Heat transfer and residence time in lean direct injection fuel galleries

In radially staged lean direct injection systems, pilot fuel plays an important role in cooling the mains fuel gallery in regions of the flight envelope where the mains fuel is stagnant. Under these conditions, managing wetted wall temperatures is vital to avoid the formation of carbonaceous particles, which become deposited on the surfaces of the fuel gallery and can lead to a deterioration in system performance. The prediction of wetted wall temperatures therefore represents an important aspect of the injector design phase. Such predictions are often based on injector thermal models, which tend to rely on the application of convective boundary conditions from empirical heat transfer correlations. The use of these correlations leads to questions over the accuracy of predicted wetted wall temperatures and therefore uncertainty over the probability of deposition. This paper seeks to improve on the current situation by applying the inverse conduction technique to determine heat transfer coefficients specific to the pilot fuel gallery. These heat transfer coefficients are crucial for determining wetted wall temperatures in the pilot and mains fuel galleries and principally govern the risk of deposition in the stagnant mains. The pilot heat transfer data is further examined alongside measurements of the pilot residence time distribution, which together control the risk of pilot deposition at low fuel flow rates. Both the heat transfer and residence time measurements demonstrate the opportunity for future fuel gallery design refinement and provide the supporting data to facilitate this.