me 322 instrumentation lecture 32 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
ME 322: Instrumentation Lecture 32 PowerPoint Presentation
Download Presentation
ME 322: Instrumentation Lecture 32

Loading in 2 Seconds...

play fullscreen
1 / 30

ME 322: Instrumentation Lecture 32 - PowerPoint PPT Presentation


  • 84 Views
  • Uploaded on

ME 322: Instrumentation Lecture 32. April 11, 2014 Professor Miles Greiner. Announcements/Reminders. Next week: Lab 10 Vibrating Beam Extra-Credit LabVIEW Workshop Friday , April 18, 2014, 2-4 PM, Jot Travis Room 125D Sign-up on WebCampus

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'ME 322: Instrumentation Lecture 32' - tuan


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
me 322 instrumentation lecture 32

ME 322: InstrumentationLecture 32

April 11, 2014

Professor Miles Greiner

announcements reminders
Announcements/Reminders
    • Next week: Lab 10 Vibrating Beam
    • Extra-Credit LabVIEW Workshop
    • Friday, April 18, 2014, 2-4 PM, Jot Travis Room 125D
    • Sign-up on WebCampus
    • If enough interest then we may offer a second session
      • Noon-2
  • Did you know?
    • HW solutions are posted on WebCampus
    • Exam solution posted outside PE 213 (my office)
  • Help wanted (see me greiner@unr.edu)
    • Summer: Help construct a convection heat transfer augmentation experiment
    • Spring 2015: ME 322r Lab Assistant
cylinder in cross flow
Cylinder in Cross Flow

V∞

Velocity

Probe

  • Speed is reduced in the wake region
  • Instability of steady flow causes periodically-shed vortices
    • Karman Vortex Street
  • Figure shows unsteady speed measured by a probe in wake
    • Fairly regular oscillations, period P ~ 0.01/6 = 0.0017 sec
    • Peak oscillatory frequency of f = 1/P ~ 600 Hz
      • Broad spectrum of frequencies
    • Can a Pitot probe measure oscillations at these high frequencies?
      • How to measure rapidly changing speeds?
strouhal number
Strouhal Number

V∞

f

Q

  • What does the vortex shedding frequency depend on?
    • Increases with
    • Decreases with
  • Dimensionless Strouhal Number
    • ;
  • For , 0.20 < < 0.21 (~constant)
    • Frequency increases linearly with speed and flow rate
    • This phenomena used to measure pipe volume flow rate Q

D

Q

example
Example
  • A car in Reno is moving at 60 miles/hour and has a ¼-inch diameter antenna. At what frequency will vortices be shed from it? The air temperature is 27°C and the atmospheric pressure is 86 kPa.
  • 0.20 < < 0.21
    • For
how to measure rapidly varying speed
How to measure Rapidly Varying Speed?
  • Pressure Method
    • Pitot probes transmit pressure to transducers using tubes
    • Ok for slowly varying speeds
    • At high frequencies, pressure response at transducer is attenuated and delayed compared to probe (2nd order system)
  • Heat Transfer Method
    • Hot Wire or Hot Film probe
      • Very small wire or metal plated quartz on a support fork
    • Electrically heat surface
    • Heat transfer to the surrounding fluid increases with fluid speed
    • Two modes:
      • Constant Current
      • Constant Temperature
circuit
Circuit

V∞T∞

R2

I

I

VO

VE

TS RS

  • Probe electrical resistance heating
    • Q = IVO (can be measured)
  • Heat is mostly removed by convection
    • Q = IVO= hA(TS-T∞)
      • Neglecting radiation and conduction
  • Convection Coefficient for small cylinders in cross flow
    • ; M and N are constants
  • If we can find TS then we can find
    • and
how to find t s
How to find TS?
  • Wire resistance depends on TS
      • Temperature Coefficient of Resistance (material property)
      • RS0 = RS at T = T0
    • ,
    • We can find
    • So, theoretically we can find TS and so
      • and
  • Two modes of operation
constant current mode
Constant Current Mode

V∞T∞

R2

I

I

VO

VE

TS RS

V0

  • VE= constant, and R2 >> RS
  • = constant
  • Probe temperature TSand resistance RSgo downs as V∞ go up
  • Measure V0 = IRS
    • V0 will decrease as V∞ increases
    • Calibrate
  • Problem: TS must reach equilibrium with surroundings
    • Takes time, ~ 0.01 sec, or frequency 100 Hz
      • Too slow!

