Chapter 7 External Forced Convection Flashcards
What does a Nu number of 100 mean
100 x more convection than conduction, 100 x more heat transfer than if fluid was not moving
What is drag
the force a flowing fluid exerts on a body in the flow direction
What makes up lift
components of the pressure and wall shear forces in the normal direction to flow tend to move the body in that direction
What does the drag force depend on
the density of the fluid, the upstream velocity V, the size, shape and orientation of the body among other things
Drag coefficient =
Cd = Fd/ 0.5rhoV^2*A
what is the part of the drag that is due directly to wall shear stress
skin friction drag (or just friction drag)
what is the part of the drag that is due directly to pressure
pressure drag
What is the total drag coefficient equal to
Cd = Cdfriction + Cdpressure
What is most drag due to at low reynolds numbers
friction drag
What is friction drag proportional to
surface area
what is pressure drag proportional to
frontal area and the difference between pressures acting on the front and back of the immersed body
Pressure drag is usually dominant and negligible for what type of bodies
dominant for blunt bodies and negligible for streamlined bodies
When a fluid separates from a body it forms a
separated region between the body and the fluid stream
What happens as the separated region increases
the pressure drag increases
Critical reynolds number for flat plate
5e5, where the distance is from the start of the plate
Local nusselt number for laminar =
isothermal and smooth
0.332 Re(x)^0.5 * Pr ^1/3 for Pr >0.6
Local nusselt number for turbulent=
isothermal and smooth
0.0296 Re(x)^085 * Pr ^1/3 for 0.6<= Pr <= 60 and 5e5 <= Re <= e7
How do the local friction and heat transfer coefficient compare in laminar and turbulent flow
both are higher in turbulent flow
Draw the diagram of convection heat transfer coefficient and local friction coefficients over a flat plate
(isothermal and smooth)
see diagram
Average nusselt number for laminar =
isothermal and smooth
0.664 Re(L)^0.5 * Pr ^1/3 for Re(L) < 5e5
Average nusselt number for turbulent=
isothermal and smooth
0.037 Re(L)^0.8 * Pr ^1/3 for 0.6 < Pr < 60 and 5e5 < Re(L) < e7
Laminar and turbulent Nu
isothermal and smooth
(0.037 Re(L)^0.8 - 871) Pr^1/3 for 0.6 < Pr < 60 and 5e5 < Re(L) < e7
Why do we normally not consider the transition region
as it occurs so quickly we cant see it
What is the peclet number
Pe(x) = Re(x)*Pr
Nu for laminar
flat plate subject to uniform heat flux
0.453 Re(x)^0.5 * Pr ^1/3 for Pr > 0.6 Re(x) < 5e5
Nu for turbulent
flat plate subject to uniform heat flux
0.0308Re(x)^0.8 * Pr ^1/3 for 0.6 < Pr < 60 and 5e5 < Re(L) < e7
When heat flux is prescibed the rate of heat transfer to form the plate and the surface temperature at a distance x can be determined from
q.s = h(x) (Ts(x) - Tinf)
Critical reynolds number for flow across a cylinder or sphere
2e5, but laminar boundary layer separation with a turbulent wake flow over a cirular cylinder at 2000
What two things are significant for flow over a cylinder or sphere
friction and pressure drag
Effect of surface roughness on drag coefficient
increases it in turbulent flow in general, however for blunt bodeies such a circular clylinder of sphere an increase in surface roughness may increase or decrease the drag coefficient depending on the reynolds number
What is it difficult to use an analytical approach to find the heat transfer coefficient of a cylinder or sphere
generally involved flow separation which is difficult to analyse, instead several empirical correlations have been developed for the heat transfer coefficient
At what temperature are fluid properties evaluated for when calculating Nu over a cylinder
film temperature Tf = 1/2 (Tinf + Ts)
