Combustion Flow Field Research Articles

Study on ignition process of air/liquid oxygen/ethanol tripropellant coaxial jets in a subscale air heater

Up to the present, no studies have been published to investigate the ignition process in air heater. In this paper, spray combustion flow field during ignition process in air heater was first visualized by high-speed shadowgraph technique. High-speed shadowgraph movies and pressure data indicated that ignition-quenching–reignition phenomenon occurred during one test run. The experimental results demonstrated that combustion of the accumulated propellant mixture during a very short period led to a peak pressure at the moment of last ignition. This process caused reverse flow of flame and ethanol spray into air injector annulus, which was one of the reasons leading to local ablation of injector. In order to obtain reliable ignition, effective improvements were necessary.

Research on combustion flow field imaging method based on ray casting algorithm

Measurement of flame or combustion is an important method to estimate combustion quality, study the performance of engine and develop renewable energy sources. This paper presents a method to detect the combustion flow field in a confined space using Positron Emission Tomography (PET) technology. Radionuclides are labelled in aviation kerosene, and the nuclides are distributed in the confined space along with the kerosene in the process of combustion. γ photons generated by the decay of the nuclides in the combustion region are detected by the detectors around the object. Each pair of γ photons with 511keV travelling in opposite directions within a short time window are detected as a line of response (LOR). The distributions of nuclide concentrations in the combustion region, which are related to the combustion flow field, are estimated through the counts of LORs at different angles and positions. In order to display the combustion flow field based on human visual features, ray casting algorithm is used to reconstruct the 3D image with the slice images got from PET. Due to the complexity of ray casting algorithm, the interpolation algorithm in ray casting algorithm is optimized. The experiment results show that the image construction speed is improved with the optimized ray casting algorithm. The constructed 3D image can display the combustion flow field from different viewpoints.

Numerical prediction of the Flame Describing Function and thermoacoustic limit cycle for a pressurised gas turbine combustor

ABSTRACTThe forced flame responses in a pressurized gas turbine combustor are predicted using numerical reacting flow simulations. Two incompressible1large eddy simulation solvers are used, applying two combustion models and two reaction schemes (4-step and 15-step) at two operating pressures (3 and 6 bar). Although the combustor flow field is little affected by these factors, the flame length and heat release rate are found to depend on combustion model, reaction scheme, and combustor pressure. The flame responses to an upstream velocity perturbation are used to construct the flame describing functions (FDFs). The FDFs exhibit smaller dependence on the combustion model and reaction chemistry than the flame shape and mean heat release rate. The FDFs are validated by predicting combustor thermoacoustic stability at 3 and 6 bar and, for the unstable 6 bar case, also by predicting the frequency and oscillation amplitude of the resulting limit cycle oscillation. All of these numerical predictions are in very good agreement with experimental measurements.

Statistical behavior of shear layers of reactive oxygen/kerosene spray

Reactive gaseous oxygen/kerosene spray was visualized by a shadowgraph imaging technique in a lab-scale rocket engine combustor. Dynamic behaviors of inner and outer shear layer were observed morphologically and analyzed quantitatively by image processing. Correlation methods were used to statistically analyze dynamic behaviors of each shear layer and interaction between two shear layers. Relationship between shear layer dynamics and combustion flow-field was investigated by analyzing periodicity of jet-core length, lump-detachment, and luminous flame intensity. The results of correlation and frequency analysis show that dominant frequency of behaviors and interaction of shear layers is 270 Hz and the degree of correlation becomes stronger along the flow direction. The dominant frequency of core length variation and luminous flame formation is around 270 Hz and, which is the same as that of behaviors and interaction of shear layers. This proposes that the surface behaviors and interaction of the inner and outer jets in the upstream affect the downstream combustion flow-field.

