For a 300 MW heat supply steam turbine unit after retrofitting with zero-output technology of low pressure(LP) cylinder

a three-dimensional model is established and meshed according to the actual dimensions of the connecting pipe of intermedium pressure(IP) and LP cylinder

the hydraulic control butterfly valve of the connecting pipe

the heating steam extraction pipe

the cooling steam bypass

the cooling steam flow orifice and the cooling steam control valve. The Realizable k-ε model and Scalable wall function method are used to carry out numerical simulation for the internal flow of the connecting pipes and the cooling steam bypass under different heating conditions. The simulation results show that

the complex strucutre of the connecting pipe of IP and LP cylinder and the cooling steam bypass system will separate the steam flow

making the flow in the whole pipeline system unstable and the flow at inlet of the LP cylinder uneven. Under zero output conditions of the LP cylinder

the steam will reach supersonic speed in expansion section at outlet of the cooling steam regulating valve and produce strong shock and Mach disk at the subsequent connected elbow. The simulation results reflect the complex steam flow field and state in the connecting pipe of IP and LP cylinder under zero output conditions of the LP cylinder

which provides a theoretical basis for the direction of the subsequent technical transformation.