Analysis of Self-Excited Combustion Instabilities Using Two- and Three-Dimensional Simulations

Gravity currents produced from a full-depth lock release propagating at the base of a linearly stratified ambient are investigated by means of newly conducted three-dimensional high-resolution simulations in conjunction with corresponding two-dimensional simulations and laboratory experiments. A passive tracer is implemented in the simulations to quantitatively measure the energies associated with the current and the ambient. The density of heavy fluid within the lock is ρ̃C, the density in the ambient of depth H̃ varies linearly from ρ̃b at the bottom to ρ̃0 at the top, and the ambient has an intrinsic frequency Ñ. Attention is focused on the initial slumping stage, during which the gravity currents propagate at a constant velocity Ṽ and the internal Froude number is defined as Fr=Ṽ/ÑH̃. The dynamics of the subcritical gravity currents, i.e., Fr<1/π, and the supercritical gravity currents, i.e., Fr>1/π, are qualitatively different and are examined with the help of three-dimensional and two-dimensional high-resolution simulations. For the subcritical gravity currents, the flow is dominated by the internal wave, the Kelvin–Helmholtz vortices are inhibited, and the two-dimensional simulation agrees well with and serves as a good surrogate model for the three-dimensional simulation in the slumping stage. For the supercritical gravity currents, the Kelvin–Helmholtz vortices are pronounced and prone to breakup into three-dimensional structures in the slumping stage. On the one hand, for the supercritical gravity currents, the kinetic energy associated with the current and the potential energy associated with the ambient are accurately captured by the two-dimensional simulation. On the other hand, the transition distance for the slumping stage and dissipation rate in the system are underpredicted while the kinetic energy associated with the ambient and the potential energy associated with the current are overpredicted by the two-dimensional simulation for the supercritical gravity currents. Therefore, information derived from the two-dimensional simulation for the supercritical gravity currents must be treated with care. The high-resolution simulations in this study also complement the existing shallow-water formulation, which has been reported to agree well with the two-dimensional simulations with good physical assumptions and simple mathematical models.

Proper orthogonal decomposition and dynamic mode decomposition are evaluated for the study of self-excited longitudinal combustion instabilities in laboratory-scaled single-element gas turbine and rocket combustors. Since each proper orthogonal decomposition mode comprises multiple frequencies, specific modes of the pressure and heat release are not related, which makes the analysis more qualitative and less efficient for identifying physical mechanisms. On the other hand, dynamic mode decomposition analysis generates a global frequency spectrum in which each mode corresponds to a specific discrete frequency so that different dynamics can be correlated. In addition, proper orthogonal decomposition results are found to be inaccurate when only a limited amount of spatial information is provided in contrast with dynamic mode decomposition results, which provide more reliable results. Overall, dynamic mode decomposition analysis proves to be a robust and systematic method that can give consistent interpretations of the periodic physics underlying combustion instabilities.

A numerical scrutiny was performed to analyze the effect of driving forces on molten pool in gas metal arc welding. In addition to the basic governing equations required for a general heat and mass transfer analysis by computational fluid dynamics, a volume-of-fluid equation for free surface tracking and additional conservation equations for calculating the distribution of alloy elements were used. Driving forces—buoyancy, drag force, arc pressure, electromagnetic force, Marangoni pressure, and droplet impingement—and the arc heat source were mathematically modeled and applied to the simulation. In order to examine the effect of driving forces, a two-dimensional axisymmetric simulation was performed, and the effect of each driving force was analyzed using the velocity components. In the radial velocity component, the effect of droplet impingement and the Marangoni force was large, and in the vertical velocity component, the droplet impingement effect was dominant. A three-dimensional simulation was also performed considering all the driving forces together, and the result was verified by comparison with experimental results. Relatively high alloying element contents were found at the bottom of the fusion zone, which was due to the droplet impingement generating a high vertical velocity.

