Miniaturization of devices has received a rapid expansion in the last years and a great attention of research activities is given to microflow due to its new applications of microfluidic systems and components. In the present paper a transient investigation on natural convection in parallel-plate vertical microchannels is carried out numerically. The vertical microchannel is considered asymmetrically or symmetrically heated at uniform heat flux. The first-order model for slip velocity and jump temperature is assumed in microscale conditions. The analysis is performed in laminar boundary layer assumption for different values for different values of Knudsen number, Rayleigh number and the ratio of wall heat flux in order to evaluate their effects on wall temperatures, mass flow rate and Nusselt number. Wall temperature overshoots are detected for the different conditions. These values increase increasing the Knudsen number, Kn, at high Rayleigh number, Ra, whereas for lower Ra the lowest wall temperature are obtained for Kn = 0.05. Mass flow rate increases increasing Kn whereas Nusselt number decreases increasing Kn.

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