Constant Volume Combustion Research Articles

Determination of Cetane Numbers Via Chemical Kinetic Mechanism

Abstract Minimizing global warming is a major task of todays' society. For air transport, sustainable aviation fuels (SAF) produced from renewable sources are a promising key solution. While electric flight is intriguing for short distances, SAF are required for mid- and long-distance flights and in addition, enable fuel design strategies to minimize environmental effects. The qualification and approval for SAF are standardized in the ASTM D4054, which include fuel properties as an essential part. Among others, lean blow-out (LBO) limits are a key performance parameter. The experimental determination of LBO is very time-consuming and cost-effective. The LBO of a specified engine is highly dependent on the fuel properties affecting evaporation, mixing, and ignitability. Therefore, prediction tools are desired to identify early promising SAF for decreasing the certification cost. Due to the correlation between LBO and derived cetane numbers (DCN), a tool for the prediction of the DCN is presented in this study. The DCN model uses chemical kinetic ignition delay time (IDT), simulated in a constant volume combustion chamber based on the ASTM D6890 standard, and seven representative physical properties of a fuel. A high agreement of the predicted DCN to the literature DCN with root-mean-square errors of 4.7 and correlation coefficients of 0.95 was found.

Electrostatic Spraying Microspheres of Nano Boron and Its Constant-Volume Combustion with Potassium Nitrate

ABSTRACT The nanometer boron (nB) powders have become the most promising high-energy additive in the field of fuel-rich solid propellants due to their extremely high gravimetric and volumetric calorific value. However, further applications are limited by its disadvantages such as easy agglomeration, easy oxidation, high ignition temperature, and incomplete energy release. Herein, nB powders were pre-agglomerated into microspheres by electrostatic spray method using fluoropolymer (F2602) as binders. The morphology and structure of the samples were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray energy spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and Brunauer-Emmett-Teller (BET). Results showed that the pre-agglomerated nB microspheres had good dispersion, high sphericity, and no surface oxide. The thermal performance and combustion performance of the samples were characterized by a synchronous thermal analyzer and combustion pressure test device. Results showed that the nB microspheres had the highest oxidation weight gain within 1000°C and lowest activation energy compared to the same scale micron boron. In addition, the microspheres significantly shorten the combustion time and improve the combustion efficiency.The pressurization rate of nB microspheres is 4.12 times that of micron boron powders and 2.85 times that of nB powders. These excellent properties may be related to its unique spherical porous structure. Therefore, the preparation of nB microspheres with controllable structure by electrostatic spraying has a good application prospect.

Achieving superior ignition and combustion performance of Al/I2O5 biocidal nanoenergetic materials by CuO addition

It was found that all iodine-containing biocidal energetic materials has a relatively long ignition delay upon combustion. A shorter ignition time (from the thermal trigger to the peak pressure) for Al/iodine oxides might further boost their combustion performance due to their high reactivity and high gas release rate. To achieve this goal, a secondary oxidizer, CuO, is incorporated into Al/I2O5 at different mass content keeping the overall thermite stoichiometry constant. The ternary Al/I2O5/CuO thermites were characterized in ignition using a T-jump ignition temperature set up, and in combustion in a constant volume combustion cell. Consequently, all ternary thermites outperform traditional Al/I2O5 counterpart with an optimum for 80/20 wt% of I2O5/CuO. This later composition ignites in 0.01 ms (30 times shorter than Al/I2O5) and produces peak pressure and pressurization rate of ∼4 and 26 times greater than those produced by Al/I2O5. A series of additional characterizations using Fourier-transform infrared spectroscopy, Differential Scanning Calorimetry, Electrical/Thermal conductivity measurement, etc., permitted to unravel the cause of such improvement and to propose a reaction mechanism for this ternary Al/I2O5/CuO system. From an applications point of view, this study proposes a facile, inexpensive and efficient way to enhance the combustion performance of Al/I2O5 biocidal nanoenergetic materials.

