chap 3 heat transfer 热传递 - ecaa, ntu.pdfVIP

1 環控農業工程學教材 台大生機系方煒 Chap 3. Heat Transfer 熱傳遞 3-1. 介紹  熱力學：Based on the concept of equilibrium or infinitesimal deviations from equilibrium.  熱傳學：Arises only from non-equilibrium, specifically, from finite differences of temperature.  熱傳 (heat transfer) 、質傳(mass transfer) 、動量傳遞(momentum transfer) 、 電流輸送等均屬輸送現象 (transport phenomena) 之研究領域，其有共通的計 算公式。  Heat Transfer may be  Steady-state 穩態( ) ：Temperature and heat fluxes do not change as functions of time.  Steady-periodic-state (階段重現性穩態) ：conditions change with time in a regular fashion and periodically return to their starting conditions.  Transient-state暫態( ) ：不屬以上二者的狀態。  For most engineering designs of agricultural buildings, steady-state analysis is adequate at least as a close approximation. 3-1.1 熱傳導 (Thermal Conduction)  Thermal Conduction is the elastic collision in a fluid or the oscillations of atoms and transport of free electrons in a solid, continuous medium.  熱傳導是唯一可透過不透明固體的傳熱方式，其亦可發生於流體中，但僅限 於平流 (laminar flow)或擾流邊界層 (turbulent boundary layer)極靠近物體表 面的平流次層 (laminar sublayer)中。 3-1.2 熱對流 (Thermal Convection)  與流體相關，涉及流體內由一處傳至另一處的能量傳遞以熱對流 (Thermal Convection) 稱之，另有涉及流體與固體表面的能量傳遞以對流熱傳遞 (Convective heat transfer ) 稱之。  熱對流 (Thermal Convection) 主要係透過渦流運動 (Eddying motion) ，對流 熱傳遞 (Convective heat transfer ) 主要在邊界層 (boundary layer)中涉及渦 流運動與熱傳導。See Figure 3-1.  渦流運動主要發生於擾流層與局部的邊界層 又( 稱緩衝層，Buffer layer) ，熱 傳導則發生於平流次層 (laminar sublayer) 。 1 Chapter 3. 熱傳遞  透過外力如風扇或幫浦去帶動流體者稱之為強制對流 (Forced convection) ， 透過由於溫差所造成的密度差而產生能量傳遞者稱之為自然