Constant Volume Combustion Chamber Research Articles

Effects of density ratio and differential diffusion on flame accelerative propagation of H2/O2/N2 mixtures

The effects of density ratio and differential diffusion on premixed flame propagation of H2/O2/N2 mixtures are investigated by constant volume combustion chamber. The density ratio and differential diffusion are controlled independently by adjusting the O2/N2 ratio and equivalence ratio. Results show that the density ratio has no effect on turbulent burning velocity while the differential diffusion has a promotion effect on turbulent burning velocity. The onsets of laminar flame acceleration are promoted by both density ratio and differential ratio. The turbulent flames perform a continuous acceleration propagation and the dependence between flame propagation speed and flame radius can be characterized as (dR/dt)/(σ·SL) ∼ R0.33∼0.37, which is lower than the 1/2 power law. The acceleration parameters of laminar flames and turbulent flames (u’/SL = 1) are around 0.17 and 0.36 respectively, and both of them are not affected by density ratio and differential diffusion. The empirical formula m = 0.19·(u’/SL)0.4+0.17 is concluded to quantitatively describe the accelerative characteristics of laminar and turbulent flames. The current study indicates that the acceleration of laminar flames is mainly induced by flame intrinsic instability, and the latter can affect the acceleration onset but not affect the fractal excess. The acceleration of turbulent flames is dominated by turbulent stretch, while the effects of density ratio and differential diffusion can be ignored.

Visualization study on combustion characteristics of direct-injected hydrous methanol ignited by diesel in a constant volume combustion chamber

Aiming to regulate the combustion and emission of diesel/methanol dual-fuel engine more effectively, the using of hydrous methanol was proposed in the current study. The ignition and combustion characteristics of direct-injected hydrous methanol ignited by diesel were investigated by optical diagnostics. Visualization experiments were conducted on a constant volume combustion chamber (CVCC) test bench with a dual-direct injection system. Diesel was injected into CVCC prior to methanol. The results show that adjusting water content in methanol and injection interval between diesel and methanol injection are two of the most important strategies that can control the ignition and combustion process. Higher water content in methanol results in much more heat absorption during water evaporation, which should be corresponding to longer ignition delay, longer flame lift-off length (FLOL) and lower spatial integrated natural luminosity (SINL). However, the decomposition of water under high temperature as well as high pressure condition can produce OH groups, which accelerates the combustion process. Moreover, the injection interval can alter the collision timing and position of spray plumes, which are very significant for diesel ignition and methanol combustion. There are almost no monotonic effect laws when water is introduced under different injection interval conditions. Suitable design and calibration of injection strategy is crucial for effective and clean combustion in diesel/hydrous methanol dual-direct injection mode.

Potential of kerosene-diesel blends as alternative fuels for diesel engines: Perspectives from spray combustion characteristics

To explore the application potential of kerosene (RP-3)/diesel blends as alternative fuels for diesel engines, the spray combustion characteristics of neat diesel, neat RP-3 and RP-3/diesel blends (varied from 25 % to 75 % of RP-3) were systematically compared in an optical constant volume combustion chamber under non-evaporating (0 % O2, 293 K), evaporating (0 % O2, 900 K) and combustion (21 % O2, 900 K) conditions. Their spray, evaporation, combustion and soot emission characteristics were visualized using various high-speed imaging techniques. The results show that RP-3/diesel blending ratio has significant effects on the spray and combustion processes. Under non-evaporating condition, higher RP-3 contents slightly decrease the spray penetration but increase the corresponding spray angle and volume. Under evaporating condition, the spray angle and volume follow the similar trend as those under non-evaporating condition except for slight fluctuations, but the liquid length decreases with the increase of blending ratio. Unexpectedly, the vapor penetrations of neat diesel and RP-3 are similar and longer than those of their blends. Under combustion condition, both the ignition delay and flame lift-off length increase with the RP-3 blending ratio. In addition, RP-3 component reduced the peak soot mass by 9.78 %, 14.7 % and 17.56 % for R25, R50 and R75, respectively. In summary, this study suggests that RP-3/diesel blends are suitable alternative fuels for diesel engines, in terms of faster evaporation and lower soot emissions. We recommend a blending ratio of 25 % as the most promising fuel for further investigations in real engines.

