Increase In Mixing Rate Research Articles

Evolution mechanism of double-layer heavy gas column interface with sinusoidal disturbance induced by convergent shock wave

Based on Navier-Stokes equations, combining the fifth-order weighted essentially non-oscillatory scheme with the adaptive structured grid refinement technique, the interactions between converging shock and annular SF<sub>6</sub> layers with different initial perturbation amplitudes and thickness are numerically investigated. The evolution mechanism of shock structure and interface are revealed in detail, and the variations of the circulation, mixing rate and turbulent kinetic energy are quantitatively analyzed. The dynamic mode decomposition method is used to analyze the dynamic characteristics of the vorticity. The results show that in the case with large initial perturbation amplitude, the transmitted shock wave forms Mach reflection structures both inside and outside of the inner interface of SF<sub>6</sub> layer, and multiple shock focusing phenomena occur in the center. After the transmitted shock wave penetrates the outer interface, the circulation increases faster, and the “spike” and “bubble” structure on inner interface develop faster, so that the amplitude of the inner and outer interfaces and the gas mixing rate increase. As for the case with larger thickness of the gas layer, the phase of the transmitted shock wave changes inside the layer, which forms “bubble” at the crest of the inner interface and “spike” at the trough. When the thickness of the gas layer decreases, the crest of the inner interface does not move inside after being impacted, and “spike” and “bubble” are generated in the late stage. The dynamic modes show that the main structure of vorticity and the exchange of positive and negative vorticity on the main structure are determined by the modes with weak growth and low frequency, but the modes with weak growth and high frequency determine rapid exchange of positive and negative vorticity at the interface in the cases with weak coupling effect.

Effects of CH4 mixing on the laminar burning velocity and Markstein length of RP-3/air premixed flame

Experiments have been carried out in a constant volume combustion bomb to investigate the effects of initial temperature (450 and 480 K), initial pressure (0.1 and 0.3 MPa) and CH4 mixing ratio (0, 0.2, 0.4, 0.6, 0.8, 0.85, 0.9 and 0.95) on the laminar burning velocity and Markstein length of homogeneous, quiescent CH4/RP-3/air mixture over wide equivalence ratio from 0.7 to 1.4. Results show that with the increase of initial temperature from 450 to 480 K, no cellular structure in flame is observed, but obvious cellular structure is observed when initial pressure increases to 0.3 MPa at equivalence ratio of 1.2. The laminar burning velocity reaches its maximum in slightly rich mixture (ϕ = 1.1) and with the increase of initial temperature and decrease of initial pressure the peak laminar burning velocity of CH4/RP-3/air mixture increases. It is found that the laminar burning velocity of CH4/RP-3/air mixture decreases slowly when methane mixing rate increase from 0 to 0.8, but decreases rapidly from 0.8 to 0.95. Analysis results show that at equivalence ratio of 1.1, the Arrhenius factor exp-Ta/2Tad has greater effect on the laminar burning velocity of CH4/RP-3/air mixtures when the methane mixing ratio is larger than 0.8. At equivalence ratio of 0.8 (lean mixture), with the increase of CH4 mixing ratio the Le-1 decreases, which leads to the decrease of Markstein length and make the premixed flame tends to be unstable, while at equivalence ratio of 1.4 (rich mixture), with the increase of CH4 mixing ratio the Ze and δ increase, which stabilizes the premixed flame.

