Arch. Min. Sci., Vol. 53 (2008), No 1, p. 87–96

Arch. Min. Sci., Vol. 53 (2008), No 1, p. 87–96

Arch. Min. Sci., Vol. 53 (2008), No 1, p. 87–96
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PAWEŁ LIGĘZA*, ELŻBIETA POLESZCZYK*, PRZEMYSŁAW SKOTNICZNY*
MEASUREMENTS OF VELOCITY PROFILE IN HEADINGS WITH THE USE OF INTEGRATED
HOT-WIRE ANEMOMETRIC SYSTEM
POMIAR PROFILU PRĘDKOŚCI W WYROBISKU GÓRNICZYM Z WYKORZYSTANIEM
ZINTEGROWANEGO SYSTEMU TERMOANEMOMETRYCZNEGO
In this paper we have presented a hot-wire anemometric system constructed in Strata Mechanics
Research Institute, Polish Academy of Sciences, which was applied in practice to measure velocity profile
in a selected cut of heading.
Hot-wire anemometric system for multi-point measurements of velocity field is based on sensors
integrated with a signal conversion module. A Sensor matrix, so-called integrated heads, is connected
to a mobile measurement data collection system and placed in a given cut of a heading. Arrangement of
measuring probes in selected velocity fields and subsequent registration and conversion of signals from
given probes allow for a simultaneous measurement of temperature and velocity field, turbulence intensity
and also for determination of volume stream.
Performed measurements had as an objective to evaluate metrological suitability of the measurement
system based on hot-wire anemometric integrated probes, in which a measuring element consists of miniature platinum-ceramic sensors in specific conditions of heading. Hot-wire anemometric system was
used for measurement of distribution of air flow velocity module in a selected cut of a heading.
Measurements were performed in a cut of a cross heading 713, “Borynia” coal mine. Cross heading
was a major heading with a cut of 14 m2 – cross cut dimensions were 5 × 2.5 m. Four hot-wire anemometric sensors placed on a special extension arm were used for measurement of instantaneous distribution
of velocity module .
Despite some discrepancies with literature sources results described in this paper confirm that used
hot-wire anemometric measurement system allows for measurements within logarithmic substrata of
velocity distribution of stringing layer, which consequently allows for further assessment of local flow
phenomena in headings. In assessment of properties and metrological usefulness of applied hot-wire
anemometric system of integrated heads it must be emphasized that despite 90 minute long measurement
and data registration the system worked smoothly.
There may be certain conclusions on practical improvements of the system, however elements
used in the heads, which measure velocity are characterized with high mechanical resistance in special
conditions.
Keywords: heading, hot-wire anemometry, integrated head, velocity vector module, stringing layer
*
STRATA MECHANICS RESEARCH INSTITUTE, POLISH ACADEMY OF SCIENCES, UL. REYMONTA 27, 30-059 KRAKÓW,
POLAND
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W pracy przedstawiono skonstruowany w Pracowni Metrologii Przepływów IMG PAN termoanemometryczny system pomiarowy, który został praktycznie zastosowany do pomiaru profilu prędkości
w wybranym przekroju wyrobiska górniczego.
Termoanemometryczny system pomiarowy do wielopunktowych pomiarów pola prędkości jest oparty
na czujnikach zintegrowanych z układem przetwarzania sygnału. Matryca takich czujników tzw. głowic
zintegrowanych, rozmieszczonych w badanym przekroju wyrobiska współpracuje z przenośnym, komputerowym systemem akwizycji danych pomiarowych. Rozmieszczenie sond pomiarowych w wybranych
punktach badanego pola prędkości, a następnie rejestracja i przetwarzanie sygnałów z poszczególnych
sond umożliwiają współczasowy pomiar pola temperatury i prędkości oraz intensywności turbulencji,
a także wyznaczenie strumienia objętościowego.
