Experimental investigation of thermal boundary layers and associated heat loss for transient engine-relevant processes using HRCARS and phosphor thermometry

Abstract

Design of efficient, downsized piston engines requires an understanding of transient wall heat losses. Measurements of the spatially and temporally evolving thermal boundary layer (TBL) are required to facilitate this knowledge. This work takes advantage of hybrid fs/ps rotational coherent anti-Stokes Raman spectroscopy (HRCARS) to measure single-shot, wall-normal gas temperatures, which provide exclusive access to the TBL. Phosphor thermometry is used to measure wall temperature. Experiments are performed in a fixed-volume chamber that operates with a transient pressure rise/decay to simulate engine-relevant compression/expansion events. This simplified environment is conducive for fundamental BL studies associated with engine-relevant processes. The TBL development and corresponding heat losses are evaluated within two engine-relevant regimes: (1) an unburned-gas regime comprised of gaseous compression and (2) a burned-gas regime, which includes high-temperature compression/expansion processes. The time-history of important BL quantities such as gas / wall temperatures, TBL thickness, wall heat flux, and relative energy lost at the wall are evaluated in these regimes. During the mild compression, Tcore increases by 30K and a TBL is initiated with {\delta}T ~ 200 {\mu}m. Wall heat fluxes remain below 6 kW/m2, but corresponds to ~6% energy loss per ms. In the burned-gas regime, Tcore resembles adiabatic flame temperatures, while Twall increases by 16K. A TBL rapidly develops as {\delta}T increases from 290-730 {\mu}m. Energy losses in excess of 25% occur after flame impingement and slowly decay to ~10% at the end of expansion. Measurements also resolve thermal mixing of fresh- and burned gases during expansion, which yield strong temperature reversals in the TBL. Findings are compared to canonical environments and demonstrate the transient TBL nature during engine-relevant processes.