Combustion characteristics for a hydrogen-fuelled low-pressure-ratio split-cycle engine

Investigation of hydrogen-air combustion characteristics for low-pressure-ratio split-cycle engines is performed for the first time. Ignition delay and combustion duration data are presented for low initial pressures below 5 bars and temperatures from ambient level till 550 °C. A simplified constant-volume combustion chamber experiment with glow-plug ignition and timed intake and exhaust valves is constructed in order to simulate the real operation. Ignition delay is found to be inversely proportional to the temperature, and unexpectedly directly proportional with pressure. Equivalence ratio has a weak influence on the ignition delay. Combustion duration exhibits the same behaviour as the ignition delay and represents an average 40% portion of the total combustion delay. Ignition delay results are bounded to the selected glow plug surface temperature range and complements with data in literature. Based on the results, the engine rotational speed would be limited to a maximum of 4500 RPM for an intake duration of 30° at 550 °C and 3.5 bars pre-combustion conditions.

The combustion performance of sustainable aviation fuel with hydrogen addition

Hydrogen (H2) aircraft have been proposed for the “Fly Net Zero” target. The combination of jet fuel and H2 was offered to successfully operate the jet engine with H2 without further combustor modification. The current work used hydro-processed renewable jet fuel (HRJ) and experimentally investigated its combustion properties and H2 addition in the constant volume combustion chamber (CVCC). Experiments were performed at the initial temperature (Tini) from 600 K to 818 K, initial pressure (Pini) 10, 15 bar and equivalence ratio (φ) = 0.5. Normal spray ignition (SPI) and direct injection spark ignition (DISI) were studied. Overall, HRJ has a shorter ignition delay (ID) for SPI. At 600 K, adding 10% and 20% H2 to HRJ increased ID to 12.68% and 38.46%, respectively. As Tini rose to 725 K, increment in ID was shortened to 5.96% and 20.47% with 10% and 20% H2 addition, respectively. For DISI, at 600 K, adding 10% and 20% H2 to HRJ shortens ID to 3.56% and 4.92%, respectively. When comparing the ID of DISI with the SPI, ID was shortened to 84.64% and 91.77% for 10% and 20% H2 addition, respectively. Emission results showed that adding H2 reduced CO2, while NOx emissions were increased. Ansys Chemkin-Pro was used to run numerical simulations of a zero-dimensional (0-D) closed homogeneous batch reactor model (CHBR). An existing HRJ mechanism was adopted, and H2 elementary reaction rate constant factors were updated for better model predictions. The updated model was under prediction with the experimental ID periods. The simulation results showed that the reaction H2+OH = H + H2O is the primary cause for consuming OH radicals with H2 addition, leading to an increase in fuel ID. At higher temperatures, two major reactions (HO2 + H2 = H2O2 + H, and HO2+HO2 = H2O2 + O2) followed by H2O2 = OH + OH are accountable for reproducing OH radicals. Thus, the fuel ID was shorter.

Effects of nozzle diameter on the characteristic time scales of diesel spray two-stage ignition under cold-start conditions

Diesel engine cold-start performances are closely related to ignition delay time which in turn is strongly influenced by the nozzle diameter (Dnoz). In this study, ignition delay time and ignition location of No. −50 diesel spray at different Dnoz are investigated by high-speed direct photography in a constant volume combustion chamber (CVCC) under cold-start conditions. The experimental results clarify the non-monotonic variation of the high-temperature ignition delay time (τHTI) with decreasing nozzle diameter. The minimum experimental high-temperature ignition delay time of about 1.85 ms is obtained when Dnoz = 160 μm. To investigate the mechanism that the influence of nozzle diameter on high-temperature ignition delay time, numerical simulations of the n-heptane spray ignition process are performed under the same conditions as optical diagnostics. Five characteristic time scales for the two-stage ignition process are defined as: physical preparation time (τphy), low-temperature pre-reaction time (Δτ1), low-temperature ignition delay time (τLTI), high-temperature pre-reaction time (Δτ2), and high-temperature ignition delay time. The physical preparation time decreases linearly with nozzle diameter caused by better spray atomization, evaporation, and air-fuel mixing. Changes in nozzle diameter barely affect low-temperature pre-reaction time, indicating that the linear shortening of low-temperature ignition delay time is attributed to the acceleration of the physical preparation process. The high-temperature pre-reaction time initially decreases and then increases with decreasing Dnoz, which is the result of a smaller scalar dissipation rate promoting the decomposition of H2O2 and inhibiting flame propagation. The combined effect of decreasing Dnoz at each stage leads to a non-monotonic trend in high-temperature ignition delay time.

