1. Heat Transfer Overview

2. How Do We Define Heat Transfer?
Energy in transit due to a temperature difference
Electrical analogy
Concerned with time rate of change
In this we’ll calculate things such as
What rate of heat transfer will occur in a certain situation?
What temperatures can we expect?
How long will it take to heat/cool something?
What are some specific applications?

3. Examples of Where You May Use Heat Transfer in Industry
Estimating the temperature of components on a circuit board to determine if failure rates are acceptable and then designing the cooling system
Choosing a heat sink and fan that will cool a transistor appropriately
Determining the orientation and size of solar panels needed to power a micro-satellite
Designing the cooling system for an ice-hockey table
Choosing the type and size of heat exchanger to be used to heat a chemical used in semi-conductor manufacturing

4. More Examples…
Choosing the type and size of radiator needed for a new type of automobile
Calculating the optimal amount of insulation to be placed on a steam pipe to reduce heating costs
Determining how much power must be applied to an antennae on an airplane to keep water from freezing on it
Designing a quick-freezing system for a meat distributor to seal in flavor and juices
Calculating heat losses and gains from a building to size their heating and air conditioning systems

5. Types of Heat Transfer
Conduction: due to interaction between particles; usually through a solid
Convection: gas to solid (or vice versa)
Includes both conduction between particles and advection (heat transfer due to mixing)
Radiation: radiated energy (no medium needed)

6. Why not just use computer programs? Why take a class?
How will you set up the model? What boundary and initial conditions?
Are the simplifications chosen OK?
3-D convection at moderate/high velocities is impossible to solve without significant simplifications
How will you interpret the results? Are the results accurate?

7. Conduction
Energy transfer from more to less energetic particles due to particle interactions – diffusion of energy due to molecular activity
Examples
Usually involves solids
Rate = f( )
What is the driving potential?

8. Fourier’s Law of Heat Conduction
k=thermal conductivity (W/m°C or Btu/h ft °F)
-- a measure of how fast heat flows through a material
-- k(T), but we usually use the value at the average
temperature
q can have x, y, and z components; it’s a vector quantity

9. Special Case If T(x) is linear, Fourier’s Law for the 1-D case becomes
When will this happen?
Example

10. Conduction Definitions
Heat capacity = rcp (J/m3°C)
Amount of heat needed to raise a unit volume of material one degree
Thermal diffusivity = a = k/rcp (m2/s)
How fast heat diffuses through a material

11. Convection Energy transfer due to both Convection=diffusion+advection
molecular motion (diffusion, like conduction) and
bulk motion of fluid (motion of gas or liquid)
Advection
Convection=diffusion+advection
Three kinds
Forced convection – external fluid motion
Natural (free) convection – motion due to buoyancy effects
Latent heat exchange – due to phase change – condensation, boiling (covered in ME 211 but not ME 114)

12. Newton’s Law of Cooling
h=heat transfer coefficient (W/m2°C)
Ts=solid surface temperature
T =temperature of fluid far from surface
h=f( )

13. Boundary Layer

14. Example
A 0.4 cm x 2 cm computer chip must dissipate 5 W of heat. Air with a heat transfer coefficient of 80 W/m2K and a temperature of 20°C blows over the chip. The chip is in danger of overheating if it reaches 90°C. Is the chip in danger? Should you attach a heat sink?

15. Thermal Radiation Emitted by all matter above 0 Kelvin
Due to changes in electron configurations
Requires no medium
Emissive power of a blackbody (ideal radiator)
Ts=surface temp in Kelvin
s=Stefan-Boltzmann Constant 5.67x10-8 W/m2K4

16. Thermal Radiation, Cont.
e=emissivity: how efficiently a surface emits compared to a blackbody
a=absorptivity: percent of incident flux absorbed
e,a =f(temp, wavelength, surface condition)

17. Thermal Radiation, cont.
Special case: e=a if surface temperatures of all surfaces in an enclosure are close
Special case: surface completely surrounded by another isothermal surface, no intervening medium

18. Total Heat Transfer
Only conduction, convection, or radiation can occur or else a combination can occur simultaneously
Qconv+Qrad or Qcond+Qrad