Parallel Jets Research Articles

Optimal disturbances in round submerged jets

In this paper, optimal growth analysis of spatial disturbances in round submerged jets is performed. Optimal energy growth is studied for various Reynolds numbers Re and frequencies ω. Different velocity profiles proposed by Michalke [“Survey on jet instability theory,” Prog. Aerosp. Sci. 21, 159–199 (1984)] are investigated by varying the shear layer momentum thickness δ, while the jet radius R is considered fixed (as characteristic length). In the case of stationary disturbances, there are no amplified eigenmodes, so eventually the energy of such disturbances decays downstream. Such disturbances are characterized by optimal energy growth as a function of streamwise coordinate z. At certain location downstream zmax this value reaches its maximum Gmax and then decays, although not monotonically, unlike the result of the temporary setting [Jimenez-Gonzalez et al., “Modal and non-modal evolution of perturbations for parallel round jets,” Phys. Fluids 27, 044105 (2015); Jimenez-Gonzalez and Brancher, “Transient energy growth of optimal streaks in parallel round jets,” Phys. Fluids 29, 114101 (2017)]. The well-known scaling Gmax∝Re2 and zmax∝Re is confirmed for stationary disturbances. Conversely, for nonstationary disturbances there is a range of frequencies in which an amplified mode is present. In this case, optimal perturbations far downstream consist only of the amplified mode. The main effect of non-modality here is “the energy pumping” of the amplified mode, so the disturbances are characterized by the “energy ratio” G/Em, i.e., the ratio of the energy of an arbitrary perturbation to the energy of the amplified mode. The influence of the shear layer momentum thickness on optimal disturbances is studied; increasing the shear layer momentum thickness of the base flow “smears” the area occupied by the optimal disturbance, although it does not dramatically affect energy gain values, slightly decreasing maximum energy as shear layer momentum thickness increases. Spatial oscillations in the energy of stationary optimal disturbances in jet flows are discovered. The spectrum structure examination shows that the frequency of these oscillations corresponds to the frequencies of the two least damped discrete eigenmodes.

Assessment and enhancement of thermal performance for ring roughened finned jet impingement solar air heater for low-temperature applications

The current study investigates the synergetic impact of artificial roughness with fins and jet impingement on the performance of flat plate solar air heater. The impingement jet Solar Air Heater (SAH) achieves roughening through the incorporation of conical ring shapes, strategically placed between the straight fins of the absorber plate. The analytical work focuses on maximizing the thermal and effective thermal performance by considering the design parameters: ring thickness ratio (TR/DH) ranging from 0.07 to 0.13, ring diameter to thickness ratio (Do/TR) from 1.67 to 2.67, streamwise pitch to diameter ratio (X/Do) from 4.71 to 7.71, ring diameter ratio (Do/Db) from 1.30 to 2.30, and a number of fins (n) ranging from 4 to 7 based on spanwise pitch ratio (Y/Do) from 2.92 to 5.14. A MATLAB code is created to solve governing equations and validated by comparing results with existing literature. The results indicate that the new configuration of SAH is most effective up to the Reynolds number of 10850 and produces the maximum energy and thermo hydraulic efficiency is 81.4 % and 77.4 %, respectively. It enhances the thermo hydraulic performance by 15.1 % and 4.1 % compared with conventional single pass and parallel pass jet impingement SAH. The system has the capability to increase the air temperature per unit of supplied solar heat flux, is ranging between 0.006 and 0.038 Km2/W.

