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Transport Processes

Transport Processes. Slides prepared by Venkatesh Merwade. Sun. Energy. Energy. Energy & Mass. Energy & Mass. Energy & Mass. Mass. Land. Ocean. Energy & Mass. Transport Processes. Sun is the major source of energy in hydrologic cycle Energy transport takes place through: Conduction

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Transport Processes

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  1. Transport Processes Slides prepared by Venkatesh Merwade

  2. Sun Energy Energy Energy & Mass Energy & Mass Energy & Mass Mass Land Ocean Energy & Mass Transport Processes • Sun is the major source of energy in hydrologic cycle • Energy transport takes place through: • Conduction • Convection • Radiation

  3. Flux • The rate of flow of extensive property per unit area of surface through which it passes is called the flux. Volumetric flux Mass flux Mass flux = density x volumetric flux Momentum flux Momentum flux = mass flux x velocity Energy flux

  4. Conduction • Heat is transferred as molecules with higher temperature collide with lower temperature molecules • Results from random molecular motion in substances • Eg. Heating of earth’s land surface

  5. Conduction of mass, momentum and energy • Flux is proportional to the gradient of a potential Momentum flux (laminar flow) Newton’s law of viscosity Mass flux Fick’s law of diffusion Energy flux Fourier’s law of heat conduction m is dynamic viscosity, D is diffusion coefficient, and k is heat conductivity. Dynamic viscosity (m) is related to kinematic viscosity (n) as m = rn The direction of transport of extensive properties is transverse to the direction of flow.

  6. Convection • Energy transfer through the action of turbulent eddies or mass movement of fluids with different velocities. • Turbulence – mechanism causing greater rate of exchange of mass, energy, and momentum than molecular exchanges • Unlike conduction, convection requires flowing fluid • Eg. Convection causes vertical air circulation in which warm air rises and cool air sinks, resulting in vertical transport and mixing of atmospheric properties

  7. Convection of mass, momentum and energy Momentum flux (turbulent flow) • Km is 4-6 orders of magnitude greater than n. • tturb is the dominant momentum transfer in surface water flow and air flow. Km is momentum diffusivity or eddy viscosity Mass flux Kw is mass diffusivity Energy flux Kh is heat diffusivity The direction of transport of extensive properties is transverse to the direction of flow.

  8. Velocity Profile • Determining momentum transfer requires knowing velocity profile • Flow of air over land or water – log velocity profile Von Karman constant Roughness height Shear velocity Wall shear stress Velocity gradient

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