Étude du refroidissement des gaz recyclés dans un moteur diesel
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It has been shown by several researchers that EGR is more efficient when it is cooled before it is introduced into the cylinder. Therefore, a heat exchanger must be properly designed to ensure proper cooling of the EGR. The present work aims to study by numerical simulation the cooling of the recycled gases through three models of shell-type exchanger, intended for heavy duty diesel engines. The first is a 19 tubes model of diameter of 10 mm, the second is a 61 tubes model of diameter of 6 mm and the third is a 19 tubes model with helical baffles on the shell side. In order to estimate the performance of these EGR coolers, two factors were taken into account: cooling efficiency and pressure drop. ANSYS FLUENT code was used to solve the governing equations. The temperature-dependent physical properties of the recycled exhaust gases have been incorporated via ""User Defined Functions"" (UDF), feature of FLUENT. The exhaust gas inlet temperature is set at 523.15 K and the input mass flow rate varies from 0.07 to 0.2 kg /s. The computed performance results were compared to existing experimental measurements. The comparison of the computed results for the three models allowed distinguishing the EGR cooler model consisting of 19 tubes with helical baffles as having the best performance in terms of cooling efficiency and pressure drop. Finally, another numerical simulation study was presented to elucidate the effect of injection timing and EGR on engine performance and NOx emissions from a natural gas-powered direct injection engine. In a first step, three injection moments in degrees of crankshaft as 9, 4 and 0 °CA bTDC were chosen and three EGR ratios as 10, 20, 30% were applied in the second stage. To model turbulent non-premixed combustion, the transported PDF model (Probability Density Function) was chosen. The results showed that the advance of injection has an advantageous effect on the engine performance but accompanied by a considerable increase of NOx. The application of the EGR rate of 20 % has largely reduced NOx and thus meets the emission standards allowed, without much degraded engine performance.