**1. Introduction**

442 Mechanical Engineering

All of the exposed diagrams are related to non dimensional ratio system coordinates, where are: k1 piston position, pa static pressure in actuator supply chamber, pr static pressure in actuator return chamber, r control servo valve throttle position, ratio of power reserve corresponding to applied external load. Presented system model enables its compatibility corresponding to the various initial conditions. On figure 14 are shown system simulation for incompressible fluid flow and corresponding model non compatibility of initial conditions. Possible pressure difference between supply and return actuator chambers which is not compensated by external force produces piston "shock" motion, which can not be described by incompressible flow system modeling. Pressure difference can be caused by various effects which produces fast changes of fluid static pressure and/or external load. Actuator locked position for longer time period also is the reason for described effects. Pressure drop or surge caused by fluid compressibility and initial condition discontinuity as result of closed control servo-valve throttle position are shown on the diagrams on figures 16 and 17. Piston position difference in relation with its incompressible model motion is defined on figure 44. Initial piston acceleration produces practical piston "shock" motion , expressed in the later as increasing static error of its position (less than 1% for usual types of fluids). Mentioned effects are of high interest for digitally driven actuators. Supply and return pressure surge is presented on figure 45. Supply and return pressure drop are

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**4.1.8 Conclusion** 

presented on figure 46.

Italy, 1996.

Symposia AIA '98, Ankara, 1998.

međunarodni, objavljen

Canada 1955.

**5. References** 

In power station practice, work is extracted from expanding steam in three stages namely High Pressure(HP), Intermediate Pressure(IP) and Low Pressure(LP) turbines. During the expansion process in the LP turbine, the steam cools down and at some stages, it nucleates to become a two-phase mixture. It is well-acknowledged in the literature that the nucleating and wet stages in steam turbines are less efficient compared to those running with superheated steam. With the advent of water-cooled nuclear reactor, the problem becomes more prominent due to the fact that in water-cooled nuclear reactor, the steam generated is in saturated condition. This steam is then supplied to the HP steam turbine which therefore has also to operate on wet steam. One of the tangible problems associated with wetness is erosion of blading. The newly nucleated droplets are generally too small to cause erosion damage but some of the droplets are collected by the stator and rotor blades to form films and rivulets on the blade and casing walls. On reaching the trailing edges or the tips of the blades, the liquid streams are re-entrained into the flow in the form of coarse droplets. It is these larger droplets that cause the erosion damage and braking loss in steam turbine. However, the formation and behaviour of the droplets have other important thermodynamic and aerodynamic consequences that lower the performance of the wet stages of steam turbines.

Interest in wet steam research was sparked by the need for efficient steam turbines used in power generation. The subject has become increasingly important in current decades with the steep increase in fuel cost. The importance of steam turbine in society is obvious considering that most of the world's power generation takes place using steam-driven turbines. Even though the importance of these machines is obvious, very little attention is given by researchers to understanding the flow behaviour inside steam turbines in comparison with other prime movers. Considerable progress has been made in the investigation of flow in gas turbines because of their applications in the aeronautical field. The findings from gas turbine research are applicable to the dry stage in steam turbine only. However, attention must also be paid to the wet stages as a significant proportion of the output is generated by them. In recent years, work in wet steam research has gained interest with the advent of high performance computing machines and measurement devices. Most of the works aim to accurately model the droplet formation using different calculation

Numerical Modeling of Wet Steam Flow in Steam Turbine Channel 445

was progressing with remarkable speed and the problems associated with the presence of liquid were being experienced. One of the particular serious consequences resulting from the presence of liquid in steam turbines is blade erosion. However, with the introduction of reheat cycles after the Second World War, the problem was temporarily alleviated. Following new developments and designs of larger steam turbine, the steam velocity could reach much higher values. This led to considerable impact velocities which brought renewed interest in wetness problems. Among the many researchers who have reported their investigations into the field are (Gyarmathy, 1962, Pouring, 1965, Hill, 1966, Puzyrewski, 1969, Wegener, 1969, Campbell & Bakhtar, 1970, Barschdorff, 1970, Filippov & Povarov, 1980). In addition to these investigations, a number of studies aimed at measuring the size of droplets formed by spontaneous nucleation were reported by (Gyarmathy & Meyer, 1965, Krol, 1971, Deich et al., 1972). The measurements have provided further data

An expansion of steam from superheated to wet condition in a typical convergent-divergent nozzle is illustrated in Fig. 1. The process can be also illustrated on an *h − s* diagram as shown in Fig. 2. Steam enters the nozzle as dry superheated vapour at point (1) and during its passage through the nozzle, it expands to the sonic condition represented by point (2). At point (3) in Fig. 2, the saturation line is crossed which may occur before or after the throat and droplet embryos begin to form and grow in the vapour. The nucleation rates associated with these early embryos are so low that the steam continues to expand as a dry singlephase vapour in a metastable, supercooled or supersaturated state. Depending on the local conditions and the rate of expansion, the nucleation rate increases dramatically and reaches

Fig. 1. Axial pressure distribution in nozzle with spontaneous condensation.

for comparison with the theoretical solutions.

**2.3 Condensation in nozzle** 

method such as quadrature method of moments (Gerber & Mousavi, 2007). Attempt was also made to use commercial CFD package to calculate the three dimensional steam properties (Dykas et al., 2007, Nikkhahi et al. 2009, Wroblewski et al., 2009).
