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Flow and Thermal Considerations

Flow and Thermal Considerations. When a moving fluid comes into contact with a surface at some temperature difference (e.g. heated surface, cold fluid) the fluid will transfer heat to/from the surface in a process analogous to conduction (random motion of the fluid )

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Flow and Thermal Considerations

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  1. Flow and Thermal Considerations • When a moving fluid comes into contact with a surface at some temperature difference (e.g. heated surface, cold fluid) the fluid will • transfer heat to/from the surface in a process analogous to conduction (random motion of the fluid) • advectheat away from the point of contact by its motion (bulk motion of the fluid) The combined effect of these two phenomena is called convection,which is described by Newton’s Law of Cooling fluid temperature [K] heat flux [W/m2] heat transfer coefficient [W/m2-K] surface temperature [K] • Implications: • the “direction” of heat flux can be considered normal to the surface • the heat transfer coefficient is related of the nature of the fluid flow • in order to study convection heat transfer we must also study fluid dynamics

  2. Convection Coefficients • There are local and average heat transfer coefficients • because flow conditions vary along a surface both the local heat flux (q”) and local heat transfer coefficient (h or hx) vary along the surface • The local heat transfer coefficient is defined as • The average heat transfer coefficient is defined as

  3. Fluid Dynamics: Boundary Layers approximate boundary layer thickness shear stress for Newtonian fluids spherical drag force over surface area μis the dynamic viscosity of the fluid • Velocity Boundary Layer (External Flow) • consequence of viscous effects associated with relative motion between a fluid and a surface • region of the flow characterized by shear stresses and velocity gradients • region between the surface and free stream whose thickness δ increases in the flow direction • manifested by a surface shear stress τsthat generates a drag forceFD.

  4. Fluid Dynamics: Boundary Layers approximate boundary layer thickness heat flux at the surface spherical heat transfer coefficient • Thermal Boundary Layer (External Flow) • consequence of heat transfer between the surface and fluid • region of the flow characterized by heat fluxes and temperature gradients • region between the surface and free stream whose thickness δt increases in the flow direction • manifested by a surface heat flux, q”sthat provides a convection heat transfer coefficient, h. kfis the thermal conductivity of the fluid

  5. Fluid Dynamics: Boundary Layers • Boundary Layer Equations • 2D, steady, incompressible flow • negligible body forces • constant & uniform fluid properties (μ,C,k) • Apply conservation of mass, momentum, and energy to differential control volume (dxdy)

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