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

Abstract This study provides empirical insights into transient fluid dynamics and their consequential impact on heat transfer augmentation in reciprocating machinery. Using the Transient Air System Rig (TASR) developed at Imperial College, realistic enging 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. Testing sinusoidal and real valve lift-off profiles revealed key differences in heat transfer performance. Sinusoidal profiles, which are often used for testing simplicity but 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 and Reynolds number). 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. The results show that these valve profiles have a large potential for augmenting thermal performance due to the presence of local flow reversal. 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.