Test Results of an Experimental Pneumatic Reversible Two-Stage Turbine

S. I. Shubovich, V. I. Lvov, and R. D. Sharapov

(Presented by Prof. V. T. Yurinskii, Doctor of Technical Sciences)

In an earlier published article [1] it was shown that, for the purpose of reversing mine turbine engines, a reversible turbine can be used while still retaining comparatively high technical and economic performance indicators of the machine.

A reversible turbine, unlike a conventional one, is made with two blade rows on one working disk and, correspondingly, with two independent nozzle apparatuses directed oppositely. Changing the direction of rotation of the shaft of a pneumatic motor equipped with such a turbine is achieved by switching the supply of compressed air from one nozzle apparatus to the other.

The advantages of the proposed pneumatic method of reversing, in comparison with the known method of reversing by switching gears in the engine reduction gear¹, are quite obvious and consist of the following: 1) the direction of rotation of the engine shaft can be changed while running; 2) it becomes possible to brake the engine during stopping by briefly supplying compressed air to the reverse-running wheel; 3) engine control is simplified, since in this case reversing of the mining machine is accomplished by turning the handle of a three-way valve.

The disadvantages of pneumatic reversing include the greater complexity of the turbine design and a certain reduction in efficiency compared with a similar turbine of the nonreversible type. The decrease in efficiency of the reversible turbine arises as a result of additional ventilation losses caused by the rotation of the reverse-running wheel in the air medium.

For an experimental verification of the proposed method of reversing and to determine the traction and economic characteristics of the reversible turbine, at the proposal and with the participation of the authors an experimental reversible turbine of radial type was designed and manufactured at the Tomsk Electromechanical Plant named after V. V. Vakhrushev (Fig. 1); the results of its tests are given in the present article.

The radial type of turbine was chosen in the design for the reason that, in this case, the blade rows and nozzle apparatuses for right-hand and left-hand rotation can be made identical in form and dimensions. In addition to technological advantages, such a turbine ensures identical characteristics of the pneumatic motor for both right-hand and left-hand rotation of the shaft.

In designing the experimental turbine, the additional task was also set of reducing the nominal rotational speed of the turbine rotor; therefore its flow passages were made with two velocity stages, of the type used in single-wheel multistage turbines (Fig. 2). The choice of a turbine with two velocity stages was further justified by the fact that, with a limited nominal rotational speed and a limited rotor diameter¹, such a turbine

Fig. 1. Experimental reversible turbine.

in comparison with a single-stage turbine, has a greater starting torque and more favorable speed characteristics.

Principal design parameters of the experimental turbine

¹ In mine turbine engines, the rotational speed of the rotor is limited by the service life of the rolling bearings, and its diameter by the permissible overall dimensions of the engine. Calculations show that, with this limitation of rotational speed and diameter, the use of more than two stages leads to a sharp decrease in turbine efficiency.

Fig. 2. Diagram of the flow passage of the reversible turbine

Fig. 3. Diagram of the experimental installation. 1—reversible turbine; 2—speed reducer; 3—hydraulic brake; 4—tachometer; 5—oil cascade of the hydraulic brake; 6—water line; 7—air line; 8—thermometer; 9—pressure gauge for measuring the pressure upstream of the throttling orifice; 10—differential pressure gauge of the throttling device; 11—three-way valve for reversing the turbine; 12—pressure gauge for measuring the pressure upstream of the nozzle box; 13—U-shaped pressure gauge for measuring the pressures in the turbine chambers; 14—switching valve.

Testing of the turbine was carried out on a stand (Fig. 3) equipped with a hydraulic brake, a speed reducer, a throttling device for measuring air consumption, a tachometer, and instruments for measuring pressures and temperatures.

Before the main tests to obtain the characteristics, the turbine was subjected to tests to determine reliability and reversing time. For this purpose, while the turbine (disconnected from the brake) was operating at nominal speed, the air supply was switched to the reverse-running blade ring by turning the handle of a three-way valve (Fig. 3). The reversing time of the engine, i.e., the time during which the engine stopped, changed the direction of shaft rotation, and accelerated to the nominal reverse speed at the design compressed-air pressure \(p = 3.5\) ati, was 10–15 sec. The reliability of reversing in this case gave no cause for doubt.

The main tests of the reversible turbine consisted in taking speed characteristics at the design compressed-air pressure. The characteristics were taken for right-hand rotation of the rotor in two cases:

1) with the inlet and outlet openings of the chamber of the left-rotation blade ring closed;

2) with the chamber openings closed and the channels of the left-rotation blade ring soldered shut.