Laminar combustion characteristics of methane/methanol/air mixtures: Experimental and kinetic investigations

Methanol has been studied to solve the problems of insufficient combustion and unstable operation of natural gas engines. To explore the influence of methanol addition on natural gas/air mixture, the laminar combustion characteristics of stoichiometric methane/methanol/air mixture with methanol volume fractions of 0–100% are measured in a constant volume combustion chamber under the initial pressures of 1–4 bar and initial temperatures of 353–393 K. The experimental results show that the laminar combustion characteristics are linearly enhanced by methanol addition. When the methanol volume fraction varies from 0% to 100% at 393 K and 4 bar, the change rates are 11.7%, 120.4%, - 28.1% and 61.7% for combustion peak pressure, maximum pressure rise rate, combustion duration and laminar burning velocity (LBV) respectively. At high pressure, the influence of methanol addition is enhanced. Chemical kinetics analysis shows that CH2OH, the intermediate product of methanol oxidation, generates HO2 through R134: CH2OH + O2<=>CH2O + HO2, which contributes from 14% to 56% to the generation of HO2, so that HO2 replaces part of the O2 to participate the reaction with H, resulting in the transformation of part of R1: H + O2<=>OH + O to R10: HO2+H<=>2OH, thereby generating more OH and promoting the reaction.

Experimental observation of turbulent jet ignition of pre-chamber with scavenging system for carbon dioxide diluted mixtures

The exhaust gas recirculation (EGR) method can suppress knock and improve the thermal efficiency of engines. But it will also deteriorate the combustion stability and engine power. Turbulent jet ignition (TJI) is a reliable ignition resource for improving ignition stability and burning rate. However, the residual productions in the pre-chamber will worsen the performance of the TJI. To this end, a self-designed pre-chamber with a scavenging system has been proposed. In this study, the ignition process and flame propagation phenomena under different EGR dilution ratios for H2/N2/O2 and CH4/N2/O2 mixtures were conducted in a constant-volume combustion chamber. The results suggested that the increase in EGR dilution weakens the influence of cellular instability and causes buoyancy instability, the latter of which could be mitigated by the passive TJI method. For the passive TJI mode, the exit time of the hot jet was delayed, and the turbulent flame speed decreased with the increase of EGR dilution ratio. Four ignition phenomena, namely jet re-ignition, flame buoyancy, re-ignition failure, and misfire, were distinctly identified. However, EGR tolerance cannot be extended by passive pre-chambers. Therefore, the pre-chamber with a scavenging system that can effectively extend the lean combustion tolerance with EGR dilution compared to SI and passive TJI was proposed. The effects of air and fuel injection quantities on ignition and flame propagation were investigated. The flame propagation velocity was positively related to the air injection mass, whereas an optimum fuel mass was required to achieve fast flame propagation. The EGR limit based on dual injections in the pre-chamber was obviously extended. Moreover, under near EGR tolerance conditions, a leaner fuel injection in the pre-chamber was required to realize successful ignition in the main chamber, as strong turbulence could cause high heat transfer loss with the cool unburnt mixture and suppress the occurrence of re-ignition.

Experimental study of the effect of physical and chemical properties of alcohols on the spray combustion characteristics of alcohol-diesel blended fuels

To reveal the effect of different long carbon-chain alcohols blended into diesel on combustion characteristics and soot emissions under different ambient temperatures, the spray combustion processes of diesel and four sets of blends (the volume ratio of propanol/diesel, butanol/diesel, pentanol/diesel, and hexanol/diesel being 20%/80%) were investigated in a constant volume combustion chamber (CVCC) using optical techniques. It was discovered that the physical (cooling effect, dilution effect, diesel evaporation inhibition) and chemical reaction properties of alcohols have a competitive effect on the spray combustion characteristics of alcohol-diesel blended fuels. The combustion pressures of alcohol-diesel blended fuels were lower than that of pure diesel due to lower heating values, and this difference became more obvious under higher ambient temperature. Alcohols blended into diesel fuel could improve the combustion efficiency. The ignition delay times (IDTs) of alcohol-diesel blended fuels decreased with increased carbon chain lengths of alcohols under low ambient temperature conditions. However, this trend was reversed with increasing ambient temperature, and the IDTs of alcohol/diesel blends became lower than that of pure diesel at higher ambient temperatures. Propanol or hexanol blended into diesel could significantly inhibit the generation of soot at low ambient temperatures, but other alcohols promoted it. Interestingly, all alcohol-diesel blend fuels inhibited the generation of soot under high ambient temperature conditions, and propanol was the most significant.

