Wall roughness

Friction factor Describes the relationship between the wall roughness, Reynolds number, and pressure drop per unit length of duct or pipe run. 

Calculation of pressure drops in steam lines is a time-consuming task and requires the use of a number of somewhat arbitrary factors for such functions as pipe wall roughness and the resistance of fittings. To simplify the choice of pipe for given loads and steam pressures. Figure 22.4 will be found sufficiently accurate for most practical purposes. 

Chapter 4 is devoted to single-phase heat transfer. Data on heat transfer in circular micro-tubes and in rectangular, trapezoidal and triangular ducts are presented. Attention is drawn to the effect of energy dissipation, axial conduction and wall roughness on the thermal characteristics of flow. Specific problems connected with electro-osmotic heat transfer in micro-channels, three-dimensional heat transfer in micro-channel heat sinks and optimization of micro-heat exchangers are also discussed. 

More accurate correlations, which take factors like wall roughness into account, are readily available, but the form used here is adequate for most purposes. It has a simple, anal5Tical form that lends itself to conceptual thinking and scaleup calculations, but see Problem 3.14 for an alternative. 

For very rough tubes, the flame acceleration is much more rapid as shown in the previous section. Transition to detonation is also clearly marked by a local explosion and abrupt change in the wave speed. The wall roughness controls the propagation of the wave by providing [5] ... 

You must determine the horsepower required to pump a coal slurry through an 18 in. diameter pipeline, 300 mi long, at a rate of 5 million tons/yr. The slurry can be described by the Bingham plastic model, with a yield stress of 75 dyn/cm2, a limiting viscosity of 40 cP, and a density of 1.4 g/cm3. For non-Newtonian fluids, the flow is not sensitive to the 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 actual size of the roughness elements on the conduit wall obviously varies from one material to another, with age and usage, and with the amount of dirt, scale, etc. Characteristic values of wall roughness have been determined for various materials, as shown in Table 6-1. The most... 

At high Reynolds numbers (high turbulence levels), the flow is dominated by inertial forces and wall roughness, as in pipe flow. The porous medium can be considered an extremely rough conduit, with s/d 1. Thus, the flow at a sufficiently high Reynolds number should be fully turbulent and the friction factor should be constant. This has been confirmed by observations, with the value of the constant equal to approximately 1.75 ... 

It is implicit in these methods that the wall slip behaviour in the pipe is similar to that in the tubes. There is evidence [Fitzgerald (1990)] that the wall roughness can have a dramatic effect on the flow of some suspensions, making the assumption of similar slip behaviour very dangerous. 

Steam is generated in a high pressure boiler containing tubes 2.5 m long and 12.5 mm internal diameter. The wall roughness is 0.005 mm. Water enters the tubes at a pressure of 55.05 bar and a temperature of 270°C, and the water flow rate through each tube is 500 kg/h. Each tube is heated uniformly at a rate of 50 kW. 

V is cross-sectional mean velocity, v is kinematic viscosity, e is wall roughness, and D is pipC di IUCtel 4Rh. Rh is hydraulic radius, A/P, where A is cross-sectional area and P is wetted perimeter (Moody,... 

Thus the transition is not a continuous process as is combstn. This shows that combstn and deton differ not only in magnitude of propagation but also in character. Transition from combstn to deton is facilitated by wall roughness inside the pipes... 

Another factor which can lead to BET areas slightly higher than those from porosimetry is pore wall roughness. Slight surface roughness will not alter the porosimetry surface area since it is calculated from the pore volume while the same roughness will be measured by gas adsorption. 

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. 

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