Combustion Science and Technology

SELAMET, AHMET; ARPACI, VEDAT S.

doi: 10.1080/00102209108951747pmid: N/A

Abstract A spectral absorption coefficient for soot in the visible range, whose first approximation given by is proposed in terras of the Penndorf expansion of the Lorenz-Mie theory. Here λ, m and D are the wavelength of radiation, the complex refractive index and the diameter of particles, respectively. The study provides a fundamental base for the experimental data hitherto correlated by the empirical relation Kaλ ∼ λ−1.39.

GIRIMAJI, S. S.

doi: 10.1080/00102209108951748pmid: N/A

Abstract Many engineering calculations of turbulent reacting flows employ the assumed-pdf approach. On the form of the assumed pdf, however, there has been little consensus and the choice has often been ad hoc. The objective of this work is to investigate the validity of the assumed β-pdf model for the case of the inert mixing of two scalars and extend the applicability of the model to multiple scalar mixing. By comparing the β-pdf model favorably with the two-scalar mixing data obtained from direct numerical simulations (DNS), it is demonstrated that the use of this model is justified for all stages of the mixing process in statistically-stationary, isotropic turbulence. It is also shown analytically that during the final stages of mixing the model β-pdf reduces to a Gaussian-pdf, consistent with the observations from experiments and DNS, The suggested multivariate β-pdf model for multiple-scalar mixing relates algebraically, The mean scalar concentrations and the turbulent scalar-energy—to the joint pdf of the scalar concentrations. The model requires that only one extra transport equation—for the turbulent scalar-energy—be solved, apart from the mean equations. The model can be used in conjunction with any model for the turbulence velocity field and is expected to provide a simple method for calculating scalar mixing in turbulent flows.

TREVIÑO, C.; MÉNDEZ, F.

doi: 10.1080/00102209108951749pmid: N/A

Abstract In this paper we have analyzed the steady-state process leading to ignition of a combustible mixture of hydrogen, oxygen and nitrogen by a hot flat plate in a boundary layer flow. For plate temperatures larger than the crossover temperature dictated by the competition between the chain branching reaction H + 02 → OH + O and the chain breaking reaction H + 02 + M → H02 + M, the ignition event corresponds to a typical chain branching explosion with negligible heat release, in a first approximation. The boundary layer equations are solved using the fact that the activation energy of the chain branching reaction H + 02 → OH + O is relatively large, employing the reduced kinetic mechanism appropriate for this regime. The equations reduce to a single integro-differential equation for the concentration of atomic hydrogen. The ignition condition can be assumed to be reached when one of the shuffle reactions reaches partial equilibrium. On the other hand, for low plate temperatures, the ignition event is characterized by a thermal runaway. The governing equations reduce to a single one-parameter integro-differential equation, to be solved numerically.

FUNG, YOUN-TIH; YANG, VIGOR; SINHA, ALOK

doi: 10.1080/00102209108951750pmid: N/A

Abstract A theoretical analysis of the active control of combustion instabilities with spatially distributed actuators is presented in this paper. The model simulates unsteady motions in a combustion chamber, with feedback control produced by the injection of secondary fuel into the chamber. The mass flow rate of the injected fuel is modulated by a proportional-plus-integral (PI) controller between the sensor and the fuel injector. The formulation is based on a generalized wave equation which accomodates all influences of combustion, mean flow, and acoustic oscillation on the system behavior. Control actions arising from the distributed combustion of the fuel are modeled by an assembly of point actuators; the power output of each acuator is determined by its position, the local burning characteristics of the fuel, and the time delay with respect to the instant of fuel injection. The analysis and design of both analog and digital PI controllers are presented. The stability of the system has been examined by means of the frequency-domain method developed by Nyiquist. In addition, a formulation is constructed to determine the eigenvalues of the closed-loop system in discrete time. Several aspects of the distributed control processes, including the relationships among time delays, actuator dimension, controller gains, and combustion response, are investigated systematically.

