Higher Jet Velocity Research Articles

Numerical investigation on the liquid jet and the dynamics of the near-wall cavitation bubble under an acoustic field

The dynamics of the near-wall cavitation bubble in an acoustic field are the fundamental forms of acoustic cavitation, which has been associated with promising applications in ultrasonic cleaning, chemical engineering, and food processing. However, the potential physical mechanisms for acoustic cavitation-induced surface cleaning have not been fully elucidated. The dynamics of an ultrasonically driven near-wall cavitation bubble are numerically investigated by employing a compressible two-phase model implemented in OpenFOAM. The corresponding validation of the current model containing the acoustic field was performed by comparison with experimental and state-of-the-art theoretical results. Compared to the state without the acoustic field, the acoustic field can enhance the near-wall bubble collapse due to its stretching effect, causing higher jet velocities and shorter collapse intervals. The jet velocity in the acoustic field increases by 80.2%, and the collapse time reduces by 40.9% compared to those without an acoustic field for γ = 1.1. In addition, the effects of the stand-off distances (γ), acoustic pressure wave frequency (f), and initial pressure (p*) on the bubble dynamic behaviors were analyzed in depth. The results indicate that cavitation effects (e.g., pressure loads at the wall center and the maximal bubble temperature) are weakened with the increase in the frequency (f) owing to the shorter oscillation periods. Furthermore, the maximum radius of bubble expansion and the collapse time decrease with increasing f and increase with increasing p*. The bubble maximum radius reduces by 12.6% when f increases by 62.5% and increases by 20.5% when p* increases by 74%.

Optimization of collection efficiency for a dual-row jet mining machine based on CFD-DEM coupling

Deep-sea mineral resources possess vast reserves; yet, the depositional environment is extremely harsh, and the mining challenges are significant. Overcoming the technical hurdles of deep-sea mining and protecting the marine environment to achieve sustainable development and utilization of seabed resources are major challenges in the field of marine resource development. In this work, we employ a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) approach with the objective of enhancing the collection efficiency of deep-sea polymetallic nodules. The research focuses on adjusting and optimizing the collection parameters of a dual-jet collector. We examine the influence of the collector's jet angle and velocity on the collection efficiency. We conduct a numerical analysis of the movement characteristics of nodules, the distribution of the flow field, and the efficiency of nodule collection. The results indicate that when the rear jet angle is too small, the stripping and blocking effects on the nodules deteriorate. Conversely, when the jet angle is too large, the upwelling flow velocity decreases. The optimal collection effect is achieved with a rear jet angle of 55°. Lower jet velocities result in poorer nodule stripping, collection, and lifting. Higher jet velocities, however, increase seafloor disturbance and reduce the device's energy utilization efficiency. The collection efficiency is optimal at jet velocities ranging from 8 to 9 m/s. This research provides valuable insights for the design of high-efficiency dual-jet collector devices.

Effect of pre-chamber volume and nozzle number on turbulent jet combustion characteristics of GDI engines

There are few studies on the effect of turbulent jets on the combustion characteristics of passive pre-chamber (PC) gasoline engines, and their mechanism needs to be elucidated. Therefore, a 3-D simulation was performed using CONVERGE software. The results reveal that a modest expansion in the PC volume contributes to combustion due to the increased jet energy, but when it is too large, the above effect is weakened. In this study, at a smaller 0.8 ml volume, the combustion is still similar to traditional spark plug ignition. At middle 1.6 ml volume, the mixture concentration in the PC increases and the uniformity is better, resulting in the highest jet velocity and shortest jet duration, thus achieving the fastest flame propagation and the highest heat release rate (HRR). However, at a larger 1.8–2.0 ml volume, the mixture concentration near the spark-plug is sharply reduced, thus the reduced combustion rate in the PC leads to insufficient jet energy in the subsequent jets and resulting deterioration in combustion. An appropriate increase in nozzle number also improves combustion by introducing more ignition sources. But more nozzles not always improving the combustion. In the eight-nozzle case, three nozzles are close to the exhaust valve so that the intake is inhibited, resulting in a worsening of the ignition. The combustion under six-nozzle shows the highest HRR and shortest combustion duration. However, the combustion deteriorates at eight-nozzle, since the increase in the ignition point is not enough to compensate for the negative impact of the reduced jet intensity on combustion.