: Combustion instability in rocket chambers is strongly influenced by acoustic interactions at the boundaries of the configuration. Many CFD simulations employ approximate boundary conditions in order to simplify the geometry but the impact that they have on the solution is not well understood. The present study focuses on the use of detailed (exact) boundary representations and an approximate boundary condition in a given longitudinal mode test chamber. The actual inlet boundary of the injector is comprised of a series of choked slots while the approximate boundary condition is a uniform constant mass flow. Both two and three-dimensional simulations are carried out. Differences in the flowfield are evident in the combustion region away from the inlet, including the size of the recirculation region and location of the peak heat release. The amplitudes of the acoustic modes are well predicted for the first two modes especially in three-dimensional simulation, while higher modes are poorly predicted. These results suggest that such boundary condition approximations must be judiciously used and having access to more detailed treatments is important to verify accuracy.

Combustion instability analysis in an ethylene-fueled scramjet combustor under various fuel penetration height conditions using an image-based nonlinear dimensionality reduction method

Diesel engines are a major source of nitrogen oxides, carbon dioxide, and particulate matter emissions. While biodiesel blends offer potential emission reductions, challenges remain in viscosity, flash point, cold weather performance, and combustion characteristics. This study examines the performance and emission behavior of biodiesel–diesel blends to enhance efficiency and reduce environmental impact. Multiscale entropy analysis was employed to assess combustion instability and complexity, focusing on the influence of fuel composition and engine speed at full load. Experimental investigations were conducted using biodiesel derived from tomato, papaya, and apricot, blended with diesel. The tested fuels included binary and ternary biodiesel–diesel blends with varying levels of complexity. Results indicate that at optimal operating conditions, biodiesel blends enhance energy and exergy efficiency while exhibiting higher carbon dioxide and nitrogen oxide emissions compared to pure diesel. Hydrocarbon emissions decreased under optimized conditions, while fuels with lower chemical complexity demonstrated improved torque output. Diesel, characterized by greater combustion complexity, resulted in lower exergy and useful work. These findings contribute to the understanding of biodiesel application in compression ignition engines, providing insights into performance optimization and emission control strategies. Keywords: Biodiesel blends; combustion stability; engine emissions; energy efficiency; exergy analysis.

Influence of two- and three-dimensional simulations on bubble behavior in gas–solid fluidized beds with and without immersed horizontal tubes

Numerical study of combustion instability in liquid propellant rocket engine is presented in the paper. Time-depending two-dimensional Navier-Stokes equations were employed to describe the fluid flow in the combust chamber and solved by finite difference method with MacCormack scheme. Turbulent viscosity was taken into consideration by the subgrid scale turbulence model. Propellant spray was treated with a lagrangian tracking method and droplet source terms were incorporated into the gas phase calculation by PSIC method. Droplet initial conditions were determined by a stochastic spray model . Spray burning rate is calculated by the classic D2 vaporization law with Ranz-Marshall correlation. The coupling of pressure disturb and burning rate variation was described with Prime's twoparameter model. The actual axisymmetric combustor was transferred into equivalent annular geometry as the references do. Combustion instability analysis was performed by imposing a bomb pressure pulse on the computed steady flow field to trigger unstable combustion. By observing the pressure wave propagation , we can asses the damping capability of the engine.The computational analysis show that numerical schemes and physical models used to describe unstable combustion are the most important factors to the success of numerical study. d NOMENCLATURE a sound velocity b annular combustor width constant pressure spcific heat CP C,, drag coefficient d droplet diameter D mass diffusivity e total energy f frequency h enthalpy 1 L~ heat of evaporation f i transverse length of the annular combstor mass flux or evaporation rate M number of computational droplet NU Nusselt number pr Prantdl number Ice Reynolds number s source terms time T lempeerature X,Y coordinates p density h coductivity P I velocity components

We have performed two-dimensional (2-D) and three-dimensional (3-D) computer simulations of random dopant fluctuations in 25-nm planar n-channel metal-oxide-semiconductor field effect transistor (MOSFET) with superhalo channel doping. Our study shows that 2-D simulations that neglect lateral percolation of the carriers can overestimate the impact on threshold voltage (V/sub T/) fluctuations by as much as a factor of four. Fundamental differences in the way the 2-D and 3-D models describe subthreshold and near-threshold conduction are highlighted in our study. Our models reveal that surface percolation of carriers is an effective agent for reducing V/sub T/ fluctuations. In addition, the halo only enhances the V/sub T/ fluctuations by approximately 10%. Though the influence of the superhalo in the device may be overwhelmed by atomistic granularity according to the 2-D model, 3-D simulations show that the halo continues to function coherently for the MOSFET ensemble when charge percolation is accounted.

