Constant Volume Reactor Research Articles

Volatile matter characterization of birch biochar produced under pressurized conditions

The volatile matter (VM) content and composition of birch biochars produced at 320 °C under elevated pressure (0.1–11 MPa) and constant pressure or constant volume reactor conditions were characterized by thermogravimetry/mass spectrometry (TG/MS) and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). Some of the thermal properties of the biochars and the composition of the VMs varied as a function of the maximal pressure applied during carbonization. The samples prepared at higher pressures released more volatiles up to 320 °C, while the maximal rate of thermal decomposition at around 440 °C showed decreasing tendency with the carbonization pressure. In terms of VM composition, the most apparent effect was the significant increase of the amounts of apoallobetulins from biochars prepared at elevated pressures, which were formed by dehydration, ring closure and rearrangement from the betulin content of birch. The change in the ratio of the evolved guaiacol and 4-methylguaiacol as well as that of syringol and 4-methylsyringol as a function of the maximal pressure of carbonization indicated a modification of the lignin decomposition mechanism.

A wide ranging experimental and kinetic modeling study of TMEDA pyrolysis and oxidation

Traditional liquid propellants like unsymmetrical dimethylhydrazine include several issues, such as toxicity, short storage time and preservation difficulty at low temperatures. For this reason, exploring green and stable propellant fuels is imperative in the development of propellants. Tetramethyl ethylenediamine (TMEDA) is considered to be a promising green propellant fuel with high specific impulse, non-toxicity, and long-term storage stability. Thus, investigating the combustion characteristics of TMEDA and constructing a detailed chemical kinetic mechanism are of fundamental importance in describing its combustion in computational fluid simulation. For this work, ignition delay times of TMEDA were measured in a shock tube at equivalent ratios of 0.5, 1.0 and 2.0, at pressures of 2, 10, and 20 bar, and in the temperature range of 870–1500 K with fixed fuel concentration at 2 %. After the reflected shock at 990–1500 K, the pyrolysis products of 2000 and 5000 ppm TMEDA in Argon were also measured at 5 and 10 bar conditions in a single pulse shock tube. In addition, laminar flame speeds of TMEDA were measured at initial pressures of 1 bar, initial temperatures of 333 and 353 K, and equivalence ratios ranging from 0.8 to 1.5 in a constant volume reactor. The experimental results were simulated using a detailed TMEDA kinetic mechanism. Reaction path analysis and sensitivity analysis were provided to clarify the chemical kinetics of TMEDA oxidation and pyrolysis. The current work provides new experimental data and fundamental analyses for understanding the oxidation and pyrolysis characteristics of TMEDA, and sheds light on optimal directions for future models.

Implementation of an OpenFOAM solver for shock and detonation simulation at high pressure

The high pressure condition widely used in industry makes it necessary to evaluate the impact of real gas behaviour in numerical simulation, especially for shock and detonation problems. Since most open source solvers are based on ideal or perfect gas assumption, we have developed an OpenFOAM solver for Real gas Shock and Detonation simulation, referred to as RSDFoam. We have considered the nonidealities in the equation of state, thermodynamic functions and mass action law brought about by the interaction between the fluid particles and by their finite volume. Connection between blastFoam and Cantera was built to utilize the CFD capabilities from the former and the efficient chemistry solver and real gas models in Cantera. The new solver has been thoroughly validated against analytical solutions, previous numerical simulation results and experimental data. For non-reactive flow, shock tube and oblique shock problems were solved. For reactive cases, constant volume reactor, steady detonation speed, reaction zone structure, and cellular structure were numerically studied. The satisfactory agreement demonstrates the accuracy and robustness of the solver for real gas based shock and detonation simulation.

Can vibrational pumping occur via O2–N2 collisions in nonequilibrium vibrationally excited air?

