Discrete Combustion Research Articles

Thermal inertia effect of reactive sources on one-dimensional discrete combustion wave propagation

In the present work, the discrete flame model developed by Shoshin et al. (Doklady Akademii Nauk SSSR, 1986) and further scrutinized by Tang et al. (Combustion Theory and Modelling, 2009) is augmented by introducing the thermal inertia of particles in the preheating zone. The effect of particle thermal inertia on the speeds, propagation limits, and near-limits dynamics of one-dimensional discrete combustion waves is studied using the new model. It is found that, with the increase of particle thermal inertia, the propagation velocity of the discrete flame decreases due to the smaller heating rate of the particles. Besides, particle thermal inertia extends the propagation limits compared to the prediction of the old model. Furthermore, it is mathematically proven that the nonphysical branch of the solutions for the discrete flame speeds, found using the old discrete model, is a set of solutions for the propagation limits of steady-state discrete flames with particle thermal inertia included. The flame speed predicted using the new model is also compared with that determined analytically using a continuum model considering the thermal inertia of the condensed phase, attributed to Goroshin et al. (Combustion and Flame, 1996). We find that the discrete flame speeds predicted by the both models become closer to each other with increasing particle thermal inertia. Finally, the two models converge regardless of the discrete nature of the heat sources when particle thermal inertia is large enough so that can limit the flame propagation. The particle thermal inertia controlled flames could be regarded as a new kind of combustion regime.

In situ measurement of combustion wave propagation during combustion synthesis of α‐Si3N4 powder

Abstractα‐Si3N4 powder was prepared by combustion synthesis method. The propagation characteristics of combustion wave were investigated by thermocouple temperature measurement and “resistance—combustion wave displacement response device.” The results show that the combustion reaction takes place in a narrow area, and there is a maximum temperature gradient of 180°C/mm in the combustion front. The “resistance—combustion wave displacement response device” was innovatively used to realize the in situ measurement of combustion wave propagation process. The test results showed the pulse combustion mode in which the combustion front took 10 s as a cycle, and the quantitative data of Si–N2 discrete combustion characteristics were obtained for the first time.

Propagation of Non-Adiabatic Heterogeneous Solid Combustion Waves with Effect to Internal Microstructure and Heat Release

ABSTRACT Propagation of combustion front with effect to the external heat loss is studied by the two-dimensional discrete periodic and disorder systems. Discrete periodic systems are modeled by the regular arrangement of burnt and unburnt point heat sources characterized by the identical rate of heat release and different heat loss parameter. Discrete disorder systems, namely, S1, S2 and S3 are considered with the concatenation of regular burnt and unburnt point heat sources distributed by different shaping parameter of beta distribution function. The unburnt point heat sources in a discrete disorder system are characterized by the different heat loss parameter and incorporation of two categories of distributions for the rate of heat release namely (i) gamma (a = 20) and (ii) gamma (a = 2). The oscillations in ignition time delay, with the manifestation of bifurcation pattern, have been detected for discrete periodic systems. In a discrete disorder system, the external heat loss plays a crucial role in developing the fingering instabilities and determining the early onset of unstable combustion, quenching and combustion limit. Degree of randomness in the distribution of heat release for the disorder system affects the duration of combustion, burn rate without affecting the combustion limit. Computed values of burn rate and combustion limit from the discrete combustion model have been compared with the available data from experiment.

Ensemble Combustion of Liquid Oxygen / Gaseous Hydrogen Coaxial Flames with Transverse Acoustics

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames, under the influence of transverse acoustic forcing. The results are systematically compared to the results from a predecessor study of the same ensemble of flames under the same conditions, but without acoustic forcing. Both pressure nodes (PN) and pressure antinodes (PAN) were investigated as a function of the outer-to-inner momentum flux ratio. Shadowgraphs and OH* emission imaging showed that PNs distort the flames more than PANs, similar to previous observations in single flames. A wave amplification mechanism (WAM) identified previously to be present both in single flames and in the unforced ensemble of flames was found to also be present under acoustic forcing. In the predecessor study without acoustics, the WAM tended to self-organize within the ensemble into an out-of-phase varicose nesting that minimized large-scale collisions. In the present study, PN forcing tended to organize the WAM structures into a sinuous nesting, which also minimized collisions. PAN forcing, on the other hand, promoted destructive in-phase varicose nesting that promoted large-scale collisions, causing subsequent breakdown into smaller scales. Dynamic Mode Decomposition revealed natural frequencies and modes in addition to the acoustic quantities. Discrete combustion waves related to the WAM propagated at the same normalized average velocities regardless of whether acoustics were present in the ensemble or not. However, the trajectories are grouped into different curves than for single-element flames.

