combustion process was simulated by CFD and tested experimentally. 3-dimensional model for combustion simulations was used. The fuel is hydrogen and primary zone equivalence ratio of 0.9 was simulated. Reactive flow calculations were carried out with 19 reversible reactions and nine species. The analysis of the simulated results was based on the obtained velocity profiles, concentration and temperature distributions within the liner. A micro combustor was manufactured and tests demonstrated its successful operation. Measurements of temperature, pressure and velocities along the length of combustion chamber were obtained. The experimental and simulation results are compared and a good qualitative and quantitative agreement was found between the experiments and the predicted val...

Examination of mode shapes in an unstable model combustor

The coupling between the fluid dynamics, heat addition, and the acoustics of a combustor system determine whether it is prone toward combustion instability. This paper presents results from a benchmark study of the eigenmodes in an unstable experimental combustor. The axisymmetric combustor configuration is representative of a number of practical systems and comprises an injector tube, geometric expansion into a combustion chamber, and a short converging nozzle. Instability limit cycle amplitudes ranged from 5% to nearly 50% of the mean 2.2 MPa pressure. Multiple harmonics were measured for the highly unstable cases. The model combustor was designed to provide a fairly comprehensive set of tested effects: sonic vs subsonic inlets; oxidizer tube lengths that were either quarter-wave, half-wave, or off-resonant acoustic equivalents to the combustion chamber; a significant injector mean flow with Ma∼0.4; and a varied combustion chamber length. The measured mode shape data were analyzed and reduced to provide comparison with results from a linearized one-dimensional Euler model, which included the effects of real boundary conditions, entropy generation, area change, and heat and mass addition, but did not include a model for unsteady heat addition. For low-amplitude instabilities, the measured resonance frequencies agreed with those calculated by the model for the injector tube-combustion chamber system. Resonance frequencies for the high-amplitude oscillation cases corresponded to the first longitudinal frequency of the combustion chamber and its integer multiples. Good quantitative agreement was obtained between computed and measured phase difference profiles, and mode envelopes agreed qualitatively. These results provide a basis for subsequent combustion response studies on the effects of unsteady heat addition.

563 燃焼振動の制振手法に関する基礎的検討

Combustion oscillation in burning appliances such as gas turbines and boilers is often appeared as a fluctuation of pressure, flow velocity and heat release and causes damage and performance degradation of apparatus. The lean premix combustion considering recent regulation on NOx emission often brings combustion oscillation. So, it is necessary to find suppression methods of these oscillations. In this study, we express combustion oscillation as one-dimensional phenomena, and analyze pressure and flow velocity fluctuations. By varying parameters such as combustion chamber length, position of reacting zone, preheat temperature and equivalent ratio, the insta- bility conditions are evaluated based on eigenfrequency analysis. Furthermore, the experimental apparatus is set up to investigate combustion oscillation.