The occurrence of vibrational pumping in air under nonequilibrium conditions is investigated as this phenomenon is not considered in the design of the current phenomenological models. It is shown that pumping can only happen during de-excitation and when the translational temperature is below around 1000 K. O2 is the molecule that would get pumped, and pumping will not occur when the initial equilibrium temperature is greater than around 1200–1600 K due to the formation of enough O to extinguish pumping via the O2–O vibration–translation reaction. The limiting initial temperature can be increased to around 2000 K if a nonequilibrium initial condition is considered. In cases where pumping does occur, constant–volume reactor simulations showed pumping of ≈5%. Nozzle simulations representative of that in hypersonic wind tunnels are conducted for an equilibrium temperature of 1100 K at the throat; pumping of up to around 10 K (≈1%) can be observed. It can be suggested that constant–volume reactors generally overestimate the manifestation of thermochemical nonequilibrium-associated phenomena and are a better zero-dimensional analogy for the relaxation process in flows with large length scales and no further expansion after an initial rapid expansion. After examination of the uncertainties of the most important rates used in the simulations, one may suggest that the current results correspond to the upper bound for the magnitude of pumping. It may be concluded that pumping is unimportant for practical intents and purposes in nonequilibrium hypersonic flows, and phenomenological models need not be able to recreate this phenomenon.

Inference of chemical kinetics and thermodynamic properties from constant-volume combustion of energetic materials

Constant-volume combustion tests have been widely used to characterize the reactivity, the combustion efficiency, and the gas-generating performance of energetic materials. However, the currently used performance metrics (ignition delay time, pressurization rate, and maximum pressure) highly rely on the specific experimental condition, which leads to the challenge of fairly comparing the material performance tested in different conditions and laboratories. For this reason, there has been a need for a new metric independent of the experimental conditions, along with a novel analytical tool. In this work, we proposed a kinetic modeling framework to infer the chemical kinetics and thermodynamic properties from the pressure profiles in constant-volume combustion tests. In this framework, a physical model predicts the thermodynamic states of the system during the constant-volume combustion process, while an inverse model calibrates the model parameters to fit the model to the experimental observation via sensitivity analysis. We demonstrated the success of the framework in inferring the chemical kinetics and thermodynamic properties with both synthesized data and experimental measurements. Moreover, the physical model with the inferred properties enabled a comprehensive understanding of how gas release and temperature rise contribute to the pressure rise in the constant-volume reactor. Therefore, the classical constant-volume combustion experiments can be utilized to infer properties for the design of energetic materials with desired gas and heat generation profiles and for modeling the energetic behaviors in more complex systems.

Discontinuity in cubic equations of state: Impact of attraction term mixing rule on combustion simulation

High-pressure or cryogenic conditions relevant to hydrogen applications introduce non-idealities in the property of fluid, which requires using real gas equations of state in numerical simulation. However, intrinsic but non-physical discontinuities exist in the partial derivatives with respect to temperature of the molecular attraction term of popular cubic equations of state. This feature is related to the van der Waals mixing rule and results in non-physical discontinuities in the thermodynamic functions and simulation results. Thermodynamic properties of dry air were used to illustrate the impact of discontinuities on the thermodynamic functions whereas steady detonation simulation and constant volume reactor simulation were performed to discuss the impact of the discontinuities on combustion modeling. We show that the accuracy of the solution can be decreased by up to several percents. In addition, an increase of computation cost is induced in the vicinity of the discontinuities when adaptive time-stepping is adopted in the simulation. A correlation previously proposed in the literature to remove the discontinuity issue was tested and proved efficient to limit the decrease of the time-step.

