Pilot Spray Research Articles

Experiments and modeling of the thermal spray-chasing phenomenon during split injection processes

Spray chasing is a specific manifestation of the interaction between split injections, where the mass and heat transfer between the pilot and main sprays significantly influence fuel mixing and ignition processes. In this study, a spray-chasing model is developed and validated based on optical diagnostic results, aiming to theoretically support the development of injection strategies that are more environmentally adaptive and offer improved performance. A series of optical diagnoses of diesel split injection is performed in a constant volume combustion chamber using diffused back-illumination imaging and high-speed schlieren imaging. The extent of the spray-chasing phenomenon is modulated by regulating the injection dwell time (DT) and ambient temperature. For inert sprays, when the background temperature exceeds 790 K, the liquid spray-chasing phenomenon can be disregarded owing to the premature evaporation of the pilot spray. Although shortening the DT can promote the development and mixing of the main spray, the lack of evaporation in the pilot fuel pocket leads to a higher local fuel concentration in the interaction zone after spray chasing, which will hinder the subsequent ignition of the main spray. For reactive sprays, the variations in spray chasing patterns under different operating parameters are determined and their impact on the ignition process is evaluated by combining optical test results with theoretical models. Under cold start conditions, the ignition delay and ignition location of the main spray exhibit a non-monotonic trend with increasing injection dwell time. Optical diagnosis observes that compared to DT of 200 us and 1500 μs, the main spray exhibits both valley in ignition delay time and ignition location at DT = 900 μs. The spray-chasing model identifies approximately 1020 μs as the optimal theoretical DT, expected to best facilitate ignition of the main spray. However, under hot-start conditions, even though the spray-chasing mode varies with different injection dwell times, the ignition characteristics remain essentially unchanged due to the limited promotion effect of split injection on ignition.

Investigation of the dynamic interaction mechanism of split injections during the ignition process under diesel engine-like cold- and hot-start conditions

In a range of operating conditions with variable injection parameters, interactions among split sprays present uncertainties across both temporal and spatial dimensions. Understanding these dynamic interactions is crucial for optimizing injection strategies to improve both the performance and environmental adaptability of diesel engines. In this study, optical diagnostics and numerical simulations were conducted to explore the interaction mechanisms between split injections and their impact on the ignition process. Engine-like cold and hot-start operating conditions were configured, and four injection strategies, including a single injection, were examined. The findings reveal that ambient temperature influences split spray interactions by altering their ignition characteristics, while injection dwell time modifies these interactions by changing the combustion spatial and temporal phase between split sprays. Compared to a single injection, split injection promotes the ignition and combustion of the main spray such as shortening ignition delay, reducing ignition location, and increasing flame area, but this promoting effect weakens under hot-start conditions. When the main spray chases the pilot spray before ignition, it increases the local concentration and reduces the temperature upstream of the pilot spray plume. However, since the effect front fails to reach the expected ignition region before the pilot ignition occurs, the ignition of the pilot fuel pocket remains unaffected. Under cold-start conditions, the main ignition delay shows a non-monotonic trend, reaching its minimum at an injection dwell of 900 μs, where the pilot ignition provides the main spray with elevated local temperatures and an increased hydroxyl mass fraction. Under hot-start conditions, the temporal overlap of ignition in split sprays is enhanced, yet spatial interaction weakens. Despite varying degrees of pilot flame decay before the main spray ignites with increasing injection dwells, the thermal radiation emanating from their combustion elevates the local temperature in the interaction zone beyond 1200 K. Consequently, upon catching up with the pilot injection, the main spray mixture rapidly ignites through the high-temperature reaction pathway, rendering the ignition delay insensitive with injection dwells.

Analysis of product properties in counter-current spray drying with two nozzles system

Results of the experiments on drying of skin-forming material sprayed with two nozzles in counter-current pilot spray dryer are presented in the article. Phase Doppler Anemometry (PDA) measurements were conducted above, between, and below two nozzles spraying upward and downward which allowed us to track changes of particle size distribution (PSD) and particle velocities in the tower for different nozzle configurations. The analysis of PSD shows that for two nozzles pointing downwards, the drying zone was observed under the lower nozzle whereas the agglomeration zone develops between nozzles. For the upward orientation of the lower nozzle, a zone of simultaneous drying and agglomeration forms between the nozzles. Analysis of product moisture content, PSD, and bulk density showed the effect of nozzle configuration on product properties.