A parametric investigation of methane jets in direct-injection compression-ignition conditions

This work investigated the effects of ambient and injection conditions on the autoignition, flame evolution and combustion characteristics of the natural gas jets at direct-injection (DI) compression-ignition (CI) conditions. Methane (CH4, as a natural gas surrogate) was directly injected into an optically accessible constant-volume combustion chamber (CVCC). For the experiments, the ambient gas density within the CVCC was fixed at 24kgm−3, but other ambient conditions were varied, including ambient gas temperature (1060–1200K) and ambient oxygen concentration (10–21 vol.%). The effects of injection pressure (10–20MPa reservoir pressure) were also assessed at a fixed ambient temperature and ambient oxygen conditions of 1200K and 21 vol.%, respectively. High-speed schlieren imaging, heat release rate analysis and flame luminosity measurement were applied to the CH4 jet flames. The results show that the ignition delay decreases with increasing ambient temperature or ambient oxygen concentration but does not display an apparent trend with changing injection pressure. Optical images reveal that the CH4 jet combustion typically starts from a localised kernel before propagating downstream of the jet volume in most cases, other than the lower ambient oxygen concentration case. The optical results also reveal that after ignition, the CH4 jet flame recesses back towards the nozzle before stabilising at a lift-off distance. The jet becomes more lifted with a decreasing ambient temperature, a reducing ambient oxygen concentration, and an increasing reservoir pressure. The results also reveal that the heat release rate and flame luminosity profiles roughly correspond to the ignition and combustion characteristics of the CH4 jet flames.

Study of Flame Kernel Development at High EGR and High Flow Speed Using Conventional Spark Igniter and Non-Thermal Plasma Under Gasoline Engine Relevant Conditions

ABSTRACT As a novel ignition approach, nanosecond repetitively pulsed discharge (NRPD)-based non-thermal plasma offers significant benefits such as low energy consumption, and lean operating conditions. However, there is no investigation conducted on the early flame kernel formation and development induced by non-thermal plasma. Therefore, in this paper, experiments of conventional spark and non-thermal plasma ignition systems in constant volume combustion chamber (CVCC) are conducted under gasoline engine relevant conditions: wide initial ambient pressures (6.5, 8.3, 11.3 bar), a range of equivalence ratios (0.7–1.0), EGR rates (10−25%), and cross flow speeds (0–30 m/s). The discharge energy of non-thermal plasma is of around 210 mJ, while a single spark event can generate energy from 65 mJ to 83 mJ depending on ambient conditions. A consecutive spark strategy is adopted to guarantee comparable total input energy to non-thermal plasma. The ignition delay and combustion phase obtained from the chamber pressure history are calculated. In the meanwhile, flame kernel radius, flame propagation rate, and flame front length ratio via schlieren images are quantified and analyzed. Results showed that the flame initiated by non-thermal plasma can maintain a robust flame kernel and propagates fast. Under high-speed cross flow and high EGR rate conditions, non-thermal plasma can maintain a robust flame kernel at early stage supporting the initial flame kernel to survive and improve its ignition probability. Considering the effects of the EGR ratio and high-speed cross flow on flame kernel development, non-thermal plasma ignition can efficiently enhance the flame propagation and ignition probability. It is also concluded that non-thermal plasma can successfully ignite under lean (equivalence ratio between 0.7–1.0), high-diluted mixture (25% EGR), and high-speed cross flow of 30 m/s within the range of current study.

Laser-induced plasma-ignited hydrogen jet combustion in engine-relevant conditions

This work aims to investigate the mechanisms governing H2 jet flame evolution and stabilisation by precisely controlling the jet ignition location using a laser-induced plasma in compression-ignition engine-relevant conditions. The experiments examine the flame evolution with high-speed schlieren imaging and pressure trace measurements in an optically-accessible constant-volume combustion chamber over a range of ambient O2 concentration (10–21 vol%) and temperature (600–800 K) conditions. Optical results reveal that in most cases, a localised flame kernel forms at the time and location of the laser-induced plasma and grows in connected regions to engulf the entire upstream and downstream jet volume of the ignition spot. The images also reveal that the flame lift-off is sensitive to ambient O2 and temperature changes. The flame appears attached to the nozzle at 21 vol% O2, but becomes lifted at a lower ambient O2 concentration and a colder temperature. A simplified numerical analysis suggests that edge-flame deflagration into stratified premixed fuel-ambient reactant streams explains the lift-off response to ambient O2 and temperature changes. Furthermore, at the lowest tested O2 concentration, the flame stabilisation occurs in leaner mixtures at the jet peripheral where it is likely exposed to stronger turbulence-chemistry interactions.