A wide range experimental and kinetic modeling study of the oxidation of 2,3-dimethyl-2-butene: Part 1

2,3-Dimethyl-2-butene (TME) is a potential fuel additive with high research octane number (RON) and octane sensitivity (S), which can improve internal combustion engine performance and efficiency. However, the combustion characteristics of TME have not been comprehensively investigated. Thus, it is essential to study the combustion characteristics of TME and construct a detailed chemical kinetic model to describe its combustion. In this paper, two high-pressure shock tubes and a constant-volume reactor are used to measure ignition delay times and laminar flame speeds of TME oxidation. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in “air”, at pressures of 5 and 10 bar, in the temperature range of 950 – 1500 K. Flame speeds of the TME/ “air” mixtures were measured at atmospheric pressure, at a temperature of 325 K, for equivalence ratios ranging from 0.78 to 1.31. Two detailed kinetic mechanisms were constructed independently using different methodologies; the KAUST TME mechanism was constructed based on NUIGMech1.1, and the MIT TME mechanism was built using the Reaction Mechanism Generator (RMG). Both mechanisms were used to simulate the experimental results using Chemkin Pro. In the present work, reaction flux and sensitivity analyses were performed using the KAUST mechanism to determine the critical reactions controlling TME oxidation at the conditions studied.

Analysis of micro-mixing and chemical kinetic effects of ultra-lean methane-air mixture ignited by turbulent jets

The Equilibrium-PaSR and the Constant-volume reactor framework were used to construct the TJI combustion system with or without turbulence effects. Numerical investigations on micro-mixing and chemical kinetic effects of ultra-lean methane-air mixture ignited by turbulent jets were conducted. The results show that main chamber combustion (MCC) is commonly affected by the jet reactive stages and fuel mixed time. The higher mixing time leads to faster heat transfer and less temperature rise, which ultimately limits the fuel conversion. So PaSR1 fuel reactivity has a stronger effect on t50 (time of 50 % fuel conversion in MCC) than the micro-mixing effect. Besides, the methane content in the jet is particularly important for the overall combustion process in MCC. For the analysis of the chemical kinetic effects, it is known that the ignition of MCC at lower temperatures is more strongly affected by the chemical effects of the combustion products from the pre-combustion chamber (PCC). As the temperature increases, the chemical effect decreases slightly, and the physical effect presents an increasing trend. The low-carbon intermediate CH3/C2H2 radicals in the cold jet have the most significant effect on shortening the ignition delay time (IDT). The ignition ability of the hot jet is closely related to the chemical effects of H/O/OH radicals, and the chemical effects of important low-carbon intermediate species are significantly reduced. For flame instability analysis, the combustion products can reduce the flame thickness of the mixture in MCC to increase the intensity of hydrodynamic instability. With the increase of [O + OH], the destabilizing effect of the thermal-diffusion instability of methane-air premixing gradually increases.

An experimental and detailed kinetic modeling study of the pyrolysis and oxidation of DMF over a wide range of conditions

A new chemical kinetic mechanism of DMF (N, N-dimethylformamide) is proposed. To assess the reliability of the mechanism, high pressure shock tube, single pulse shock tube and constant volume reactor were used to experimentally evaluate ignition delay times, high temperature pyrolysis, and laminar flame speeds of DMF, respectively. Ignition delay times of DMF/ “air” were measured for equivalence ratios of 0.5, 1.0, and 2.0, pressures of 2.0 and 10.0 bar, and temperature ranging from 900 to 1430 K. Additionally, ignition delay times of DMF/acetone/air were measured for equivalence ratios of 0.5 and 1.0, pressures of 2.0 and 10.0 bar, and temperature ranging from 900 to 1430 K. High temperature pyrolysis products of DMF/acetone mixtures, diluted in argon, were studied at 5.0 bar, temperatures between 1000 and 1600 K. Flame speed measurements of DMF/air were obtained for pressures of 100 and 200 kPa, temperatures of 398, 423, and 448 K, and equivalence ratios from 0.7 to 1.4. Simultaneously, a chemical kinetics mechanism was developed to simulate the experimental results using ChemKin Pro, with sensitivity and flux analysis being carried out to elucidate limitations and possible refinements.

