N-heptane Combustion Research Articles

Dimensional Effect on Self-Sustaining Catalytic Combustion of n-Heptane in Micro/Meso Tubes

As one of the potential candidates for fuel surrogates, n-heptane has been the focus of numerous oxidation and combustion studies. In this work, n-heptane combustion in cylinder tubes with diameters of 2, 3, and 4 mm was investigated experimentally with the use of packed catalyst beds of Pt/Ce0.8Zr0.2O2 to study the mechanism of self-sustaining combustion and the dimensional effects of the inner diameter of a tube combustor. The region of self-sustaining stable combustion, the tube wall temperature, and the combustion efficiency were examined. n-Heptane can maintain self-sustaining stable combustion in the three tubes even if the wall temperature was below 430 K. As the equivalence ratio (Φ) increased, the stability region of the Reynolds number (Re) was expanded at the expense of fuel wastage. The lower stability limit of Re decreased as the tube diameter increased, and the value could reach as low as 12 in the tube with a diameter of 4 mm. The equivalence ratio was the dominant factor of the wall tube t...

Curve matching, a generalized framework for models/experiments comparison: An application to n-heptane combustion kinetic mechanisms

The increasing number of experimental data, accurate thermodynamic and reaction rate parameters drive the extension, revision, and update of large size kinetic mechanisms. Despite these detailed mechanisms (i.e. the models) generally allow good predictive capabilities, their management and update are critical. The usual validation procedure of a kinetic scheme consists in graphically comparing numerical simulations with the widest set of experimental data, with the goal of proving the model predictive capabilities over a broad range of temperature, pressure, and dilution conditions. At every iteration the model needs to be automatically evaluated through a quantitative methodology, without relying upon a standard graphical visualization.This work aims at proposing a method, named Curve Matching (CM), to evaluate the agreement between models and experimental data. The approach relies on the transformation of discrete experimental data and the relative numerical predictions in two different continuous functions. In this way, CM allows not only to compare the errors (i.e. the differences between the experimental and calculated values), but also the shapes of the measured and numerical curves (i.e. their first derivatives) and possible shifts along the x-axis. These features allow to overcome the limitations of Sum of Squared Error based methods. The present approach is discussed by means of a few models/experiment comparisons in ideal reactors and laminar flames. Due to the large number of both experimental data and kinetic mechanisms available in the literature, n-heptane was selected as the test fuel.

High-speed OH* chemiluminescence imaging of ignition through a shock tube end-wall

A high-speed OH* chemiluminescence imaging diagnostic was developed to image the structure and homogeneity of combustion events behind reflected shock waves in the Stanford Constrained Reaction Volume Shock Tube. An intensified high-repetition-rate imaging system was used to acquire images of OH* chemiluminescence (near 308 nm) through a fused quartz shock tube end-wall window at 10–33 kHz during the combustion of n-heptane (21 % O\(_{2}\)/Ar, \(\phi = 0.5\)). In general, the imaging technique enabled observation of the main ignition event in the core of the shock tube that corresponded to typical markers of ignition (e.g., pressure rise), as well as localized ignition near the wall that preceded the main core ignition event for some conditions. Case studies were performed to illustrate the utility of this novel imaging diagnostic. First, by comparing localized wall ignition events to the core ignition event, the temperature homogeneity of the post-reflected shock gas near the end-wall was estimated to be within 0.5 % for the test condition presented (\(T=1159\,\hbox {K}, P=0.25\,\hbox {MPa}\)). Second, the effect of a recession in the shock tube wall, created by an observation window, on the combustion event was visualized. Localized ignition was observed near the window, but this disturbance did not propagate to the core of the shock tube before the main ignition event. Third, the effect of shock tube cleanliness was investigated by conducting tests in which the shock tube was not cleaned for multiple consecutive runs. For tests after no cleaning was performed, ignition events were concentrated in the lower half of the shock tube. In contrast, when the shock tube was cleaned, the ignition event was distributed around the entire circumference of the shock tube; validating the cleaning procedure.

