Experimental Study of Three-Dimensional Natural Convection High-Rayleigh Number

Natural convection in buildings is characterized by three-dimensional flow at high Rayleigh numbers (Ra ∼ 10¹⁰). At present, little is known about natural convection heat transfer in this regime, although a better understanding would allow more energy efficient usage of buildings. This paper presents results from the first phase of an experimental program aimed at improving our understanding of heat transfer and air flow in buildings. The experimental apparatus consists of a cubical enclosure filled with water in which each wall may be heated or cooled in a controlled manner. A transparent, adiabatic top and bottom provide flow visualization capability. Average heat transfer coefficients for the walls have been measured for several configurations of heating and cooling of the vertical isothermal walls. A unified Nu-Ra correlation has been computed which collapses the heat transfer coefficients of the various configurations to within 5.7 percent. The heat transfer and flow visualization results indicate that even at Rayleigh numbers as high as 6 × 10¹⁰, the heat transfer mechanism is laminar boundary-layer convection from one wall to the bulk fluid.