Case Study CS-LDTC12 Rev. A

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Figure 5. Two-dimensional contour of H

2O

mole

fraction distribution. Left, CFD simulation; right, present

TAS measurement.1

The reconstruction used for the tomography portion

in this research is highly sensitive to the error/noise in

the integrated absorbance. Any slight fluctuation in the

distribution along the radial direction leads to noticeable

deviations in the final reconstruction. Obtaining A

int accounts

for the majority of the uncertainty. In this research the

uncertainty in A

int is estimated to be ±2.5%. 100 random

tests were run at a height of 5 mm were ran with noise-

perturbed A

int data. Other spectral parameters can contribute

to the uncertainty.

Figure 6 shows the 95% confidence

interval of the random tests and the ±2.5% uncertainty in

linestrength in the shaded area. The uncertainty of the TAS

measurement is relatively small at ΔX

H2O ≈ ±0.0025 when

considering the ambient H

2O used as the absorbing species

in the data collection at X

H2O = 0.02889.

1

Figure 6. Uncertainty

of

the

present

TAS

reconstructions of X

H2O (H = 5 mm) in the laminar jet.

1

The combined methods of absorption spectroscopy and

2D tomography allowed researchers to reveal the H

2O

transport in a non-reactive laminar jet. The water vapor

in the ambient was used as the absorbing species in the

TAS measurements, and results were well matched with

CFD simulations. Mole fraction of H

2O was mapped in

similar fashion to CFD 2D contour distribution, and the

experimental data agreed with simulation data. The

reconstruction uncertainty of ±0.0025 was considered

reasonable and successful.1 Although further improvements

can be made to upgrade the TAS system, TAS has

been proven to be a useful tool in the research of mass

transfer and the scalar field of gaseous flows. Combustion

diagnostics and other non-reactive jets can benefit from the

developed TAS technology.

WAVELENGTH'S ROLE

A distributed-feedback (DFB) laser served as the TAS

laser source for the investigation of H

2O transport in a dry

laminar jet with TAS. The laser required stable current and

temperature control in the precision setup. Wavelength

Electronics' LDTC0520 laser diode and temperature

controller provided the stable drive current and temperature

control for the DFB laser with minimal noise and high

temperature accuracy. This ensured minimal noise was

added to the photodetector after TAS. The LDTC0520

provides up to 500 mA output current to the laser as well as

up to ±2.2 A to the thermoelectric cooler or resistive heater.

With low noise of 7.5 μA RMS and current stability as low

as 50 ppm at 25ºC for 1 hour, the LDTC0520 delivered

constant current to the laser for constant center wavelength

for the tomography absorption measurements. The

precision temperature stability of the temperature controller

at 0.005ºC at 25ºC for 1 hour also provided system stability.

The laser current was modulated with a triangle current

scanning signal of 1 kHz using the LDTC0520. The

bandwidth of the LDTC0520 can be as high as 500 kHz

for a variety of applications. Both the laser driver and

temperature controller portions of the LDTC0520 have

special safety features and designs for optimal user setup

and control.

The LDTC0520 driver and temperature controller enabled

sensitive gaseous process characterization with low noise

and stable current and temperature output. This makes

the developed TAS system a reliable tool for exploring the

effects of various conditions on practical applications of

combustion and other gaseous processes.