This document summarizes research using terahertz time-domain spectroscopy and tomography to sense and image within sooty flames. A bespoke high-pressure burner was designed to produce methane diffusion flames at different air co-flow rates, allowing line-of-sight THz measurements through the flames. THz time-domain spectroscopy measurements were taken of flames at various air co-flow rates from 3-12 l/min. The time delay values extracted from the THz waveforms revealed that optical density along the sensing path initially increases linearly with co-flow, then levels off after 12 l/min, indicating changes in radial temperature distribution. Varying the air co-flow provides a way to study flame properties with potential for
Comparative Calibration Method Between two Different Wavelengths With Aureole...Waqas Tariq
A multi-stage method for calibration of sunphotometer is proposed by combining comparison calibration method between two different wavelengths with aureole observation method for long wavelength calibration. Its effectiveness in reducing the influences for calibration due to molecular and aerosolfs extinction in the unstable turbidity conditions is clarified. By comparing the calculated results with the proposed method and the existing individually calibration method, it is found that the proposed method is superior to the existing method in terms of calibration accuracy. Namely, Through a comparison between ILM and the proposed method using band 0.87um as reference, the largest calibration errors are 0.0014, 0.0428 by PM are lower than that by ILM (0.011,0.0489) for sky radiances with no error and -3~+3%, -5~+5% errors. By analyzing the observation data of 15 days with POM-1 Skyradiometer, the largest standard deviation of calibration constants by PM is 0.02016, and is lower than that by ILM (0.03858).
Early kick detection and nonlinear behavior of drilling mu…Frank-Michael Jäger
The following test measurements serve the quantification of resolution and achievable sensitivity of parameters of sound velocity and sound absorption in wellbore fluids. More precisely, these studies refer to tools and methods to identify the flow of liquids or gases, preferably hydrocarbons in the well bore in real time during the drilling. The aim is a way to show with the highly sensitive and robust tools for use in the deep ocean can be realized.
IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) is an open access international journal that provides rapid publication (within a month) of articles in all areas of mechanical and civil engineering and its applications. The journal welcomes publications of high quality papers on theoretical developments and practical applications in mechanical and civil engineering. Original research papers, state-of-the-art reviews, and high quality technical notes are invited for publications.
Detection of hydrogen sulfide above the clouds in Uranus’s atmosphereSérgio Sacani
Visible-to-near-infrared observations indicate that the cloud top of the main cloud deck on Uranus lies at a pressure level of
between 1.2 bar and 3 bar. However, its composition has never been unambiguously identified, although it is widely assumed
to be composed primarily of either ammonia or hydrogen sulfide (H2S) ice. Here, we present evidence of a clear detection of
gaseous H2S above this cloud deck in the wavelength region 1.57–1.59 μm with a mole fraction of 0.4–0.8 ppm at the cloud top.
Its detection constrains the deep bulk sulfur/nitrogen abundance to exceed unity (>4.4–5.0 times the solar value) in Uranus’s
bulk atmosphere, and places a lower limit on the mole fraction of H2S below the observed cloud of (1.0 2.5) × 10 5 − − . The detection
of gaseous H2S at these pressure levels adds to the weight of evidence that the principal constituent of 1.2–3-bar cloud is
likely to be H2S ice.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
Comparative Calibration Method Between two Different Wavelengths With Aureole...Waqas Tariq
A multi-stage method for calibration of sunphotometer is proposed by combining comparison calibration method between two different wavelengths with aureole observation method for long wavelength calibration. Its effectiveness in reducing the influences for calibration due to molecular and aerosolfs extinction in the unstable turbidity conditions is clarified. By comparing the calculated results with the proposed method and the existing individually calibration method, it is found that the proposed method is superior to the existing method in terms of calibration accuracy. Namely, Through a comparison between ILM and the proposed method using band 0.87um as reference, the largest calibration errors are 0.0014, 0.0428 by PM are lower than that by ILM (0.011,0.0489) for sky radiances with no error and -3~+3%, -5~+5% errors. By analyzing the observation data of 15 days with POM-1 Skyradiometer, the largest standard deviation of calibration constants by PM is 0.02016, and is lower than that by ILM (0.03858).
