Quiz 8

Question 1

What are the three heat transfer modes?

Answer 1

Convection, conduction and radiation

Question 2

What are the three distinct modelling strategies for the near-wall region?

Answer 2

o Integrate to the wall (very fine mesh)
o Wall-function approach (fine mesh)
o Engineering correlations (coarse mesh)

Question 3

What near-wall approach does FDS uses as standard?

Answer 3

Engineering correlations

Question 4

When is it appropriate to calculate the convective heat transfer directly from the gas and solid variables?

Answer 4

When you can run DNS, which means you can resolve the actual boundary layer

Question 5

What does the Nusselt number represent?

Answer 5

o Nu represents the enhancement of heat transfer through a fluid layer as a result of convection relative to conduction across the same fluid layer
o The greater Nu, the more effective the convection
o 𝑁𝑢 = 1 represents heat transfer across the layer by pure conduction

Question 6

What are the different layers within the boundary layer?

Answer 6

o Outer layer
o Fully turbulent region or log-layer
o Sublayer + buffer layer

Question 7

What are the drawbacks to the conventional wall functions?

Answer 7

o Poor results in non-equilibrium flows
o Near-wall cell size dependency
o Does not take into account such effects as:
▪ pressure gradients
▪ convective transport
▪ gravitational forces

Question 8

Which of the drawbacks to the conventional wall functions do you see being a problem (if any) in FDS?

Answer 8

o Cell size dependency
o Flow is often not in equilibrium
o Gravitational forces are acting (buoyancy)