Experiments on n-heptane combustion with two types of catalyst layouts

n-Heptane combustion was conducted in packed-bed micro combustors using two catalyst layouts, namely, continuous (type A) and two-sectional (type B) types. Cerium–Zirconium solid solution-supported platinum was adopted as catalyst. The two combustors demonstrated distinct combustion characteristics owing to their different catalyst layouts. Type B significantly expanded the upper limit of Reynolds number (Re) of stable combustion, maintaining stable combustion at Re = 1500 under fuel-rich conditions at an equivalent ratio of Φ = 1.6. Type A was more advantageous at low Re values, and the lower Re limit was reduced to 12. Types A and B displayed a single peak and a bimodal distribution of wall temperature, respectively. The bimodal distribution in type B not only slowed down the variation in wall temperature with Re but also proved that catalyst-free homogenous combustion occurred in the throat. As a result of the different combustion processes, type A showed different variation trends of products along the length compared with type B. Moreover, the conversion rates of the two types reached their maximum values (84.4% and 88.3%) at Re = 160. High conversion rate of type B was attributed to heterogeneous/homogenous combustion and long heat reflux zones.

Large eddy simulation of n-heptane spray combustion in partially premixed combustion regime with linear eddy model

Spray combustion of n-heptane in a constant-volume vessel under engine-relevant conditions was investigated using linear eddy model in the framework of large eddy simulation. In this numerical approach, turbulent mixing was traced by an innovative stochastic approach instead of the conventional gradient diffusion model. Chemical reaction rates were calculated with the consideration of the sub-grid scale spatial fluctuations of reactive scalars. Turbulence-chemistry interactions were represented by the separated treatments of the underlying processes including turbulent stirring, chemical reaction, and molecular diffusion. The model was validated against the experimental data of ignition delay times, chemiluminescence images, and soot images from Sandia National Laboratories. Numerical results showed that the ignition process changed from the temperature-controlled regime to the mixing-controlled regime as the initial ambient temperature increased from 800 K to 1000 K. The premixed flame and the diffusion flame coexisted, while the gross heat release rate was found to be dominated by the premixed flame. The temperature fluctuation was mainly observed around the spray jet due to the cooling effect of the fuel vaporization. The fluctuations were more significantly smoothed out by the high-temperature flame than the low-temperature flame. The mean temperature would be overpredicted if the sub-grid temperature fluctuation was neglected.

Skeletal mechanism modelling of n-heptane/oxygen laminar coflow flame structure at pressures

A skeletal reaction mechanism of n-heptane combustion is developed and validated. The axisymmetric laminar co-flow diffusion flames of n-heptane/oxygen are simulated using the skeletal mechanism at elevated pressure, and the chemical reaction paths in three reaction zones of flame are presented. The n-heptane/oxygen flame height decreases from 8.8 to 5.6mm with the increase of pressure from 0.1 to 2MPa, and the Roper’s formulation is not expected for the prediction of n-heptane/oxygen flame height. The maximum volume fraction of soot increases with pressure as fv,max∝P2. The ratio of flame height to soot oxidation length of the n-heptane/oxygen flames is close to unity (1.037–1.121).

An experimental investigation into combustion and performance characteristics of an HCCI gasoline engine fueled with n-heptane, isopropanol and n-butanol fuel blends at different inlet air temperatures

An experimental study was conducted in a single cylinder, four stroke port injection Ricardo Hydra test engine in order to determine the effects of pure n-heptane, the blends of n-heptane and n-butanol fuels B20, B30, B40 (including 20%, 30%, 40% n-butanol and 80%, 70%, 60% n-heptane by vol. respectively) and the blends of n-heptane and isopropanol fuels P20, P30, P40 (including 20%, 30%, 40% isopropanol and 80%, 70%, 60% n-heptane by vol. respectively) on HCCI combustion. Combustion and performance characteristics of n-heptane, n-butanol and isopropanol were investigated at constant engine speed of 1500rpm and λ=2 in a HCCI engine. The effects of inlet air temperature were also examined on HCCI combustion. The test results showed that the start of combustion was advanced with the increasing of inlet air temperature for all test fuels. Start of combustion delayed with increasing percentage of n-butanol and isopropanol in the test fuels. Knocking combustion was seen with B20 and n-heptane test fuels. Minimum combustion duration was observed in case of using B40. Almost zero NO emissions were measured with test fuels apart from n-heptane and B20. The test results also showed that CO and HC emissions decreased with the increase of inlet air temperature for all test fuels. Isopropanol showed stronger resistance for knocking compared to n-butanol in HCCI combustion due to its higher octane number. It was determined that n-butanol was more advantageous according to isopropanol as thermal efficiency. As a result it was found that the HCCI operation range can be extended using high octane number alcohols away from knocking combustion and autoignition can be controlled.

