New ignition delay time measurements of natural gas mixtures enriched with small amounts of n-hexane and n-heptane were performed in a rapid compression machine to interpret the sensitization effect of heavier hydrocarbons on auto-ignition at gas-turbine relevant conditions. The experimental data of natural gas mixtures containing alkanes from methane to n-heptane were carried out over a wide range of temperatures (840–1050 K), pressures (20–30 bar), and equivalence ratios (φ = 0.5 and 1.5). The experiments were complimented with numerical simulations using a detailed kinetic model developed to investigate the effect of n-hexane and n-heptane additions. Model predictions show that the addition of even small amounts (1–2%) of n-hexane and n-heptane can lead to an increase in reactivity by ∼40–60 ms at compressed temperature (TC) = 700 K. The ignition delay time (IDT) of these mixtures decreases rapidly with an increase in concentration of up to 7.5% but becomes almost independent of the C6/C7 concentration beyond 10%. This sensitization effect of C6 and C7 is also found to be more pronounced in the temperature range 700–900 K compared to that at higher temperatures (>900 K). The reason is attributed to the dependence of IDT primarily on H2O2(+M) ↔ 2ȮH(+M) at higher temperatures while the fuel-dependent reactions such as H-atom abstraction, RȮ2 dissociation, or Q˙
OOH + O2 reactions are less important compared to 700–900 K, where they are very important.

In this work, the ignition delay time characteristics of C2 – C3 binary blends of gaseous hydrocarbons including ethylene/propane and ethane/propane are studied over a wide range of temperatures (750 – 2000 K), pressures (1 – 135 bar), equivalence ratios (φ = 0.5 – 2.0) and dilutions (75 – 90%). A matrix of experimental conditions is generated using the Taguchi (L9) approach to cover the range of conditions for the validation of a chemical kinetic model. The experimental ignition delay time data are recorded using low- and high-pressure shock tubes and two rapid compression machines (RCM) to include all of the designed conditions. These novel experiments provide a direct validation of the chemical kinetic model, NUIGMech1.1, and its performance is characterized via statistical analysis, with the agreement between experiments and model being within ~ 26.4% over all of the conditions studied, which is comparable with a general absolute uncertainty of the applied facilities (~ 20%). Sensitivity and flux analyses allow for the key reactions controlling the ignition behavior of the blends to be identified. Subsequent analyses are performed to identify those reactions which are important for the pure fuel components and for the blended fuels, and synergistic/antagonistic blending effects are therefore identified over the wide range of conditions. The overall performance of NUIGMech1.1 and the correlations generated are in good agreement with the experimental data.

Methanol is a widely used engine fuel, blend component, and additive. However, no systematic auto-ignition data or laminar flame speed measurements are available for kinetic studies of the effect of methanol as a blending or additive component. In this work, both ignition delay times and laminar flame speeds of pure methanol, n-heptane and their blends at various blending ratios were measured at engine-relevant conditions. Results show that increasing methanol in a blend promotes reactivity at high temperatures and inhibits it at low temperatures, with the crossover temperature occurring at approximately 970–980 K with it being almost independent of pressure. The experimental data measured in this work, together with those in the literature are used to validate NUIGMech1.1, which predicts well the experimental ignition delay times and laminar flame speeds of the pure fuels and their blends over a wide range of conditions. Furthermore, kinetic analyses were conducted to reveal the effects of methanol addition on the oxidation pathways of n-heptane and the dominant reactions determining the fuel reactivities. It is found that competition for ȮH radicals between methanol and n-heptane plays an important role in the auto-ignition of the fuel blends at low temperatures. At high temperatures, methanol produces higher concentrations of HȮ2 radicals which produce two ȮH radicals either through the production of H2O2 and its subsequent decomposition or through direct reaction with Ḣ atoms. This promotes the high temperature reactivity of methanol/n-heptane mixtures for ignition delay times and laminar flame speeds, respectively.

Ignition delay time measurements for multi-component natural gas mixtures were carried out using a rapid compression machine at conditions relevant to gas turbine operation, at equivalence ratios of 0.5–2.0 in ‘air’ in the temperature range 650–1050 K, at pressures of 10–30 bar. Natural gas mixtures comprising C1–C7 n-alkanes with methane as the major component (volume fraction: 0.35–0.98) were considered. A design of experiments was employed to minimize the number of experiments needed to cover the wide range of pressures, temperatures and equivalence ratios. The new experimental data, together with available literature data, were used to develop and assess a comprehensive chemical kinetic model. Replacing 1.875% methane with 1.25% n-hexane and 0.625% n-heptane in a mixture containing C1–C5 components leads to a significant increase in a mixture's reactivity. The mixtures containing heavier hydrocarbons also tend to show a strong negative temperature coefficient and two-stage ignition behavior. Sensitivity analyses of the C1–C7 blends have been performed to highlight the key reactions controlling their ignition behavior.