Determining the heating characteristics of non-spherical particles in combusting flows

In order to perform efficient numerical calculations of the heating characteristics of inert, non-spherical particles, an Euler – Lagrangian approach was used for this work. The continuous phase was the combusting flow field in a lab-scale furnace. The Steady Laminar Flamelet (SLF) and the Flamelet Generated Manifold (FGM) combustion model were used for simulations of the continuous phase and the corresponding results were evaluated. It was concluded that the FGM combustion model offers superior performance and moderate computational effort in combustion modelling of air and natural gas. The second phase consisted of discrete coal slag particles which were injected into the furnace. The heating characteristics of spherical and non-spherical particles were evaluated and a model for calculating the temperature of non-spherical particles was validated. The application of the FGM in combination with the Euler-Lagrange approach and specially developed multiphase model adaptions led to very precise results. The maximum deviation between experimentally and numerically determined mean temperatures of non-spherical particles was only 6 K at a temperature level of 680 K.

Numerical Analysis of Transpiration Cooling in Annular Slinger Combustor

Wilcox k-ω turbulence model, EBU-Arrhenius turbulence combustion model and six-flux radiation model were applied to simulate turbulence combustion flowfield in a real annular slinger combustor. Structured grids of the annular slinger combustor were generated through TTM method and the anchored grid method. For considering the transpiration holes’ cooling effect, reaction flow field in whole annular slinger combustor and in the near-wall region of flame tube were analyzed by numerical simulation, respectively. A new method for data exchanging had been developed to transfer whole flow field numerical result to near-wall region by the inverse distance method, which can solve the contradiction between the requirement of vast computing resources and the computer’s processing ability. Wall temperature and cooling effect of flame tube were researched by numerical simulation method, which provides a technological support for the majorization of flame tube’s structure.

Instantaneous one-dimensional equivalence ratio measurements in methane/air mixtures using femtosecond laser-induced plasma spectroscopy.

Equivalence ratio is one of the most significant parameters in combustion flow fields. In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) technique for instantaneous one-dimensional local equivalence ratio measurements were performed. By measuring the spatially resolved spectra of FLIPS, we found that the spectral peak area ratios of CH (431 nm)/N2 (337 nm), CH (431 nm)/N2 (357 nm), and CH (431 nm)/O (777 nm) can be utilized to achieve one-dimensional local equivalence ratio measurements. Among them, the CH peak at ~431 nm and the O peak at ~777 nm are strong enough to be used to achieve single-shot measurements, which is important to turbulent flow fields. Furthermore, systematic experiments were performed by using FLIPS in both laminar and turbulent flow fields. The FLIPS technique features the abilities of instantaneous one-dimensional quantitative measurements, high spatial resolution, and no Bremsstrahlung interference.

燃烧流场波长调制光谱吸收模型的研究

燃烧流场波长调制光谱吸收模型的研究

燃烧流场羟基示踪测速的噪声去除方法

燃烧流场羟基示踪测速的噪声去除方法

The Effect of Dilution Air Jets on Aero-Engine Combustor Performance

Abstract In aero-engine combustor, the primary and secondary dilution air jets play a vital role to achieve efficient combustion and provide a satisfactory temperature profile at the combustor exit. In the current research, seven kinds of three-dome combustor sector associated with different dilution holes arrangements are simulated using Flamelet model. The influences of location, number and diameter of dilution air holes on flow field and performance of combustor are analysed in detail. The results demonstrate that the variation of airflow distribution, while keeping the total admission area constant, impact the combustion efficiency and pattern profile considerably, yet has insignificant effect on the total pressure coefficient. The maximum combustion efficiency and minimum pattern factor can be achieved simultaneously with deliberate dilution jet holes arrangement.