Early kick detection and nonlinear behavior of drilling mu…Frank-Michael Jäger
The following test measurements serve the quantification of resolution and achievable sensitivity of parameters of sound velocity and sound absorption in wellbore fluids. More precisely, these studies refer to tools and methods to identify the flow of liquids or gases, preferably hydrocarbons in the well bore in real time during the drilling. The aim is a way to show with the highly sensitive and robust tools for use in the deep ocean can be realized.
IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) is an open access international journal that provides rapid publication (within a month) of articles in all areas of mechanical and civil engineering and its applications. The journal welcomes publications of high quality papers on theoretical developments and practical applications in mechanical and civil engineering. Original research papers, state-of-the-art reviews, and high quality technical notes are invited for publications.
Detection of hydrogen sulfide above the clouds in Uranus’s atmosphereSérgio Sacani
Visible-to-near-infrared observations indicate that the cloud top of the main cloud deck on Uranus lies at a pressure level of
between 1.2 bar and 3 bar. However, its composition has never been unambiguously identified, although it is widely assumed
to be composed primarily of either ammonia or hydrogen sulfide (H2S) ice. Here, we present evidence of a clear detection of
gaseous H2S above this cloud deck in the wavelength region 1.57–1.59 μm with a mole fraction of 0.4–0.8 ppm at the cloud top.
Its detection constrains the deep bulk sulfur/nitrogen abundance to exceed unity (>4.4–5.0 times the solar value) in Uranus’s
bulk atmosphere, and places a lower limit on the mole fraction of H2S below the observed cloud of (1.0 2.5) × 10 5 − − . The detection
of gaseous H2S at these pressure levels adds to the weight of evidence that the principal constituent of 1.2–3-bar cloud is
likely to be H2S ice.
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
Gas chromatography-mass spectrometry (GC-MS) is the synergistic combination of two analytical method to separate and identify different substances within a test sample.
Gas chromatography separates the components of a mixture in time.
Mass spectrometer provides information that aids in the identification and structural elucidation of each component.
Understanding and Predicting CO2 Properties for CCS Transport, Richard Graham, University of Nottingham. Presented at CO2 Properties and EoS for Pipeline Engineering, 11th November 2014
Wall static pressure distribution due to confined impinging circular air jeteSAT Publishing House
IJRET : International Journal of Research in Engineering and Technology is an international peer reviewed, online journal published by eSAT Publishing House for the enhancement of research in various disciplines of Engineering and Technology. The aim and scope of the journal is to provide an academic medium and an important reference for the advancement and dissemination of research results that support high-level learning, teaching and research in the fields of Engineering and Technology. We bring together Scientists, Academician, Field Engineers, Scholars and Students of related fields of Engineering and Technology.
mass spectrometry for pesticides residue analysis- L1sherif Taha
This is the first lecture in series of lectures on mass spectrometry for pesticides residue analysis. This lecture (1) include Pesticides classification, introduction to mass spectrometry, vacuum system for Agilent GC MS/ MS and AB SCIEX LC MS/ MS
Diffusers are extensively used in centrifugal
compressors, axial flow compressors, ram jets, combustion
chambers, inlet portions of jet engines and etc. A small change in
pressure recovery can increases the efficiency significantly.
Therefore diffusers are absolutely essential for good turbo
machinery performance. The geometric limitations in aircraft
applications where the diffusers need to be specially designed so
as to achieve maximum pressure recovery and avoiding flow
separation.
The study behind the investigation of flow separation in a planar
diffuser by varying the diffuser taper angle for axisymmetric
expansion. Numerical solution of 2D axisymmetric diffuser model
is validated for skin friction coefficient and pressure coefficient
along upper and bottom wall surfaces with the experimental
results of planar diffuser predicted by Vance Dippold and
Nicholas J. Georgiadis in NASA research center [2]
.
Further the diffuser taper angle is varied for other different
angles and results shows the effect of flow separation were it is
reduces i.e., for what angle and at which angle it is just avoided.
Gas chromatography-mass spectrometry (GC-MS) is the synergistic combination of two analytical method to separate and identify different substances within a test sample.
