The present paper focuses on the optimization of the thermal performance for compressible laminar natural convection flow induced under high-temperature difference in an open-ended vertical channel by optimizing the channel inter-plate spacing using numerical simulation. The present investigation is conducted for a wide range of Rayleigh number (Ra) 10(4) to 10(7) in channel heated asymmetrically by uniform surface temperature with air (Pr 0.72) as working fluid. Several values of channel gap between plates, new modified preconditioned all-speed Roe scheme along with dual time stepping technique and modified Local One-dimensional Inviscid (LODI) relations as channel inlet and outlet boundary conditions suitable for compressible laminar natural convection is employed for the current simulation. Heat transfer rate in terms of average Nusselt number is obtained for all Rayleigh number and channel aspect ratio is obtained. Variation of thermal and velocity profiles, the variation of average Nusselt number and mass flow rate into the channel for the combination of each Rayleigh number and channel aspect ratio is reported. From the results obtained, a correlation for optimum aspect ratio with Rayleigh number which maximizes the heat transfer within the channel is presented. (C) 2018 Elsevier Ltd. All rights reserved.