CARTER, CAMPBELL D.; KING, GALEN B.; LAURENDEAU, NORMAND M.

doi: 10.1080/00102209108951751pmid: N/A

Abstract We have employed the TOPLIF (two-optical paths laser-induced fluorescence) method for making quenching-corrected laser-saturated fluorescence measurements of hydroxyl concentration in high-pressure laminar C2H6/02/N2 flames. With this technique, which requires detection of fluorescence along two optical paths, the ratio of the two fluorescence signals accounts for the spatial dependence of the laser irradiance and can be used to determine the influence of the excitation and quenching rate coefficients on the fluorescence signals. In an atmospheric flame, we have calibrated the fluorescence signals with absorption measurements and have generated a quenching correction function from the corresponding saturation curves. With this function, the fluorescence voltages from flames at 3·1, 6·1, and 9·2 aim have been adjusted for changes in quenching and laser power. We have thus obtained absolute concentration profiles of OH for these high-pressure flames. The TOPLIF technique is still affected by the inaccuracy of the balanced cross-rate model: at 6·1 atm, the disparity between fluorescence and absorption measurements indicates ˜25% depletion of the laser-coupled levels. Moreover, the method is sensitive to scattered and reflected laser radiation. Nonetheless. TOPLIF could be particularly useful in complex combustion environments where local variations in the fluorescence quenching rates can adversely affect the accuracy of concentrations derived from laser-induced fluorescence.

CLAVIN, PAUL; SUN, JIONG

doi: 10.1080/00102209108951752pmid: N/A

Abstract A mechanism of a strong acoustic instability is proposed for flames propagating in sprays or particle-laden gases, enclosed in a tube. A complete one-dimensional analysis is provided for the homogeneous burning regime occuring when the particles are small enough and suflficiently volatile to vaporize before they reach the combustion region. Inertia and drag of particles oscillating upstream of the flame front are responsible for the coupling of the acoustic waves and the heat release (through modifications to the flame structure). This velocity coupling mechanism leads to a vibratory instability which is much stronger than the one associated with the pressure coupling. The analysis is based on a multiscale method associated with an asymptotic expansion in the limit of large Zeldovich number. Analytical expression of the transfer function and of the growth rate are obtained in terms of different parameters: vaporization temperature, latent heat, flame temperature, particle size, diffusive properties of the gas. The instability is found to be maximum at acoustic frequencies of the order of magnitude of the inverse of the transit time across the flame. Quantitative results are provided in the discussion.

PITT, P. L.; CLEMENTS, R. M.; TOPHAM, D. R.

doi: 10.1080/00102209108951753pmid: N/A

Abstract Experimental evidence relating to the early phase of the spark ignition process is reviewed. This suggests that the evolution of the spark kernel after the initial shockwave formation is governed by well defined fluid mechanical structures. In the case of an axial spark this takes the form of a toroidal vortex pair which, depending on the discharge conditions, can be either a laminar or turbulent structure, with a rapid transitions between the two states. Plasma jet and surface discharge igniters on the other hand produce an axially propagating spherical turbulent kernel. It is proposed that igniters can be classifed into two major groups on the basis of their underlying fluid mechanical structures, either radially or axially propagating mixing elements. A mathematical model of the turbulent toroidal mixing element is developed and compared with some of the available experimental data on kernel growth rates. The consequences of the internal vorticity of the kernels for ignition in turbulent mixtures are examined and some of the relevant published data is re-interpreted in terms of the turbulent mixing model.

ROPER, F.; ARNO, J.; JAGGERS, H. C.

doi: 10.1080/00102209108951754pmid: N/A

Abstract Release velocity has a large effect on the burning time of a fuel gas cloud, changing it by a factor of up to four. Previous studies have usually ignored this aspect, as direct measurement of the velocity is difficult, due to the transient nature of the release. We measured the fuel gas release velocity from the impulse given to a ballistic pendulum. The mean velocity of the fuel gas is the impulse divided by the mass of fuel. Using this method, we studied fireballs with velocities up to 88m/sec, with fuel masses from IS to 13 grams. The release geometry was varied to simulate the flow from a ruptured fuel tank. The fireballs acted as scale models for much larger releases with velocities up to the sonic region. The results were correlated using dimensional analysis, which defines the conditions for results to be scaled to larger fuel masses. This gave an equation relating the burning time to the fuel mass and initial fuel release velocity. Burning times of other workers using pressurised LPG obeyed the same correlation. if the release velocity was calculated assuming isentropic expansion.