Micro-Channels with Rough Walls

Several investigators obtained friction factors in micro-channels with rough walls that were greater than those in smooth wall channels. These observations should be considered taking into account the entrance effects, losses from change in channel size, etc. 

The existence of roughness leads also to decreasing the value of the critical Reynolds number, at which transition from laminar to turbulent flow occurs. The character of the dependence of the friction factor on the Reynolds number in laminar flow remains the same for both smooth and rough micro-channels, i.e., X = const/Re. 

For Re 2 X 10, the relative roughness that corresponds to the boundary between the smooth and rough channels is k /rQ 0.08. The latter shows that the hydraulic characteristics of the present micro-channels (Table 3.4) are close to characteristics of smooth micro-channels. 

The deviation of the data related to flow in smooth (Table 3.3) and rough (Table 3.4) micro-channels may be a result of heterogeneity of the actual roughness, where height of some roughness peaks signiflcantly exceeds its mean value. For example, under conditions of experiment by Pfund et al. (2000) the mean heigh of 

Author Micro-channel Shape 4i [mm] Fluid Re P exp/POtheor Poiseuille number Dependence on Re Relative roughness % fe/rol RCcr 

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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. 

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