Optimization of Engine Performance through different piston shapes by Taguchi Method

The world’s energy demand has increased drastically in the past and is likely to increase even more in the upcoming years. The fossil fuels are non-renewable energy, depleted at fast rate and this fact intensifies the need to look for alternative fuels to meet our day to day energy needs in all power sectors. The consumption rate of energy has increased tremendously and it necessitates increased supply of energy in all forms. The conventional energy resources like diesel, petroleum, gas and coal will soon be depleted. Hence there is a dire requirement to generate alternative sources of the fuel. Biodiesel is one of the best alternative and renewable fuel. It is oxygenate, Sulphur free and biodegradable. Oxygen content in biodiesel helps to improve the efficiency of the engine. Combustion chamber in compression ignition engine is one of the most important roles to enhance the fuel – air mixing rate (swirl) in short possible time. The turbulence is guided by the shape of the combustion chamber. The air swirl is created in combustion chamber, when the fuel air mixing rate increases. Hence the time duration of air fuel mixing rate decreases. The overall duration of the combustion process to shorten as swirl has leads to increases mechanical efficiency. In this work, in which biodiesel is prepared by transesterification process and engine performance is optimized by different parameters such as Piston shape, Load and Blend ratio and analyzed by Analysis of Variance.

The influence of the sinusoidal shock generator on the mixing rate of multi hydrogen jets at supersonic flow

The rate of fuel mixing inside the combustor is highly significant and effective on the performance of the scramjet engine. In this study, numerical simulations are applied to study the influence of the sinusoidal shock generator on the flow structure of the multi hydrogen jet at supersonic crossflow. The primary focus of this research is to perform flow analysis to determine the role of the shock interaction produced by the presence of the shock generator on the rate of mixing in the downstream of the fuel jets. To simulate the flow, the CFD method with the K-w turbulence model is applied to capture the shock formation inside the domain. According to our results, the mixing rate increases by approximately 40% as the amplitude of the sinusoidal shock generator increases from 2 to 5 mm. Besides, it is found that the fuel distribution becomes uniform when the produced shock generator is more strengthen.

The asymmetry of the recirculation zone of the annular jet with different diameter ratio

We performed direct numerical simulations of the annular turbulent jet flow for the Reynolds number based on the bulk velocity inside the nozzle and its outer diameter Re = 8900 with three different values of the inner-to-outer diameter ratio a = d / D = 0.3, 0.5, 0.7. The size of the reversal flow region, the fluctuation level and mixing rate increase with the increase of the diameter ratio. For all the cases of geometry parameters the symmetry breaking phenomenon was detected. The degree of asymmetry increases with the increase of the diameter ratio resulting in the displacement of the global stagnation points form the axis of symmetry and the deflection of the averaged streamlines from the jet axis.

Crystallization of schoenite from saturated salt solutions

Influence of cooling and mixing rate on the process of schoenite crystallization from saturated salt solutions was investigated in laboratory conditions. It was established that as the system’s cooling rate and mixing speed increase, a significant decrease of the induction period of schoenite crystallization is observed. Fivefold increase of the cooling rate increases sizes of schoenite crystals by an average of 2.5–3.0. Mixing rate increase leads to the formation of a fine crystalline precipitate (crystal size 10−20 μ m).

Combustion flow in different advanced vortex combustors with/without vortex generator

Numerical calculation has been performed to analyze the combustion flow in different advanced vortex combustors. For the rear blunt body, when slot size is 2 mm and half open angle θ is 50°, advanced vortex combustor has the best combustion flow characteristics. The performance of advanced vortex combustor with vortex generator principle is obviously better than that without vortex generator. The effects of injection parameters on the performance in AVC were studied. With the increase of anteversion angle α, the mixing rate increases, and the degree of thermal energy conversion to the kinetic energy of the combustor outlet increases. With the increase of sloping angle β, the high temperature region in the cavity becomes wider, and the distribution is more uniform. When α=60° and β=60°, the combustion chamber can achieve stable combustion with higher temperature and lower pressure drop in the lean combustion conditions.