Nozzle zone agglomeration in spray dryers: Determination of the agglomeration efficiency in the fines return by means of agglomerate properties and residence time distribution

In multi-stage spray dryers, agglomeration is forced by recirculating fine material into the nozzle zone. Drying droplets collide with dry particles and form agglomerates. The aim of this study was to achieve a more comprehensive understanding and control of agglomerate formation. Agglomerates with different properties were produced using a pilot spray drying plant. An increased air mass flow of the fluidized bed as well as a reduced solids content of the feed solution resulted in an increased number of primary particles per agglomerate. A residence time-based approach was then applied to determine the agglomeration efficiency, defined as the number of successfully adhered primary particles per recirculation. It ranged from 0.33 to 0.71 and was correlated to collision and sticking probabilities in the nozzle zone. This value is of importance both for process optimization and for the validation of simulation approaches.

Sedimentation and rheological behavior of reactive and non-reactive magnesium hydroxide pulps for industrial spray dryer processing

Two different reactive concentrated magnesium hydroxide (HM) pulps, produced in a lab reactor mill, and a synthetic and non-reactive HM-pulp, for comparative analyses, have been prepared and studied. Sodium polyacrylate was selected as a dispersant for improving the HM pulp fluidity, with concentrations varying from 0.42% to 2.0% on a dry basis. The two reactive pulps differed from each other mainly by the impurity level and for both, a residual MgO amount was detected, implying that the hydration reaction could proceed. Results (with and without dispersant) confirmed the formation of particle clusters as HM concentration rose until a critical value, above which the pulp behaved as a solid. Reactive HM-pulps with high impurities may represent a limiting factor in the drying operation. Among all HM-pulps studied, the purest reactive pulp with 1% dispersant presented the best flow behavior making this reactive pulp the most suitable to be tested in a pilot spray dryer.

The ignition characteristics of dual-fuel spray at different ambient methane concentrations under engine-like conditions

In the present study, dual-fuel ignition process of pilot diesel spray injection into a natural gas-air mixture under engine-like conditions is numerically investigated via large-eddy simulation (LES). Methane and n-dodecane are considered as surrogates for natural gas and diesel fuels, respectively. This study aims to provide understandings of kinetic interactions for dual-fuel spray during the ignition process. The effects of ambient methane concentration on the ignition delay times (IDTs), product distributions, and heat release rates (HRRs) during dual-fuel ignition process are emphasized. The results show that methane prolongs the IDT of n-dodecane, especially for the first-stage IDT. Kinetic analyses reveal that the competition for OH between the dehydrogenation reactions of methane (CH4 + OH <=> CH3 + H2O) and n-dodecane (RH + OH <=> R + H2O) results in the increasing first-stage IDT of pilot spray. Moreover, it is found that the retarding effect positively correlates with the methane concentration. For product distributions, it is observed that the transition of CH2O from fuel-lean to fuel-rich regions is inhibited in the first-stage ignition. In the second-stage ignition, the concentration of CO is decreased as the methane concentration increases and the area with high CO concentration is gradually shifted to the downstream of the spray. In addition, HRRs of low-temperature exothermic reactions appear at the radial periphery as well as the head of the spray front; HRRs of high-temperature exothermic reactions are gradually shifted from the head to the center of the spray with the increase of ambient methane concentration. The HRRs of low- and high-temperature reactions are decreased with increasing ambient methane concentration, and the major reactions for heat release at various ambient methane concentrations are different.

Flashback-induced flame shape transition in a two-stage LPP aeronautical combustor

