Oct

2

Simcenter Flomaster crosses the saturation line

Simcenter Flomaster software has a long history of being used in superheated (dry) steam systems.

The typical model involves the distribution of the steam and determining flow rates and choking points in the systems for both steady state and transient applications.

While users have found a great deal of value in using Simcenter Flomaster in this manner, there has always been the limitation that it could only model the distribution of the steam and it could not be used to model the generation of the steam or the actual usage of the steam.

In both of these areas, the system involves phase change from liquid to vapour for steam generation or the reverse direction from vapour to liquid when the steam is consumed.

Recent developments in Simcenter Flomaster as described in the white paper “Improved transient flow modelling for non-ideal gases” have made it possible for Simcenter Flomaster to begin modeling the phase change that occurs in these processes

Challenges

Modelling phase change in piping systems brings with it some very unique challenges that are not present in single phase fluid flow.

1. Rankine cycle

The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid.

This cycle is at the heart of the two-phase process. Simcenter Flomaster must be able to model the full Rankine cycle. The process of the basic Rankine cycle is described below.

There are other variations of the Rankine cycle that involve superheat and reheating. These are typical in the actual use of steam in industry.

These will be discussed in the application section. There are four processes in the Rankine cycle; these states are identified by number in the diagram below.

  • Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
  • Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour.
  • Process 3-4: The dry saturated vapour expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur.
  • Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change.

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