Arch. Min. Sci., Vol. 53 (2008), No 1, p. 87–96
Transkrypt
Arch. Min. Sci., Vol. 53 (2008), No 1, p. 87–96
Arch. Min. Sci., Vol. 53 (2008), No 1, p. 87–96 87 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 88 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 90 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. 92 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, 96 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. REFERENCES D z i u r z y ń s k i W., T r u t w i n W., C i e r n i a k W., 1996. Stany nieustalone w kopalnianych sieciach wentylacyjnych, Prace Instytutu Mechaniki Górotworu PAN, Kraków. E l s n e r J., 1987. Turbulencja przepływów, PWN, Warszawa. L i g ę z a P., 1994. Thermoanemometric method for measuring velocity and temperature in non-isotermal flows, Archives of Mining Sciences, vol. 39, p. 367. L i g ę z a P., P o l e s z c z y k E., 2004. Termoanemometryczne metody pomiaru prędkości – aparatura pomiarowa w kopalni, Arch. of Mining Sciences, vol. 49, Special Issue, p. 141-150, Kraków. L i g ę z a P., P o l e s z c z y k E., 2005. Multi-points measurements of gas flow velocity fields. Arch. of Mining Sciences, vol. 50, iss. 4, p. 417. L i g ę z a P., P o l e s z c z y k E., 2006. Integrated Hot-Wire Probes for Measuring Gas Flow Parameters in Mining Conditions. 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