Kinetic Modeling of Nitric Oxide Sensitization of n-heptane Auto-ignition and Combustion

This article develops a new kinetic model for better modeling auto-ignition and combustion of n-heptane, which are sensitized by the presence of nitric oxides. The mechanism has been validated for temperatures between 550 and 1100 K and pressure of 10 atm. Experimental data used for validation include jet stirred reactor conditions for species evolution as a function of temperature, as well as homogeneous charge compression ignition engine experiments for auto-ignition delay time measurements. Rate-of-production analysis and sensitivity coefficient analysis of nitrogen species are performed in order to explain the impact of NOx on n-heptane oxidation.

Evaluation and optimisation of phenomenological multi-step soot model for spray combustion under diesel engine-like operating conditions

In this work, a two-dimensional computational fluid dynamics study is reported of an n-heptane combustion event and the associated soot formation process in a constant volume combustion chamber. The key interest here is to evaluate the sensitivity of the chemical kinetics and submodels of a semi-empirical soot model in predicting the associated events. Numerical computation is performed using an open-source code and a chemistry coordinate mapping approach is used to expedite the calculation. A library consisting of various phenomenological multi-step soot models is constructed and integrated with the spray combustion solver. Prior to the soot modelling, combustion simulations are carried out. Numerical results show that the ignition delay times and lift-off lengths exhibit good agreement with the experimental measurements across a wide range of operating conditions, apart from those in the cases with ambient temperature lower than 850 K. The variation of the soot precursor production with respect to the change of ambient oxygen levels qualitatively agrees with that of the conceptual models when the skeletal n-heptane mechanism is integrated with a reduced pyrene chemistry. Subsequently, a comprehensive sensitivity analysis is carried out to appraise the existing soot formation and oxidation submodels. It is revealed that the soot formation is captured when the surface growth rate is calculated using a square root function of the soot specific surface area and when a pressure-dependent model constant is considered. An optimised soot model is then proposed based on the knowledge gained through this exercise. With the implementation of optimised model, the simulated soot onset and transport phenomena before reaching quasi-steady state agree reasonably well with the experimental observation. Also, variation of spatial soot distribution and soot mass produced at oxygen molar fractions ranging from 10.0 to 21.0% for both low and high density conditions are reproduced.

Hydrocarbon Droplet Turbulent Combustion in an Elevated Pressure Environment

New experimental data on the effect of turbulence on the combustion of n-heptane and n-decane droplet at elevated pressure are reported. Turbulent flow field in the central volume of a spherical vessel was generated by a set of eight axial fans. Flow characterization demonstrated that turbulence is isotropic and homogeneous within the 40 mm central volume of the vessel. The combustion of n-heptane and n-decane droplet was examined by varying turbulence intensity via the fans rotational speed and ambient pressure and temperature inside the vessel/chamber. Results showed that droplet burning rate becomes sensitive to turbulence only at elevated ambient pressure. Droplet flame extinction was found to depend on the heat loss from the droplet envelop flame, which increases with turbulence intensity.