Influence of the angle of incident shock wave on mixing of transverse hydrogen micro-jets in supersonic crossflow

A three-dimensional numerical study has been performed to investigate the influence of angle of shock waves on sonic transverse Hydrogen micro-jets subjected to a supersonic crossflow. This study focuses on mixing of the Hydrogen jet in a Mach 4.0 crossflow with a global equivalence ratio of 0.5. Flow structure and fuel/air mixing mechanism were investigated numerically. Parametric studies were conducted on the angle of shock wave by using the Reynolds-averaged Navier–Stokes equations with Menter's Shear Stress Transport turbulence model. Complex jet interactions were found in the downstream region with a variety of flow features depending upon the angle of shock incident. These flow features were found to have subtle effects on the mixing of Hydrogen jets. Results indicate a different flow structure than for a typical micro jet, with the development of shock angle to the flow of the Hydrogen jet. According to the results, without oblique shock, mixing occurs at a low rate. When the intersection of incident shock and the lower surface is at a low angle (15°) of shock incident; significant reduction (up to 30%) occurs in the maximum concentration of the Hydrogen jet at downstream. Moreover, when the angle of shock incident increases, Hydrogen–air mixing rate increase and the concentration of the Hydrogen micro jet is uniformly distributed. Consequently, an enhanced mixing zone occurs downstream of the injection slots which leads to flame-holding.

Numerical study of shock wave interaction on transverse jets through multiport injector arrays in supersonic crossflow

In the present paper, the influence of shock wave position on sonic transverse hydrogen micro-jets in supersonic crossflow is investigated. This study focuses on mixing and shock interaction of the hydrogen jet in a Mach 4.0 crossflow with various jet conditions. Flow structure and fuel/air mixing mechanism were investigated numerically. Parametric studies were conducted on the position of the shock wave by using the Reynolds-averaged Navier–Stokes equations with Menter׳s Shear Stress Transport turbulence model. Complex jet interactions were found in the downstream region with a variety of flow features dependent on the shock position. Results show a different flow structures than for a typical micro jet, with a location of the oblique shock on the flow of the hydrogen jet. According to the results, for oblique shock on the first jet, the mixing performance in high pressure ratio increases more than 20% in downstream. Furthermore, a significant reduction (up to 20%) occurs in the maximum concentration of the hydrogen jet at downstream when the intersection of incident shock is on the top of the last jets. Moreover, hydrogen-air mixing rate increase and the concentration of the hydrogen micro jet are uniformly distributed in the surface close to shock. Thus, an enhanced mixing zone occurs in the vicinity of the intersection of the jet and the shock wave.

Effects of incident shock wave on mixing and flame holding of hydrogen in supersonic air flow

The effects of incident shock wave on mixing and flame holding of hydrogen in supersonic airflow have been studied numerically. The considered flow field was including of a sonic transverse hydrogen jet injected in a supersonic air stream. Under-expanded hydrogen jet was injected from a slot injector. Flow structure and fuel/air mixing mechanism were investigated numerically. Three-dimensional Navier–Stokes equations were solved along with SST k-ω turbulence model using OpenFOAM CFD toolbox. Impact of intersection point of incident shock and fuel jet on the flame stability was studied. According to the results, without oblique shock, mixing occurs at a low rate. When the intersection of incident shock and the lower surface is at upstream of the injection slot; no significant change occurs in the structure of the flow field at downstream. However when the intersection moves toward downstream of injection slot; dimensions of the recirculation zone and hydrogen-air mixing rate increase simultaneously. Consequently, an enhanced mixing zone occurs downstream of the injection slot which leads to flame-holding.

Thermal explosion in a stirred medium

The critical conditions of a thermal explosion in a cylindrical chemical reactor with several symmetrically positioned agitators for creating a forced convection of the reaction mixture are examined. The analysis is carried out for an arbitrary Peclet number on the assumption that the flow of the fluid is laminar. The critical value of the thermal explosion parameter (Frank-Kamenetskii parameter) is determined as a function of the number of agitators, Peclet number, and the distances between the axes of the agitators and the reactor axis. In contrast to the previous studies, the dependence of the thermal explosion parameter on the rate of stirring of the medium is considered. Calculations show that, as the number of agitators and mixing rate increases, the thermal explosion probability decreases, so that the critical value of the thermal explosion parameter can be several times greater than its classical value. It is demonstrated that, depending on the values of the parameters, the problem can have four stationary solutions, two of which are stable.