ЖИТТЯ ГАЛАКТИК У СПОСТЕРЕЖУВАНОМУ ВСЕСВІТІ. ЧАСТИНА 2

The exchange of hydrogen between galaxies and the galactic structures that ensure this process are considered. This exchange between the main objects of the Universe was given in the first part of the article, but it was not considered there as another and important process of the galactic cycle. However, the largest structures in the observable Universe are associated with the exchange of the lightest gas. This circumstance allows us to rightfully distinguish it as the next process of the galactic cycle. In the second process, determined by the phase of the quasar in the galactic core, stellar waste is disposed of. Weak quasar jets eject the results of recycling either into the halo of their own galaxy or into its immediate vicinity. From these emissions, stars are formed directly in the galaxy, or near it. As a rule, these are stars from the galaxy's own matter. High-speed jets contain particles at their bases that move at speeds faster than the speed of light. This allows jets to eject matter far beyond the boundaries of the parent galaxy. The matter of such a jet leaves the galaxy forever. Quasars turn on periodically and relatively rarely change the direction of their emissions. Therefore, they often send streams of matter in one direction. Observable intergalactic filaments are formed from energetic flows. Given that clusters contain many galaxies and each of them goes through quasar stages, several filaments may emerge from the cluster. Therefore, filaments begin and end on galaxies, or their clusters. Outgoing filaments, if they are located nearby and equally directed, can unite with each other, forming large filaments. The question of the forces contributing to the unification of almost parallel jets remains open. The filaments contain hot hydrogen, some dust, “metals,” stars and galaxies. Hydrogen in filaments is expected with a minimal percentage of “metals”. In the filaments, stars and galaxies can form from the available gas. This often happens where the threads intersect with each other. Filaments from other sources fall into the gravitational field of a galaxy, or cluster of galaxies. By delivering the necessary building material, filaments introduce new stellar life into the basic elements of the Universe. Therefore, filaments are channels that provide life to galaxies. This process of hydrogen exchange between galaxies is the fourth galactic process. He refines the model of galaxy life in the observable Universe, which is presented in the first part of the article

Review[sbnd]Recent developments on applying acoustic waves for efficiency improvements of different thermofluids systems

This review classified recent works using proper acoustic waves to develop more efficient thermofluids systems for different industrial equipment such as swirl combustor, jet in crossflow, gas flare, different types of heat exchangers, cooling mechanism, aircraft wings, and turbine blades. For the swirl combustor, the range of proper excitation Strouhal number and jet pulsation intensity depends on the type of fuel, geometry of burner, swirl number, and central and annular Reynolds numbers. The combined effects of early jet break up, and centrifugal force induced by proper acoustic waves can lead to higher jet spread rate that can induce higher levels of mixing of the fuel and air in swirl combustor, improving combustion efficiency. For the transverse jet, the range of proper excitation Strouhal number and jet pulsation intensity depends on the type of fuel, geometry, and angle and direction of the burner, and jet and cross flow Reynolds numbers. Proper acoustic excitation can force the transverse jet flow to synchronized shear layer vortices. This phenomenon can cause the new vortices to appear in the upwind shear layer region of the deflected jet; then the spread, penetration, entrainment, and dispersion of the jet significantly increases, improving combustion efficiency. For the aircraft wing, vertical acoustic excitation significantly improves the lift force when the sound frequency is equal to the period of flow fluctuation in the separated boundary layer. For the heat exchanger, the ultrasonic transducer can be installed on the walls of a heat exchanger with direct contact to the fluid, improving the heat transfer coefficient. The physics of fluids behind the efficiency improvements of all mentioned cases were also clarified. Moreover, the acoustically excited impinging jets with higher cooling efficiency applied in different areas are discussed. Proper acoustic forcing can decrease the rate of unstable frequencies of disturbances related to impinging jets over a short distance. This phenomenon can significantly enhance heat transfer and improve cooling efficiency. This review paper provides insight into how acoustic waves could improve the efficiency of thermofluids systems in a variety of applications. Acoustic excitation could have the potential capability to improve the efficiency of new applications such as wind, gas or steam turbines, some types of industrial chemical mixers with parallel plane jets, aircraft wings or those of unmanned aerial vehicles, more efficient combustion chambers, and less polluting gas flares. However, the feasibility of this concept requires further investigation by the community in the future.

