Poor Mixing Research Articles

Perfluoroalkyl-Functionalized Graphene Oxide as a Multifunctional Additive for Promoting the Energetic Performance of Aluminum.

Aluminum (Al) is a widely used metal fuel for energetic applications ranging from space propulsion and exploration, and materials processing, to power generation for nano- and microdevices due to its high energy density and earth abundance. Recently, the ignition and combustion performance of Al particles were found to be improved by graphene-based additives, such as graphene oxide (GO) and graphene fluoride (GF), as their reactions provide heat to accelerate Al oxidation, gas to reduce particle agglomeration, and fluorine-containing species to remove Al2O3. However, GF is not only expensive but also hydrophobic with poor mixing compatibility with Al particles. Herein, we report a multifunctional graphene-based additive for Al combustion, i.e., perfluoroalkyl-functionalized graphene oxide (CFGO), which integrates the benefits of GO and GF in one material. We compared the effects of CFGO to GO and GF on the ignition and combustion properties of nAl particles using thermogravimetric analysis, differential scanning calorimetry, temperature-jump ignition), Xe flash ignition, and constant-volume combustion test. These experiments confirm that CFGO generates fluorine-containing species, heat, and gases, which collectively lower the ignition threshold, augment the energy release rate, and reduce the combustion product agglomeration of nanosized Al particles, outperforming both GO and GF as additives. This work shows the great potential of using multifunctionalized graphene as an integrated additive for enhancing the ignition and combustion of metals.

Analysis of MONARC and ACTIVATE Airborne Aerosol Data for Aerosol-Cloud Interaction Investigations: Efficacy of Stairstepping Flight Legs for Airborne In Situ Sampling

A challenging aspect of conducting airborne in situ observations of the atmosphere is how to optimize flight plans for specific objectives and constraints associated with weather and flight restrictions. For aerosol-cloud interaction research, two recent campaigns utilized a “stairstepping” approach whereby an aircraft conducts level legs at various altitudes while moving forward with each subsequent leg: the 2019 MONterey Aerosol Research Campaign (MONARC) over the northeast Pacific and the 2020–2022 Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) over the northwest Atlantic. We examine the homogeneity of several atmospheric variables both vertically and horizontally in the marine boundary layer with a focus on the sub-cloud environment. In well-mixed boundary layers, there was generally good horizontal and vertical homogeneity in potential temperature, winds, water vapor mixing ratio, various trace gases, and many aerosol variables. Selected aerosol variables exhibited the most variability owing to sensitivity to humidity and near-cloud conditions (supermicrometer aerosol concentrations), coastal pollution gradients (e.g., organic aerosol mass), and small spatial scale phenomena such as new particle formation (aerosol number concentration for particles with diameter >3 nm). Illustrative cases are described when stairstepping can pose issues requiring extra caution for data analysis: (i) poor vertical mixing and layers decoupled from those below; (ii) multiple cloud layers; (iii) fluctuating cloud base/top and boundary layer top heights; and (iv) horizontal variability across specific features leading to sharp gradients such as right near coastlines and over the Gulf Stream with strong sea surface temperature changes. Results from this study provide a guide both for future studies aiming to examine these mission datasets and for designing new airborne campaigns.

A post-occupancy study of ventilation effectiveness from high-resolution CO2 monitoring at live theatre events to mitigate airborne transmission of SARS-CoV-2

Mass-gathering events were closed around the world in 2020 to minimise the spread of the SARS-CoV-2 virus. Emerging research on the transmission of SARS-CoV-2 emphasised the importance of sufficient ventilation. This paper presents the results of an indoor air quality (IAQ) monitoring study over 82 events in seven mechanically ventilated auditoria to support the UK government Events Research Programme. Indoor carbon dioxide concentration was measured at high resolution before, during, and after occupancy to allow for assessment of the ventilation systems. Generally, good indoor air quality was measured in all auditoria, with average IAQ found to be excellent or very good for 70% of spaces. In some auditoria, spatial variation in IAQ was identified, indicating poor mixing of the air. In addition, surface and air samples were taken and analysed for the presence of bacteria by culture and SARS-CoV-2 using RT-qPCR in one venue. SARS-CoV-2 RNA was detected on a small number of surfaces at very low copy numbers, which are unlikely to pose an infection risk. Under the ventilation strategies and occupancy levels investigated, it is likely that most theatres pose a low risk of long-range transmission of COVID-19.

