Convective Heat Transport

1
Convective Heat Transport

• Heat transfer
• Conduction
• Convection
• Radiation, Latent Heat
• Heat flux calculation
• Requires ? Temperature and resistance (or
  conductance) to heat flow
• Objective
• Determine resistance (conductance) to heat flow

Bare Surface
2
Organisms in the Environment

• Develop methods for controlling conductances
• Feathers, coats, fat layers, leaf hairs, etc.
• Consider two surfaces
• Flat plate leaf surface
• Cylinder animal
• Simplification but useful for analysis
• Useful because we can use standard equations for
  analysis

3
Boundary Layer Theory

• Heat conductance through a layer of air is only
  dependent on
• Wind speed (u)
• Characteristic dimension of surface (see table
  7.5)
• k (constant)
• Derived from an analysis of the flow over the
  surface
• And.

4
Radiative Conductance (gr)

• Conductance to heat through radiation
• All objects emit energy based on the fourth power
  of their temperature
• gr is based solely on the object and air
  temperature (as long as surrounding objects are
  close to Tair).
• Take an average of the two
• Look up in gr Table A.3

5
Resistance Diagram (neglecting radiative
conductance)
Wind
Thermocouple
6
Conductance Diagram
Ta
gHa
Heat Flux
Wind
Ts
Thermocouple
7
If we no longer neglect the radiative
conductance?? - draw circuit diagram - write
equation for gH
8
Todays Lab
Convective heat flow from plate, cylinder, and
furry cylinder
Fine Thermocouple
Animal Fur
Cylinder
Plate
9
Convective heat loss from a bare body
- Governing equation from class?
10
Convective heat loss from a bare body
- Now account for radiative conductance!
11
Calculating conductances from a bare plate and
cylinder
In this lab we will establish a known heat flux
and calculate real values gHa and k
Then compare theoretical ks to calculated ks
12
Now lets add some fur to the system!

• - Draw the circuit analog to the system
• Write equation for gH
• Write the governing equation for
• convective heat loss

13
Conductance of heat through fur vs. still air

• Calculate conductance through fur (gHc)
• Use gHa that we calculated in the previous part
  (not theoretical)

14
Conductance of heat through fur vs. still air

• Calculate conductance through fur
• Then use equation of radial conductance through
  equal thickness of still air to compare.
• Equation 7.6 and Example 7.1