Fuel-oxidizer Ratio Research Articles

Absolute concentration measurements of atomic hydrogen in subatmospheric premixed H_2/O_2/N_2 flat flames with photoionization controlled-loss spectroscopy

An experimental protocol, using photoionization controlled-loss spectroscopy (PICLS), has been developed for obtaining absolute number densities of atomic hydrogen from laser-induced fluorescence measurements in flames. Two laser beams are employed, the first to excite hydrogen atoms from the ground state to the second excited state via two-photon absorption and the second to strongly photoionize the excited atoms. The resulting fluorescence measurements are independent of quenching. A model is presented that assures the viability of PICLS as long as the photoionization rate is greater than or equal to the quenching rate. The model is verified in fuel-lean, stoichiometric, and fuel-rich flat premixed H(2)/O(2)/N(2) flames at pressures of 20 and 72 Torr. Over this range in pressure, the ratio of number densities obtained from PICLS to those calculated from partial equilibrium is constant to within 20%. Most of the error arises from the sensitivity of the partial equilibrium calculat ions to small uncertainties in both the fuel-oxidizer ratio and the measured OH concentration. Because of the quenching-independent nature of PICLS, quantitative fluorescence measurements can be made by calibrating at a single favorable flame condition.

A Flashback-Resistant Burner for Combustion Diagnostics and Analytical Spectrometry

A general utility burner for the production of laminar, homogenous diffusion flames, which is immune to flashbacks, is presented. Because the fuel and oxidant mix on the surface of the burner rather than within the spray chamber, the flames cannot flashback. A wide variety of gas mixtures has been investigated, including oxygen, nitrous oxide, and nitric oxide as the oxidants. Any combination of fuel and oxidant can be safely burned to produced a stable, laminar, and audibly quiet flame. Flame temperatures can be varied over a wide range either by changing the fuel-oxidant ratio or by diluting the flame gases with an inert gas. In this manner, the optimum flame temperature and composition can be achieved. These burners are of general use in analytical emission, fluorescence, and photoacoustic spectrometry, as well as combustion diagnostics.

Detonation in a cryogenic hydrogen-oxygen mixture

A closed model has been constructed for stationary detonation in a heterogeneous gas-droplet mixture; studies have been made on the structures of the subsonic and supersonic zones, and it has been found that there is substantially incomplete combustion of the c phase at the Jouguet point with certain initial oxygen-fuel ratios. It has been found that for 30<do<300 μm, the speed of the droplets relative to the front at the Chapman-Jouguet point is greater than the gas speed when there is incomplete combustion, while the detonation speed is only slightly dependent on the initial droplet diameter; there may be two stationary states or only one in accordance with the fuel-oxidant ratio. The higher detonation speed corresponds to complete burnup (in [4, 5], the number of states was only odd).

Analysis of possible transformations of hydrocarbon ions in atmospheric-pressure propane-air flames

We shall examine the prerequisites for a possible scheme of ionic transformations. A study of the ion distributions [7] has shown that they are divided according to structural elements into two groups: The first group includes the hydrocarbon ions located within the "blue" cone and the second includes those which contain oxygen and lie outside the "blue" cone. First it is necessary to examine the structural elements of a flame for developing electron--ion processes and their relationships with the fundamental physical and chemical parameters. Chromatographic studies of the distribution of neutral molecules in hydrocarbon fuel flames [8] show that the initial fuel and oxidant molecules begin to be consumed at low temperatures ahead of the combustion front. This is accompanied by the formation of substantial amounts of C02, CO, and H2 even during early stages of combustion, in good general agreement with mass spectrometric measurements [9]. According to current ideas, in the low-temperature zone up to the luminous zone (combustion front) chemical changes for which the intermediate particles are the excited radicals C2" and CH* cause the fuel molecules to break up into their simplest fragments. The simplest products of these transformations participate in the oxidation process [9-11]. At the same time, this region is distinguished by a substantial number of hydrocarbon ions [7]. The following region is characterized by the presence of a maximum number of excited particles C=* and CH* and constitutes the so-called combustion front. This region corresponds to the minimum concentration of any ions, while its inner and outer boundaries correspond to the maximum concentration of hydrocarbon and oxygen-containing ions, respectively. The "violet" zone, usually called the burnup zone, is longer than these regions. The maxima in temperature and ionization occur there. These maxima lie in a narrow space adjacent to the luminous zone. The H30 + ion has the greatest concentration of the ions. Thus, a substantial amount of H30 + and other oxygen-containing ions are formed in the violet (burnup) zone and the peak concentration of H30 + occurs next to the outer boundary of the combustion front. A large fraction of the initial fuel molecules have been consumed before the maximum H30 + concentration has been reached; thus, a very small portion of the fuel may participate directly in the formation of this ion. Given these facts, we may suppose that the particles responsible for production of Hs0 + are the CH* or C2" radicals which are widely regarded as being present in all hydrocarbon flames. A redistribution in the concentrations of these radicals during changes in the fueloxidant ratio [I] or in the initial fuel composition, as well as because of the reaction [3] C~ + OH-+ CO + CH*, (I)