Numerical Comparative Study on the In-Cylinder Mixing Performance of Port Fuel Injection and Direct Injection Gas-Fueled Engine

In recent decades, research on alternative fuel engines is becoming more and more popular. Compressed natural gas (CNG) has the advantages of abundant reserves and a lower cost. It can reduce vehicle emissions relatively quickly and has little impact on the entire transportation infrastructure. As the fourth generation of a gas fuel supply method, gas fuel direct injection (DI) technology can effectively avoid volumetric efficiency reduction and power reduction problems of the port fuel injection (PFI) method. However, the former’s mixing path and duration are shortened greatly, which often leads to poor mixing uniformity. In order to improve the in-cylinder mixing uniformity, the in-cylinder mixing process of the CNG-fueled engine is taken as the research object in this study. The computational fluid dynamics (CFDs) models of the mixing process for the PFI and DI modes are established, and their mixing uniformities are compared. Besides, based on the authors’ previous research, the influence mechanism of the piston crown shape and fuel injection angle on the mixing process of the CNG DI engine is explored. The results show that the probability distribution frequency (PDF) of the best mixture concentration region (BMCR) is as high as 72% for the PFI mode, which is much higher than for the DI mode. The shorter jet impingement distance of the flat top piston leads to higher turbulent kinetic energy (TKE) intensity, and the in-cylinder mixing uniformity will be improved. When gas fuel is injected into an area with a higher in-cylinder TKE, the average in-cylinder TKE will be higher, and the in-cylinder mixture will be more homogeneous.

Experimental study on the effect of injection pressure ratio on the propagation characteristics of hydrogen rotating detonation waves

AbstractThis study aimed to evaluate the effects of the ratio of air injection pressure to hydrogen injection pressure (pR = pA/pH) on the propagation characteristics of rotating detonation waves (RDWs) and the working range of continuous rotating detonation engines (CRDEs). To this end, experiments were conducted under different injection pressure ratios and air mass flow rates with hydrogen as the fuel and air as the oxidant. The results reveal that RDWs can propagate in a stable manner when the injection pressure ratio is 0.67–1.7; however, RDW propagation is unstable when the injection pressure ratio is 0.51–0.67 and 1.7–2.03. The formation of unstable detonation waves may be attributed to the long response times at low pressures and the poor mixing effect of hydrogen and air. Furthermore, under an injection pressure ratio of 2.08–3.09, CRDE initiation failed. As the air mass flow rate increased, the range of the injection pressure ratio increased and the CRDE operated in a stable manner. In addition, the RDW velocity first increased and then decreased as the injection pressure ratio increased. Notably, under an air injection pressure of 3.6 bar, the rotation direction of the RDW changed during propagation.

Transportability Improvement of a Gas–Liquid Rotodynamic Pump Using the Two-Step Multi-Objective Optimization Strategy

The development of multiphase pumps is restricted because of low efficiency and poor mixing transportation capacity. In this study, a two-step multi-objective optimization design system for a multiphase pump is constructed using a three-dimensional (3D) design theory and multi-objective optimization techniques. The transportability improvement for the pump impeller is achieved by implementing the optimization variables of its geometry and blade loading and the optimization objectives containing the pump efficiency and gas uniformity. The optimal results of impeller geometry show that the blade wrap angle and control parameters of the hub have remarkable effects on the pump efficiency and gas uniformity. The optimal results of impeller blade loading show that the pump efficiency is improved if the impeller with large loading at the leading edge, large slope at the middle part, and large negative high-pressure edge angle. The gas uniformity is improved if the hub loading at the middle and trailing parts is larger than the shroud loading. Compared to the original test impeller, the pump efficiency with the impeller T-Opt1 and the gas uniformity at the impeller outlet are improved by 10.81% and 6.91%, respectively. The maximum fluctuation amplitudes in T-Opt1 and its corresponding guide vane are reduced by 0.61% and 67.76%, respectively.

