Properties of Ignition Kernel Generated by a Sunken Fire Igniter

Environmental requirements imposed stringent measures such as the adoption of bio fuels as well as reduction of fuel consumption. As these are integrated, reliable engine operation for in flight re-ignition and high-altitude ground ignition must be ensured. Flame development and propagation in annular combustion chambers strongly rely on kernel characteristics and its interaction with a surrounding environment typically challenging to ignite. Therefore, a sharper understanding of kernel properties for detrimental conditions is sought. The main objective of the present study is to characterize ignition kernel properties using a real helicopter igniter for characteristic times for which fine and detailed information are lacking. These are indeed sought to properly initialize numerical simulations. Two diagnostics have been adopted for characterization: microcalorimetry and schlieren visualization, showing decreasing energy transfer efficiency and growing final volume as initial pressure decreases.

Temperature dependence of resonance energy transfer in DCM doped anthracene nanoaggregates

ABSTRACTThe effects of long term exposure to suboptimal growth temperature on the photosynthetic apparatus ofDunaliella tertiolectaButcher were investigated using carbon fixation rate versus irradiance curves and the variable fluorescence induction method. Carbon fixation rates per unite chlorophyll a at saturating (pBm) and subsaturating (αB) irradiances were 55% and 39% lower, respectively, at 12° C than at 20° C. Chlorophyll a quotas and the spectrally averaged in vivo absorption cross section normalized to chlorophyll a (a*) were not significantly different at these two temperatures. Analysis of the fluorescence kinetics revealed 1) no significant variations of the amount of PSII photoactive reaction centers per unit chlorophyll a, 2) a 14% decrease of the PSII quantum yield(+) and 3) a 29% decrease of the energy transfer efficiency between the light harvesting chlorophyll a pigment bed and the PSII reaction centers. The decrease in energy transfer efficiency between the antennae and the PSII reaction centers at 12° C was interpreted as a mechanism to avoid photoinhibition.

Spark ignition of lean and dilute fuel-air mixtures provides emission reductions of NOx. Furthermore, operation at the lean-dilute limit increases engine efficiency through reduced pumping loses and reduced heat transfer. However, ignition near the lean flammability limit becomes more stochastic and exhibits substantially decreased flame propagation rates. In this work, spark ignition and the subsequent flame kernel development and propagation are studied in a constant volume optical combustion vessel. The vessel provides full field orthogonal and line-of-site optical access via sapphire windows. Additionally, an automated process controller with a versatile gas system enables the creation of a wide range of fuel-air mixtures, including lean and dilute mixtures of hydrocarbons, oxygen, nitrogen, carbon dioxide, and other gases. Ambient conditions including in-chamber temperature and pressure levels, along with dilution conditions, can be set independently. Ignition is provided by an automotive spark plug in the chamber. Optical diagnostics including simultaneous CH* chemiluminescence and shadowgraph imaging are utilized to characterize initial kernel growth and flame development under elevated pressure conditions, from atmospheric to 17.3 bar. Chemiluminescence images are quantified to determine flame intensity and kernel radius to understand the success of initial flame kernel development and propagation. Increasing the pressure yields a slower rate of flame kernel development and propagation, with a thickening flame front, which in turn increases the effects of buoyancy and heat loss. Leaning the mixture can yield unsuccessful kernel development due to heat loss to the large electrode which may cause a failed sustaining of combustion. This knowledge on kernel development near the lean limit benefits the engine community by characterizing the importance of ambient conditions including pressure and mixture properties in sustaining flame growth and propagation.