一种提高超燃流场羟基示踪速度测量中信号提取能力的方法

一种提高超燃流场羟基示踪速度测量中信号提取能力的方法

提高复杂燃烧流场羟基示踪测速精度的方法

提高复杂燃烧流场羟基示踪测速精度的方法

燃烧流场温度二维重建多吸收谱线重建方法

燃烧流场温度二维重建多吸收谱线重建方法

The Effects of Turbulent Burning Velocity Models in a Swirl-Stabilized Lean Premixed Combustor

Abstract The effects of turbulent burning velocities in a turbulent premixed combustion simulation with a G-equation are investigated using the 3D LES technique. Two turbulent burning velocity models – Kobayashi model, which takes into account the burning velocity pressure effect, and the Pitsch model, which considers the flame regions on the premixed flame structure – are implemented. An LM6000 combustor is employed to validate the turbulent premixed combustion model. The results show that the flame structures in front of the injector have different shapes in each model because of the different turbulent burning velocities. These different flame structures induce changes in the entire combustor flow field, including in the recirculation zone. The dynamic mode decomposition (DMD) method and linear acoustic analysis provide the dominant acoustic mode.

Numerical and experimental study of the thermochemical erosion of a graphite nozzle in a hybrid rocket motor with a star grain

Hybrid rocket motors is a promising propulsion system because of its intrinsic advantages over a conventional solid rocket motor and liquid rocket engine. However, serious nozzle erosion is a key problem that prevents hybrid rocket motors from being widely used, especially for propulsion systems with long operating times. In this paper, the erosion of a graphite-based nozzle coupled with a combustion flow field is studied in a hybrid rocket motor with a star grain. As the oxidizer and fuel, 90% hydrogen peroxide and hydroxide-terminated polybutadiene are adopted, respectively. The nozzle erosion was simulated coupled with the flow field in a typical hybrid rocket motor through three-dimensional numerical simulations. The simulations are based on a pure-gas steady numerical model considering turbulence, fuel pyrolysis, oxidizer/fuel reactions, thermal conduction and solid-gas boundary interactions on the fuel and nozzle surfaces. The results indicate that the nozzle erosion is greatly influenced by the inner flow field. The flame near the grain trough is thicker than that near the grain peak. Therefore, the maximum erosion rate (0.042 mm/s) occurs near the nozzle throat corresponding to the grain trough. The OH and H2O contribute 49.8% and 45.5% to the erosion rate, respectively, in this area. Furthermore, 56.6% and 31.9% contributions are made by OH and H2O, respectively, in the area corresponding to the grain peak. The O, CO2 and O2 make much lower contributions to the total erosion. In addition, a firing test is carried out to characterize the graphite nozzle erosion on a full-scale hybrid rocket motor with star grain. The nozzle inner profiles before and after test show that the erosion behavior of the graphite material is strictly related to the fuel shape.

Experimental study on flame development and stabilization in a kerosene fueled supersonic combustor

Flame development and stabilization in a kerosene fueled supersonic combustor with air throttling were experimentally investigated in the present paper. Stagnation conditions were 1100 K and 1.0 MPa and Mach number at the isolator entrance was 2.0. Various measurements included schlieren, shadow, interferometry, flame emission and PLIF were made during the experiments in an attempt to better understand the combustion flow field. The flow structure of non-reacting flow with and without air throttling was investigated firstly, the results showed a shock train was generated due to the increased back pressure by the throttling air, shock waves of the two cases oscillated with the flow. The flame development of pilot hydrogen was studied secondly, the ER (Equivalence Ratio) of pilot hydrogen should be 0.1, because a stable flame was beneficial to ignition transient. Finally, the effects of air throttling on flame stabilization were investigated, time for ignition of the case with air throttling was nearly a third of the case without air throttling, the pilot flame was blown off by the room temperature kerosene. But the kerosene was ignited successfully by the pilot hydrogen with the aid of air throttling, and even when the throttling air was removed, the flame was still stable.