Development of compartment model for inhibition of thermal runaway in free-radical polymerization

Shortstopping reactions via injection of chemical inhibitors is a promising technique of emergency response to prevent thermal runaway, with unique advantages of cost effectiveness and process sustainability over current evacuation practices. However, computational modeling of the inhibition process for ensuring successful designs remains challenging. In this paper, a compartment model is developed to investigate the design of the inhibition system with detailed characterization in both complex kinetic mechanism and nonideal mixing effect. The reactor volume is divided into a network-of-zones (NoZ), with each zone modeled as a well-mixed reactor. The free-radical solution polymerization of methyl methacrylate (MMA) in a stirred batch reactor is used as a demonstrative example. The flow parameters and network structure of compartment models are validated with the CFD simulation. The developed compartment models are employed to investigate interactions of poor mixing and exothermic chemistry that lead to thermal runaway and key design factors including injection location, quantity, kinetic requirement, and timing on the effectiveness of quenching thermal runaway.

Structural behaviors of steel fiber-reinforced concrete-filled geotextile tube stub columns under axial compression

Geotextile materials are recently widely used for concrete casting formwork system; however, concrete is brittle whose inherent abrupt failure property needs to be improved. In this study, a novel composite stub column that consists of external geotextile tube and internal steel fiber reinforced concrete (SFRC) core is proposed. A total of 36 circular concrete stub columns are tested to analyze the effects of fiber volume fraction and cross-sectional diameter on structural behaviors of geotextile-tubed SFRC under axial compression. Based on the test results, geotextile-tubed SFRC stub columns showed superior properties in terms of strength characteristic, strain capacity, modulus of elasticity, ductility and toughness, while the effect of cross-sectional diameter on structural behaviors of the stub columns became less obvious with increasing fiber volume fraction. Furthermore, the experimental data of SFRC stub columns was collected to evaluate the effect of formwork systems. It is evident that the geotextile tube can effectively solve problems of poor mixing and consolidation of SFRC with a high fiber volume fraction to ensure the concrete properties. Finally, a non-linear calculation model was also developed to evaluate the compressive behaviors for geotextile-tubed SFRC. Therefore, use of geotextile is recommended to applied as formwork system of SFRC members, which can provide researchers with more insights into the construction technique of concrete structures.

Controllable preparation of thio-functionalized composite polysilsesquioxane microspheres in a microreaction system

Controllable preparation of monodispersed composite polysilsesquioxane microspheres (CPSQs) with scalability is required but difficult to realize in a conventional stirred batch reactor because of the reactors’ poor mixing performance and scaling-up effects. A new semi-continuous microreaction system integrating a microreactor with a stirred batch reactor was developed for the synthesis of monodispersed thio-functionalized CPSQs by employing a two-step sol–gel method. Methyltrimethoxysilane and (3-mercaptopropyl)trimethoxysilane were used as silicon sources. The effects of the synthesis variables were systematically studied. The particle size could be adjusted between several hundreds of nanometers and several micrometers with a narrow size distribution (the coefficient of variation was <10%), and the sulfhydryl group (SH) concentration reached 14.13 at.%. Compared with the batch reactor, the semi-continuous microreaction system showed higher synthesis reproducibility and higher potential for the large-scale production of CPSQs.