Effect of turbulent coflows on the dynamics of turbulent twin jets

The impact of turbulent coflows on the dynamics of turbulent twin round jets is investigated experimentally. Parallel twin jets, at three jet spacing values and two Reynolds number/jet-to-coflow velocity ratios, were released into turbulent coflows with two distinct levels of turbulence intensity. Velocity measurements were made using acoustic Doppler velocimetry. An increase in the coflow turbulence intensity leads to an earlier merging and combining of the jets and also accelerates the rate of decay with downstream distance of the mean centerline excess velocity of the jets. The mean velocity on the symmetry line, for different values of jet spacing, ratios of jet exit velocity to coflow mean velocity, and coflow turbulence intensity, is self-similar when scaled by the maximum mean velocity on the symmetry line and the corresponding streamwise distance. Moreover, as the turbulence level of the coflow intensifies, the turbulence intensity along the symmetry line of the jets increases. The longitudinal integral length scale on the symmetry line of the twin jets decreases as the coflow turbulence intensity increases. The energy spectra of the coflowing twin jets show that the turbulence in the coflow transfers the energy contained by the larger scales to the smaller scales at a greater rate than that which occurs for jets in a quiescent background. However, as the jet spacing increases, less energy is transferred to the smaller scales.

Numerical Simulation Study of Energy Separation in Jet Shear Layer

In order to investigate the rational utilization of energy in the fluid jet process, a free dual jet of parallel air is used as a research object. Simulation of turbulence was carried out based on the realizable k-epsilon equations with pressure-velocity coupling, PISO method, discrete method with second-order windward format, and first-order implicit transient solution. The energy separation phenomenon within the jet shear layer and its influencing factors were investigated under dual-jet Reynolds number differences of 12,800, 19,200, 25,600, 32,000, 41,500, and 51,100, respectively. The simulation is in transient format and the jet inlet velocity is given by udf. Calculations show that the pressure perturbation in the air shear layer, which gives rise to the pressure work exchange between the jet and the surroundings, is the main reason for the coexistence of high- and low-temperature regions formed within the jet shear layer, and the larger the Reynolds number, the stronger the energy separation effect. The center streamline path of a parallel air free dual jet can be fitted by a circular arc, whose radius of curvature increases with the Reynolds number, and the curvilinear development path of the dual jet can offset the energy separation effect to some extent. This paper confirms the existence of energy separation in the shear layer of a parallel air free double jet, reveals the mechanism of energy separation in the fluid domain, and explores the flow characteristics of two parallel jets.

Mode Transition Induced by Gas Heating Along the Discharge Channel in Capacitively Coupled Atmospheric Pressure Micro Plasma Jets

The effects of neutral gas heating along the direction of the gas flow inside the discharge channel of a parallel plate micro atmospheric pressure plasma jet, the COST-jet, on the spatio-temporal dynamics of energetic electrons are investigated by experiments and simulations. The plasma source is driven by a single frequency sinusoidal voltage waveform at 13.56 MHz in helium with an admixture (0.05–0.2%) of nitrogen. Optical emission spectroscopy measurements are applied to determine the spatio-temporally resolved electron impact excitation dynamics from the ground state into the He I (3s)3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}S1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_1$$\\end{document} state and the rotational temperature of nitrogen molecules at different positions along the direction of the gas flow inside the 30 mm long discharge channel. The gas temperature, which is assumed to be equal to the N2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} rotational temperature, is found to increase along the discharge channel. This effect is attenuated as the nitrogen concentration is increased in the gas mixture, leading to an eventually constant temperature profile. The experimental data also reveal a plasma operating mode transition along the discharge channel from the Ω\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Omega$$\\end{document}- to the Penning-mode and show good agreement with the results of 1d3v kinetic simulations, which spatially resolve the inter-electrode space and use the gas temperature as an input value. The simulations demonstrate that the increase of the gas temperature leads to the observed mode transition. The results suggest the possibility of using the nitrogen admixture and the feed gas temperature as additional control parameters, (i) to tailor the plasma operating mode along the direction of the gas flow so that the production of specific radicals is optimized; and (ii) to control the final gas temperature of the effluent. The latter could be particularly interesting for biological applications, where the upper gas temperature limit is dictated by the rather low thermal damage threshold of the treated material.

