Possible Methods and Features of Measuring the Performance of Mine Auxiliary Fans Under Operating Conditions
Abstract Generated abstract
This paper analyzes methods for measuring the capacity of adjustable mine auxiliary ventilation fans under operating conditions, with emphasis on axial fans controlled by rotation of inlet guide apparatus blades. It compares measurement by static pressure, dynamic pressure, partial air flow, and total air flow, assessing each method in relation to swirl, nonuniform velocity fields, changing operating modes, air properties, and practical installation constraints. The study argues that static pressure and partial-flow methods are unreliable for regulated fans, while total-flow measurement is impractical in service conditions. It concludes that the most suitable approach is to determine fan capacity from dynamic pressure measured in the interblade passage of the guide apparatus using a streamlined flowmeter with a rotating blade, while accounting for the aerodynamic and operational factors that affect accuracy.
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Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov
Volume 158
1968
Possible Methods and Features of Measuring the Performance 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)
A radical way to increase the economy of operation of mine auxiliary-ventilation fan installations is the use of adjustable fans.
Operation of adjustable fans under optimal conditions is possible if there is an instrument by means of which it would be possible to determine the performance of fans under operating conditions.
In designing such an instrument, it is necessary first of all to analyze the possible methods and features of measuring the performance of mine fans under operating conditions.
This analysis is carried out with reference to an axial fan with performance control by turning the blades of the guide apparatus (GA).
In principle, the following methods are possible for determining fan performance:
- By the static pressure of the fan.
- By dynamic pressure.
- By measuring the partial air flow rate $q$.
- By the total air flow rate passing through the fan.
By the first method (Fig. 1), the static pressure downstream of the straightening apparatus (SA) is subject to direct measurement.
The advantage of this method is the simplicity of taking the static pressure itself, which is done through holes in the fan casing downstream of the SA. But the method under consideration for measuring fan performance has substantial shortcomings. In this case the air flow rate can be determined only from the previously known (passport) characteristic of the fan, expressing the dependence between the fan performance $Q$ and the static pressure created by it:
\[ Q = f(H_{\mathrm{st}}). \]
Fig. 1. Determination of fan performance by static pressure
When the fan performance is regulated by turning the blades of the GA, this dependence is not stable; consequently, the дан-
... method of determining capacity is possible only for unregulated fans.
The static pressure of a fan, as is known, is determined by the difference between the total pressure \(H\) of the fan and the dynamic pressure at the outlet from the impeller. In axial-flow fans the latter is determined from the axial discharge velocity beyond the guide apparatus in the fan delivery duct:
\[ H_{\text{st}} = H - \rho \frac{c_a^2}{2}, \qquad \frac{\text{kg}}{\text{m}^2}, \]
where
\[ c_a = \frac{Q}{F_a} \]
is the axial discharge velocity, m/sec;
\(F_a\) is the area of the outlet section of the delivery duct, \(\text{m}^2\).
As studies have shown, the air flow leaves the impeller strongly swirled, and the guide apparatus does not straighten it sufficiently. In this case the static pressure measured in the delivery duct will be less than the static pressure of the fan by the amount of the swirl pressure:
\[ h_{\text{st}} = H - \rho \frac{c^2}{2} = H_{\text{st}} + \frac{\rho}{2}\left(c_a^2 - c^2\right) = H_{\text{st}} - \rho \frac{c_u^2}{2}, \qquad \frac{\text{kg}}{\text{m}^2}, \]
where
\[ c = \sqrt{c_a^2 + c_u^2} \]
is the total velocity vector at the measurement point;
\(c_u\) is its circumferential component;
\[ H_{\text{dyn}}^{\text{swr}} = \rho \frac{c_u^2}{2} \]
is the swirl pressure, \(\frac{\text{kg}}{\text{m}^2}\).
Determining the capacity of a fan from its characteristic curve and the measured \(h_{\text{st}}\) leads to an error, since the fan characteristic expresses the dependence between \(Q\) and \(H_{\text{st}}\). Therefore the method considered for measuring fan capacity is not advisable.
Liquid and spring manometers may be used to measure static pressure. Under fan operating conditions, the use of liquid manometers is generally undesirable, while the use of spring manometers is very limited because of their sensitivity to vibration, the complexity of their design, and other reasons.
When determining the capacity of a fan by the second method, the dynamic pressure is subject to direct measurement. When the dynamic pressure is taken with the aid of a pneumometric tube, the fan capacity can be determined relatively simply:
\[ Q = k_c \cdot c_0 \cdot F, \qquad \text{m}^3/\text{sec}, \]
where
\(k_c\) is the velocity-field coefficient;
\[ c_0 = \sqrt{\frac{2H_{\text{dyn}}}{\rho}} \]
is the flow velocity at the point where the dynamic pressure is measured, m/sec;
\(F\) is the area of the section in which the mean velocity is determined:
\[ c_{\text{avg}} = k_c \cdot c_0. \]
It should be noted that this method of determining capaci—
…will be sufficiently accurate if, when the operating mode of the fan is changed, the velocity-field coefficient remains constant.