Przeprowadzone pomiary miały na celu sprawdzenie przydatności metrologicznej systemu pomiarowego opartego na termoanemometrycznych głowicach zintegrowanych, w których element mierzący
stanowiły miniaturowe sensory platynowo-ceramiczne w specyficznych warunkach wyrobiska górniczego.
Termoanemometryczny system pomiarowy użyto do zmierzenia rozkładu modułu prędkości przepływu
powietrza w wybranym przekroju poprzecznym wyrobiska.
Pomiary wykonano w wybranym przekroju w przekopie 713, KWK „Borynia”. Przekop był głównym
wyrobiskiem wentylacyjnym o przekroju 14 m2, przy wymiarach przekroju poprzecznego 5 × 3,5 m. Do
pomiaru chwilowych rozkładów modułu prędkości użyto czterech czujników termoanemometrycznych
umieszczonych na specjalnie skonstruowanym wysięgniku.
Mimo pewnych rozbieżności w porównaniu ze źródłami literaturowymi, opisane w artykule wyniki
badań świadczą o tym, że zastosowany termoanemometryczny system pomiarowy umożliwia pomiary
w obrębie subwarstwy logarytmicznego rozkładu prędkości warstwy przyściennej, co z kolei daje możliwość głębszego zbadania lokalnych zjawisk przepływowych w wyrobiskach górniczych. Oceniając
właściwości i przydatność metrologiczną zastosowanego termoanemometrycznego układu głowic zintegrowanych należy podkreślić, że czas trwania pomiarów wynosił 90 minut i w trakcie rejestracji wyników
zainstalowana aparatura działała bez zarzutów.
Nasuwają się pewne wnioski dotyczące praktycznego udoskonalenia systemu, jednakże zastosowane w głowicach elementy mierzące prędkość wykazują dużą odporność mechaniczną w warunkach
specjalnych.
Słowa kluczowe: wyrobisko górnicze, termoanemometria, głowica zintegrowana, moduł wektora
prędkości, warstwa przyścienna
1. Introduction
Measurements of gas flow velocity are important in the assessment of condition and
status of mines ventilation systems. Efficacy and reliability of the measurement system
of ventilation impacts the process of bed exploitation and safety at work in a mine.
Due to special metrological conditions (non-isomeric flows, non-stationary, highly
polluted and humid medium) (Roszczynialski, 1992) there have been developed several
methods of gas flow velocity measurements. Vane anemometer due to its time-constant
is suitable primarily to static measurements and is the most commonly used system in
mine ventilation.
Hot-wire anemometers are used in time-variable measurements of transient states
in ventilation systems and other dynamic measurements. This type of anemometers are
characterized by a broad spectrum of transmitted frequencies up to several hundred
kilohertz. Miniature dimensions of measuring probe allow for measurements close to
point measurements and slightly disturb velocity and temperature field.
89
In Strata Mechanics Research Institute, Polish Academy of Science we have designed
and constructed hot-wire multi-point anemometric system for simultaneous measurements of velocity and temperature fields in special conditions. Measuring probes and
the system which is based on them was developed for multi-point measurements of
velocity profile in a heading cut with particular emphasis on stringing layer. Obtained
results in a mine constitute starting point for experimental numerical verification of air
flow simulation in heading.
2. Integrated hot-wire anemometric head
Hot-wire anemometric system for multi-point measurements of velocity field is based
on sensors integrated with a signal transformation module. A Sensor matrix, so-called
integrated heads, is connected to a mobile measurement data collection system and
placed in a given cut of a heading. It allows for simultaneous multipoint measurement
of air flow (Waluś, 2003; Ligęza & Poleszczyk, 2004) in the assessed cut. Original
measurement system was used with constant temperature bridge system sensors with
temperature compensation. This new technological solution involves also the use of
miniature thin-film platinum-ceramic sensors as anemometric and compensative sensors
for flow velocity. These sensors are time and temperature stable and highly mechanical
resistant, which is of particular importance in the use of hot-wire anemometric systems
in mine measurements (Fig. 1).
