REFINED PHYSICAL MODEL OF STEAM FLOW IN THE LOW-PRESSURE PART OF A HIGH-POWER TURBINE IN LOW-FLOW RATE MODES
Abstract
Analytical methods for calculating the performance of the turbine are operational as long as the “layered” character of the steam flow is preserved in the flow part. However, as shown by experimental studies, the character of the flow changes significantly in low-flow rate modes. In these cases, existing methods of three-dimensional calculations require reliable formation of boundary conditions. The purpose of this study is to analyze the characteristics of low-pressure cylinder (LPC) stages operating in high-power stationary steam turbines, and to form physical models of the flow of the working medium (steam) in such stages at low-flow rate modes, taking into account their location in the flow part. It is advisable to imagine the physical model of the movement of steam in the turbine, starting from the last stage of the LPC and the outlet nozzle, in which separation of flow from the hub the develops. Its formation occurs when the relative volumetric flow rate decreases, starting with = 0.8, depending on the angle b2к at the root of the working blades. A decrease in the operating mode of the LPC turbine stages is also accompanied by a decrease the flow speed in the peripheral zone and a negative angle of attack at the flow entry into the channels of the impeller. On the concave surface of the profile part of the blade, a separation area is formed, which rotates in the rim clearance in the circumferential direction. Reverse flow from the inlet part of the channels of the impeller into the rim clearance occur when the static pressure in it exceeds the full pressure of the flow coming out of the channels of the guide apparatus. Turbulization of the flow of the rotating vortex leads to large irreversible losses of the mechanical energy of the moving flow, which are converted into heat, heating the mass of the working medium of the vortex and the main flow averaged through the channels of the working blade. Thus, the analysis of the gas-dynamic structure of the flow and its thermal state made it possible to clarify the physical model of the steam flow in the LPC at low-flow rate modes.
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