Determination of the mass-flow coefficient of jet-reactive turbine’s feed-in nozzle

The possibility of adjusting of the jet-reactive turbine’s head parameters through changing of mass-flow of the working body with the help of adjustable feed-in nozzle is examined. The mass-flow coefficient of the feed-in nozzle is determined experimentally.


Introduction
Nowadays, the requirements to the efficiency, environmental friendliness, reliability, and safety of running engines, operating in the national economy, are becoming much more severe. It fully refers to the drives of ball valves of compressor stations (CS) and cross-country pipelines (CCP).
Following the latest demands of the customers the drives of such ball valves should use raw gas (right from the pipe) as the working body and operate the valve under the drive inlet pressure from min 2MPa to maximum pressure (16MPa). Herewith, the drive inlet gas temperature can vary within the range from -60 o C till +80 o C depending on the ball valve placement and environmental climatic conditions.
For ball valves with flow path diameter DN>300mm the most wide-spread are pneumohydraulic piston-type drives, which are more simple, safer and cheaper than electric motor drives. To provide drive's operation two kinds of fluids are necessary: raw gas, running right from the gas pipe-line through the control unit to pneumatic cylinder and nonfreezable oil running to the hydraulic cylinder and manual pump. The main disadvantages of such drives are complexity and clunkiness of the construction, low operating efficiency, difficult operation, as constant presence of oil in the system, control over it and its' refilling (in case of oil leakage emergency conditions appear) are required.
Production of air-powered drives of volumetric operational mode for the required settings of ball valves with big flow-path diameter is quite complicated. It is determined by the necessity of dramatic increase of dimensions of drive's power section in order to provide the workability of drive under low pressure.
The solving of this problem is possible through the production of the air-powered drive with an engine, making no excess forcing within the whole range of pressures and being safely operated. As such type of engine we offer a simple, relatively cheap, easily reversing turbine's engine based on the jet-reactive turbine (JRT) [1].
Air-driven drives operating turbine's air-driven engines are sensitive to the change of the load and are basically suitable for the devices with the stable load and settings of the working body at the inlet. However, in case of necessity, they can be quite easily mechanically adjusted to the computation parameters with the help of regulators of mass-flow, pressure, rotating frequency, etc. It should be noted that while the usage of turbine engine as the air-driven engine there appears the possibility for the efficiency usage of the great difference of pressures, and taking into account that real pressure in the pipelines is more than 5MPa, turbine engines can compete with the piston ones even according to the coefficient of efficiency). While the usage of turbine-driven engines for the examined series of valves it should also be taken into account that the critical sections of flow section of the turbine-driven engine should have as small dimensions as possible to provide the smallest flow, as energy losses in the supply pipeline are equal to the quadrupled flow. Taking into account all ideas expressed above (possibility for efficiency usage of the great difference of pressures, small dimensions of critical sections) jet-reactive engine and turbine-driven engine, using classical one-nozzle turbines (axial-flow and inward-flow), show themselves up among the series of turbine-driven engines. Jet-reactive engine is easily reversed, has lower flywheel effect and integral pipeline ( Figure 1) that decreases the possibility of its freezing, though the coefficient of its efficiency can be lower. The main advantages of the turbine-driven engines comparing to the piston ones is excluding of the emergencies and smaller weight-anddimensional characteristics.

Objectives settings
According to the information stated above it is clearly seen that jet-reactive drives should work with the maximum efficiency or close to it. However, the inlet parameters are not always stable; as a result of this the fast reaction to these changes (regulation) is required. As it has already been mentioned above, the basic methods of JRT parameters regulation is by inlet pressure, rotor spinning frequency, and mass-flow. The last method we shall study in details, as the change of mass-flow can be achieved without any additional expenditures and complication of the drive's construction.

