Non-Boussinesq effects on natural convection airflows simulated in a tall rectangular cavity

Abstract

Natural convection of air in a tall rectangular cavity is explored by solutions of the fully compressible transient Navier–Stokes equations in two-dimensional form, without the Boussinesq approximation. A temperature difference is imposed on the two vertical side walls, each isothermal, with the two horizontal boundaries adiabatic. Thermo-physical properties of air, including density, viscosity, thermal conductivity and specific heat, are all variable with temperature. Contrary to the conclusions in most previous numerical studies invoking the Boussinesq approximation, this study finds the instability of the conduction regime to be a traveling wave drifting downward in the cavity. The wave-drift speed is a strong function of the dimensionless wall temperature difference, ε, defined as the temperature difference between the vertical walls divided by twice the mean wall temperature. Wave-drift speeds and wave numbers are calculated over a range of ε less than 0.5 and Rayleigh numbers less than 9000. The dimensionless temperature difference ε is also found to have significant effects on local heat transfer and transient flow structure inside the cavity.

Keywords:

• Boussinesq limit
• critical conditions
• natural convection
• non-Boussinesq conditions
• tall rectangular cavity
• temperature-dependent air properties
• transient flow
• two-dimensional flow

Acknowledgments

This work was facilitated though the use of advanced computational, storage, and networking infrastructure provided by the Hyak supercomputer system, funded by the Student Technology Fee at the University of Washington. This article is an expanded version of an article titled “A Numerical Study of Natural Convection of Air in a Tall Rectangular Cavity” presented at the 10th Australasian Heat and Mass Transfer Conference (AHMT2016) in Brisbane, Australia, July 14–15, 2016.

Additional information

Funding

These results were obtained as part of the research supported by the National Science Foundation through Grant CMMI-1301491.