In order to study the unstable characteristics of vapor-liquid two-phase flow in water-cooled wall pipe of an opposed combustion natural circulation boiler during deep peak shaving

a frequency domain mathematical model suitable for different working conditions is established. By performing small perturbation linearization on the mass

energy

and momentum equations

eliminating high-order infinitesimal perturbations and steady-state quantities

and conducting Laplace transformation

the transfer function used to describe stability of the vapor-liquid fluid flow in the pipeline is obtained through the Nyquist diagram. The graphical method is used to judge the stability of the working fluid flow in the pipe. The calculation results show that

the critical heat flux densities of typical circuits operating at 25% BMCR and 50% BMCR are 182.20 kW/m2 and 240.13 k W/m2.Moreover

this model is used to calculate the unstable boundary of the water wall pipe section of a 350 MW natural circulation boiler and study the influence of parameters such as inlet subcooling

mass flow rate

pipe length

inclination angle and inlet throttling coefficient on the flow instability characteristics. The calculation results show that

the influence of inlet subcooling on critical heat flux density is non-unique

and the unstable boundary diagram shows a “double-C” shape. Increasing the mass flow rate reduces the density difference between the inlet and outlet of the fluid

which is beneficial to the flow stability. Increasing the heat flow density increases the density difference between the inlet and outlet of the fluid

which is not conducive to the stability of the flow. Enhancing the inlet throttling coefficient can suppress the pulsation of the flow at the inlet

which is conducive to the stability of the flow. Increasing the inclination angle of the pipe will increase the weight pressure drop and increase the disturbance caused by it

which is not conducive to the stability of the flow.