Diesel engines are extensively used in heavy-duty transportation, power generation, and marine vehicles due to their superior thermal efficiency and extended high-load operability compared to spark ignition (SI) engines. However, combustion in diesel engines is generally characterized by locally rich fuel-air mixtures and high combustion temperatures, causing significant amounts of soot and NOx emissions from these engines. Utilizing carbon-free alternative fuels and enhancing fuel efficiency represent promising strategies to mitigate greenhouse gas (GHG) and other emissions in the heavy-duty transportation sector. In this context, ammonia (NH3), as a hydrogen carrier, has received significant attention as a viable substitute for hydrocarbon fuels, due to its carbon-free composition, relatively high energy density, and well-established infrastructure. Many previous studies have considered combustion and emission characteristics of ammonia-hydrocarbon fuel blends in engines and simplified flames. However, detailed investigations on the effect of ammonia on the performance of hydrocarbon fuels under engine conditions are lacking. In the present study, we perform large-eddy simulations (LES) of the ignition and flame processes in a constant volume combustion reactor, where n–heptane is injected in an ammonia/air ambient mixture in a diesel-like environment. A detailed and validated reaction mechanism containing 302 species and 1981 reactions is employed. The ECN Spray H experimental data is used to validate the spray model under non-reacting and reacting conditions. Dual-fuel combustion is simulated using the well-stirred reactor (WSR) approach. Results are presented for two spray cases: (1) Single fuel (SF) with n-heptane injected into a mixture of air and combustion products; (2) Dual-fuel (DF) with the injection of n-heptane in a mixture of air, ammonia, and combustion products. It is observed that the presence of ammonia has a significant effect on the ignition and flame development processes. With ammonia addition, both the first and second stage ignition delay times increase, but the effect of ammonia on the second stage ignition is significantly more prominent. In addition, the ignition kernel size and growth rate decrease noticeably. For SF spray, the main ignition is characterized by multiple ignition kernels near spray tip, whereas for DF spray, a single relatively small ignition kernel forms and grows slowly in the downstream direction. The flame development and the final quasi-steady flame structure are also modified due to ammonia. The outcome of this research would enable better understanding of ammonia-diesel dual-fuel spray flame behavior and guide development of associated engine combustion strategies.

A visualization study using the schlieren method is adopted in an optically-accessible, cylindrical constant volume combustion chamber to identify the mechanism of ignition energy and ignition system interaction in spark ignited, lean gasoline-air mixture. In order to research the effects of ignition system on flame propagation, two kinds of ignition system are designed, and several kinds of spark plugs are tested and evaluated. To control the discharge energy, the dwell time is varied. The initial flame development is quantified in terms of 2-D images which provides information about the projected flame area and development velocity as a function of ignition system and discharge energy. The results show that high ignition energy and extended spark plug gap can shorten the combustion duration in lean mixtures. The material, diameter and configuration of electrodes the flame development by changing the transfer efficiency from electrical energy to chemical energy and discharge energy. However these factors do not affect of flame development as much a ignition energy or extended gap does.

Computational insights into flame development and emission formation in an ammonia engine with hydrogen-assisted pre-chamber turbulent jet ignition

In order to investigate the effects of three different electrostatic discharge energies on gas explosions, a high-speed PIV system has been applied. The present study paid attention to the flame initiation by the gas explosions and its propagation at the existence of an obstacle within a chamber. Three different ignition energies such as 0.56 mJ, 52.87 mJ and 112.5 mJ were used. It is found that the ignition kernel is bent by the electrostatic discharge during the flame initiation. Tangential velocities of unburnt mixture ahead of initially propagating flame fronts are increased with increasing ignition energy, which makes the flame propagation faster before it reaches the obstacle. Although the flame speed was found to be less sensitive to the ignition energies, the flame developments were different. The effects of the energies on explosion pressures were also discussed.

