Convection

Question 1

Equation for Resitiance (conv)

Answer 1

R(conv) = 1 / hAs

As - surface area
h - thermal convection

Question 2

Equation for shear stress, τ

Answer 2

τ = μ du/dy

Question 3

Definition and equation of Reynolds number

Answer 3

Definition:
Ratio of inertial and viscous forces

Re = Inertia forces / viscous forces

Re = ρuL / μ

Question 4

General equation for Q.conv

Newton’s law of cooling

Answer 4

Q.conv = hA(s) * (Ts - T∞)

h = heat transfer coefficient (found experimentally)

Question 5

Name three types of flow

Answer 5

Laminar
Transitional
Turbulent

Question 6

Definition and equation of Prandtl number

Answer 6

Definition:
Pr = Thickness of momentum boundary layer /
Thickiness of thermal boundary layer

Pr = μc / k (in LBOTF)

Question 7

What is the Prandtl number for liquid metals due to heat diffusing very quickly

Answer 7

Pr &laquo_space;1

Question 8

Definition and equation of Nusselt number

Answer 8

Definition:
Ratio between convection heat transfer and conduction heat transfer

Nu = hL/k

for flat plate:
Nu = 0.332Pr^1/3 Re^1/2

Question 9

What does it mean when Nu = 1

Answer 9

Heat transfer is pure conduction

Question 10

Coefficient of friction, Cf = f(Re)

what is this for a flat plate

Answer 10

Cf = 0.664Re^(-1/2)

Question 11

What is film temperature

Answer 11

All properties of fluid are calculated at film temp

Tf = (T∞ + Ts) / 2