19 research outputs found
Physics of puffing and microexplosion of emulsion fuel droplets
The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.This article has been made available through the Brunel Open Access Publishing Fund
Ramjet compression system for a hypersonic air transportation vehicle combined cycle engine
This report assesses the performance characteristics of a ramjet compression system in the application of a hypersonic vehicle. The vehicle is required to be self-powered and perform a complete flight profile using a combination of turbojet, ramjet and scramjet propulsion systems. The ramjet has been designed to operate between Mach 2.5 to Mach 5 conditions, allowing for start-up of the scramjet engine. Multiple designs, including varying ramp configurations and turbo-ramjet combinations, were investigated to evaluate their merits and limitations. Challenges arose with attempting to maintain sufficient pressure recoveries and favourable flow characteristics into the ramjet combustor. The results provide an engine inlet design capable of propelling the vehicle between the turbojet and scramjet phase of flight, allowing for the completion of its mission profile. Compromises in the design, however, had to be made in order to allow for optimisation of other propulsion systems including the scramjet nozzle and aerodynamics of the vehicle; it was concluded that these compromises were justified as the vehicle uses the ramjet engine for a minority of the flight profile as it transitions between low supersonic to hypersonic conditions
Puffing-enhanced fuel/air mixing of an evaporating n-decane/ethanol emulsion droplet and a droplet group under convective heating
Pu ng of a decane/ethanol emulsion droplet and a droplet group under convective
heating and its e ects on fuel/air mixing are investigated by direct numerical simulation
(DNS) that resolves all the liquid/gas and liquid/liquid interfaces. With distinct
di erences in the boiling point between decane and ethanol, the embedded ethanol subdroplets
can be superheated and boil explosively. Pu ng, i.e. ejection of ethanol vapour,
occurs from inside the parent decane droplet, causing secondary breakup of the droplet.
The ejected ethanol vapour mixes with the outer gas mixture composed of air and vapour
of the primary fuel decane, and its e ects on fuel/air mixing can be characterised by the
scalar dissipation rates (SDRs). For the primary fuel SDR, the cross-scalar di usion due
to ethanol vapour pu ng plays a dominant role in enhancing the micromixing. When the
vapour ejection direction is inclined toward the wake direction, the wake is elongated,
but the shape of the stoichiometric mixture fraction iso-surface is not changed much,
indicating a limited e ect on droplet grouping in a spray. On the other hand, when the
ejection direction is inclined toward the transverse direction, the stoichiometric surface is
pushed further away in the transverse direction and its topology is changed by the pu ng.
The trajectories of ejected ethanol vapour pockets can be predicted by the correlation
obtained for a jet in cross
ow, and the vapour pockets may reach a few diameters
away from the droplet. Therefore, in a multiple-droplet con guration, the transverse
ethanol vapour ejection due to pu ng may transiently change the droplet grouping
characteristics. In simulation cases with multiple droplets, the interaction changing the
droplet grouping due to pu ng has been con rmed, especially for droplets in the mostupstream position in a spray. This implies that pu ng should be accurately included in the mixing and combustion modelling of such a biofuel-blended diesel spray process.Financial support from the Engineering and Physical Sciences Research Council (EPSRC), grant No. EP/J018023/
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Experimental understanding on the dynamics of micro-explosion and puffing in ternary emulsion droplets
Dynamics of puffing and micro-explosion phenomena occurring in ternary fuel emulsion droplets under high temperature environment were explored using high speed backlight imaging technique. A single droplet composed of diesel-biodiesel-ethanol emulsion was placed at the tip of a 75 µm gauge thermocouple and introduced rapidly into a furnace maintained at 500 °C. Several interesting features such as oscillation of suspended droplets, physical transformations occurring within the droplet, vapour expulsion, puffing, micro-explosion, sheet formation, perforations, growth of perforations, sheet disintegration and rotation of secondary droplets were observed. High resolution image analysis revealed separation of emulsion components within the core of the suspended droplet, which appeared either as a single nucleus or multiple nuclei. Two distinct types of micro-explosion were identified. For droplets encountering a single nucleus at the core resulted in a stronger vapour expulsion followed by intense micro-explosion. For droplets having multiple nuclei at the core resulted in a weaker vapour expulsion and slower growth of droplet prior to micro-explosion. Both types of micro-explosion process resulted in a number of child droplets. For the case of strong vapour expulsion nearly 80% of its child droplets have their sizes distributed within 150 μm compared to 60% for weaker vapour expulsion. The child droplets that were generated from the primary events of both puffing and micro-explosion cascaded further into secondary and tertiary events of puffing and micro-explosion in freely suspended environment.EPSR
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Spray characteristics of air-assisted urea -SCR sprays of sub-atmospheric temperatures
EPSR
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Spatio-temporal Evolution of Diesel Sprays at the Early Start of Injection
The impact of injector life on the spatio-temporal evolution of fuel spray quality was optically investigated using high speed imaging techniques. Both new and used injectors, which had been used in intense operation for up to 90,000 miles, were considered in this investigation. Used injectors are prone to wear, deposit formation, and altered internal nozzle flow. High resolution SEM images clearly portray the presence of carbonaceous deposits both at the injector tip as well as within the holes of used injectors. Investigations revealed that used injectors tend to produce a chaotic hole-to-hole variation at the start of each injection, resulting in an asymmetric early fuel spraypenetration pattern in the first 500μs. Often those sprays thatsuffereda reduced spray tippenetration rate at the start of injection also showed off-axis transient expansions, and those sprays appeared to be bulky compared to other sprays. Following the early asymmetric spray penetration phase of injection the retarded sprays undergo rapid acceleration with time, and this transformed the early asymmetric spray pattern into a nearly uniform spray pattern from all the orifices in the quasi-steady state regime. The hole-to-hole penetration variations and the resultant asymmetric spray structure at the early start are therefore short lived transient phenomena. However if the radial expansion of the spray is large during the early phase, the radially expanded plume remains almost at the same radial location due to lack of local axial momentum, even after different time instants of spray tip propagation. This appears as a bulge to the spray and can eventually end up as a stationary local pocket of fuel vapor close to the nozzle for the entire duration of injection. This may alter the ignition, flame lift-off and entrainment characteristics of sprays injected from used or deposit rich injectors