The Characteristic of Flow Field and Emissions of a Concentric Staged Lean Direct Injection (LDI) Combustor

The characteristic of flow field and emissions was investigated by numerical simulation for a concentric staged lean direct injection combustor, the non-reaction and reaction flow fields were studied under idle and takeoff conditions, and the effect of combustion heat release on flow field was analyzed, then the influence of injection angle on emissions were obtained. The investigation results indicated that the steady central recirculation zone could be formed under non-reaction and reaction conditions by medium swirl strength which the swirl angle is 32°, however, the size and reverse mass flow at reaction conditions is significantly increased than non-reaction conditions; the combustion reaction zone moves toward the outlet as the decrease of fuel injection angle, which reduce the resistant time of hot gas in combustor, and then the production of nitrogen oxides would be decreased.

Experimental study on the effect of equivalence ratio and injector position on flow structure and flame development in the scramjet combustor

In order to understand the combustion characteristic clearly in the scramjet combustor, the effect of equivalence ratio and injector position on flow structure and flame development in a hydrogen fueled scramjet combustor was investigated experimentally in the present paper, various measurements included schlieren, shadow, differential interferometry, flame emission and OH*-PLIF were made during the experiments in an attempt to better understand the combustion flow field. The results were obtained under the inflow condition of Ma2.0, total temperature and total pressure were 950 K and 0.82 MPa, respectively. The combustion induced backpressure had not spread into the isolator when the equivalence ratio was 0.1, the flow structure was stable and the flame was located in the cavity shear layer in a line shape. But the backpressure of the cases when the equivalence ratio was 0.3 had spread into the isolator, the flow structure was unstable and the flame existed both in the cavity and in the core flow in a shredded paper shape. The hydrogen injected from the upstream injector burnt more intensively than that injected from the downstream injector under the same equivalence ratio condition. The above mentioned non-intrusive measuring methods had their own limitations, but the synchronous measurement method could well show a more comprehensive coupling information of the flow structure and combustion characteristics.

Structure conditioned velocity statistics in a high pressure swirl flame

A piloted, partially premixed, liquid-fueled swirl burner is operated at high pressure (1 MPa). High-speed (6 KHz) stereoscopic PIV is used to investigate the characteristics of the stagnation line separating the pilot jet and the central recirculation zone (CRZ) with varying pilot-main ratio and global equivalence ratio. The mean curvature of the stagnation line displayed a large spatial scale pattern that was present for all operating conditions. All three components of velocity, in-plane shear, and swirling strength are conditioned upon the instantaneous stagnation line. Mean distributions of the velocity normal to the stagnation line show that velocity is oriented towards the CRZ when the stagnation line is found nearer the centerline of the combustor. The conditioned out-of-plane velocity (w) shows a distinct concentration of large mean and fluctuation RMS values towards the center of the measurement domain. Varying fuel flow does not significantly change this spatial structure, only the magnitudes of the w statistics. The in-plane shear stress was the largest for the pilot biased condition as a stronger shear layer develops. For the leanest flame, large fluctuation RMS values of shear stress were confined to a region where the pilot jet begins to interact more heavily with the main jet. Operating with less pilot fuel flow enhanced the mean conditional swirling strength indicating that the pilot shear layer was shedding more intense eddies. Disregarding spatial relations, a scatter plot of w, shear stress, and swirling strength displayed trends between the variables. The largest swirling strength values coincide with highest magnitude shear stresses and the widest range of w. These conditioned statistics highlight how certain aspects of the combustor flow field are invariant with fuel distribution. This is desirable for aeropropulsive combustors that must maintain stable ignition from a range of conditions from landing/take-off to cruise.

Numerical Study on Configuration of Scramjet Combustor

The numerical simulation has been carried out to further investigate the cold flow characteristics for mixing efficiency and flame stability of the composite structure of pylon and cavity in the supersonic combustor and optimize the supersonic combustor flow field. It is found that the pylon can greatly improve the penetration depth of the jet and enhance the blending, and it will not bring the greater total pressure loss. Cavity can promote the diffusion of ethylene and improve the penetration depth of fuel, but also cause greater total pressure loss. The combination of the pylon and cavity can make good use of the advantage of the each single device and the geometry dimension and the combination mode of the pylon and cavity can be further analyzed from to improve the performance of the scramjet engine in next step.