Experimental study on microscopic characteristics of transcritical spray for high density environment

As the power density of diesel engines increases, the in-cylinder spray process appears supercritical phase transition, but the transition boundary and judgment criteria are unclear. This study explores the supercritical phase transition of spray using a constant volume combustion chamber and a high-speed optical system, considering ambient temperatures of 700 K to 1600 K and densities of 12 kg/m3 to 22.8 kg/m3. The results show that under subcritical conditions, the droplets at the end of the spray remain spherical during the evaporation process due to the existence of surface tension, and eventually evaporate. Under supercritical conditions, the droplets begin to be spherical during the evaporation process. As time goes on, the wake appears and falls off, oscillates and deforms, and finally diffuses and disappears, which is regarded as the occurrence of supercritical phase transition. The temperature limit of supercritical phase transition decreases with the increase of ambient density, and the density limit decreases with the increase of ambient temperature. This is because as the ambient density or temperature increases, the heat absorption required to destroy the gas–liquid interface (reducing the surface tension, thickening the interface, and reducing the average free molecular path) decreases. The supercritical phase transition time is shortened with the increase of ambient temperature or ambient density. The reason is that the ambient density and temperature increase under supercritical conditions, the heat and mass transfer is accelerated, and the temperature rise of the droplet surface is accelerated. As the droplet size increases, the supercritical phase transition boundary remains unchanged, and the supercritical transition time is prolonged, which is due to the decrease of the surface volume ratio and the slow heating of the droplet.

Effects of energy-share and ambient oxygen concentration on hydrogen-diesel dual-fuel direct-injection (H2DDI) combustion in compression-ignition conditions

This study investigates the ignition and combustion characteristics of interacting hydrogen (H2) and diesel surrogate jets under simulated compression-ignition engine conditions. The experimental setup includes two converging single-hole injectors in an optically accessible constant-volume combustion chamber (CVCC). The parameters varied in the study are fuel injection durations and ambient O2 concentrations (10 to 21 vol.%). The results show that a longer interaction between the diesel products and the H2 jet is required to achieve ignition of the H2 jet at lower O2 concentrations. Once ignited, the flame stabilises near or at the nozzle, except under the lowest ambient O2 condition of 10 vol.% where a lifted flame is observed. The lift-off response, however, is influenced by the relative injection duration of the fuels, with the interaction between the incoming H2 jet and the diesel combustion recession products possibly playing a role. The interaction between the jets also affects the recorded intensity and the distribution of the diesel fuel jet soot zone.

Experimental Study on Macroscopic Spray and Fuel Film Characteristics of E40 in a Constant Volume Chamber

In the modern industrial field, there is a strong emphasis on energy-saving and emission reduction. Increasing the amount of ethanol in ethanol–gasoline blends has the potential to replace fossil fuel gasoline more effectively, improving energy efficiency and lowering emissions. The interaction between liquid fuel film generation on the piston crown and spray impingement in the combustion chamber in the setting of GDI engines has a substantial impact on particle emissions and engine combustion. In this study, 92# gasoline and ethanol by volume are combined to create the ethanol–gasoline blend E40. The spray characteristics and film properties of both gasoline and the intermediate proportion ethanol–gasoline blend E40 were researched utilizing a constant volume combustion platform and the schlieren method and refractive index matching (RIM) approach. The results show that, for 0.1–25 operating conditions, gasoline consistently displays greater macroscopic spray characteristic parameters than E40. This shows that gasoline fuel spray evaporation is superior to E40. Similar results are seen in the analysis of wall-attached fuel films, where the volume and thickness of the gasoline film are less than those of the E40 film under the given operating conditions. In contrast, E40 consistently exhibits stronger macroscopic spray characteristic values than gasoline under the 0.1–150 and 0.4–150 operating conditions, along with lower film thickness and volume. As a result, under these two operating conditions, E40 fuel performs better during spray evaporation.

