Centrifugal Compressor Casing Treatment Design for a Turbocharged Hydrogen Combustion Engine

Rapid decarbonisation of energy-intensive industries such as heavy-duty transport and stationary power generation is essential to meet the emissions targets set by legislative authorities around the globe. Emission-free battery technologies are promising but they do not yet achieve the energy density needed for larger machines and the suitability of fuel cells to operate in harsh environments remains uncertain. Hence, Hydrogen internal combustion engines (ICEs) emerge as an attractive solution. Existing Diesel and Gasoline ICEs can be readily adapted to Port Fuel Injection (PFI) Hydrogen ICEs using lean-burn combustion or Exhaust Gas Recirculation (EGR) to achieve high efficiency and low emissions. Such engine configurations typically demand turbocharger compressors with high pressure ratio and efficiency. However, compressor map width tends to decrease with increasing pressure ratio and this can sometimes limit the engine operating range. The literature review identified the ported shroud (or bleed slot) casing treatment as an effective add-on technology to improve compressor map width, particularly at high pressure ratios. This paper details the design of a ported shroud casing treatment for a highly loaded centrifugal compressor equipped with a vaneless diffuser. A parametric study was conducted using a validated Computational Fluid Dynamics (CFD) model to determine the sensitivity of compressor pressure ratio, efficiency, and recirculation ratio to the downstream slit position, width and meridional angle. The angle of the struts necessary for structural support of the ported shroud inside the compressor housing was also considered. The findings of the parametric study were subsequently used to inform a design optimisation with the goal of maximising the potential stability enhancement offered by a ported shroud casing treatment.