Thermodynamics- External Flow Over Bluff Objects

Question 1

What is a bluff object?

Answer 1

One which a fluid flows across

Question 2

What does Pr actually measure?

Answer 2

Viscous diffusivity over heat diffusivity. Equal to thickness of velocity boundary layer over thermal boundary layer.

Question 3

What does Nu actually measure?

Answer 3

Heat transfer due to convection over heat transfer from conduction.

Question 4

Formula for Stanton number, St and what is means

Answer 4

St=Nu/RePr=h/ρuCp

Heat transfer due to convection over heat capacity

Question 5

What is the stagnation point for a flow over a bluff object?

Answer 5

Location of zero velocity and maximum pressure

Question 6

Describe the formation of a separation point for flow over a cylinder

Answer 6

Initially round the cylinder, there is a favourable pressure gradient as the pressure decreases from the stagnation point and the free stream velocity increases. Pressure reaches minimum and increases around the cylinder again so decelerates. At point where velocity gradient is 0 on surface is separation point and boundary layer separation occurs. Beyond this there is flow reversal and a downstream wake.

Question 7

Does separation occur nearer or further back with turbulence compared to laminar flow

Answer 7

Further round with turbulence

Question 8

When can the formula for average Nu by Churchill and Bernstein be used?

Answer 8

For flow over cylinders with all properties at film temperature.
Formula in week 5 notes page 7

Question 9

What area do you use to convert heat flux into heat exchange for flow over cylinder?

Answer 9

Area around the long face, not the end faces

Question 10

For flow across tube banks, what do ST, SL, SD and D mean?

T, L and D subscript

Answer 10

ST is transverse pitch (vertical distance between tube centres in same bank)
SL is longitudinal pitch (horizontal distance between tube centres in adjacent banks)
SD is absolute difference between centres of closest tubes in adjacent banks
D is diameter of tubes

Question 11

Where to find formulae for maximum velocity for aligned and staggered banks of tubes

Answer 11

Week 5 page 14

Question 12

Is there a formula for average Nu for entire tube bank?

Answer 12

Yes on week 5 page 15
Constants needed are in table 7.5 of incroprera
Properties evaluated at temperature inlet+outlet over 2 apart from Prs at surface temperature.