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CHEMICAL KINETICS AND INJECTION MODELING FOR HP LOX/CH4 JET FLAMES
No full textThe aim of the present work is to compare different approaches for modeling combustion in cryogenic
LOx/CH4 rockets. The oxidizer is liquid oxygen, delivered to combustion chambers as a spray of droplets; the fuel
is gaseous methane. Pressure and temperature are well above thermodynamic critical points of both the
propellants and so the reactants show liquid-like characteristics for density and gas-like characteristics for
diffusivity. A so complex behavior leads to some difficulties in choosing the most appropriate modeling approach
for phenomenon description.
In literature it is possible to find a large number of experimental works concerning cryogenic combustion of
liquid oxygen and gaseous hydrogen under sub-critical and trans-critical conditions but also for methane as fuel.
In the present work LOx-methane combustion for G1 and G2 cases of RCM3-VO4 test plane of ONERA have
been investigated to compare different approaches in numerical modeling of cryogenic flame. The most important
aspects taken into account are combustion models, kinetics descriptions and thermodynamic properties. The Eddy
Dissipation combustion model and the Probability density function PDF model were compared; the Jones-
Lindstedt and the Skeletal mechanisms were used for kinetics description for the Eddy Dissipation model; for
thermodynamic approach, in the current work gas was modeled using both ideal gas and real gas equation of state
(Peng-Robinson and SRK) and liquid oxygen was treated with a Lagrangian approach in a hybrid Eulerian-
Lagrangian approach for mixture. Numerical predictions were compared with experimental data from literature
Different Combustion Models Applied to High PressureLOX/CH4 Jet Flames
No full textThe combustion phenomena in liquid-propellant rocket engines are highly complex. The combustion occurs at
operating conditions well above of the thermodynamic critical points of the fluid where reactants properties
show liquid-like densities, gas-like diffusivities, and pressure-dependent solubility. Actually, there is a great
interest in the development of reusable liquid rocket engines that operates with methane and liquid oxygen as
propellants. In the numerical study of LOX/CH4 jet flames there are some critical aspects to be taken into
account.
The choice of the combustion model is a critical point: it should be accurate in the phenomena description but
it should also characterized by a low computational cost. In the present study different combustion models
were used as the Eddy-dissipation finite-rate approach based on Arrhenius chemical kinetics, the equilibrium
mixture fraction model (PDF) and the Steady State Flamelet approaches.
In the case of reacting models based on chemical kinetics, both simplified and more complex kinetics models
can be used to numerically describe the flames but the critical point in the choice is the individuation of the
best compromise between computational cost and accuracy. In this work different chemical kinetics schemes
were used, as the Skeletal mechanism and the Jones- Lindstedt mechanism, that permit to limit the number of
reactions and species but taking into account also the intermediate species in the flame.
Finally a purpose of this work is also to develop a pure Eulerian (i.e., single-phase) methodology by using
both ideal gas and real gas equation of state and to compare with the discrete phase approach that uses an
Eulerian description of the gas phase and Lagrangian equations for the dilute spray. For all the models used, a
comparison with experimental data from literature was performed
Experimental and Numerical Investigations on the Effect of Different Air-Fuel Mixing Strategies on the Performance of a Lean Liquid Fueled Swirled Combustor
Full text linkIn the present work the performance of a multipoint lean direct injection strategy for low emission aero-propulsion systems has been experimentally and numerically investigated, and compared with the single point injection strategy. A swirler liquid fueled combustor was designed and used in experiments to investigate the flame behavior in lean and ultra-lean conditions for both the single-point and the multi-points injection strategies. Multipoint injection has been realized injecting an amount of fuel upstream the swirler inlet and using also the central injector as a “pilot” injection.
As regarding the experimental facilities, the combustor is equipped with four optical accesses for high speed flame imaging and with pressure and temperature sensors. Experimental data on flame characteristics and pollutant emissions are obtained. The characterization of the flame was realized using intensified high rate CCD camera for the acquisition in the ultraviolet spectral range. In front of the camera various combinations of optical filters were installed to selectively record the respective chemiluminescent species (OH* and CH*).
