熱對流

熱對流(Convection)是指溫度不同的固體表面與宏觀流動的流體(氣體或是液體)之間的熱傳現象,同時也涵蓋流體本身因為溫度差異造成的密度差,致的流體混合與能量傳遞現象。下圖所示是流體流經不同溫度的固體表面,此間的熱傳機制。假設兩端的溫度穩定,則局部的熱通量(Local heat flux )可記為

Convection-1

局部的熱傳量和熱對流係數h 會隨著流體和固體表面的狀態而變化,而總對流熱傳量可以整理為

Convection-2

其中

Convection-3

 

熱對流係數用來表示對流熱傳的強弱,物理意義是流體和固體表面有每度的溫差,單位面積(m²)的熱能傳量(W)。 影響熱對流係數的因素包括:

  • 流體運動狀態,流速和方向等等
  • 流體的種類與其物理特性
  • 熱傳固體的形狀和表面狀態

下表為熱對流係數的範圍與參考

Convection-4

Fluids Convection coefficient (w∕m² ∙K)
Air / Natural convection
5~25
Air / Forced convection
20~120
Water / Natural convection
100~1200
Water / Forced convection
300~10,000
Water / Phase change
3,000~20,000

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