Lecture 6 - External Forced Convection and Natural Convection

Question 1

What are external flows?

Answer 1

Flows over objects

Question 2

What is natural or free convection?

Answer 2

In the absence of an external means of pushing the fluid to flow, such as pumps and fans, the convection is termed natural or free.

Question 3

Examples of natural?

Answer 3

1. The weather and its daily seasonal changes are the result of natural heat convection between the surface of the earth and the atmosphere.
2. Any electronic device without a fan.
3. Heating and air conditioning.

Question 4

Examples of external flows?

Answer 4

Gas turbine blade cooling, heat exchangers.

Question 5

In all forced convection problems what is Nu a function of?

Answer 5

Nu(Re,Pr)

Question 6

For flows over a flat plate what is Re?

Answer 6

Re = Rex=Ux/v where x is the distance from the leading edge of the plate.

Question 7

What is film temperature?

Answer 7

The arithmetic average of the bulk flow temp and the solid surface temp (Tf = (Ts + Tinfinity)/2).

Question 8

What is Rec?

Answer 8

Critical reynolds number, the value of Rex for which transition begins. Rec = rho.u_inf.x_c/mu = 5x10^5. If Rex < Rec, flow is laminar.

Question 9

For laminar flow boundary layer over an isothermal plate Nux=?

Answer 9

Nux = h_x.x/k = 0.332Rex^0.5Pr^0.333

Question 10

For turbulent flow boundary layer over an isothermal plate Nux=?

Answer 10

Nux= 0.0296Rex^(4/5)Pr^(1/3) where 0.6</=60

Question 11

What does the flow field over a cylinder involve?

Answer 11

DRAW DIAGRAM. 1. A forward stagnation point in which flow comes to rest

2. A laminar boundary layer
3. Transition to turbulent boundary layer
4. A wake region behind the cylinder in which flow is completely chaotic and disordered.

Question 12

For overall heat transfer from/to the cylinder ave Nu_d=?

Answer 12

Nu_d= hD/k = CRe^(m)Pr^(1/3)

Question 13

What is maximum Reynolds number defined as?

Answer 13

Red,max=rho.Vmax.D/mu

Question 14

For alligned configurations what is Vmax?

Answer 14

Vmax= (St/St-D).V. Draw diagram

Question 15

For staggered configuration Vmax is?

Answer 15

Vmax is the maximum of the previous relation and: Vmax = (St/2(Sd-D))V. Draw diagram

Question 16

How does convection occur without any external means of pushing the fluid?

Answer 16

The rise and sink of fluids with different temperatures sets convection currents (fluid motion). The required force for motion is supplied by buoyancy due to the difference in density of hot/cold fluid and the surrounding fluid.

Question 17

What is Grashof Number?

Answer 17

Motion of a fluid induced by natural convection forms a boundary layer over the solid surface. This boundary layer is characterised by the so called Grashof number which is defined as: Grx = gBeta(Ts-Tinf)x^3/vu^2 where Beta is the volumetric thermal expansion coefficient and is a thermophysical prop of the fluid. Grashof number is a measure of relative strength of buoyancy and viscous forces.

Question 18

For ideal gases what is beta?

Answer 18

Beta = 1/T

Question 19

What is Gr equivalent to in forced convection?

Answer 19

Re ==> in natural convection: Nu=Nu(Gr,Pr).

Question 20

What is Rayleigh number?

Answer 20

Ra_x=Gr_x.Pr=g.Beta.(Ts-Tinf).x^3/vu.alpha