Liquid breakup and droplets behavior of free triple-impinging jets with different impinging distance

The breakup characteristics and droplet behavior of free triple-impinging jets (FTIJs) were investigated using a high-speed camera. The liquid sheet and droplets breakup mechanism, liquid sheet breakup length, width, droplet diameter and velocity, were studied under different jet velocity and horizontal impinging distance and perpendicular impinging distance. The results revealed that with increasing jet velocity from 1.42 m/s to 6.61 m/s, different breakup modes were observed: closed-rim mode, open-rim mode, rimless mode, and wave or ligament mode. At low jet velocities, increasing impinging distance delayed the development of breakup mode. Based on experimental observations, the sheet breakup mechanism is categorized into two main regimes with four subregimes. Droplet breakup occurs in two stages: growth and necking, however, at high jet velocities complete breakup was achieved. Increasing jet velocity led to an increase in liquid sheet breakup length, width, and droplet velocity but a decrease in droplet diameter. Conversely, increasing impinging distance resulted in a decrease in liquid sheet breakup width and droplet radial velocity but an increase in droplet diameter. The effect of perpendicular impinging distance on axial velocity and breakup length was found to be more significant than that of horizontal impinging distance due to the effect of perpendicular nozzle. The findings indicate that the perpendicular jet greatly enhances atomization. This study provides a theoretical basis for designing and optimizing FTIJs.

Dynamics of regime transition of autoignitive jet flames from conventional to MILD combustion

Turbulent combustion of jet flames in a hot diluted coflow of combustion products has been studied across a range of jet Reynolds numbers for propane, ethylene, and an ethylene-propane blend as fuels. The study revealed a transition from conventional autoignitive combustion to a regime of Moderate or Intense Low-oxygen Dilution (MILD) combustion, which is characterized by a nearly invisible flame. The flames studied are luminous at low jet velocities and become MILD at higher jet velocities. Planar Laser-Induced Fluorescence (PLIF) of formaldehyde (CH2O), a key intermediate species in hydrocarbon combustion, is combined with CH*-chemiluminescence imaging and Rayleigh scattering to investigate the phenomena. The transition to MILD combustion is found to accompany a broadening of the formaldehyde region, indicating a broader low-temperature reaction zone. As the transition is approached, the appearance of holes in the chemiluminescent front is also observed. Correspondence between the location and structure of these holes with regions of formaldehyde in the flame are illustrated. These regions are proposed to be signatures that precede the transition to a fully MILD flame. In other words, the transition to MILD combustion begins at a local level and the MILD region spreads to engulf the entire combusting mixture. Scalar gradients were imaged to further unravel the local turbulence/chemistry interactions that yield the MILD inception. The measured scalar gradients were found to be commensurate with scalar gradients obtained from chemical kinetic simulations at the stoichiometric mixture fractions, while being an order of magnitude larger at other locations; this suggests that the inception of MILD combustion occurs near the stoichiometric region.

Characterization of the hypergolic ignition of green fuels with hydrogen peroxide by drop test and jet impingement

This work presents an experimental study on the hypergolic behavior of variants of N,N,N’,N’-tetramethylethylenediamine (TMEDA)/N,N-dimethylethanolamine (DMEA) system, named PAHyp 0, with 93 wt% hydrogen peroxide. Two different copper-based catalyst and one cobalt-based catalytic agent were dissolved in the fuels to trigger the pre-ignition reactions. When employing copper chloride dihydrate, the resultant fuel requires methanol (ternary system) as a stabilizing agent, whereas both carboxylic salts of copper and cobalt dissolved in TMEDA/DMEA deliver a stable formulation (binary system) without the need for any organic solvent. Shadowgraph high-speed imaging was used to capture the ignition process and pressure wave development of over 30 fuel samples in a modified drop test. An ignition/stability envelope was proposed to map the safe and unstable formulations. It was demonstrated that the concentration of TMEDA should be no more than 50% in the binary system, while the amount of methanol should fall within a concentration window of 33 to 50% in the ternary system. Lower loadings of catalyst, ranging from 0.5 to 1.0 wt%, are required to deliver stable fast-igniting fuels. Three promising formulations catalyzed with 1 wt% copper salts were selected to perform impinging jet fire experiments. Impinging jets yielded significantly lower ignition delay times than drop tests due to the better mixing conditions. However, it was verified that ignition may fail for low values of jet velocities, below 0.8 m/s, and for high jet velocities, exceeding 14 m/s. An attempt to explain the ignition and non-ignition events was made by using the fundamentals of explosion limits of the hydrogen/oxygen system and oxidation reactions at low temperature. Remarkably, ultra-fast ignitions of about 10 ms were measured by the impinging jet setup within near-optimal operational conditions. Therefore, TMEDA/DMEA-based fuels with reduced catalyst loadings (0.5–1.0 wt%) offer new opportunities in aerospace propulsion and should be further studied.

