Axial Back Conduction through Channel Walls During Internal Convective Microchannel Flows

Recent developments during the last decade in the field of manufacturing and development of many high-power mini-/micro-devices led to increased interest in microfluidic devices involving heat transfer. Since the pioneering work by Tuckerman and Pease (IEEE Electron Device Lett 2:126–129, 1981) on the use of microchannels for high heat flux removal, certainly a lot of developments has been witnessed through ever-increasing analytical, experimental, and highly sophisticated numerical studies by many researchers across the globe. In spite of this progress, many fundamental understandings of flow and heat transfer phenomena in mini-/microchannel systems are still obscure. One such phenomenon is the flow of heat in the solid wall of microchannel systems by means of conduction normally in a direction opposite to that of internal convective mini-/microchannel flow of fluid, called “axial wall conduction” or “axial back conduction.” Axial back conduction is not a new phenomenon, rather mostly neglected unintentionally because of its convincingly smaller influence on heat transfer in conventional-size channels. As the hydraulic diameter of a channel decreases, the coupling between the substrate and bulk fluid temperatures becomes significant because of the relative size of the fluid to the solid wall. Unlike in conventional-size channels, negligence of axial back conduction along the solid walls of micro heat exchangers frequently leads to erroneous conclusions and inconsistencies in the interpretation of transport data. Thus, it is important to explicitly identify the thermofluidic parameters of interest which lead to a distortion in the boundary conditions and thus the true estimation of species transfer coefficients. In this chapter, we focus our attention on the axial back conduction in the solid substrate/channel wall as against the axial back conduction in the liquid flow domain; thus, a detailed review of the state-of-the-art on axial back conduction in both conventional as well as mini-/microchannel systems is presented.