Computational fluid dynamic (CFD) simulations were also performed for a deeper understanding of the flame characteristics under the two injection strategies. The typical combustor operations were reproduced to more deeply understand the differences between the injection modes and the related flame patterns. The numerical results show different temperature and species fields predicted for the non-premixed and the partially premixed cases and furnish relevant information about the fluid dynamics in the combustion chamber in both the injection conditions
Experimental characterization of near-blowout instabilities in a lean liquid-fuelled combustor
No full textIn the present work, an experimental investigation was performed to characterize lean combustion flames in a liquid fuel burner. Two different regimes were investigated: non-premixed and partially-premixed combustion modes. The fuel mass flow rate was fixed and the air mass flow was reduced until the blow-out limit was reached. A high rate CCD and a PMT tube equipped wit1h an OH* filter were used for the acquisitions. Statistical and spectral post-processing methods were applied obtaining variance maps, trends of the averaged value of variance with respect to the equivalent fuel/air ratio and trends of the wavelets energy contents with respect to the frequency ranges. Results underline that the onset of flame instability occurs at higher fuel/air ratio in the non-premixed combustion regime compared to the partially premixed mode. Furthermore the rise of CO emissions starts at the leaner conditions in the case of partially premixed combustion. The present work also shows that imaging techniques are suitable to individuate this instability incipience using spectral and statistical parameters extracted by the temporal series of flame images, hence they might be implemented in online monitoring systems
Flame Instability in A Liquid Fuel Burner: Comparisons Between Single And Multipoint Injections
Full text linkWith the aim to characterize the flame behavior when ultra-lean combustion
conditions are reached, an experimental investigation was performed on a liquidfuel
gas turbine derived burner, at different fuel/air ratios and comparing different
fuel injection modes. Ultra lean conditions have a negative impact on combustion
efficiency for the instabilities insurgence. High speed acquisitions by a CCD
camera were performed to investigate the behavior of the spray and the flame close
to lean blowout. Statistical and spectral analyses were also applied to the flame
acquisitions to extract suitable parameters for blowout recognition
SPRAY AND COMBUSTION MODELLING IN HIGH PRESSURE CRYOGENIC JET FLAMES
Full text linkThe aim of the present work is the investigation of the combustion
phenomenon in liquid-propellant rocket engines. The
combustion of liquid oxygen and gaseous methane of a shear
coaxial injector under supercritical pressure was analyzed. To
realize an efficient numerical description of the phenomena, it
is important to treat the LOx jet in a manner which takes into
account its real behavior. In the present work different kinetics,
combustion models and thermodynamics approaches were
used in association with the description of the jet as a discrete
phase. For all the approaches used, a comparison with experimental
data from literature was performed
Image processing for the characterization of flame stability in a non-premixed liquid fuel burner near lean blowout
No full textAbstract In the present work, an experimental investigation was performed by varying the fuel/air ratio of a liquid-fuel gas turbine derived burner in the non-premixed mode, until an ultra-lean combustion condition was reached. In this condition, flame instabilities occur with negative impacts on combustion efficiency. Two high speed visualization systems in the visible range and in the infrared spectral region were used. Moreover, they were supported by an OH⁎ chemiluminescence measurement and by gas exhaust measurements. Different techniques were used starting from the luminosity signal of each pixel: the Wavelet Decomposition to calculate the wavelet energy, the frequency analysis of pixel intensities of the flame images to estimate the dominant frequency, finally the statistical analysis to calculate the pixel intensity variance. Both the statistical and frequency analyses were applied to the OH⁎ chemiluminescence data. One of the most important results of the present work regarded the capability of imaging techniques to individuate the instability insurgence and to be used as a predictive tool. Furthermore 2D maps of some parameters, extracted by the wavelet-based analysis of flame images, permitted to investigate local unsteadiness in the flame area
Ultra Lean Combustion Characterization in a Pilot-Scale Gas Turbine Burner Using Image Processing Techniques
No full textThe aim of the present investigation is the characterization of the behavior of a lean partially-premixed liquid fuel gas turbine near lean blowout limit.