ABSTRACTInterchain interactions can play a positive role in reaching amplified spontaneous emission in an interesting core–polymer system where the donor (side chains) and the acceptor (core) are chemically linked together. Different degree of interchain interactions modifies the photophysical characteristics of the polymer. By means of transient absorption spectroscopy we show that the stimulated emission from the core decreases passing from solid state to concentrated solution and it is almost absent in the diluted solution. The conformational rearrangements of the core–polymer chain in solution limits the efficiency of the intrachain Förster energy transfer mechanism. The free chain rotations decrease the exciton hopping along the conjugated chains, the ratio between donor and acceptor moieties in the polymer, and change the relative orientation of the transition dipoles of the donor and acceptor causing a strong decrease of energy transfer efficiency and subsequently of the gain. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 965–969

Marine organisms are currently, and will continue to be, exposed to the simultaneous effects of multiple environmental changes. Plankton organisms form the base of pelagic marine food webs and are particularly sensitive to ecosystem changes. Thus, warming, acidification, and changes in dissolved nutrient concentrations have the potential to alter these assemblages, with consequences for the entire ecosystem. Despite the growing number of studies addressing the potential influence of multiple drivers on plankton, global change may also cause less obvious alterations to the networks of interactions among species. Using inverse analyses applied to data collected during a mesocosm experiment, we aimed to compare the ecological functioning of coastal plankton assemblages and the interactions within their food web under different global change scenarios. The experimental treatments were based on the RCP 6.0 and 8.5 scenarios developed by the IPCC, which were extended (ERCP) to integrate the future predicted changes in coastal water nutrient concentrations. Overall, we identified that the functioning of the plankton food web was rather similar in the Ambient and ERCP 6.0 scenarios, but substantially altered in the ERCP 8.5 scenario. Using food web modelling and ecological network analysis, we identified that global change strengthens the microbial loop, with a decrease of energy transfer efficiency to higher trophic levels. Microzooplankton responded as well by an increased degree of herbivory in their diet and represented, compared to mesozooplankton, by far the main top-down pressure on primary producers. We also observed that the organisation of the food web and its capacity to recycle carbon was higher under the ERCP 8.5 scenario, but flow diversity and carbon path length were significantly reduced, illustrating an increased food web stability at the expense of diversity. Here, we provide evidence that if global change goes beyond the ERCP 6.0 scenario, coastal ecosystem functioning will be subjected to dramatic changes.

The propagation of premixed ethylene-air mixtures (of 0.5, 0.525, 0.55, and 0.65 equivalence ratios) in a divided-chamber combustor was investigated. The vessel, divided by a small cylindrical prechamber, had optical access (for laser-schlieren videography) and was instrumented by a pressure transducer. For the Reynolds numbers of 1870, 2300, and 2830, the observed spatial development of the laminar flames showed that the flame position and shape could be scaled by a characteristic time, based on the burned gas flame speed and the length of the prechamber. Above a Reynolds number of 4330, this scaling breaks down the appearance of Kelvin-Helmholtz instabilities. The observed flame propagation was compared with predictions obtained with a numerical model of flame propagation. The calculated spatial and temporal development of the flame in the main combustion chamber agreed with the experimental observations only for the lowest Reynolds number (1870).

Investigation into engine performance of a hydrogen-dimethyl ether spark-ignition engine under various dimethyl ether fractions

Flame development analysis in a diesel optical engine converted to spark ignition natural gas operation

Propagation of a premixed flame in a divided-chamber combustor

Two types of supersonic combustion instabilities identified in a M = 2 reacting flow at direct injection of gaseous hydrocarbon fuel and quasi-direct-current (Q-DC) plasma assistance have been considered: a global instability triggered by a mechanism of flow-combustion-plasma coupling and a fast instability of a thermoacoustic nature. The experiments were performed at SBR-50 supersonic combustion facility at variable conditions: pressureP0 = 1-4 bar, temperature T0 = 300-750 K, and fuel mass flow rate m‘ = 1-8.5 g/s. Diagnostics includedpressure measurements, filtered fast camera imaging, schlieren visualization, and spectroscopic observations. The global instability develops with a characteristic time of about 10 ms and is related to the interaction of the combustion-based separation zone with the reflected shock wave (SW) and electric discharge. It is shown that this instability could be effectively controlled by electrical discharge power. The thermoacoustic instability is developed with a characteristic time less than 1 ms. The analysis of pressure data reveals a resonant acoustic wave presence in the combustion zone between the fuel injection ports and the test section diffuser.