An Intermediate-level Model of External Convection for Building Energy Simulation

Abstract An ‘intermediate-level’ computer code for simulating convective heat exchange at the external facades of a building is described. The code is intermediate in complexity between the traditional data correlations found in guidebooks, and high-level air flow models based on techniques developed in the field of computational thermo-fluid dynamics. It takes into account most of the key dependent variables, including wind speed and direction, the change in shape and height of the atmospheric boundary layer over different terrains, and relative dimensions of the building. The computer is used to generalize available data correlations for the individual flow regimes that prevail around buildings, such as stagnation, boundary layer, and separated flows. Although external convection is normally wind-induced, the code also takes account of the influence of buoyancy-driven motion at low wind speeds. The computations of this improved model compare favorably with the albeit limited experimental data available for real buildings. The improved external convection model has been incorporated as a subroutine in a building energy simulation code: the NBSLD (U.S. National Bureau of Standards Load Determination) program. Simulations were performed for two ‘test houses’ subject to U.K. and U.S. weather conditions respectively. The variation in the computed heating/cooling loads due to the replacement of the original external convection algorithm by the intermediate-level one is reported. These results form the basis for a discussion of the likely sensitivity of dynamic building thermal models to the treatment of external convection with regard to the simulation of both energy consumption and internal environmental conditions. Intermediate-level convection models are advocated as offering the best prospect for meeting the needs of building energy simulation programs in terms of accuracy, economy, and user-friendliness.