Hot Wire Anemometer

1
Hot Wire AnemometerIntroduction to Heat
Transfer
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Four Central Ideas of Engineering

• The Dual Variables of Effort and Flow
• how power flows between interacting objects,
  regardless of their domain (e.g. Electrical,
  Mechanical, Thermal, Biological, etc).
• State
• how systems remember the past, and which results
  from the time integration and storage of energy.
• Transduction
• the bidirectional transformation of effort and
  flow from one domain to another.
• Feedback
• used in almost all engineered devices to bring
  about desired behavior despite undesired
  disturbances.

3
What is heat?

• First Law of Thermodynamics
• Change in energy heat put in system work
  done on system.
• Attributed primarily to Joule in
  mid-1800s-overturned caloric theory
• Heat transfer energy in transit due to a
  temperature difference.
• So what is energy?

4
Conduction

• Transfer of energy from more energetic to less
  energetic particles of a substance due to
  interactions between particles.
• Gas T molecular motion, direct particle
  collisions equilibrate energy.
• Liquid same as gas, but molecular interactions
  are stronger and more frequent.
• Solid lattice vibrations.

5
Temperature distribution in a solid
heat flow
Inside of house is warm
Outside is cold
298 K
273 K
Temperature distribution is linear
k thermal conductivity (Watts/ m K) T
temperature (K) q heat flux vector (Watts/m2)
L distance (m)
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Convection
Tair
y
Cool air flow
q
Tplate
T
Hot Plate
h convection coefficient Watts/m2
(function of everything)
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Combined modes
Thot
T2
T1
Tcold
T
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Radiation

• Energy emitted by matter that is at a finite
  temperature.
• Emission attributed to changes in electron
  configurations.
• Energy transported by electromagnetic waves.
• No medium needed.

9
Conservation of Energy
Energy in
Change in Energy stored
Energy out
Energy generated
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We know how to compute the energy terms
Total energy out due to surface heat transfer.
Total energy generated in an electrical resistor.
Change in energy stored in the solid m is mass,
C is specific heat.
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Model of cooling hot coffee in air
Control volume
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Model Cooling hot coffee in air
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Model Heating the stove coils
hA
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Simulation

• What determines final T?
• What is the role of thermal mass?
• What determines the rate of T increase?

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Hot wire/hot film anemometer
Hot film for liquids
Hot wire for gases
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Why do we want the probe so small?

• Resolution in space
• Resolution in time (high frequency)
• Dont want to disturb what we are measuring.

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Uses are found anywhere fluids flow

• Aerodynamics lift, drag
• Combustion IC, gas turbine engines
• Meteorology
• Fires and fire safety
• Ocean currents
• How bugs fly and how fish swim
• Turbulence (Richard Feynman, "the most important
  unsolved problem of classical physics." )
• Ordinary measurement tools, i.e. HVAC probes

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The stove
Heat flow out
Power in
hA
T (K)
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Results from changes..
T (K)
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Constant power device
P
A
Response time dictated by the response of the
system.
Constant power mode needs to wait for steady
state to correlate T to h
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Constant T device
P
Power is function of time
A
Constant T mode always at steady state -
instantly correlate P to h
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Proportional Control
kp
Power input
dT/dt
T
hA(T-Tair)
h
T-Tair
Physical system
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Proportional control
Kp1
Kp0.1
Kp0.01
Why cant p-gain be set to be very large?
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Time delay can cause instability
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What is happening?

• Cant make P-gain too high, time delay can cause
  instability.
• If P-gain is too low there is high offset.

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Our lab

• Tungsten filament has resistance that changes
  with temperature if we measure the resistance we
  measure the T.
• Build a circuit to measure the filament
  resistance.
• Create a controller to provide a known amount of
  power (Voltage control does not work since R
  varies)
• Create a proportional controller to keep R
  constant.

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Circuit to measure R and P
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How do we get the actual power?
P
x
-1/R1
Circuit
simulink
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Virtual op-amp
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Power control
P
x
i lamp
-1/R1
Circuit