Future zero carbon ammonia engine: Fundamental study on the effect of jet ignition system characterized by gasoline ignition chamber

Ammonia is a carbon-free fuel with tremendous potential for clean internal engine applications in the future. However, the combustion and emissions limitations of ammonia fuel have impeded the development of ammonia engine. As a combustion enhancement technology, the ignition chamber (pre-chamber) jet ignition system has emerged as an effective solution to address the challenges associated with ammonia combustion. This study utilized a high-speed camera to capture the evolution of jet and the combustion processes of ammonia. The combustion method entailed the injection of gasoline into the ignition chamber, while ammonia was injected into the main chamber. The experimental results demonstrated that ignition chamber jet ignition system significantly enhanced ammonia combustion and shortened the combustion duration as compared to spark plug ignition system. The study involved evaluating the ammonia combustion performance under different equivalence ratios (1.0 and 0.8) while comparing it to various gasoline energy percentages (2.5%, 2.0%, 1.5%, and 1.0%). The results revealed that the combustion performance at 1.5% was superior to other gasoline energy percentages. Additionally, in comparison to ignition chamber outlet diameter of 4.5 mm and 6.0 mm, it was found that the 3.0 mm diameter exhibited weak ignition capability, resulting in a 107.1% and 40.3% increase in ignition delay at the equivalence ratio of 0.8, and ignition failure at the equivalence ratio of 0.6. However, its high jet velocity induced a more homogeneous mixing of radicals with ammonia/air, leading to a 30.6% reduction in rapid combustion and a 43.1% decrease in combustion duration at the equivalence ratio of 0.8. Additionally, the investigation of equivalence ratios (0.8, 1.0, and 1.1) demonstrated that the fastest initial combustion of ammonia occurred at the equivalence ratio of 0.8, with an ignition delay of only 4.7ms. Therefore, an appropriate reduction in the equivalence ratio could enhance the ammonia combustion efficiency to some extent. The findings of this study provide a fundamental ignition technique for future applications of zero carbon ammonia engines.

Mechanism of jet velocity affecting surface quality based on electrode dynamics under temperature field

Electrolyte jet machining (EJM) is a rapid method for preparing metal surface microstructures. The local temperature changes at the anode interface caused by electrochemical heat and jet velocity are to be determined, and the mechanism of the effect of different jet velocities on the surface quality of the anode is not clear. Therefore, an EJM simulation model considering temperature was developed, and the etching profiles were compared with and without the effect of temperature. The simulation results show that considering the temperature field can improve the prediction accuracy of the simulation. In addition, simulation and experimental results show that high electrochemical heat is favourable for increasing the conductivity of the electrolyte and generating a high-quality surface. Enhanced convective heat dissipation at high jet velocities reduces the conductivity of the electrolyte in the machining area, generating low-quality surfaces. It is worth noting that excessively low jet velocity can accumulate anode electrolysis products at the interface, affecting machining accuracy. This paper analyzes the mechanism of the influence of different jet velocities on the surface quality of an anode in combination with the electrode dynamics under the influence of the temperature field. This analysis is crucial for achieving high surface quality in EJM.

