NUMERICAL SIMULATION OF CONDENSATION INDUCED WATER HAMMER

Abstract

A direct contact of subcooled water and steam leads to condensation induced water hammer (CIWH). Intensive condensation on water-steam interface causes the water column movement towards the steam and acceleration of water mass that replenishes the volume of condensed steam. Water column accelerates until it splashes onto the closed end of the pipe, closed valve or onto another water column leading to a high pressure increase in the pipeline or vessel system. The impact pressure increase can cause mechanical damage of equipment and endanger the safety of employees at the facility. According to available data of plant accidents and carried out experiments, pressure increases over 10 MPa can occur due to CIWH in a piping system that is initially at a low pressure close to the atmospheric pressure. Prediction of possible occurrence of CIWH is necessary to prevent accidents that this phenomenon can cause. A numerical model for the simulation and analysis of the water hammer in the pipe two-phase flow is developed. The modelling approach is based on one-dimensional compressible flow, tracking of the interface between steam volume and water column and modelling of the direct condensation of steam on subcooled liquid. Mass, momentum and energy conservation equations are solved by the method of characteristics. Fluid particle and the steam-water interface tracking are obtained through a third-order accurate solving of the energy equation in space. The thermodynamic quality, calculated from the enthalpy value, is used to determine whether the observed computational region is filled with water, two-phase mixture or steam. The condensation rate is strongly influenced by the negative or positive acceleration of the water column. Under higher acceleration the steamwater interface (water column head) bursts into huge amount of droplets. The area of steam-water interface, i.e. the area for heat transfer exchange, suddenly increases, which causes a higher condensation rate. The new model is introduced that relates the interface area and the condensation heat transfer with the acceleration of water column. Also, the transient friction is included in the momentum equation due to intensive accelerations of the water column. The model is applied to simulations and analyses of CIWH conditions in experimental facilities. Numerical results are validated with available measured data.