![]() In other words, N u cannot be less than N uT in a thermally developing flow or in a flow under uniform surface heat flux accompanied by axial conduction. The N uT listed in the table is the lower limit of N u because the axial conduction effect is absent when the surface temperature is uniform. The ratio of N uq to N uT is larger than 1. Table 1 lists the typical N u and P o values for a fully developed laminar flow in various channels 6, where N uT is the N u at a uniform surface temperature, and N uq is the N u at a uniform heat flux. Because αD h is nearly a constant, the forced convection heat transfer coefficients in fully developed laminar channel flows are expected to increase with a decrease in the microchannel size. In a fully developed laminar channel flow, the classical theoretical solutions for the Nusselt number (hereafter, N u, where α is the convective heat transfer coefficient, D h is the hydraulic diameter of a channel, and λ is the thermal conductivity) and Poiseuille number (hereafter, P o P o = fR e, where f is the Fanning friction factor and R e is the Reynolds number) are constants and are independent of the Reynolds number. Continuum theory is applicable to single-phase liquid flow in microchannels, which implies that the equations developed for macroscale applications, such as the Navier–Stokes equation and convective heat transport equations, will be applicable to even small channels 1, 2, 3, 4, 5. Heat and mass transfer in microchannels has been extensively investigated over the past several decades. Using the theoretical Poiseuille and Nusselt numbers derived under the slip boundary condition at the solid–liquid interface, we estimate the slip length and thermal slip length at the interface. The forced convective heat transfer characteristics of single-phase laminar flow in a parallel-plate microchannel are investigated. We demonstrate that the deviation from classical theory with a reduction in hydraulic diameters is due to the breakdown of the continuum solid–liquid boundary condition. Here we show the scale effect of the boundary condition at the solid–liquid interface on single-phase convective heat transfer characteristics in microchannels. Given the larger surface/volume ratio of microchannel, the surface effects increase as channel scale decreases. However, the inconsistencies between experimental and classical theoretical predictions for the liquid flow in microchannel remain unclarified. Rapid advances in microelectromechanical systems have stimulated the development of compact devices, which require effective cooling technologies (e.g., microchannel cooling). ![]()
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