Toughening of glass fiber reinforced Nylon 66 with EPDMMAH: Dynamic mechanical, rheological, and morphological investigations

AbstractIn this research the variation of mechanical and rheological properties of the composite as a function of different processing parameters and ethylene propylene diene monomer (EPDM) rubber content was studied. The optimum amount of dicumyl peroxide (DCP) to achieve the highest percentage of grafting of maleic anhydride to EPDM was obtained by Fourier transform infrared (FTIR) spectroscopy. In general, immiscibility between nylon 66 and EPDM, and their high interfacial tension makes their blending difficult. Maleic anhydride was grafted to EPDM to reduce interfacial tension between two phases. Also the blends were prepared at highest possible speed of mixing (screw speed in extrusion) and lowest temperature to reach a well mixed sample which can be seen in scanning electron microscopy (SEM) images and impact strength results. Dynamic mechanical thermal analysis (DMTA) tests were conducted to evaluate the miscibility of the components and to measure the glass transition temperature of the blends. Rheological measurement showed that the viscosities of the components of the blend getting closer to each other when EPDM content, time of mixing and shear rate increase.

Some Counterintuitive Dependencies of Tropical Cyclone Frequency on Parameters in a GCM

AbstractHigh-resolution global climate models (GCMs) have been increasingly utilized for simulations of the global number and distribution of tropical cyclones (TCs), and how they might change with changing climate. In contrast, there is a lack of published studies on the sensitivity of TC genesis to parameterized processes in these GCMs. The uncertainties in these formulations might be an important source of uncertainty in the future projections of TC statistics.This study investigates the sensitivity of the global number of TCs in present-day simulations using the Geophysical Fluid Dynamics Laboratory High Resolution Atmospheric Model (GFDL HIRAM) to alterations in physical parameterizations. Two parameters are identified to be important in TC genesis frequency in this model: the horizontal cumulus mixing rate, which controls the entrainment into convective cores within the convection parameterization, and the strength of the damping of the divergent component of the horizontal flow. The simulated global number of TCs exhibits nonintuitive response to incremental changes of both parameters. As the cumulus mixing rate increases, the model produces nonmonotonic response in global TC frequency with an initial sharp increase and then a decrease. However, storm mean intensity rises monotonically with the mixing rate. As the strength of the divergence damping increases, the model produces a continuous increase of global number of TCs and hurricanes with little change in storm mean intensity. Mechanisms for explaining these nonintuitive responses are discussed.

Mixing of single-component hydrocarbon droplets and water at supercritical or near-critical conditions

The mixing process of a single component hydrocarbon droplet, either toluene or n-decane, in a reservoir of supercritical or near-critical water is modeled. The 1D droplet model provides microscale information, such as droplet mixing times, that can be used to help understand large scale mixing in applications that include supercritical water oxidation and supercritical water desulfurization. Transport, thermodynamic, and phase equilibrium sub-models are used to estimate relevant physical properties, e.g., diffusion coefficients and fugacities. The upper critical solution temperature (UCST) acts as the controlling parameter in the transport and mixing process as demonstrated by the solutions. When the initial water temperature is below the UCST, complete miscibility defined as the dissolution of the interface can be never achieved. The droplet initially swells due to thermal expansion and net diffusion of water into it, but starts to shrink when it becomes water saturated. A sudden increase of miscibility and mixing rate is observed as the interface temperature increases from below to above the UCST point for the water–toluene system. The mixing rate increases noticeably with increasing water temperature until instant miscibility occurs, after which much slower increase is observed.

