Those responsible for mine ventilation have to monitor the flow rate and average flow velocity in galleries on the online basis. Typically, those parameters are measured with hand-operated anemometers, by traversing the gallery cross-sections. To be more specific, the flow rate ought to be understood as the volume flow rate in the gallery's cross section. This flow rate can be determined on the basis of local velocity measurements, by integrating the velocity distribution over the cross-section area. Establishing the velocity field distribution in the gallery cross-section is the key element in the measurement procedure, allowing for computing the volumetric flow rate of air in the branches of the ventilation network in an underground mine. Moreover, knowledge of velocity distributions in the galleries is most useful in analyses of local airing conditions, associated with the occurrence of gas, dust and climatic hazards.
Experts' opinions and research efforts now often rely on computer simulations of 3D flows in critical sections of the ventilation network. The quality of simulation results largely depends on precise definition of boundary conditions, for instance the velocity field distribution at the inlet to the investigated gallery section. Comparison of measured and predicted values allows us to evaluate the reliability of simulation data. Computer modelling, therefore, has extended the applicability of the concept of 'velocity field' beyond the mere flow rate calculations.
Velocity fields can be investigated by several measurement methods, most of them making use of anemometric sensors placed in the investigated flow. Depending on the flow type and metrological requirements, various types of sensors can be employed.
The research team of the Laboratory of Measurement Systems has developed a portable system allowing the velocity fields to be measured and the flow rate determined accordingly.
The system relies on the network of vane anemometers transmitting the measurement data by radio to the computer system for data acquisition and processing.
The range of velocities handled by the sensors is from ± 0.2 to ± 20 m/s. Apart from sensor readouts, the information is required about the sensors' geometric position in the plane determined by the gallery's cross-section and the boundary points have to be determined on the gallery contours. This information can be obtained by taking survey measurements in the defined coordinate system, with the use of a tape measure. Another method is referred to as photogrammetry. The knowledge of the surface area of the gallery's cross – section is not required.
Dedicated calculation algorithms implemented to the computer program allows the volumetric flow rates to be determined accordingly. Velocity distributions are estimated by the linear triangulation method, whereby the cross-section area is divided into smaller areas and apart from the measured velocity, additional velocity values are introduced, either calculated or assumed. Hence, the grid of measurement points becomes denser. For instance, given a system of 16 sensors placed in the gallery's cross-section, the grid can be made denser, to yield 55 control points. The system is a mobile device, intended for use in underground mines.
More detailed information can be found in the monograph:
Krach A., Krawczyk J., Kruczkowski J., Pałka T.: Variability of Velocity Fields and Volumetric Flow Rates in Deep Mines. Archives of Mining Sciences. Monograph 1. Kraków 2006.
Selected arrays of the system in various points of the ventilation network in a mine are shown in photos.
Variations of the volumetric flow rate in a gallery under the steady-state airing conditions are shown in the photo below.
The multi-point system for velocity field measurement (SWPPP system) is a high-precision device intended for volumetric flow rate measurements. This fact has been confirmed by the validation procedure conducted in the wind tunnel at the IMG PAN Laboratory of Calibration of Measurement Devices for Ventilation. The SWPPP system assembled in the large chamber of the wind tunnel for the purpose of validation tests is shown on the photo below.