Spray-evaporation characteristics of n-pentanol/n-dodecane binary fuel at ultra-high injection pressure

The spray and evaporation characteristics of the fuel directly affect the combustion and emission performance of the engine. In this work, the atomization and evaporation characteristics of the n-pentanol/n-dodecane binary fuel were investigated by varying the injection pressure (100–300 MPa). Process of breaking, atomization, evaporation and oil-gas mixing were measured by schlieren technique. The liquid phase length of the spray was measured using the Mie scattering method. Numerical simulations were performed in a constant volume combustion chamber to further investigate the effects of injection pressure. The findings demonstrate that n-pentanol/n-dodecane binary fuel has slightly more liquid penetration than pure n-dodecane fuel, which may be attributed to n-pentanol's higher density. The results also show that the ultra-high injection pressure notably improved the spray penetration and reduced the droplet size that led to fragmentation of the fuel droplets and effectively improved the atomization characteristics. Moreover, a substantial reduction of over 20% in droplet size was observed with the increase in injection pressure from 100 to 200 MPa. However, it has been observed that the reduction in droplet size due to an increase in injection pressure diminishes when the pressure exceeds 200 MPa. In such cases, the size reduction is less than 10%, which implies that the impact of ultra-high injection pressure on size reduction is restricted. Although the impact of ultra-high injection pressure on fuel evaporation under evaporative conditions is limited, it can enhance fuel air mixing performance through intensified turbulence activity within the spray, thereby promoting greater air roll-up.

Experimental study of the effect of structural differences in the composition of kerosene-like blended fuels on low-temperature autoignition in a constant volume combustion chamber

The cold start performance of compression ignition engines at high altitudes is expected to be improved by means of fuel design involving kerosene-like blended fuels. In this study, the autoignition characteristics of ten blended fuels with different chemical compositions and physical properties were investigated in a constant volume combustion chamber. Steady-state conditions in a low thermal atmosphere are considered at varying temperatures (723, 773, 823 K), pressures (2.2, 2.8, 4 MPa), injection pressures (40, 60, 80 MPa), and injection pulse widths (1.5, 2.5, 5 ms). The relative importance and relevance of the physical and chemical properties of the fuel to the ignition delay were evaluated based on characteristic parameters e.g. total ignition delay, physical delay, chemical delay, combustion delay, pressure and heat release rate evolution. It was found that adding ether-oxygenated component content to kerosene-like significantly shortened the ignition delay, followed by chain alkanes, naphthenes, and aromatics, in that order. The effect of oxidation chemistry changes on the ignition performance of fuels in low-temperature-pressure environments is more significant than physical properties. As the thermal atmosphere or fuel activity intensifies, the percentage of physical delay in the total ignition delay gradually exceeds that of chemical delay as the main part, and the variability in the ignition qualities of fuels with different physicochemical properties diminishes. There is a good linear correlation between ignition delay and cetane number (ranging from 44 to 60), density, viscosity, and recovery temperature, whilst the role of cetane number and density is more prominent.

Inhibition effect and mechanism of CF3CHCl2 on methane flame in oxygen-enriched environment

In oxygen-enriched environment (oxygen concentration above 21%), flammable materials become even more flammable with higher net heat release rate, burning velocity and combustion intensity. Unfortunately, most investigations concentrated on the characteristics and mechanisms of oxygen-enriched combustion, the methods to control or weaken their perniciousness of oxygen-enriched fire have been rarely reported. CF3CHCl2 (HCFC-123, R123) has been demonstrated excellent inhibition performance for oxygen-normal combustion in the fire suppression experiment, was considered as the potential fire extinguishing agents of aircraft cargo. This study performed the comparison between the inhibition performance and kinetic mechanism of CF3CHCl2 for the CH4-N2-O2 (21% of O2) flame and the flames enriched up to 23.1%, 25.2%, 27.3% of oxygen with constant-volume combustion chamber and numerical prediction. Even for the oxygen-enriched flames, CF3CHCl2 showed substantial inhibition effect on the burning velocities and adiabatic flame temperature. The chain-radicals production/consumption rate and normalized sensitivity coefficients were investigated to analyze the in-depth inhibition mechanisms of CF3CHCl2 for oxygen-enriched flames. The mechanism analysis found that the inhibition effect of CF3CHCl2 and relevant halogenated species on oxygen-enriched flame are smaller than that in normal flame.

