Investigation of high-pressure dual-fuel combustion of ammonia and n-heptane using OH <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"> <mml:msup> <mml:mspace width="0.1em"/> <mml:mo>∗</mml:mo> </mml:msup> </mml:math> and NH

This study presents an experimental investigation into the high-pressure dual-fuel combustion of ammonia spray flames ignited by n-heptane pilot injections. The campaign explores the influence of ambient thermodynamic conditions and pilot injection timing on combustion behavior, flame development, and emissions. High-speed imaging techniques, namely shadowgraph imaging, OH ∗ , and NH 2 ∗ chemiluminescence, were simultaneously employed to characterize the spray and flame structure, alongside pressure-based heat release rate calculations and exhaust gas measurements. The results show that higher ambient temperatures led to earlier ammonia ignition, shorter and more stable flame lift-off lengths, higher OH ∗ and NH 2 ∗ chemiluminescence signals, increased heat release, and reduced ammonia slip and N 2 O emissions, but higher NO x emissions. Pilot injection timing strongly affected ignition and flame development: early timing reduced ammonia interaction and delayed ignition, while later timings increased overlap. For a certain degree of pilot-main overlap, the increasing ambient air temperature/pressure did not affect any of the global parameters. The NH 2 ∗ and OH ∗ chemiluminescence measurements showed that ignition occurred along the periphery of the ammonia spray, although strong interaction with the central part of the spray was ensured. During the flame development phase, OH ∗ was found to extend across the entire ammonia spray width, while NH 2 ∗ was concentrated in the spray core. After the end of ammonia injection, the OH ∗ signal continued to increase, while NH 2 ∗ decreased, which was due to ammonia being consumed and more air mixing into the spray. • High combustion efficiency is achieved for 95 % ammonia energy shares. • NH 2 ∗ is located in the ammonia spray core, while OH ∗ covers the entire spray width. • In the spray core, the NH 2 ∗ and OH ∗ levels spatially depend on each other. • Ambient temperature does not affect emissions for overlapping pilot and main injections. • Ammonia flame lift-off lengths increase over time with strong pilot-main overlap.