A numerical study on discrete combustion of polydisperse magnesium aero-suspensions

In this numerical study, discrete combustion of polydisperse magnesium dust clouds was investigated. A numerical model accounting for the effects of ignition, thermal conduction, and radiation was formulated to simulate the spatiotemporal distribution of temperature. Three distribution models, i.e., Dagum, log-normal, and Beta prime, were used to describe the magnesium particle-size polydispersity. The numerical model was first validated by comparison against experimental data on discrete combustion of both mono-sized and polydisperse magnesium aero-suspensions. Subsequently, the flame propagation characteristics of mono-sized and log-normally polydisperse cases at two different mean magnesium particle sizes were compared. The comparison shows that polydisperse magnesium dust clouds have higher flame propagation speeds than their mono-sized counterparts. Finally, the differences among the polydisperse cases with different size distributions were compared, revealing that magnesium powders with a higher percentage of small particles give rise to higher flame propagation speeds. Furthermore, results show that in comparison with the Dagum and Beta prime distributions, the log-normal distribution results in a lower flame propagation speed and a higher minimum ignition energy. As either the particle size decreases or the dust-cloud concentration increases, the flame propagation speed increases and the minimum ignition energy decreases.

Preparation of Ni/SiO2 catalyst via novel plasma-induced micro-combustion method

Abstract Ni/SiO2 catalysts are prepared via plasma-induced micro-combustion method in this study. The process during micro-combustion is analyzed and the catalysts are applied for syngas methanation. Discrete combustion is observed in the process of plasma-induced micro-combustion. X-ray diffraction, Temperature programmed reduction together with X-ray photoelectron spectroscopy disclose that highly dispersed NiO strongly interacting with SiO2 generates when prepared via micro-combustion method. X-ray diffraction and transmission electron microscope verifies the smaller metal Ni particles after reduction. Thus, NiO aggregation is prevent and strong interaction between NiO and SiO2 restrained metal Ni sintering during reduction via moderate micro-combustion. The as-prepared Ni catalysts also own higher surface areas, higher pore volumes and pore diameters. These characters prominently enhance the activity and stability of catalysts in syngas methanation. This work provides a promising method to synthesize nanoscale materials.

Structure and Behavior of Gasless Combustion Waves in Powders

ABSTRACTThe method of simulation of the structure of powder mixture, which allows calculating the samples with a relative density from 15% up to practically close packing, is described. The 3D model of discrete combustion wave in powder mixture, which uses the actual structure of the sample, is considered. The parametric investigations of combustion of powder mixture in the wide range of relative density and particle ignition temperature are carried out. The model predicts the steady-state combustion, oscillating combustion, and extinction depending on the parameters of powder mixture. The results of the simulation are compared with experimental data.

A three-dimensional simulation of discrete combustion of randomly dispersed micron-aluminum particle dust cloud and applying genetic algorithm to obtain the flame front

Discrete combustion of micron-sized aluminum particle dust cloud in air was investigated by a three-dimensional simulation. In light of presence of radiation and conduction heat transfer mechanisms, simulations became more realistic. Particles were randomly dispersed within a rectangular control volume using two different methods. An average flame front speed was obtained at each particle concentration using both methods. The second method yielded more accurate average flame front velocities. At lower dust concentrations, the results were in a good agreement with experimental outcomes. In the control volume composed of particles distributed using the second method, flame front was approximated with a fitted surface obtained via genetic algorithm. The temperature profiles of four random particles were plotted to provide insight into preheating time order of unburned particles. Later, the effect of particle diameters on flame front velocities was demonstrated. In the end, the flammability lean limit, as a significant parameter in safety issues, was calculated for different particle diameters.