A bio-inspired flexible squeezing reactor for efficient enzymatic hydrolysis of lignocellulosic biomass for bioenergy production

Wheat straw is a promising source that can be used to obtain clean energy through its bioconversion into biofuel. Enzymatic hydrolysis is considered as a key step in biofuel production. However, the poor mixing of enzymes and lignocellulosic substrates caused by the high viscosity of biomass slurry significantly astricts such a process. In this study, a novel bio-inspired flexible tube reactor was developed for enhancing mass transfer through squeezing at different parts to imitate the intestinal peristalsis. The results of the visualization experiment showed that the apparent vertical diffusion velocity at different position were about 1.4–4.2 times faster than the control without squeezing, when the concentration of model solution (carboxymethyl cellulose solution) was 5 g/L. More importantly, the apparent vertical diffusion velocity increased by 5.2 times as the concentration of model solution increased to 20 g/L at squeezing mode. When the total solid (TS) concentration of lignocellulosic slurry was increased to 7.5%, the bionic reactor showed an excellent performance on the enzymatic hydrolysis of wheat straw. The highest reducing sugar yield achieved 20.2 g/L with 71.8% of carbohydrate conversion after 170 min at 40 rpm, which was 2.1 times higher than the control without squeezing.

Thermal Stress Analysis and Optimization of FSW Process on UHMWPE by Using Triangular Tool Profile

Abstract: The use of polymeric materials has grown widely in various sectors such as packaging, building, electronic, automotive, and aerospace industries. Particularly, Ultra-High Molecular Weight Polyethylene has wide engineering applications and is used in large quantities in automotive oil pans, gears, slides, cams, bearings, fluid reservoirs, and the sports industry. Friction Stir Welding (FSW) is a solid-state process in joining thermoplastic materials. In this investigation, FSW process must be applied to join a UHMWPE plate of 10 mm thickness with specially designed triangular tool pin profile. The aim of this study is to examine the effect of main friction stir welding (FSW) parameters on the quality of UHMWPE plate welds. FSW machine, using a tool with a stationary shoulder and no external heating system. The welding parameters studied were the tool rotational speed which varied between 1300 and 1500 (rpm); the traverse speed which varied between 15 and 25 (mm/min), and the axial force ranging from 8 to 10 (KN). Good quality welds are achieved without using external heating, when the tool rotational speed and axial force are above a certain threshold. For high rotational speed and axial force welds have poor material mixing at the retreating side and mild voids at the nugget, tensile strength also obtained very poor. The hardness angle distortion and bead geometry also evaluated. Taguchi design optimum parameters and ANNOVA were found. Keywords: FSW, Tool Profile, Taguchi, HRM, Polyethylene

Spray analysis of Palm-Based biodiesel to correlate performance and combustion analysis of a compression ignition engine

Palm oil diesel (POD) is derived from palm oil using an esterification process. POD has similar physicochemical properties to fossil diesel, and this allows POD to be the primary substitute to diesel. However, existing research has found that the combustion of POD in CI engines produce lower performance standards than pure diesel. This is due to POD having a higher viscosity, density and presence of heavier molecules resulting in poor evaporation and mixing with the air leading to higher brake specific fuel consumption (BSFC) and reduced power. This research aims to observe the fuel spray behaviors inside the combustion chamber and corelate these with the lower performance of POD than diesel. POD is tested at 100% (B100) and, blended with diesel at a ratio of 50/50 (B50) by volume to reduce the viscosity. The study of spray properties in real engines is expensive and difficult, with most studies being performed in constant volume transparent chambers using Mie-scattering and Schlieren imaging techniques. Therefore, the engine simulation is conducted using ANSYS Forte software to observe the in-cylinder spray behaviors. A 26-kW diesel engine used in a generator is modelled using SolidWorks with respective performance parameters noted from simulation data. The CHEMKIN-PRO software is used to create POD by solving thousands of reaction combinations. Spray analysis of fuels shows longer penetration lengths with POD, higher Sauter Mean Diameter (SMD) and heavier mass of fuel molecules than diesel. These along with a lower calorific value of POD resulted in lower power and a higher brake specific fuel consumption than diesel. Furthermore, B50 performs better than B100.

