NUMERICAL STUDY OF FLOW IRREGULARITY IN A NEW TYPE CONTROL SECTION OF STEAM TURBINE HIGH-PRESSURE MODULE

effectiveness


Introduction
Most steam turbines, including 200 and 300 MW units operating on alternating load schedules, must be designed to operate not only at full load but also at significantly (up to 50%) reduced load.Under these conditions, it is more efficient to use a nozzle steam distribution system [1].In such turbines, all major changes in the flow structure associated with the operating mode occur in the control section that consists of the first (control) and second (first pressure stage) stages, as well as of the equalization chamber located between them.In [2] it is shown that the irregularity of the relative total pressure at the control system (CS) outlet can be more than 2% for the mode of a half mass flow rate.Pressure losses in the equalization chamber at the CS are about 2% of the total inlet pressure [3].
In order to improve the flow part of the CS section and to enhance the energy performance, a three-stage control section of the steam turbine K-325-23.5HP module that has no equalization chamber was developed at the Institute of Mechanical Engineering Problems NAS of Ukraine.To determine the efficiency of gas-dynamic improvement of the CS section, it is necessary to study the spatial structure of the turbulent flow [2], [4], including the methods of mathematical modeling.In order to estimate the pressure and the mass flow rate irregularity at the CS outlet, the IPMFlow software package [5] was updated.The software package was developed by the Institute for numerical modeling of spatial turbulent flows in various power machines, in particular, steam turbines.A software module that allows to create a calculation area, which contains an arbitrary amount of blades, indicating the number and position of closed and open channels, has been added to the software package.This allowed the flow simulation in the CS with partial steam supply in all the turbine operating modes.
The results of the flow numerical studies in the steam turbine control section at partial and full supply are presented in this paper.The purpose of the study was to identify circumferential irregularity of pressure and steam mass flow at the control section outlet and the partiality effect on the irregularity.

Object of research
The Institute has developed a 3D flow part model of the three-stage control system of the steam turbine K-325-23.5HP module.The turbine is equipped with a nozzle control system (Fig. 1), which means blocking of a certain number of channels for each operating mode.The flow part of the developed control system is devoid of the equalization chamber and consists of three stages (Fig. 1, c), unlike the original twostage section (Fig. 1, b).The final version of the developed HP flow part efficiency, according to the results of the conducted calculations that don't include the steam takeoffs, was 94.7% at the rated operating mode.
The circumferential irregularity of steam flow for the operating modes of 100%, 70% and 50% of mass flow rate was studied.The 70% and 50% operating modes are characterized by two closed control valves out of four, which corresponds to 37% of the open blade channels.The studies were performed using methods and software developed by the Institute for simulation of 3D flows in power machines flow parts.Methods of research 3D calculations of steam flow in the studied flow parts were carried out using the IPMFlow software package developed by the Institute [5].The calculations of steam flow in the studied flow parts were performed on H-type computational mesh.The modeling of viscous turbulent flow was based on the Reynolds-averaged Navier-Stokes equations numerical integration.Turbulent phenomena were accounted by using the two-equation differential Menter's SST turbulence model [6].For the description of thermodynamic dependences, the two-term Tamman's equation was used [7].

Results and discussion
The researched control section flow part consists of three stages, in contrast to the original structure with two stages.The pressure magnitude and mass flow rate irregularity at the outlet of control section in modes of 100%, 70% and 50% of mass flow rate was investigated.The developed software application was used to construct calculation areas corresponding to 100% mode (all valves open) and 50-70% (two valves open, two valves closed).Fig. 2 shows the view of the calculation area, which was used to calculate the flow in the control section for the mode of 100% power.The calculation area has a total number of elementary volumes (cells) of about 10 million.
Using the IPMFlow software, numerical simulation of the flow in the control section was performed in modes corresponding to 100%, 70% and 50% of the rated mass flow.For this purpose, the following boundary conditions were set: total inlet pressure -22.73 MPa; 22.73 MPa; 16.1 MPa, total inlet temperature -808.5 K, static outlet pressure -16.578MPa; 11.628 MPa; 8.306 MPa, respectively.Quasi-steady-state conditions were used to reduce flow steadying time at the computational grid inter-row boundaries, which ensure proper transfer of flow parameters between the rows without shifting rows in time relative to each other.As a result of numerical simulation, the distribution of pressure and relative velocity fields for the 100% of mass flow mode was (Fig. 4).The figure clearly shows that the irregularity extends only to the first stage of the control section.
Also, as a result of numerical simulation, the distribution of pressure and relative velocity fields for the 70% of mass flow mode was obtained (Fig. 5).The figure clearly shows that the irregularity of pressure subsides at the last row, and the flow irregularity extends further beyond the control section.There are also significant ventilation flows in the inter-row gaps of the first stage turbine wheel.
Finally, as a result of numerical simulation, the distribution of pressure and relative velocity fields for the 50% of mass flow mode was obtained (Fig. 6).The figure shows a pattern similar to the 70% mode.Fig. 11 shows graphs of pressure distribution along the semicircle in the middle cross-section behind the control section rows for the mode of 70% flow rate.The graph clearly shows that as the flow passes rows, the pressure irregularity gradually decreases.
Fig. 12, 13, 14 show graphs of the specific flow rate distribution along the semicircle in the middle cross-section along the control section rows for the mode of 70% power.The graphs clearly show that the irregularity of channel-averaged specific flow rate remains to the last stage.Fig. 15 presents graphs of pressure distribution along the semicircle in the middle cross-section along the control section rows for the mode of 50% power.The graph also clearly shows that in almost all stages there is a significant irregularity of pressure, except for the values at the section outlet, same as for the 70% mode.
Fig. 16, 17, 18 show graphs of the specific flow rate distribution along the semicircle in the middle cross-section along the control section rows for the mode of 50% power.The graphs clearly show that the irregularity of channel-averaged specific flow rate remains to the last stage.

Conclusions
The results and analysis of steam flow calculations in the K-325-23.5steam turbine control section for three operating modes in the form of flow rate and pressure distributions in the inter-row gaps and at the section outlet are presented.The simulation results analysis shows a rather low irregularity of the steam flow parameters at the control section outlet in the partial supply modes and a slight irregularity in the rated mode.Based on the results of the analysis, a conclusion regarding the effectiveness of the developed control section of the HP module use for steam turbine K-325-23.5 modernization was made.However, to deploy the new design of the control section, it is reasonable to conduct additional studies of unsteady loads on the nozzle and turbine blades of the HP module, caused by circumferential irregularity of flow parameters at partial supply.
These results confirm the possibility of the updated software package IPMFlow usage for study more complicated features of the flow in the steam turbines stages, including effects caused by circular variations in the flow part geometry.section of the HP module of a steam turbine K-325-23.5 at a partial mode of 0.4].Aviatsionno-kosmicheskaya tekhnika i tekhnologiya − Aerospace Technic and Technology, no. 9, pp.75-80 (in Russian).4

Introduction
Solution of inverse heat conduction problems (HCP) of identifying the parameters of mathematical models is of particular importance as an important step in ensuring the adequacy of these models in the presence of experimental information about the thermal process being studied.This article discusses the nonlinear internal inverse HCP of identifying thermophysical characteristics.These can be temperature-dependent coefficient of heat conductivity, heat capacity, internal heat sources, etc.The authors of [1][2][3][4][5][6] propose classifications

Fig. 1 .
Fig. 1.The turbine nozzle control system view: a -cross-section of the K-300 type turbine steam admission system; b -original structure of control section; c -new structure of control section