A comprehensive experimental and kinetic modeling study of the ignition delay time (IDT) characteristics of some binary blends of C1–C2 gaseous hydrocarbons such as methane/ethylene, methane/ethane, and ethane/ethylene was performed over a wide range of composition (90/10, 70/30, 50/50%), temperature (∼800–2000 K), pressure (∼1–40 bar), equivalence ratio (∼0.5–2.0), and dilution (∼75–90%). An extensive literature review was conducted, and available data were extracted to create a comprehensive database for our simulations. Based on the existing literature data, an experimental matrix was designed using the Taguchi approach (L9) in order to identify and complete the experimental matrix required to generate a comprehensive experimental IDT set necessary for the validation of a chemical kinetic model. The required high- and low-temperature IDTs were collected using low-/high-pressure shock tubes and rapid compression machines, respectively. The predictions of NUIGMech1.0 are examined versus all of the available experimental data, including those taken in the current study using the IDT simulations and a correlation technique. Moreover, the individual effect of the studied parameters, including mixture composition, pressure, equivalence ratio, and dilution on IDT, is investigated over the studied temperature range. Correlations that were developed based on NUIGMech1.0 are presented for each specific blended fuel over the conditions studied. These correlations show an acceptable performance versus the experimental data.

Using of shrouded valve creates high swirl ratio inside the engine cylinder. Moreover, using of swirl generation device in the inlet port can improve this swirl ratio. In this paper, both twisted tap and guide vanes devices inserted in the inlet port are used individually for enhancing the generated swirl by the shrouded valve. In addition, the effect of these combinations on the volumetric efficiency and the turbulent kinetic energy (TKE) is studied. A three-dimensional simulation model based on SST k- ω model was used for predicting the air flow characteristics through the inlet port and the engine cylinder in both intake and compression strokes. The results showed that the using of twisted tap and guide vanes with the shrouded valve combinations increases the swirl ratio by 5.2% and 2%, respectively, at the start of injection. They also increase the TKE by 145% and 86.5% but they decrease the volumetric efficiency by about 3%.

Air swirl motion inside the engine cylinder improves the air-fuel mixing which has a great effect on the thermal efficiency, soot formation, and engine emissions. In this work, experimental and numerical investigations were performed on a diesel engine cylinder having a configuration of a helical-spiral inlet port and shrouded valve under steady flow condition. Four shrouded valves having different shroud angles were used; the shroud angles are 90 deg, 120 deg, 150 deg, and 180 deg. With each shroud angle, four orientation angles were selected; they are 0 deg, 30 deg, 60 deg, and 90 deg. The experimental and numerical analyses were performed under a constant vacuum pressure of 350 mm H2O. In addition, numerical analysis, using the SST k − ω model, is performed on the engine cylinder using shrouded valve having shroud angle of 90 deg as a case study. The results showed that using of shrouded valve decreases the mass flow rate and the discharge coefficient while it increases the swirl number at all valve shroud and orientation angles except valve shroud of 180 deg and orientation angle of 90 deg. The numerical simulation analysis showed reasonable agreement with the experimental work. Therefore, the virtual test rig can be used for studying the influence of the valve and the port configurations on the flow characteristics.

The in-cylinder airflow motion is an important factor that severely affects combustion efficiency and emissions in diesel engines. It is greatly affected by the inlet port and valve geometries. A diesel engine cylinder with a helical–spiral inlet port is used in this study. An ordinary inlet valve and shrouded inlet valve having different shroud and orientation angles are used to study the shroud effect on the swirl and tumble motion inside the engine cylinder. Four shroud angles of 90 deg, 120 deg, 150 deg, and 180 deg are used. With each shroud angle, four orientation angles of 0 deg, 30 deg, 60 deg, and 90 deg are also used. Three-dimensional simulation model using the shear stress transport (SST) k–ω model is used for simulating air flow through the inlet port, inlet valve, and engine cylinder during both the intake and compression strokes. The results showed that increasing the valve shroud angle increases the swirl, and the maximum increase occurs at a valve shroud angle of 180 deg and orientation angle of 0 deg with a value of 80% with respect to the ordinary valve. But it decreases the volumetric efficiency, and the maximum decrement occurs at valve shroud of 180 deg and orientation angle of 90 deg with a value of 5.98%. Variations of the shroud and orientation angles have very small effect on the tumble inside the engine cylinder.

