Effect of Combustor Geometry on the Performance of an Airblast Atomizer under Sub-Atmospheric Conditions

摘要

One of the certification requirements that a jet engine has to fulfil is its altitude relight capability. Relighting an aero gas turbine engine at high altitudes is more challenging than at sea level conditions. The pressure, air velocity, and temperature within the combustor at such conditions, are very low, hindering the fuel atomization and evaporation process. After ignition, combustion efficiency can be relatively low due to the poor fuel atomization quality, leading to slow shaft acceleration rates. Further studies in this field can help determine and predict the fuel spray characteristics which limit the relight and pull-away capability of the engine at these sub-idle conditions. Reported in this paper is the CFD analysis of a typical airblast atomizer, simulated at different sub-idle operating conditions. Two sets of models were used; one with a simple liner-only combustor with co-flow air, and the other with a more detailed geometry, including wall cooling slots, primary and secondary dilution holes, and co-flow air. For the simpler model, three different inner and outer liner wall spacing were modelled to examine the effect of the chamber volume on the fuel spray behaviour. The CFD models were then run at a chamber pressure of 101, 41 and 31kPa, typical of sub-idle conditions. The effect of such conditions on the atomization quality of the fuel spray was analysed. The study carried out indicates how the chamber pressure, chamber volume and AFR (through amount of co-flow air introduced) significantly affect the resulting spray characteristics. A parametric analysis was performed to extract a correlation between the spray SMD (Sauter Mean Diameter) and fuel flow rate.