Gas chromatography separates the components of a mixture in time.
Mass spectrometer provides information that aids in the identification and structural elucidation of each component.
Understanding and Predicting CO2 Properties for CCS Transport, Richard Graham, University of Nottingham. Presented at CO2 Properties and EoS for Pipeline Engineering, 11th November 2014
Wall static pressure distribution due to confined impinging circular air jeteSAT Publishing House
IJRET : International Journal of Research in Engineering and Technology is an international peer reviewed, online journal published by eSAT Publishing House for the enhancement of research in various disciplines of Engineering and Technology. The aim and scope of the journal is to provide an academic medium and an important reference for the advancement and dissemination of research results that support high-level learning, teaching and research in the fields of Engineering and Technology. We bring together Scientists, Academician, Field Engineers, Scholars and Students of related fields of Engineering and Technology.
mass spectrometry for pesticides residue analysis- L1sherif Taha
This is the first lecture in series of lectures on mass spectrometry for pesticides residue analysis. This lecture (1) include Pesticides classification, introduction to mass spectrometry, vacuum system for Agilent GC MS/ MS and AB SCIEX LC MS/ MS
Diffusers are extensively used in centrifugal
compressors, axial flow compressors, ram jets, combustion
chambers, inlet portions of jet engines and etc. A small change in
pressure recovery can increases the efficiency significantly.
Therefore diffusers are absolutely essential for good turbo
machinery performance. The geometric limitations in aircraft
applications where the diffusers need to be specially designed so
as to achieve maximum pressure recovery and avoiding flow
separation.
The study behind the investigation of flow separation in a planar
diffuser by varying the diffuser taper angle for axisymmetric
expansion. Numerical solution of 2D axisymmetric diffuser model
is validated for skin friction coefficient and pressure coefficient
along upper and bottom wall surfaces with the experimental
results of planar diffuser predicted by Vance Dippold and
Nicholas J. Georgiadis in NASA research center [2]
.
Further the diffuser taper angle is varied for other different
angles and results shows the effect of flow separation were it is
reduces i.e., for what angle and at which angle it is just avoided.
Varying Effects of Temperature and Path-length on Ozone Absorption Cross-sectionTELKOMNIKA JOURNAL
Inconsistencies in the absorption cross section of ozone have been observed. Hence, for accurate measurement, we have reported the combined effects of varying optical path-length and temperature on the ozone gas absorption cross section (OACS) at 334.15nm. Adopting optical absorption spectroscopy, results of the (OACS) have been simulated using spectralcalc simulator with HITRAN 12 has the latest line list. OACS increased by 52.27% as the temperature increased from 100K to 350K while it was slightly affected by a 0.007% decrease varying the path-length from 0.75cm-130cm.
The objective is to analyze and propose a methodology to manage with the attenuating effect promoted by carbon dioxide - CO2 on the performance of ultrasonic flow meter in gas flaring applications. Such methodology is based on experiments performed in a wind tunnel with a Reynolds number about 10^4 and concentration of CO2 above 60%. The results indicate that the ultrasonic meter exhibited measurement readings failures, especially in stages of abrupt changes in gas concentration, whose contents were above 5%. It is verified, as well, that the approximation of ultrasonic transducers tends to reduce such measurement failures.
Electro-optic Dendrimer is used to generate milliwatts of terahertz power by difference frequency
method. A terahertz time-domain spectrometer (THz-TDS) has been designed around this source that
exhibits wide broadband terahertz range, 0.1 to 35 THz. Examples of molecular characterization are discussed
for three common explosives and the vibrational states of Fullerenes. The explosives’ spectra are
unique for each explosive that allow detection and identification of the species. The Fullerenes C60 and
H2@C60 also exhibit distinctively different spectra and absorbance states indicating that the THz-TDS is
suitable for probing increased number of vibrational states expected from molecular vibrations.
