Renewal mechanism

Marchello and Toor (M2) proposed a mixing model for transfer near a boundary which assumes that localized mixing occurs rather than gross displacement of the fluid elements. This model can be said to be a modified penetration-type model. Kishinevsky (K6-K8) assumed a surface-renewal mechanism with eddy diffusion rather than molecular diffusion controlling the transfer at the interface. 

The first type of model considers the heat transfer surface to be contacted alternately by gas bubbles and packets of closely packed particles. This leads to a surface renewal process whereby heat transfer occurs primarily by transient conduction between the heat transfer surface and the particle packets during their time of residence at the surface. Mickley and Fairbanks (1955) provided the first analysis of this renewal mechanism. Treating the particle packet as a pseudo-homogeneous medium with solid volume fraction, e, and thermal conductivity (kpa), they solved the transient conduction equation to obtain the following expression for the average heat transfer coefficient due to particle packets,... 

Assuming that this packet renewal mechanism accounts for all the heat transfer associated with the particle phase, h can be equated to hd in Eq. (3). 

The experimental evidence of the importance of the tube renewal mechanism when short molecules are added to a high polymer is provided by blends of narrow polystyrene (Mg = 8 500 and Ml = 900 OOO) [28]. 

The stirrer must be very powerful because the mixture becomes lumpy and finally very viscous. If the material agglomerates so badly that the stirrer will not operate, hand stirring may be necessary temporarily until the mass liquefies sufficiently to renew mechanical stirring. The checkers tried one run in which carbon tetrachloride was added to facilitate stirring, but, though it accomplished this purpose, the yield was only about half of that obtained without a liquid medium. 

In a series of papers between 1949 and 1954 Kishiuevskii and cowotkers24 27 proposed a surface-renewal mechanism which, in contrast to the theories described above, postulates that transfer into an eddy at the interface occurs predominantly by convective mass flow and not by molecular diffusion. The authors also dispute the noggestion drat the probability of replacement of a surface element is independen of its age. 

Comparisons between experimental observation and the predictions of (he Him theory on (he ons hand and various forms of surface-renewal theory on the other are reviewed briefly in Ref. 33 (Chap. 5). It is interesting that, although the evidence generally appears to favor the surface-renewal mechanisms, the two-film theory contributes to the design of complex processes in a manner Ihel continues to be very useful. An example of (his will be given later in the formulation of transfer unit relationships for pecked column design. 

Wasan DT, Ahluwalia MS. Consecutive film and surface renewal mechanism for heat and mass transfer from a wall. Chem Eng Sci 24 1535-1542, 1969. 

The use of wind as a renewable energy source involves the conversion of power contained in moving air masses to rotating shaft power. These air masses represent the complex circulation of winds near the surface of Earth caused by Earth s rotation and by convective heating from the sun. The actual conversion process utilizes basic aerodynamic forces, ie, lift or drag, to produce a net positive torque on a rotating shaft, resulting in the production of mechanical power, which can then be used directly or converted to electrical power. 

World War I and the interest in aviation gave rise to a rapid development of fluid mechanics and aerodynamics. This, in turn, gave a renewed... 

In recent years there has been a renewed appreciation of potential beneficial effects of roughness on a macroscale. For example Morris and Shanahan worked with sintered steel substrates bonded with a polyurethane adhesive [61]. They observed much higher fracture energy for joints with sintered steel compared with those with fully dense steel, and ascribed this to the mechanical interlocking of polymer within the pores. Extra energy was required to extend and break these polymer fibrils. 

Taxol s journey to the clinic was slow and arduous. Initial difficulties with aqueous solubility and lack of knowledge regarding its mechanism, of action delayed its development until 1979 when, in another seminal paper in the field, S.B.Horwitz and her collaborators disclosed their findings on the interaction of taxol with microtubules.4 Taxol s unique biological action, which includes promotion of microtubule formation and microtubule stabilization, stimulated a renewed interest in taxol as a potential drug candidate. The problem of procuring adequate supplies of taxol became even... 

The diversity of these subcellular actin structures is remarkable and appears to be determined by the interactions of many actin-binding proteins (ABPs) as well as by changes in the concentrations of intracellular signaling molecules such as Ca and cAMP, by small GTP-binding proteins, and by signals arising from mechanical stress. Approximately 50% of the actin molecules in most animal cells are unpolymerized subunits in the cytosolic pool and exist in a state of dynamic equilibrium with labile F-actin filamentous structures (i.e., new structures are formed while existing structures are renewed) (Hall, 1994). 

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