External Convection - C HAPTER 7: E XTERNAL C ONVECTION In...

Info iconThis preview shows pages 1–6. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: C HAPTER 7: E XTERNAL C ONVECTION In external convection, boundary layers will always form freely since there is no adjacent surface to restrict boundary layer development b there is always fluid outside the boundary layer where the velocity and temperature are constant External convection: Internal convection: u u u u max < u u Midline b Examples: Flow over flat plate (inclined or parallel), around cylinders/spheres/turbine blades, banks of tubes, etc. Goal of this chapter: to derive h for external convection geometries The typical form of the expression is: n m C &u Pr Re = Prandtl number (Pr) : = k c p Pr Dimensionless value based only on fluid properties (i.e. independent of flow conditions or geometry) Normally found in tables Reynolds number (Re): = VL Re Relates fluid properties to the geometry of the object over (or through) which the fluid passes. The characteristic length L is the length of a plate ( x ) or diameter ( D ) of a sphere, cylinder or duct (boundary layer development) Nusselt Number: = k hL &u Relates the relative importance of convection versus conductive heat transfer from the vantage point of the fluid (i.e. k = k fluid ) Or: relative contribution of advective heat transfer versus conductive heat transfer in determining h Relates convection (advection b fluid flow + conduction) to conduction ( b fluid properties) Defines thermal boundary layer Defines material properties of fluid Relates velocity and thermal boundary layers Relates flow and object geometry to the physical fluid properties C f b Defines velocity boundary layer n m C &u Pr Re = The terms C , m , and n are determined experimentally ( empirically ) using an electrical heater to maintain constant T s ( T s > T ) and measuring T s , T , and P = IV required to keep T s constant By energy balance: q elec = q conv IV= h L A s ( T s-T ) Nu L = C Re L m Pr n Perform experiments over a range of different u , L ( Re ), and fluids ( Pr ) and plot log( Nu L /Pr n ) vs. log( Re L ) to find C (slope) and m,n values which give a linear relationship Log( Nu L ) Log( Nu L /Pr n ) Log( Re L ) Log( Re L ) Pr 1 Pr 2 Pr 3 IMPORTANT NOTE: All empirical correlations have significant errors (up to 25%) and constraints on their use (i.e. the range of conditions over which experimental data was linearly correlated with the given C, m, and n ) b Apply only over specific ranges of Re and/or Pr (or Pe = Re*Pr , where Pe = Peclet number = VL/ ) b Apply only to local vs. average convection coefficients b Apply only when fluid properties ( k , c p , , , Pr ) are evaluated at specific temperatures defined in the correlation definition; i.e....
View Full Document

Page1 / 30

External Convection - C HAPTER 7: E XTERNAL C ONVECTION In...

This preview shows document pages 1 - 6. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online