2011 Elsevier B.V. All rights reserved.
technoloTwo dimensional numerical simulation of the combined heat transfer in...ijmech
A numerical investigation was conducted to analyze the flow field and heat transfer characteristics in a vertical channel withradiation and blowing from the wall. Hydrodynamic behaviour and heat transfer results are obtained by the solution of the complete Navier–Stokesand energy equations using a control volume finite element method. Turbulent flow with "Low Reynolds Spalart-Allmaras Turbulence Model" and radiation with "Discrete Transfer Radiation Method" had been modeled. In order to have a complete survey, this article has a wide range of study in different domains including velocity profiles at different locations, turbulent viscosity, shear stress, suctioned mass flow rate in different magnitude of the input
Rayleigh number, blowing Reynoldsnumber, radiation parameter, Prandtl number, the ratio of length to width and also ratio of opening thickness to width of the channel. In addition, effects of variation in any of the above non-dimensional numbers on parameters of the flow are clearly illustrated. At the end resultants had been compared with experimental data which demonstrated that in the present study, results have a great accuracy, relative errors are very small and the curve portraits are in a great
agreement with real experiments.
Underwater Sound Generation Using Carbon Nanotube Projectorschrisrobschu
The application of solid-state fabricated carbon nanotube sheets as thermoacoustic projectors is extended from air to
underwater applications, thereby providing surprising results. While the acoustic generation efficiency of a liquid immersed nanotube
sheet is profoundly degraded by nanotube wetting, the hydrophobicity of the nanotube sheets in water results in an air envelope
about the nanotubes that increases pressure generation efficiency a hundred-fold over that obtained by immersion in wetting alcohols.
Due to nonresonant sound generation, the emission spectrum of a liquid-immersed nanotube sheet varies smoothly over a wide
frequency range, 1-105 Hz. The sound projection efficiency of nanotube sheets substantially exceeds that of much heavier and thicker
ferroelectric acoustic projectors in the important region below about 4 kHz, and this performance advantage increases with decreasing
frequency. While increasing thickness by stacking sheets eventually degrades performance due to decreased ability to rapidly transform
thermal energy to acoustic pulses, use of tandem stacking of separated nanotube sheets (that are addressed with phase delay) eliminates
this problem. Encapsulating the nanotube sheet projectors in argon provided attractive performance at needed low frequencies, as
well as a realized energy conversion efficiency in air of 0.2%, which can be enhanced by increasing the modulation of temperature.
2. or air flow rates, results in the change of different aspects of
the flame properties, i.e. flame geometry, combustion stability,
soot emission and temperature field. Although much work has
been reported in the literature relating to the combustion of
fuel jets in still air or in parallel co-flowing streams [5,6], the
effect of air co-flow on the laminar non-lifted diffusion flame
properties is left almost unattended to. Darabkhani and Zhang
[5] reported that the flame dynamics and combustion
characteristics of a co-flow diffusion flame are strongly
affected by the co-flow air velocity. It was observed that when
the co-flow velocity reaches a certain value, the buoyancy-
driven flame oscillations may become completely suppressed.
That work demonstrated that the co-flow of air is able to push
the location of the instability initiation point beyond the
visible flame to create a very steady laminar flow region in the
reaction zone.
The resolution and sensitivity of THz imaging and
spectroscopy are still rather limited compared to existing
shorter wavelength optical and infrared diagnostics. At the
same time, the alternative diagnostic techniques such as laser
induced fluorescence (LIF), polarization spectroscopy and
Coherent anti-Stokes Raman Spectroscopy (CARS) [7] are
difficult, and sometimes near to impossible, to apply in
moderately to heavily sooted combustion environments, owing
to strong absorption, spectral interference from particulate
scattering, and fluorescence from large molecules. Therefore,
the case for applying THz-TDS of combustion species under
such conditions remains strong. As shown below, the major
combustion product with which the THz photons interact is
water vapor. To our knowledge, the observations in this work
are the first in literature on the effect of air co-flow velocities
on the transmission of THz radiation (0.2 – 2.5 THz) through
the flame zone.
