Joint Research Projects

Study In Thailand

The Ignition Characteristics of Petro- and Renewable Fuels in a Spray Combustion Chamber





Regarding the novel fuel research, attentions are paid on the combustion power performance and pollution emission. This will happen when people are trying to upgrade the recycled waste oil into high value and low combustion pollution fuel through the combustion examination. To develop the fundamental knowledge of liquid fuel combustion, this project studies the ignition characteristics of petro- and renewable diesels in a spray combustion chamber. The NCKU Fuel & Combustion Laboratory (F&A Lab) is now working with the professors from 3 countries, including Malaysia (U. of Nottingham and Universiti Tunku Abdul Rahman), Thailand (Kasetsart University) and Vietnam (Ho Chi Minh City University of Technology), concentrating on optical combustion analysis, ignition analysis, nozzle design, combustion chemical kinetics, and combustion emission analysis. Totally 7 Ph.D. students are participating in this project based on their expertise, and are expected to improve their international visibility and exchange during this project.

Fig 1. Schematic diagram of ignition testing device located in Fuel and Combustion Laboratory, NCKU



The spray ignition experiments were carried out in a constant volume combustion chamber (CVCC), as shown in Fig. 1. A combustion chamber with a volume of 500 mL was built from ultra-high temperature stainless steel material that afforded high temperature operation up to 873 K and was developed for handling high operating chamber pressures as high as 80 bar. The fuel tested in this study is a renewable fuel produced “in-house” through catalytic hydro-conversion technology, named hydro-processed renewable jet (HRJ). HRJ has contains 8% aromatics, which conforms to the current standard for alternative jet fuels. The jet fuels used in civil and military aviation sectors, Jet A-1 and JP-5, are also tested in this study for comparison. A typical jet fuel has a lower normal-to-iso alkane ratio necessary for achieving the minimum cold flow properties. The higher fuel density of Jet-A1 may cause better spray atomization and affect the ignition properties. JP-5 has a lower cetane number, as compared to Jet-A1 and HRJ, which may contribute to its combustion behavior. HRJ has higher latent heat evaporation compared to Jet-A1 and JP-5.



Measurements of the spray ignition delay for the petroleum jet fuels (JP-5 and Jet-A1) and the alternative jet fuel (HRJ) were carried out at various chamber pressures of 10, 15, and 20 bar and at chamber temperatures ranging from 600 K to 818 K. The large distinction among the total ignition delays for HRJ, JP-5, and Jet-A1 was more obvious at low chamber temperatures at all chamber pressures. A moderate discrepancy was found at intermediate to high chamber temperatures. At a low chamber pressure (10 bar), HRJ fuel had shorter total ignition delays by approximately 74% and 67% as compared to those for the petroleum jet fuels (JP-5 and Jet-A1), respectively, at low temperature. Additionally, at the same temperature, the cool-flame ignition delays were 2 times longer than the total ignition delays for all of the tested fuels at all chamber pressures. It was noted that the cool-flame ignition delay was among the essential parameters for determining the time required for the injected fuel to react with the oxidizer and become a combustible mixture.



Fig 2. (a) Total ignition delay; (b) cool-flame ignition delay for three jet fuels



The main ignition or total ignition delays contributed to the chemical kinetic reaction, which was mostly dominated by the degenerate chain branching rates in the exothermic reaction. The longer time from cool-flame ignition to main ignition at a lower chamber temperature was due to the ignition process taking place during oxidation kinetics at low temperature. At lower chamber temperatures, the slower reactivity of the given fuels at cool-flame ignition could be attributed to the fact that more specific energy is required for the heat release rate to compete against heat vaporization of the fuels in the endothermic reaction. Although HRJ had the highest latent heat vaporization compared to the petroleum jet fuels, the cool-flame ignition delays of HRJ were shorter than those of Jet-A1 and JP-5. This was because the higher H/C ratio of the HRJ fuels makes it possible to have more specific energy during the chain initiation process, thus enhancing the reactivity of the HRJ fuels as compared to Jet-A1 and JP-5. The longer ignition delay was caused by a weak reaction rate of H atom abstraction; R + O2 -> RO2 and rate of isomerization; RO2-> QOOH. The size of the C–H bonds in the fuels strongly influenced the H atom abstraction stability.