Lecture 1 & 2- Principles of convection (KARIMI)

Question 1

What is convection?

Answer 1

If a moving fluid is in contact with a solid or fluid at a different temperature, heat is convected by the moving fluid.

Question 2

Does conduction still exist during convection?

Answer 2

Yes but usually in moving fluids convection is much stronger

Question 3

What is newtons law of cooling?

Answer 3

q=hAdeltaT
q = rate of heat transfer [W]
h= convection coefficient [W/m^2.K]
A= area over which heat is transferred [m^2]
deltaT = temp difference between the solid surface and the fluid

Question 4

What is a thermo-physical property?

Answer 4

as soon as the material type and its temp are known, the property can be found in tables.

Question 5

Is the convection coefficient a thermo-physical property of the fluid?

Answer 5

No, it also depends on the nature of the flow

Question 6

What happens when a uniform flow with constant velocity and temp flows over a flat wall (fluid is viscous)?

Answer 6

The first row of molecules sticks to the solid surface (viscosity) u_(y=0)=0. Friction between the first and second rows: retardation (loss of momentum). Friction between the third and second rows: less retardation. Other layers get similarly retarded. At boundary layer thickness u/u(original)=0.99

Question 7

Are viscosity effects very significant inside the boundary layer?

Answer 7

YES, there is a large velocity gradient within the boundary layer

Question 8

Equation for viscous stree?

Answer 8

tau = mu.du/dy

Question 9

Equation for skin friction coefficient?

Answer 9

Cf = tau/0.5rhou(original)^2

Question 10

Are viscosity effects significant outside the boundary layer?

Answer 10

No viscous effect, no velocity gradient, no shear stress.

Question 11

How does boundary layer thickness change along the surface?

Answer 11

As the molecules get retarded more fluid penetrates into the BL and so boundary layer thickness increases along the surface

Question 12

Does the bernoulli-eqn hold inside and outside the boundary layer?

Answer 12

only holds outside the BL

Question 13

What happens to the temp of the stationary layer of molecules?

Answer 13

Take the wall temp

Question 14

What happens between the retarded fluid layers?

Answer 14

There is conduction between the retarded fluid layers. Therefore there are strong temp gradients inside the thermal boundary layer

Question 15

What is the temp at the edge of the thermal boundary layer?

Answer 15

(Ts-T)/(Ts-T(original)=0.99

Question 16

does the thermal boundary layer grow or shrink along the wall?

Answer 16

Grows

Question 17

Describe laminar boundary layer?

Answer 17

Flow is highly ordered and along well organised streamlines.

Question 18

What is the transition zone?

Answer 18

Flow starts to become irregular and shows complex behaviour of a mix of laminar and turbulent flow.

Question 19

Describe turbulent flow BL?

Answer 19

Disorderness, chaotic motion, high level or mixing and velocity fluctuations but the flow remains almost laminar in the vicinity of the wall (viscous or laminar sub-layer)

Question 20

What happens to velocity gradient and shear stress/viscous force as BL thickness increases?

Answer 20

Velocity gradient decreases and so does the surface shear stress/ viscous force.

Question 21

In the turbulent boundary layer is there a higher or lower convection coefficient and why?

Answer 21

Higher because velocity fluctuations in the turbulent boundary layer greatly enhance the transfer of heat and momentum.

Question 22

Can you draw the diagrams comparing laminar and turbulent boundary layers?

Answer 22

YES OR NO

Question 23

Equation for Reynolds number?

Answer 23

Re_x=rho.u(original).x/mu

Question 24

Equation for critical Reynolds number?

Answer 24

Re_c=rho.u(original).x_c/mu

where x_c is the critical length for which transition to turbulence begins

Question 25

What is Re_c?

Answer 25

The value of Re_x for which transition begins, Re_x flow is laminar.

Question 26

Combining Newton’s law of cooling and conduction heat flux in the stationary layer gives?

Answer 26

h = -k_f.(dT/dy)/(Ts-T(original))

Question 27

What is the non-dimensionalised coefficient of convection?

Answer 27

Nusselt Number : 
Nu=h.L/K_f
h = coefficient of convection
L = characteristic length defined according to the problem configuration
K_f = fluid thermal conductivity

Question 28

In forced convection, what is nusselt number a function of?

Answer 28

Nu = Nu(Re,Pr)

Question 29

Re is defined as what in relation to Nu?

Answer 29

Nu = U.L/v <–(nu)

Question 30

What is Prandtl number defined as?

Answer 30

Pr = v <–(alpha)

Question 31

What is alpha?

Answer 31

Thermal diffusivity, alpha = k_f/rho.Cp

Question 32

Types of properties are Pr and Re?

Answer 32

Pr - thermo-physical property of the fluid, Re - characteristic of the flow