# Hot wirehot film anemometer

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```					Hot wire/hot film anemometer

Hot film for liquids

Hot wire for gases
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
Use of hot wire in turbulence
The stove
Heat flow out

P                                      1/mC
1                              Power in
Net Power                        s      Temperature
constant                                              Integrator
power in                               Thermal mass                                Constant
1/mC
298
Step in convection
hA(T-Tair)
coefficient
hA(T-Tair)

Area        Product

A
hA
hA                        T-Tair
600

550

dE
 Net power
500

power (W)
dt                                                                        T (K)
450

dT (t )                                                                          400
mC p          P  hA(T (t )  Tair )
dt                                                                              350

300

250
0   20   40        60    80   100
time (sec)
Constant power device
1
P
-C-
Net Power
-K-
s      Temperature
constant                                      Thermal mass       Integrator
power in                                         1/mCp                                          Constant

298
Step in convection
coefficient
hA(T-Tair)

Area        Product

A                                                                     mC

-K-
T-Tair

hA
402

400

398
T (K)

396
Response time dictated
394
by the response of the
392                                                                       system.
390
0          2    4            6     8            10
time (sec)
Constant T device

Constant power mode needs to
 P  h(t ) AT (t )  Tair 
dT (t )                                 correlate T to h
mC
dt

 0  P (t )  h(t ) AT  Tair 
dT                                       Constant T mode “always” at
mC
correlate P to h
Proportional Control
proportional gain kp
Power input
-K-            Set Point

400

dT/dt         1
-K-
s                    T
1/mCp             Integrator
Constant

298
Step
hA(T-Tair)
h
Area              Product

-K-
T-Tair

Physical system
Proportional control
0.072
0.07
Kp=1
power (W)   0.068                            Kp=0.1
0.066
0.064                               Kp=0.01                mC
        seconds
0.062                                                      kp
0.06
4.8    4.9     5       5.1      5.2   5.3   5.4   mC  0.001 J/K
time (sec)

402

400

398
T (K)

396

394

392
4.8     4.9      5      5.1       5.2   5.3   5.4
time (sec)
proportional gain kp

1/1                             Set Point

400

1
-K-
s
1/mCp                Integrator
Pulse           Constant
Generator
298

Area                         Product
100 Constant1
-K-

0.07

0.069
power (W)

0.068

0.067

0.066

0.065                                                               Why can’t p-gain be
1.2   1.25      1.3       1.35     1.4   1.45
set to be very large?
time (sec)
Time delay can cause instability
proportional gain

1                                       Set Point

400

Transport
Delay

-K-
1                                                               0.08
s
1/mCp             Integrator
Constant           0.075
298
0.07

Power
0.065
Pulse
Generator
0.06

0.055

Product                100 Convection Coeff                                   0.05
Area
1   2    3     4   5
-K-                                                                                                     time
What is happening?
• Can’t make P-gain too high, time delay
can cause instability.
• If P-gain is too low there is high offset.
PI Control
Integral gain        Integrator1
1
-K-
s

proportional gain

-K-                                   Set Point

400
1     Out1

1   In1

Out1 In1

Out1
Controller
In1 1                                                                         1

1
-K-
s
1/mCp              Integrator
Constant
Power
In1 Out1
Temperature                                                                                                  298
0.1
0.01
Physical System
Random       0.01 Kelvin!
Noise
Pulse
Generator

Product                100 Convection Coeff
Area

-K-
Result with Integral Control

0.075
With proportional &integral control

0.07
power (W)

0.065

0.06                                            Steady state value (energy balance)
Proportional only

3        3.5          4          4.5            5       5.5          6         6.5
time (sec)
Results from changes..
600

550
Increase the power
Step change in h
Increase the power
500
T (K) (W)

450
power

400

350            Increase the thermal mass
Increase the thermal mass

300

250
0   20             40            60               80       100
time (sec)

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