The dynamic pressure of the flow can be measured in front of the fan inlet guide vanes (IGV) (Fig. 2, a), in the interblade channels of the IGV (Fig. 2, v) or downstream of the straightening device (SD), in the fan discharge duct (Fig. 2, b).
When the operating mode of the fan changes, the swirl imparted to the flow by the impeller changes. A straightening device with fixed blades cannot ensure complete deswirling of the flow at different fan operating modes. Different swirl at the outlet from the SD makes it difficult to measure the dynamic pressure in the discharge duct and to determine the fan capacity.
The distribution of velocities in the flow cross-section before its entry into the IGV depends mainly on the conditions under which air enters the fan. The absence of a collector, or the use of a poor-quality collector (which is most often encountered under fan operating conditions), causes flow separation at the walls of the fan IGV casing. In addition, the IGV blades retard and split the flow into parts. For these reasons the velocity field before the IGV is nonuniform and uneven, which considerably complicates determination of the velocity-field coefficient and establishment of proportionality between the velocity determined from the dynamic pressure at some point of the flow and the actual capacity of the fan.
The dynamic pressure of the flow can be determined from the force action of the flow on a solid body washed by this flow. The operation of a flowmeter of the drag type with a pivoting blade is based on this principle. This instrument must be installed in a flow channel with the most uniform and even velocity field, since the accuracy of the flowmeter readings depends on this.
Fig. 2. Determination of fan capacity from dynamic pressure
In our opinion, in an axial fan the velocity field will be most uniform in the interblade channels of the IGV. It is here that it is expedient to install a flowmeter with a pivoting blade.
Investigations confirmed the correctness of this conclusion and made it possible to draw conclusions concerning the features of determining fan capacity from the dynamic pressure measured by means of a flowmeter with a pivoting blade in the interblade channel of the IGV. These features are as follows:
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The velocity fields in the flow channels of the IGV are nonuniform, since, as the flow passes through the interblade channel, a boundary layer is formed at the walls of the latter. The aerodynamics of entry into the IGV and the flow around its blades do not ensure uniformity of the velocity field in its flow channels.
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When the fan capacity is regulated by turning the IGV blades, the kinematics of the flow in the IGV flow channels changes substantially.
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The air flow entering the fan is unsteady. This is explained by the fact that in the network supplying the electric dri…
fan, the voltage often changes; the latter leads to a change in the number of revolutions of the fan and, consequently, to a change over time in the velocities in all flow passages of the fan.
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Under operating conditions of the fan, the temperature of the mine air and its pressure may change significantly, which causes changes in the density and viscosity of the air drawn in by the fan.
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Installing a flowmeter in the interblade passage of the HA somewhat changes the aerodynamics of the flow around the HA cascade of profiles and results in a flow rate \(q < \dfrac{Q}{z}\) passing through this passage (where the number of flow passages is \(z\)).
According to the third method, what is subject to direct measurement is not the total air flow rate passing through the fan, but a certain partial flow rate \(q\). For example, the air flow passing through a flow-measuring device and drawn in by the operating fan through an opening in the casing before the HA (Fig. 3, a) or after the HA (Fig. 3, b).
Fig. 3. Determination of fan capacity by partial air flow rate.
The advantage of this method is the possibility of direct measurement of the flow rate \(q\) by any flowmeter of the streamlined type. This flow rate is determined by the pressure difference:
\[ \Delta P = P_a - P, \]
where
\[ \begin{aligned} P_a &\text{— pressure before the fan;}\\ P &\text{— pressure before the HA or after the HA, depending on the location of air intake.} \end{aligned} \]
In principle, it is possible to establish a relationship between the partial flow rate and the actual capacity of the fan, \(q = f(Q)\), since \(\Delta P = \varphi(Q)\).
The application of the method of determining fan capacity considered here may be regarded as expedient if the dependence \(q = f(Q)\) is proportional under the various operating modes of the fan.
The obvious change in the aerodynamics of the inlet to the HA and of the flow around the HA cascade of profiles when the fan capacity changes does not allow one to count on proportionality of the dependence \(q = f(Q)\). Therefore, under operating conditions of the fan, determining its capacity by measuring the partial flow rate cannot be recommended.
Measuring the fan capacity by the total air flow passing through the fan is, in principle, possible. For this it is necessary to use special fan installation schemes employed in their laboratory tests. Under operating conditions of the fan this is, obviously, impractical.
On the basis of the above, the following conclusion may be drawn:
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Under operating conditions it is expedient to determine fan capacity from the dynamic pressure measured in the HA interblade passage, using a streamlined flowmeter with a rotating blade.
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In designing and calculating such a flowmeter, the above-noted features of measuring fan capacity under its operating conditions should be taken into account.