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Investigation of Spray Angle Measurement Techniques
The in-cylinder fluid-dynamic processes of fuel injection and air entrainment influence the structure and shape of evolving fuel sprays, which can subsequently alter the ignition, combustion and pollutant formation processes in diesel engines. Different spray angle detection methods have been used in the literature to investigate the global and local spray characteristics. In this work, the five most widely used diesel spray angle detection methods were identified, and used to evaluate the characteristic features of each detection method: methods with a detection range based on the spray penetration length, methods with a fixed detection range in the near and far-field spray region, triangular-based methods, and methods based on averaging local data points. The sprays were acquired from our spray chamber and processed with different thresholding techniques to explore the differences between spray angle detection methods. All five methods generated a similar global trend of spray angle variation for temporally evolving sprays over the complete injection period. However, the actual spray angle values detected by each method were not always comparable. The differences in spray angle values between the different detection methods were larger during the early start of injection, and these differences systematically decreased as the spray approached a steady-state. The methods that detected the angle in the far-field demonstrated lower spatiotemporal variability when compared to the methods that detected the angle in the near-field. An assessment ofthecomparabilitybetweenangledetectionmethodswasmadeandtheoutcomeprovidesguidanceforthe selection of the spray angle detection method.Engineering and Physical Sciences Research Council in the UK (EPSRC
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Evaluating the potential of a long carbon chain oxygenate (octanol) on soot reduction in diesel engines
Copyright © The Authors 2019. The automotive industries are facing challenges to meet stringent CO2 and air-quality regulations. One of the factors affecting emissions performance is the fuel’s chemical composition. The presence of oxygen in a fuel offers the potential to lower soot emissions, however, these oxygenates must be compatible with existing hardware and fuels. To this end, the combustion of a long carbon-chain oxygenate, 1-octanol, was studied. It was injected at high pressures into a constant volume chamber, and a high-speed, two-colour pyrometry system was used to evaluate the spatial and temporal variations of soot and temperature. A significant reduction of soot was obtained whilst the flame temperature remained similar to that for diesel.EPSR
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Experimental study of the effect of C8 oxygenates on sooting processes in high pressure spray flames
© 2020 The Authors. Oxygenated compounds have the ability to reduce soot emissions and to improve the combustion efficiency in engines. Most studies have focused on the soot reduction potential of shorter carbon chain oxygenates, whilst longer carbon chain oxygenates are still relatively unexplored. In this work, the soot reduction potential of long carbon chain oxygenates having similar thermo-physical and chemical properties to those of diesel have been studied, viz., 2-octanone (ketone), 1-octanol (alcohol), hexyl acetate (ester) and octanal (aldehyde). These oxygenates were injected at high pressure into a constant volume chamber maintained at high ambient temperature conditions, and their spray flames were investigated using a high-speed, two-colour pyrometry system. It was found that for the same injected fuel mass, the oxygenates reduced the overall soot when compared to diesel. Small differences in sooting tendencies were observed between different oxygenated moieties but these were smaller than those relative to diesel. The absence of aromatic groups and the presence of oxygen directly bonded to carbon atoms seemed to have a larger effect on soot reduction than the oxygenated functional group. The oxygenates altered the local oxygen equivalence ratio in the spray, influencing the soot formation and its distribution in the flame. Under the high pressure conditions studied, the average sooting tendency of the long carbon-chain oxygenates studied increased in the order of: ester< aldehyde ~ alcohol<ketone.Engineering and Physical Science Research Council (EPSRC
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Combustion of Ligaments and Droplets Expelled after the End of Injection in a Multi-hole Diesel Injector
Experimental investigations were carried out to study the end of injection spray characteristics using a number of production grade multi-hole common-rail injectors. These injectors were taken from light-duty diesel vehicles that are currently in operation on the UK roads and have done different mileages. All the production injectors suffered expulsions of ligaments and droplets after the end of injection(aeoi). It is shown that injector age/mileage has very little effect on the amount of ligaments and droplets ejected from production injectors compared to injection-to-injection variations in the amount of post-injection expulsions. Brand new production injectors also show the presence of these post-injection expulsions after every injection, which is not a desired feature of the modern solenoid actuated common-rail fuel injection system. Subsequent combustion of these post-injection ligaments and droplets lasted up to 25ms after the end of fuel injection in our high pressure, high temperature experiments, and this would contribute to engine-out soot and unburned hydrocarbon (ubhc) emissions in a firing engine.This work has been sponsored by shell Global Solutions (UK) and EPSRC