Measurements in Fluid Mechanics
058:180 (ME:5180)
Time & Location: 2:30P - 3:20P MWF 3315 SC
Office Hours: 4:00P – 5:00P MWF 223B-5 HL
Instructor: Lichuan Gui
lichuan-gui@uiowa.edu
Phone: 319-384-0594 (Lab), 319-400-5985 (Cell)
http://lcgui.net
Lecture 22. Composition measurement
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Composition measurement
Sample analysis
Orsat analyzer
- classical, old-fashioned device
- volume change measured after
absorption of each gas
- simple and inexpensive
- manual, slow and tedious
Electronic testers
- contain electrochemical gas sensors, each measuring the concentration of a particular type of molecule.
Continuous-emission monitoring systems (CEMs)
- continuous monitoring of exhaust gas from
industrial plants
- automatic systems including three main
methods of sample analysis
•
absorb spectroscopy
•
luminescence techniques
•
electroalanalytical methods
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Composition measurement
Sample analysis
Gas chromatography - method for separation and analysis of gas and volatile liquid mixtures
signal output
sample injection
flow regulator
detector
column oven
carrier gas
- A small gas sample is injected into a stream of an inert carrier gas, e.g. helium or nitrogen (liquid samples are
vaporized before injection into the carrier stream)
- The gas stream is passed through the packed column, through which the components of the sample move at
velocities that are influenced by the degree of interaction of each constituent with the stationary nonvolatile
phase.
- The substances having the greater interaction with the stationary phase are retarded to a greater extent and
consequently separate from those with smaller interaction.
- As the components elute from the column they can be quantified by a detector and/or collected for further
analysis.
4
Composition measurement
Sample analysis
Gas chromatography
Typical chromatogram
- appears as a sequence of peaks, each associated with a gas component and separated by specific
time intervals.
- the components of the gas mixture in the test sample are identified by timing of the peaks.
- the concentrations of the identified components can be found by measurement of the areas
under each peak.
5
Composition measurement
Sample analysis
Absorption spectrophotometry
Light absorption following Beer’s law:
For two or more types of molecules:
I0 – radiant intensity of incident light
I – radiant intensity of passing light
 – molar absorptivity coefficient
l – length of path
C – concentration of molecules
- A particular molecule can be identified by analyzing spectrum of the absorbed radiation because
each type of molecule absorbs radiation at particular wavelengths
- Concentrations of gas mixture components can be determined by radiation of different wavelengths
Example of absorption spectrophotometry
6
Composition measurement
Sample analysis
Mass spectrometry
- utilize electric and magnetic fields to separate ions
according to mass and charge
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Composition measurement
Thermal probes
- thermal conductivity of a fluid mixture depends on the mass fractions of its constituents.
- the composition of a binary mixture of fluids can be estimated from heat transfer
measurements by use of thermal sensors.
Interfering thermal probe
- two hot-wire-hot-film sensors
Aspirating thermal probe
- thermal sensor inside small sampling tube
Electric conductivity probes
- used to measure local concentration of electrolyte in liquid solution according to electric resistance
of a path between two electrodes
- single-electrode type sensors commonly used
- calibration in liquid solutions with known
uniform concentrations of electrolyte
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Composition measurement
Light-scattering methods
Basic experimental arrangement
- laser beam or other collimated beam of light
- focused on measuring volume of 1 mm3 or less
- scattered light collected by collecting lens
- separated from other radiation with slit or pinhole
- projected to photodetector for subsequent analysis
Mie-scattering methods
- used to measure local concentration of smoke or mists contained in gases
Rayleigh-scattering methods
- used to measure local concentration of chemical species in pure gas
Many other related & similar methods
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Homework
- Read textbook 13.1-13.4 on page 307 - 316
- Questions and Problems: 1 on page 323
n 1  K
𝐾 = 𝐾1 𝜌1 + 𝐾2 𝜌2 /
1+2=
??
- Due on 10/17
10
Learn to write a Matlab program
•
to select image samples in a 32×32-pixel window from two images at x=400, y=200
Example of Matlab program:
clear;
A1=imread('A001_1.bmp');
A2=imread('A001_2.bmp');
G1=img2xy(A1);
G2=img2xy(A2);
M=32;
N=32;
x=400;
y=200;
g1=sample01(G1,M,N,x,y);
g2=sample01(G2,M,N,x,y);
g1=g1-mean(mean(g1));
g2=g2-mean(mean(g2));
c=xcorr2(g1,g2);
[cm Sx Sy]=peaksearch(c,20)
C=xy2img(c);
imwrite(C,'C.bmp','bmp');
http://lcgui.net/ui-lecture2012/hw/00/A001_1.BMP
http://lcgui.net/ui-lecture2012/hw/00/A001_2.BMP
•
to remove mean gray values of the 2 image samples
•
to determine cross-correlation function
•
to determine particle image displacement
Sx = -2.2969
Sy = 2.1598
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Learn to write a Matlab program
•
to select an image sample in a 64×64-pixel window from a double exposed PIV
recording at x=400, y=200
Mean removed
http://lcgui.net/ui-lecture2012/hw/00/D001_1.BMP
•
to remove mean gray values of the image sample
•
to determine auto-correlation function
Mean not removed
clear;
D1=imread('D001_1.bmp');
G1=img2xy(D1);
M=64;
N=64;
x=400;
y=200;
g1=sample01(D1,M,N,x,y);
g1=g1-mean(mean(g1));
c=xcorr2(g1,g1);
C=xy2img(c);
imwrite(C,'C.bmp','bmp');
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