External Flow: Flow over Bluff Objects Cylinders, Sphere, Packed Beds and Impinging Jets

1
External FlowFlow over Bluff Objects
(Cylinders, Sphere, Packed Beds)andImpinging
Jets

• Chapter 7
• Sections 7.4 through 7.9

2
Cylinder in Cross Flow
The Cylinder in Cross Flow

• Conditions depend on special features of
  boundary layer development, including
• onset at a stagnation point and separation,
  as well as transition to turbulence.

3
Cylinder in Cross Flow (cont.)
and is accompanied by flow reversal and a
downstream wake.

• Location of separation depends on boundary
  layer transition.

4
Cylinder in Cross Flow (cont.)

• What features differentiate boundary
  development for the flat plate in
• parallel flow from that for flow over a
  cylinder?

• Force imposed by the flow is due to the
  combination of friction and form drag.

The dimensionless form of the drag force is
5
Cylinder in Cross Flow (cont.)

• Heat Transfer Considerations

• The Local Nusselt Number

• Churchill and Bernstein Correlation

• Cylinders of Noncircular Cross Section

6
Tube Banks
Flow Across Tube Banks

• A common geometry for
• two-fluid heat exchangers.

• Aligned and Staggered Arrays

Aligned
Staggered
or,
7
Tube Banks (cont.)

• Flow Conditions

How do convection coefficients vary from
row-to-row in an array?
How do flow conditions differ between the two
configurations?
Why should an aligned array not be use for ST/SL
lt 0.7?

• Average Nusselt Number for an Isothermal Array

8
Tube Banks (cont.)

• Fluid Outlet Temperature (To)

• Total Heat Rate

• Pressure Drop

9
Spheres and Packed Beds
The Sphere and Packed Beds

• Flow over a sphere

• Boundary layer development is similar to that
  for flow over a cylinder,
• involving transition and separation.

• What are the limiting values of the Nusselt
  number and the convection
• coefficient for slow flows over small
  spheres?

• Gas Flow through a Packed Bed

• Flow is characterized by tortuous paths through
  a bed of fixed particles.

• Large surface area per unit volume renders
  configuration desirable for
• the transfer and storage of thermal energy.

10
Spheres and Packed Beds (cont.)

• For a packed bed of spheres

11
Jets
Gas Jet Impingement

• Characterized by large convection coefficients
  and used for cooling and heating
• in numerous manufacturing, electronic and
  aeronautic applications.

• Flow and Heat Transfer for a Circular or
  Rectangular Jet

Significant Features

• Mixing and velocity profile development in the
  free jet.

• Stagnation point and zone.

• Velocity profile development in the wall jet.

12
Jets

• Local Nusselt number distribution

• Average Nusselt number

• Jet Arrays

Slot Jets

• What is the nature and effect of jet
  interactions and discharge conditions?

• Nusselt number correlations for arrays of
  circular and slot jets ? Section 7.7.2.

13
Problem Extrusion Process
Problem 7.63 Cooling of extruded copper wire
by convection and radiation.
14
Problem Extrusion Process (cont.)
15
Problem Extrusion Process (cont.)
16
Problem Extrusion Process (cont.)
17
Problem Extrusion Process (cont.)
18
Problem Thermocouple Junction
Problem 7.78 Measurement of combustion gas
temperature with a spherical thermocouple
junction.
19
Problem Thermocouple Junction (cont.)
20
Problem Thermocouple Junction (cont.)
21
Problem Thermocouple Junction (cont.)