Optical Measurements of Soot Formation in Premixed Flames

Abstract The growth of soot particles in rich premixed flames was measured using the diffusion broadening particle-sizing technique. This dynamic-light-scattering technique reduces the dependence of size measurements on particle complex index of refraction which is a problem with most light-scattering size measurements. Size measurements were made in fuel-rich premixed methaneoxygen flat flames with fuel-oxidizer ratios ranging from 2.2to 2.9 times sloichiometric values,and in propane-oxygen flames at an equivalence ratio of 2.6. Particles as small as 23nm in diameter were observed 5 mm above the burner surface (within about a millimeter of the flame front), growing t0 60-120nm (depending on the equivalence ratio) 14mm above Ihe burner. Measurements of the intensity of scattered light were used in conjunction with the size measurements to determine the soot number density in the methane-oxygen flames, which was found to be decreasing with height above the burner at positions 5 mm above the burner and high...

Burning Parameters of Premixed Oxygen-Hydrogen and Oxygen-Acetylene Flames

Parameters affecting the stability of premixed oxyhydrogen and oxyacetylene flames are investigated. The results of this study show the effect of port diameter, fuel-oxidant ratio, total gas flow, and burner temperature on the stability of these flames. Data were obtained by the use of a specially designed and constructed one-hole insert burner. The results of this study provide the required design criteria and operational parameters for utilization of these fuel mixtures.

Temperature coefficient of burning rate of condensed mixtures at different component ratios

1. The dependence of the burning rate u on the initial temperature T0 has been studied for model mixtures of ammonium perchlorate and polystyrene, polymethyl methacrylate, polyformaldehyde, and bitumen. In all cases, with increase in T0 the burning rate increases monotonically. The relation u(T0) may conveniently be characterized by the temperature coefficient β=(d ln u)/(d T0). 2. The temperature coefficient strongly depends on the fuel-oxidizer ratio α. The curve β(α) has a minimum, whose position coincides with that of the maximum burning rate. 3. For mixtures not too far from stoichiometric the temperature coefficient increases with increase in the oxidizer particle size. 4. The experimental results are in agreement with the notion that the value of β is determined by the temperature Tg in the zone that influences the burning rate; if Tg is large, β is small and, conversely, if Tg is small, β is large.

Burning rate of composite systems as a function of the particle size of the components at different fuel-oxidizer ratios

1. The u(d) curve for mixtures of NH4ClO4 plus polystyrene and polymethyl methacrylate has been observed to display anomalous behavior at sufficiently large d. In this case as d increases, the burning rate falls only to a certain limit, passes through a minimum, and then slowly increases. A minimum on the u(d) curve was observed at fuel percentages not too high and at pressures not too low. 2. An explanation of the observed effect is offered on the basis of ideas relating to the influence zone.

Combustion of condensed mixtures with polydisperse components

1. A study has been made of the combustion of model mixtures of polystyrene, plexiglas and bitumen with coarse, fine, and mixed (half coarse, half fine) ammonium perchlorate. 2. The burning rate of compositions with mixed oxidizer is not generally equal to (uc+uf)/2. Depending on the nature of the fuel and the fuel-oxidizer ratio and also on the pressure, um may approach uf (and even exceed it) or uc (and even fall below it). 3. The burning rate of compositions with mixed oxidizer is similar to that of a homogeneous mixture of the fuel and the fine oxidizer filling the gaps between the coarse oxidizer crystals. The presence of coarse oxidizer only slightly reduces or increases the burning rate. However, if this homogeneous mixture is itself incapable of burning or burns very slowly, the presence of coarse oxidizer is important (in this case um≤uc).

A Simplified Combustion Analysis System

Knowledge of the chemical composition of the inlet and exhaust gases of combustion engines aids in evaluating engine performance. Jet engine research requires rapid gas analysis unobtainable by conventional analytical methods. A convenient and rapid gas analysis system which combines thermal conductivity measurements with a chemical analytic scheme and which has been employed extensively on ramjet engines is described. Gases are analyzed for fuel-oxidizer ratio and for chemical composition. With simplifying assumptions, this analysis permits the calculation of other performance data such as combustion efficiency, flame temperature, Mach number, etc.