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1. Laminar flame characteristics of ammonia dimethyl ether mixtures during the autoignition period

   https://doi.org/10.1016/j.fuel.2024.133522  

   Goyal, Tushar; Samimi-Abianeh, Omid (February 2025, Fuel)

   The ammonia (NH3) and dimethyl ether (DME) mixture is a promising alternative fuel that offers the potential for cleaner combustion. This study presents an investigation of the autoignition-assisted flame speeds of stochiometric NH3/DME mixtures under conditions relevant to practical combustion systems. Experiments were conducted at pressures of 5 and 10 bar, gas temperatures ranging from 625 to 708 K, and three DME concentrations (10, 20, and 30%, mole fraction basis) in NH3/DME fuel mixtures using a rapid compression machine-flame (RCM-Flame) apparatus. For the majority of the autoignition experiments, first-stage ignition delay time was observed. Thus, the flame experiments were performed by igniting the spark both before and after the first-stage ignition delay time. The results are presented in terms of the Beta-Damköhler Number, defined as the ratio of spark ignition time to the first-stage ignition delay. The flame speed changes depending on the Beta-Damköhler Number, pressure, gas temperature, and DME concentration. The flame speed increases by increasing the temperature, decreasing the pressure, and increasing DME concentrations. However, the effect of Beta-Damköhler Number on flame speed is complicated: with 10% DME in the mixture, the flame speed is independent to Beta-Damköhler Number, and slight observed slight decrease of flame speed is due to the temperature drop during the post-compression period; with 20% DME in the mixture, at both pressures, the flame speed jumps after the first ignition delay (or Beta-Damköhler Number of one) , and remains constant before and after; similar behavior was observed with 30% DME in the mixture at 5 bar, however, at some temperatures, the flame speed increases at Beta-Damköhler Number of greater than one, and at 10 bar, the first ignition delay was short and flame speed was not measured at Beta-Damköhler Number of less than one. For all studied conditions, a linear trend was observed between burning velocity and stretch rate. Positive Markstein lengths were observed at most conditions, except for two specific gas temperatures (664 K at 5 bar and 671 K at 10 bar) with 30% DME, where negative Markstein lengths are found. One-dimensional laminar flame speed simulations agreed with measured data for Beta-Damköhler Numbers. less than one, but underpredicted the measured data at other conditions. 

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2. Combustion Characteristics of Low DCN Synthetic Aviation Fuel, IPK, in a High Compression Ignition Indirect Injection Research Engine

   https://doi.org/10.4271/2023-01-0272  

   Soloiu, Valentin; Weaver, Amanda; Smith, Richard; Rowell, Aidan; Mcafee, John; Willis, James (April 2023, SAE Technical Paper Series)

   The Coal-To-Liquid (CTL) synthetic aviation fuel, Iso-Paraffinic Kerosene (IPK), was studied for ignition delay, combustion delay, pressure trace, pressure rise rate, apparent heat release rate in an experimental single cylinder indirect injection (IDI) compression ignition engine and a constant volume combustion chamber (CVCC). Autoignition characteristics for neat IPK, neat Ultra-Low Sulfur Diesel (ULSD), and a blend of 50%IPK and 50% ULSD were determined in the CVCC and the effects of the autoignition quality of each fuel were determined also in an IDI engine. ULSD was found to have a Derived Cetane Number (DCN) of 47 for the batch used in this experimentation. IPK was found to have a DCN of 25.9 indicating that is has a lower affinity for autoignition, and the blend fell between the two at 37.5. Additionally, it was found that the ignition delay for IPK in the CVCC was 5.3 ms and ULSD was 3.56 ms. This increase in ignition delay allowed the accumulation of fuel in the combustion chamber when running with IPK that resulted in detonation of the premixed air and fuel found to cause high levels of Ringing Intensity (RI) when running neat IPK indicated by the 60% increase in Peak Pressure Rise Rate (PPRR) when compared to ULSD at the same load. An emissions analysis was conducted at 7 bar Indicated Mean Effective Pressure (IMEP) for ULSD and the blend of 50% ULSD and 50% IPK. With the addition of 50% IPK by mass, there was found to be a reduction in the NO_x, CO₂, with a slight increase in the CO in g/kWh. 

