60239 - Numerical Analysis of Non-Radial Blading in a Low Speed-Low Pressure Turbine for Electric Turbocompounding Applications 

This paper presents a CFD- based numerical analysis on the potential benefits of non-radial blading turbine for specific low speed-low pressure application. Electric turbocompounding is a waste heat recovery technology consisting of a turbine coupled to a generator that transforms the energy remaining in engine exhaust gases, typically at low pressure, into electricity. Turbines designed to operate at low specific speed are ideal for these applications since the peak efficiency occurs at lower pressure ratios than conventional high speed turbines and make the generator coupling easier and more efficient. The baseline design consisted of a vaneless radial fibre turbine, operating at a design point of 1.2 pressure ratio and 28,000 rpm rotational speed. Experimental low temperature tests were carried out with the baseline radial blading turbine at design and off-design operating conditions to validate the numerical simulations. The baseline turbine incidence angle effect was studied and positive inlet blade angle impact is assessed in the current paper. Four different turbine rotor designs of 20, 30, 40 and 50 degrees of positive blade inlet angle are presented and compared in the current study, with the aim to reduce the losses associated to positive incidence, specially at midspan. The volute domain was included in all the CFD calculations to take into account the volute-rotor interactions.  The results obtained from the numerical simulations of the modified designs were compared with those from the baseline at design and off-design conditions. Total-tp-static efficiency improved in all the non-radial blading designs at all operating points considered, by maximum of 1.5 % at design conditions and of 5 % at off-design conditions, particularly at low pressure ratio. As non-radial fibre blading may be susceptible to high centrifugal and thermal stresses, structural analysis was also performed to assess the feasibility of each design. Most of the non-radial blading designs showed acceptable levels of stress and deformation, although higher than the baseline at the studied conditions for electric turbocompounding applications, low speed and low pressure.