Imaging of Temperature Variations in the Near-Wall Region of an Optical Reciprocating Engine using Laser-Induced Fluorescence

Abstract

Understanding engine heat transfer can enable the design of more efficient engines with advanced operational strategies that reduce net carbon emissions. However, accurate predictions of in-cylinder heat transfer processes require a significant investigation into the transient thermal boundary layer effects within the near-wall region (NWR). This work investigates the development of thermal stratification at two measurement locations near the cylinder head surface of an optical reciprocating engine using laser-induced fluorescence (LIF). Temperature images are obtained from high-speed toluene LIF measurements using the one-color detection technique, and the calibration procedure is based on predicted in-cylinder temperature from an engine simulation software (GT-Power). Precision uncertainty is assessed within a 1 x 1 mm2 calibration region, and found to be within ±2 K. First measurements examine temperature variations within a 20 x 12 mm2 field-of-view that includes the cylinder head and the piston top surfaces under motored operating conditions, while a second set of measurements examine temperature variations within an 8 x 6 mm2 field-of-view at the cylinder head surface under motored and fired operating conditions and at two different engine speeds. The near-wall temperature measurements of this work provide unique insights into the spatial and temporal temperature variations in the NWR of an optical reciprocating engine, which were enabled by a rigorous experimental effort and attentive post-processing steps to quantitatively process the LIF images to yield temperature fields. These measurements add to continuous effort to extend the fundamental understanding of near wall engine heat transfer, and aid in achieving a more comprehensive characterization of the NWR by complementing previously-collected near wall velocity measurements and a parallel effort in Large Eddy Simulations (LES) that focus on wall heat transfer. Future work should address improvements to the experimental methodology to better resolve the region within 0.5 mm from the surface, and perform simultaneous velocity field and surface temperature imaging to fully quantify turbulent heat fluxes, which are critical to understand heat transfer under transient high pressure, high-temperature conditions.