In modern civil aero-engine gas turbine combustors, lean burn technology is widely adopted to achieve low NOx emission target. As such, most of the flow issued from the compressor is expected to flow into the combustor dome, compared with a typical value of 30% in conventional combustors. To accommodate this increased mass flow rate, lean module fuel injectors should be significantly larger than their conventional counterparts. This will change the combustor external aerodynamic layout such as a deeper flame tube together with an enlarged dump gap, which is the distance between the pre-diffuser outlet and the flame tube. The modification will potentially increase the total pressure loss due to enlarged turning within the dump region. Thus it is important to investigate the influence of the dump gap on the aerodynamic performance of the diffuser. Experiments have been carried out and presented in this paper. The tested geometry comprises a pre-diffuser, followed by a sudden expansion through which the main flow is divided into three passages, i.e., the combustor dome, the outer passage, and the inner passage. Up to 60% of the airflow issued from the pre-diffuser flows into the dome. It is found that the loss coefficient of pre-diffuser decreases as dump gap increases. The overall loss coefficient is relatively high when the dump gap ratio is smaller than 1.2 or larger than 2.8, and is relatively low and insensitive to dump gap with intermediate dump gaps. It is also found that the proportion of the pre-diffuser loss to the overall loss is larger than conventional dump diffuser.

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