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

Design of efficient, downsized piston engines requires a thorough understanding of transient near-wall heat losses. Measurements of the spatially and temporally evolving thermal boundary layer 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 thermal boundary layer. Phosphor thermometry is used to measure wall temperature. Measurements 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 boundary layer and heat transfer studies associated with engine-relevant processes. The thermal boundary layer 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 and expansion processes. The time-history of important boundary layer quantities such as gas / wall temperatures, boundary layer thickness, wall heat flux, and relative energy lost at the wall are evaluated through these regimes. During the mild unburned-gas compression, T_core increases by 30 K and a thermal boundary layer is initiated with thickness δ_T ~ 200 μm. Wall heat fluxes remain below 6 kW/m², but corresponds to ~6% energy loss per ms. In the burned-gas regime, T_core resembles adiabatic flame temperatures, while T_wall increases by 16 K. A thermal boundary layer rapidly develops as δ_T increases from 290 to 730 μm. 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 boundary layer. Findings are compared to canonical environments and demonstrate the transient thermal boundary nature during engine-relevant processes.