Large-Eddy Simulations were performed to study the flashback-induced flame shape transition of a lean premixed M flame in a staged liquid-fuelled aeronautical lean-burner, as observed experimentally. The BIMER combustor is a Lean Premixed Prevapourised (LPP) burner composed of two stages, each with its own injector and swirler: the main outer stage, called multipoint, uses jet-in-crossflow injection to achieve the LPP regime, while the central stage, called pilot, uses a pressure swirl injector to create a hollow cone spray to stabilise the flame. During LPP operation, this M flame presents a strong acoustic activity, promoting a periodic flashback of its leading edge. When, aiming to stabilise the flame, the pilot injection is increased and the multipoint injection decreased, the oscillating leading edge (due to the longitudinal acoustic perturbations) attaches to the pilot spray, changing the flame into a Tulip shape. Two phenomena were identified as being the most relevant causes of this flame shape transition. First, the leading edge position and the thermoacoustic instability amplitude are directly linked to the combustion chamber final temperature. The higher the temperature in the chamber, the more upstream the leading edge stabilises, and the higher the acoustic oscillation amplitude, both increasing the risk of a successful flashback. Second, the injection regime with high pilot injection allows the leading edge to attach to the pilot spray, as the flame only transitions when the pilot spray is sufficiently high. The higher the pilot fuel flow, the higher the amount of fuel sprayed in the critical region where the flame might attach for a transition to the Tulip shape. Therefore, as the change in injection regime is the main mechanism lean staged burners use to reduce emissions while increasing operability, this works shows that an M flame is unsuitable to such burners with similar aerodynamic topology and properties.

Large-eddy simulation of diesel pilot spray ignition in lean methane-air and methanol-air mixtures at different ambient temperatures

In dual-fuel compression-ignition engines, replacing common fuels such as methane with renewable and widely available fuels such as methanol is desirable. However, a fine-grained understanding of diesel/methanol ignition compared to diesel/methane is lacking. Here, large-eddy simulation (LES) coupled with finite rate chemistry is utilized to study diesel spray-assisted ignition of methane and methanol. A diesel surrogate fuel ( n-dodecane) spray is injected into ambient methane-air or methanol-air mixtures at a fixed lean equivalence ratio [Formula: see text] = 0.5 at various ambient temperatures ([Formula: see text] = 900, 950, 1000 K). The main objectives are to (1) compare the ignition characteristics of diesel/methanol with diesel/methane at different [Formula: see text], (2) explore the relative importance of low-temperature chemistry (LTC) to high-temperature chemistry (HTC), and (3) identify the key differences between oxidation reactions of n-dodecane with methane or methanol. Results from homogeneous reactor calculations as well as 3 + 3 LES are reported. For both DF configurations, increasing [Formula: see text] leads to earlier first- and second-stage ignition. Methanol/ n-dodecane mixture is observed to have a longer ignition delay time (IDT) compared to methane/ n-dodecane, for example ≈ three times longer IDT at [Formula: see text] = 950 K. While the ignition response of methane to [Formula: see text] is systematic and robust, the [Formula: see text] window for n-dodecane/methanol ignition is very narrow and for the investigated conditions, only at 950 K robust ignition is observed. For methanol at [Formula: see text] = 1000 K, the lean ambient mixture autoignites before spray ignition while at [Formula: see text] = 900 K full ignition is not observed after 3 ms, although the first-stage ignition is reported. For methanol, LTC is considerably weaker than for methane and in fully igniting cases, heat release map analysis demonstrates the dominant contribution of HTC to total heat release rate for methanol. Reaction sensitivity analysis shows that stronger consumption of OH radicals by methanol compared to methane leads to the further delay in the spray ignition of n-dodecane/methanol. Finally, a simple and novel approach is developed to estimate IDT in reacting LES using zero-dimensional IDT calculations weighted by residence time from non-reacting LES data.

Thermal and intermediates' effects of pre-injection on the auto-ignition of main-injection in cold-start conditions

• The pilot injection inhibits the ignition of main injection at Ta ≤ 722.5 K. • The effect of split-injection on IDT changes at turning point of Ta = 725 K. • The thermal effect of pilot injection is dominant. • Equivalence ratio of ignition position of split injection is reduced by pilot spray. Spray misfire is highly likely to happen when diesel engines are operated in extremely low-temperature or high-altitude conditions. In this study, single and split injections of n-dodecane spray are compared to identify the role of pilot injection in main spray autoignition near spray misfire. Two different effects of the pilot injection are identified, depending on the relative importance of cool flame and evaporation heat absorption: (1) when ambient temperature (T a ) is lower than 722.5 K, pilot spray inhibits the autoignition of the main spray because the heat release of the cool flame cannot overcome evaporation heat absorption; (2) when T a > 722.5 K, the cool flame reactions in the pilot spray create a hot atmosphere that is rich with cool-flame intermediates for the main spray. In such a case, the pilot spray could promote the autoignition of the main spray. When T a > 725 K, the benefits of split injection to ignition delay reduction are weakened because the cool flame of the pilot injection becomes more similar with increasing T a . Moreover, the effects of thermal and cool-flame intermediates on the main spray are investigated separately in kinetical analyses. It is found that the thermal effects of cool flame are dominant in promoting the autoignition of the main spray under cold- start conditions.