Numerical study on explosion limits of ammonia/hydrogen/oxygen mixtures: Sensitivity and eigenvalue analysis

The study aims to investigate explosion limit characteristics of NH3/H2/O2 mixtures and construct a compact mechanism for explosion limits prediction. To study effects of H2 addition, simulations of mixtures with varied H2 percentages were performed in an adiabatic, constant-volume reactor based on detailed mechanisms over a wide range of pressures and temperatures. Results showed that the monotonic explosion limit curves for NH3/O2 mixtures transits to non-monotonic Z-shaped curves with H2 addition, and higher H2 percentage shifts the explosion limits to lower temperatures. Moreover, sensitivity analysis was conducted to identify the dominant reactions in explosion limits. It showed that reactions involving H, NH2 and NH3 are dominant at low pressures, and with elevated pressures, the effects of oxidation reactions involving NO, NO2, HNO and H2NO are enhanced. To construct a compact mechanism for explosion limit prediction, the detailed mechanism was firstly reduced by the directed relation graph method. Then, a linear mechanism was established based on eigenvalue analysis, consisting of molecule–molecule and radical-molecule reactions in the reduced mechanism. Compared with the simulation results of the detailed mechanism, the simulation results of the linear mechanism are satisfactory at low and medium pressures, but less satisfactory at high pressures. Considering the significant effects of radical–radical reactions at high pressures, four nonlinear reactions were incorporated into the linear mechanism, finally building the compact mechanism, which is able to well reproduce the explosion limits of NH3/H2/O2 mixtures.

Role of low-temperature oxidation in non-uniform end-gas autoignition and strong pressure wave generation

This study highlights the importance of heat release rate in low-temperature oxidation (LTO) on non-uniform end-gas autoignition and strong pressure wave generation, which are substantially relevant to knocking combustion. The simulations are conducted using the compressible Navier–Stokes equations with detailed transport and chemical kinetics models in a one-dimensional constant-volume reactor. Four fuel/air stoichiometric mixtures, n-butane, i-octane, n-heptane, and dimethyl ether (DME)/air mixtures, are simulated. The results show that larger knocking intensities are produced with n-heptane and DME in their negative temperature coefficient (NTC) regimes because of the stronger non-uniformity of end-gas autoignition. The non-uniformity of end-gas autoignition is enhanced by a pressure wave disturbance that is caused by the rapid temperature rise of the end-gas region in LTO. In particular, the high heat release rate with the DME/air mixture generates a distinct pressure wave disturbance in the reactor, which considerably enhances the non-uniformity of end-gas autoignition through the reflection of the wave at the wall. In contrast, the heat release rate in the n-heptane case is milder than that in the DME case, and therefore, the knocking intensity in the n-heptane case is smaller compared to that of DME due to less enhancement of the non-uniform end-gas autoignition. No large knocking intensities are produced with n-butane and i-octane, which have weak NTC, because of the absence of a temperature rise in LTO. Thus, this study concludes that the high heat release rate in LTO and the generated pressure wave disturbance play a significant role in the generation of large knocking intensities through the enhancement of non-uniform end-gas autoignition.

Effects of Globally Stratified Temperature Distributions and NTC Characteristics on End-gas Combustion Modes

ABSTRACT This study numerically investigates the effects of stratified temperature distributions on the end-gas combustion mode in a constant volume reactor. The initial temperature in the reactor was globally stratified with linear gradients. The effects of negative temperature coefficient (NTC) characteristics are addressed through a comparison between the results of a non-NTC fuel and an NTC fuel. The compressible Navier–Stokes equations are solved with detailed chemistry. The results show that the addition of large temperature gradients such as −10 or −5 K/cm can prevent the pressure wave development associated with end-gas autoignition, and the knocking intensity is significantly reduced. In contrast, small temperature gradients such as −1 K/cm lead to the generation of the developing detonation mode and thus to large knocking intensities. In the NTC fuel case, the knocking intensity becomes relatively large because the spatial gradient of the ignition delay time decreases even with the addition of large temperature gradients, especially in the NTC regime. Based on all the results, a correlation is presented between the initial temperature and the reaction front speed (the spatial gradient of the ignition delay time) for the prediction of end-gas combustion mode. The correlation suggests that the possibility of preventing large knocking intensities by temperature gradients is reduced under high-pressure conditions because of the increase in the reaction front speed. Consequently, this study suggests that very large temperature gradients would be required under high-pressure conditions to prevent large knocking intensities under the concept of thermal stratification.