Effect of Turbulence and Diffusional–Thermal Instability on Hydrogen‐Rich Syngas Turbulent Premixed Flame

As a kind of fuel with low‐carbon emissions, hydrogen‐rich syngas has a large resource potential and is a promising alternative energy resource. A series of experiments in a constant volume combustion bomb are carried out to investigate the effects of turbulence and diffusional–thermal (DT) instability on a hydrogen‐rich syngas turbulent premixed flame. The flow field in the constant volume combustion bomb is calibrated, and the turbulent flow field's homogeneity and isotropy were investigated. The effects of different speeds of fan (1366–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0–3.0 bar), and equivalence ratios (0.6–1.0) on the hydrogen‐rich syngas turbulent premixed flame are studied. According to the analyses, the flow field in the experimental device exhibits good homogeneous and isotropic characteristics. With the increase of turbulence intensity, equivalence ratio, hydrogen fraction, or pressure, the turbulent flame propagation speed increases gradually. As the hydrogen fraction increases or the equivalence ratio decreases, the effective Lewis number decreases, and the effect of DT instability on the flame increases. The research in this article is crucial for the clean and efficient utilization of hydrogen‐rich syngas and the design of related burners.

Investigation of ammonia cracking combined with lean-burn operation for zero-carbon combustion and NO/N2O/NO2 improvements

Ammonia has received significant attention as a carbon-free fuel and a promising carrier of hydrogen. Ammonia cracking presents an effective method for on-line hydrogen production, eliminating the need for hydrogen transportation and storage. This paper aims to study the comprehensive impacts of ammonia cracking ratio (CR) and equivalence ratio (ER) on combustion performance and NO/N2O/NO2 behaviors under lean-burn operation mode. The ammonia/hydrogen mechanism was updated and simplified to enhance the calculation efficiency in 3D simulation. Visualization measurements for flame propagation and NO and 3D simulations were simultaneously performed in a constant volume combustion chamber (CVCC). The results showed that ammonia blending with the cracked gas of H2/N2 for CR can significantly enhance the combustion rate under lean-burn operation at an ER of 0.5. Nevertheless, this enhancement leads to an increase in NOx due to elevated OH and HNO formation, regardless of N2 separation. In the context of varying ER, ranging from 0.5 to 0.3, the study revealed that increasing CR combined by decreasing ER at a certain point can sustain a higher combustion rate while reducing NOx emissions. For instance, CR45ER0.4 yields a much shorter combustion duration and lower NOx compared to pure ammonia at an ER of 1.0. However, a further increase in CR, such as CR80ER0.3, results in higher NOx emissions again. Moreover, complete cracking or separation from the non-cracked ammonia proves favorable for NOx elimination due to the absence of fuel-NO. The current study provides insights on high-efficiency and clean zero-carbon combustion technology.

Experimental and simulation study on the combustion characteristics of ammonia/n-dodecane mixtures

Laminar burning velocity is a crucial characterization parameter in fuel combustion progress. Investigating the combustion properties is significant to improve combustion efficiency. This paper employs a constant volume combustion bomb and high-speed ripple shadow camera technology to explore the influence of equivalence ratios (0.8–1.4) and ammonia energy shares (0%–40%) on the laminar burning velocity of n-dodecane/ammonia mixture at an ambient temperature of 400 K and a pressure of 1 bar. The results illuminated that the unstretched laminar burning velocity of n-dodecane increased from 46 cm/s at ϕ = 0.8 to 61 cm/s at ϕ = 1 and then decreased to 32 cm/s at ϕ = 1.4. When the ammonia energy ratio reaches 40%, the maximum laminar burning velocity experiences a reduction of 44% because the blended ammonia inhibited the formation of H and O radicals based on kinetic analysis in the fast reaction zone. With the increase of ammonia ratio, the Markstein length of mixed burned gas gradually increases. Particularly under a 1.4 equivalent ratio condition, the Markstein length increases from 0.2 mm to 0.8 mm, indicating that ammonia components significantly enhance flame stability. To gain a deeper understanding of the influence mechanisms of laminar burning velocity, a simplified Yao-Otomo (Y-O) model with 92 species and 355 reactions is proposed, coupling the mechanisms of n-dodecane and ammonia. The Y-O model has demonstrated a predictive trend that is consistent with more detailed WUT-NH3 model (176 species and 2839 reactions), and the Y-O model reduces the amount of computation to save calculation time, which has a higher engineering applicability. The turbulent kinetic energy (TKE) induced by premixed fuel in engine cylinders is estimated through comprehensively analyzing the laminar burning velocity and flame thickness characteristics. The findings indicate a 45% reduction in the peak value of TKE as the ammonia energy ratio increases.

