Spreading velocity

Spreading velocities v are on the order of 15-30 cm/sec on water [39], and v for a homologous series tends to vary linearly with the equilibrium film pressure, it", although in the case of alcohols a minimum seemed to be required for v to be appreciable. Also, as illustrated in Fig. IV-3, substrate water is entrained to some depth (0.5 mm in the case of oleic acid), a compensating counterflow being present at greater depths [40]. Related to this is the observation that v tends to vary inversely with substrate viscosity [41-43]. An analysis of the stress-strain situation led to the equation... 

Interesting pattern formations also occur in surfactants spreading on water due to a hydrodynamic instability [52]. The spreading velocity from a crystal may vary with direction, depending on the contour and crystal facet. There may be sufficient imbalance to cause the solid particle to move around rapidly, as does camphor when placed on a clean water surface. The many such effects have been reviewed by Stemling and Scriven [53]. 

Contact angle Drop height or volume Spreading velocity... 

Calculate the upward spread velocity at 0.5 m from the floor. The flame height from the floor is 1.8 m and the heat flux from the flame is estimated at 3 W/cm2. 

The maximum effect of interfacial turbulence on the mass transfer coefficient can be estimated using the correlation of Davies and Rideal (D6) for the initial spreading velocity, of a surface tension-lowering material spreading at the interface between two fluid phases ... 

Equation 3.32 provides an expression for the flame spread velocity based only on the parameters of the problem and on the experimental conditions, but still relies on the presence of an unknown constant that needs to be determined experimentally. 

In a similar manner, the flame spread velocity can be obtained for thermally thin materials ... 

Perrins, L.E. and Pettet, K., Measurements of flame spread velocities, Journal of Fire and Materials, 5, 85, 1974. 

As the characteristic wave dimension, 1, the wave crest or the distance between two crests can be taken. A doubling of this length leads to the fourfold travelling or spreading velocity of the waves. 

Let q denote an energy per unit area per unit time transferred across the surface. As a first approximation, let us assume that a critical enthalpy increase Ah per unit mass of fuel is needed for ignition to occur. If p is the fuel density and L is a spread velocity, then, according to an energy balance,... 

This equation may be employed to calculate a spread velocity if q, p, and Ah can be estimated. The concept of a critical temperature 7 for ignition of the fuel in the presence of a flame may be introduced to provide an approximate expression for Ah, namely. 

Estimates of spread velocities by the two mechanisms of liquid motion are readily developed. Consider buoyancy-driven flow first. The pressure difference in the horizontal direction between hot and cold liquid is of the order of gh(T — TQ)(dpp/dT), where h is the depth of the heated layer, 7 now is the temperature of the liquid surface beneath the flames, typically slightly less than the normal boiling point, and the temperature derivative of the liquid density is to be evaluated at constant pressure. If the resulting horizontal pressure gradient is balanced against viscous forces for steady flow at a liquid-surface velocity V, then it is found that... 

Spreading of drops on powder surface spreading Contact angle goniometer Contact angle Drop height or volume Spreading velocity References Kossen and Heertjes, Chem. Eng. Sd, 20, 593 (1965). Pan et al., Dynamic Properties of Interfaces and Association Structure, American Oil Chemists Society Press, 1995. 

Fig. 4. Spreading velocities [cm/s] in dependence on the distance from the source [cm] for pure oleyl alcohol (OLA top), a 75 mmol solution of OLA in ethanol (OLA/ethanol middle), and pure oleic acid methyl ester (OLME bottom) note the different ordinate scales...

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