Test Engineer – Experimental Characterization of Small Radial Turbomachinery

Decarbonization requires significant evolutions in the architecture of transportation systems. Many sub-systems, such as the propulsive unit or the air conditioning systems, are impacted, and the turbomachinery involved in those subsystems needs to be adapted. In the electrification process, the efficient operating range requirement for turbines and compressors must be significantly increased compared with the current state of the art. The chair CASTOR is a research program of cooperation between Liebherr Aerospace, a recognized air conditioning system provider for the aeronautic industry, and ISAE-Supaero, which has acknowledged research activity in the off-design operation of radial turbomachinery. The main objective of the chair is to increase the operability range of centrifugal compressors and radial turbines. A research team of ISAE-Supaero faculty members, Liebherr experts, post-doctoral researchers, PhD students, technical staff, and students in internship will be dedicated to this ambitious four-year program.

A reliable measurement of efficiency is mandatory to meet the objectives of Castor. However, the stages developed by Liebherr induce difficulties that forbid a strict comparison with numerical simulations: (i) the small size of the stages exacerbates the intrusivity of the probes, (ii) the outlet flow heterogeneity is difficult to measure, (iii) the heat transfer distorts the work exchange measurement based on total temperature variations, (iv) the tip clearance depends on the operating point, and (v) the flow inlet properties (humidity) can change during operation. These difficulties are recurrent on the test benches of Liebherr and also appeared on the two test benches devoted to radial turbomachinery.

Work description:
The candidate will help define and contribute to the different test campaigns which will overcome the experimental difficulties.

• Regarding the flow heterogeneity, a dual approach will be conducted and applied to the turbine outlet, which naturally provides a radially stratified flow. The first part involves accurate measurements of local pressure and temperature with miniaturized five-hole probes and hot wire traverse to help identify the stratified flow. Original conditioning of two wire probes, one in hot wire configuration and the other in cold wire setting, will be attempted to reach fluctuating passive devices mixing the stratified flow. A preliminary calibration with fluctuating pressure and temperature will be developed for that purpose. This campaign should provide accurate knowledge of the real flow. The second part is more pragmatic and uses passive devices that mix the stratified flow before measuring the gas state. Such a measurement will be compared to different averaging of the previously measured stratified flow. The consequences on the overall efficiency measurement will be established;

• Regarding heat transfers, the large database of Liebherr will be explored. Also, a reference situation, as close as possible to adiabatic, is required. For that purpose, a specific campaign for which the temperature at the inlet will be regulated according to the value of the compressor outlet since most of the heat transfer comes from metallic diffusion. Extensive metal temperature probes will characterize it. Insulation of the casing will limit heat exchange with the atmosphere. Tests conducted in diabatic conditions with the same geometry will characterize the deviation induced by heat transfer. The use of nodal models, available at Liebherr, fed with the metal temperature recording will also help that characterization. Finally, pragmatic corrections based on the literature survey will be attempted;

• Regarding the tip clearance evolution, fast response displacement laser sensors will be set in the same fashion as fast response sensors. This implies prototype casings with optical access. The minimum distance recorded by the signal will represent the tip of the blade and be tracked by phase-locked analysis. Simultaneous measurement of the casing deformation will give access to the tip clearance value. Those tests will take place first at the lab, then some measurement will be attempted on the test benches of Liebherr.

• Regarding the inlet conditions, many variations will be attempted by changing the treatment of the supplied air. The possibility to heat the inlet air or use the vacuum system of DAEP to suck instead of blowing offers a large possibility of exploitation. On the Liebherr side, the test bench dedicated to fuel cells enables control of the moisture of the air. A parametric study distributed on the two sites will then quantify the efficiency measurement’s sensitivity to the air’s exact state at the inlet of both compressors and turbines. The influence of outlet icing on the accuracy of the different measurements will be quantified.