Application of a semi-detailed soot modeling approach for conventional and low temperature diesel combustion – Part I: Model performance

In the present study a previously developed soot model has been tested extensively for conventional and low temperature diesel combustion engine simulations. The soot model framework, which was implemented in the Computational Fluid Dynamics (CFD) code KIVA-CHEMKIN, is based on four fundamental steps, viz., soot inception through a four ring polycyclic aromatic hydrocarbon species, soot surface growth through acetylene and benzene, soot coagulation, and oxygen and OH-induced soot oxidation. The proposed model can be used not only for soot mass predictions, but also for soot number density and soot particle diameter predictions. Diesel combustion was simulated using reduced n-heptane/primary reference fuel (PRF) chemistry mechanisms and a reduced polycyclic aromatic hydrocarbon (PAH) chemistry mechanism, while the n-heptane chemistry mechanism was used for modeling constant volume n-heptane combustion. Soot model performance was evaluated by comparing the model predictions with available constant volume combustion chamber optical diagnostic experiments, optical engine in-cylinder soot data, and a light-duty single cylinder metal engine-out smoke data. In particular, a variety of combustion regimes were tested, including conventional diesel, premixed charge compression ignition (PCCI), and high exhaust gas recirculation (EGR) low temperature combustion (LTC). In general, the model was able to predict well the trends in soot mass over the range of operating conditions. With increasing degree of premixing, the model was able to predict relatively lower soot concentrations compared to diffusion combustion. In terms of soot particle diameter, the model computed particle size was seen to increase with increasing simulated EGR when the ambient oxygen concentration was varied from 21% to 8% by volume and with increasing in-cylinder density (14.8kg/m3 vs. 30kg/m3 at the same in-cylinder O2 level) under steady-state constant volume combustion conditions. It is also observed from the optical and metal engine studies that the soot particle size is larger for high temperature combustion conditions that are marked with high soot formation rates during the combustion process and may or may not lead to smaller particles at exhaust valve opening (EVO) depending on the oxidation rates. The soot size for LTC is generally lower through-out the combustion process including EVO conditions. All but one data-point show uni-modal type soot particle size distribution with respect to both soot mass and particle number.

A comparative study of the physical properties of in-cylinder soot generated from the combustion of n-heptane and toluene/n-heptane in a diesel engine

This study examines the similarities and dissimilarities in the physical properties of in-cylinder soot obtained during combustion of n-heptane and a toluene/n-heptane mixture (TRF20, 20% toluene by volume) in a heavy-duty diesel engine. A total cylinder sampling system was used to sample the in-cylinder soot during different combustion phases. For each sample, the morphology, size, fractal dimension and nanostructure of the soot particles were analyzed using high-resolution transmission electron microscopy and Raman scattering spectrometry. The oxidation reactivity of soot particles was also evaluated in terms of apparent activation energy using thermogravimetric analysis. Throughout the combustion process, soot aggregates from the combustion of TRF20 generally exhibited more primary particles and larger-sized clusters than the n-heptane soot aggregates. Despite the different fuel formulations, both n-heptane and TRF20 showed similar trends during the combustion process in terms of changes in fractal dimension of soot aggregates, mean primary particle size and nanostructure (as characterized by fringe length, separation distance and tortuosity). The presence of toluene, however, led to a decrease in the fractal dimensions of the aggregates and a concurrent increase in the mean size of primary particles. Moreover, in the same combustion stages, the mean fringe length for the TRF20 soot was smaller than that for the n-heptane soot, while the mean separation distance and tortuosity for the TRF20 soot were both larger than those for the n-heptane soot. Relative to the n-heptane soot, the TRF20 soot had less graphitic organization and lower resistance to oxidation.

Experimental and Computational Study of n-Heptane Autoignition in a Direct-Injection Constant-Volume Combustion Chamber

The purpose of this study was to characterize experimental n-heptane combustion behavior in a direct-injection constant-volume combustion chamber (DI-CVCC), using chamber pressure to infer ignition delay and heat-release rate. Measurements generally displayed expected trends and indicated entirely premixed combustion with no mixing-controlled phase. A significant finding was the observation of negative temperature coefficient (NTC) behavior. Comparing results with CHEMKIN-PRO simulations, it was found that a homogeneous combustion model was reasonably accurate for ignition delays longer than 5 ms. The combination of NTC behavior and homogeneous fuel-air mixtures suggests that this DI-CVCC can be useful for validation of chemical-kinetic mechanisms.