Experimental Studies of Combustion and Emission Characteristics of Biomass Producer Gas (BPG) in a Constant Volume Combustion Chamber (CVCC) System

Most of the world’s energy requirements are still derived from natural resources. This will result in a catastrophic energy crisis with negative environmental consequences. The increased energy supply will result in greater consumption of non-renewable sources. The production of biomass producer gas (BPG) from biomass gasification has received significant attention as an alternative fuel due to the depletion of non-renewable resources. This experimental study aimed to determine the flame propagation, flame propagation speed, and chamber pressure trace of BPG at different equivalence ratios. Understanding the characteristics of the BPG’s combustion, finding lower greenhouse gas emissions of BPG, and minimizing the use of fossil fuels is necessary to mitigate these problems. Using the direct visualization technique, an optical constant volume combustion chamber (CVCC) was developed to measure combustion characteristics. Liquid petroleum gas (LPG) was used to compare the flame propagation speed in the CVCC calibration. In comparison to wood pellet (WP), coconut husk (CH), and palm kernel shell (PKS), the chamber peak pressure at ϕ equal to 1 of CH for the combustion of BPG was the lowest at 20.84 bar. At ϕ of 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, and 1.3, the chamber peak pressure of CH was discovered to be around 17.77, 18.12, 18.81, 20.84, 20.39, 17.25, and 16.37 bar, respectively. Compared to the other two types of BPG, CH produced the lowest emissions of CO2 and CO at 2.03% and 0.022%, respectively. In conclusion, the CH had the lowest chamber peak pressure and emissions due to the lower heating value (LHV) being relatively lower.

Experimental investigation of transition process from LTC to ITC and HTC during diesel spray combustion at low ambient temperatures

The dynamics of low temperature combustion (LTC) and intermediate temperature combustion (ITC) dramatically affect the combustion limits and properties of high temperature combustion (HTC) and engine performance. To reveal the transition from LTC to ITC and HTC and the characteristics of heat release in spray combustion under low ambient temperatures, optical experiments were conducted in a constant volume combustion chamber (CVCC). The high-speed natural luminosity method and the high-speed schlieren method were used to obtain images of the evolution of spray ignition and vapor-phase spray combustion, respectively. The results reveal a significant increase in CVCC pressure when the flame luminosity is not captured by the high-speed natural luminosity method at a low ambient temperature (740 K), which is attributed to heat release by the LTC process. The cumulative heat release obtained from the first law of thermodynamics based on pressure attains 29.72 % of the total lower heating value of the fuel. The transition from LTC to ITC and HTC is diagnosed based on the luminosity variation of the vapor-phase spray tip region in the schlieren images. Furthermore, the transition from LTC to ITC and HTC can be monitored based on the average luminosity of the spray tip. Finally, with the decrease in ambient temperature, the end of LTC is gradually delayed by a smaller magnitude and rate of change. These results demonstrate that the transition from LTC to ITC and HTC is sensitive to ambient temperature.

Hydrogen-diesel dual-fuel direct-injection (H2DDI) combustion under compression-ignition engine conditions

This study investigates the ignition and combustion characteristics of interacting diesel-pilot and hydrogen (H2) jets under simulated compression-ignition engine conditions. Two converging single-hole injectors were used to inject H2 and diesel-pilot jets into an optically accessible constant-volume combustion chamber (CVCC). The parameters varied include fuel injection sequence, timing between injections, and ambient temperature (780–890 K). The results indicate that when diesel-pilot is injected before H2, with increasing time separation, the burnt diesel products mix and cool down, requiring longer jet-jet interaction to ignite the H2 jet. When H2 is injected before diesel-pilot, the H2-air mixing amount prior to pilot-fuel igniting impacts the combustion spreading through the H2 jet. If ignition of the H2 jet occurs beyond its end-of-injection (EOI), the H2 mixture zone where the pilot-diesel interacts with becomes too lean for combustion. At lower ambient temperatures, the combustion variability increases, attributed to the diesel-pilot lean out.

