International Journal of Heat and Mass Transfer, 80 (2015) pp. 529-538 Corresponding author: Suvash C. Saha. Email: s_c_saha@yahoo.com Phone: +61403526512, Fax: +61731381469 Natural convection in a triangular enclosure heated from below and non-uniformly cooled from top Suvash C. Saha * , Y. T. Gu School of Chemistry, Physics & Mechanical Engineering, Queensland University of Technology 2, George Street, GPO Box 2434, Brisbane QLD 4001, Australia Abstract This study is concerned with transient natural convection in an isosceles triangular enclosure subject to non-uniformly cooling at the inclined surfaces and uniformly heating at the base. The numerical simulations of the unsteady flows over a range of Rayleigh numbers and aspect ratios are carried out using Finite Volume Method. Since the upper inclined surfaces are linearly cooled and the bottom surface is heated, the flow is potentially unstable. It is revealed from the numerical simulations that the transient flow development in the enclosure can be classified into three distinct stages; an early stage, a transitional stage, and a steady stage. The flow inside the enclosure depends significantly on the governing parameters, Rayleigh number and aspect ratio. The effect of Rayleigh number and aspect ratio on the flow development and heat transfer rate are discussed. The key finding for this study is to analyse the pitchfork bifurcation of the flow about the geometric centre line. The heat transfer through the roof and the ceiling as a form of Nusselt number is reported in this study. Keywords: Natural convection; triangular enclosure; non-uniform cooling; pitchfork bifurcation; instability. Nomenclature A Aspect ratio of the enclosure (h/l) t Time [s] g acceleration due to gravity [ms -2 ] T Temperature [K] H height of the attic [m] T 0 ambient temperature [K] h eff heat transfer coefficient [Wm -2 K -1 ] u, v velocity components [ms -2 ] K Kelvin [K] x, y Coordinates [m] k thermal conductivity [Wm -1 K -1 ] l half horizontal length of the attic [m] Greek symbols Nu Nusselt number  thermal expansion coefficient [T -1 ] P pressure [Pa] T temperature difference between the surface and the ambient [K]