에탄올 혼합율이 엔진성능에 미치는 영향

A rapid growth of automobile industry has become a major cause for the environmental pollution of big cities, which has driven the emission regulation into extreme. The study of alternative fuel is one of the many researches for improving car emissions.<BR> In this study, the effect of an ethanol mixing rate on the engine performance of exhaust emissions, fuel consumption and a maximum torque is assessed for a gasoline engine without any retrofit.<BR> The result shows that maximum torque is not reduced in the range of ethanol mixing rate of 10 to 15%. CO and NOx is reduced with the increase of ethanol mixing rate and the fuel consumption remains in similar level.

초음속 유동장 내 이중 수직분사의 특성에 관한 연구 I-혼합특성

단일 분사 노즐을 갖는 연소기 내부의 혼합과정에 대한 이해를 바탕으로 연료 분사 노즐을 이중으로 갖는 연소기의 혼합과정에 대한 수치연구를 수행하였다. 수치연구를 위하여 3차원 Navier-Stokes 방정식과 k-<TEX>$\omega$</TEX> SST난류 모델을 이용하여 연료 이중 분사 유동을 모사하였다. 이중 분사구 사이의 거리 변화에 따른 혼합특성의 변화를 살펴보기 위하여 파라메터 연구를 수행하였다. 연료 이중 수직분사에서 두 분사기의 유동 및 혼합특성은 서로 상당히 다른 경향을 보이며, 후방 분사기의 유동 및 혼합특성은 전방 분사류의 영향을 받아 더 크게 팽창하고 침투거리가 증가하는 것으로 나타났다. 어떤 특정 거리가 되기 전까지 분사기 사이의 거리가 증가할수록 전체적인 혼합률과 침투거리가 증가하는 등 혼합특성이 개선되지만, 특정 거리보다 크게되면 오히려 혼합특성이 악화되는 것으로 나타났다. 이는 이중분사기에서 최적인 혼합특성을 위한 두 분사기 사이의 거리가 존재함을 의미하는 것으로 판단된다. Based on the analyses of the single transverse injection in supersonic flow fields, the mixing characteristics of dual transverse injection of hydrogen in supersonic air flow are studied with computational methods. Three-dimensional Navier -Stokes and the k-<TEX>$\omega$</TEX> SST turbulence model were used. A parametric study is conducted with the variation of the distance between two injectors. The flow patterns and the mixing characteristics of two injection flows are very different from each other, and the flow patterns and the mixing characteristics of the rear injection flow are strongly influenced by those of the first injection flow. The increase of the distance between two injectors up to a specific distance results in the increase of mixing rate and penetration of fuel. However, the increase of the distance over the specific distance results in the decrease of mixing rate and penetration of fuel. From the results it can be stated that there exists a distance between two injectors for optimum mixing characteristics.

Limits of mass transfer enhancement in lithium bromide-water absorbers by active techniques

A model to predict the theoretical limits of mass transfer enhancement in a falling film absorber using LiBr aqueous solution has been developed and it provides a means of comparative absorber performance evaluation. During vapor absorption, the vapor- liquid interface becomes saturated immediately after it is exposed to vapor. Further absorption can only be sustained by mass diffusion and heat conduction into the fluid bulk, assuming a stationary interface. Due to the relatively small heat and mass diffusivities of LiBr aqueous solution, the mass absorption rates sustained by pure diffusion and conduction processes are small. To increase the mass absorption rates, mixing of the solution is favored. Therefore, every mass transfer enhancement technique, passive or active, consists of basically disturbing the film and causing mixing of the solution near the interface. The higher the mixing rate, the higher the mass absorption rate. To assess the effect of a mixed interface, a mathematical model is formulated. This model solves one-dimensional heat and mass differential equations coupled at the interface. An asymptote of the mass transfer rate as the mixing rate increases is derived. The model, which takes into account all relevant parameters such as temperatures, pressures, concentrations, mass flow rates, and geometry, is an ideal tool for rating absorber performance. Results show that, under typical operating conditions found in commercial chillers, the theoretically possible maximum mass absorption rate is 0.049 kg m 2 · s −1, and that, at a mechanically feasible mixing frequency of 1000 Hz, a mass absorption rate of 0.0256 kg m 2·s −1 is possible. The latter rate is about an order of magnitude larger than that found in commercial chillers.

