Chapter 7 External Forced Convection

Question 1

What does a Nu number of 100 mean

Answer 1

100 x more convection than conduction, 100 x more heat transfer than if fluid was not moving

Question 2

What is drag

Answer 2

the force a flowing fluid exerts on a body in the flow direction

Question 3

What makes up lift

Answer 3

components of the pressure and wall shear forces in the normal direction to flow tend to move the body in that direction

Question 4

What does the drag force depend on

Answer 4

the density of the fluid, the upstream velocity V, the size, shape and orientation of the body among other things

Question 5

Drag coefficient =

Answer 5

Cd = Fd/ 0.5rhoV^2*A

Question 6

what is the part of the drag that is due directly to wall shear stress

Answer 6

skin friction drag (or just friction drag)

Question 7

what is the part of the drag that is due directly to pressure

Answer 7

pressure drag

Question 8

What is the total drag coefficient equal to

Answer 8

Cd = Cdfriction + Cdpressure

Question 9

What is most drag due to at low reynolds numbers

Answer 9

friction drag

Question 10

What is friction drag proportional to

Answer 10

surface area

Question 11

what is pressure drag proportional to

Answer 11

frontal area and the difference between pressures acting on the front and back of the immersed body

Question 12

Pressure drag is usually dominant and negligible for what type of bodies

Answer 12

dominant for blunt bodies and negligible for streamlined bodies

Question 13

When a fluid separates from a body it forms a

Answer 13

separated region between the body and the fluid stream

Question 14

What happens as the separated region increases

Answer 14

the pressure drag increases

Question 15

Critical reynolds number for flat plate

Answer 15

5e5, where the distance is from the start of the plate

Question 16

Local nusselt number for laminar =

isothermal and smooth

Answer 16

0.332 Re(x)^0.5 * Pr ^1/3 for Pr >0.6

Question 17

Local nusselt number for turbulent=

isothermal and smooth

Answer 17

0.0296 Re(x)^085 * Pr ^1/3 for 0.6<= Pr <= 60 and 5e5 <= Re <= e7

Question 18

How do the local friction and heat transfer coefficient compare in laminar and turbulent flow

Answer 18

both are higher in turbulent flow

Question 19

Draw the diagram of convection heat transfer coefficient and local friction coefficients over a flat plate
(isothermal and smooth)

Answer 19

see diagram

Question 20

Average nusselt number for laminar =

isothermal and smooth

Answer 20

0.664 Re(L)^0.5 * Pr ^1/3 for Re(L) < 5e5

Question 21

Average nusselt number for turbulent=

isothermal and smooth

Answer 21

0.037 Re(L)^0.8 * Pr ^1/3 for 0.6 < Pr < 60 and 5e5 < Re(L) < e7

Question 22

Laminar and turbulent Nu

isothermal and smooth

Answer 22

(0.037 Re(L)^0.8 - 871) Pr^1/3 for 0.6 < Pr < 60 and 5e5 < Re(L) < e7

Question 23

Why do we normally not consider the transition region

Answer 23

as it occurs so quickly we cant see it

Question 24

What is the peclet number

Answer 24

Pe(x) = Re(x)*Pr

Question 25

Nu for laminar

flat plate subject to uniform heat flux

Answer 25

0.453 Re(x)^0.5 * Pr ^1/3 for Pr > 0.6 Re(x) < 5e5

Question 26

Nu for turbulent

flat plate subject to uniform heat flux

Answer 26

0.0308Re(x)^0.8 * Pr ^1/3 for 0.6 < Pr < 60 and 5e5 < Re(L) < e7

Question 27

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

Answer 27

q.s = h(x) (Ts(x) - Tinf)

Question 28

Critical reynolds number for flow across a cylinder or sphere

Answer 28

2e5, but laminar boundary layer separation with a turbulent wake flow over a cirular cylinder at 2000

Question 29

What two things are significant for flow over a cylinder or sphere

Answer 29

friction and pressure drag

Question 30

Effect of surface roughness on drag coefficient

Answer 30

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

Question 31

What is it difficult to use an analytical approach to find the heat transfer coefficient of a cylinder or sphere

Answer 31

generally involved flow separation which is difficult to analyse, instead several empirical correlations have been developed for the heat transfer coefficient

Question 32

At what temperature are fluid properties evaluated for when calculating Nu over a cylinder

Answer 32

film temperature Tf = 1/2 (Tinf + Ts)