Heat transfer, pressure drop, and instability phenomena in water-steam two-phase flows are important research areas with regard to thermal power plants. Here, demineralized water is vaporized to operate steam turbines that drive generators, converting the energy from fuel that is burned into electricity. To design the evaporation process, it is important to know the heat transfer coefficients of water-steam-mixtures and the pressure drop due to friction, elevation and momentum in terms of steam quality. Moreover, occurring instabilities need to be understood to prevent unstable and unsafe operation conditions. The aim of this research work is to design and construct a test rig to analyze these thermal-hydraulic phenomena. Furthermore, the obtained results will be applied to validate the heterogeneous flow model of the dynamic simulation software APROS (Advanced Process Simulation Software) from VTT (Technical Research Center of Finland).
In contrast to previous studies in this field, the fluid used for the experiments is demineralized water — instead of usually used refrigerants — to eliminate possible side effects related to different fluid properties. Furthermore, the heated test section of the test rig is a thermal-hydraulic scale down from a thermal power plant’s low-pressure evaporator tube. The focus lies on a huge parameter variability in terms of operation conditions (sub-cooling, inlet pressure, mass flow, heating power, upstream compressible volume) and test rig layouts (vertical and horizontal test section, different lengths and inner diameters, single and parallel tubes) to analyze their impact on the different instability phenomena (density-wave oscillations, pressure-drop oscillations). Literature dealing with experimental investigation of two-phase flow in horizontal evaporator tubes exposed to natural circulation at low pressure is rare. This work aims to contribute especially to this field of research.
Currently, the designed test rig is under construction. Once the experiments are performed, the results will be analyzed by means of stability maps and fast Fourier transformation (FFT) amongst others. The results will then be used to validate the heterogeneous six-equation model of the dynamic simulation software APROS with regard to heat transfer, adiabatic pressure drop and flow instabilities. The six-equation flow model (also known as Euler-Euler approach) implies a considerably higher accuracy than the usually regarded homogeneous three-equation model.