This study reports new ignition delay time (IDT) measurements of ethane (C2H6)/‘air’ mixtures with NOx (nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O)) addition in the range 0 – 1000 ppm at stoichiometric fuel to air (φ) ratios, at compressed temperatures (TC) of 851 – 1390 K and at compressed pressures (pC) of 20 – 30 bar. In addition, new IDT measurements of three highly diluted C2H6/NO2 mixtures at φ = 0.5, TC = 805 – 1038 K, and pC = 20 – 30 bar are also studied. These new experimental data, together with data already available in the literature, are used to validate NUIGMech1.2 with an updated NOx sub-mechanism. Although the addition of 200 ppm of NO or NO2 to ethane shows a minimal promoting effect, the addition of 1000 ppm significantly promotes its reactivity. The similarity of the effect of the addition of both NO and NO2 addition is due to the fast conversion of NO into NO2 in the presence of molecular oxygen. However, the 1000 ppm NO doped ethane mixtures exhibit ∼20% shorter IDTs compared to the NO2 blended ones. The addition of 1000 ppm of N2O exhibits no effect on ethane oxidation at the conditions studied. The NUIGMech1.2 predictions can reproduce the sensitisation effect of NOx on ethane with good agreement over a wide range of pressure, temperature, equivalence ratio, and percentage dilution. Sensitivity and flux analyses of C2H6/NOx are performed to highlight the key reactions controlling ignition over the different temperature regimes studied. The analyses show that there is a competition between the reactions Ṙ + NO2 ↔ RȮ + NO and Ṙ + NO2 (+M) ↔ RNO2 (+M). This governs NOx sensitization on C2H6 ignition.

The oxidation of propane (C3H8), with the addition of different oxides of nitrogen (NO, NO2, and N2O) in concentrations of 0 – 2000 ppm, has been investigated for stoichiometric mixtures, at compressed temperatures of (TC) = 690 – 1420 K, and at compressed pressures of (pC) = 2.0 – 3.0 MPa using both a rapid compression machine and a high-pressure shock tube. These new ignition delay time (IDT) measurements, together with C3/NOx data available in the literature, provide a direct validation of NUIGMech1.3 which includes an updated C3/NOx sub-mechanism. The experimental results show that the mixtures with NO2 and NO/NO2 added have longer IDTs, inhibiting reactivity at TC〈 800 K, and shorter IDTs, promoting reactivity at TC〉 800 K, compared to the base C3H8/‘air’ mixtures indicating the complex chemical interactions involved. Both the inhibiting and prompting effects depend on the concentrations of NO and NO2 added and on the temperature regime. The addition of 1000 ppm NO2 significantly reduces the negative temperature coefficient (NTC) behavior of C3H8 in the temperature range 715–800 K compared to the addition of 200 ppm. Model predictions with 1000 ppm NO added, assuming no conversion of NO to NO2, are significantly slower than for both the 0 and 1000 ppm NO2 addition cases at TC < 800 K. Although NO and NO2 addition have different impacts on C3H8 oxidation at low-, intermediate, and high-temperatures, the addition of 1000 ppm N2O did not show any chemical effect at the conditions studied. NUIGMech1.3, with the updated C3/NOx sub-mechanism, reproduces the sensitisation effect of NOx on C3H8 with generally good agreement. Sensitivity and flux analyses have been performed to highlight the key reactions controlling ignition. The analyses show that competition between the reactions Ṙ+NO2↔RȮ+NO and Ṙ +NO2 (+M) ↔ RNO2 (+M) governs NOx sensitization on propane ignition. The inhibiting effect of NO and NO2 addition to propane stems from the nĊ3H7+ NO2↔ nC3H7Ȯ + NO and nC3H7Ȯ2 + NO ↔ nC3H7Ȯ + NO2 reactions, which compete for nC3H7Ȯ2 radicals, reducing the rate of isomerization of nC3H7Ȯ2 into Ċ3H6OOH1–3 (RȮ2 ⇌ 
OOH).