Simulation studies on the boot shape injection of a giant magnetostrictive injector

Giant magnetostrictive injector using giant magnetostrictive material acting an electronic controlled injector may be one new promising injector to acquire adjustable injection rates while maintaining large injection quantity. An electronic controlled injector driven by a giant magnetostrictive actuator was designed through combining the driving requirement and output characteristics of the material. To promote responding speed of the coil current, the driving voltage with open-hold-fall type waveform was employed just like using in an electromagnetic injector. Simulation model for the injection characteristic of the injector was established using AMEsim software and verified using experimental results collected by the single injection meter. From simulation and experimental results, designed giant magnetostrictive injector showed good performances as maximum spray rate of 4.5 L/min and minimum spray pulse width of 0.21 ms, and realized the boot shape injection when generated by the designed voltage wave. Furthermore, duration time and amplitude of the pilot spray part in a boot shape injection were respectively adjusted through changing the dwell time and opening time. The boot shape injection reached by the giant magnetostrictive injector can reach quite accurate control of fuel injection and then promote fuel efficiency effectively.

Experimental study on tri-fuel combustion using premixed methane-hydrogen mixtures ignited by a diesel pilot

A comprehensive investigation on diesel pilot spray ignited methane-hydrogen (CH4–H2) combustion, tri-fuel combustion (TF), is performed in a single-cylinder compression ignition (CI) engine. The experiments provide a detailed analysis of the effect of H2 concentration (based on mole fraction, MH2) and charge-air temperature (Tair) on the ignition behavior, combustion stability, cycle-to-cycle (CCV) and engine performance. The results indicate that adding H2 from 0 to 60% shortens the ignition delay time (IDT) and combustion duration (based on CA90) up to 33% and 45%, respectively. Thereby, H2 helps to increase the indicated thermal efficiency (ITE) by as much as 10%. Furthermore, to gain an insight into the combustion stability and CCV, the short-time Fourier transform (STFT) and continuous wavelet transform (CWT) methodologies are applied to estimate the combustion stability and CCV of the TF combustion process. The results reveal that the pressure oscillation can be reduced up to 4 dB/Hz and the CCV by 50% when MH2 < 60% and Tair < 55 °C. However, when MH2 > 60% and Tair > 40 °C, abnormal combustion and knocking are observed.

Breakup and Coalescence of Oil Droplets in Protein-Stabilized Emulsions During the Atomization and the Drying Step of a Spray Drying Process

The goal of this study was to investigate the changes in oil droplet size in whey protein–stabilized emulsions during the atomization and the subsequent drying step of a spray drying process. For this purpose, experiments were performed in an atomization rig and a pilot spray dryer with two commercial pressure swirl atomizers. By comparing the oil droplet size before atomization, after atomization, and after spray drying, the changes in oil droplet size during each process step were quantified. The effect of oil droplet breakup during atomization was isolated by atomizing emulsions with 1 wt.% oil content and a protein to oil concentration ratio of 0.1. At 100 bar, the Sauter mean diameter of oil droplet size was reduced from 3.13 to 0.61 μm. Directly after breakup, coalescence of the oil droplets was observed for emulsions with a high oil content of 30 wt.%, leading to a droplet size after atomization of 1.15 μm. Increasing the protein to oil concentration ratio to 0.2 reduced coalescence during atomization and oil droplets with a mean diameter of 0.92 μm were obtained. Further coalescence was observed during the drying step: for an oil content of 30 wt.% and a protein to oil concentration ratio of 0.1 the mean droplet size increased to 1.77 μm. Powders produced at high oil contents showed a strong tendency to clump. Comparable effects were observed for a spray drying process with a different nozzle at 250 bar. The results confirm that droplet breakup and coalescence during atomization and coalescence during drying have to be taken into consideration when targeting specific oil droplet sizes in the product. This is relevant for product design in spray drying applications, in which the oil droplet size in the powder or after its redispersion determines product quality and stability.