Ignition characteristics of the high-velocity pulverized coal jet in MILD combustion mode: Experiments and prediction improvements

The influence of high jet velocity on the ignition characteristics of pulverized coal is very important for the Moderate or Intense Low-oxygen Dilution (MILD) combustion technology of pulverized coal. This paper conducts experimental and numerical simulation research on the ignition characteristics of the pulverized coal jet in a high-temperature environment based on the Hencken-type flat flame burner. The results show that when the ambient temperature is 1873 K and the ambient oxygen mole fraction is 5 %, the ignition delay distance of the pulverized coal jet decreases as the jet velocity increases from 15 m/s to 100 m/s for the formation of MILD combustion. The adoption of the turbulent particle heat transfer model and the non-spherical particle motion model effectively improves the prediction of particle dispersion, ignition delay, and reaction zone in the high-velocity coal jet flame. The enhancement of gas–solid heat transfer by turbulent fluctuation plays a dominant role in advancing the coal ignition in the high-velocity jet, while the non-spherical shape of coal particles mainly improves the uniformity of reaction distribution. The influence of ignition characteristics in the high-velocity jet on the MILD combustion formation of pulverized coal has been also analyzed in detail.

Effect of ammonia addition on combustion characteristics of hydrogen/air using passive turbulent jet ignition

Hydrogen (H2) and ammonia (NH3) are ideal carbon-free fuels, and the application of NH3/H2 in internal combustion engines offers the potential to reduce carbon emissions. And turbulent jet ignition (TJI) is an advanced ignition strategy that can effectively enhance the combustion. This work aims to experimentally investigate the effect of NH3 addition on the combustion characteristics of H2/air mixtures under passive TJI conditions. Combustion images and instantaneous pressure were collected for processing and analysis. The effect of NH3 substitution and equivalence ratio were investigated. The results indicate that the peak combustion pressure decreases with the addition of NH3, accompanied by longer combustion duration and ignition delay, as well as lower jet velocity and flame area growth rate. The hot jet will be quenched by the strong heat transfer, causing the change in the ignition mechanism as the NH3 substitution ratio increases. In addition, compared to lean conditions, the combustion appears to be less sensitive to the equivalence ratio on the rich side and always shows high jet velocity and flame propagation speed. However, a relatively small amount of NH3 addition changes the ignition mechanism on the rich combustion side compared with lean conditions, resulting in a longer ignition delay.

Modeling of reactive species interphase transport in plasma jet impinging on water

The interaction between low-temperature atmospheric pressure plasma and water is of primary relevance to an increasing number of applications, from water treatment to medicine. The interaction between an argon plasma jet and water is investigated using a three-dimensional (3D) time-dependent computational model encompassing turbulent gas flow and induced liquid motion, gas–water interface dynamics, multiphase species transport, and gas- and liquid-phase chemical reactions. A single-field approach based on the volume-of-fluid (VoF) method together with conditional volume averaging (CVA), is used to consistently describe the dynamics of the interface together with interfacial reactive mass transfer. Three CVA-based interface species transport models, based on arithmetic, harmonic, and unified mixture species diffusivities, are evaluated. Simulations of a plasma jet impinging on water at different gas flow rates are presented. The resulting deformation of the interface and the production and accumulation of hydrogen peroxide, reactive oxygen, and nitrogen species corroborate prior findings in the research literature showing that higher jet velocities and associated increased interface deformation led to the enhanced transport of reactive species across the plasma-water interface. The VoF-CVA approach appears promising for the modeling of general plasma-liquid multiphase systems.

Computational analysis of influence of CFJ components on aerodynamic performance

This research aims to explore the effect of the co-flow jet (CFJ) airfoil injection jet velocity, injection height, and injection mass flow rate on the aerodynamic coefficient through numerical analysis. Five distinct circumstances are generated by adjusting the jet velocity, injection height, and mass flow rate, and each of the cases underwent numerical investigation. For this computational analysis, Reynolds averaged Navier–Stokes (RANS) solver has been utilized by employing the Spalart–Allmaras turbulence model, and the flow is treated as incompressible. The higher injection slots reduce the coefficient of lift (CL) due to poor aerodynamic shape, which can be overcome by injecting higher jet velocities, whereas lower jet velocities injecting into lower injection slots do not raise the CL even though they have streamlined aerodynamic shape. Hence, even if CFJ airfoils with poor geometry perform less aerodynamically, this problem is not intractable because it can be solved by injecting high jet velocities; meanwhile, injecting low velocities will result in reduced aerodynamic performance for CFJ airfoils with good geometry. The CFJ airfoil with the highest jet velocity and lowest injection slot increases the CL by a maximum of 65% compared to the baseline airfoil, which is higher than all other CFJ airfoils taken into consideration.

