1098 CHINESE OPTICS LETTERS / Vol. 8, No. 11 / Novemb er 10, 2010
On-line measurement of temperature and water vapor in
CH
4
/air premixed flame using near-infrared diode laser
Bo Tao (陶陶陶 波波波)
∗
, Xisheng Ye (叶叶叶锡锡锡生生生), Zhiyun Hu (胡胡胡志志志云云云),
Lirong Zhang (张张张立立立荣荣荣), and Jingru Liu (刘刘刘晶晶晶儒儒儒)
State Key Lab oratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi’an 710024, China
∗
E-mail: nktaob o@yeah.net
Received April 9, 2010
We establish a single dio de laser sensor system to obtain temperature and water concentration in CH
4
/air
premixed flame. Line-of-sight properties are analyzed, but line-of-sight results are not path average values
for temperature measurements. The measurements are performed on a flat burner based on scanned-
wavelength direct absorption spectroscopy using two adjacent water lines at 7153.75 and 7154.35 cm
−1
.
Real-time results are acquired using a data acquisition card with a Labview data processing program.
The standard uncertainties of the temperature and water concentration measurements are 2.3% and 5.1%,
resp ectively.
OCIS co des: 300.1030, 300.6260, 300.6340.
doi: 10.3788/COL20100811.1098.
Tunable diode laser absorption spectroscopy (TDLAS)
has become one of the most powerful tools for combus-
tion diagnostics because of its excellent characteristics
such as high spectral resolution, fast response, compact
structure, and low cost. Sensor systems based on ab-
sorption spectroscopy can offer significant advantages to
on-line measurements of multiple flow field parameters,
such as temperature, species concentrations, and velocity.
Many practical implementations for measuring tempera-
ture and species concentrations in engines and detecting
atmospheric trace gases have been reported
[1−5]
.
Water is often selected as the absorbing species for tem-
perature sensing because it is a major combustion prod-
uct. It also has strong rovibrational spectra in the near-
infrared region, from which telecommunication lasers and
fiber optic technology are well developed
[1−3]
. In this let-
ter, two adjacent water lines (7153.75 and 7154.35 cm
−1
)
within a single laser scan are selected
[6]
. We discuss
the theory and experimental techniques behind TDLAS,
which is essential for resolving the problems in on-line
measurements of combustion temperature and water con-
centration.
The wavelength of distributed feedback (DFB) lasers
can be tuned by varying their temperature or injection
current. For scanned-wavelength direct absorption spec-
troscopy, we use linear injection current ramps (sawtooth
waveforms at kilohertz rates) to scan the DFB diode laser
both in wavelength and intensity, so that the laser wave-
length can cross selected absorption lines in every scan.
Various combustion parameters can be inferred from the
absorption strength and line shap es. A discussion of the
advantages and disadvantages of this method compared
with others can be found in Ref. [7].
Figure 1 shows a typical detected signal in a direct ab-
sorption scan. The laser intensity changes in response
to an injection current ramp. A polynomial fit to the
non-absorbing wings of the absorption feature is used to
extrapolate a zero absorption baseline
[7]
.
According to the Beer-Lambert law, the relationship
of transmitted signal I and baseline I
0
can be expressed
as
[6]
I = I
0
exp[−P XS(T )φ(ν)L], (1)
where P (atm) is the pressure, X is the mole fraction of
absorption species, S(T ) (cm
−2
·atm
−1
) and φ(ν) (cm)
are the line strength and line shape (generally the Voigt
function) of the absorption lines, respectively, and L (cm)
is the path length. The signal intensity lies on the sunken
depth displayed in Fig. 1, and it can be estimated using
d = I
0
− I ≈ I
0
P XS(T )φ(ν)L. (2)
Using Eq. (2) as basis, the signal intensity is de-
termined by laser intensity I
0
, absorption coefficient
PXS (T)φ(ν), and path length L. For a selected line, the
signal intensity can be increased using a higher I
0
and a
longer L. However, the maximum I
0
in direct absorption
measurement is confined by the saturation intensity of
the detector, and using a longer L via multiple reflec-
tions may introduce more system measurement errors.
Therefore, one should choose appropriate parameters to
acquire the desired signal intensity.
The combustion temperature is usually obtained by
comparing the line strength of two different absorp-
tion lines that have different temperature dependence
values
[7]
:
Fig. 1. Typical detected signal in a direct absorption scan
(signal intensity lies on the sunken depth).
1671-7694/2010/111098-04
c
° 2010 Chinese Optics Letters