Verification of diesel spray ignition phenomenon in dual-fuel diesel-piloted premixed natural gas engine

Dual-fuel (DF) engines, in which premixed natural gas and air in an open-type combustion chamber is ignited by diesel-fuel pilot sprays, have been more popular for marine use than pre-chamber spark ignition (PCSI) engines because of their superior durability. However, control of ignition and combustion in DF engines is more difficult than in PCSI engines. In this context, this study focuses on the ignition stability of n-heptane pilot-fuel jets injected into a compressed premixed charge of natural gas and air at low-load conditions. To aid understanding of the experimental data, chemical-kinetics simulations were carried out in a simplified engine-environment that provided insight into the chemical effects of methane (CH4) on pilot-fuel ignition. The simulations reveal that CH4has an effect on both stages of n-heptane autoignition: the small, first-stage, cool-flame-type, low-temperature ignition (LTI) and the larger, second-stage, high-temperature ignition (HTI). As the ratio of pilot-fuel to CH4entrained into the spray decreases, the initial oxidization of CH4consumes the OH radicals produced by pilot-fuel decomposition during LTI, thereby inhibiting its progression to HTI. Using imaging diagnostics, the spatial and temporal progression of LTI and HTI in DF combustion are measured in a heavy-duty optical engine, and the imaging data are analyzed to understand the cause of severe fluctuations in ignition timing and combustion completeness at low-load conditions. Images of cool-flame and hydroxyl radical (OH*) chemiluminescence serve as indicators of LTI and HTI, respectively. The cycle-to-cycle and spatial variation in ignition extracted from the imaging data are used as key metrics of comparison. The imaging data indicate that the local concentration of the pilot-fuel and the richness of the surrounding natural-gas air mixture are important for LTI and HTI, but in different ways. In particular, higher injection pressures and shorter injection durations increase the mixing rate, leading to lower concentrations of pilot-fuel more quickly, which can inhibit HTI even as LTI remains relatively robust. Decreasing the injection pressure from 80 MPa to 40 MPa and increasing the injection duration from 500 µs to 760 µs maintained constant pilot-fuel mass, while promoting robust transition from LTI to HTI by effectively slowing the mixing rate. This allows enough residence time for the OH radicals, produced by the two-stage ignition chemistry of the pilot-fuel, to accelerate the transition from LTI to HTI before being consumed by CH4oxidation. Thus from a practical perspective, for a premixed natural gas fuel–air equivalence-ratio, it is possible to improve the “stability” of the combustion process by solely manipulating the pilot-fuel injection parameters while maintaining constant mass of injected pilot-fuel. This allows for tailoring mixing trajectories to offset changes in fuel ignition chemistry, so as to promote a robust transition from LTI to HTI by changing the balance between the local concentration of the pilot-fuel and richness of the premixed natural gas and air. This could prove to be a valuable tool for combustion design to improve fuel efficiency or reduce noise or perhaps even reduce heat-transfer losses by locating early combustion away from in-cylinder walls.

Modeling Drying Behavior of an Aqueous Chitosan Single Droplet Using the Reaction Engineering Approach.

Spray drying of Chitosan solutions to prepare microparticles either using pilot or industrial scale spray dryer is a complex process; tracking morphological changes and obtaining drying kinetics of a single droplet would be very difficult. The acoustic levitator being a non-intrusive method is a useful experimental apparatus that enables particle/droplet suspension in the gaseous medium and capable of mimicking the drying process in a spray dryer. The drying of chitosan aqueous solutions into solid particles was investigated. The prediction of the size and drying kinetics until the formation of the solid structure was performed in an acoustic levitator. Studying the drying of single droplets is crucial for revealing the influence of the drying process parameters on the formation of dried particles. Droplets with initial chitosan concentration (10, 20, and 30mg/ml) were investigated at different air-drying temperatures. A Reaction Engineering Approach (REA) model was developed and compared with the experimental drying curves, a very well agreement was found between the drying experiments and the REA model with a relative error of about 3% between the initial droplet mass and predicted droplet mass by the REA model.

