風場對養殖池的曝氧研究

Wind Effects on Oxygen Transfer in Aquaculture Pond

陳昇(Sheng Chen)

指導教授 ： 張文亮

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00977

摘要

養殖魚塭在一般的環境當中會因太陽的照射產生分層現象，造成水體的密度改變，阻隔水層間的物質傳輸。以養殖業者角度來說，密度的層化會造成溶氧上
混合的限制，使其無法傳輸至底層水體，也會使有效養殖區降低。因此養殖業者會使用曝氣設備破除水體分層現象，提供水體擾動的機會。然而，水體的混合不
一定要透過曝氧設備，可以藉由自然風產生風趨流使水體混合。
本研究以現地實驗連續監測 24 小時無養魚無曝氣、無養魚有曝氣以及有養魚有曝氣的溶氧與溫度。研究結果發現無養魚無曝氣的魚池在白天會產生分層現象，但到下午 6 時至下午 10 時這段時間受到風的影響使得水體發生擾動，將表水層的溶氧傳輸至底部溶氧並快速混合。以 250 公分的水深來說，溶氧傳輸係數可達 0.156/hr。在無養魚有曝氣池中，因曝氣設備將水體混合並穩定增加溶氧，影響夜晚溶氧降低的主因來自於浮游生物與藻類的呼吸作用，溶氧傳輸係數的分層變化幾乎一致，整體的溶氧傳輸係數為-0.046~0.052/hr。在有養魚有曝氣池中，因魚類的對氧的消耗量大，需要曝氣設備穩定的供氧，整體的溶氧傳輸係數較前者來的變化大(-0.034~-0.252/hr)，但在分層上的改變差異很小。
透過室內實驗探討風對於水體混合與氧氣傳輸的效率，實驗結果發現隨著風速增加，溶氧傳輸係數也越大。從風 速造 成的溶 氧傳 輸倍 數來看 ，在風速
0.28-2.93m/s(U10:0.46~4.83m/s)時，就能在 DO 0-2mg/L 產生均勻的混合。但若要在 DO 2-4mg/L 的區間產生均勻的混合，風速須增至 2.93-3.66m/s(U10:4.83~6.04 m/s)。溶氧的區間越接近飽和溶氧區，風對水體的溶氧傳輸效果會降低，如需達到更均勻的混合，風速須提升。溶氧傳輸係數產生變化 不只出現在表水層，水深50與100公分也有明顯增加的現象。從室內實驗可了解風所造成水體的擾動最少可以到達水深100公分。

關鍵字

層化作用 ； 風趨流 ； 溶氧傳輸係數

並列摘要

In general, the separation in the fish ponds may occurred due to the density inversion caused by the solar radiation, which the interrupted the mass transportation between different layer of the water in the fish ponds. For the fishery, the limitation of the oxygen dissolution emerged once the water layers of density inversion were taken place, which decreased the efficiency of pond fishery. Therefore, the aeration system is equipped to promote the water circulation and to avoid the density inversion in the fishery. However, the aeration system the way, the wind-based mixing is another prospective way out.
In the research site, this study monitored dissolved oxygen and temperature continuously in 24 hours in 3 different states of fish ponds, no fish and unexposed oxygen pond, no fish and exposed oxygen pond, exposed oxygen fish pond. The results showed that there has the stratification in the no fish and unexposed oxygen pond in the daytime. However, the disturbance of the water body affected by the wind occurs at 18:00 to 22:00, and makes the dissolved oxygen mix from the surface to the bottom of the water body rapidly. At 250 cm depth, the oxygen transfer coefficient is up to 0.156/hr. In the no fish and exposed oxygen pond, the oxygen exposure devices blended the water and the dissolved oxygen increase steadily. The reason of the dissolved oxygen reduction in the night is the respiration of the plankton and algae. Oxygen transfer coefficients almost the same in different layer of water body, and the overall oxygen transfer coefficients is -0.046 ~ 0.052/hr.。In the exposed oxygen fish pond, there need the oxygen exposure devices because of the big oxygen consumption of fish. The change of the overall oxygen transfer coefficients(-0.034 ~ -0.252/hr) is large than the former pond, but the change is small in the different layer of water body.
The other part of the study conducted in the defined condition in the tank, to evaluate the efficiency of oxygen transportation and the wind-based mixing of the water layers. The oxygen transfer coefficient in the pond increased with the velocity of the wind. When the velocity reach 0.28-2.93 m/s(U10:4.83~6.04 m/s), the DO concentration interval 0-2 mg/L was evenly mixed; while the DO concentration interval 2-4 mg/L evenly mixed, the velocity of the wind should be set at 2.93-3.66 m/s(U10:4.83~6.04 m/s). When the oxygen concentration reached saturation, thus, the efficiency of the wind-base mixing decreased. The elevated velocity of wind is needed to gain DO of the system more even. The phenomenon is consistent in the surface water layer and in the shallow water layer (50-100 cm). The wind-based mixing is proved to be effective in mixing the DO concentration in the water tank to the extent of 100 cm deep.

並列關鍵字

stratification ； wind driven flow ； oxygen transfer coefficient

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