III. EXPERIMENTAL SETUP
The context of the sensing task requires a dedicated burner,
capable of high pressures where soot often becomes a
problem, as well as allowing providing he views necessary for
tomographic imaging. The air co-flow burner used in this
study was designed to produce a classic Burke–Schumann [8]
laminar diffusion flame. This burner was specifically designed
and fabricated for the THz-TDS experiments and THz
tomography experiments in flames. The burner was capable of
pressures up to 30 bar and has eight 45 mm diameter
windows, as shown schematically in Figure 1, to enable THz
tomography based on 4 simultaneous views with no moving
parts. The window material is 20 mm thick high-resistivity
float-zone silicon (HRFZ-Si), which allows access for
wavelengths up to 1000 µm, and is commonly used for THz
measurements and spectroscopy [4]. The nitrogen port shown
in Figure 1 is used for pressurizing the chamber. It was
completely shut during the atmospheric pressure experiments.
The high pressure capability of the burner was not used to
obtain the results in this paper as the reported work is at
atmospheric pressure. Since the measurements are based on
simple LOS transmission, only two opposite windows were
used.
Gaseous methane (CH4) fuel was supplied from a
compressed gas cylinder regulated by a needle valve and
measured by a calibrated mass flow meter with 1% full scale
accuracy.
Co-flow air from a compressed dry air cylinder was
supplied into the burner and diffused using a layer of glass
beads, after which a honeycomb structure with 1.5 mm
diameter holes was used to straighten the flow. The air co-
flow was controlled by a needle valve to access a range of
mass flow rates from 1 to 20 l/min through a coaxial air exit
nozzle with a shroud diameter of 37.8 mm. The flame on a
fuel nozzle with an exit diameter of 4.6 mm was stationary
and stable under all co-flow conditions. Over the range of
examined flow conditions, the RMS flame tip flicker was less
Pressure Gauges
Filter
Cooling Block
Back-pressure Valve
View Port
Flow Metres
Optical Accesses
Figure 1. Schematic of the high-pressure burner facility. In
this experiment the burner was run at atmospheric pressure
and only one opposite pair of windows were used for line-of-
sight transmission measurements.
than 1% of the flame height
THz-TDS measurements on the methane diffusion flames
were taken at different air flow rates (5, 7.5, 10, 12.5 and 15
l/min) under a constant fuel co-flow rate of 200 ml/min. To
distinguish the effects of combustion (combustion products,
temperature gradients, etc.) from possible changes in optical
density and lensing effects in the cold methane/air co-flow
itself, two reference groups of data were collected from the
flows without a flame (henceforth referred to as cold flows):
a) Air flow only, with flow rates of 5, 7.5, 10, 12.5, 15 l/min
through the air jacket nozzle in the burner; b) Co-flow of air at
3. 5, 10, 15 l/min and methane at 200 ml/min through their Griot) with a 500 nm step accuracy. Each THz field scan has a
Gas
l/min
Rate
m3
/s
Flow
Rate
mg/s
Velocit
m/s
y
Fr No.
Methane 0.2 3.33E-06 2.27 0.230 61.2 1.249
Air 5 8.33E-05 100 0.077 171.27 0.019
Air 7.5 1.25E-04 150 0.115 256.91 0.043
Air 10 1.67E-04 200 0.153 342.55 0.076
Air 12.5 2.08E-04 250 0.192 428.18 0.119
Air 15 2.50E-04 300 0.230 513.82 0.171
respective nozzles in the burner, without ignition.
Table 1 shows the physical parameters of the fuel and air
streams for the reported experiments, including the reference
cases of cold co-flows. The mean fuel jet exit velocity is
determined by the flow rate and the nozzle cross-section 0.23
m/s with Reynolds number (Re) of 61. The air exit velocities
are in the range from 0.077m/s to 0.23 m/s with Re from 171
to 514. This substantiates the assumption that all flows were in
a laminar mode during all sets of experiments. The maximum
fuel jet Froude number (Fr), as a measure to compare inertia
and gravitational forces in the flow stream, is calculated to be
1.249 and the maximum Fr of the co-flow air is 0.171 at 15
l/min of air flow rate. Both Re and the Fr of the air streams
have been calculated based on the hydraulic diameter of the
annulus air port (DH= Do-Di=31.5 mm).