Laplacian‐P‐splines for Bayesian inference in the mixture cure model

The mixture cure model for analyzing survival data is characterized by the assumption that the population under study is divided into a group of subjects who will experience the event of interest over some finite time horizon and another group of cured subjects who will never experience the event irrespective of the duration of follow‐up. When using the Bayesian paradigm for inference in survival models with a cure fraction, it is common practice to rely on Markov chain Monte Carlo (MCMC) methods to sample from posterior distributions. Although computationally feasible, the iterative nature of MCMC often implies long sampling times to explore the target space with chains that may suffer from slow convergence and poor mixing. Furthermore, extra efforts have to be invested in diagnostic checks to monitor the reliability of the generated posterior samples. A sampling‐free strategy for fast and flexible Bayesian inference in the mixture cure model is suggested in this article by combining Laplace approximations and penalized B‐splines. A logistic regression model is assumed for the cure proportion and a Cox proportional hazards model with a P‐spline approximated baseline hazard is used to specify the conditional survival function of susceptible subjects. Laplace approximations to the posterior conditional latent vector are based on analytical formulas for the gradient and Hessian of the log‐likelihood, resulting in a substantial speed‐up in approximating posterior distributions. The spline specification yields smooth estimates of survival curves and functions of latent variables together with their associated credible interval are estimated in seconds. A fully stochastic algorithm based on a Metropolis‐Langevin‐within‐Gibbs sampler is also suggested as an alternative to the proposed Laplacian‐P‐splines mixture cure (LPSMC) methodology. The statistical performance and computational efficiency of LPSMC is assessed in a simulation study. Results show that LPSMC is an appealing alternative to MCMC for approximate Bayesian inference in standard mixture cure models. Finally, the novel LPSMC approach is illustrated on three applications involving real survival data.

Exponential mixing by orthogonal non-monotonic shears

Non-monotonic velocity profiles are an inherent feature of mixing flows obeying non-slip boundary conditions. There are, however, few known models of laminar mixing which incorporate this feature and have proven mixing properties. Here we present such a model, alternating between two non-monotonic shear flows which act in orthogonal (i.e. perpendicular) directions. Each shear is defined by an independent variable, giving a two-dimensional parameter space within which we prove the mixing property over open subsets. Within these mixing windows, we use results from the billiards literature to establish exponential mixing rates. Outside of these windows, we find large parameter regions where elliptic islands persist, leading to poor mixing. Finally, we comment on the challenges of extending these mixing windows and the potential for a non-exponential mixing rate at particular parameter values.

New Insights into the Gas Dispersion and Mass Transfer in Shear-Thinning Fluids Inside an Aerated Coaxial Mixer via Analysis of Flow Hydrodynamics and Shear Environment

The performance of an agitated bioreactor is hampered by an intense shear environment and low mass-transfer coefficient (kLa). The gas hold-up, kLa, and shear environment achieved by a coaxial mixer comprising a central impeller and an anchor impeller for various central impeller types, impeller speeds, pumping directions, and rotating modes were explored using tomography, dynamic pressure, and computational fluid dynamics methods. A novel method was introduced to explore the performance of aerated coaxial mixers in enhancing the mass transfer through analyzing the gas and liquid flow patterns. The results showed that the gas flow pattern created by the down-pumping pitched blade-anchor (PBD-anchor) mixer led to higher gas hold-up compared to that for the up-pumping pitched blade-anchor (PBU-anchor) mixer. However, the liquid flow pattern generated by the PBD-anchor mixer demonstrated poor mixing, resulting in a lower kLa compared to that for the PBU-anchor mixer. For all configurations, the mass-transfer efficiency attained in the co-rotating mode was higher than that in the counter-rotating mode. The shear environment generated by a coaxial mixer was comprehensively analyzed for the first time. Due to a lower shear environment, a bioreactor furnished by the co-rotating PBD-anchor is more suitable for shear-sensitive applications.

