Effective tube roughness

One of the possible ways to account for the effect of roughness on the pressure drop in a micro-tube is to apply a modified-viscosity model to calculate the velocity distribution. Qu et al. (2000) performed an experimental study of the pressure drop in trapezoidal silicon micro-channels with the relative roughness and hydraulic diameter ranging from 3.5 to 5.7% and 51 to 169 pm, respectively. These experiments showed significant difference between experimental and theoretical pressure gradient. 

The effect of roughness on pressure drop in micro-tubes 620 and 1,067 pm in diameter, with relative roughness of 0.71, 0.58 and 0.321% was investigated by Kandlikar et al. (2003). For the 1,067 pm diameter tube, the effect of roughness on pressure drop was insignificant. For the 620 pm tube the pressure drop results showed dependence on the surface roughness. 

Claassen (CIO), 1918 Experimental studies of film flow of water, NaCl solutions, and molasses on vertical tubes. Measurements of film thickness, liquid adhering after draining, effects of roughness. 

Mott and Bott illustrated the effect of different materials on the accumulation of Pseudomonas fluor-escens biofilms on the inside of tubes under identical operating conditions (see Fig. 9). The differences between the effects of the materials occur for two reasons roughness and surface electrical properties. The quality of the surface, in terms of roughness, on which microorganisms attach, can affect the biofilm accumulation as discussed earlier. The effect of roughness is illustrated in Fig. 9 by the difference of biofilm accumulation between electropolished and as received 316 stainless steel. The rougher stainless steel is seen to be more hospitable to biofilm growth. 

EFFECT OF ROUGHNESS. The discussion thus far has been restricted to smooth tubes without defining smoothness. It has long been known that in turbulent flow... 

Walker, R.A. and Bott, T.R., 1973, Effect of roughness on heat transfer in exchanger tubes. Chem. Engr. No. 271, March, 151-156. 

The influence of the Reynolds number Re is negligible, because in contrast to a free falling sphere, the flow profile is dominated by the wall effects. The diameter of the sphere should not be too small. If the gap between the tube and the sphere gets smaller than 0.1 mm, the effects of roughness of the tube and sphere gain too much influence on the results. An evaluation of the shear rate is not possible, so that an application of this type of viscosimeter for fluids with a strong non-Newtonian flow behavior is not advisable. An attempt to empirically determine the shear rate in a micro falling sphere viscosimeter is described in [23]. 

EFFECT OF TUBE ROUGHNESS — THE OVERALL FRICTION FACTOR CORRELATION... 

The friction factor derivation culminating in the relationship of equations (3-18) and/or (3-22) specified that the tube be smooth. This raises a natural question as to what the effect of tube roughness would be on frictional heating. 

Consider tubes with various roughness (see Figure 3-2). If a fluid were to flow in such tubes in laminar flow, the streamlines would conform in such a way as to minimize the effect of tube roughness on frictional heating. On the other hand. 

Figure 15.4 Effect of tube roughness on the fluid slug velocity.

Dispersion In tubes, and particiilarly in packed beds, the flow pattern is disturbed by eddies diose effect is taken into account by a dispersion coefficient in Fick s diffusion law. A PFR has a dispersion coefficient of 0 and a CSTR of oo. Some rough correlations of the Peclet number uL/D in terms of Reynolds and Schmidt numbers are Eqs. (23-47) to (23-49). There is also a relation between the Peclet number and the value of n of the RTD equation, Eq. (7-111). The dispersion model is sometimes said to be an adequate representation of a reaclor with a small deviation from phig ffow, without specifying the magnitude ol small. As a point of superiority to the RTD model, the dispersion model does have the empirical correlations that have been cited and can therefore be used for design purposes within the limits of those correlations. 

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. 

This chapter has the following structure in Sect. 3.2 the common characteristics of experiments are discussed. Conditions that are needed for proper comparison of experimental and theoretical results are formulated in Sect. 3.3. In Sect. 3.4 the data of flow of incompressible fluids in smooth and rough micro-channels are discussed. Section 3.5 deals with gas flows. The data on transition from laminar to turbulent flow are presented in Sect. 3.6. Effect of measurement accuracy is estimated in Sect. 3.7. A discussion on the flow in capillary tubes is given in Sect. 3.8. 

Judy J, Maynes D, Webb BW (2002) Characterization of frictional pressure drop for liquid flows through micro-channels. Int J Heat Mass Transfer 45 3477-3489 Kandlikar SG, Joshi S, Tian S (2003) Effect of surface roughness on heat transfer and fluid flow characteristics at low Reynolds numbers in small diameter tubes. Heat Transfer Eng 24 4-16 Koo J, Kleinstreuer C (2004) Viscous dissipation effects in microtubes and microchannels. Int J Heat Mass Transfer 47 3159-3169... 

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