Experimental investigation of wall heat transfer due to spray combustion in a high-pressure/high-temperature vessel

Abstract

Combustion chamber wall heat transfer is a major contributor to efficiency losses in diesel engines. In this context, thermal swing materials (adapting to the surrounding gas temperature) have been pinpointed as a promising mitigative solution. In this study, experiments are carried out in a high-pressure/high-temperature vessel to (a) characterise the wall heat transfer process ensuing from wall impingement of a combusting fuel spray, and (b) evaluate insulative improvements provided by a coating that promotes thermal swing. The baseline experimental condition resembles that of Spray A from the Engine Combustion Network, while additional variations are generated by modifying the ambient temperature as well as the injection pressure and duration. Wall heat transfer and wall temperature measurements are time-resolved and accompanied by concurrent high-speed imaging of natural luminosity. An investigation with an uncoated wall is carried out with several sensor locations around the stagnation point, elucidating sensor-to-sensor variability and setup symmetry. Surface heat flux follows three phases: (i) an initial peak, (ii) a slightly lower plateau dependent on the injection duration, and (iii) a slow decline. In addition to the uncoated reference case, the investigation involves a coating made of porous zirconia, an established thermal swing material. With a coated setup, the projection of surface quantities (heat flux and temperature) from the immersed measurement location requires additional numerical analysis of conjugate heat transfer. Starting from the traces measured beneath the coating, the surface quantities are obtained by solving a one-dimensional inverse heat transfer problem. The present measurements are complemented by CFD simulations supplemented with recent rough-wall models. The surface roughness of the coated specimen is indicated to have a significant impact on the wall heat flux, offsetting the expected benefit from the thermal swing material.

Optimization of thermal barrier coating performance and durability over a drive cycle

“…This, however, requires that the gas temperature is not significantly affected by the change in heat loss due to the coating; the constant h assumption arises from an incomplete understanding of coating roughness effects. 9,[28][29][30][31]33,62 Figure 1 shows a comparison of the fully coupled baseline (uncoated) case to the fully coupled coated case, and the uncoupled coated results. The data are for a steady-state cycle of the production John Deere 4045 engine.…”

“…This, however, requires that the gas temperature is not significantly affected by the change in heat loss due to the coating; the constant h assumption arises from an incomplete understanding of coating roughness effects. 9,28–31,33,62…”

“…Additionally, the air-fuel mixing process can be affected, which in turn, affects chemistry. 28 Minor efficiency improvements 9,29,30 have been shown for polished 31 thermal barrier coatings at high loads compared to the as-sprayed condition. The heat transfer coefficient has been suggested by some authors to increase [32][33][34][35] with the addition of a thermal barrier coating in comparison to an all-metal baseline engine.…”

Special issue: THIESEL 2020 Conference on Thermo- and Fluid-Dynamic Processes in Direct Injection Engines

“…Splitter et al 4 analyze the effect on soot production of using EGR and late intake valve closing in a single-cylinder SI high compression ratio engine and conclude that depending on the fuel characteristics (gasoline or LPG), combustion efficiency may be significantly improved when compared to a conventional Diesel engine. Keskinen et al 5 measured heat fluxes in an engine-relevant environment to determine if the use of thermo-swing coating materials helps significantly reduce heat transfer losses. However, they concluded that it is rather difficult to improve the thermal efficiency of the engine by these means.…”

Improvement of diesel combustion with suppression of mutual fuel spray flame interactions with staggered nozzle hole arrangement and a spatially divided combustion chamber

“…Recently, in view of demands for reductions in carbon dioxide emissions, improvements in thermal efficiency as well as establishment of ultralow emissions in diesel engines are attracting attention and many papers related to the thermal efficiency of diesel engines have been reported in the recent 4 years. The papers can be classified into three categories, including papers on the cooling loss reduction with suppression of heat transfer to the combustion chamber wall, [1][2][3][4][5][6][7][8][9][10][11] applications including the Miller cycle and cylinder deactivation, [12][13][14][15][16] and combustion improvement with optimizations of combustion chamber shapes and fuel injection strategies. 1, Among the fuel injection strategies, highly distributed fuel sprays with increases in the number of nozzle holes as well as with higher fuel injection pressures and smaller nozzle hole diameters are being developed to establish improvements in diesel combustion.…”