Investigation of the Properties of Compositions Based on Modified Polymer Mixtures

The properties of vulcanized compositions based on mechanical mixtures of Ethylene propylene rubber (EPR)and Ethylene propylene terpolymer (EPTR)with PVC deteriorate compared to the properties of EPR and EPTRperoxide vulcanizers, which is explained by the poor mixing of PVC with these copolymers. In the extruder of the specified polymer mixtures -145 ° C, 36 MPa, n = 200 qra. The thermal and mechanical effects (MIC) under these conditions cause a significant improvement in the properties of the compositions based on these mixtures. The formation of EPR –PVC, and EPTR -PVX copolymers as a result of IMT in these polymer mixtures was confirmed by the construction of these copolymers with peroxide-free, PVC clusters with methemtal oxides - MgO, ZnO, and PbO (mass fraction 8-18). Modification of ethylene-propylene copolymers with PVC was carried out at a temperature higher than the flow temperature of PVC, which ensures its good dispersion in the mixture and chemical bonding with the elastomer phase, the effect of strengthening the elastic phase with the dispersed PVC phase occurs

The Possibility of Minimizing Rutting Distress in Asphalt Concrete Wearing Course

The excessive permanent deformation (rutting) in asphalt-concrete pavements resulting from frequent repetitions of heavy axle loads is studied in this paper. Rutting gradually develops with additional load applications and appears as longitudinal depressions in the wheel path. There are many causes of the rutting of asphalt roads, such as poor asphalt mixing and poor continuous aggregate gradation. All factors affecting the mixture resistance to permanent deformation must be discussed, and all must be properly considered to reduce the rutting propensity of asphalt-aggregate mixtures. In this study, several mixtures were produced with the most common techniques in rutting resistance (using the most effective additives for each mixture), and their performance was compared with the (conventional) mixture currently used in Iraq. The tests focused on the asphalt-concrete mixture for wearing courses. Different mixtures types were tried, namely, dense hot asphalt mixture (HMA) with two different asphalt contents (4.7% and 5.3%), Open-Grade Friction Course (OGFC) mixture, Stone Mastic Asphalt (SMA) mixture, and Beton Bitumineux a Module Eleve (BBME). The modifiers included natural Sisal Fibers (SFs), Carbon Fibers (CFs), and mineral filler (hydrated lime, HL). Marshall test was carried out to find stability and flow values. Rutting was evaluated by the repeated load test for cylindrical specimens under two temperatures (40°C and 60°C) to obtain the permanent deformation parameters. The parameters were used as input to the VESYS 5W software to evaluate the rut depth during different times of design life under 7×10^6 Equivalent Single Axle Loads (ESALs). The results of the selected mixtures were compared with the mixture designed in the laboratory dense gradation mix Job-Mix Formula (JMF)) within the limits of the Iraqi specification (SCRB,2003). Manipulation of the aggregate gradation that is customary in the implementation of the local mixture showed that the best performance regarding rutting resistance was exhibited by JMF, which decreased the rut depth at 40°C and 60°C by 21.63mm and 44.304mm respectively, in comparison with the conventional mixture. Changing the aggregate gradation of the local mixture gives better performance in rutting resistance without additives or changing the percentage of asphalt, at the same cost.

Performance of continuous-flow micro-reactors with curved geometries. Experimental and numerical analysis

