Study on the impact of buildings on the outdoor thermal environment based on a coupled simulation of convection, radiation, and conduction

ASHRAE Transactions, July, 2007 by Hong Chen, Ryozo Ooka, Hong Huang, Madoka Nakashima

INTRODUCTION

Outdoor thermal environments represented by the urban heat island phenomenon have deteriorated markedly in recent years due to changes in land cover and increases in the release of artificial heat in summer, symptomatic of urbanization in Japan. Artificial coverage of the ground in urban areas, ventilation obstacles coupled with increased absorption rates of solar radiation due to dense building arrangements, and artificial heat released from buildings are currently considered the most important factors related to the formation of heat islands. In order to implement effective countermeasures for outdoor thermal environments, therefore, it is very important to quantitatively understand the influences of these factors on such environments. However, very few studies have made quantitative comparisons with respect to the influences of these factors on thermal environments. In this research, an impact factor index of the sensible heat flux from building walls and the artificial heat release from buildings as they relate to the increase in the air temperature of the outdoor environs is proposed. Moreover, an impact factor index of the differences of artificial heat release positions and building block patterns, such as homogeneous and heterogeneous, is calculated using a coupled simulation of radiation, convection, and conduction.

OUTLINE OF IMPACT FACTOR INDICES FOR OUTDOOR THERMAL ENVIRONMENTS

The impact factor indices for outdoor thermal environments evaluate the influence on the rise in air temperature at each point in an open space according to the distribution of increasing (or decreasing) air temperature caused by each heat source, such as building walls, the ground, and the artificial heat release from buildings. Here, impact factors on the outdoor thermal environment are considered as each heat source described above, and the indices of these impact factors can be regarded as a contribution rate by the heat flux from each heat source to the air temperature formation at the arbitrary point of the outdoor space. Then, if the impact factor indices of each heat source are obtained, the dominant factor on the outdoor thermal environment can be understood. This is quite useful for carrying out effective countermeasures for the hot outdoor environment.

The impact factor index of the outdoor thermal environment of each building wall evaluates the influence on the air temperature of each point in the open space by the sensible heat flux from each building wall (Figure 1). That is, it corresponds to the air temperature rising at each point caused by the existence of each building wall. In order to calculate the impact factor index of each building wall, first a computational fluid dynamics (CFD) analysis is performed as usual, and then the spatial distributions of the wind velocity and air temperature are calculated. Second, the wind velocity field is fixed, the surface temperature of the wall is raised by about [T.sub.wall], and only the air temperature is recalculated. In this research, the assumption that the air temperature equation can be considered a linear equation under some temperature changes is employed. In previous research, a comparison between the case that the wind velocity field is fixed and the case that the wind velocity field is coupled was performed. It was confirmed that there was not a large difference in distribution of air temperature under some temperature changes, e.g., the difference was only about a 5[degrees]C change, which corresponds to the Bulk Richardson number, [ or -]1.33, in the study. Then, the impact factor index of each building wall is calculated by Equation 1 using the new spatial distributions of the air temperature obtained in the second step calculation.

[FIGURE 1 OMITTED]

[imp.sub.jw]=([DELTA][T.sub.[airj]]/[DELTA][T.sub.[walli]])x(T.sub.i]-[T.sub.ai]) [[degrees]C] (1)

where [imp.sub.jw] is the impact factor index at cell j of each building wall, [delta][T.sub.airj] is the degree of rising air temperature at cell j, [delta][T.sub.walli] is the degree of rising surface temperature at wall i (i = north wall, south wall, east wall, west wall, and roof), [T.sub.i] is the surface temperature at wall i, and [T.sub.ai] is the mean air temperature of the first cell in the wall i neighborhood.

Here, [delta][T.sub.airj]/ [delta][T.sub.wallj] indicates the degree of rising air temperature at adjacent points around the building when the surface temperature of the wall rises by 1[degrees]C. In this research, the impact factor index of the outdoor thermal environment of a building wall is defined as a product of the difference between the building wall temperature and the air temperature

at the adjacent points [delta][T.sub.airj]/ [delta][T.sub.walli] This factor indicates the heating ability of each building wall on the surrounding air. (See Appendix A.)

The impact factor index for outdoor thermal environments with regard to artificial heat release from buildings is calculated by the air temperature change at each point in open space between the areas with and without artificial heat release.