壁内中空層の自然換気による日射熱排除効果 : 第1報-中空層中に生ずる気流および熱流についての実測結果 [in Japanese] Natural Ventilation of Wall Air Cavity for Solar Heat Gain Reduction : Part 1-Experiment on Heat and Air Transfer in Cavity [in Japanese]
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Ventilation of an air cavity in a building envelope by natural force is expected to be an effective measure to release solar irradiation in a hot or arid district. An external surface of a wall absorbs solar irradiation, and transfers it to the air in the cavity. The warmed air gets buoyant force. So when openings are provided at the top and bottom of the cavity, the warmed air is released through the top opening and cooler outside air replaces the space in the cavity. This reduces the further heat transmission into the built environment. This natural ventilation effect seems to be steady and strong. So if the width of a cavity and the openings are properly designed, the cooling load reduction by natural ventilation is believed to be considerable. An experimental model of an air cavity was constructed to examine the natural ventilation effect. The model has height 2.4m, width 70mm and depth 0.45m. Electric heating panels were hurried in the both sides of the cavity. Slit shaped openings were provided at the top and bottom of the cavity. Temperature and velocity measuring facilities were prepared in the experimental model. A number of measurements were carried out by changing the combinations of heat production and slit width. When heat production on the both sides were 78W/sq.m, and slits were set to 40mm, the average air velocity was 0.28m/s. The average convective heat transfer coefficient was 2.4W/sq.m K. It was affected by the surface heat production rate, but it was little affected by the slit size and the air velocity. When slit width was gradualy reduced the air temperature rose, and the air velocity fell suddenly between slit widths 10 and 20mm. The influence of dynamic loss seemed to become critical between these slit widths. When heat was produced on one of the two surfaces, a strong upward air stream was caused at the near region to the heated surface. The velocity fell steeply in the central region, and the velocity was nearly null at the unheated surface. The velocity boundary layer was much thinner than the half of the cavity width. When the both sides were heated symmetrically, the air temperature was highest at the directly attached regions to the surfaces, and the lowest temperature was observed at the center. The buoyant force was strongest at the attached regions to the surfaces, and it was weakest at the center. The buoyant force, frictional resistance and velocity pressure loss influence each other, and they form the velocity distribution. The peaks of the velocity distribution were observed in the regions 5 to 10mm from the two surfaces. The velocity distribution showed a trough at the center. The effect of momentum difused and disappeared in a relatively short distance from the entrance, it was shorter than 550mm. In the longer distance, the two thin velocity boundary layers were formed along the two sides, and the air in the center was induced by the air movement of the velocity boundary layers. This experiment indicated that if a cavity width and opening size are appropriately arranged, the natural ventilation performs effectively to release solar irradiation to the outside.
- Transactions of the Society of Heating,Air-conditioning and Sanitary Engineers of Japan
Transactions of the Society of Heating,Air-conditioning and Sanitary Engineers of Japan 11(30), 91-102, 1986
The Society of Heating, Air-Conditioning & Sanitary Engineers of Japan