At this combustion regime the onset of instability will occur with negative impacts on combustion efficiency.
The identification of the instability occurrence permits an efficient flame control adjusting the combustion parameters (as fuel or air mass flow, temperature, pressure, etc.) to stabilize the flame or designing opportunely flame control system.
High-speed images of the flame under stable and near blowout condition were captured in conjunction with simultaneous optical data in order to better understand the phenomenology of the flame blowout process and the onset of instability.
In particular the experimental characterization was performed through a High Speed Digital Camera, an Infrared camera and a Photomultiplier Tube (PMT) in association with the use of optical filter (OH*). The data collected with these instrumentations produce useful features for the development of an efficient tool for the flame control in industrial and aeronautical burners. The images acquired by the different cameras were processed considering the luminosity signal of each pixel and evaluating the frequency behavior, the variations of amplitude of the signals and some other descriptive parameters able to define the regime of the flame. Spectral analysis and Wavelet transform of pixel intensities of flame images were used and entropy and energy contents were evaluated. The spatial maps of the different spectral and statistical parameters were shown at different fuel/air equivalence ratio. The OH* emissions data measured by the PMT were processed and compared with the data obtained from the images processing
Comparisons between different combustion models for Highpressure LOX/ CH4 jet flames
Full text linkThis paper presents an investigation of LOX/CH4 Liquid Rocket Engines injector flames
to investigate the impact of real gas effects and the combustion models on the predictions. In
liquid-propellant rocket engines the combustion occurs at operating conditions well above of
the thermodynamic critical points of the fluid where reactants properties show liquid-like
densities, gas-like diffusivities, and pressure-dependent solubility. So using real gas
properties as accurately as possible is a key issue in the preliminary design of LRE injectors
and combustion chambers. In the numerical study of LOX/CH4 jet flames a critical aspect is
the choice of the combustion model, that should be accurate and a well compromise between
phenomena description and computational costs. Complex, simplified and reduced kinetics
scheme could be implemented in the CFD modeling of cryogenic spray and to make a well
choice of the modeling approach it is necessary to estimate the mixing and kinetic time scales
in the case of study. In the rockets it’s generally possible to assume that the chemistry is
infinitely fast and that burnt gas conditions can be approximated like similar to the chemical
equilibrium condition. Under these conditions a simple finite rate combustion model is less
realistic; otherwise an Eddy Dissipation Approach or Flamelet model could more
realistically model phenomena. In this work LOX/CH4 jet flames at high pressure have been
simulated by implementing different kinetic mechanisms usually presented in literature for
the methane/air flame, starting from the simple one step mechanism up to the detailed
Skeletal model derived from the Grimech 3.0. Then results obtained using some simplified
mechanisms, as the Modified Jones-Lindstedt kinetics model, more accurate for combustion
in presence of pure oxygen have been compared with the previous ones. EDC and Non-
Premixed Combustion Model, including flamelet approaches are implemented in the CFD
Fluent v.13.0 code, like in the present work, it is not possible to take into account real gas
effetcs with the non-premixed model. Real gas effects have been also considered in the case
of EDC combustion model. In a liquid rocket engine at high pressures real gas effect needs to
be modelled by using a Real Gas Equation of State (RG EOS). In the current work the real
gas effects have been modelled by the Soave-Redlich-Kwong (SRK) real gas model
Effect of Actuation Parameters on Stabilization of Methane Diffusive Flames Using Plasma Actuators
Full text linkThe reduction of nitric oxides (NOx) in aircraft engines, gas turbines, or internal combustion engines is a main issue in the design of novel combustion systems. The reduction of the NOx emissions might be reached by lean combustion. However, the major issue is the stabilization of the flame under lean conditions.
In this context, the present work investigates the possibility of increasing the combustion efficiency of a lean flame through the employment of a plasma actuator, operated by both nanosecond repetitively pulsed high voltage (NRPP) and sinusoidal DBD high voltage (HV). Different actuation conditions have been tested to stabilize and improve the efficiency of a lean non premixed methane/air flame in a Bunsen-type coaxial burner with central fuel jet. An image processing approach was used to characterize the flame behavior near blowout condition
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