A localized thickened flame model for simulations of flame propagation and autoignition under elevated pressure conditions

The present study proposes a localized thickened flame (LTF) model for the accurate prediction of flame propagation and autoignition timing. The unresolved-scale terms appeared in spatially-filtered governing equations due to thin flame structures are constructed under a physical constraint in which laminar flame speed is maintained. A high-order derivative is introduced to dynamically localize the effects of the LTF in the regions of unresolved propagating flame. The model is also designed such that the thickened flame is resolved by the same number of grid points for any grid size used. Therefore, a user-specified constant in the model does not need to be adjusted depending on the employed grid size. Laminar flame propagation problems are used to validate the performance of the proposed LTF model and determine the appropriate value of the user-specified constant. The results using a one-dimensional constant-volume reactor demonstrate that the LTF successfully captures the accurate flame propagation behaviors under elevated pressure conditions, while not affecting the end-gas autoignition timing, even on relatively coarse grid resolutions. The high-order derivative in the LTF serves as a dynamic parameter for detecting the thinning flame under elevated pressure conditions.

Impact of the surrogate formulation on 3D CFD engine knock prediction using detailed chemistry

For engine knock prediction, surrogate fuels are often composed of iso-octane and n-heptane since they are the components of the Primary Reference Fuel (PRF). By definition, a PRF has no octane sensitivity (S = RON-MON). However, for a commercial gasoline fuel holds RON > MON and therefor S > 0. More complex surrogates are Toluene Reference Fuels (TRF) and Ethanol containing Toluene Reference Fuels (ETRF). In this work, the impact of the surrogate formulation on the prediction of flame propagation and auto-ignition in the unburnt gases are investigated. The surrogates are composed such that the Research Octane Number is the same. The auto-ignition events ahead of the flame front are predicted using 3D CFD and a combustion model based on the ETRF mechanism by Seidel (2017). The strength of the auto-ignition is determined using the detonation diagram by Bradley and co-workers (2002, 2003). Applying the different surrogates, ignition kernels of different size and reactivity are predicted. The results indicate a dependency on the local temperature history and the low temperature chemistry of the fuel species. The comparison of homogenous constant volume reactor and transient simulations show that the analysis of ignition delay time and octane rating solely from homogenous simulations is not sufficient if the knock tendency of a surrogate in engine simulations needs to be characterized.

Assessment of detailed reaction mechanisms for reproduction of ignition delay times of C2–C6 alkenes and acetylene

The main aim of the work was to assess the performance of selected detailed reaction mechanisms (DRM) with regard to calculating the ignition delay times (IDTs) of C2–C6 alkenes and acetylene. To carry out the analysis an extensive literature review was done in order to collect ignition delay time data. The data included IDTs obtained with shock tubes only, giving in total 1206 experimental points for acetylene, ethene, propene, 1-,2-, iso-butene, 1-penten and 1-hexene. To minimize the influence of facility effects, only IDTs shorter than 1 ms were taken into account. 15 DRMs were selected with a view to assessing their ability to predict experimental IDTs with Cantera code and constant volume reactor model assumptions. As some research groups provided DRMs that were designed in a hierarchical way, the Pearson linear correlation coefficient was calculated for each group of DRMs to assess whether the results were similar within the group. Newly developed assessment techniques were used to assess the IDT prediction capabilities of the DRMs. Box-whisker plots summarized the statistical dispersion of the applied error function across pressure, temperature and equivalence ratio. Additional heat maps of the error function on pressure–temperature plots provided more insight into the performance of mechanisms and highlighted areas of uncertainty and possible improvement.

