Pronounced Effects in MicroChannel Heat Transfer

The heat transfer in microchannels is expected to agree with conventional theory provided that the discussed continuum assumptions can be made. For example, under fully developed laminar flow conditions at low Re, Nu is constant. However, many experimental data show large deviations between each other and inconsistency with classical theory exists. There is an increase in Nu with increasing Re measured. According to Herwig and Hausner [37], a common theoretical basis on forced convection for macro- and microchannels can be used to describe forced convection of liquids in the laminar regime. However, there are effects which are more pronounced and which are of more importance on the microscale, such as surface tension, viscous forces and electrostatic forces [38]. These effects are called scaling effects with respect to standard macroscale analysis. 

There have been several approaches to identify these scaling effects of flow in microchannels, which lead to a discrepancy between classical theory and heat transfer in microchannels below 1 mm [39]. The deviations may be explained by general effects which are neglected in macroscale calculations but are of increasing importance as the characteristic size decreases. 

A lower limit for hydraulic diameter of 1.2 mm was proposed by Adams et al. [23] for the applicability of standard turbulent single-phase Nusselt correlations. 

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