Fundamental Experiment of Ignition and Flame Development in Turbulent Jet Ignition Ignited Methane Jet Flame

ABSTRACT In heavy-duty natural gas engines, diesel is usually injected before natural gas to initiate methane combustion. However, such an approach requires additional fuel and precise injection strategies. In the present work, an alternative approach of pre-chamber turbulent jet ignition (TJI) ignited methane jet flame is performed experimentally. The underlying mechanism of TJI-HPDI jet combustion is investigated in an optically visualized constant-volume combustion chamber with an active pre-chamber. It is found that with the increase of the injection-ignition delay (ti), the combustion process undergoes several transitions. In addition, the success ignition region and ignition failure region separated by ti and T MI are discerned on the ti-T MI diagram. As ti increases, there is an initial rise in both the mean flame front velocity and peak pressure, followed by a subsequent decrease This behavior highlights the TJI-HPDI system’s notable advantages in expanding the lean combustion limit and enhancing overall combustion characteristics compared to premixed combustion. These advantages can be attributed to the mixture stratification and turbulence. Furthermore, pre-chambers fueled with hydrogen can further improve the intensity and energy of the turbulent hot jet, which further decreases the critical injection-ignition delay to initiate global flame propagation in the TJI-HPDI system. The present work deals with real engineering technologies, and it will provide new insights into the design and control of TJI-HPDI engines, especially for heavy-duty natural gas engines.

Impact of n-butanol addition to hydrogenated catalytic biodiesel fueled a constant volume combustion chamber; a computational study

ABSTRACT The continuous quest for renewable, efficient, and clean energy sources has led to significant advances in the field of biodiesel production. Hydrogenated catalytic biodiesel (HCB) seems to be a promising option among various renewable biodiesel fuels which can be obtained from waste cooking oil. However, its extreme ignitability limits its application in existing engines. Therefore, a blend of n-butanol and HCB has been proposed in the present study, aiming to explore the spray combustion and soot formation behavior of these blends as a step toward the best possible utilization of this blend in engine. The objective was to examine the impact of adding n-butanol to HCB fuel on spray combustion and soot particles. A new skeletal chemical kinetic model of HCB/n-butanol blend was introduced and validated through measured data reported in the literature. Numerical studies have been performed in a constant volume combustion chamber under reactive spray environments. The simulation results concluded that, blending 20%, 40%, and 60% n-butanol with HCB fuel increased the heat release rates by 31.6%, 47.8%, and 87%, respectively. Furthermore, introducing a 20% of n-butanol into HCB fuel resulted in a 35.7% increase in ignition delay time and a 35.3% increase in flame liftoff length. These results demonstrated the reactivity of n-butanol/HCB mixture experienced a decrease when increasing the n-butanol fraction in the blend, while the fuel atomization was improved. The simulation results also demonstrated a notable reduction in the equivalence ratio when the n-butanol addition ratio was increased, which was a result of the entrainment effect that enhanced the mixing of fuel spray and surrounding fresh air. The simulation results also demonstrated that, adding 20%, 40%, and 60% n-butanol to HCB fuel resulted in a 25%, 43%, and 50% reduction in soot mass of HCB, respectively. This was mainly attributed to the less amount of C2H2, A1, and A4 species with n-butanol addition. Consequently, this sequential reduction leads to suppression of PAHs growth, nucleation, surface growth, and soot coagulation. Besides, a longer flame length resulting from the increased blending ratio of n-butanol could also have an additional soot suppression effect by improving air-fuel mixing and thus suppressing the overall soot mass.