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

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

Micro-nano morphology parameters and mechanical properties of soot particles sampled from high pressure jet flames of diesel from direct coal liquefaction

Diesel from direct coal liquefaction (DDCL) is a new type of engine alternative energy. In this study, a premixed constant volume combustion chamber system with soot particle sampling devices were built. High resolution transmission electron microscope (HR-TEM) and atomic force microscopy were applicated to characterize the micro-nano morphology parameters and mechanical properties of soot particles sampled from high pressure jet flames of DDCL. There were two in-house automatic processing codes were developed to process the HR-TEM images and extract the micro-nano morphology parameters of the soot particles. This study has systematically studied the effect of injection pressures on the micro-nano morphology parameters and mechanical properties of soot particles. The test results illustrated that there was a correlation between the micro-nano morphology parameters and mechanical properties of soot particles. Under the same combustion conditions, DDCL soot particles demonstrated smaller size, compacted structure, lower graphitization degree and weaker antioxidant capacity than that of diesel soot particles. The average adhesive force and energy dissipation of diesel soot particles were much higher, which indicated the diesel soot particles have stronger agglomeration performance. The average elastic modulus of DDCL soot particles was smaller than that of diesel, which caused the weaker ability to resist deformation for DDCL soot particles.

Explosion characteristics of n-decane/hydrogen/air mixtures

Hydrogen can be used in conjunction with aviation kerosene in aircraft engines. To this end, this study uses n-decane/hydrogen mixtures to investigate the explosion characteristics of aviation kerosene/hydrogen in a constant volume combustion chamber with different hydrogen addition ratios (0, 0.2, 0.4), wide effective equivalence ratios (0.7–1.7), an initial temperature of 470 K, and initial pressures of 1 and 2 bar. The results show that the explosion pressure and explosion time decrease linearly with increasing hydrogen addition ratio. The effect of initial pressure is also discussed. A comparison of the adiabatic explosion pressures indicates that the hydrogen addition effect varies at different initial pressures and effective equivalence ratios owing to heat loss. In addition, the maximum pressure rise rate and deflagration index increase with increasing hydrogen concentration, which is more obvious for rich mixtures and high hydrogen concentrations.

Experimental characterization of spark ignited ammonia combustion under elevated oxygen concentrations

Due to its nature as a carbon free fuel and carrying hydrogen energy ammonia has received a lot of attention recently to be used as an alternative to fossil fuel in gas turbine and internal combustion engines. However, several barriers such as long ignition delay, slow flame speed, and low reactivity need to be overcome before its practical applications in engines. One potential approach to improve the ignition can be achieved by using oxygen enriched combustion. In this study, oxygen-enriched combustion of ammonia is tested in a constant volume combustion chamber to understand its combustion characteristics like flame velocity and heat release rates. With the help of high speed Schlieren imaging, an ammonia-oxygen flame is studied inside the combustion chamber. The influence of a wide range of oxygen concentrations from 15 to 40% are tested along with equivalence ratios ranging from 0.9 to 1.15. Ammonia when ignited at an oxygen concentration of 40% with an equivalence ratio of ϕ= 1.1 at 10 bar has a maximum flame velocity of 112.7 cm/s. Reduced oxygen concentration also negatively affects the flame velocity, introducing instabilities and causing the flame to develop asymmetrically due to buoyancy effects inside the combustion chamber. Heat release rate (HRR) curves show that increasing the oxygen concentration from 21 to 35% of the mixture can help reduce the ignition delays. Peak HRR data shows increased sensitivity to air fuel ratios with increased oxygen concentrations in the ambient gas. HRR also shows an overall positive dependence on the oxygen concentration in the ambient gas.

Experimental investigation of the two-stage ignition delay and flame structure of pentanol/n-dodecane binary fuel

High thermal efficiency and low emissions are achievable with pentanol direct injection compression ignition, but there are issues of difficulty and unstable combustion at low loads. To solve the above problem, a binary fuel is obtained by blending high-activity n-dodecane with pentanol. In a constant-volume combustion chamber, the natural flame luminosity, schlieren, formaldehyde planar laser-induced fluorescence, and the OH* chemiluminescence methods were used to investigate the combustion characteristics of the two-stage ignition delay, the formaldehyde, and the flame liftoff length. The results show that, compared with n-dodecane, the two-stage ignition delay of pentanol/n-dodecane binary fuel is longer; and the initial time of the formaldehyde signal of pentanol/n-dodecane binary fuel is delayed by about 100 μs? Additionally, under all operating conditions, the initial position of formaldehyde varies concurrently with changes in the flame lift-off position, and the formaldehyde signals of the pentanol/n-dodecane binary fuel and n-dodecane gradually decrease downstream from the flame liftoff position. The pentanol/n-dodecane combustion model was built with experimental data, and it can be helpful in further numerical simulation.