Constant-volume Combustion Vessel Research Articles

The laminar burning velocity of propyl acetate at high pressures and temperatures

Since propyl acetate has the potential to be used as a biofuel in heat engines, it is necessary to ascertain and comprehend its combustion properties. Using equivalence ratios of 0.7–1.4, the laminar burning velocity of propyl acetate was studied in the constant volume combustion vessel at the initial pressures of 1, 2, and 4 bar and initial temperatures of 390 K, 420 K, and 450 K. The constant volume method was used to determine the burning velocity via extrapolations at the initial condition. In addition, a ten-fitted empirical correlation was formulated to calculate the burning velocity for pressures and temperatures as high as 8 bar and 534 K, respectively. The extrapolated burning velocity showed good agreement with the numerical simulation burning velocity, and the correlated burning velocity agreed well with the extrapolated burning velocity. The laminar burning flux was also investigated and was ascertained that laminar burning flux increases as the initial pressure and temperature increase. The fuel mixture density significantly influences the burning flux as the initial pressure rises, whereas the laminar burning velocity has a notable impact on the burning flux as the initial temperature rises. In summary, this study advances our knowledge of propyl acetate combustion, particularly concerning its flame propagation at higher pressures and temperatures and its potential application in combustion engines, and the data are of great engineering value for developing and understanding the combustion science of propyl acetate.

Analysis of effects of pre-chamber orifices on torch flame behaviours in lean-burn gas engines

Fuel conversion from heavy oil to natural gas is favoured as a quick remedy for GHG re-duction from industrial engines in ship propulsion and power generation fields thanks to fewer carbon contents in natural gas. In medium-speed gas engines, the ejection of torch flame from a pre-chamber is often used to promote flame propagation within a main com-bustion chamber. Although orifice specifications affect the ejection behaviour of the torch flame and the following combustion in the main chamber, the effects are difficult to fully understand. Some of the authors developed a constant-volume combustion vessel that simu-lated the combustion chamber around the top dead centre of a pre-chamber type gas engine and observed the effects of orifice specifications on the torch flame ejection and the com-bustion in the main chamber. Still, the correlation was not concluded due to the lack of physical examination. In the paper, the above measurement results were confirmed by using RANS-type CFD considering the larger scale of the target engines, and the physical back-ground of the effects of orifice specifications was successfully reproduced.

Exploration of NH3 and NH3/DME laminar flame propagation in O2/CO2 atmosphere: Insights into NH3/CO2 interactions

Cofiring with reactive hydrocarbon and oxygenated fuels is an effective approach to enhance ammonia (NH3) combustion, which raises growing interest in understanding CN interactions. Among these fuels, dimethyl ether (DME, CH3OCH3) has attracted extensive attention due to its renewable nature and potential for large-scale synthesis from CO2 feedstock. This work reports an experimental and modeling investigation on the laminar flame propagation of NH3 and NH3/DME in O2/CO2 atmosphere, with an emphasis on evaluating the enhanced NH3/CO2 interactions under flame conditions. Laminar burning velocities (LBVs) were measured at elevated pressures up to 5 atm in a constant-volume combustion vessel. A kinetic model was developed and can well reproduce the measured LBVs in this work, as well as the LBVs and speciation data in literature. Modeling analyses and the modified fictitious diluent gas method were used to explore the effects of DME cofiring, pressure and CO2 dilution on LBVs. The results reveal that the chemical effect plays a crucial role in promoting the laminar flame propagation of NH3 with DME cofiring, while the reduction of LBVs with CO2 dilution is predominantly influenced by the thermal effect. LBVs of NH3/DME present a stronger pressure dependency than that of pure NH3, which is attributed to the enhanced pressure-sensitive three-body reactions, including CH3 + H (+M) = CH4 (+M) and H + O2 (+M) = HO2 (+M). Phenomenological analysis of LBVs was conducted by varying the diluent gas from pure N2 to pure CO2, complemented by modeling analyses with adjusted rate constants, to provide insights into NH3/CO2 interactions. The results show that CO2 reduction reactions, including NH + CO2 = HNO + CO and CO + OH = CO2 + H, compete for reactive radicals like H and NH with chain-branching reactions H + O2 = OH + H and consequently prohibit the laminar flame propagation.