End-gas autoignition behaviors under pressure wave disturbance

The present study addresses effects of pressure wave disturbance on end-gas autoignition behavior and subsequent pressure wave development, which are of particular interest for understanding a fundamental process of knocking combustion in spark-assisted ignition engines. Such pressure wave disturbance is initiated by a compression wave generated from spark-like ignition and continuously exists in a combustion chamber. The investigation is conducted through a direct numerical simulation with a 1-D constant volume reactor, where the compressible Navier–Stokes equations are solved with detailed reaction mechanisms of n−C7H16 and n−C4H10/air mixtures. The width of an ignition kernel parametrically changes in order to control the strength of an initial compression wave and thus pressure wave disturbance. The result showed that stronger compression waves with larger kernel widths enhance the progress of the autoignition process at the wall through the transient increase in pressure and temperature due to the wave reflection. Consequently, stronger pressure waves such as a developing detonation are generated with the enhanced inhomogeneous end-gas autoignition starting from the wall. In contrast, weak pressure wave disturbance produces no strong pressure waves due to the lower inhomogeneity of autoignition timing in an end-gas region. An analysis with temperature variation in an end-gas region indicates that cool flame generation helps maintain the superiority of the wall for earlier autoignition through the heat release at an earlier stage. Therefore, a stronger pressure wave associated with inhomogeneous end-gas autoignition is likely generated around 650 K in the case of an n−C7H16/air mixture, which corresponds to a distinct peak generation of knocking intensity in the negative temperature coefficient regime. For lower temperature conditions such as 500 K, the end-gas autoignition first takes place in a region away from the wall. This is because the initially induced temperature inhomogeneity in the end-gas region is considerably weakened due to wave interactions and dissipation effects during longer ignition delay times. Thus, multiple end-gas points including the wall can become a preferred point for earlier end-gas autoignition, resulting in a variation of end-gas autoignition locations. The result for an n−C4H10/air mixture is finally shown and the difference from that for the n−C7H16/air mixture is highlighted.

A low temperature natural gas reaction mechanism for compression ignition engine application

This paper demonstrates the improvement of a low temperature natural gas (LTNG) mechanism for simulating fuel ignition and combustion emissions under engine-like conditions. The LTNG mechanism was compared and validated in both the simplified zero-dimensional (0D)/one-dimensional (1D) reactors and three-dimensional (3D) engine models. In a 0D premixed constant volume reactor, the LTNG mechanism can more accurately predict the ignition delay than the well-developed GRI-3.0 natural gas mechanism, when the temperature is lower than 1300 K, compared to the experimental data. The results also indicate that the LTNG mechanism can well predict the formation of important carbon-containing gaseous species, such as acetylene and carbon dioxide, and nitrogen oxides emissions under typical engine conditions with and without technologies that implement low temperature combustion. In a 3D computational fluid dynamic model with a range of temperatures and pressures, the matched ignition delay curves from the numerical simulation and the experimental measurements indicate that the LTNG mechanism can improve the prediction of natural gas ignition and combustion compared to the GRI-3.0 mechanism. A further validation conducted in a direct-injection natural gas engine ignited by a hot surface illustrates the reliability of the LTNG mechanism for predicting natural gas ignition and combustion inside compression ignition engines.

Detonation onset in a thermally stratified constant volume reactor

Understanding detonation development from a flame kernel initiated by a pre-ignition event is important for modern internal combustion (IC) engines operating at boosted conditions. To provide fundamental insights into the effects of bulk gas temperature stratification on the characteristics of detonation development, one-dimensional high fidelity simulations were conducted for a constant volume reactor filled with a thermally stratified reactive stoichiometric hydrogen/air mixture. A linear temperature variation in the upstream end-gas was introduced to represent the thermal stratification of the bulk mixture, and the evolution from the initial deflagration flame front to detonation development was examined. The results showed that the bulk-gas temperature gradient has a significant effect on the run-up time and intensity of the developing detonation. Detailed analyses further revealed that the mechanism of detonation development is qualitatively different for the positive and negative temperature gradient cases. In the former, the detonation development is initiated by the end-gas autoignition at the wall, while the latter exhibits detonation development following the process of the self-acceleration of the flame similar to the deflagration-to-detonation transition. This behavior is attributed to the longer residence time in the end-gas allowing the reinforcement by the interaction of incident and reflected pressure waves during the flame propagation, and results in the peak pressure even higher than the case with the same level of positive temperature gradient. Furthermore, yet another detonation development pattern was observed for the negative temperature gradient condition in the presence of a uniform temperature region just ahead of the flame. In this case, autoignition was found to start in the middle of the bulk end-gas, and subsequently leads to the transition to detonation. The results demonstrate the importance of the bulk gas conditions in predicting the detonation development, which corroborate the existing theoretical framework.

