No abstract

SynopsisThe flow properties of concentrated polymer dispersions which exhibit extreme shear thickening have been investigated in steady and transient shearing flows using various types of rheometers. A phenomenological characterization based on few fluid parameters is proposed and pertinent test modes for their determination are discussed. The dispersions consist of monodisperse, electrostatically stabilized, solid spherical particles (diameter <0.5 /Lm) dispersed in glycols. They show a reversible steplike viscosity transition of nearly three powers of ten when an apparent critical shear rate 'Ye is exceeded. This jump from a low to a high viscosity state shows typical features of a shear-rate-induced phase transition, including hysteresis and the existence of a metastable low viscosity state above re. The viscosity step can be traced continuously by using stress controlled rheometers. Pronounced fluctuations of the rate of deformation with time are observed at the critical shear rate. The apparent critical shear rate depends on the rheometer geometry. Its change with the gap width in Couette flow and with the radius of circular dies can be interpreted by taking into account a true critical shear rate Yo and a wall slip velocity V S ' Periodic switching to the high viscosity state in oscillatory shear only occurs if both a critical rate amplitude and a critical shear amplitude are reached during the cycle. Applications of these dispersions for simple speed controlling or selectively damping mechanical elements are briefly introduced.

An investigation of the surface-temperature variation during nucleate pool boiling at atmospheric pressure was conducted. The effect of surface temperature, heat flux, and heating-surface material was investigated. The temperature variation of the surface was found by a specially fabricated thermocouple placed in contact with it. The average maximum temperature variation may be found from the following equation: ΔTavgmax=c[q(f/α)1/2]−αqΔTsur−satk The exponent a is a function of surface roughness. The rate of heat flow was denoted by q in Btu/sq ft-hr, α the thermal diffusivity in sq ft/hr, f the frequency of variations in cphr, k the thermal conductivity in Btu/ft-hr-deg F, and ΔTsur−sat the temperature difference between the heating surface and the saturation temperature of water. The coefficient c and exponent a were determined experimentally for various heating-surface materials and surface finishes.

A curious phenomenon of significant summertime downslope movements, observed at other sites where trees grow on the lower slope of clay fill embankments, has been recorded during 10 years of downstream slope monitoring at Aldenham embankment dam, UK. The data from four monitored sections of the slope are described. At two locations, downslope movements were observed in high soil moisture deficit (SMD) conditions, where it appeared that high SMD and shrinkage caused by trees growing on the lower slope allowed the grassed upper slope with lower SMD to slip downwards under its own weight. This led to a seasonal, ratcheting accumulation of crest settlement — a type of mechanism that presents a continual serviceability problem to infrastructure embankments as well as dams. This mechanism was simulated in a qualitative way by finite difference analysis. At a third monitored section, a sparser distribution of lower water-demand trees, lower plasticity clay fill, and a moisture content close to or below the plastic limit appeared to result in insignificant slope movements. At a fourth low-height monitored section, a slow accumulation of downslope movements, as well as tension cracking, was observed.

No abstract