Design improvement of compressed natural gas (CNG)-Air mixer for diesel dual-fuel engines using computational fluid dynamics

摘要

This project carried out a computational fluid dynamics (CFD) analysis using ANSYS Fluent 14.5 software to design a compressed natural gas (CNG)-air mixer for CNG-diesel dual fuel engine. This study aimed to examine the performance of existing secondary fuel premixing controller (SFPMC) commercial mixer and modify its design in terms of air fuel ratio (AFR) and CNG-air mixture homogeneity (CAMH). The design modification of the mixer involved changing the surface of control valve shaft, gas manifold, position and directions of the CNG outlet holes. Results from simulation indicated that the original mixer was unable to control AFR since it changed from 8 at the engine speed of 1000 rpm to 176.39 at the engine speed of 3600 rpm when gas inlet pressure is fixed. However, AFR for the optimized mixer is between 17.21 and 17.4, a range close to stoichiometric AFR with various engine speeds and specific locations of the control valve. In terms of CNG-air mixture homogeneity, the uniformity index (UI) of methane mass fraction (M_(ch4)) at the outlet of the original mixer is between 0.555 when engine speed is 1000 rpm and 0.578 when engine speed is 3600 rpm. However UI of M_(ch4) at the outlet of optimized mixer is between 0.995 when engine speed is 1000 rpm and 0.961 when engine speed is 3600 rpm. That means the CNG and air are homogeneously mixed at the outlet of optimized mixer in comparison with the original mixer. In summary, changing the surface of control valve shaft from cylindrical to conical shape lead to gradually opening the gas way and allow to rising the gas flow rate with increasing of air mass flow rate due to growing of engine speed, so the AFR is controlled in comparison with the origin mixer. Besides, changing the asymmetrically distributed seven gas injection holes near the mixer outlet to equal distribution 10 gas injection holes far from the mixer outlet perpendicularly to the air flow stream resulted in excellent homogeneity of the air and gas mixture in the optimized mixer.