Experimental Characterization of Heat Transfer and Fluid Dynamics in Pulsating Exhaust Flows

This study provides empirical insights into transient fluid dynamics and their consequential impact on heat transfer augmentation. Using the Transient Air System Rig (TASR) developed at Imperial College, realistic engine exhaust flow conditions are generated by controlling the frequency and lift-off profiles of active valve trains to replicate a wide range of pressure waveforms. Heat transfer characteristics are then measured in a straight heated pipe (HEAT-TRAP), which replicates two waste heat recovery scenarios: restricted outlet conditions for industrial power generation and open-ended conditions for heavy-duty propulsion. Sinusoidal profiles, used for testing simplicity deviate from real engine dynamics, showed discrepancies between predicted and actual heat transfer rates. These profiles produced limited or no heat transfer enhancement due to their low amplitudes in an inertia-dominated flow regime (high mass flow). In contrast, real-valve profiles–representative of practical engine operation–demonstrated consistent heat transfer enhancement driven by the onset of flow reversal, a phenomenon not achievable with sinusoidal waveforms. The measured Nusselt number augmentation ratio ranges found for sinusoidal profiles were: [0.87, 1.06] for restricted exhaust and [0.90, 1.01] for open-ended exhaust, respectively. Conversely, real AVT profiles yielded an augmentation range of [1.05, 1.35], providing enhancement across all frequencies and mass flows tested. These findings demonstrate that to achieve accurate heat transfer predictions in reciprocating systems, reliance on oversimplified sinusoidal shapes is insufficient. Instead, empirical solutions, which better capture the complex fluid dynamics using correlations, offer a reliable alternative.