Measurements and Correlations of Local Cylinder-Wall Heat-Flux Relative to Near-Wall Chemiluminescence across Multiple Combustion Modes 2020-01-0802
Minimizing heat transfer (HT) losses is important for both improving engine efficiency and increasing exhaust energy for turbocharging and exhaust aftertreatment management, but engine combustion system design to minimize these losses is hindered by significant uncertainties in prediction. Empirical HT correlations such as the popular Woschni model have been developed and various attempts at improving predictions have been proposed since the 1960s, but due to variations in facilities and techniques among various studies, comparison and assessment of modelling approaches among multiple combustion modes is not straightforward. In this work, simultaneous cylinder-wall temperature and OH* chemiluminescence high-speed video are all recorded in a single heavy-duty optical engine operated under multiple combustion modes. The cylinder-wall HT is derived from the measured transient temperature and compared with Woschni HT correlation predictions using both bulk and estimated local gas-temperatures. The local Woschni correlation predictions of heat flux and the HT coefficient for spark ignition (SI) and homogeneous charge compression ignition (HCCI) match surprisingly well with measurements. Uncertainty analysis shows that the modeled results falls in the measurements uncertainty. For SI, both the measured and the predicted heat transfer coefficient decrease rapidly when the flame arrives at the cylinder-wall. Per the correlation, this is mainly due to the rapid rise of gas temperature that reduces the Reynolds number and hence near-wall turbulence. For HCCI, both the measured and predicted heat transfer coefficient increase at the start of high temperature heat release, chiefly because pressure increases rapidly with temperature, yielding an increase in near-wall turbulence per the correlation. Further improvements of HT predictions for these modes would likely require even lower experimental uncertainty. For impinging jets of conventional diesel combustion (CDC), the Woschni correlation vastly under-predicts both heat flux and the heat transfer coefficient. The Woschni correlation underpredictions are due to its basis on internal pipe flow, for which the flow is generally parallel rather than perpendicular to the wall as for impinging jets. Hence, for CDC, the predictions of local heat flux by empirical correlations like Woschni can be improved by using an impinging-jet basis.
Zheming Li, Mark Musculus, Zachary Shechtman