The Effects of Equivalence Ratio on Pressure Wave Development during Knocking Combustion

A knocking combustion in a one-dimensional constant volume reactor has been simulated with a detailed chemical kinetic mechanism of n-heptane premixed gases by using the compressible Navier-Stokes equations. This study focuses on the impact of the various equivalence ratios (0.6–1.4) in the pressure wave development during knocking combustion in an account of different initial temperature (600–900K). The result demonstrates the autoignition process in the end-gas region and explains the knocking phenomenon on different equivalence ratio. In an adiabatic wall condition, the largest knocking intensity occurs in equivalence ratio 1.4 whereas the lowest knocking intensity found in equivalence ratio 0.6. Regarding to initial temperatures, a strong peak of knocking is generated around 650K in all the equivalence ratios (0.8–1.4). However, yet a small perceptivity of knocking is found in around 750K for each equivalence ratio. In case of smaller equivalence ratios, a weak knocking occurs, which can be identified by the behavior of the pressure wave generation in the end-gas region.

Effects of Fuel Unsaturation on Transient Ignition and Flame Development in Sprays

ABSTRACTFundamental studies using homogeneous mixtures in shock tube and rapid compression machine (RCM) have shown that the fuel reactivity and ignition characteristics are significantly affected by the presence and number of double bonds in the fuel molecular structure. However, this phenomenon has not been investigated in turbulent reacting sprays under realistic conditions. In this article, we numerically examine the effects of fuel unsaturation and sensitivity on the transient ignition and flame evolution in n-heptane and 1-heptene sprays in the Sandia constant volume reactor. 3D simulations of two-phase reacting flows are performed using a commercial CFD code, CONVERGE. The ignition and combustion chemistry of n-heptane and 1-heptene is modeled using a reduced CRECK mechanism, which has been previously validated against the shock tube ignition delays and reacting spray measurements. Results indicate that, at intermediate temperatures, the ignition in both homogeneous mixtures and sprays is strongly influenced by fuel unsaturation. First of all, the ignition delay is much higher for 1-heptene compared to that for n-heptane, especially at low temperatures. Secondly, ignition in n-heptane sprays occurs in fuel rich mixtures, and is characterized by a two-stage ignition process. On the other hand, ignition in 1-heptene sprays occurs in lean mixtures. Results also indicate the existence of multiple ignition locations in sprays, which is in contrast to ignition in homogeneous mixtures. The flame evolution in sprays correlates strongly with the ignition kernel evolution. Consequently, the n-heptane spray flame contains two reaction zones, namely, a rich premixed zone (RPZ) and a non-premixed reaction zone (NPZ), while the 1-heptene flame is characterized by three reaction zones, i.e., a lean premixed zone in addition to NPZ and RPZ. The difference is a direct consequence of longer ignition delay in 1-heptene spray compared to that in n-heptane spray, which facilitates increased fuel-air mixing in the former. The flame lift-off location correlates with the ignition delay and ignition kernel location. Thus, at lower temperatures (1000 K and 1100 K), the lift-off length of 1-heptene flame is greater than that of n-heptane flame, which is due to longer ignition delays for 1-heptene sprays. In contrast, at higher temperatures (1200 K) the lift-off length of 1-heptene flame is smaller, which is due to the fact that the ignition kernel and, thus, lean premixed zone are located closer to the injector for 1-heptene spray compared to n-heptane spray.