An experimental and kinetic modeling study on NH3/air, NH3/H2/air, NH3/CO/air, and NH3/CH4/air premixed laminar flames at elevated temperature

Ammonia combustion helps to realize the off-site use of renewable energy. To understand the combustion characteristics of NH3, laminar burning velocities (SL) of NH3/air, NH3/H2/air, NH3/CO/air, and NH3/CH4/air with various mole fractions of H2, CO, and CH4 in fuel (α), equivalence ratios (ϕ = 0.7∼1.4) at elevated initial temperature (Ti = 298∼423 K) were measured in a constant-volume combustion vessel. Based on updated chemistry of NH2, NNH, N2H2, and NOx, a detailed kinetic model with 169 species and 1268 elementary reactions was developed and validated against the SL, NOx emissions, NH3/NO interaction, and ignition delay times measured by this work and literature. The experimental results show that SL of NH3/H2/air, NH3/CO/air, and NH3/CH4/air flames have different dependence on α. It is nearly exponential for NH3/H2/air flame, non-linear for NH3/CO/air flame, and nearly linear for NH3/CH4/air flame. NH3/air flame has the strongest temperature dependence, followed by NH3/H2/air, NH3/CH4/air, and NH3/CO/air flames. The updated NH3 chemistry, especially NNH chemistry (NNH = N2 + H, N2H2 + H = NNH + H2, and N2H2 + O = NNH + OH), enables the present model obtain better temperature dependence of NH3/air flame. Kinetic modeling analysis shows that the production and reduction of NOx are preferred by the lean and stoichiometric flames of NH3/H2/air, NH3/CO/air, and NH3/CH4/air because of the increased concentrations of NHi and H/O/OH radicals. In the lean flames of NH3/H2/air, NH3/CO/air, and NH3/CH4/air, H2, CO, and CH4 can promote the conversion of NH2 to NO, and relatively inhibit the NO reduction by NH2, resulting in the increase in NO emissions. In the rich flames of NH3/H2/air and NH3/CO/air, NH2 and NH can produce N2H2 and N2H3 through reactions of NH2 + NH = N2H2 +H and NH2 + NH = N2H3, which dominate the promotion and inhibition of laminar flame propagation, respectively.

Role of CH2O moiety on laminar burning velocities of oxymethylene ethers (OMEn): A case study of dimethyl ether, OME1 and OME2

Oxymethylene ethers (OMEn) are an important family of e-fuels that can be produced sustainably from carbon dioxide and hydrogen via renewable electricity. In this work, laminar flame propagation of dimethyl ether (DME, which can be deemed as OME0), dimethoxymethane (OME1) and methoxy(methoxymethoxy)methane (OME2) was investigated in a constant-volume cylindrical combustion vessel. Laminar burning velocities (LBVs) of the three fuels were derived at 423 K, 1–10 atm and equivalence ratios of 0.7–1.5. A kinetic model for the high-temperature oxidation of the three fuels was developed with the isomerization and decomposition reactions of OME2 radicals theoretically calculated. Reasonable predictions can be achieved by the present model during the validation against the new data in this work and previous data in literature. Based on the modeling analysis, fuel-specific flame chemistry of the three fuels was analyzed, especially for the key formation pathways of major intermediates including formaldehyde, methyl formate and CH3. Special attentions were paid on the role of CH2O moiety, which is demonstrated by the variation of LBV and flame chemistry with the ratio (α) of CH2O moiety to the rest moiety in the fuel molecule (α = 1, 2 and 3 for DME, OME1 and OME2). It is observed from the experimental and simulated results that as α increases, the LBV profile has close peak values and peaks towards rich conditions, which results in the crossings of profiles and ascending LBV values under the richest conditions. Reactions involving fuel-specific radicals HCO and CH3 result in the peak shift of H profile and different LBV values, especially under the richest conditions. Furthermore, extended α values at 0 and ∞ by using methane and formaldehyde respectively were also explored with kinetic modeling to provide more insight into the effects of fuel molecular structures.