Association of haemolysis markers, blood viscosity and microcirculation function with organ damage in sickle cell disease in sub-Saharan Africa (the BIOCADRE study).

Sickle cell anaemia (SCA) is a monogenic disease with a highly variable clinical course. We aimed to investigate associations between microvascular function, haemolysis markers, blood viscosity and various types of SCA-related organ damage in a multicentric sub-Saharan African cohort of patients with SCA. In a cross-sectional study, we selected seven groups of adult patients with SS phenotype in Dakar and Bamako based on the following complications: leg ulcer, priapism, osteonecrosis, retinopathy, high tricuspid regurgitant jet velocity (TRV), macro-albuminuria or none. Clinical assessment, echocardiography, peripheral arterial tonometry, laboratory tests and blood viscosity measurement were performed. We explored statistical associations between the biological parameters and the six studied complications. Among 235 patients, 58 had high TRV, 46 osteonecrosis, 43 priapism, 33 leg ulcers, 31 retinopathy and 22 macroalbuminuria, whereas 36 had none of these complications. Multiple correspondence analysis revealed no cluster of complications. Lactate dehydrogenase levels were associated with high TRV, and blood viscosity was associated with retinopathy and the absence of macroalbuminuria. Despite extensive phenotyping of patients, no specific pattern of SCA-related complications was identified. New biomarkers are needed to predict SCA clinical expression to adapt patient management, especially in Africa, where healthcare resources are scarce.

A numerical study on the structure and dynamics of fuel-rich premixed H2–air jet flames above a divergent micro-nozzle

In this study, the flame structures of a fuel-rich premixed H2–air mixture issued from a divergent micronozzle were numerically studied and compared with their counterparts. The flame dynamics of the divergent micronozzle at relatively low jet velocities were also analyzed. A flame pattern of “dual flames with a smaller one surrounded by a larger one” was discovered for both the straight and divergent micro-nozzles at high jet velocities. This flame structure began to appear at a lower jet velocity (Vin) as the divergence angle (θ) decreased. Specifically, it first appeared at Vin = 7.5 and 20 m/s for θ = 0° and 5°, respectively. In addition, a flame mode consisting of dual flames with a smaller one inside the nozzle occurred only for divergent micronozzles under low and medium jet velocities. This flame structure could emerge at a higher jet velocity as the divergence angle increased. For example, it could occur until Vin = 5 m/s and 17.5 m/s for θ = 1° and 5°, respectively. A global map of all flame patterns was drawn based on the results obtained by varying the jet velocity and divergence angle. Moreover, periodic flame dynamics appeared when the jet velocity was less than the critical value (<1.7 m/s). Specifically, a small flame (i.e., secondary flame) split from the jet flame (i.e., main flame) and propagated towards the nozzle inlet; however, its propagation direction was inverted and it finally extinguished owing to the heat loss to the nozzle wall. The next cycle was initiated after a short duration. The analysis revealed that the periodic flame dynamics are a result of the thermal interactions between the flame and nozzle wall.

Inhibiting flow-accelerated copper corrosion under liquid jet impingement by utilizing nanobubbles

Flow-accelerated corrosion can be very destructive, leading to rapid loss of component efficiency and eventual failure of the system; however, finding inexpensive, effective, and chemically benign inhibitors is still challenging. This paper studied for the first time the use of nanobubbles/ultrafine bubbles as a “green” functional nanostructured material for inhibiting flow-accelerated corrosion under the impingement of a turbulent liquid jet. We examined the corrosion inhibition of the copper specimen for 5 h in the flow regime at a Reynolds number of from 6.1 × 103 to 3.0 × 104, with adding ∼6 × 107 bubbles/mL of air-nanobubbles to the tested liquid of 0.25% CuCl2 solution at 40 °C. We measured weight losses, analyzed microstructure using SEM, and acquired maximum erosion depths and roughness curves of tested copper specimens. We found that nanobubbles mitigated flow-accelerated corrosion under jet impingement. The inhibition effectiveness was more significant at higher jet velocity and longer test time, with up to 43.9 ± 0.1% and 69.5 ± 1.8% based on weight losses and erosion depths, respectively. Nanobubbles play a role in reducing wall shear stress on the copper surface, likely by generating bubble mattresses. We suggest that nanobubbles inhibit the corrosion in highly erosive/corrosive turbulent conditions where most inhibitors could not work well.

