Effect of Wall Roughness

For rough tubes in turbulent flow (7VRc > 4000), the von Karman equation was modified empirically by Colebrook to include the effect of wall roughness, as follows ... 

The flow of thin liquid films in channels and columns has also served as the basis of fundamental studies of wave motion (M7), the effects of wall roughness in open-channel flow (R4), the effects of surface-active materials (T9-T12), and the like. 

Although nearly all of the theoretical and experimental studies of film flow have dealt with the flow of films along hydrodynamically smooth surfaces, it is of interest to review the limited information available on the effects of wall roughness in view of their possible importance. 

It seems that it would be of practical interest to investigate the effects of wall roughness in greater detail, since it might be possible by means of suitably arranged small roughness elements to increase the rates of heat and mass transfer in film-type equipment. 

Keulegan (Kl3), 1938 Extension of Prandtl-von KdrmSn turbulent flow theories to turbulent flow in open channels. Effects of wall roughness, channel shape, and free surface on velocity distribution are considered. 

Reinius (R4), 1961 Studies of water flows in open channels at small slopes, Nr, = 50-13,000. Data on film thicknesses, film friction factors, effects of wall roughness. 

Determination of ArRe rit for film flow is made from measurements of absorption of CO2 in water film. Effects of wall roughness on iVReorit were also studied. 

There are some difFerences between two-phase flows in microreactors and conventional reactors. The importance of surface over volume forces increases. Reynolds number is usually small and laminar flow is established, where viscous forces dominate over inertial ones. The effects of wall roughness, wettability, and flow confinement become important. 

Barth Cyclones and swirl tubes Considers the frictional losses in the body. The effects of wall roughness and solids loading are accounted for in the value of the friction factor, /. 

Muschelknautz improved the model of Barth in a nnmber of ways. As we discussed in the previous chapter, he measured friction factors in both cylindrical and conical bodied cyclones and, on basis of this work, accounted for the effects of wall roughness and solids loading npon the cut-point and the pressure drop. We will discuss the latest version of his model for cyclone... 

The ability to account for the effects of wall roughness due to both the physical roughness of the materials of construction and to the presence of collected solids. 

C. Kleinstreuer and J. Koo, Computational analysis of wall roughness effect for liquid flow in micro-conduits, Journal of Fluids Engineering 126, 1-9... 

Effects of surface roughness are also evident in the boundary layer mean velocity profiles shown in Fig. 6.46. The profiles still exhibit a near-wall logarithmic behavior, but with a dependence on the roughness Reynolds number k = ksu lv. The law of the wall for a rough surface may be written as... 

In laminar flow/ = 16/Re, whereas in turbulent flow the dependence of/ on Re is a function of the specific rheological behavior of the fluid and roughness of the walls of the inside of the drill pipe (91). A number of functional relationships between / and Re have been proposed for turbulent flow. Ignoring the effects of the roughness of the surface of the drill pipe, / can be approximately related to Re by a generalized form of the well-known Blasius equation for Newtonian fluids (90, 95)... 

The laminar regime in a channel holds until the Reynolds number reaches a critical value above which the laminar motion becomes unstable and a transition to a turbulent flow will generally occur. It has been demonstrated experimentally that for a microchannel the critical value of the Reynolds number depends on the entrance conditions, on the cross-sectional geometry, and on the wall roughness. The effect of the roughness on the laminar-to-turbulent transition is very important, and it can be evidenced by observing Fig. 8 in which the experimental values of the Poiseuille number (f Re) are shown as a function of the Reynolds number for two microtubes made in stainless steel and in fused silica. The stainless steel microtube has an internal... 

We have used our pore models to discuss the effect of surface roughness and structural defects on the adsorption mechanism and on the nature of the dense phases. In Fig. 4 we present plots of the local density profile p r, z), as well as representative simulation snapshots for the three pore models at different values of P/Pq and r= 100 K. For pore model A, p also depends on the angular coordinate 0 nevertheless, a plot of pUp, z) can provide a suitable measure of the local state of the confined phase. As P/Po increases, the pore walls are covered by an adsorbate film whose thickness increases gradually with P/Po, until it reaches a point when there is formation of a condensate bridge between low density regions of adsorbate in... 

Fotir granular products were employed as test materials. Their particle properties are listed in Table 1. Note that the wall friction angle listed in Table 1 was measured by using Jenike shear tester based on the similarity between the test pipeline internal condition and the shear plate (i.e. a piece of bright mild steel plate). Such similarity was determined manually, not by measuring surface roughness. Since the effect of wall friction on the slug performance is very complicated, further research is needed to address this issue. 

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