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3. Investigations of Low-Temperature Heat Release and Negative Temperature Coefficient Regions of Iso-Paraffinic Kerosene in a Constant Volume Combustion Chamber

   https://doi.org/https://doi.org/10.1115/ICEF2021-68203  

   Valentin Soloiu, Richard Smith (October 2021, Proc. ASME. ICEF2021, ASME 2021 Internal Combustion Engine Division Fall Technical Conference,)

   ASME; iCEF (Ed.)

   Research was conducted to observe the correlation of ignition delay, combustion delay, the negative temperature coefficient region (NTCR), and the low temperature heat release region (LTHR), in a constant volume combustion chamber (CVCC) in relation to blended amounts of iso-paraffinic kerosene (IPK) by mass with Jet-A and their derived cetane numbers (DCN). The study utilizes the ASTM standard D7668-14.a in a PAC CID 510 CVCC. The DCN was calculated using the ignition delay and combustion delay measured over 15 combustion events. The fuel blends investigated were 75%Jet-A blended with 25%IPK, 50%Jet-A with 50%IPK, 25%Jet-A with 75%IPK, neat Jet-A, and neat IPK. The ignition delay of neat Jet-A and IPK was found to be 3.26ms and 5.31ms, respectively, and the combustion delay of the fuels were 5.00ms and 17.17 ms, respectively. The ignition delay for 75Jet-A25IPK, 50Jet-A50IPK, 25Jet-A75IPK, fuel blends were found to be 3.5ms, 3.8ms, and 4.2ms, respectively. The combustion delay between the 75Jet-A25IPK, 50Jet-A50IPK, 25Jet-A75IPK, blends are 5.8ms, 7.0ms, and 9.4ms, respectively. The DCNs for 75Jet-A25IPK, 50Jet-A50IPK, 25Jet-A75IPK 43.1, 38.7, and 33.5, respectively. The DCN of the fuel blends compared to neat Jet-A was lower by 10.16% for 75Jet-A25IPK, 19.37% for 50Jet-A50IPK, 30.50% for 25Jet-A75IPK and 46.03% for neat IPK. Blends with larger amounts by mass of IPK resulted in extended ignition and combustion delays. It is concluded that the fuels that have larger amounts of IPK blended within them have extended NTC regions, LTHR regions, and decreased ringing intensity during combustion. 

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4. Combustion characteristics of F24 compared to Jet A in a Common Rail Direct Injection Research Compression Ignition Engine

   https://doi.org/10.1115/ICEF2022-91113  

   Soloiu, Valentin; Smith, Richard Collins; Weaver, Amanda; Parker, Lily; Brock, Dillan; Rowell, Aidan; Ilie, Marcel (October 2022, Proc. ASME. ICEF2022, ASME 2022 ICE Forward Conference,)

   ASME (Ed.)

