Natural Ventilation of Wall Air Cavity for Solar Heat Gain Reduction : Part 4-Simulation of Transient Heat Transfer in Cavity Wall including Heat and Mass Transfer in Cavity

  • KADOYA Terunori
    Department of Regional Planning, Toyohashi University of Technology
  • HOMMA Hiroshi
    Department of Regional Planning, Toyohashi University of Technology

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Other Title
  • 壁内中空層の自然換気による日射熱排除効果 : 第4報-中空層付き壁体の中空層内気流および熱流計算を含めた非定常熱流計算
  • 壁内中空層の自然換気による日射熱排除効果-4-中空層付き壁体の中空層内気流および熱流計算を含めた非定常熱流計算
  • ヘキナイ チュウクウソウ ノ シゼン カンキ ニ ヨル ニッシャ ネツ ハイジ

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Description

Natural ventilation of a wall cavity is expected to be an effective measure to reduce solar heat penetration through a building envelope into the indoor environment. In the present part, a numerical simulation model was composed to discuss the effect of the cavity ventilation on the heat penetration reduction. The performance of the simulation was examined by comparing its results with the experimental results of the weather exposed models of a cavity wall. The simulation model consisted of the three parts as shown in Figure 3. The first part corresponds to the external cover plate, which receives the solar irradiation, dissipates some of the irradiation to the external air, and transfers the rest of the heat to the air in the cavity and also to the main wall surface. The second part simulates the heat and air mass transfer in the cavity. The third part simulates the transient heat conduction through the main part of the wall. The division pattern of the heat and air flow balance simulation in the cavity wall is shown in Figure 3. The structure of one of the weather exposed models is shown in Figure 2. The main part of the model is a massive concrete plate. The plate was arranged to direct one of its wide surfaces to the South. This surface was erected vertically to simulate a southern facade of a building. In front of this surface, a thin steel plate coated with dark brown paint was set placing a cavity of a thickness of 70mm between the surface and the cover plate. The temperature changes of the cover plate and those in the concrete plate are compared in Figures 6 and 7. The calculated temperatures were slightly lower than the experimental result in the daytime. The maximum difference in them was 4℃. The temperature change in the center of the concrete plate is shown in Table 1. The simulated temperature rise in the daytime (from 6 a.m. to 6 p.m.) was 8.4℃, while the corresponding value of the experimental result was 7.7℃ for the model of the slit width of 10mm. The simulated and experimental temperature rises of the same duration were 7.0 and 7.2℃, respectively, for the model of the slit width of 40mm. The temperature change in the cavity air is compared in Figures 8 and 9. The difference between the simulated and experimental temperatures of the cavity air was 7℃ at the maximum. This difference seemed to be caused mainly by the difference in turbulence intensities of the experimental condition and the simulation condition of the air flow in the cavity. The heat flux absorbed at the concrete surface is compared in Figures 10 and 11. The simulated heat flux appeared to be smaller than the experimental result through out the day. The simulated heat flux was smaller by 11% than the experimental heat flux at 11 a.m., when the heat flux showed its highest value, for the slit width of 10mm. This difference enlarged to 18% for the slit width of 40mm. This simulation method indicated good agreement of temperature change in the concrete plate with the experimental results. This simulation method seemed to be satisfactorily to examine the effect of cavity ventilation on the thermal performance of cavity walls under various climatic conditions.

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