Mass-flow hypothesis

The mechanism of transport in the sieve tubes has still not been elucidated. Nonetheless most of the presently known findings argue in favor of the correctness of the mass flow hypothesis put forward by Munch in 1926. According to it, convection or mass flow is responsible for the transport in the sieve tubes just as for transport in the xylem or in the blood vessel system of animals. The driving force of this mass flow in the sieve tubes is a concentration gradient of osmotically active substances decreasing in the direction of transport. 

Up to now we have postulated what conditions must hold. Now we must point out something that may not happen if the mass flow hypothesis is to have validity, a bidirectional transport in one and the same sieve tube. 

Bidirectional transport in one and the same plant is completely compatible with the mass flow hypothesis and has also been demonstrated. Thus, assimilated materials from leaves can be conducted both upward into the shoot meristem and also downwards in the direction of the root. However, bidirectional transport in one and the same sieve tube would contradict the reality of the mass flow hypothesis. 

However, Eschrich provided an explanation that is compatible with the mass flow hypothesis (Fig. 227). In one sieve tube fluorescein streams upwards and in another C " -urea downward. The two sieve tubes make contact with each other through transverse connections. Via the latter the substance in one sieve tube can pass over into the other and both substances are then swept along by mass flow in both directions. 

Transport in both the xylem and the phloem is a long distance transport. We have just mentioned that the sieve plates still raise problems for the mass flow hypothesis. According to some hypotheses transport over the short distance through the sieve plates is not accomplished by mass flow as in the sieve tubes but by an active transport. This active transport is over a short distance and its essential characteristic is that it requires the expenditure of energy. 

Fig. 7.6. Diagram of an osmotic model illustrating the principle of the Miinch mass-flow hypothesis of translocation. (For discussion see text.)...

The hypothesis of constant CP is fundamental in PPA. If the enthalpy-temperature relation is not linear, then the stream must be segmented . Note that in the relation (10.1) CP is in fact the term FxCP (mass flow rate by the mass heat capacity). 

In the other case the vapour mass fiaction at the relief device entrance must be estimated taking into account the flushing flow hypothesis. This allows the calculation of the so-called entropy parameter ro, which is required for the determination of the critical mass flow density G. ... 

To test this Working Hypothesis, 0%, 0.5%, 1% and 2% cysteine were added to wheat flour as a radical scavenger during extrusion (3). The original Bouncer flour (14% protein) was extruded at process moisture 16% (w/w), constant die temperature 185 C, 225 g/min mass flow rate, and screw speed 500 rpm conditions that provided optimum wheat flour expansion, flavor and textural quality in the earlier experiments. 

The sustenance of particles in suspension under the action of turbulence is modeled in identical fashion to what has been done for Brownian agitation Under the same hypothesis whereby no particles are fed into the bottom or top of the fluid layer, the steady-state distribution of particles along the vertical axis of the fluid layer corresponds to an average mass flow of particles that is zero at all points. We can therefore return to [15.21], replacing the diffusive flux associated with Brownian motion with the turbulent concentration flux, namely ... 

Prandtl s mixing length hypothesis (Prandtl, 1925) was developed for momentum transport, instead of mass transport. The end result was a turbulent viscosity, instead of a turbulent diffusivity. However, because both turbulent viscosity and turbulent diffusion coefficient are properties of the flow field, they are related. Turbulent viscosity describes the transport of momentum by turbulence, and turbulent diffusivity describes the transport of mass by the same turbulence. Thus, turbulent viscosity is often related to turbulent diffusivity as... 

The temperature, composition, and density are presumed to have only radial variations. The pressure, however, is allowed to vary throughout the flow, but in a very special way as will be derived shortly. Also the magnitude of the pressure variations is assumed to be small compared to the mean thermodynamic pressure. Using these assumptions, and invoking the Stokes hypothesis to give X = —2p./3, we can reduce the mass-continuity and Navier-Stokes equations to the following ... 

By the end of 1970s most objections against the dark matter hypothesis were rejected. In particular, luminous populations of galaxies have found to have lower mass-to-luminosity ratio than expected previously, thus the presence of extra dark matter both in galaxies and clusters has been confirmed. However, the nature of dark matter and its purpose was not yet clear. Also it was not clear how to explain the Big Bang nucleosynthesis constraint on the low density of matter, and the smoothness of the Hubble flow. 

Since the Navier s slip hypothesis of the last century, most experiments have failed to obtain positive evidence for a slip boundary condition on macroscopic scales in low molar mass liquids. However, Navier s notion of slip turns out to be extremely useful and convenient for the latest description of flow anomalies of highly entangled polymer melts including linear polyethylenes (LPE). The ability of a melt/solid interface to possess two profoundly different states as shown by Fig. 4a,b clearly reveals the potential role of interfacial slip in governing various melt flow phenomena in high pressure extrusion. Before reviewing recent experimental studies that have elucidated the molecular origins of different flow... 

The hypothesis advanced here is that the observed enhancement is due to thermocapillarity arising from the temperature dependence of the interfacial tension. Although we know of no theory for such a Marangoni effect in binary condensation per se, theory does exist for both mass transfer (M. 11) and heat transfer (16, 12) aeross an interface in the absence of bulk flow. We note that the sign of the derivative of the interfacial tension with respect to temperature is positive near a lower consolute point and that this is in the correct direction to sustain disturbances in condensation rate. Thus, in retrograde condensation, provided a critical temperature gradient normal to the interface is exceeded, a local increase in condensation flux toward the vapor liquid interface will result in its cooling. 

---