Flame detection via active plasma probing

Combustion diagnostics of highly diluted air fuel mixtures are of great importance for reducing the carbon footprint of various types of combustion systems. Flame detection techniques, such as ion sensing and optical diagnostics, have been reported for diagnosing combustion status. In this paper, a flame front detection technique based on active plasma probing is introduced and analyzed. Unlike the conventional ion sensing used in internal combustion engines, a separate electrode gap is used to detect the flame front arrival. Further, the voltage potential across the electrodes of the spark plug probe is modulated actively to be slightly below the breakdown threshold prior to flame arrival. At the arrival of the flame front, the ions in the flame tend to decrease the breakdown voltage threshold and trigger a breakdown event. An optically accessible constant volume combustion vessel is employed to investigate the efficacy of such active plasma probing for the detection of the flame front under quiescent and flow conditions. Three types of fuels are used in the tests, including methane, propane, and DME, with an air fuel ratio sweep (from stoichiometric to extremely lean) for each fuel. Efforts are made to characterize the probing criteria of minimum, yet adequate voltage to succeed in the detection of the arrival and departure of the flame front most sensitively and reliably, for all three types of fuels under various mixture strengths. For comparison, conventional ion current measurements are conducted under identical background conditions to benchmark the efficacy of flame detection. Results show that the active plasma probing can promptly detect the arrival and departure of the flame front, robustly regardless of the fuel type and excess air ratios. The precise control of the detecting voltage is key for reliable flame detection with high sensitivity.

Large Eddy Simulation of a Pistonless Constant Volume Combustor: A New Concept of Pressure Gain Combustion

Abstract Classical gas turbine thermodynamic cycle has undergone no change over the last decades. The most important efficiency improvements have been obtained by reducing thermal losses and raising the overall pressure ratio and peak temperature. Pressure gain combustion (PGC) represents an increasingly interesting solution to break out current technological limits. Indeed cycle models show that a pressure raise across the combustion process would reduce fuel consumption, increasing efficiency. Providing an efficiency close to the corresponding detonative technological concepts, constant volume combustion (CVC) represents a viable solution that still needs to be studied. In this work, the CV2 (constant-volume combustion vessel) installed at the Pprime laboratory (France) is numerically investigated using the high-fidelity compressible large eddy simulation (LES) solver AVBP. All the successive phases of the CVC cycle, i.e., air intake, fuel injection, spark-ignited combustion, and exhaust, are considered in the LES. Intake and exhaust valves are properly represented by novel boundary conditions able to mimic the valves impact on the flow without the need to directly consider their presence and dynamics during the simulation, reducing the computational costs. The spark ignition is modeled as an energy deposition term added to the energy equation. The combustion phase is treated by the dynamic version of the thickened flame model (DTFLES) extended to deal with nonconstant pressure combustion. Time-resolved particle imaging velocimetry (PIV) and pressure measurement inside the chamber reveal that cold and reactive turbulent flow are well captured in all the phases, showing the reliability of the approach and the models used.

On the laminar flame propagation of C5H10O2 esters up to 10 atm: A comparative experimental and kinetic modeling study

Biodiesel is a family of renewable engine fuels with carbon-neutral nature. In this work, three C5H10O2 esters (methyl butanoate, methyl isobutanoate and ethyl propanoate), which can serve as model compounds of biodiesel and represent linear and branched methyl esters and linear ethyl esters, were investigated to characterize their laminar flame propagation characteristics up to 10 atm and unravel the effects of isomeric fuel structures. A high-pressure constant-volume cylindrical combustion vessel was used to achieve laminar burning velocity measurements at 1–10 atm, 423 K and equivalence ratios of 0.7–1.5, while comparative experimental work was performed on a heat flux burner at 1 atm, 393 K and equivalence ratios of 0.7–1.6 for methyl butanoate and ethyl propanoate. The laminar burning velocity generally decreases with increasing pressure and increases in the order of methyl isobutanoate, methyl butanoate and ethyl propanoate, which shows distinct fuel isomeric effects. A kinetic model of C5H10O2 esters was developed and validated against the new data in this work and previous data in literature. Modeling analyses were performed to provide insight into the fuel-specific flame chemistry of the three esters isomers. Remarkable differences in radical pools of three ester isomers are concluded to be responsible for the observed fuel isomeric effects on laminar flame propagation. The feature of two ethyl groups connected to the ester group in ethyl propanoate facilitates the ethyl production and inhibits the methyl and allyl production, making it propagate fastest among the three isomers. The branched structure feature of methyl isobutanoate with methyl and i-propyl groups connected to the ester group prevents the ethyl formation and results in considerable CH3 and allyl production, which decelerates its laminar flame propagation.

