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Heat and mass transfer -Convective transfer.pptx
- 1. INTRODUCTION: • Conduction and convection are similar in that both mechanisms require the presence of a material medium. But they are different in that convection requires the presence of fluid motion. • Heat transfer through a solid is always by conduction, since the molecules of a solid remain at relatively fixed positions. • Heat transfer through a liquid or gas, however, can be by conduction or convection, depending on the presence of any bulk fluid motion. • Heat transfer through a fluid is by convection in the presence of bulk fluid motion and by conduction in the absence of it.
- 2. Convection: The process of heat transfer between a surface and a fluid flowing in contact with it, is called convection. Newton’s Law Of Cooling: The rate equation for the convective heat transfer between a surface and an adjacent fluid is prescribed by Newton’s law of cooling Q = hA(Ts - T) Where, A –Area exposed to heat transfer in m2, h - heat transfer coefficient in W/m2K Ts – Temperature of the surface in K, T - Temperature of the fluid in K
- 3. Forced Convection: If the flow is caused by an external device like a pump or blower, it is termed as forced convection.
- 4. Free Convection: If the flow is caused by the buoyant forces generated by heating or cooling of the fluid the process is called as natural or free convection.
- 5. Forced External flow Reynolds Number Critical Reynold's Number Nusselt Number from DB Correlation u N = ℎ𝐿 𝑘 h Internal flow Reynolds Number Critical Reynold's Number Nusselt Number from DB Correlation u N = ℎ ℎ𝐷 𝑘 h
- 6. Free or Natural flow in closed Enclosures Grashoff Number Nusselt Number from DB Correlation u N = ℎLc 𝑘 h External flow Grashoff Number Nusselt Number from DB Correlation u N = ℎLc 𝑘 h Critical Rayleigh Number
- 17. Dry air at 80°C blows over a copper sphere at 320°C at a velocity of 14 m/s. The diameter of the sphere is 25 mm. Determine the heat transfer rate at that instant.
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