Metrological properties of measuring elements used in heads were evaluated in a laboratory (Ligęza, 1994; Ligęza & Poleszczyk, 2005, 2006). These sensors may be used
in double systems with detection of a velocity vector turn. Described probes and a measuring system may also be used in other research applications of flow metrology.
Fig. 1. Integrated probe (head)
Rys. 1. Sonda (głowica) zintegrowana
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Measuring system is composed of hot-wire anemometric probes integrated with signal
conversion system and acquisition system of measurement data from specific probes.
Data acquisition system works on the basis of multi-functional module DAQ for USB
type: NI USB-6009 by National Instruments of the following parameters:
number of measuring channels – 8,
conversion resolution A/C – 14 bit,
sampling frequency – 48 kS/s.
Two modules may be used to increase the number of measuring channels. Modules
cooperate with Toshiba Satellite L10 – 118 notebook. The system has autonomous power supply and is fully mobile. Arrangement of measuring probes in selected velocity
fields and subsequent registration and conversion of signals from given probes allow for
a simultaneous measurement of a temperature and velocity field and turbulence intensity
and also for determination of volume stream.
3. Measurements of velocity profile in a heading cut with the use of integrated
hot-wire anemometric system
3.1. Selected measurement area
Measurements were performed in a cut of a cross heading 713, „Borynia” coal
mine. This cross heading was a major ventilation bed, whose cut surface was 14 m2
and dimensions 5 × 3.5 m. To eliminate obsolete turbulent structures in this cut, which
could affect the quality of results, the measurements were performed 10 meters away
from double timber O1. Extended diagram of measurement site was presented in Fig. 2.
Flow conditions were stationary, where the medium flow velocity was Um = 5 m/s, at
the air temperature of ta = 24°C and humidity of f = 95%.
Four hot-wire anemometric sensors placed on a specially constructed extension
were used for measurement of transient distribution of velocity modules. This method
of placement allows for simultaneous measurement of velocity modules. Fig. 3, 4, 5.
shows extension with installed hot-wire anemometric heads.
Fig. 6 demonstrates sensors distribution and measurement sites in a cut. 2.
3.2. The objective and course of measurements
Performed measurements had as an objective to evaluate metrological suitability in
specific conditions of heading of the measurement system based on hot-wire anemometric
integrated probes, in which a measuring element consists of miniature platinum-ceramic
sensors. Hot-wire anemometric system was used for measurement of distribution of air
flow velocity module in a selected cut of a heading.
91
reference cross-section
to the outlet
40
°
V1
o2
4m
5m
o1
contaminated air
10 m
T1
co
nn
fre
sh
ec
tin
g
dr
ift
V2
ai
r
Fig. 2. Diagram of the bed in which measurements were conducted.
Measurement cut is indicated with a break line
Rys. 2. Schemat wyrobiska, w którym wykonywano pomiary.
Linią przerywaną zaznaczono przekrój pomiarowy
Hot-wire anemometric measuring heads were connected to multi-channel data acquisition system composed of:
• analog to digital converter
• power supply module (integrated with A/C converter)
• notebook with acquisition software.
• Battery power supply allowing for 3 hours of measurements.
Listed measuring devices allowed for registering of time-courses at a given sampling
frequency. The use of four measuring heads allowed for multi-point and simultaneous
measurement of velocity vector.
The measurements were performed in four planes inclined at the angles of 0, 50, 70
and 90° to heading floor. In each position of extension arm 4 time courses of velocity
module vector were measured simultaneously. The minimal distance of the first head
from side wall was 4 cm, and of the last head was 92.4 cm. The distribution of specific
heads on the probing plane was shown in the Fig. 6.
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Fig. 3-5. Hot-wire anemometric heads matrix in stringing layer
Rys. 3-5. Matryca głowic termoanemometrycznych w warstwie przyściennej
93
P4
P3
pipelines
P2
90°
P1
1
2
3
4
70°
50°
measuring device
40
140
240
924
USB
PC
Fig. 6. Distribution of measuring sensors in a heading cut
Rys. 6. Schematyczne rozmieszczenie czujników oraz badane miejsca w przekroju
3.3. Results
First important thing that had to be done prior to actual measurements was to determine minimal frequency of signal sampling. As fs – minimal frequency determined
accordingly to Shannon’s theorem
fs =
1
2h
(1)
where h indicates sampling period.
