H&M CH 7: External Forced Convection

Q: What is free-stream velocity?

A: The velocity of fluid far upstream from a body, unaffected by the presence of the body.

Q: What happens to fluid velocity at a solid surface?

A: It becomes zero due to the no-slip condition.

Q: What is drag?

A: The force exerted by a fluid on a body in the direction of flow.

Q: What are the two main components of drag?

A: Friction drag (skin friction) and pressure drag.

Q: What causes friction drag?

A: Shear stress along the surface due to viscosity.

Q: What causes pressure drag?

A: Pressure differences between the front and back of a body.

Q: Drag force acting on a flat plate parallel to the flow depends on…

wall shear only

Q: Drag force acting perpendicular/normal to the flow depends on…

pressure only

Q: What factors affect drag force?

A: Fluid density, velocity, and body size, shape, and orientation.

What is drag coefficient?

A: A dimensionless number representing drag characteristics of a body.

What is the relation between friction drag and pressure drag to get the drag coefficient?

The part of drag that is due directly to wall shear stress t_w is called the skin friction drag (or just friction drag) since it is caused by frictional effects, and the part that is due directly to pressure P is called the pressure drag.

Q: What type of drag dominates at low Reynolds numbers?

A: Friction drag.

Q: What type dominates at high Reynolds numbers?

A: Pressure drag.

Q: What is a separated region?

A: A low-pressure region behind a body where flow recirculates.

Q: What is a wake?

A: The region behind a body where velocity is disturbed.

Q: Where are viscous effects most significant?

A: In the boundary layer, separated region, and wake.

Q: What is film temperature?

A: The average of surface and free-stream temperatures used for property evaluation. It is used to find table A-15 Values to complete problems in chapter 6 and 7.

Q: What determines flow regime over a flat plate?

A: The Reynolds number

Q: What is the typical critical Reynolds number range for a flat plate?

A: Between 10⁵ and 3 × 10⁶ and the generally accepted value for the Critical Reynold number is 5 X 10⁵

To use laminar flow equations, Reynolds number should be…

Re < 5 × 10⁵, w

To use turbulent flow equations, Reynolds number should be..

5 × 10⁵ ≤ Re ≤ 10⁷

Q: How do heat transfer coefficients compare between laminar and turbulent flow?

A: They are higher in turbulent flow.

Q: How does heat transfer coefficient change along a flat plate?

A: It decreases with distance from the leading edge.

Q: How does uniform heat flux affect heat transfer compared to isothermal surfaces?

A: It increases values (~36% for laminar, ~4% for turbulent).

Q: What is an unheated starting length?

A: A region at the beginning of a plate where no heat transfer occurs.

Q: What complicates analysis of flow over cylinders and spheres?

A: Flow separation and wake formation.

Q: What contributes to drag on cylinders and spheres?

A: Both friction and pressure drag.

Q: When is friction drag dominant for cylinders?

A: At very low Reynolds numbers (Re < 10).

Q: How does surface roughness affect drag?

A: It generally increases drag in turbulent flow.

Q: Where is cross-flow over tube banks commonly used?

A: In heat exchangers.

Q: What are the two main tube bank configurations?

A: In-line (square arrangement) and staggered (equilateral arrangement).

Q: What is the characteristic length for tube banks?

A: The outer tube diameter.

Q: What affects heat transfer in tube banks?

A: Tube arrangement and turbulence created by upstream tubes

Q: When is a correction factor needed in tube banks?

A: When the number of tube rows is less than 16.

Q: What determines pressure drop across tube banks?

A: Friction factor and correction factors for geometry.

What is log mean temperature difference used for?

Calculating heat transfer in heat exchangers.