Novel Experiments for the Investigation of Non-Ideal Compressible Fluid Dynamics: The ORCHID and First Results of Optical Measurements

The vast number of pioneering theoretical developments inthe area of non-classical gas dynamics together with the rising number ofapplications of organic Rankine cycle (ORC) technology have shaped a new branchof fluid mechanics called non-ideal compressible fluid dynamics (NICFD). Thisfield of fluid mechanics is concerned with flows of dense vapors, whoseproperties do not comply with the ideal gas model. These types of flows occurin dense vapors, supercritical fluids and liquid-vapor mixtures.

NICFD is encountered in a variety of industrial processes,the most relevant in the power and propulsion sector, and is of interest forfundamental research.  Chapter 2documents an extensive literature review which covers the main developments inthe numerical, theoretical and experimental areas. In particular, the review ofcurrent and past experiments was aimed at summarizing the lessons learned.Despite the relevance of NICFD applications, experimental information regardingthis type of flows is scarce, due to the challenges inherent with the operatingconditions of such experiments.

The main objective of the research documented in thisdissertation was to perform accurate measurements of NICFD flows which can thenbe used to assess the predictive capabilities of state-of-the art numericaltools. To this end, the design and commissioning of suitable and fullyinstrumented facilities capable of generating NICFD flows in a multitude ofsteady, controlled conditions is necessary in order to provide high-quality andwell characterised data. Arguably, state-of-the-art optical techniques are mostsuited for this goal, together with more conventional temperature, pressure andmass flow rate measurements.

Advanced laser diagnostic techniques such as particle imagevelocimetry (PIV) are possibly the measurement technique of choice whenaccurate measurements with a high spatial and temporal resolution are needed.However, the use of PIV or other optical techniques capable of providing localand instantaneous information within the flow is not documented. Therefore, afeasibility study was conducted by means of a simpler experiment:  the planar PIV technique was applied tocharacterise the dense vapor of an organic fluid (D4, a siloxane) stirred withina transparent container. Chapter 3 documents the successful results of thisexperiment. The optical properties of the dense vapor make PIV possible.Titanium dioxide (TiO2) seeding particles were used to track the low-speed motionof the fluid around a rotating disk. Vector fields of the natural convectionflow and of the superposition of natural convection and rotating flow wereacquired and studied as exemplary cases. The particles adequately trace theflow since the calculated Stokes number is 6.5×10^-5. The quality ofthe experimental data was assessed by means of particle seeding density andparticle image Signal to Noise ratio (S/N). The results are deemed acceptablein view of envisaged high-speed flow experiments.

In order to obtain measurement data of high-speed vaporflows in the NICFD regime, new experimental facilities must be conceived,designed, realized and tested. Chapter 4 presents the organic Rankine cyclehybrid integrated device (ORCHID), which was designed, built and commissionedat the Delft University of Technology. The facility can operate continuouslyand with a wide range of operating conditions. The maximum operating pressureand temperature are 25 bara and 350 °C. The facility has been designedto operate with siloxane MM as the working fluid, but it was numericallyverified that it may also be operated with other working fluids such as MDM, MD₂M,D₄, D₅, D₆, pentane, cyclopentane, NOVEC649, PP2, PP80, PP90, and toluene. Twotest sections allow to operate the ORCHID either as a supersonic/transonicvapor tunnel or as an ORC turbine test bed. Currently, a supersonic nozzlefeaturing a throat of 150 mm² with optical access allows to perform gasdynamic experiments for the validation of numerical simulation codes. A secondtest section, a test-bench for mini-ORC expanders,  is being designed and will accommodate afully instrumented 10 kWe machine, however machines of any configuration andwith a rated power of up to approximately 80 kWe can be tested.

Chapter 5 documents the achievements reached during thecommissioning of the ORCHID. The successful commissioning of the setup with MMas the working fluid is detailed and discussed based on the recordings ofseveral test runs, including the start up and shut down of the facility.  Data were acquired during the operation atsteady state at the two main operating conditions typical of supersonic nozzleand ORC turbine tests. The operation of the facility is characterized withregards to the process stability, moreover process variables are assessed fortheir uncertainties. The correct operation of the nozzle test section wasverified with a mass flow rate of fluid of m = 1.15 kg / s, and at a thermodynamic state at the nozzle inletcorresponding to T = 252 °C and P =18.36 bara. The test sectionconditions typical of a turbine experiment were T = 275 °C, P =20.8 bara, with a mass flow rate of m = 0.17 kg / s. All therelevant process variables of the test section are affected by a relativeuncertainty that is lower than 0.6 %.

Chapter 6 reports the results of the first supersonic nozzleexperiments. Schlieren images of the MM flow through the two-dimensionalconverging-diverging nozzle with the inlet in the NICFD regime were recorded,together with the static pressure profile along the nozzle. A series ofschlieren photographs displaying Mach waves in the supersonic flow wereobtained and are documented at two operating conditions, namely, for inletconditions corresponding to a stagnation temperature and pressure of T₀  = 252 °C and P₀ = 18.4 bara, and to a back pressure of 2.1bara. Furthermore, static pressure values were measured along the expansionpath for operating conditions given by T₀ = 252 °C and P₀ = 11.2 bara at the nozzle inletand by a back pressure of 1.2 bara. The two inlet conditions of the fluidcorrespond to a compressibility factor of Z₀ = 0.58 and Z₀ = 0.79.

These Mach number values together with the values of thestatic pressure along the top and the bottom profile of the nozzle were usedfor a first assessment of the capability of evaluating NICFD effects occurringin dense organic vapor flows by comparison with the results of CFD simulations.The outcome of this initial comparison was deemed satisfactory.

Chapter 7 introduces the first steps towards the validationof a CFD solver for non-ideal compressible flows. In particular, anindustry-standard validation method was used together with syntheticexperimental data (experimental data were not available yet) to illustrate, asan exercise, how the uncertainty of NICFD software can be quantified. Theassessment is limited to determining how the uncertainties in model inputs,e.g., fluctuations in boundary conditions and thermodynamic property modelsinfluence the overall accuracy of NICFD simulations. The assessment of theuncertainty of the other sub-models is left for a successive phase of thisresearch program. The validation exercise confirmed the applicability of theproposed method, but also pointed out that the adopted validation metricsshould be complemented with additional statistical indicators. The errorsources in the designed experiment are identified and all the uncertainties areadequately quantified.

The final chapter summarizes the answers to the researchquestions listed in the first chapter, above all that the ORCHID facility wassuccessfully commissioned and can generate stable dense vapor flows of organicfluids for both fundamental gas dynamic experiments and ORC turbine testing.The first experiments demonstrate that it can be used to obtain accurateoptical and non-optical measurements of supersonic nozzle flows for thevalidation of CFD codes, including flows in the NICFD regime.  An overview of the next phases of theresearch program is also provided.