Lecture 5 - Internal Flows - Heat Convection Coefficient (KARIMI)

Question 1

For fully developed, laminar flow with constant wall heat flux in a circular pipe, Nu=?

Answer 1

Nu = h.D/k_f = 4.36

Question 2

For fully developed, laminar flow with constant surface temperature in a circular pipe, Nu=?

Answer 2

Nu = h.D/k_f = 3.66

Question 3

Why is constancy of Nu limited to the fully developed region?

Answer 3

Within the entrance region the boundary layers (thermal and hydrodynamic) are growing axially and therefore the heat transfer coefficient varies with x.

Question 4

Why does the heat transfer rate decrease as x increases?

Answer 4

DRAW DIAGRAM. The thinner the boundary layer, the sharper the velocity gradient => it is easier for heat to be extracted from the surface when the boundary layer is narrow.

Question 5

How is the heat transfer coefficient in the entrance region usually expressed?

Answer 5

In terms of Graetz number: Gzd = (D/x)Re.Pr.

Question 6

For constant surface temp, the average Nu number from entrance to point L in the tube can be estimated by?

Answer 6

Nu = 3.66 + (0.065(D/L)Re.Pr)/(1+0.04[(D/L)Re.Pr]^(2/3))

Question 7

For analysing turbulent flow, convective heat transfer coefficient must be found by?

Answer 7

Using experimental correlations.

Question 8

When convection transport is in non-circular tubes, what should be defined?

Answer 8

An effective diameter, often regarded as Hydraulic Diameter, defined as: Dh = 4Ac/P where Ac =cross sectional area, P = wetted perimeter.

Question 9

How should Nu be found for laminar flow in non circular tubes?

Answer 9

From tables.

Question 10

To enhance heat transfer rate what should be changed?

Answer 10

From newton’s law of cooling: q=hAdelT.. either convective heat transfer coefficient or heat transfer surface area must increase. (delT usually specified by application and cannot be changed).

Question 11

What are a few methods of heat transfer enhancement?

Answer 11

1. Introducing more turbulent flow and therefore increasing the convective heat transfer coefficient, such as increasing surface roughness to enhance turbulence by machining the surface or through insertion of a coil-spring wire.
2. Increasing the heat transfer surface area, which can be done by internal fins
3. Introducing a swirl through insertion of a twisted tape. The insert consists of a thin strip that is periodically twisted through 360 degrees. Introduction of tangential velocity component increases the fluid velocity, particularly near the tube.

Question 12

Disadvantage of these techniques?

Answer 12

Increased turbulence means increased pumping power is required due to friction.