Investigation of flow characteristics and velocity fields of excited two parallel plane jets

An experimental investigation was conducted to study the flow characteristics and velocity fields of excited two parallel plane jets. The experiments were carried out at a jet Reynolds number of 200. A loudspeaker system was used to create the jets pulsation and to vary the intensity of jet pulsations at a constant excitation frequency of 40 Hz. A hot-wire anemometer was used to measure the velocities of the jets as they exited. The flow patterns were visualized using a laser-light sheet technique combined with smoke flow visualization. The jet spread widths were determined from images taken with a long-exposure method using binary edge detection. A particle image velocimetry measurement technique was used to render the flow field behaviors of the parallel jets. The introduction of jet pulsation by the speaker led to the roll-up of coherent vortices along the shear layers of the jets. These vortices became more prominent as the intensity of jet pulsations increased. These coherent vortices broke apart into turbulent eddies, resulting in wider jet spread with higher pulsation intensities. Two counter-rotating vortices were detected at the jet exit. These vortices moved closer to the jet exits as the jet pulsation intensity was increased. The intensity of turbulence and the presence of vortices were both influenced by the magnitude of the jet pulsation. Greater jet pulsation led to higher turbulence levels, a more pronounced vorticity field, and a more efficient transfer of momentum, consequently enhancing the mixing process.

Features of the dynamics of several elevated jets in a cross-flow

Using the thermoanemometric method, the effect of turbulent cross-flow on vertical low-speed elevated axisymmetric jets with a relative velocity of approximately 2.45 has been experimentally investigated. Processing and qualitative analysis of experimental data on velocity profiles for the case of two and four identical isothermal turbulent jets propagating in a uniform cross-flow are carried out. The analysis of velocity contours in different cross-sections showed that the interaction of the jets proceeds in a complex way; and in this interaction, in addition to the jets and the cross flow, the trace under the jets and tubes – the sources of the jets – are involved. It is obtained that the trajectory of a twin parallel jet shows less penetration than that of a single jet. At the same time, the trajectory of the tandem jet after the merging of the rows shows significant penetration into the transverse flow compared to the single one.

Numerical Study of Mixed Convection of Buoyant Twin Jet

Abstract The effect of mixed convection of twin vertical jets is investigated numerically in this paper. The results are presented specifically for flows affected by buoyancy for two parallel jets of same velocities ranging between 0.25 m/s and 5.0 m/s and temperatures between 295 K and 320 K. Both jets generate a slow flow with a temperature difference (with the ambient flow) less or equal to 32 °C (305 K). Predictions of dynamical and thermal parameters are obtained for the merging and combining regions. This study reveals that the trajectory of the two jets is strongly influenced by the ratio of buoyancy to inertial forces. Results indicate that, relative to isotherm jets, the location along the vertical symmetry plane at which the two jets merge (merging point) decreases with increasing jet inlet temperature. It was also found that the location of the merging point is shifted toward the confining wall as the velocity of the jets increases. The behavior law (linear regression), relating to the expansion of the jet, is not verified in the developed region for each value of the inlet velocity and temperature. This is explained by the fact that natural convection is more predominant than forced convection. The results show that the self-similarity of the cross profiles of the mean velocity and the behavior law relating to the expansion of the jet are obtained throughout the developed region.

Multi-scale proper orthogonal decomposition (mPOD) analysis of vortex evolution and viscous dissipation in a circular-cylinder wake controlled by parallel symmetric jets