Holistic performance evaluation of a turbofan featuring pressure gain combustion for a short-range mission

Aviation is at the center of public interest because of its environmental impact, such as noise and pollutant emissions. Evolutionary technologies are not expected to be sufficient to contribute significantly to the 2050 CO2 emissions reduction targets. This could change if the constant pressure combustion of the Joule cycle could be replaced with a constant volume combustion (pressure gain combustion, PGC). However, PGC comes with a major drawback of a periodic, highly unsteady process that potentially reduces the stability and efficiency of the adjacent compressor and turbine. Furthermore, changes to the secondary air system (SAS) are required since the driving force of SAS stems from the pressure loss in the conventional combustor. Despite the lasting prevalence of pressure gain combustion, there is no whole system analysis taking into account not only the beneficial aspects of PGC, but also the detrimental effects on the SAS and turbo components. To close that gap, a zero-dimensional whole engine model is created. That model accounts for the individual effects of PGC. A comprehensive design methodology identifies optimum engine specifications for a short-range mission. By comparing the results of the PGC turbofan to a conventional turbofan, an SFC improvement of 3.3% was identified.

Effect of the β-hydroxy group on ester reactivity: Combustion kinetics of methyl hexanoate and methyl 3-hydroxyhexanoate

One of the major biofuels used today is biodiesel, composed of fatty acid methyl esters. The fats and oils feedstocks used to make biodiesel are in relatively short supply. To meet increasing global demand, engineered microorganisms have been developed to catalyze conversion of sugar to fatty acid esters that contain unique β‑hydroxy-esters. This study investigated the effect of a hydroxyl group on the combustion characteristics of methyl esters by comparing the chemical behavior of methyl hexanoate (MHx) and methyl 3-hydroxyhexanoate (M3OHHx)—used as surrogates for diesel-boiling-range esters with longer fatty acid chains. The oxidation of these esters was studied experimentally in a flow reactor at 0.84 and 10 bar, 600 to 1,100 K, and stoichiometric conditions; and in a constant volume combustion chamber at 5 and 10 bar, 600 to 900 K, and equivalence ratios of 0.3 and 0.6. MHx was more reactive in the constant volume chamber at temperatures below 800 K at 10 bar and equivalence ratio of 0.6. MHx also exhibited a higher indicated cetane number (16.4) than M3OHHx (8.1). We investigated this reactivity trend using an updated MHx kinetic model and M3OHHx model developed as part of this work. The kinetic models predicted that radicals formed from MHx were consumed by low-temperature chemistry reactions, whereas the M3OHHx reactivity was significantly governed by the β-radical (on the same carbon as the OH group) chemistry which mainly terminated via the less reactive chain propagation pathway to methyl-3-oxohexanoate + HO2. This study extends our understanding of structural effects on ester reactivity, which will allow for accurate surrogate formulation and performance simulations.

Development of Physical-Chemical Surrogate Models and Skeletal Mechanisms for the Combustion Simulation of Several Jet Fuels

Abstract The physical–chemical surrogate models for S-8, Jet-A, and RP-3 fuels to capture their physical and kinetics properties have been developed in this study. n-dodecane (nC12H26), 2,5-dimethylhexane (C8H18-25), and toluene (C6H5CH3) were chosen as candidate surrogate components and formulated by the function group based surrogate fuel methodology. Some important physical properties and spray characteristics for S-8, Jet-A, and RP-3 surrogate models were validated. The results indicate that present surrogate models can well emulate various physical properties to accurately reproduce the spray characteristics. Then, a minimal and high-precision surrogate skeletal mechanism that can be suitable for computational fluid dynamics (CFD) simulations was developed and validated against some fundamental combustion experiments for each surrogate component. Furthermore, the performances of surrogate models that contain the surrogate formulation and associated skeletal mechanisms were validated against the experimental data on ignition delay times (IDTs), species concentration profiles, and laminar flame speeds (Su0) in a wide range of conditions. Finally, the surrogate fuels were used to combustion CFD simulations to model the spray combustion process in a constant volume combustion chamber. It can be seen that the agreements between the simulation and experiment in fundamental and spray combustion characteristics are reasonably good, which proves that present surrogate models are accurate and robust to be applied in CFD simulations.

