Experimental analysis of thermohydraulic oscillations during the temperature control of a heat transfer fluid test rig

Kurzfassung

The characterization and qualification of novel heat transfer fluids HTF) also in combination with different thermal-hydraulic components requires a technical scale test facility providing controlled temperature and flow conditions exceeding typical industrial applications. The mobile pump and HTF test facility (MoPuW) at DLR’s Institute of Future Fuels offers a suitable environment for this purpose but is limited regarding the realisable negative temperature gradients due to lack of active cooling. As a solution for enhancing cooling capacities, a temperature control using the cooler HTF from the expansion tank was investigated and implemented in this thesis. By controlling the flow through the tank, gradients up to -150 K/h can now be realised down to a temperature of 230 °C for a maximum fluid temperature of 130 °C inside the tank. The implemented control loop also significantly improves the control behaviour for positive gradients as well as steady state temperatures. In order to ensure stable operating conditions with the new cooling setup which strongly influences the expansion tank related arameters, the observed instabilities of the temperature control had to be analysed. The design of experiments (DOE) method was used to systematically investigate the influence of various system parameters on the instabilities of the system and their underlying causes. Although no clear correlation was found between the parameters tudied and the magnitude of the instabilities, the cause was identified as being related to the control of the electrical resistance heating that unfavourably splits the required power between to installed devices causing sudden temperature drops. As a prerequisite for investigating the instabilities and as a general extension of the capabilities of the test facility a mass flow rate control was also implemented using established tuning rules for PID parameters without the need of a mathematical model of the test facility. Based on an improved measurement of the mass flow rate using a pressure difference sensor combined with an orifice plate it is now possible to precisely set a required mass flow rate for experiments with changing HTF temperatures.