This paper describes a novel flow control scheme over circular-cylinder wakes using parallel symmetric jets. The effects of different jet momentum coefficients (Cm) at θ = ±30° on vortex evolution and viscous dissipation in the circular-cylinder wake are experimentally investigated using high-speed particle image velocimetry (PIV) in an open-jet wind tunnel at a Reynolds number of 1.01 × 104. Multiscale proper orthogonal decomposition (mPOD) is used to analyze the evolution of coherent structures and establish the relationship between flow structures and viscous dissipation. At Cm ≤ 0.0089, the vortex formation length is elongated and the scale and intensity of the vortex are suppressed. Small-scale jet vortices are then generated by the interaction between the jet shear layers at Cm = 0.0174, whereupon the turbulent kinetic energy attains a minimum and the Reynolds shear stress in the wake is basically eliminated. At higher values of Cm, band-like jet oscillations appear in the wake, increasing the turbulence intensity. The inhibition effect of the jets on the wake vortex decreases the viscous dissipation in the wake, with optimal viscous dissipation control obtained at Cm = 0.0089. When Cm > 0.0089, the coupling between the jets increases the viscous dissipation of small-scale, high-frequency turbulent structures, and this effect gradually becomes more obvious with increasing Cm.

Simulation of the effect of pulsatile flow on flow features dynamics during abrupt contraction into two parallel rectangular jets

The analysis of the converging region of two parallel jets under unsteady flow conditions is important in its design and safety. Numerical simulation was used to investigate the flow features before the contraction plane. Pulsatile flows with a fixed flow period of 5 s, mean flow velocities between 0.033–0.045 m/s, and flow amplitudes of 0.2–0.4 were simulated in this study. The changes at the contraction were characterized by repeated changes in the velocity profile, recirculation area, and corner vortices. The velocity profile in the contraction region remains axisymmetric throughout the cycle of pulsation. The corner vortices increased with a decrease in the mean velocity when the vortex length was longer than the length from the corner to the stagnation point but began to reduce when the mean velocity started increasing in the pulsation cycle. The results provide more explanation of flow feature characteristics at channel corners and abrupt contraction planes.

Experimental investigation of dual jet flow past a heated surface: Effect of Reynolds number

A dual jet flow comprises a wall jet, discharged immediately parallel to a solid boundary, flowing alongside a parallel jet whose exit is offset from the solid boundary by some distance. Despite the widespread industrial application of dual jet flows, experimental analysis of their flow and heat transfer characteristics is severely underrepresented in the published literature. As a result, the heat transfer capabilities and driving flow mechanisms associated with dual jet flows are not yet fully understood, meaning they cannot be optimised towards specific applications. The present study is thus concerned with characterising the transfer of heat to a dual jet flow for varying Reynolds number through experimental means. Using air as the working fluid, each jet Reynolds number is varied in the range 5500≤Re≤12,000 and the offset ratio and velocity ratio are each maintained at a constant value of 1. A constant heat flux generation of 1670W/m2 is maintained in the bounding wall and the surface Nusselt number distribution is measured using infrared thermography techniques. A distinct Nusselt number profile is observed with respect to downstream distance, which remains consistent regardless of the Reynolds number. The results indicate that increasing Re elevates the Nusselt number proportionally at all downstream locations, where the surface-averaged Nusselt number rises linearly with increasing jet Reynolds number.

On the role of low frequency modes in the thermal and momentum mixing in parallel plane jets

Parallel plane jet mixing has been the subject of extensive study and its physics are characterized well in literature. However, the physics of parallel plane jet mixing in relatively small confined regions is not well known. In this study we investigate the effects of re-circulatory flow on thermal and momentum mixing in confined parallel plane jets. Studying confined jet interaction is important for a variety of engineering applications. One of these applications is sodium cooled fast reactors (SFR). It is common in SFRs to have coolant stream mixing in a confined domain, often the upper plenum. The Reactor Cavity Cooling System (RCCS) facility at the University of Michigan models this problem in a simplified enclosure with isothermal parallel plane jets as the incoming coolant streams. A Large Eddy Simulation (LES) of this simplified plenum was developed using optimal boundary conditions, proven mesh convergence, and consistent resolution. This model was then validated against experimental results from the RCCS facility. After model validation, flow structures in the jet interaction were discerned by means of Proper Orthogonal Decomposition (POD). Comparison of 3D POD to a clipped 2D POD region reveals that re-circulatory flow can lead to dominant oscillations within the jet mixing region. Subsequently, Power Spectrum Density (PSD) and Continuous Wavelet Transforms (CWT) were used to ascertain frequency-based phenomena that might occur in this mixing region. Frequency analysis uncovered a dominant low-frequency oscillation in both velocity and temperature PSDs. These frequencies fall within the known ranges of thermal striping in sodium. The data generated in this study can be used for the benchmarking of less computationally intensive approaches. This data will be made available upon request to ebm5351@psu.edu.

