Flow-Around Flowmeter with a Pivoting Vane for Measuring the Capacity of Mine Local Ventilation Fans under Operating Conditions
Abstract Generated abstract
This paper addresses the need for a simple, reliable instrument to measure the capacity of adjustable mine auxiliary ventilation fans under operating conditions, where vibration, humidity, dust, and temperature variation limit conventional devices. It examines a flow-around flowmeter with a pivoting vane mounted in the fan guide apparatus, deriving its operation from the balance between aerodynamic force and gravity and calibrating prototypes on a test rig using reference airflow measurements. Experiments compared several vane body shapes and found that spherical and curved S-shaped bodies gave nearly linear relationships between flow rate and scale reading, while pointer oscillations depended on body shape, inlet flow formation, and unsteady fan operation. The study concludes that pivoting-vane flowmeters are suitable for this application and that damping devices can substantially reduce oscillations.
Full Text
FLOW-AROUND FLOWMETER WITH A PIVOTING VANE FOR MEASURING THE CAPACITY OF MINE AUXILIARY VENTILATION FANS UNDER OPERATING CONDITIONS
I. Yu. Sokolova
(Presented by the scientific seminar of the Department of Mining Machines, Mine Transport, and Mining Mechanics)
One of the ways to improve the economy of operation of auxiliary ventilation fan installations is to create adjustable fans. Regulation of fan capacity under operating conditions makes it possible to reduce electricity consumption by 18–20%.
At present, of all the existing methods for regulating the capacity of auxiliary ventilation fans, the simplest in design and the most economically advantageous is regulation by rotating the blades of the guide apparatus (GA).
To ensure optimal operating modes of an adjustable fan, an instrument is needed by means of which it would be possible to determine the capacity of the given fan under its operating conditions.
The basic technical requirements for the design of such an instrument are determined by the operating conditions of the fan in the mine. The instrument must be simple in design and form a single unit with the fan. In addition, it is necessary to ensure reliable operation of the instrument under vibration of the fan casing, relative air humidity up to 100%, air temperature from \(-10\) to \(+30^\circ\text{C}\), and air dustiness usual under mine conditions. The readings of the instrument must remain stable throughout the guaranteed service life of the fan.
Of the existing types of flowmeters, the flow-around flowmeter with a pivoting vane (Fig. 1) best meets the requirements set forth above. The principle of operation of such a flowmeter is based on the force interaction between the air stream and a solid body. The aerodynamic force acting on the body being flowed around is determined, as is known, by the formula:
\[ P = \kappa \cdot \rho \cdot \frac{c^{2}}{2} \cdot F,\ \text{N}, \]
where
\(\kappa\) — drag coefficient, depending on the shape of the body being flowed around;
\(\rho\) — air density, \(\text{kg}/\text{m}^3\);
\(c\) — velocity of air motion, \(\text{m}/\text{s}\);
\(F\) — the “midship” section of the body, i.e., the area of the projection of the body onto a surface normal to the direction of the flow, \(\text{m}^2\).
The moment of the aerodynamic force acting on the streamlined body, relative to the latter’s axis of rotation, is balanced by the moment of the force of gravity of this body relative to the same axis. Therefore, when the flow velocity is measured, the angle of deflection of the streamlined body from its initial (vertical) position also changes. Frictional forces in this case may be disregarded, since they are negligibly small, and the presence of vibration excludes their influence on the instrument readings.
Fig. 1. Schematic of a flow-around flowmeter with a pivoting vane
The flow-around flowmeter with a pivoting vane is installed in the inter-blade channel of the fan guide apparatus (Fig. 2). The body of the instrument is fastened directly to the casing of section NA.
Fig. 2. Schematic of flowmeter installation
For the investigation and calibration of such a flowmeter, a special test rig was built. The operating conditions of the fan and the flowmeter here corresponded to service conditions. The actual airflow rate passing through the fan was determined by means of an orifice plate and a total-pressure nozzle.
The streamlined body had to be given such a shape and dimensions that: a) the instrument scale would be used completely; b) the dependence between the actual airflow rate passing through the fan and the angle of deflection of the streamlined body and of the instrument pointer located on the same axis with it would be as linear as possible, since only in this case can the instrument scale be uniform; c) the instrument readings would be stable in time, i.e., so that oscillations of the instrument pointer would either be eliminated altogether or would be insignificant. The latter, as will be shown below, depends not only on the shape of the streamlined body itself. To determine the shape of the streamlined body most consistent with the requirements stated above, a series of experiments was carried out.
The dependence between the actual airflow rate passing through the fan and the instrument readings in units of a conventional uniform scale for streamlined bodies of various shapes is shown in Fig. 3. For a streamlined body in the form of a sphere and in the form of a curved S-shaped plate, this dependence may in practice be considered linear. For a rectangular plate and a cylinder, this dependence is not linear.
In the course of studying the operation of streamlined bodies of various shapes, the amplitude of oscillations of the instrument pointer was also determined. The investigations showed that these oscillations are less significant with a streamlined body in the form of a rectangular plate. When the flow acts on a sphere, the oscillations of the instrument pointer were the greatest.
To reduce the oscillations of the instrument pointer, a damper was used in the form of a weight fastened to the instrument pointer. The use even
such a simple damping device significantly reduced the oscillations of the instrument pointer. The causes of the oscillations of the instrument pointer can apparently be explained by the following:
- The flow entering the fan’s guide apparatus was not properly formed;
- This flow was not steady.
- Separation phenomena occurred when the body was flowed around by the air stream.
Fig. 3. Fan capacity and flowmeter readings with a body of different shape being flowed around: 1—cylinder, 2—rectangular plate, 3—curved S-shaped plate, 4—sphere
As is known from hydrodynamics, the inlet conditions have a great influence on the formation of the flow at the entrance to a pipeline. In order for the flow entering the fan’s guide apparatus to be well formed, a suction pipeline of a certain length with a collector is needed.
On the experimental installation (in accordance with operating conditions), there is no suction pipeline in front of the fan guide apparatus, where the flowmeter is located. Therefore the air flow entering the guide apparatus is not properly formed, and separation phenomena may occur in the inlet section. To check how the inlet conditions of the flow into the fan guide apparatus affect the oscillations of the streamlined body and of the instrument pointer, the following experiment was conducted. A suction pipeline 5 diameters long with a collector was installed in front of the fan guide apparatus, which made it possible to obtain a formed flow. The amplitude of oscillation of the instrument pointer in this case decreased by approximately a factor of two.
The second cause is explained by the fact that the voltage in the network often changes; the latter leads to a change in the number of fan revolutions and, consequently, to a change over time in the velocity of air movement in all flow passages of the fan.
Under operating conditions of the fans, the first and second causes of oscillation of the instrument pointer cannot be eliminated. Separation phenomena when a solid body is flowed around by a stream can be reduced by giving this body the most streamlined shape.
On the basis of the foregoing, the following conclusion may be drawn.
- For measuring fan capacity under operating conditions it is advisable to use flow-around flowmeters with po-
with a pivoting vane, as being the simplest in design and reliable in operation.
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The streamlined body may have the form of a sphere or of a curved S-shaped plate, since in these cases the relationships between the flow rate and the instrument readings may be considered linear. The advantage of the sphere is that it reacts less than the S-shaped plate to deflection of the flow when the guide-vane blades are turned in the process of regulating the fan capacity. The disadvantage of the sphere is its oscillations, which are more significant than those of the S-shaped plate.
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The oscillations of the streamlined body and of the instrument pointer connected with it can be substantially reduced by using a damping device.