Ozone-Assisted Combustion—Part I: Literature Review and Kinetic Study Using Detailed n-Heptane Kinetic Mechanism

In this first paper, the authors undertake a review of the literature in the field of ozone-assisted combustion in order to summarize literature findings. The use of a detailed n-heptane combustion model including ozone kinetics helps analyze these earlier results and leads into experimentation within the authors' laboratory using a single-cylinder, direct-injection compression ignition engine, briefly discussed here and in more depth in a following paper. The literature and kinetic modeling outcomes indicate that the addition of ozone leads to a decrease in ignition delay, both in comparison to no added ozone and with a decreasing equivalence ratio. This ignition delay decrease as the mixture leans is counter to the traditional increase in ignition delay with decreasing equivalence ratio. Moreover, the inclusion of ozone results in slightly higher temperatures in the cylinder due to ozone decomposition, augmented production of nitrogen oxides, and reduction in particulate matter through radial atomic oxygen chemistry. Of additional importance, acetylene levels decrease but carbon monoxide emissions are found to both increase and decrease as a function of equivalence ratio. This work illustrates that, beyond a certain level of assistance (approximately 20 ppm for the compression ratio of the authors' engine), adding more ozone has a negligible influence on combustion and emissions. This occurs because the introduction of O3 into the intake causes a temperature-limited equilibrium set of reactions via the atomic oxygen radical produced.

Experimental study of the influence of λ and intake temperature on combustion characteristics in an HCCI engine fueled with n-heptane

Combustion characteristics fueled with n-heptane were investigated experimentally in a retrofitted engine to realize homogeneous charge compression ignition (HCCI) combustion mode. The influence of intake temperature and λ on chemical reactions was analyzed. In-cylinder pressure, temperature, heat release rate and ignition timing profiles were obtained with variations of the intake temperature and λ. The oxidation process was divided into three phases and the main reactions were analyzed. The results show that the combustion of n-heptane is more sensitive to intake temperature than to λ. The reason is that the intake temperature has a more profound effect on the oxidation process than the effect of λ. Moreover, the increase of intake temperature can extend the range of λ, and an optimal intake temperature exists corresponding to the value of λ.

Development of an n-heptane-n-butanol-PAH mechanism and its application for combustion and soot prediction

A reduced chemical reaction mechanism was developed for modeling the combustion process and soot emissions for both non-oxygenated and oxygenated hydrocarbon fuels. A detailed poly-aromatic hydrocarbon (PAH) mechanism was reduced and embedded into a reduced n-heptane mechanism for describing the formation of PAH up to four rings (A4) and for soot prediction. A reduced n-butanol mechanism was combined with the n-heptane-PAH mechanism to investigate of effects of oxygenated n-butanol fuels on combustion and soot emissions. The final mechanism consists of 76 species and 349 reactions. The mechanism was validated with experiments in shock tubes, constant volume chambers and test-bed engine data. New experiments were also conducted and reported in current investigation and have been used to validate the proposed mechanism. The effects of oxygenated additives on combustion and soot emissions under diesel-like conditions were also investigated. The results show that the present simulations give reliable predictions of combustion and soot emissions. The results also agree with the general soot formation processes near the lift-off length in mixing controlled diesel fuel jets, and the present mechanism can be used to predict the combustion and soot emissions of diesel, n-heptane and n-butanol fuels in 3D CFD simulations.

Numerical Simulation of the Nitrogen-Enriched Air Combustion and Emission in Diesel Engine

Combustion intermediate products in nitrogen-enriched intake air (NEA), such as CH2O and H2O2, are researched in this paper with chemical reaction kinetics software. Besides that, combustion of n-Heptane in NEA at different intake temperature and different rotate speed are researched. Nitrogen-enriched air combustion process in diesel engine is simulated with CAE software. The temperature trace peak and NOx emission at NEA and different rotate speed are studied in this paper. The increment of nitrogen in the intake air greatly impacts on the n-Heptane combustion intermediate products CH2O and H2O2; and the results show that the nitrogen is of fire-retardant properties. The peak-cylinder combustion pressure and the relative pressure-increase rate will decrease with the increment of nitrogen in intake air when the temperature is higher than certain value. NOx emissions decrease with the increment of nitrogen in intake air.