Interactions between two parallel gas jets on the surface of an underwater vertical moving body

In the present study, interactions between two parallel bubbles after the gas jets inject into a liquid cross flow are studied experimentally. For bubble coalescence on the surface of a moving body, the water layer between the twin parallel bubbles possesses initial momentum and bubble coalescence is difficult to conduct. Based on the verification of the repeatability of the experiments, the coalescence mechanism of the twin parallel bubbles subjected to a cross-flow is studied and the effect of re-entrant jet is analyzed, which can reduce the initial momentum of the water layer and promote the coalescence of bubbles. After that, the influences of the initial pressure inside the air chamber and motion speed of the body have been analyzed. The bubble length and its growth rate are proportional to the motion speed of the body and the initial pressure inside the air chamber has little influence on the growth rate of the bubble length. Moreover, only after the stream-wise length of the water layer reaches critical value, bubble coalescence can occur. According to parametric studies, the critical value of stream-wise length of the water layer depends on the motion speed of the body and a correlation has been estimated.

The velocity and thermal fields of two parallel plane jets using simultaneous PIV and two-color LIF measurements

This paper presents the results of simultaneous measurements of the velocity and thermal fields of two parallel plane jets. Jets with two different distance ratios having Reynolds numbers of 850 and 910 are reported, with distance ratios selected based on whether or not they have a periodic vortex shedding phenomenon, and were experimented using simultaneous particle imaging velocimetry and two-color laser induced fluorescence. Results showed that jets with a distance ratio of 1.4 have a periodic vortex shedding phenomenon, which are absent in jets with a distance ratio of 2.0. The overall structure of the temperature fields are dissimilar to the velocity fields and major differences in Reynolds shear stress and turbulent heat flux were also apparent, showing that transfer processes of momentum and heat flux differ greatly. The spreading of turbulence statistics showed the mixing capabilities of the periodic vortex shedding phenomenon.

Effect of parallel magnetic field on growth and properties of jet electrodeposited Ni-Co-SiC composite coatings

Ni-Co alloy and composite coatings were fabricated by a parallel magnetic field-assisted jet electrodeposition (PMAJE), and the effect of nano SiC particles on the properties of Ni-Co alloy coating was studied. Crystal structure, micromorphology and 3D profile, microhardness, adhesion and corrosion resistance of the Ni-Co alloy and composite coatings were studied respectively. The deposition mechanism of Ni-Co alloy and composite coatings in PMAJE was analyzed. It was found that the parallel magnetic field (PM) significantly affected the surface morphology and flatness of Ni-Co alloy and composite coatings in jet electrodeposition. The microprotrusions of Ni-Co alloy and composite coating gradually increased with the magnetic flux density. PM refined the grain growth of Ni-Co alloy and composite coating when the magnetic flux density was 50 mT. Compared with jet electrodeposition, the Ni-Co-SiC composite coating exhibited a high mechanical properties and corrosion resistance in PMAJE. The microhardness of Ni-Co-SiC composite coating was 582.4HV, the adhesion was 27.2 N, the corrosion current density of Ni-Co-SiC composite coating was as low as 5.824 μA·cm−2 and the polarization resistance reached 6471 Ω·cm2, which was better than that of jet electrodeposition.