Polymer electrolytes with a dual-phase structure composed of poly(acrylonitrile- co-butadiene)/poly(styrene- co-butadiene) blend films impregnated with lithium salt solution

Dual-phase polymer electrolytes that possess good mechanical strength and high ionic conductivity were prepared by mechanically mixing a poly(acrylonitrile- co-butadiene) rubber (NBR) and poly(styrene- co-butadiene) rubber (SBR) binary solution and casting polymer blend films. The films were swollen with lithium salt solutions (e.g. 1 M LiClO 4 in γ-butyrolactone) to obtain dual-phase polymer electrolyte films. As the mixing rate increases, the average domain size of the NBR and SBR in the film decreased, levelling off at a mixing rate of > 10 000 rev min −1 as it approached the value of 5 μm. A mechanically strong film was obtained by reducing the domain size to less than one-fifth of the film thickness. On the other hand, the ionic conductivity depended on the fraction of NBR in the NBR/SBR matrix rather than on the domain size of the film. Thus high ionic conductivity (> 10 −4 S cm −1) could be achieved with an NBR weight fraction of over 50% (w/w). Additionally, transmission electron microscope observation and differential scanning calorimetric analysis showed evidence that a dual-phase structure was created, in which the NBR phase provided an ion-conductive pathway and the SBR acted as a mechanically supportive matrix. Quantitative analysis of ionic conductivity suggested that a ‘free’ lithium salt solution absorbed in the matrix caused the high ionic conductivity of the polymer electrolyte.

On the use of a simple deciduous forest model for the interpretation of climate change effects at the level of carbon dynamics

Abstract In this publication an approach is described to model the Net Carbon Exchange (NCE) between the vegetated surface—atmosphere interface of a deciduous forest. In that context it is not only important to simulate effects of climatology (temperature, CO 2 ) on forest evolution, but also to evaluate its NCE. For climatological conditions representative for the mid latitudes the influences of changes in atmospheric temperature and CO 2 levels are simulated. Four compartments are defined in the model, two compartments simulating phytomass functional elements with large temporal biomass fluctuations (green compartment) and phytomass temporal fluctuations that are relatively small (non-green compartment). The litter compartment is also incorporated in the model and its fluxes evaluated. Model runs were executed for two CO 2 scenarios and a climatological record reconstructed for the period covering 1833 till 1989 at a latitude and longitude of 51°N, 2.5°E. It has to be remarked that the addition of 4.5°C to a climatological record for all seasons is a simplification when simulating the effects of a temperature rise. The model outcome however suggests that the CO 2 mixing ratio rate increase after World War II (WW II) has induced a phytomass increase in young forests. The constraints of the model and its application in Remote Sensing-derived data sets is discussed.

Chemical models for pdf modeling of hydrogenair nonpremixed turbulent flames

Development of simplified chemical models is necessary for probability density function (pdf) modeling of turbulent flames with multiple species. Two promising approaches, the reduced mechanism method and the constrained equilibrium method, have been extensively studied to develop six plausible chemical models for nonpremixed combustion of hydrogen with air. Examination of the thermodynamic properties predicted by these chemical models in the allowable domains reveals that in regions where temperatures are above 1400 K, all six models yield similar results. In regions near the pure mixing limits, the partial equilibrium assumption for bimolecular reactions breaks down and yields unrealistic results. Reasonable agreement between the measured temperatures and those from Monte Carlo simulations with all six chemical models has been obtained for a nonpremixed turbulent jet flame at low mixing rates. As mixing rate increases, flame blowout has been predicted only by a chemical model that treats the hydrogen atom H by its kinetics rate equation, which is based on the reduced mechanism method. The predicted jet velocity at flame blowout is much higher than that observed in experiments. One of the implications from the present results is the need for improved submodels for mixing processes.