Turbulent flame speed of NH3/CH4/H2/H2O/air-mixtures: Effects of elevated pressure and Lewis number

This study investigated the turbulent flame speed (ST) of NH3/CH4/H2/air mixtures subjected to differential-diffusion effect characterized by sub-unity Lewis number (Le) at elevated pressures (1 and 5 atm), temperature (373 K), lean equivalence ratios (ϕ = 0.6–0.91) with and without 10% volumetric H2O dilution. The experiments were conducted in a fan-stirred constant volume combustion vessel, with a homogeneous turbulence intensity of u′ = 0 - 2.34 m/s. Schlieren imaging was used to derive the instantaneous turbulent flame speed. Morphology analysis shows that at atmospheric-pressure conditions, the laminar diffusional-thermal unstable flame surface is subjected to cellular wrinkling, this instability is increased at elevated-pressure conditions. Whereas, external turbulence will aggravate the wrinkling but the increase in turbulence intensity suppressed differential-diffusion effects on the flame structure. Flame speed development at sub-unity Le cases show differential diffusion effect and propagates faster than unity Le cases even though they have similar laminar flame speed. Then, the turbulent spherical flame speeds are scaled with a power-law fitting of flame Reynolds number, ReT,flame. The normalized turbulent propagation speed (ST/SL) is much higher for the sub-unity Le flames but it decreases as hydrogen mole fraction (XH2) increases in the fuel mixture. This was explained by the synergistic effect of differential diffusion and turbulence. Water dilution and elevated pressure via increase of Karlovitz number (Ka) and turbulent ReT,flow affect the ST/SL increasing. At sufficiently low Damköhler numbers, Da, a ratio of normalized turbulent flame speed ST/SL is mainly controlled by ReT and scales as a half power law, in line with the classical hypothesis by Damköhler. Additional turbulent stretch modification with Ka or Da can correlate with different hydrogen content conditions, this is because the independent pair of scaling parameters from a set of (u′/SL, lT/lf, Da, Ka, ReT) is considered. The scaling parameter ((u′/SL)α(lT/lf)β(Le)γ) is insensitive to Lewis number, because of the dominance of relative turbulent intensity and turbulent length scale.

Effect of single and split injection on combustion process of diesel spray injected into 2D piston cavity

The flame characteristics of wall-impinging diesel fuel spray at single injection and split injection strategies were investigated in a high-temperature high-pressure constant volume combustion vessel. The 2D piston cavity has been used to form the impingement spray flame and the two-color method has been used to analyze the flame process. Results reveal that 2nd ignition delay can be defined in a specific area surrounded by single and split injections when the 1st injection proportion is from 25% to 75%. The KL factor and distribution area of a single injection are smaller than that of a split injection at EOI + 0.2 ms. The soot oxidation area is far away from the wall surface. However, the soot formation area is mainly concentrated along the cavity groove. The soot oxidation area proportion plays a decisive role in the emission of soot. The split injection interval has a key role in the soot formation dominant process at the initial stage of ignition. The initial mean flame temperature changes slightly, the mean flame temperature decreases earlier, the soot oxidation degree becomes higher, no interaction or small interaction between the 1st and 2nd injection, and the maximum degree of soot formation and oxidation dominant process occur when the 1st injection proportion equals 50%. The mean flame temperature drop time of the single injection is advanced. The total flame area and the soot formation and oxidation dominant process before and after EOI + 0.2 ms were also discussed. The total flame area of split injection is larger than single injection after EOI + 0.2 ms.