Assessing the Structure-Based Turbulence Model Performance for Thermal Striping Applications Using Symmetric Jet Experiments

Turbulent mixing of coolant streams can result in an oscillatory mixing phenomenon called thermal striping. These fluctuations have the potential to lead to anticipated thermal fatigue failures in advanced nuclear reactors. To predict thermal striping, robust and computationally affordable modeling tools that are capable of accurately representing complex turbulence are needed. Hybrid turbulence approaches, such as detached-eddy simulation and scale-adaptive simulation, have shown some success in resolving complex unsteady turbulence for massively separated flows, however the applicability of these models to internal flows is limited. A STRUCTure-based (STRUCT) second-generation Unsteady Reynolds-Averaged Navier–Stokes turbulence model was recently proposed at the Massachusetts Institute of Technology to robustly extend the applicability of hybrid closures. In this work, the STRUCT model is evaluated using experimental data taken at the Reactor Cavity Cooling System separate-effects test facility at the University of Michigan. The experiments observed the interaction of parallel symmetric rectangular jets, and include measurements for mean profiles of velocity and Reynolds stresses. In the present work, the simulation results are assessed against mean profiles of velocity and Reynolds stresses, demonstrating the ability to reproduce the unsteadiness of the jets in close agreement with the measurements at considerably reduced computational cost.

Four-step global kinetics mechanism for diluted combustion fueled with kerosene

The aim of the present study was to develop a global kinetics mechanism capable of predicting characteristics of the combustion of kerosene diluted with combustion gases. A four-step model of the Aachen mechanism was developed via modification of a mechanism for lower molecular weight hydrocarbons. A simplified fuel was used to simulate kerosene, the thermodynamic properties of which were estimated based on the surrogate fuel in the Aachen mechanism. Combustion of this model fuel in a perfectly stirred reactor (PSR) operating under high-temperature air combustion (HiTAC) conditions was simulated using inflow mixtures preheated and diluted with nitrogen. In this new mechanism, the activation energy for the fuel consumption reaction and the pre-exponential factor for the hydrogen oxidation reaction were adjusted. The resulting model accurately predicted variations in temperature and major species over time. The four-step mechanism was also validated under conventional flame and moderate or intense low oxygen dilution (MILD) combustion conditions. The PSR simulations with the four-step mechanism accurately predicted variations in temperature and major species under MILD combustion conditions. In addition, this four-step mechanism was confirmed to be superior to other global mechanisms with regard to predicting the mole fractions of various species. Experimental trials were carried out with diluted kerosene flames formed above parallel jet burners composed of a fuel spray nozzle and an oxidizer nozzle, and it was found that NOx emission decreased with increasing distance between nozzles even for spray combustion. These diluted flames were numerically simulated using the eddy dissipation concept model combined with the four-step mechanism. The temperature distributions obtained from the simulations showed good agreement with the experimental data in the downstream region.

Effect of Air Inlet Spacing on Air Distribution and Ventilation Performance of Stratum Ventilation Using Multiple Parallel Jets

An appropriate layout of air terminals can improve the thermal comfort and air quality for room occupants. Stratum ventilation (SV) can efficiently provide thermal comfort and quality of inhaled air. However, investigations into the impact of the supply air inlet spacing on performance under SV are lacking. This study conducted experimental measurements and numerical simulations to analyze the effect of the supply inlet spacing on the air distribution and ventilation performance of the SV. Six theoretical axial velocity attenuation formulas were verified using experimental results. The effective draft temperature for SV (EDTS), air diffusion performance index (ADPI), the local mean age of air (LMAA), and energy utilization coefficient (EUC) were used as the evaluation indexes. The results indicated that the modified Abramovich formula was the most suitable for predicting the attenuation of the axial velocity of multiple parallel jets under SV. At an air supply spacing of 650 mm, the position near the central air supply inlet experienced a cold thermal environment with high air velocity and low air temperature; the positions on both sides of the room were warmer owing to insufficient airflow. The air distribution at these positions improved with increasing air inlet spacing, but some areas could not be covered by the supply airflow. The optimal value of ADPI corresponded to an air inlet spacing of 800 mm, whereas optimal LMAA and EUC corresponded to a spacing of 950 mm. Analysis of a multi-objective optimization method indicated that an air inlet spacing of 800 mm provided optimal overall performance.