Experimental Investigation on Cross-Impingement Characteristics Under Various Biodiesel-Butanol Blended Proportions and Ambient Conditions

Abstract The cross-impingement phenomenon always appears in several diesel engines with two or more injectors. Meanwhile, the application of biofuels has a great potential in realizing clean and efficient combustion. Therefore, the investigation aims to explore the cross-impingement characteristics at small (10%), middle (30%), and large (50%) biodiesel-butanol blended proportions. Experiments are conducted in a constant-volume combustion chamber with twin injectors. Spray images are captured by optical diagnosis techniques. Several macroscopic parameters are obtained, including diffusion length, collision width, and spray area. Results show that the cross-impingement accelerates the droplet interaction, and the spray presents a “fan-shaped” behavior after the collision, which promotes a more uniform mixing between the fuel and ambient gas. As the twin sprays collide at 120 deg, the vapor-phase vertical diffusion rate is close to the vertical component of the single spray, and the horizontal diffusion rate is about 1.2 times the vertical diffusion rate. The cross-impingement is likely to decrease the spray-wall impingement owing to a change in the diffusion direction. At various blended fuels, the biodiesel blended with 30% n-butanol displays the smallest liquid-phase diffusion length, width, and area. The further increase in the n-butanol mixing ratio leads to larger liquid-phase parameters. Contrary to the biodiesel blended with 10% n-butanol, the biodiesel blended with a higher proportion of n-butanol presents faster vapor-phase diffusion, which promotes fuel-gas mixing.

Manipulating hydrogen oxy-combustion through carbon dioxide addition

Hydrogen (H2) is increasingly viewed as an attractive carbon-free fuel due to its potential compatibility with the existing transportation and conversion infrastructure. However, one of the major challenges facing large-scale deployment is the fundamentally different combustion properties it has in comparison to commonplace hydrocarbon fuels such as natural gas. For example, the laminar burning velocity (LBV) of a combustible mixture has a direct impact on how it can be used in internal combustion engine or gas turbines; the LBV of hydrogen is over 7 times greater than that of natural gas when combusted in air. Carbon dioxide (CO2) can be used as a working fluid, as opposed to nitrogen (in air), to reduce the flame speed of hydrogen combustion. A mixture of hydrogen, oxygen, and carbon dioxide as a working fluid can provide LBVs comparable to natural gas in air, which potentially enables existing conversion architectures. Furthermore, by replacing nitrogen (N2) with CO2 in the mixture, NOx emissions are avoided and opportunities for carbon sequestration or closed-cycle processes are possible. This study experimentally explores fundamental premixed oxy-combustion properties of H2-CO2 mixtures in a constant volume combustion chamber (CVCC) across a range of initial pressures (1, 1.5, and 2bar) and equivalence ratios (0.4, 0.6, 0.8, and 1). The spherically expanding flames are examined to determine the flame speed, LBV, and lower flammability limits (LFL) with respect to different CO2 concentrations (40%, 60%, 65%). Furthermore, at an initial pressure of 1bar and stoichiometric conditions, it was found that the flame speed of the 65% CO2 case was lower than that of CH4-air combustion.

Autoignition Characterization of Hydrogen Directly Injected into a Constant-Volume Combustion Chamber through a Heavy-Duty Injector

One factor limiting the exploitation of hydrogen as a fuel in internal combustion engines is their tendency to autoignition. In fact, on one hand, its low activation energy facilitates autoignition even with low compression ratios; on the other hand, this can become uncontrollable, due, for instance, to the presence of hot spots in the combustion chamber or to the collision of hydrogen on close surfaces. This represents a limit to the use of hydrogen at medium–high loads, therefore limiting the power density of the engine. In this work, hydrogen was injected at a pressure ranging between 15 and 25 bars into a constant-volume combustion chamber in which the temperature and pressure were increased by means of a previous combustion event. The phenomena taking place after hydrogen injection were observed through fast image acquisition and characterized by measuring the chamber pressure and temperature. In particular, ignition sites were established. The physical system was also modeled in Ansys Fluent environment, and the injection and mixture formation were simulated in order to evaluate the thermo-fluid dynamic field inside the combustion chamber just before autoignition.