Visualization of diesel spray and combustion from lateral side of two-dimensional piston cavity in rapid compression and expansion machine

Effect of spray/wall interaction in a rapid compression and expansion machine on mixture formation, ignition location, and soot generation was investigated. A two-dimensional piston cavity designed as the cross section of a reentrant piston was utilized to observe the spray and combustion process from the lateral side. The experiment was conducted at 120 MPa injection pressure under single and split injection strategies with an ambient gas of 15% O2 concentration. A shadow methodology was applied to investigate the interaction between the fuel spray and the piston cavity. Combined with the natural flame luminosity captured by a high-speed color video camera, the behaviors of the impinging spray and the combustion process were studied. The combustion characteristics of the in-cylinder pressure, heat release and combustion phase were recorded and analyzed simultaneously. The results showed that the split injection strategies effectively softened the heat release trace and promoted the onset of the main combustion. The cool-flame phenomenon was captured by using the high-speed color video camera, and the intense ignition was observed when the pilot spray was controlled to impinge on the lower lip of the piston rim. Moreover, results also showed that further extending the mixing process of the pilot spray is inclined to form a homogeneous mixture which was beneficial for the promotion of low-temperature combustion and the reduction of soot generation. This research provides a detailed investigation on the spray and combustion process and it highlights the significant effect of spray/wall interaction on the subsequent combustion process.

Spray technology applications of xanthan gum-based edible coatings for fresh-cut lotus root (Nelumbo nucifera)

In this study, the effect of spraying method as an application technique for xanthan gum-based edible coatings was investigated, based on its barrier and microbial properties on fresh-cut lotus root. Xanthan gum solutions (0.1%, 0.3%, and 0.5%) were prepared and incorporated with 2% (w/w) citric acid as an anti-browning agent and 1% (w/w) glycerol as plasticizer. The coatings were then sprayed using a pilot spray system to 5 mm-thick slices of fresh-cut lotus root for 20 s, packed in polyethylene bags, stored for 16 d at 5 °C and analyzed for color, pH, morphology and microbial counts. It was found that spray-coated fresh-cut lotus root samples had significant reduction in the total color changes as compared to non-coated samples. The experimental results suggested that the spray coating treatments were effective in decreasing the enzymatic browning of fresh-cut lotus root during storage which could potentially increase its shelf-life in the market. In addition, we have also found that the xanthan gum-based spray coated treatments were also effective against inhibiting the growth of Bacillus subtilis during 24 h of incubation which were indicated by the lower microbial counts recorded as compared to non-coated fresh-cut lotus root samples. In this part of the work, the author highlighted the spray coating technique of xanthan gum-based edible coatings as a promising strategy in improving the storage stability of fresh-cut lotus root during post-harvest storage. Overall, the application of edible coatings is a promising strategy in extending the shelf life of fresh-cut lotus root. In the future, the author aims to widen the scope of the application of these coatings to other agricultural products which are prone to degradation during storage in the market.

Stickiness and agglomeration of blackberry and raspberry spray dried juices using agave fructans and maltodextrin as carrier agents

The present study shows the effect of agave fructans as a carrier agent compared with maltodextrin to evaluate the particle stability of blackberry and raspberry juices. A pilot spray dryer was used with feed flow of 20 mL h–1 and atomization rate of 28 000 rpm. The inlet air temperature of 180 °C and outlet air temperature of 80 °C were used as parameter constants. Only the parameters of the carrier agent concentration of 5, 7.5 to 10% (w/v) were changed. The concentration of 10% agave fructans was high enough to recover the higher yields of 89% only for blackberry, for raspberry the concentration of 7.5% agave fructans was sufficient to recover the yield of 67%. The stability diagrams show the conditions of the particles that should not be exceeded when leaving spray drying, as well as the storage conditions that must be followed to avoid agglomeration.

X-Ray Phase Contrast Imaging of Liquid Film and Spray Development Inside an Aircraft Engine Swirler

