Browsing by Author "Dai, Liming"

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    Numerical study of premixed PODE3-4/CH4 flames at engine-relevant conditions
    (MDPI, 2024-03-12) Leng, Yupeng; Ji, Xiang; Zhang, Chengcheng; Simms, Nigel; Dai, Liming; Yu, Chunkan
    Polyoxymethylene dimethyl ether (PODEn, n ≥ 1) is a promising alternative fuel to diesel with higher reactivity and low soot formation tendency. In this study, PODE3-4 is used as a pilot ignition fuel for methane (CH4) and the combustion characteristics of PODE3-4/CH4 mixtures are investigated numerically using an updated PODE3-4 mechanism. The ignition delay time (IDT) and laminar burning velocity (LBV) of PODE3-4/CH4 blends were calculated at high temperature and high pressure relevant to engine conditions. It is discovered that addition of a small amount of PODE3-4 has a dramatic promotive effect on IDT and LBV of CH4, whereas such a promoting effect decays at higher PODE3-4 addition. Kinetic analysis was performed to gain more insight into the reaction process of PODE3-4/CH4 mixtures at different conditions. In general, the promoting effect originates from the high reactivity of PODE3-4 at low temperatures and it is further confirmed in simulations using a perfectly stirred reactor (PSR) model. The addition of PODE3-4 significantly extends the extinction limit of CH4 from a residence time of ~0.5 ms to that of ~0.08 ms, indicating that the flame stability is enhanced as well by PODE3-4 addition. It is also found that NO formation is reduced in lean or rich flames; moreover, NO formation is inhibited by too short a residence time.
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    Numerical study on the combustion and emissions characteristics of liquid ammonia spray ignited by dimethyl ether spray
    (MDPI, 2024-12-31) Leng, Yupeng; Dai, Liming; Wang, Qian; Lu, Jiayu; Yu, Ouqing; Simms, Nigel John
    Ammonia has attracted considerable attention as a zero-carbon fuel for decarbonizing energy-intensive industries. However, its low reactivity and narrow flammability limit efficient ignition and efficient combustion. By using CONVERGR software, this study numerically investigates the ignition and combustion characteristics of liquid ammonia spray ignited by dimethyl ether spray in a constant-volume chamber at an ambient temperature of 900 K. Critical parameters, including injection angles (90°–150°), liquid ammonia injection pressures (60–90 MPa), and ambient pressures (2.8–5.8 MPa), were systematically analyzed to evaluate their effects on ignition conditions and emissions. Results indicate that increasing the injection angle improves mixing between liquid ammonia and dimethyl ether sprays, enhancing combustion efficiency and achieving a maximum efficiency of 92.47% at 120°. Excessively large angles cause incomplete combustion or misfire. Higher liquid ammonia injection pressures improve atomization and promote earlier interactions between the sprays but reduce combustion efficiency, decreasing by approximately 2% as injection pressure increases from 60 MPa to 90 MPa. Higher ambient pressures improve combustion stability but decrease ammonia combustion efficiency. Post-combustion NO emissions at 5.8 MPa are reduced by 60.48% compared to 3.8 MPa. The formation of NO is strongly correlated with the combustion efficiency of liquid ammonia. A higher combustion rate of liquid ammonia tends to result in elevated NO. Based on these findings, an injection angle of 120°, an NH3 injection pressure of 75 MPa, and an ambient pressure of 3.8 MPa are recommended to optimize combustion efficiency.