   Research was conducted to determine combustion characteristics such as: ignition delay (ID), combustion delay (CD), combustion phasing (CA 50), combustion duration, derived cetane number (DCN) and ringing intensity (RI) of F24, for its compatibility in Common Rail Direct Injection (CRDI) compression ignition (CI) engine. The first part of this study is investigating the performance of Jet-A, F24, and ultra-low sulfur diesel #2 (ULSD) using a constant volume combustion chamber (CVCC) followed by experiments in a fired CRDI research engine. Investigations of the spray atomization and droplet size distribution of the neat fuels were conducted with a Malvern Mie scattering He-Ne laser. It was found that the average Sauter Mean Diameter (SMD) for Jet-A and F24 are similar, with both fuels SMD droplet range between 25–29 micrometers. Meanwhile, ULSD was found to have a larger SMD particle size in the range of 34–40 micrometers. It was observed during the study, utilizing the CVCC, that the ID and CD for neat ULSD and Jet-A are nearly identical while the combustion of F24 is delayed. F24 was found to have longer durations of both ID and CD by approx. 0.5 ms. This results in a lower DCN for the fuel of 43.5, whereas ULSD and Jet-A have DCNs of 45 and 47 respectively. The peak AHRR for ULSD and Jet-A are nearly identical, whereas F24 has a peak magnitude of approx. 20% lower than ULSD and Jet-A. It was found that both aviation fuels had significantly fewer ringing events occurring after peak high temperature heat release (HTHR), a trend also observed in the CRDI research engine. Neat F24, Jet-A and ULSD were researched in the experimental engine at the same thermodynamic parameters: 5 bar indicated mean effective pressure (IMEP), 50°C (supercharged and EGR) inlet air temperature, 1500 RPM, start of injection (SOI) 16°BTDC, and 800 bar of fuel rail injection pressure as the baseline parameters in order to observe their ignition behavior, low temperature heat release, combustion phasing, and combustion duration. It was found that the ignition delay of F24 and Jet-A was greater than ULSD, approx. 5% for both aviation fuels. This ignition delay also affected the combustion phasing, or CA 50, of the aviation fuels. The CA 50 of the aviation fuels was delayed by approx. 2% compared to ULSD. Jet-A had a nearly identical combustion duration compared to ULSD, however F24 had an extended combustion duration which was approx. 3% longer than that of ULSD and Jet-A. It was discovered with the accumulations of these delays in ID, CD, CA50, that the RI of the aviation fuels were reduced. F24 was discovered to have more delays, and the RI correlates with these results having a 70% reduction in RI compared to ULSD. 

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5. An experimental investigation of flame and autoignition behavior of propane

   https://doi.org/https://doi.org/10.1016/j.combustflame.2020.12.001  

   Burnett, Miles A. (February 2021, Combustion and flame)

   null (Ed.)

   Autoignition delay time data are one important means to develop, quantify, and validate fundamental understanding of combustion chemistry at low temperatures (T<1200 K). However, low-temperature chemistry often has higher uncertainties and scatter in the experimental data compared with high-temperature ignition data (T>1200 K). In this study, autoignition properties of propane and oxygen mixtures were investigated using the University of Michigan rapid compression facility in order to understand the effects of ignition regimes on low-temperature ignition data. For the first time for propane, autoignition delay times were determined from pressure histories, and autoignition characteristics were simultaneously recorded using high-speed imaging of the test section through a transparent end-wall. Propane mixtures with fuel-to-O2 equivalence ratios of ϕ = 0.25 and ϕ = 0.5 and O2-to-inert gas molar ratios of 1:3.76 were studied over the pressure range of 8.9 to 11.3 atm and the temperature range of 930 – 1070 K. The results showed homogeneous or strong autoignition occurred for all ϕ = 0.25 experiments, and inhomogeneous or mixed autoignition occurred for all ϕ = 0.5 experiments. While a limited temperature range is covered in the study, importantly the data span predicted transitions in autoignition behavior, allowing validation of autoignition regime hypotheses. Specifically, the results agree well with strong-autoignition limits proposed based on the Sankaran Criterion. The autoignition delay time data at the strong-ignition conditions are in excellent agreement with predictions using a well-validated detailed reaction mechanism from the literature and a zero-dimensional modeling assumption. However, the experimental data at the mixed autoignition conditions were systematically faster than the model predictions, particularly at lower temperatures (T< ~970 K). The results are an important addition to the growing body of data in the literature that show mixed autoignition phenomena are important sources of the higher scatter observed in the low-temperature autoignition data for propane and other fuels. The results are discussed in terms of different methods to capture the effects of pre-autoignition heat release associated with mixed autoignition conditions and thereby address some of the discrepancies between kinetic modeling and experimental measurements. 

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