Gas-turbine combustion systems based on premixed flames, for certain operating conditions, exhibits unacceptable high pressure oscillations due to combustion instabilities. In this paper, a nonlinear mathematical model for the description of these phenomena is presented. The model is built via a thorough description of both the gasdynamics and the combustion process, which is concisely outlined. The core of the paper is devoted to the use of the model for the study of the flame limit-cycle behaviour and on the corresponding flame front oscillations within the combustor chamber. The developed model is intended for active control of combustion instabilities usage, and is therefore a description of the leading dynamics.

ABSTRACTThe main aims of this research are, at first, combustion instability study based on equivalence ratio oscillation, and, secondly, investigation various frequency modes of combustion instability, taking combustion chamber geometry into account. Considering the configuration of the simulated combustion chamber, excitation probability of the longitudinal modes is higher than that of transversal modes. The reason of this fact is that the resonance frequency values of the longitudinal modes are less than those of transversal modes. So, the most important frequency mode, during combustion instability, is the first longitudinal mode. In this paper thermo-acoustic instability model is utilized for pre-mixed gas turbines combustion chamber, founded on equivalence ratio oscillation. For this purpose Lieuwen method is developed in order to attain the phase difference between pressure and heat release oscillations. Results concluded from combustion instability simulation for the first longitudinal mode, considering its importance, are compared with experimental data and good agreement is observed.

FDF-based combustion instability analysis for evolution of mode shapes and eigenfrequency in the multiple flame burner

We present a single-explosion model study of the 3C 400.2 supernova remnant (SNR), made with two-dimensional axisymmetric simulations. The numerical simulations were made with the adaptive grid code YGUAZU-A. Several initial scenarios have been tested in order to reproduce the centrally peaked X-ray emission observed for this remnant. This study reveals that the explosion of the SN inside a pre-existing stellar wind bubble successfully generates the observed morphology, when thermal conduction is included in the model. The best morphological fit is obtained at an evolution time of 21 000 yr, when the total luminosity is 1.2 x 10 34 erg s -1 .

The Hyperloop system is a new concept that allows a train to travel through a near-vacuum tunnel at transonic speeds. Aerodynamic drag is one of the most important factors in analyzing such systems. The blockage ratio (BR), pod speed/length, tube pressure, and temperature affect the aerodynamic drag, but the specific relationships between the drag and these parameters have not yet been comprehensively examined. In this study, we investigated the flow phenomena of a Hyperloop system, focusing on the effects of changes in the above parameters. Two-dimensional axisymmetric simulations were performed in a large parameter space covering various BR values (0.25, 0.36), pod lengths (10.75–86 m), pod speeds (50–350 m/s), tube pressures (~100–1000 Pa), and tube temperatures (275–325 K). As BR increased, the pressure drag was significantly affected. This is because of the smaller critical Mach number for a larger BR. As the pod length increased, the total drag and pressure drag did not change significantly, but there was a considerable influence on the friction drag. As the pod speed increased, strong shock waves occurred near the end of the pod. At this point, the flows around the pod were severely choked at both BR values, and the ratio of the pressure drag to the total drag converged to its saturation level. At tube pressures above 500 Pa, the friction drag increased significantly under the rapidly increased turbulence intensity near the pod surface. High tube temperatures increase the speed of sound, and this reduces the Mach number for the same pod speed, consequently delaying the onset of choking and reducing the aerodynamic drag. The results presented in this study are applicable to the fundamental design of the proposed Hyperloop system.

Thermal recycling analysis in regenerative cooling channels based on liquid rocket engine cycles