Experimental investigation of wall heat transfer and Nusselt–Reynolds correlation in a diesel-like combustion chamber using a rapid compression and expansion machine

The enhancing thermal efficiency and reducing emissions in internal combustion engines could be obtained by efficient control of wall heat transfer. In diesel-like combustion chamber, high gases temperature and flame wall interaction significantly affect to the thermal balance, combustion stability, as well as energy efficiency. Therefore, this experimental study aimed to investigate the wall heat transfer phenomena using local Nusselt-Reynolds (Nu-Re) correlations within a two-dimensional stepped-lip piston cavity employing a Rapid Compression and Expansion Machine (RCEM). To enable detail observation of flame wall propagation and interaction under diesel spray combustion, a unique geometry of piston cavity was implemented in this experiment. These unique piston and Nu-Re correlations become the novelty due to the researcher engaged in a limited scope of investigation. The flame behavior was observed by high-speed video camera integrated in diffused backlight illumination (DBI) and direct photography (DP) method, while local wall heat transfer at multiple locations was measured by heat flux sensors. The experiments were conducted under diesel-like spray combustion with a common-rail pressure variations of 75–150 MPa for two-holes injector and 30–180 MPa for-six-holes injector, and wall heat flux was evaluated at the cylinder head, cavity lip, and cavity bottom regions. The experimental data produced local convective heat transfer coefficients and dimensionless Nu–Re correlations, Nu_x=0.173Rex^0.8. The derived correlation exhibits a turbulent convection characteristic with a Reynolds-number exponent of 0.8, indicating dominant wall-jet–controlled heat transfer following flame impingement. Heat convection controlled the regime governed by turbulent wall-jet motion, and this provided adequate understanding of heat transfer in local areas. The results provide quantitative insight into regional heat-transfer behavior in stepped-lip combustion chambers and serve as experimental reference data for validating high-fidelity combustion and heat-transfer models. It was also used as data for verifying exceptionally effective models responsible for producing less pollution engines.