Diffusion Combustion of Liquid Heptane in a Small Tube with and without Heat Recirculating

In order to understand diffusion flame characteristics in a small tube, combustion of liquid n-heptane and air was experimentally and numerically studied. A tube of ID 4 mm and OD 6 mm made of quartz was used as the burner. Liquid n-heptane was delivered into a capillary from a syringe pump. Stable flames were established inside the burner with and without heat recirculating. Additionally, numerical simulations were conducted, and effects of equivalence ratio and external heat loss coefficient on diffusion flame were studied. Results show that, for a diffusion flame of liquid n-heptane in a small tube, as fuel flow rate increases, the flammable limits increase. The diffusion flame position moves downstream with increasing air flow, eventually stabilizing at the bottom of the outer tube until extinction. When the flame passes in the tube, the peak temperature would occur on the wall. If there is heat recirculating, the wall temperature of the inner tube is higher than the boiling point of liquid n-heptane. It is conducive to the pre-evaporation of liquid n-heptane. In contrast, if there is no heat recirculating, liquid fuel will be accumulated in the tube. The heat loss coefficient has a great influence on flammable limits of the tube burner without heat recirculating.

Influence of ozone on the combustion of n-heptane in a HCCI engine

Homogeneous charge compression ignition (HCCI) experiments were performed in a single-cylinder diesel engine fueled with n-heptane at constant equivalence ratio of 0.3, intake temperature of 300K, and engine speed of 1500rpm. The intake manifold was seeded by ozone produced by a dielectric barrier discharge reactor with a wire–cylinder configuration. Experimental results showed that low ozone concentrations, i.e., <50ppm, have an important impact on the phasing of the cool and main flame in the engine. To interpret the engine results, the modeling of constant volume combustion at 20 and 40bars was performed using a detailed chemical kinetic reaction mechanism. The kinetic modeling indicated that the fuel starts to oxidize via reaction with O-atoms produced by ozone decomposition (n-C7H16+O→C7H15+OH) and that the ozone-accelerated production of OH speed up both cool and main flame. The production of O-atoms by decomposition of ozone during the compression stroke could be a way to control, cycle to cycle, two stage ignition fuels of HCCI engine.

Sub-millimeter sized methyl butanoate droplet combustion: Microgravity experiments and detailed numerical modeling

Combustion characteristics of isolated sub-millimeter sized methyl butanoate (MB) droplets are studied at low gravity (10−4m/s2) in a 1.2s drop tower. In the experiments, droplets were grown and deployed onto the intersection of two 14μm silicon carbide fibers in a cross-string arrangement and exposed to symmetrically placed spark ignition sources. The initial droplet diameter was fixed at 0.54±0.01mm, and experiments were carried out in room temperature air at atmospheric pressure. Detailed measurements of the evolution of droplet diameter, flame standoff ratio and burning rate are reported. The experimental results are compared against predictions from a comprehensive time-dependent, sphero-symmetric droplet combustion simulation that includes detailed gas phase chemical kinetics, spectrally resolved radiative heat transfer, multi-component diffusive transport, full thermal property variations and tether fiber perturbation effects. The predicted combustion characteristics of MB are also compared with n-heptane droplets of nearly identical sizes over a range of oxygen concentrations.The results show that predicted burning histories, burning rates and flame standoff ratios are in excellent agreement with the measurements. The average burning rates and flame temperatures for both fuels were found to be similar even though the heat of combustion of n-heptane is higher by a factor of ∼1.6. However, the average flame standoff ratio for MB was found to be significantly smaller than for n-heptane, due to the presence of additional oxygen atoms in the parent fuel. Important differences in the diffusion flame chemistries of the two fuels are also discussed.