TABLE I
FUEL AND AIR PARAMETERS IN CO-FLOW AIR EXPERIMENTS
range of approximately 67 ps, sampled at 2048 points,
corresponding to a 10 μm step. To improve the signal-to-
noise, the antenna bias is modulated at 10 KHz and the
balanced detector output is processed by a lock-in amplifier
(EG&G 7265). The measurement system is integrated in a
LabView environment. The transmitted THz spectra are
obtained from the electrical field time domain waveform by
applying a Fast Fourier Transform (FFT) with appropriate
zero padding. This results in a useful bandwidth of 0.2-2.5
THz with a signal-to-noise ratio of 30 dB.
Volume Flow Mass
Re No.
We use a standard set-up for THz-TDS [4], shown
schematically in Figure 2. Ultra-short pulses of 80 fs duration,
spectrally centered at 800 nm, are delivered by a Coherent
Mira 900D Ti:Sapphire laser, pumped by a Verdi-V18
Nd:YVO4 laser. The total average power of 300 mW is split
90/10 between the pump and probe beams respectively. The pump
beam is focused on a biased photoconductive SI GaAs antenna
[9] to generate vertically polarized THz pulses. The optical
path of the THz beam is shown in Figure 2 as a thicker green
streak between “Photoconductive THz emitter” and “ZnTe
crystal”. The THz radiation is collected and collimated by a
gold-coated parabolic mirror into a 10 mm diameter beam,
which is then transmitted through the co-flow burner. The
transmitted THz beam is focused by a second gold-coated
parabolic mirror onto a 1 mm thick (110)-oriented ZnTe
crystal, where it is combined with the collinearly propagating
horizontally polarized and delayed probe beam, entering
through a hole in the second parabolic mirror. The THz-
induced changes in the polarization of the probe beam are
analyzed by a quarter-wave plate followed by a Wollaston
prism and recorded with a balanced detector (Nirvana 2017) as
a function of delay time, yielding the electrical field time
variation of the transmitted THz pulse. The time delay is
controlled by passing the probe beam through a hollow retro-
reflector mounted on a mechanical translation stage (Melles
Figure 2. Experimental setup for THz-TDS. Ultra-short
pulses are split in a pump beam to excite the THz emitter and
a probe beam with controlled delay, to allow time correlation
between the IR (probe) and transmitted THz pulses.
To reduce the effects of ambient water in the beam path,
two purging boxes were installed in the path of the THz beam,
as shown in Figure 2, just before and after the silicon
windows, and purged by a flow of dry air. Reference spectra,
with no fuel supply to the chamber, were taken for the 5 dry
air flows specified in Table 1.
IV. RESULTS AND DISCUSSION
The time domain signals recorded for methane diffusion
flames at different air-flow rates (see Table 1) are shown in
Figure 3 over a time window of 3 ps, shifted with 5.5 ps from
the origin. The no-flame reference measurements, in the
presence of an air-flow (5, 7.5, 10, 12.5 and 15 l/min) only and
an air-fuel cold co-flow (methane at 0.2 and air at 5, 10, 15
l/min), exhibited a negligible spread in the time at which the
signal crosses the zero and therefore are not shown.
4. 5, 10, 15 l/min and methane at 200 ml/min through their Griot) with a 500 nm step accuracy. Each THz field scan has a
Signal(V)
7.0E-02
5.0E-02
3.0E-02
1.0E-02
-1.0E-02
-3.0E-02
-5.0E-02
Ref
Meth200-air5 l/min M
eth200-air7.5 l/min Me
th200-air10 l/min Me
th200-air12.5 l/min Me
th200-air15 l/min
5.5 6.0 6.5 7.0 7.5 8.0 8.5
Time (ps)
effect of increasing the air flow rate, observed in Figure 4, is
unlikely to be caused by the methane-air co-flow per se. The
most likely cause is the radial gradient in the refractive index
due to the thermal gradients across the flame. Indeed, at higher
co-flows the diffusion rate increases, resulting in a shorter
optical path through hot gas; this causes variation in the
optical path length through hot and cold gas, and consequently
in the distortion of the beam as it traverses the flame. Thus the
decrease in the swing amplitude in Figure 3 can be ascribed to
the increased THz beam steer or divergence as it passes
through the flame at higher flow rates. Such beam “walk”
and/or changes in the filling of the detector aperture results in
a drop in transmission. Substantiation of this speculation can
be sought by acquiring additional evidence on the change in
the optical path through cold and hot gas, e.g. through
Figure 3. THz time domain signals of reference case and
methane diffusion flames at different air flow rates (5, 7.5, 10,
12.5 and 15 l/min
The waveforms in Figure 3 exhibit a clear trend towards
reduction of the swing amplitude between the negative and
positive peak, simultaneous with a time shift towards longer
delays, as the air co-flow is increased. For a more precise plot
of the estimated time delay, we applied additional processing
on the THz field waveforms, which was introduced and
explained in detail in earlier work [4]: the values for the pulse
delay for each value of the air flow were calculated using
parabolic interpolation on the time-integrated waveforms in
Figure 3. The result is plotted in Figure 4, showing a linear
relationship between the time delay of the THz pulse and the
co-flow values. After an initial linear increase, the optical
Figure 4. THz pulse delay as a function of the air flow.