V∞

constant temperature anemometer cta
Constant Temperature Anemometer (CTA)
  • Incorporates hot sensor into a Wheatstone bridge
  • If V∞ increases, TS and RS“start” to go down
  • This decreases VBridge, but Feedback amplifier (op-amp) very quickly increases VO to increase current to sensor and restore its temperature and resistance (RS = RR)
  • The current and power to sensor adjusts to make its temperature constant
  • Output is VCTA (voltage across sensor)

VCTA

V∞

RR

TS RS

VBridge

cta transfer function
CTA Transfer Function
  • Convection Heat Transfer from probe to fluid
  • )
  • So
    • )
    • )
    • Or find constants a and b by calibration
  • Feedback amplifiers respond very quickly
    • Accurate for up to f = 400,000 Hz
    • Requires feedback control (Lab 12)
  • To use CTA, measure VCTA.
    • Calculate ,

Constants

hot film system calibration
Hot Film System Calibration
  • The fit equation VCTA2 = aSA0.5+b appears to be appropriate for these data.
  • The dimensional parameters are a = 1.366 volts2s1/2/m1/2 and b = 2.2057 volts2
lab 11 unsteady speed in a karman vortex street
Lab 11 Unsteady Speed in a Karman Vortex Street
  • Use the same wind tunnels as Lab 6
    • Sign up for 1.5 hour periods with your partner in lab next week
  • Two steps
    • Statically calibrate hot film CTA using a Pitot probe
    • Measure unsteady speed downstream from a cylinder
      • Measure “steady” speed without cylinder V
      • Perform spectral analysis and find frequency with peak amplitude, fP
      • Calculate StD= DfP/V and compare to expectations
setup
Setup

myDAQ

Variable Speed

Blower

VCTA

Barometer

PATM

TATM

Plexiglas

Tube

CTA

  • Add CTA and cylinder in cross flow
  • Do not use Venturi tube or Gage Pressure Transducer
    • Assume Pstat = PATM (Pgage= 0)
  • Tunnel Air Density

DTube

Cylinder

Static

Pitot-Static Probe VC

Total

PP

-

+

3 in WC

IP

before experiment
Before Experiment
  • Construct VI (formula block)
  • Measure PATM, TATM, and cylinder D
  • Find m and r for air
  • Air Viscosity from A.J. Wheeler and A. R. Ganji, Introduction to Engineering Experimentation, 2nd Edition, Pearson Prentice Hall, 2004, p. 430.
calibrate cta using pitot probe
Calibrate CTA using Pitot Probe
  • Remove Cylinder
  • Align hot film and Pitot probes (carefully)
    • 4 probes cost $600
  • Measure VCTA,AVG and IPitot for different blower speeds
table 2 calibration data
Table 2 Calibration Data
  • The initial and final no-wind hot film voltages and Pitot transmitter currents are the same.
standard error of the estimate
Standard Error of the Estimate

x

x

  • Find best fit line
  • Find Standard Error of the Estimate
  • Now measure VCTA to determine

x

x

VCTA2

x

x

x

x

measure v cta to determine
Measure VCTA to determine
    • Invert
  • Uncertainty
  • But we want
slide24

Cylinder in cross flow

Wake: region of reduced speed

Frequency

Strughold #:

Constant

Page 360 to 361

Measure flow rate in a pipe

For

example1
Example

A car antenna D = 0.25 in and car s=60 mph

What will the frequency of the shed vortices be?

slide26

Before we used: Pressure Method

Pito-probe/pressure transmitter (too slow)

Heat transfer method:

-hot film or hot wire probe

-small electrically heated surface

Probe:

Acid etched wire (hot wire)

-small but brittle

Metal plated quartz cylinder (hot film).

slide27

Probe electrical resistance heating

→ Can measure I, V0 Q [watts]

Heat is mostly dissipated by convection

For small cylinders in cross flow

slide28

How to find TS:

Wire resistance changes with its temperature TS:

α ≡ material property

So theoretically by measuring: A, I, V0, & known α.

slide29

Tow modes of operation:

  • 1) Constant current

VE ≡ constant & R2 >> RS

As U↑, h↑, TS↓, RS↓

Problem:

TS must reach equilibrium with surroundings.

Takes time

Max frequency Response

slide30

2) Constant Temp Anemometer (CTA)

    • Uses electronic feedback (op-amp) to very VE so TS (and RS)
    • stay constant.

Wheat stone bridge circuit