Due to the fact that the analysis of the signal in a mine is difficult mainly because of
a limited battery power supply the height fpr was determined on the basis of precious
measurements in a similar bed (Dziurzyński et al., 1996). According to the data derived
in this paper the highest frequency present in amplitude-frequency spectrum was about
10 Hz. On this basis it was assumed that the measurement frequency fs = 40 Hz would
be at least twice as high as the maximum frequency observed in the spectrum.
Fig. 7 presents examples of signal power density. As it may be observed, transient
structures present in measured signals are fully represented. Therefore, it may be assumed
that the sampling frequency for this flow conditions was properly set.
On the basis of performed measurements charts (Fig. 8) were prepared, which present
the distribution of velocity vector module in above shown probing planes.
94
120
P, m 2/s 2
probe 1
80
40
0
0
1
2
3
4
f, Hz
120
P, m 2/s 2
probe 4
80
40
0
0
1
2
3
4
f, Hz
Fig. 7. Spectra of signal power density in probe 1 and 4
Rys. 7. Widmowa gęstość mocy sygnału z sondy 1 i sondy 4
8
U, m/s
6
4
2
0
0
0.2
0.4
I, m
0.6
0.8
Fig. 8. Measured velocity profiles for four probing lines
Rys. 8. Zmierzone profile prędkości dla 4 linii sondowania
1
95
Solid line represents a curve of logarithmic adjustment, break line represents line
adjustment. Function describing profile within the zone of logarithmic distribution of
velocity for lines 1, 2, 3, and 4 has the following form:
1: Y = 1.15 * ln(X) + 6.67
2: Y = 1.10 * ln(X) + 6.67
3: Y = 0.86 * ln(X) + 6.23
4: Y = 0.72 * ln(X) + 5.29
According to literature references (Elsner, 1987) the function describing velocity
distribution in logarithmic zone for smooth plate has the following form:
Y=2.5 ln(X) + 5.
(2)
Resulting discrepancies between literature data and measurements may be explained
by high roughness of the bed walls and differences in starting conditions of analyzed
flow. In a discussed case we have to deal with flow round infinitely vast plate where
a bed is a closed duct.
Despite these discrepancies, the most important information obtained from described
measurements is that the discussed measuring systems allows for measurements within
logarithmic substrata of velocity distribution in boundary layer, which consequently
allows for further assessment of local flow phenomena in headings.
4. Conclusions
In assessment of properties and metrological usefulness of applied hot-wire anemometric system of integrated heads it must be emphasized that despite 90 minute long
measurement and data registration the system worked smoothly.
There are however certain observations on the broader application of a measuring
system. It would be advantageous to increase power supply capacity, which would allow for extended measurements and would produce more detailed time observations of
distribution of velocity vector module.
Mechanical stability of the assembly and distribution of measuring heads in a heading
cut could be improved by connecting long and numerous cables into a single bundle,
which would improve spatial handle of elements of a measuring system.
The following conclusions were drawn:
1. on the basis of the data received it is possible to assess phenomena occurring
in flows within borders between flow axis and logarithmic substrata of velocity
distribution in a stringing layer,
2. discussed system allows for simultaneous measurements of air flow velocity in
a selected measurement cuts,
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3. transfer band of hot-wire anemometric sensors allows for non-stationary measurements of air flow in a heading,
4. Thin-film platinum-ceramic sensors used in heads are mechanically highly resistant
in mine conditions,
5. obtained assessment results have to be supplemented with further data from measurements conducted in various geometric configurations of beds.
This paper received a grant for science for the years of 2006-2008 within the
research project 4 T12A 008 30.
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Received: 14 November 2007