The delay values are calculated from the maxima of the
integrated THz waveforms shown in Figure 3. The exact
values are shown as labels next to the experimental points.
density along the sensing path levels out above 12 l/min.
The power spectra (not shown) of the THz pulses
transmitted through the methane flame at different air flow
rates, normalized to their recorded reference spectra without a
flame (cold flow), confirm that the main interaction of the
incident THz radiation with the flame is due to water vapor, as
expected from the polar character of the H2O molecule.
schlieren imaging.
The observations in this work indicate substantial challenges
for hard-field THz tomography in flames: beam steering can
induce cross-talk in detector arrays, potentially compromising
the precision of the Radon transform. However, we have
recently demonstrated THz tomography in time-of-flight
contrast [4], which overcomes problems caused by the
“softening” of the probe field. Therefore, it is conceivable
that such tomography can be implemented with measurements
similar to those reported above.
REFERENCES
[1] Wood, M.P. and Ozanyan, K.B., Temperature mapping from molecular
absorption tomography, Proc. IEEE SENSORS 2011, pp. 865-869, 2011.
[2] York T., McCann H. and Ozanyan K.B., Agile Sensing Systems for
Tomography, IEEE Sensors J .2011, vol.11 pp. 2507
[3] Bassi, J., Naftaly, M., Miles, B., and Zhang, Y., The investigation of
sooty flames using terahertz waves. J. Flow. Meas. Instrum.,.vol. 16
pp. 341-345 (2005).
[4] Ozanyan, K.B.; Wright, P.; Stringer, M.R.; Miles, R.E. Hard-Field THz
Tomography, , IEEE Sensors J. 2011, vol.11, no.10, pp. 2507-2512,
2011.
[5] Gohari Darabkhani, H. and Zhang, Y. Suppression Dynamics of a
Laminar Oscillating Diffusion Flame with Co-flow Air. in the
Proceedings of the World Congress on Engineering (WCE) 2010, The
International Conference of Mechanical Engineering (ICME), 30 June -
2 July, 2010, Imperial College, London, UK,.
[6] Chen, Y.-C. and Bilger, R. W., Stabilization mechanisms of lifted
laminar flames in axisymmetric jet flows. Combust. Flame, 2000. 123(1-
2), p. 23-45.
[7] Tobai, J. and Dreier, T., Measurement of relaxation times of NH in
atmospheric pressure flames using picosecond pump-probe degenerate
four-wave mixing. Journal of Molecular Structure, 1999. 480-481, p.
307-310.
[8] Burke, S. P. and Schumann, T. E. W., Diffusion flames. Proc. Combust.
Inst., 1948. 1(2), p. 2-11. (2011).Tomography Shu, Z., Aggarwal, S. K.,
Katta, V. R., and Puri, I. K., Flame-vortex dynamics in an inverse
partially premixed combustor: The Froude number effects. Combust.
Flame, 1997. 111(4), p. 276-286.
[9] Nahata A., Weling A. S., and Heinz T. F., A wideband coherent
terahertz spectroscopy system using optical rectification and electro-
optic sampling. Applied Physics Letters, , 1996. 69(16), p. 2321-2323.