Effect of the primary oxidizer stream jet velocities on the ignition and combustion characteristics of pulverized coal under MILD oxy-coal combustion conditions

MILD oxy-coal combustion technology is a prospective technology with the potential for CO2 capture and storage, and it is characterized by the intense flue gas recirculation to preheat the fuel and to dilute the oxidizer due to the oxidizer or fuel jet with high velocity. The effect of the primary oxidizer stream jet velocities ranging from 1.7 m/s to 11.2 m/s on the coal ignition and combustion characteristics were experimentally investigated on a flat flame pulverized coal burner at various oxygen concentrations of 5%-21% and gas temperatures of 1473–1873 K under O2/CO2 and O2/N2 atmospheres. The coal flame images were recorded by the digital camera, and the flame structure evolution and ignition delay times of coal particles were analyzed at different primary oxidizer stream jet velocities and hot coflow conditions. The coal combustion radiation spectrum was monitored by an optical fiber spectrometer and the coal particle temperatures were derived with the two-color pyrometry. The results showed that the bright yellow soot flame gradually disappeared with the decrease of O2 concentration when CO2 replaced N2. The difference of ignition delay times between O2/CO2 atmosphere and O2/N2 atmosphere decreased from 2.8 ms to 0.2 ms when the primary air jet velocity increased from 1.7 m/s to 11.2 m/s at 1473 K and 5% O2. This meant the elevated jet velocity was beneficial to offset the delayed effect of the physicochemical properties of CO2 on coal ignition. The peak coal particle temperature increased by 68 K at 1873 K and 5% O2 under O2/CO2 atmosphere with the highest jet velocity owing to the accelerated diffusion of O2 into coal particle stream. Moreover, a dimensionless coal particle temperature parameter η was proposed to characterize the variation of the peak coal particle temperature caused by the improved heat transfer and mass transport between the coflow and coal particles under high jet velocity. The maximum value of η was 1.053 at 1873 K and 21% O2 under O2/CO2 atmosphere with the jet velocity of 11.2 m/s, indicating the increased jet velocity had the predominant effect on the increase of peak particle temperature at high gas temperature and O2 concentration.

A Promising Needle-Free Pyro-Drive Jet Injector for Augmentation of Immunity by Intradermal Injection as a Physical Adjuvant.

Current worldwide mRNA vaccination against SARS-CoV-2 by intramuscular injection using a needled syringe has greatly protected numerous people from COVID-19. An intramuscular injection is generally well tolerated, safer and easier to perform on a large scale, whereas the skin has the benefit of the presence of numerous immune cells, such as professional antigen-presenting dendritic cells. Therefore, intradermal injection is considered superior to intramuscular injection for the induction of protective immunity, but more proficiency is required for the injection. To improve these issues, several different types of more versatile jet injectors have been developed to deliver DNAs, proteins or drugs by high jet velocity through the skin without a needle. Among them, a new needle-free pyro-drive jet injector has a unique characteristic that utilizes gunpower as a mechanical driving force, in particular, bi-phasic pyrotechnics to provoke high jet velocity and consequently the wide dispersion of the injected DNA solution in the skin. A significant amount of evidence has revealed that it is highly effective as a vaccinating tool to induce potent protective cellular and humoral immunity against cancers and infectious diseases. This is presumably explained by the fact that shear stress generated by the high jet velocity facilitates the uptake of DNA in the cells and, consequently, its protein expression. The shear stress also possibly elicits danger signals which, together with the plasmid DNA, subsequently induces the activation of innate immunity including dendritic cell maturation, leading to the establishment of adaptive immunity. This review summarizes the recent advances in needle-free jet injectors to augment the cellular and humoral immunity by intradermal injection and the possible mechanism of action.

Effect of Jet Velocity on the Formation of Moderate or Intense Low-Oxygen Dilution Combustion of Pulverized Coal.

Moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal is regarded as a new combustion technology with great potential due to its advantages of reducing NO x emission and improving the uniformity of heat flux in the furnace. Increasing the jet velocity has been proved to be an important technical means to achieve MILD oxy-coal combustion, especially without a high level of preheat, but the transition mechanism of coal combustion modes with the increase of jet velocity is not clear enough. In this work, a high-velocity coal-laden jet combustion system on the basis of a flat-flame burner was designed to study the effect of jet velocity on the formation of MILD combustion of pulverized coal. The combustion mode transition of the pulverized coal jet is revealed by analyzing the flame structure, temperature, radiation, and reaction intensity through a variety of optical measurement methods. Theoretical criteria for combustion mode were applied to predict the formation of MILD combustion and were validated by the experimental data. In the environment with an ambient temperature of 1600 °C, the transition velocity of the coal jet into the MILD combustion regime is about 50 m/s and about 100 m/s at 5 and 15% O2 molar fraction, respectively. The dilution effect, jet entrainment, and turbulent mixing in the high-velocity jet, as the key factors to achieve an MILD combustion regime, were analyzed theoretically and experimentally. The dilution effect inherent in the jet significantly reduces the reactant concentration and ultimately reduces the reaction intensity and flame brightness while the entrainment of the jet promotes the radial dispersion of particles and the flame uniformity, which is dominant at lower jet velocities. Strong turbulent mixing promotes the ignition and volatile combustion, which is dominant at higher jet velocities.

Optimized Film Cooling Flow on a Contoured Endwall Within a Transonic Annular Cascade

AbstractFilm cooling was measured on the endwall of a five-vane annular cascade in a blowdown wind tunnel at an exit Mach number of 0.9. The adiabatic film cooling effectiveness was calculated from the partial pressure of oxygen measured with binary pressure-sensitive paint (BPSP). Cylindrical film cooling holes were located in the upstream and passage regions with the coolant-to-mainstream mass flow ratio (MFR) independently varied for each region. One row was located upstream of the vanes and supplied by an upstream plenum. Two rows were located in the passage between two vanes and supplied by a downstream plenum. Three total MFRs were investigated: 1%, 1.5%, and 2%. For a given total MFR, four combinations of upstream and downstream MFRs were compared to an even split of coolant. Coolant-to-mainstream density ratios (DRs) of 1.0 and 2.0 were investigated. The most efficient use of coolant hinged on balancing the downstream MFR for the second row due to the endwall pressure gradient preventing coolant from exiting the holes or a high jet velocity causing liftoff. For this row, selecting the optimum MFR increased the area-averaged film cooling effectiveness by up to 200% with a reduction in row 1 of less than 25%. At high downstream MFRs, increasing the density ratio delayed liftoff and increased film cooling effectiveness in row 2 by 65%. However, at low MFRs, increasing the density ratio reduced film cooling effectiveness in row 2 by 60%.

Numerical investigation of the effect of reactive gas jets on the flame acceleration and DDT process

Jet obstacles can quickly induce the deflagration to detonation transition (DDT) process and reduce the thrust loss of an engine. However, there are few studies on the use of combustible premixed gases as the reactive jet obstacle. Based on the OpenFOAM platform, a detailed numerical investigation of the flame acceleration and DDT process is carried out with different initial jet velocities and the number of jet obstacles. The results show that, although the stronger flame generated by the reactive jet obstacles will reduce the virtual blockage ratio to a greater extent, the turbulence and combustion heat release effect generated by them still significantly promote the flame acceleration. In terms of flame acceleration, initially, increasing the initial jet velocity has no obvious effect on the flame acceleration. Later, turbulence and combustion heat release begin to dominate the flame acceleration and significantly promote the flame acceleration. Since the jets in this study adopt the same combustible premixed gases, increasing the number of reactive jet obstacles downstream does not improve the upstream flame acceleration. In DDT, increasing the initial velocity of the reactive jet can improve the detonation initiation, but the effect of the obstacle number on DDT is greatly affected by the initial jet velocity. Specifically, at a low initial jet velocity, more jet obstacles can significantly promote the detonation initiation, while at a high initial jet velocity, the enhanced turbulence caused by the jet is the dominant factor for the flame acceleration. Therefore, compared with the number of jet obstacles, the detonation initiation process of the premixed gases is more sensitive to the change in the initial jet velocity.