The inlet and outlet openings of the chamber of the left-rotation blade ring were closed in order to reduce the ventilation losses of this ring. Closing the above-mentioned openings gives a substantial effect in reducing the ventilation losses of the reverse-running ring [1], and therefore mine engines with a reversible turbine should include a special device for automatically closing the openings during reversing.

Tests with the channels of the left-rotation ring soldered shut were carried out in order to determine the magnitude of the losses caused by rotation of the reverse-running blade ring in the chamber with the openings closed.

To determine the magnitude of the starting torque, additional experiments were carried out. For this purpose, a lever was fastened to the turbine shaft, at the end of which, by means of weights, the force was determined when air at a pressure of 3.5 ati was supplied to the turbine.

Measurements and calculations of torque, effective power, and air consumption were carried out according to the generally accepted procedure. Losses in the reducer were included in the turbine losses in the calculations. Therefore, the test results obtained below are considered as the results of tests of a turbine reversible engine consisting of the turbine and the speed reducer.

The test results are presented by the graphs in Fig. 4. From these graphs it can be seen that the power expended on rotating the reverse-running blade ring in the closed chamber at the speed corresponding to maximum power is 0.6–0.7 hp, which amounts to 3–3.5% of the power developed by the turbine. Such a value of the losses, caused by rotation of the ring with the leading edges of the blades forward in the chamber with the inlet and outlet openings closed, agrees well with the theoretical premises [1] that served as the basis for creating the reversible turbine.

For comparison of the obtained indicators of the reversible turbine engine with those of engines of other types, a graph is given (Fig. 5) in which the torque values and the number of revolutions are presented as fractions of their nominal values. The characteristics of the DTR-20 engine, equipped with a single-stage nonreversible turbine, and of the PRSh-10 gear motor (Fig. 5) are plotted from data of tests conducted earlier at the Tomsk Electromechanical Plant.

From Fig. 5 it can be seen that the minimum specific consumption of compressed air of the engine tested, in comparison with the consumption of the DTR-20 engine, is greater by 4%. Such a slight increase in air consumption, caused by additional ventilation losses of the reverse-running crown, cannot diminish the advantages of pneumatic reversing; therefore the use of reversible turbines in mine pneumatic engines should be regarded as entirely expedient.

c

Figure annotations: \(N_e\), hp; \(M\), kgm; “with closed return-running channels”; \(q\), m³/hp·min; \(n_t\), rpm; \(n_e\), rpm.

Fig. 4. Speed characteristics of the experimental reversible turbine engine.

Comparison of the curves of specific consumption (Fig. 5) shows that the economy of the reversible turbine engine changes with variation of the speed regime to a lesser degree than the economy of the DTR-20 engine. The more favorable course of the curve of specific consumption of the reversible engine is explained by the fact that in this engine a turbine with two velocity stages is used. As can be seen from Fig. 5, the use of a two-stage turbine also ensured an increase in the starting coefficient.

moment coefficient¹ by 28% compared with the starting-moment coefficient of the DTR-20 engine.

From the comparison presented it follows that the use of turbines with two velocity stages in mine pneumatic motors is fully justified, since in this case not only is a reduction in the rotational speed of the turbine rotor achieved, but the tractive properties of the motor and its economic characteristics are also noticeably improved. If one compares—

Fig. 5. Comparative graph of the characteristics of turbine and gear motors

—the characteristics of the reversible turbine motor with the characteristics of the reversible gear motor PRSh-10 (Fig. 5), it is easy to see that the gear motor is inferior to the turbine motor not only in economic indicators, but is inferior to an even greater degree in its tractive properties.

In conclusion it should be noted that, in the course of testing the reversible turbine, shortcomings were observed in the selected form of the air bypass duct between the first and second velocity stages. There is reason to believe that, after an optimal duct shape has been found, the economic indicators of the reversible turbine will increase somewhat further.

Conclusions

1. The use of a reversible turbine in mine pneumatic engines in place of the usual nonreversible turbine does not cause any appreciable reduction in the efficiency of the engine. For the experimental reversible engine, the reduction in efficiency is 4%.

2. In mine pneumatic turbine engines that, under operating conditions, are limited by overall dimensions and rotational speed, it is advisable to use two-stage turbines with velocity stages. Compared with a single-stage turbine, such a turbine has a somewhat lower rotational speed, a higher starting-torque coefficient, and a better course of the efficiency characteristic as a function of rotational speed.

References

1. Shubovich S. I. Experimental investigation of friction and ventilation losses in a turbine stage. Izvestiya TPI, vol. 75, 1954.

1. By the starting-moment coefficient is meant the ratio of the moment at starting \((n_T = 0)\) to the moment at nominal operating conditions. ↩↩