Exploration on laminar flame propagation of 3,3-dimethyl-1-butene, 2,3-dimethyl-1-butene and 2,3-dimethyl-2-butene

Laminar flame propagation of branched hexene isomers/air mixtures including 3,3-dimethyl-1-butene (NEC6D3), 2,3-dimethyl-1-butene (XC6D1) and 2,3-dimethyl-2-butene (XC6D2) was investigated using a high-pressure constant-volume cylindrical combustion vessel at 1–10 atm, 373 K and equivalence ratios of 0.7–1.5. The measured laminar burning velocity (LBV) decreases in the order of NEC6D3, XC6D1 and XC6D2, which indicates distinct fuel molecular structure effects. A kinetic model was constructed and examined using the new experimental data. Modeling analyses were performed to reveal fuel-specific flame chemistry of branched hexene isomers. In the NEC6D3 and XC6D1 flames, the allylic CC bond dissociation reaction plays the most crucial role in fuel decomposition under rich conditions, while its dominance is replaced by H-abstraction reactions under lean conditions. The H-abstraction and H-assisted isomerization reactions are concluded to govern fuel consumption in the XC6D2 flame under all investigated conditions. Both C0C3 reactions and fuel-specific reactions are found to be influential to the laminar flame propagation of the three branched hexene isomers. Fuel molecular structure effects were analyzed with special attentions on key intermediates distributions and fuel-specific reactions in all flames. Due to the formation selectivity of key intermediates such as 2-methyl-1,3-butadiene and 2,3-dimethyl-1,3-butadiene, the production of reactive radicals especially H follows the order of NEC6D3 > XC6D1 > XC6D2, which results in the same order of fuel reactivities and LBVs.

Experimental and numerical study on the laminar flame characteristics for PODE3 and PODE3/iso-octane blends under elevated and sub-ambient initial pressures

• The laminar burning speeds for PODE 3 under different initial pressures are tested. • The laminar burning speeds for PODE 3 / iso -octane blends are investigated. • The accuracy of mechanism for PODE 3 and PRF/PODE n is validated. • The Markstein length and laminar flame instability are analyzed in detail. Polyoxymethylene dimethyl ether (PODE n ) is a promising alternative fuel for diesel due to its high oxygen contents (>40%) and high cetane number, thereby improving the thermal efficiency and reducing the pollutant emissions caused by the internal combustion engine. This study investigated the laminar burning speeds and Markstein length for PODE 3 and PODE 3 / iso -octane blends at wide equivalence ratios (0.7–1.6) and initial temperature of 408 K under elevated and sub-ambient initial pressures (0.5–4 bar) in a cylindrical constant-volume combustion vessel with high-speed Schlieren photography method combined with the nonlinear extrapolation model. The numerical simulation was also carried out to validate the accuracy of the mechanisms of PODE 3 and primary reference fuel (PRF)/PODE n blends and analyze the reaction sensitivities and the mole fraction profiles of key species. In addition, flame instability was discussed experimentally and theoretically. The results show that the laminar burning speed for PODE 3 decreases with the initial pressure, and the laminar burning speed for PODE 3 / iso -octane basically accords with the linear relationship to the PODE 3 volume fractions. The findings indicate that the kinetic effect has a dominant impact on the laminar burning speed for PODE 3 when compared with the thermal effect. The results also reveal that the Markstein length is negative only at an equivalence ratio between 1.5 and 1.6 under P = 2 bar, indicating that PODE 3 has more enhanced flame stability. Furthermore, the flame instability decreases with the increase of PODE 3 volume fractions, increases with the increase of initial pressure, and increases with the increase of the equivalence ratio. The differences between experimental results and theoretical results including the Markstein length, the critical radius and the critical Peclet number might be explained by the stretch rate results.

Turbulent flame speed measurement of NH3/H2/air and CH4/air flames and a numerical case study of NO emission in a constant volume combustion chamber (C.V.C.C.)

