都市街谷內噴霧降溫效果之模擬

Abstract

都市熱島效應使得都市空調需求劇烈增加，為了降低降溫的耗電需求，有許多研究探討除了冷氣以外的降溫方式，然而噴霧降溫被認為是眾多方法中較有效率且彈性的降溫方式。 噴霧顆粒與空氣的動量、能量、質量交換為二相流(two-phase flow)問題，可以利用計算流體力學的Lagrange-Eulerian 耦合模型模擬。本研究利用水槽實驗驗證街谷內流場，搭配已被驗證的粒子模型，模擬噴霧在都市街谷內的降溫效果。本研究主要的目標為模擬在高濕度 (相對溼度為70% 及 80%)、不同街谷高寬比的街谷內降溫情形，以符合台北都會區夏季的平均濕度情況。 模擬結果顯示不同街谷高寬比下，當相對溼度大於70%時，小顆粒的水珠噴出後會在短時間使空氣達到飽和，而大顆粒的水珠亦會使噴霧正下方空氣非常接近飽和，隨著噴霧高度增高(由2.5公尺增高至3.5公尺)，噴霧正下方空氣會達到飽和，並降溫至濕球溫度，也就是蒸發降溫的極限溫度。因此在台北都會區內，水珠粒子與噴霧高度並非需要考量的變因。由於都市街谷內風速較慢，無法帶走水珠粒子以及降溫後的空氣，因此受到噴霧影響最大的區域就是噴霧下方；另外在窄街谷內，由於冷空氣較容易聚集在街道內，因此街道中央的人可能可以感受到噴霧降溫的效果。

Urban heat islands rapidly increase energy demand for air conditioning. To reduce the energy demand for cooling the environment, some possible solutions have been studied and applied. Among these methods, the water spray system is considered most effective and flexible with its dynamic controls. To simulate the cooling effect of water spray system, numerical simulation with Computational Fluid Dynamics (CFD) is used. This simulation was validated with water channel and wind tunnel experiments. The goal of this study is to simulate the cooling effect in the street canyon with different aspect ratio in high relative humidity (70% and 80%) environment, which is often the case in Taipei city. The results showed that if relative humidity is larger than 70%, the air cooled by small water droplets was easily saturated. Large water droplets almost saturated the air just under the nozzles. If the nozzle height was increased from 2.5 m to 3.5 m, the air under the nozzles was completely saturated, and reached wet bulb temperature, which is the lowest bound of temperature. The coolest region is just below the nozzles because the wind in street canyon is too weak to blow the cold air away. However, in a narrow street, people may feel the cooling effect in the middle of the street because the accumulation of the cold air.