Fuels and oxidants for

Gutmark, E., T. P. Parr, and D. M. Hanson-Parr. 1995. Synchronized acoustic excitation of fuel and oxidizer for efficient combustion. 16th CEAS/AIAA Aeroacoustics Conference Proceedings. Munich, Germany. 

Several interesting visual effects can be achieved by careful selection of the fuel and oxidizer for a spark-producing composition. 

Pyrotechnics will take an important step forward by making use of several nanosized fuels and oxidizers for pyrotechnic formulations in the near future. As a result, the performance of such pyrotechnic formulations will become considerably better and thus the problem of availability of space for pyrotechnic devices will not remain as critical as it is now, because several metal powders and oxidizers are commercially available at the nanoscale these days. Before we discuss nanosized fuels, oxidizers and their formulations, it is considered essential to describe in brief nanomaterials (NMs)including carbon nanotubes (CNTs), their methods of preparation, their properties in general, and some important applications. 

Note that this definition differs slightly from the one commonly used in industry in which the stoichiometry is usually defined as the total oxidizer flow divided by the fuel flow. The problem with the definition commonly used in industry is that the stoichiometry must be recalculated whenever the oxidizer composition changes and that stoichiometric conditions change for each oxidizer composition. This is not a concern if air is always used as the oxidizer, which is the case for the vast majority of combustion processes. The benefit of the definition used here is that stoichiometry is independent of the oxidizer composition, so stoichiometric conditions are the same for any oxidizer composition. In Equation 1.2, S = x/1 = x. Theoretically, for the complete combustion of CH4, S = 2.0. Actual flames generally require some excess 02 for complete combustion of the fuel. This is due to incomplete mixing between the fuel and oxidant. For the fuel rich combustion of CH4, S < 2.0. For the fuel lean combustion of CH4, S > 2.0. 

The basic structure and principle of all fuel cells is similar the cell consists of two electrodes which are separated by an electrolyte. The electrodes are connected through an external circuit. The electrodes are exposed to gas or liquid flows to supply fuel and oxidant (for instance hydrogen and oxygen). The electrodes have to be gas or liquid permeable and therefore possess a porous structure. The electrolyte should have a gas permeability as low as possible. For fuel cells with an acid electrolyte, hydrogen is oxidized at the negative electrode (the anode) according to the following equation. The protons formed enter the electrolyte and are transported to the cathode ... 

In addition to catalysts, stabilizers, opacifiers (to reduce heat radiation ahead of the flame), flash depressants, plasticizers, and binders, the main fuel and oxidizers for rocket propellants include... 

Data for the several flame methods assume an acetylene-nitrous oxide flame residing on a 5- or 10-cm slot burner. The sample is nebulized into a spray chamber placed immediately ahead of the burner. Detection limits are quite dependent on instrument and operating variables, particularly the detector, the fuel and oxidant gases, the slit width, and the method used for background correction and data smoothing. 

Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air-acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame. 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

The bipolar plate material of the PAFC is graphite. A portion of it has a carefully controlled porosity that sei ves as a resei voir for phosphoric acid and provides ffow channels for distribution of the fuel and oxidant. The plates are elec tronically conductive but impervious to gas crossover. 

Optimum Mixture A specific mixture of fuel and oxidant that yields the most rapid comhustion at a specific measured quantity or that yields the lowest value of the minimum ignition energy or that produces the maximum deflagration pressure. The optimum mixture is not always the same for each comhustion property that is measured. 

Premixed Flame. For this type of flame, the fuel and oxidizer—both gases—arc mixed together before flowing to the flame zone (the thin region of the flame). A typical example is the inner core of a Bunsen burner (Figure 1), or combustion in an auto-... 

A fuel cell system also needs ancillaries to support the stack, just as an IC engine has many of the same type of ancillary subsystems. Major subsystems are needed for providing adequate humidification and cooling, and for supplying fuel and oxidant (air) with the correct purity and appropriate c uantity. 

One extremely important point to realize is that different propellant types may have different rate-controlling processes. For example, the true double-base propellants are mixed on a molecular scale, since both fuel and oxidizing species occur on the same molecule. The mixing of ingredients and their decomposition products has already occurred and can therefore be neglected in any analysis. On the other hand, composite and composite modified-double-base propellants are not mixed to this degree, and hence mixing processes may be important in the analysis of their combustion behavior. 

The basic approach taken in the analytical studies of composite-propellant combustion represents a modification of the studies of double-base propellants. For composite propellants, it has been assumed that the solid fuel and solid oxidizer decompose at the solid surface to yield gaseous fuel and oxidizing species. These gaseous species then intermix and react in the gas phase to yield the final products of combustion and to establish the flame temperature. Part of the gas-phase heat release is then transferred back to the solid phase to sustain the decomposition processes. The temperature profile is assumed to be similar to the situation associated with double-base combustion, and, in this sense, combustion is identical in the two different types of propellants. 

Because the activation energy and preexponential factor for the fuel and oxidizer pyrolysis reactions are not identical, the only way for Eq. (30) to be valid is for Ta t Tf. 

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