Heat Transfer

Question 1

Three main heat transfer mechanisms

Answer 1

Conduction, convection, radiation

Question 2

Reaction enthalpy

Answer 2

<0 egzothermic

>0 endothermic

Question 3

Relationship between heat generation and transfer

Answer 3

Heat is generated by egzothermic reaction (exponential), while heat transfer increases linearly with T = heat management is crucial because the system is unstable if heat generation isn’t equal to heat removal rate.

Question 4

Heat transfer surface area density

Answer 4

How much m2 for heat tr. area we can get per m3 of equipment.

Question 5

Heat exchanger types

Answer 5

Parallel flow
Cross flor
Cocurrent
Counter current

Total efficiency is highest in counter flow

Question 6

What is the Nu?

Answer 6

Ratio of heat transfer by convection and conduction. Constant for a fully developed flow.

Question 7

Why do studies show that Nu does depend on Re?

Answer 7

Because classical correlations assume the boundary layer is fully developed and that there is no conductive heat transfer. But there is, an additional heat flux exists opposite to the direction of flow and proportional to the thickness of the cross section.

Question 8

Heat transfer

Answer 8

A combination of conduction in the metal, convection in the fluid and surface roughness.

Question 9

What does conjugate heat transfer depend on?

Answer 9

Wall and fluid conductivity
Wall thickness / diameter ratio
Péclet number

Question 10

Pr

Answer 10

Ratio of thermal and hydrodynamic entrance region. If =1 then momentum transfer equals heat transfer.