Aim of research JRT
The aim of the work is to receive experimental dependences of the mass-flow and the flow coefficient of feed-in nozzle depending on the JRT inlet pressure, what, being based on these dependences, will allow in future using the feed-in nozzle even as the flow-meter.
As the first stage of researches in Sumy State University (Technical Thermophysics Department) there was made a test stand for the researching of the jet-reactive turbines and based on them pneumatic units, for the improving of the calculation methodology of JRT characteristics and receiving basic nondimensional dependences.
The model of the jet-reactive turbine as the basic element of the test stand is presented in Figure 2. Gas is running through the feed-in nozzle to the hollow rotor shaft and further through the gas path to the HN. In the feed-in nozzle the potential energy of compressed gas is transformed into the kinetic energy releasing together with sonic and supersonic speed of gas flow, which in its turn creates jet reaction and as a consequence rotation moment on the turbine shaft, which in its turn is calculated according to the famous dependence [2]: in which: M U -moment, which is defined according to the theorem of moment of momentum of the gas flow relatively to the spinning axis, based on the co-operation of gas flow with JRT flow part; M r.r -rotor resistance moment in the environment (aerodynamic moment); K r.r -aerodynamic resistance coefficient; depends on density of the environment, rotor outside diameter, shape, quantity and mutual position of pipes of shoulder tubes of rotor; rotation frequency of the turbine shaft; it can be defined only experimentally; Т  -angular rate of turbine rotation.
For the no-load conditions, when 0 Т M  , we shall receive (not taking into account mechanical losses in bearings) For the calculation of the head parameters and characteristics of JRT and pneumatic units, based on it, it is necessary to know actual gas flow through feed-in nozzle and that is the coefficient of this nozzle flow. Besides, the receiving of experimental dependencies of mass-flow and coefficient of feed-in nozzle flow, depending on the pressure on the JRT inlet will allow in future using the feed-in nozzle also as a flow-meter. That is why the experimental researches on the determination of the coefficient of feed-in nozzle flow in different positions of valve needle and pressures at the inlet were conducted. (see Figure 3) In its structure feed-in nozzle consists of: supply pipe with pressure gage placed on it for inlet pressure measurements; casting, in which needle seat with expansion angle 60º, constructing outer nozzle closure, is placed; regulation needle, spinning along the thread of screw in the casting and having possibility to move in the axial direction tightening to the seat. Thus, nozzle flow path is reduced. Pith of metric screw thread in the casting for the needle is equal 1mm.
Position, in which, the needle was completely tightened to the seat (nozzle fully closed) was considered to be the starting mode. For this needle position h=0 mm (see Figure 3). Then, needle was gradually moving back to h=16.
For the conduction of measurements and obtaining basic gas-dynamic dependencies there was constructed the test stand, structural drawing of which, is presented in Figure 4. The test stand allows examining starting and no-load modes of jet-reactive turbine. The figure shows: F -air-intake filter; C -compressor; V -shut-off valve; EV -emergency valve; R -receiver; RV -regulation valve, G 1 , G 2 -pressure gages; PV -pressure-control valve of vesseltype; T 1 , T 2 -temperature meters (electronic multi-meter); AR -air-flow regulator (feed-in unit of JREM); LG -load gage of weight type; JREM -jet-reactive expansion machine; T -tachometer; FM -flow-meter; DG -differential pressure gages. Figure 5 shows the results of the measurements. As it is seen from the graphical curves, while the increasing of the needle move h>10 mm lines    Diagrams character proves theoretical dependencies of the gas flow through the nozzle and the nozzle head [4]. Gas mass-flow while the process of isentropic flow through the short nozzle head from the hollow of unlimited volume is described by two equalizations: for the pre-critical mode suits the equalization:  In the result the graphical diagram of changes   fh   was received. It is presented in Figure 9.

Conclusions
Experimental dependences of mass flow and flow coefficient for the regulating feed-in nozzle of jetreactive turbine in different positions of regulating needle and with different pressure values in the nozzle inlet were received, that allowed clarifying of techniques for calculation of turbine head settings and characteristics. It was defined that while the needle move from the stop mode into the casting 2 h mm  flow coefficient of feed-in nozzle remains actually constant and equal to 0,9.
Obtained dependencies also allow refusing from installing flow-meters, and defining actual gas flow right by the diagram for jet-reactive turbines with similar construction of supply nozzle.