The wettability of polymeric tape surfaces such as polystyrene, polyethylene, and polypropylene does not significantly affect air entrainment velocities. However, rougher surfaces, as suggested by Scriven ( I 982), show significantly higher air entrainment velocities.Air entrainment velocities are related to maximum coating speeds in coating operations and have been studied using flexible tapes plunging vertically into pools of liquid by Perry (1 967), Burley and Kennedy (1 976 a, b), Kennedy and Burley (1977), Burley and Jolly (1982, 1984), and Gutoff and Kendrick (1982). These studies indicate that liquid viscosity has the most significant effect on the air entrainment velocity, which decreases with the 0.67-0.78 power of the viscosity. Gutoff and Kendrick found no significant effect on surface tension, but Burley and his coworkers believe that air entrainment velocities increase with the 0.38 power of surface tension. The wettability of the polymers they studied did not vary enough to determine surface effects. This work was carried out to study the effects of surface wettability and roughness on air entrainment velocities.A number of plastic and paper tapes were slit I6 rnm wide and tested in the apparatus shown in Figure 1. The tape first contacted grounded tinsel, and then passed over grounded metal rollers to reduce any static charges (Burley and Jolly, 1984) before plunging vertically into the pool of liquid. Various aqueous solutions and pure organic liquids were used. The tape velocity was increased (using a variable speed D C motor) until air entrainment was observed for the particular side of the tape studied. The velocity a t this point was determined by measuring the length of tape passing through the bath in a fixed time interval 10-30 s, and was found to vary between 0.07-1 .O m/s at air entrainment.To compare the tape surface roughness and wettability effects, all air entrainment velocity data were normalized to that for the polystyrene tape for each particular solution. The root mean square roughnesses of the tapes were measured on a Dektak surface Profilometer. These results are tabulated in Table 1. Although the data show more scatter than one would like, it is very clear that the rough surfaces of the uncoated sides of the paper tapes and the rough surface polyethylene coated paper show very significantly higher air entrainment velocities than do the smoother tapes. The air entrainment velocities seem to increase with root mean square roughness. This agrees with the suggestion of Scriven (1982) that near the dynamic wetting line with rough surfaces, air can "escape" through the valleys between peaks in the surface.Although not as clearly shown, the data also indicate that surface wettability, which varies from the more wettable polysty-
Np,. = Prandtl number N R~ = Reynolds number 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.NOTATION U A Ax AT B C De Dl = coefficient in experimental correlation, ~p = u.un = coefficient in experimental correlation, N E~ = = cross-sectional area of any nontriangular rib, sq. in. = cross-sectional area of any triagular rib, sq. in. = dimension of rib base, in. = one-half the number of corrugations on a rib sur-= equivalent diameter of channel, ft. = developed length of plate surface, ft.
P Pc S wc U Y = transverse base dimension of a rib, in. = Blasius friction factor, 2.Ap.g;De/p.u2.D1, di-= rib frequency or distance between corresponding = gravitational constant, 32.2 ft.lb.,/lb.f sq. sec. = height of a rib or internal geometry, in. = coefficient in empirical correlation, ~p = K.ue = exponent in experimental correlation, Ap = U . U " = number of ribs in a transverse row succeeding n2 = number of ribs in a transverse row succeeding n1 = exponent in experimental correlation, N E~ = = Euler number, Ap.g,/p.u2, dimensionless = Reynolds number, De.u.p/p, dimensionless = pressure drop, Ib./sq.in. = lineal perimeter of a noncorrugated rib, in. = lineal perimeter of a corrugated rib, in. = average plate spacing for fluid flow, in. = lineal fluid velocity in channel, ft./sec. = transverse width of plate channel between gas-= plate separation or compressed gasket thickness, mensionless points of successive ribs, in. A . N R e -N kets, in. in. Greek Letters a p' p B r 6 = transverse rib angle for cross diagonal ribs, o ( = = exponent correlation for rib base angle effects = base angle of ribs, radians = coefficient correlation for rib base angle effects = exponent correlation for rib corrugation effects = coefficient correlation for rib corrugation effects = exponent correlation for plate separation effects 0, radians A E e x A p u 2 r T 4 $ = coefficient correlation for plate separation effects = exponent in empirical correlation, A p = K.u'= transverse rib angle for diagonal ribs, radians = exponent correlation for rib frequency effects = coefficient correlation for rib frequency effects = density of fluid, lb./cu.ft. = exponent correlation for rib shape effects = coefficient correlation for rib shape effects = exponent correlation for transverse rib angle ef-= coefficient correlation for transverse rib angle ef-= exponent correlation for protrusions = coefficient for protrustions = rib shape geometrical parameter, A d A T -1.0, ects fects dimensionless LITERATURE CITED 1. Troupe, RHeat transfer and pressure drop data were taken on commercial plate heat exchange equipment. Nusselt and Euler correlations were determined for each of the six commercial heat exchangers investigated. These correlations were combined to establish a single heat transferpressure drop relationship for any plate type of heat exchanger channel. The results of this investigation were tested by using the correlations developed in Part I of this series to predict pressure drop data for the commercial unit based on their channel geometries. These predicted pressure drops were then used with the results of this part of the series to predict and compare heat transfer data. The correlations developed in this work allow one to determine the heat transfer characteristics in a ribbed rectangular channel from the pressure drop characteristics of the channel in question.The effects of internal geometries on the pressure drop in ribbed rectangular channels have been correlated as functions of the rib and channel geometry in Part I of this seri...
Demonstration of laminar and turbulent flow using water in one experimental unit has always been a challenge. One can achieve one of the two defined flow regimes by varying tube diameter; however, the versatility to move across a decade or more in Reynolds number with a single tube diameter is generally difficult. A unit operations fluid flow experiment composed of a two ¾-inch ID glass tubes, 36 inches long, has been developed that allows demonstration of flow in all flow regimes with ease. One of the tubes is empty and contains no flow elements (typical flow inside a pipe); the other tube contains a multi-element, 33-inch long, static mixer. Using a secondary dye injection system, students conduct experiments in which the various flow regimes (laminar, transition, or turbulent) may be observed in the empty tube. The effects of the static mixer blending the dye into the water stream can be observed in the other tube. Students record the flow effects in their experiments using still and motion digital photography. Pressure transducers, located at the entrances and exits of the tubes, allow quantitative measurement of pressure drop across each tube to be observed. Students can then compare their results with pressure loss predictions using information found in the literature such as a Fanning Friction Chart. The experiment has been technically successful and is very popular with our students. This paper presents the evolution of this experiment and on the results that students are able to observe and evaluate. Nomenclature D Inside diameter of pipe or tube, m F Frictional pressure losses in flow systems, m 2 /s 2 f Fanning friction factor, dimensionless f M Moody friction factor, dimensionless L Length of tubing, m L eq Equivalent length of tubing for similar pressure drop, m P System pressure, N/m 2 Re Reynolds number (defined in Equation 1), dimensionless v velocity, m/s V Volumetric flow rate, m 3 /s Subscripts 1 Entrance condition 2 Exit condition ref Reference condition • Session 2313
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