One of the major challenges in the design of micro-devices, when very fast reactions are carried out, is to overcome the limited performance due to the poor mixing efficiency of the reactants. Here, we report a holistic analysis of reactants mixing and reaction rate in liquid phase flow micro-reactors with curved geometries. In this sense, a mathematical model that accounts for momentum and mass conservation equations, together with species transport and chemical reaction rate under isothermal conditions, has been developed using computational fluid dynamics techniques (CFD). To validate the predictive model, four micro-reactor geometries with different radius and curved length (straight reactor, two types of serpentines and an Archimedean spiral) have been evaluated. Simulated results proved that mixing is promoted through the formation of Dean vortices as a consequence of the reduction of the radius of curvature and at the same time of the extension of the curve. Thus, the overall performance of the micro-reactor is improved because mass transport limitations are minimized and the process kinetics are greatly enhanced. Accordingly, the spiral micro-reactor reported the best performance by reducing by half the time required to obtain 95 % conversion when compared with the straight reactor. Simulated findings have been confirmed with the experimental analysis of the reaction between aqueous ammonium and hypochlorite ions. Very good agreement between simulated and experimental results has been achieved with an error lower than 10 %. Therefore, the robust model herein reported is a novel and valuable tool to assist in the optimum design of micro-reactors for fluid-phase isothermal applications.

Effects of inter-stage mixing on the NOx emission of staged ammonia combustion

Rich-lean staged combustion is a promising method to achieve low NOx emission for NH3 fuel. The NOx emission of the rich-lean staged combustion heavily relies on inter-stage mixing, which has not been explicitly explored for NH3 fuel. Therefore, this paper presents a modeling study concerning the effects of the inter-stage mixing on the NOx emission of NH3 fueled rich-lean staged combustion. The rich-lean staged combustor is modeled by chemical reactor networks. The non-uniform mixing is modeled by assuming Beta distribution, which is a suitable model for the random behavior of percentages and proportions that is featured in the jet-in-cross-flow mixing of the secondary air and the primary-stage combustion products. The NOx emission is obtained at different inter-stage mixedness, ηl, and rich-burn stage equivalence ratios, ∅r, with constant combustor outlet temperature (constant global equivalence ratio for lean stage, ∅g). The major impact of inter-stage mixing on NOx for staged NH3 combustion is strongly dependent on ∅r. The most significant NOx increase caused by poor inter-stage mixing occurs at around ∅r=1.25 and is due to the increase of thermal NOx, which is similar to hydrocarbon fuels. This finding has important implications for practical applications because the ∅r=1.25 point has the lowest NOx emission and is considered to be the desired operating point for staged gas turbine combustor. Therefore, complete mixing of the primary-stage combustion products and the secondary air needs to be done to ensure low NOx emission for rich-lean staged combustion of NH3. In contrast, at ∅r>1.4, poor mixing may lead to NOx reduction due to the reduction in fuel NOx. However, the NOx at this point is still high and should be avoided in practical operation.

Numerical simulation of similarities between rotating detonation and high-frequency combustion instability under two mixing schemes

This paper presents an experimental study on rotating detonations in a hollow combustor with the slit-orifice nozzle. The experimental results reveal that the propagation speed of detonation waves increases with the rise of mass flow rates and is greater than the Chapman–Jouguet detonation speed (VC−J). Furthermore, numerical simulations of rotating detonation in a non-premixed three-dimensional cylindrical combustor have been conducted based on a multispecies reacting code. The influence of two mixing schemes—that is, slit-orifice and coaxial injector—on detonation waves are studied to determine whether the characteristics of detonation waves tend toward high-frequency combustion instability due to changes in the mixing scheme. It is found that the slit-orifice scheme’s detonation speed, pressure, and temperature are significantly higher than those of the coaxial injector scheme. In particular, the detonation speed of the former reaches 124% of the VC−J, while that of the latter is only 80.5% of the theoretical value. The numerical results reveal that the low-speed detonation is caused by the deterioration of the hydrogen (H2)/air mixing conditions. Moreover, the flow-field structures of two mixing schemes were comparable, both containing transverse detonation waves, oblique shocks, contact surfaces, and wedge-shaped reactant regions. Furthermore, the Rayleigh index analysis showed that the unsteady heat release was in phase with the pressure fluctuations, amplifying the pressure. Therefore, it is suggested that high-frequency combustion instability may be a manifestation of rotating detonation waves under poor mixing conditions.