orders of magnitude at a higher stress(behavior reminiscent of a pseudo yield-stress.

Clearly, the rheology of associative polymer solutions is much more complex than the simple cases we have considered thus far. This is because hydrophobic interactions can be either intrapolymeric (between hydrophobes on the same chain) or interpolymeric (between hydrophobes on adjacent chains), with a combination of both mechanisms at any given stress level. We believe that the dominant mode of interaction is intrapolymeric at low stresses and interpolymeric at intermediate stresses (English et al., 1997). The interpolymeric interactions contribute to shear thickening and to the viscosity plateau (Ballard et al., 1988). At very high stresses, all hydrophobic interactions are precluded, as the hydrodynamic forces in the system begin to dominate. The transition from a state of appreciable interactions to negligible interaction occurs at the pseudo yield-stress.

Another significant effect in these systems is caused by the presence of surfactant. Consider an aqueous solution of HASE polymer at a polymer concentration of 0.6 g/dl (Figure 6-14). The elastic modulus (G´) for this system varies with frequency as ω^0.8 (at low ω) and exceeds the viscous modulus (G´´) only at high frequencies. When we add 1.5 g/dl of a non-ionic surfactant to the system, we find that the levels of both G´ and G´´ are increased. Moreover, the elastic modulus (G´) shows plateau-like behavior over most of the frequency range and is higher than the viscous modulus (G´´) over the entire spectrum. This indicates that the addition of surfactant renders the system more elastic. We conjecture that this behavior is caused by enhanced

hydrophobic interaction in the system, with the surfactant molecules acting as links between polymer hydrophobes. Note that the molecular structure of the surfactant is crucial to its behavior; other types of surfactants can produce the reverse effect, i.e., they can reduce the elastic character of the system.

Currently, the fuels used in the gas-turbine engines of commercial aircraft are kerosene-based jet fuels (Hutchinson, 1995). The two main types are Jet A fuel, used in North America, and Jet A-1, used in most other regions of the world. Jet fuels are among the most tightly specified products of oil refineries, the specifications involving boiling point, water content, aromatic content, etc. The important qualities jet fuels must generally exhibit under all operating conditions are as follows (Kroes and Wild, 1995):

• pumpability and ease of flow, with negligible volatility

• efficient combustion and high calorific value

• adequate lubrication for the moving parts of the engine

• minimal corrosive effects, fire hazards, etc.

The viscosity of an aviation fuel is a factor in calculating pressure drops in fuel lines, through its relationship to the Reynolds number (Bird et al., 1987). A lower viscosity corresponds to smaller pressure drops and lower pumping requirements. The viscosity increases with decreasing temperature, and when the freezing point of the fuel is approached, waxy particles begin to form. Many fuel specifications, therefore, include a maximum viscosity limit at low temperatures to ensure pumping and flow capabilities (CRC report, 1984). Research on aviation fuels has also led to the development of additives for specific purposes, e.g. to prevent ice and bacterial contamination in the fuel or to reduce the buildup of static charge (Kroes and Wild, 1995).

One class of additives is intended to reduce the flammability of aviation fuels in the event of an accident or crash. These additives, called "antimisting" (AM) agents, are essentially linear polymers of high molecular weight (> 10⁶) (Chao et al., 1984). A small amount of polymeric additive (100 ppm) is sufficient to reduce the formation of atomized droplets or "mist." Droplets in sprays of antimisting fuels tend to be larger and, in some cases, deformed to strings or filaments (Hoyt et al., 1980). The reduction of surface area available for vaporization, combined with the greater distance between droplets, inhibits flame propagation.

The antimisting behavior of fuels containing polymeric additives is a consequence of the viscoelastic nature of the fuel system (Hoyt et al., 1980; Chao et al., 1984). Accordingly, it is important to study the rheology of AM fuels. Under shear flow, however, non-Newtonian effects cannot

Front Matter (R1-R16)

I. Summary of Workshop (1-16)

II. Presented Papers: Fuel and Additive Technologies (17-58)

III. Presented Papers: Aircraft Fuel System Requirements (59-78)

IV. Presented Papers: Characterizing Fuel Fires (79-122)

Appendix A: Workshop Participants (123-124)

Appendix B: New Concepts in Fuel Fire Research: Final Summary Report of Short-Term Advisory Services (STAS) Team (125-140)

Appendix C: Biographical Sketches of the Committee Members and Technical Consultant (141-141)