Ammonia combustion is a meaningful method to retrieve stored amounts of excess variable renewable energy. The practical combustors like engines and gas turbines fired by the common hydrocarbon fuels need to be operated by the carbon-free like ammonia or ammonia/hydrogen blends shortly. Thus, in this study, the turbulent flame speed (ST) and NO emission potentials of the NH3/H2 blend (NH3/H2 = 40/60, 50/50, and 60/40 vol%) are investigated in comparison with the methane/air flames in a constant volume combustion vessel. The effects of Lewis number (Le), turbulent intensity (u′=0.78–2.34 m/s), and pressures (1, 5 bar) on normalized ST/SL are considered and validated against literature proposed correlations. Flame morphologies show that the turbulent flame wrinkling characteristics are mainly dominated by the turbulent intensity (u′/SL), turbulent length scale (lT/lF) at different pressures, and differential-diffusion term (Le). The differential-diffusion and turbulent stretch, jointly determine the ST/SL of NH3/H2/air and CH4/air mixtures. The scaling parameter ReT,flame/Le2 can describe the self-similar propagation characteristics of ST/SL of spherical flame and consider the differential-diffusion effect. The further Damköhler number, Da modification on the general turbulent flame speed correlation proposed as: ST/SL ∼ ADaB·(ReT,flame/Le2)0.5 unifies different hydrogen content cases by considering the turbulent stretch effect. When literature experimental data are also correlated together, the scaling law becomes: ST/SL ∼ Re0.5/Le ∼ (u'/SL)0.5(lT/lF)0.5Le-1 with equal weighting factors of u'/SL and lT/lF terms, this follows the Damköhler's second hypothesis at highly turbulent regimes. The impact of turbulent intensity and residence time on NO emissions in the flue gas of NH3/H2/air and CH4/air flames are independent of turbulent intensity due to the long residence time. Simulated results show that residence time has contrary effects on NO emissions of NH3/H2/air and CH4/air flames due to the contrary role of the thermal-NOx pathway. Practical utilization of NH3/H2/air in gas turbine combustors could increase the flame residence time by increasing the back-flow zone in the post-flame zone and swirl number to suppress the NOx emissions.

Influence of Elevated Fuel Temperatures On the Spray Characteristics of Gasoline - Ethanol Blends

Abstract In this study, the effect of elevated fuel temperatures on the spray characteristics of gasoline-ethanol blends were studied in an optically accessible constant volume spray and combustion vessel. MAHLE SmartHeat® is a fuel heater located directly upstream of the fuel injector. High speed images of the spray injected from a six-hole gasoline direct injection injector typical of a side-injection engine were captured with shadowgraph imaging technique. Two fuel blends, gasoline with 10% ethanol (E10) and 85% ethanol (E85) were investigated at ambient conditions of 1 bar, 45°C and 4 bar, 180°C at an injection pressure of 100 bar. Fuel temperatures were varied from 75 to 250°C. A comparison of the near nozzle and the global spray characteristics was made for the two fuels in the range of fuel temperatures selected. Results showed that flash boiling leads to two primary effects for the two fuel blends: (i) an appreciable increase in spray angle near the exit of the nozzle followed by (ii) a decrease in spray angle downstream of the nozzle due to the interaction of the plumes and the collapsing of the spray. Furthermore, for both fuel blends, upon flash boiling, entrainment and mixing were reduced downstream of the nozzle because of the collapse of the spray. To reduce this effect, nozzle orientations and geometries should be modified.

Characterizing methane and nitric oxide interaction in oxygen-free outwardly propagating spherical flame

Co-firing methane (CH4) and ammonia (NH3) has attracted growing concerns as a feasible greenhouse gas reduction strategy in gas turbine-based power generation, which raises the need to better understand the interaction of methane and nitric oxide (NO) under flame conditions. In this work, laminar flame propagation of CH4/NO mixtures at initial pressure (Pu) of 1 atm, initial temperature (Tu) of 298 K and equivalence ratios of 0.4–1.8 was experimentally investigated using a constant-volume combustion vessel. Laminar burning velocities (LBVs) and Markstein lengths were experimentally determined. A kinetic model of CH4/NO combustion was developed with rate constants of several important reactions updated, presenting reasonable predictions on the measured LBVs of CH4/NO mixtures. The modeling analyses reveal that the reduction of NO can proceed through two mechanisms, i.e. the hydrocarbon NO reduction mechanism and non-hydrocarbon NO reduction mechanism. Among the two mechanisms, the non-hydrocarbon NO reduction mechanism which includes reactions NO+H = N+OH, NO+O = N + O2 and NO+N = N2+O has a higher contribution to NO reduction at the equivalence ratio of 0.6, while the hydrocarbon NO reduction mechanism with hydrocyanic acid (HCN) as the key intermediate plays a more important role at the equivalence ratio of 1.8. NO+H = N+OH and CH3+NOHCN+H2O are found to be the two most sensitive reactions to promote the flame propagation, while the LBVs measured in this work are demonstrated to provide strong constraint for these reactions. Furthermore, previous CH4/O2/NO oxidation data measured in flow reactor and rapid compression machine were also simulated, which provides extended validation of the present model over wider conditions.

