Abstract
Previous research indicated that the surface-averaged forced convective heat transfer coefficient (CHTC) at a windward building facade can vary substantially as a function of building width and height. However, existing CHTC expressions generally do not consider the building dimensions as parameters and are therefore strictly only applicable for the building geometry for which they were derived. Most CHTC expressions also categorize facades only as either windward or leeward. This indicates the need for new and more generally applicable CHTC expressions. This paper presents new generalized expressions for surface-averaged forced CHTC at building facades and roofs that contain the reference wind speed, the width and the height of the windward building facade as parameters. These expressions are derived from CFD simulations of wind flow and forced convective heat transfer for 81 different isolated buildings. The 3D Reynolds-averaged Navier-Stokes equations are solved with a combination of the high-Re number realizable k-ε model and the low-Re number Wolfshtein model. First, a validation study is performed with wind-tunnel measurements of surface temperature for a reduced-scale cubic model. Next, the actual simulations are performed on a high-resolution grid with a minimum near-wall cell size of 400 μm to resolve the entire boundary layer, including the viscous sublayer and the buffer layer, which dominate the convective surface resistance. The new CHTC expressions are analytical formulae (trivariate polynomials) that can easily be implemented in Building Energy Simulation (BES) and Building Envelope Heat-Air-Moisture (BE-HAM) transfer programs. The accuracy of the expressions is confirmed by in-sample and out-of-sample evaluations.
Highlights
Wind flow around buildings is very complex, as it is characterized by flow impingement, separation, recirculation, reattachment and von Karman vortex shedding in the wake (Fig. 1)
This paper presented new generalized expressions for surface-averaged forced convective heat transfer coefficient (CHTC) at building facades and roofs that contain the reference wind speed, the width and the height of the windward building facade as parameters
These expressions were derived from three groups of CFD simulations of wind flow and forced convective heat transfer around 81 different isolated buildings
Summary
Wind flow around buildings is very complex, as it is characterized by flow impingement, separation, recirculation, reattachment and von Karman vortex shedding in the wake (Fig. 1). Concerning the accurate and reliable CFD simulation of CHTC, the first difficulty is strongly amplified, because of the necessity to resolve the entire thermal boundary layer at all building surfaces, including the very thin viscous sublayer and the buffer layer, which dominate the convective surface resistance This requires a y* value smaller than 5 and preferably equal to 1 [50,51] which implies a very high near-wall grid resolution, yielding wall-adjacent cell sizes that can go down to 300 mm [22,23]. This refers to the use of analytical expressions (often called “correlations”) that have been established mostly based on previous onsite measurements or wind-tunnel measurements or on CFD simulations Many of these expressions are implemented in Building Energy Simulation (BES) programs [3,4,55] and BE-HAM (Buildings Envelope Heat, Air an Moisture transfer) computational codes [5,7,56e58].
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