Stephan flow

Stephan flow The flow of molecules towards or away from the surface of a volatile liquid due to either evaporation or condensation. 

Aerosol particles move by diffusion when there is a particle concentration gradient or when there is a concentration gradient in the continuous gas phase (diffusio-phoresis). Diffusion is described further in Section 5.5. A specific kind of diffusion flow - called Stephan flow - occurs when the continuous phase gas molecules are themselves diffusing either towards a condensing surface or away from an evaporating surface. In either case, the suspended particles are carried along with the gas and they move perpendicular to the surface. This can occur in any wet scrubber and can influence the particle collection efficiency. 

Knaff and Schlunder [9] studied the evaporation of naphthalene and caffeine from a cylindrical surface (a sintered metallic rod impregnated with the solute) to high-pressure carbon dioxide flowing over an annular space around the rod. They studied the diffusion flux within the bar and in the boundary layer. The mass-transfer coefficient owing to forced convection from cylinder to the gas flowing in the annular duct was correlated, using the standard correlation due to Stephan [7]. For caffeine, it does not require a free-convection correction, as the Reynolds dependence is that expected by a transfer by forced convection. This is... 

Previously, Durst and Stephan Q) observed some enhancement in heat transfer coefficients in natural convection in mixtures of n-heptane -- methane as the two-phase region was entered. We reported heat transfer results earlier ( ) for free convection and cross flow about a heated horizontal cylinder in a supercritical n-decane--C02 mixture. These also showed enhancement in the two-phase region relative to single phase. At that time, analysis was hindered by the unavailability of estimates or measurements of some crucial mixture properties. Happily, this situation has been remedied somewhat and we can now draw some conclusions. 

For X+ oo, the known end value of 3.6568 for the Nusselt number for thermal fully developed flow is obtained. The calculation of the Nusselt number for small values of the length X+ = L/(dPe) requires many terms of the series for the adiabatic mixing temperature (3.249). Therefore the exact solution has been approximated by empirical equations. According to Stephan [3.30] through... 

In the fourth chapter the heat and mass transfer in condensation and boiling with free and forced flows is dealt with. The presentation follows the book, Heat Transfer in Condensation and Boiling (Berlin Springer-Verlag 1992) by K. Stephan. Here, we consider not only pure substances condensation and boiling in mixtures of substances are also explained to an adequate extent. 

For pipe flow, which is neither developed concerning flow nor temperamre profile, and which shows negligible viscosity differences near the wall, the following is valid (Stephan 1959) ... 

But, if the pipe flow is developed concerning fluid dynamics but not concerning temperamre or concentration profile, then the following correlation for short pipes holds (Stephan 1959 Hausen 1959) ... 

Vesselinov, H.H., Stephan, B., Uwe, H., Holger, K., Gtlnther, H., and Wilfried, S. (2008), A study on the two-phase flow in a stirred tank reactor agitated by a gas-inducing turbine, Chemical Engineering Research Design, 86(1) 75-81. 

The apparatus contains an empty sampler isolated from the main vessel of the bomb by a valve needle until equilibrium is established. Before the liquid phase is sampled, the bomb is set in a vertical position with the sampler at the bottom. The sampler channel is opened for a few minutes and the liquid phase, after flowing through a dense filter, is injected into the empty sampler. Stainless steel mesh or fiit (Schroeder et al., 1935 Stephan and Miller, 1962 Ulmer, 1971 Susarla et al, 1987), sintered Teflon (Dickson et al., 1963), porous ceramics (Voigt et al, 1985) or a fine silver wire (Bemshtain and Matsenok, 1961, 1965 Itldna, 1973 Ravich, 1974 Valyashko, 1990) pressed into the sampler channel have all been used as filtering materials. Extension... 

Finally, another important heat transfer meehanism is through radiation. Here the heat transfer is determined by the Stephan-Boltzmann Law that relates the heat flow to the temperature of the radiating body ... 

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