A white paper on analyzing two-phase pressure drop and heat transfer using Mentor Graphics’ Flowmaster simulation software was recently published. In the white paper, the authors provide explanation as to how the software works — something that users don’t always get.
Users need to be aware of the assumptions made by their software, especially when those assumptions are designed to ensure the safety and property of end-users.
Flowmaster was originally designed to evaluate water hammer, the sudden waves of pressure caused when liquids or gases abruptly stop or change directions. With empirical data and multiple correlations, though, Flowmaster is also able to simulate two-phase systems. In theory, this makes Flowmaster the perfect program for analyzing a power plant Rankine cycle, but the question is how?
To reduce calculation time when simulating a Rankine cycle pressure model, users can choose between two assumptions: a homogenous or a separated mixture system.
For homogeneous calculation, Flowmaster assumes that both the gas and liquid phases have equal velocity and properties. These properties are calculated by averaging the actual properties of each separate phase. The simulation is then calculated using pressure drop, static, and momentum calculations as if it contained only the one averaged phase.
In the separated mixture system, Flowmaster assumes an equal temperature between the two phases, as well as that each phase is separated into different (imaginary) pipes. The sizes of these separate pipes are estimated by multiplying the actual pipe size by the void fraction (the fraction of the actual pipe occupied by gas) at any given point. This method is more complicated, as it deals with density changes between the phases as well as void fraction calculations at any given point.
As for the calculations of frictional pressure drop across the various pipe orientations, there are a number of different correlations. To allow flexibility, Flowmaster includes the four pressure drop calculation correlations from Friedel, Lockhart & Martinelli, Chisolm, and Müller-Steinhagen & Heck.
As for heat transfer, the calculations are more complex and take into account pipe orientation, flow, and the predicted heat transfer method at any given point. Without accurate calculations of all fluid temperatures, heat fluxes, and pipe walls, simulations are impossible. To analyze these values between saturation temperature and critical heat flux, Flowmaster provides three correlation options: Gungor-Winterton, Shah, and Chen.
To a chemical engineer, this under-the-hood explanation should sound familiar, having done similar (though simplified) calculation by hand in a unit operations, fluid dynamics, or thermodynamics course. Where Flowmaster is effective is in the speed and freedom of calculation method, making something like the design of a Rankine cycle turning 20 turbines both quick and accurate.
To read the Mentor Graphics article in full, see here.