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 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, increases by 30 K and a thermal boundary layer is initiated with thickness ~ 200 μm. Wall heat fluxes remain below 6 kW/m , but corresponds to ~6% energy loss per ms. In the burned-gas regime, resembles adiabatic flame temperatures, while increases by 16 K. A thermal boundary layer rapidly develops as increases from 290-730 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 freshand 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.