Transition Criteria for Entropy Reduction of Convective Heat Transfer from Micropatterned Surfaces

Abstract

This paper develops an entropy-transition number for characterizing irreversibilities of external flow past micropatterned surfaces with controlled surface roughness. It is shown that embedded surface microchannels can reduce flow irreversibilities below a classical boundary-layer limit, due to drag reduction of slip-flow conditions within the microchannels. A surface-irreversibility ratio establishes the proportion of entropy production of slip-flow relative to no-slip conditions. Then an entropy-transition number is defined to characterize flow regimes in which the ratio decreases below unity, thereby indicating conditions in which surface micropatterning has an overall beneficial impact on energy conversion efficiency. Results are presented for various micropatterned surfaces. It is shown that slip-flow conditions within the embedded surface microchannels can overcome the additional friction irreversibility of more surface area, reducing the total entropy production up to 20% below the boundary-layer flow without microchannels. At lower Reynolds numbers, it is shown that the irreversibility ratio decreases (for example, down to 0.79) at a Reynolds number of 1200. The newly defined surface-irreversibility and entropy-transition numbers are shown to provide useful new parameters to characterize the boundary-layer irreversibilities of convective heat transfer.