Experimental diesel spray characterization of the medium-duty injector with single- and multi-hole nozzle configurations under non-reacting, non-vaporizing conditions

Characteristics of diesel sprays injected through Cummins medium-duty ISB injectors were studied experimentally in an optically accessible constant-volume combustion vessel. The experiments were performed with ultra-low-sulfur diesel (ULSD) under non-reacting and non-vaporizing conditions, including different ambient gas densities (23–65 kg/m3), injection pressures (500–1,500 bar), and injection duration times (0.5–1.5 ms). The ambient temperature of the vessel was maintained at a room temperature of 313 K for all the tests. A systematic comparison was made between single-hole (SH) and multi-hole (MH) injector configurations. A plume-to-plume variation in spray penetration length was observed for various operating conditions. A substantial deviation was observed for a specific hole against the averaged plume, indicating that arbitrary selection of the plume index may result in inaccurate spray characterization of the MH injector. The penetration length of the MH injector was shorter than that of the SH injector under the same operating conditions, indicating that a spray model calibrated on SH injector data may not accurately predict the transient spray behavior of the MH injector in practical engine simulations. A square-root correlation of the spray penetration length was applied for both the SH and MH injectors. The spray penetration length and dispersion angles of the ISB SH injector were also compared with those of the heavy-duty Cummins ISX SH injector. While the ISX SH injector showed a faster penetration than the ISB SH injector, the dispersion angle was similar. The differences in spray penetration between ISB and ISX injectors followed the expected trend based on their nozzle hole diameters.

Laminar flame propagation of acetone and 2-butanone at normal to high pressures: Insight into fuel molecular structure effects of ketones

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of acetone and 2-butanone at normal to high pressures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at 1–10 atm, 423 K and equivalence ratios of 0.7–1.5. A kinetic model of acetone and 2-butanone combustion was developed from our recent pentanone model [Li et al., Proc. Combust. Inst. 38 (2021) 2135–2142] and validated against experimental data in this work and in literature. Together with our recently reported data of 3-pentanone, remarkable fuel molecular structure effects were observed in the laminar flame propagation of the three C3C5 ketones. The laminar burning velocity increases in the order of acetone, 2-butanone and 3-pentanone, while the pressure effects in laminar burning velocity reduces in the same order. Modeling analysis was performed to provide insight into the key pathways in flames of acetone and 2-butanone. The differences in radical pools are concluded to be responsible for the observed fuel molecular structure effects on laminar burning velocity. The favored formation of methyl in acetone flames inhibits its reactivity and leads to the slowest laminar flame propagation, while the easiest formation of ethyl in 3-pentanone flames results in the highest reactivity and fastest laminar flame propagation. Furthermore, the LBVs of acetone and 3-pentanone exhibit the strongest and weakest pressure effects respectively, which can be attributed to the influence of fuel molecular structures through two crucial pressure-dependent reactions CH3 + H (+M) = CH4 (+M) and C2H4 + H (+M) = C2H5 (+M).

