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convective heat transfer coefficient correlations obtained by ASHRAE (1975), ..... for a wind-to-surface angle of 135°, which appears to be lower than those ...

PUBLISHED IN: M.G. EMMEL, M.O. ABADIE, N. MENDES, Numerical Determination of External Convective Heat Transfer Coefficients for Low-Rise Buildings. In: 13th International Heat Transfer Conference, 2006, Sydney.

NUMERICAL DETERMINATION OF EXTERNAL CONVECTIVE HEAT TRANSFER COEFFICIENTS FOR LOW-RISE BUILDINGS M Emmel1, M Abadie1, 2, N Mendes1 1

LST - Pontifical Catholic University of Paraná, Curitiba, Brazil 2 LEPTAB - University of La Rochelle, La Rochelle, France

Abstract Building energy analysis are very sensitive to external convective heat transfer coefficients so that some researchers have conducted sensitivity calculations and proved that depending on the choice of those coefficients, energy demands estimation values can vary from 20 to 40 %. In this context, computational fluid dynamics calculations have been performed to predict convective heat transfer coefficients at the external surfaces of a simple shape low-rise building. Effects of wind velocity and orientation have been analyzed considering different surface-to-air temperature differences. Results show that the convective heat transfer coefficient value strongly depends on the wind velocity, that the wind direction has a notable effect for vertical walls and almost none for roofs and that the surface-to-air temperature difference has a negligible effect for wind velocity higher than 5 m/s. Keywords: External Convective Heat Transfer Coefficient, Wind Velocity, Wind Orientation, Building Physics, Computational Fluid Dynamics.

Nomenclature dloc hc k Ln Rf Uloc U10

distance from the wall to the velocity evaluation point (m) convective heat transfer coefficient at building external wall surfaces (W/m2.K) turbulent kinetic energy (m2/s2) characteristic length for natural convection (area-to-perimeter ratio) (m) surface roughness factor local wind velocity (m/s) wind velocity at z = 10 m (m/s)

x y+     

distance along wind direction from the roof edge to hc evaluation point (m) dimensionless wall normal coordinate turbulent kinetic energy dissipation rate (m2/s3) weighting factor for natural convection thermal conductivity (W/m.K) wind direction (North = 0°, East = +90°, South = +180° and West = -90°) turbulent kinetic energy specific dissipation rate (/s)

1. Introduction Building Simulation tools need to better evaluate convective heat exchanges between external air and wall surfaces. Previous analysis demonstrated the significant effects of convective heat transfer

coefficient values on the room energy balance. Awbi (1998) and Beausoleil-Morrison (1999) have pointed out that large discrepancies observed among widely used building thermal models can be attributed to the different correlations used to calculate or impose the value of the convective heat transfer coefficients. By performing sensitivity calculations, the latter author proved that the choice of convective heat transfer coefficient values can lead to differences from 20 % to 40 % of energy demands. In-situ measurements performed by McAdams (1954), Ito (1972), Sharples (1984) and Clear et al. (2003) permit the elaboration of current convective heat transfer coefficient correlations. A clear limitation of these in-situ experiments is the need of taking into account and measuring all parameters that can affect the evaluation of the studied coefficient. As stated by the last cited researcher, an important issue is the evaluation of the radiative exchange between the wall surfaces and the sky because no accurate correlation is available. With the increase of computational resources, Computational Fluid Dynamics (CFD) simulation becomes a valuable alternative way of evaluating airflows around buildings and, as a consequence, external convective heat transfer coefficients. The present study describes simulations that have been performed using CFD techniques to predict convective heat transfer coefficients at external building surfaces. In a first part, main correlations for vertical walls and roof are summarized. Studied geometry and simulation procedures are then described. All results are presented and commented in a third part.

2. Literature review Many research works have been conducted since early eighties focused on the convection heat transfer problems inside buildings. Heat transfer coefficient correlations in enclosure can be found in the literature review made by Khalifa (2001). On the other hand, not much research has been focused on the determination of external heat transfer coefficients. The main reason lies in the variability of wind characteristics in urban environments and more precisely around buildings. Nevertheless, experimental works have been conducted by McAdams (1954), Sturrock (1971) and especially Ito (1972) whose results have been incorporated for energy calculations by the ASHRAE Task Group (1975). These relationships that are commonly used in building energy simulations link convective heat transfer coefficient values to local wind velocity near the surface (evaluated from the meteorological wind velocity at 10 m) and surface tilt (horizontal or vertical) and wind-surface angle through the definition of windward and leeward surface. A wall is considered as windward wall if the angle of incidence between the normal to the wall surface and the wind direction is less than  90° and leeward for all other directions. It should be noticed that other relationships can be found in Nicol (1977) and CIBS (1979) to name a few. From the results of his experimentations in situ, Sharples (1984) concluded that the existence of numerous relationships mainly lies on the evaluation of the local wind velocity. Loveday and Taky (1996) remarked that turbulent intensity of the airflow has to be studied too and that flow separation at building edges should be taken into consideration to better define what is a windward and a leeward surface. Table 1 summarizes convective heat transfer coefficient correlations obtained by ASHRAE (1975), Sharples (1984) and Loveday and Taky (1996) for windward and leeward walls. These correlations have been chosen as they give relations between the convective heat transfer coefficient and the local wind velocity and between the local and meteorological wind velocities. Table 1: Convective heat transfer coefficient correlations for windward and leeward walls.

hc

ASHRAE (1975)

Sharples (1984)

Loveday and Taky (1996)

18.6U loc0.605

1.7U loc  5.1

16.21U loc0.452

Uloc, windward

0.25U10 if U10>2 m/s 0.5 if U10