Heterogeneous fuels

Proton-conduction mechanisms that occur in the aforementioned proton solvents, when present as a homogeneous phase and as a component of heterogeneous fuel cell separator materials, are described more specifically in the following two sections. 

The fuel-bed composition has a significant effect on the thermochemical conversion behaviour. The composition is in reality a very complex variable and can be divided into fuel chemistry and fuel physics of the individual fuel particles making up the bed. These authors will make a drastic simplification and define only two degrees of fuel-bed composition, namely a homogeneous or a heterogeneous fuel bed. In this review the terms homogeneous and heterogeneous are used in a relative and practical way. However, they could be described as two extreme cases as well [9]... 

Unfortunately, most of the elements chosen as promising propellant ingredients are metals and hence solids rather than liquids. The endeavor to incorporate solid-phase metals and metal compounds into liquid propellants has led to a whole new area of propellant technology. Heterogeneous fuels are discussed in detail later. 

Theoretical Basis. Heterogeneous fuels are suspensions of finely divided metals or metal compounds in appropriate liquid fuels and represent one approach for using the low molecular weight metals (and certain of their derivatives) that have high heats of combustion with typical rocket oxidizers. The theoretical basis for the interest in metal-containing fuels is discussed in the introduction. Glassman (16) showed that all... 

Table VI. Maximum Theoretical Specific Impulse Values of Heterogeneous Fuels with Nitrogen Tetroxide...

The use of a heterogeneous fuel, in which the metal compound is suspended in a liquid fuel, avoids a third storage vessel because it is used with the oxidizer as in a conventional bipropellant system. The technical problems are then associated only with the stabilization of the suspension, with the rheological properties of the stabilized fuel, and with the reactivity of the suspended solid with its carrier. 

Mechanical Stabilization. Four methods may be considered for stabilizing heterogeneous fuels mechanically. The use of the metals in the form of sols is not practical because the powders, when subdivided to the required degree, are expensive and hazardous to handle. If they are allowed to become coated with the metal oxide, to eliminate their pyrophoricity, their metal content is reduced to an unacceptable level. Another approach involves the use of a liquid whose density equals that of the solid phase. Such an approach is also impractical because there are few liquid materials that have appropriate densities, and those that do are inappropriate as fuel components for chemical or thermodynamic reasons. Furthermore, the density equivalence is lost at reduced and elevated temperatures, causing settling or floating of the solid phase. 

The fourth approach to the stabilization of heterogeneous fuels—by gelation of the liquid carrier—has been developed more thoroughly than the emulsification method. Hence, the gelled fuels are discussed in greater detail than emulsified fuels. However, some aspects of heterogeneous fuels are not related to mechanical stabilization when these are discussed in connection with gelled fuels, it will be apparent that the comments apply also to emulsified fuels. 

Chemical Stability of Fuels. Although the principal problem associated with the development of a gelled heterogeneous fuel is the mechanical stabilization of the suspended solids, the solid phase must be chemically compatible with the carrier fuel. The hydrazine-type carriers utilized in these fuels are thermodynamically unstable and decompose to produce gaseous products by mechanisms that are sensitive to catalysts. Special attention must therefore be given to the problem of decomposi-... 

Status of Technology. At present only one heterogeneous fuel, aluminum in gelled hydrazine (42), has been thoroughly developed and evaluated extensively in a static propulsion system. Other metal-containing fuels are being developed, but fuels based on the metal hydrides are... 

In the given form, the Butler-Volmer equation is applicable rather broadly, for flat model electrodes, as well as for heterogeneous fuel cell electrodes. In the latter case, concentrations in Eq. (2.13) are local concentrations, established by mass transport and reaction in the random composite structure. At equilibrium,/f = 0, concentrations are uniform. These externally controlled equilibrium concentrations serve as the reference (superscript ref) for defining the equilibrium electrode potential via the Nernst equation. 

For heterogeneous fuels such as coal-oil mixtures, control of the oil s potential for coating the coal particles requires treatment of its interaction energy [1.21]. 

Experimental reactivity results are summarized in Figure 1 as are the core specifications. All of the cores were rectangular parallelpipeds, divided into four equal quadrants by two mutually perpendicular cross water gaps (rod channels) and were characterized by extreme heterogeneity, fuel plate spacing being about 1 in. 

Fuel cycle options, alternative Radkowsky Thorium Fuel (RTF) once-through fuel cycle with heterogeneous fuel layout bimetallic enriched uranium (<20%) seed subassemblies thorium dioxide blanket subassemblies ... 

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