Abstract Modern aircraft engines combine liquid fuel and air using an intricate flow device with many fuel and air flow passages. To date, the process by which the fuel atomizes within this swirler set has not been examined directly due to optical access limitations. In this work, high-speed X-ray phase-contrast imaging of a liquid spray inside a gas turbine engine swirler geometry is presented. Measurements were carried out at the 7-BM beamline of the Advanced Photon Source at Argonne National Laboratory using the high-energy broadband X-ray beam. The synchrotron X-ray source provides the necessary photon energy and flux to capture time-resolved fluid phenomena within the confines of the relevant geometry while liquid and air are flowing. Spray nozzle hardware and geometries were provided by the National Jet Fuels Combustion Program (NJFCP), allowing for characterization of the spray using a commercially relevant configuration. Modified swirlers were three-dimensional printed with acrylic to improve imaging access while maintaining influential internal features. Water was used as a surrogate fluid for these studies to demonstrate the visualization capabilities. The experiments were conducted at atmospheric exit pressure conditions with a pressure drop of 6% across the swirler. High-speed imaging of the pilot spray cone revealed sheet breakup several millimeters downstream of the orifice exit, upon interaction with the radial assist air flow. These droplets and ligaments were observed to impinge on the inner filming surface of the swirler and flow toward the exit while developing a tangential flow. Under these conditions, the liquid film grows up to several hundred microns in thickness on the filming surface, and subsequently forms ligaments up to several millimeters in length before breaking up. This work demonstrates the capability of X-ray diagnostics in visualizing liquid flows within solid geometries of technical relevance. Furthermore, the spatial quantification of filming flows and liquid interaction with the swirler air provides validation data for modeling of the multiphase flows and surface interactions within the swirler.

Transient prediction capabilities of a novel physics-based ignition delay model in multi-pulsed direct injection diesel engines

This work is an extension of a novel physics-based ignition delay modeling methodology previously developed by the authors to predict physical and chemical ignition delays of multiple injections during steady operations in diesel engines. The modeling methodology is refined in this work to consider the influence of additional operating parameters such as volumetric efficiency, exhaust temperature and pressure on the ignition delay of multiple injections. Computational fluid dynamics predictions on two different engines indicated that the main spray encounters local temperatures about 60 K above average temperatures for about 1 mg of pilot. Hence, the modeling methodology was further refined to include this effect by considering the air mass trapped in pilot spray, computed based on the spray penetration and cone angle and tuned using results of the computational fluid dynamics studies. Comparisons of the ignition delay predictions with the stock boost temperature sensor and a specially incorporated, transient-capable fine wire thermocouple indicated that the measurements with stock sensor could be satisfactorily used for transients. Cycle-by-cycle changes in ignition delay could be predicted accurately when transients were imposed in boost pressure, rail pressure and main injection quantity in a turbocharged intercooled diesel engine controlled with an open engine control unit. Further validations were done even under a transient cycle when the engine was controlled by its stock engine control unit. The same tuning constants could be used for the prediction of the ignition delay under transients on another naturally aspirated engine. This indicates the suitability of the model for application in different engines. Finally, the model was incorporated within an open engine controller, and cycle-by-cycle prediction of ignition delays of the pilot and main injections were done in real time. It was possible to compute the ignition delays in less than 2 ms within engine control unit using the already available sensor inputs within an error band of ±60 µs.

A large-eddy simulation study on the influence of diesel pilot spray quantity on methane-air flame initiation

The present study is a continuation of the previous work by Kahila et al. (2019), in which a dual-fuel (DF) ignition process was numerically investigated by modeling liquid diesel-surrogate injection into a lean methane-air mixture in engine relevant conditions. Earlier, the injection duration (tinj) of diesel-surrogate exceeded substantially the characteristic autoignition time scale. Here, such a pilot spray ignition problem is studied at a fixed mass flow rate but with a varying tinj. The focus is on understanding the influence of pilot quantity on spray dilution process and low- and high-temperature chemistry. In total, ten cases are computed with multiple diesel pilot quantities by utilizing a newly developed large-eddy simulation/finite-rate chemistry solver. The baseline spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration, enabling an extensive validation of the present numerical models and providing a reference case for the DF computations. Additionally, experimental results from a single-cylinder laboratory engine are provided to discuss the ignition characteristics in the context of a real application. The main results of the present study are: (1) reducing tinj introduces excessive dilution of the DF mixture, (2) dilution lowers the reactivity of the DF mixture, leading to delayed high-temperature ignition and slow overall methane consumption, (3) low enough pilot quantity (tinj < 0.3 ms) may lead to very long ignition delay times, (4) cumulative heat release is dominated by low/high-temperature chemistry at low/high tinj values, (5) analysis of the underlying chemistry manifold implies that the sensitivity of ignition chemistry on mixing is time-dependent and connected to the end of injection time, and 6) long ignition delay times at very low tinj values can be decreased by decreasing injection pressure.