Thermodynamics-based mean-value engine model with main and pilot injection sensitivity

This paper presents a thermodynamics-based mean-value engine model that predicts the temperature and pressure at the end of each discrete combustion phase, the peak temperature and pressure, and the indicated mean effective pressure. The model has sensitivities to the engine speed, the main and pilot injection quantities and timings, and the temperature and pressure conditions in the intake and exhaust manifolds. Mathematical relationships are established between the input variables and the output variables through the thermodynamic principles of the modified ideal-gas-limited pressure cycle. Modifications to the ideal-gas cycle include the pilot combustion stage, the effective compression ratio based on the fuel injection timing and start of combustion, the effect of the constant-volume burn ratio on combustion, and the exhaust valve opening timing. The model is also augmented with empirical correlations for the ignition delay, the constant-volume burn ratio, and the heat transfer in order to improve the fidelity of the model. It is validated using test data from a Ford 6.7 l diesel engine. Unlike most data-regression-based mean-value engine models, the model presented in this paper does not sacrifice essential details of the underlying physics while providing a computationally efficient simulation tool.

Discrete combustion waves of two-dimensional nanocomposites

A combustion model for a thin heat-conducting film with discrete, chemically active hot spots distributed on its surface is proposed. An equation describing the combustion of such a system is derived, and exact analytical solutions of this equation for periodic arrangement of hot spots on the film surface are found. The sensitivity of burning rate to changes in the initial temperature of the system depending on its main parameters is determined. It is shown that the system has critical parameters (film thickness, hot spot concentration, and initial temperature) that define the limits of its combustion. With large concentrations of hot spots on the film surface, it is theoretically possible to reach burning rates that significantly exceed the speed of sound in the ambient gas. The proposed combustion model provides a qualitative explanation of the high burning rate of mechanoactivated nanocomposites and allows one to understand the influence of mechanoactivation on combustion of powder mixtures.

Dynamical and statistical behavior of discrete combustion waves: A theoretical and numerical study

We present a detailed theoretical and numerical study of combustion waves in a discrete one-dimensional disordered system. The distances between neighboring reaction cells were modeled with a gamma distribution. The results show that the random structure of the microheterogeneous system plays a crucial role in the dynamical and statistical behavior of the system. This is a consequence of the nonlinear interaction of the random structure of the system with the thermal wave. An analysis of the experimental data on the combustion of a gasless system (Ti + xSi) and a wide range of thermite systems was performed in view of the developed model. We have shown that the burning rate of the powder system sensitively depends on its internal structure. The present model allows for reproducing theoretically the experimental data for a wide range of pyrotechnic mixtures. We show that Arrhenius' macrokinetics at combustion of disperse systems can take place even in the absence of Arrhenius' microkinetics; it can have a purely thermal nature and be related to their heterogeneity and to the existence of threshold temperature. It is also observed that the combustion of disperse systems always occurs in the microheterogeneous mode according to the relay-race mechanism.

Effects of oxidation on reaction front instabilities and average propagation speed in Ni/Ti multilayer foils

Vapor-deposited, equiatomic Ni/Ti multilayer foils exhibit low-speed, self-propagating formation reactions that are characterized by a spin-like reaction front instability. In addition to the intermetallic reaction between Ni and Ti, reactions performed in air can also exhibit a discrete combustion wave associated with the oxidation of Ti. In general, the oxidation wave trails the complex intermetallic reaction front. Multilayers that have a large reactant layer periodicity (≥200 nm) exhibit a decrease in net reaction speed as air pressure is reduced. Oxidation has a much smaller effect on the net propagation speed of multilayers with small layer periodicity (<100 nm). The net propagation speed of the multilayers is increased when air is present, due to the added energy release of Ti oxidation. High-speed optical microscopy shows that the increased front speed is associated with an increased nucleation rate of the reaction bands that typify the spinning reaction instability of the Ni/Ti system.

A CFD-applicable discrete combustion model for thermally large particles

The objective of this paper is to present a comprehensive new single-particle combustion model that can be used effectively in Computational Fluid Dynamics (CFD) applications. As a major difference from standard particle sub-models used in CFD software packages, the new model is suitable for simulating thermally large particles, which means that it considers the intra-particle temperature gradients and overlapping stages of drying, pyrolysis and char conversion. The model was validated by comparison with experiments and also with a more detailed model presented in our earlier work. Although the model was applied here for black liquor combustion, it is also suitable and can be used for other different applications, such as the combustion of biomass, coal or heavy fuel oil and metallurgical processes.