Effect of ethanol addition on the laminar burning velocities of gasoline surrogates

Laminar burning velocities (LBV) and Markstein lengths of spherical flames of n-heptane (H), iso-octane (O), and ethanol (E) and their binary and ternary mixtures (in equal volume fractions) at elevated pressures and temperatures are experimentally investigated. Measurements are performed in a constant volume combustion vessel using two distinct techniques: flame front imaging and pressure rise rate. For pure components, ethanol shows a higher LBV compared to n-heptane and iso-octane at same conditions. A pressure dependence on LBV of each pure component is observed where ethanol is found to have the strongest pressure dependence. This is also indicated by the higher negative coefficients in three-body termination reactions through a sensitivity analysis. For binary mixtures, the EH mixture was found to have the highest LBVs compared to other blends. The pressure dependence on LBV of ethanol is also present in binary mixtures where at a higher pressures the EO blend is found to have a lower LBV compared to the OH blend due to competing reactions against the H radical pool. Non-linear behaviour of the EH mixture’s LBV was observed and verified through the chemical kinetics suggesting the widely used mixing rules for blends might need additional improvements, especially at higher temperatures. An equal volume EHO ternary mixture shows the LBV to be similar to the OH mixture at 380 K. At 450 K, the LBVs differ by 7% at atmospheric pressure, but due to ethanol’s pressure dependence, this difference decreases to zero as the pressure increases to 4 bar.

Comparison of air and EGR with different water fractions dilutions on the combustion of hydrogen-air mixtures

Lean combustion and exhaust gas recirculation (EGR) are key strategies for controlling NO and heat release process of hydrogen engines. This paper firstly compared these two strategies on combustion behaviors of hydrogen-air mixtures within dilution ratio ranges from 0 to 50%. Then, the water vapor effects in EGR on combustion of hydrogen-air mixtures are further investigated. All tests are conducted in a constant volume combustion vessel at the initial temperature of 373 K and pressure of 1 bar. CHEMKIN PRO is adopted to predict the hydrogen-air mixtures combustion behaviors under different conditions with one-dimensional premixed adiabatic planner flame model. The unburnt hydrogen concentration and the NO mole fraction in the final products were measured. The results show that laminar burning velocity decreases more obviously with the EGR ratio than air dilution ratio. EGR dilution has much obvious influence on reducing NO mole fraction in the final products. However, both dilution strategies adversely cause the increase of incompletely combusted H2. To balance the NO control and unburned H2 emission, 30% EGR rate with 20% water vapor fraction is recommended based on the testing ranges of this study.

Unraveling combustion chemistry of tetramethoxysilane in flow reactor pyrolysis and laminar flame propagation

Tetramethoxysilane (TMOS, Si(OCH3)4) is a widely used precursor in the vapor-fed aerosol flame synthesis (VAFS) of silica nanoparticles. This work reports the first attempt to investigate the flow reactor pyrolysis and laminar flame propagation of TMOS. Pyrolysis species were detected using gas chromatography (GC) at 1.09 atm and 930–1250 K, such as methane, methanol, ethylene, ethane and carbon monoxide, and their mole fraction profiles versus the heating temperature were also measured. The laminar flame propagation of TMOS/air mixtures was investigated in a constant-volume cylindrical combustion vessel at the initial pressure (Pu) of 1 atm, initial temperature (Tu) of 373 K and equivalence ratios from 0.7 to 1.4. TMOS has generally lower laminar burning velocities (LBVs) than many hydrocarbon fuels due to the high oxygen content and consequently low adiabatic flame temperature of TMOS. A kinetic model of TMOS combustion was developed and validated against both the new data in this work and the shock tube pyrolysis data in literature. Rate of production (ROP) analysis and sensitivity analysis were performed to provide insight into the key pathways in TMOS decomposition. In the flow reactor pyrolysis, the methanol elimination reaction and the H-abstraction reactions dominate the decomposition of TMOS. Among the primary decomposition products, CH2OSi(OCH3)2 mainly isomerizes to CH3OSi(=O)OC2H5, while CH2OSi(OCH3)3 and OSi(OCH3)3 mainly decompose via β-Si-O scission reactions producing OSi(OCH3)3 and OSi(OCH3)2, respectively. The primary decomposition pathways of TMOS can all eventually lead to the formation of SiO2. In the laminar flame propagation, the modeling analyses indicate formaldehyde and methyl radical are abundantly produced, making many reactions involving the two intermediates influential on the laminar flame propagation of TMOS.