One-Dimensional Discrete Combustion Waves in Periodical and Random Systems

The authors consider propagation of burn wave along a one-dimensional system of point heat sources connected by inert heat-conducting elements for periodical and disordered systems. Dependence of burn rate of such systems on burn temperature is determined. It is shown that under certain conditions a steady-state combustion mode in periodical systems becomes unstable and it is replaced by oscillatory modes, which consistently lose stability in the form of doubling period bifurcations. It is established that the burn rate of disordered system is many times less than the burn rate of a periodical system at identical average parameters. Results of modeling are compared with experimental data on combustion of thermite systems.

Robust Engine Torque Control by Discrete Event Disturbance Observer

A model-based approach is applied to robustly control torque generation in four-stroke spark ignition (SI) engines. Discrete event engine model (DEM) is adopted to describe the torque generation process consisting of discrete combustion strokes. Disturbance observer (DOB) is utilized to achieve robust stability and performance of the torque generation process. For a single-input, single-output model with throttle air intake as input and generated torque as output, the desired plant behavior is stably realized by the DOB over a desired frequency band which is sufficient for powertrain control applications. Numerical and experimental results show the effectiveness of the proposed DOB scheme.

Inherent limitations and control design for camless engine idle speed dynamics

AbstractThe idle speed control problem of a spark‐ignited engine equipped with a camless valvetrain is considered. The camless valvetrain allows control of the individual intake and exhaust valves of each cylinder and can be used to achieve unthrottled operation, and consequently, optimize the engine performance. We formulate the speed control problem for this engine and show that it exhibits unstable open‐loop behaviour with a significant delay in the feedback loop. The instability is intrinsic to the unthrottled operation and specific to the camless actuation used to achieve the unthrottled operation. The delay is caused by the discrete combustion process and the sensor/computer/actuator interface. We demonstrate the inherent system limitations associated with the unstable dynamics and the delay and provide insight on the structural (plant) design that can alleviate these limitations. Finally, stabilizing controllers using classical and modern robust design techniques are presented and tested on a nonlinear simulation model. Copyright © 2001 John Wiley & Sons, Ltd.

ANALYSIS OF A CONCEPT FOR INCREASING THE EFFICIENCY OF A BRAYTON CYCLE VIA ISOTHERMAL HEAT ADDITION

A thermodynamic study indicates that the hypothetical modification of gas-turbine engines to include two heat additions rather than one may result in significant efficiency improvements of over 4% compared with conventional engines. Specifically, the usual constant pressure heat addition would be constrained to a given temperature and then further heat addition carried out in a manner approaching an isothermal process. Owing to the limited peak combustion temperature of the overall heat addition process, the emissions of NOx may be reduced by as much as 50%, thus offering an environmental benefit as well as an efficiency advantage. This paper details the analysis of a proposed combustion chamber in which an isothermal heat addition is approximated. The combustion chamber would consist of a converging duct featuring discrete combustion sites positioned along the streamwise direction. A numerical analysis developed to assess the deviation from isothermal flow in the combustion chamber shows that a reasonable approximation of such a heat addition may be possible with two or more combustion sites. Moreover, a simplified treatment of the combustion process implies that flame stabilization at these sites is feasible. © 1997 by John Wiley & Sons, Ltd.

Turbocharged Diesel Engine Modeling for Nonlinear Engine Control and State Estimation

Engine models that are used for nonlinear diesel engine control, state estimation, and model-based diagnostics are presented in this paper. By collecting, modifying, and adding to current available engine modeling techniques, two diesel engine models, a mean torque production model and a cylinder-by-cylinder model, are summarized for use in the formulation of control and state observation algorithms. In the cylinder-by-cylinder model, a time-varying crankshaft inertia model is added to a cylinder pressure generator to simulate engine speed variations due to discrete combustion events. Fuel injection timing and duration are control inputs while varying engine speed, cylinder pressure, and indicated torque are outputs from simulation. These diesel engine models can be used as engine simulators and to design diesel engine controllers and observers.