Near-Field Flow Structure and Entrainment of a Round Jet at Low Exit Velocities: Implications on Microclimate Ventilation

This paper explores the flow structure, mean/turbulent statistical characteristics of the vector field and entrainment of round jets issued from a smooth contracting nozzle at low nozzle exit velocities (1.39–6.44 m/s). The motivation of the study was to increase understand of the near field and get insights on how to control and reduce entrainment, particularly in applications that use jets with low-medium momentum flow like microclimate ventilation systems. Additionally, the near field of free jets with low momentum flow is not extensively covered in literature. Particle image velocimetry (PIV), a whole field vector measurement method, was used for data acquisition of the flow from a 0.025 m smooth contracting nozzle. The results show that at low nozzle exit velocities the jet flow was unstable with oscillations and this increased entrainment, however, increasing the nozzle exit velocity stabilized the jet flow and reduced entrainment. This is linked to the momentum flow of the jet, the structure characteristics of the flow and the type or disintegration distance of vortices created on the shear layer. The study discusses practical implications on microclimate ventilation systems and at the same time contributes data to the development and validation of a planned computational turbulence model for microclimate ventilation.

Numerical Simulation of Flow Field in Air-Jet Loom Main Nozzle

To improve airflow injection capacity of the main nozzle and decrease backflow phenomenon, a new main nozzle structure with two throats is designed. Negative pressure value and negative pressure zone length are first proposed evaluating the strength of backflow phenomenon. Commercial computational fluid dynamic (CFD) code “Fluent” is performed to simulate the flow field inside and outside the main nozzle. Exit velocity increases about 10 m/s in new main nozzle. Airflow core length of the new main nozzle is 35% higher than that of commonly used main nozzle. Smaller negative pressure value and shorter negative pressure zone length mean a weaker backflow phenomenon in the new main nozzle. Bigger air drag force indicates stronger weft insertion ability in the new main nozzle.

Stochastic Simulation of Soot Formation Evolution in Counterflow Diffusion Flames

Soot generally refers to carbonaceous particles formed during incomplete combustion of hydrocarbon fuels. A typical simulation of soot formation and evolution contains two parts: gas chemical kinetics, which models the chemical reaction from hydrocarbon fuels to soot precursors, that is, polycyclic aromatic hydrocarbons or PAHs, and soot dynamics, which models the soot formation from PAHs and evolution due to gas-soot and soot-soot interactions. In this study, two detailed gas kinetic mechanisms (ABF and KM2) have been compared during the simulation (using the solver Chemkin II) of ethylene combustion in counterflow diffusion flames. Subsequently, the operator splitting Monte Carlo method is used to simulate the soot dynamics. Both the simulated data from the two mechanisms for gas and soot particles are compared with experimental data available in the literature. It is found that both mechanisms predict similar profiles for the gas temperature and velocity, agreeing well with measurements. However, KM2 mechanism provides much closer prediction compared to measurements for soot gas precursors. Furthermore, KM2 also shows much better predictions for soot number density and volume fraction than ABF. The effect of nozzle exit velocity on soot dynamics has also been investigated. Higher nozzle exit velocity renders shorter residence time for soot particles, which reduces the soot number density and volume fraction accordingly.

Analysis of Jet Characteristics Among Various Cold Spray Nozzles

This research is conducted mainly to analyze the jet characteristics of various cold spray nozzles. This study presents the theoretical and practical aspects of Cold Spray process modeling, discusses multiple numerical analysis research areas, and determines the significant parameters to be considered while developing a custom cold spray setup and exhibits analysis-based correlations. The simulations were performed on some meshes of different density using the SST turbulent model in Star CCM+ solver. For the first time, in this work, the jet characteristics inside a step drilled nozzle was presented; Furthermore, shock diamond formation was found inside the divergent section of step drilled nozzle which strongly influence the flow regime with sharp fluctuations. The comprehensive comparison between step drilled nozzle, conical nozzle and curved nozzle indicates that curved nozzle results in slightly higher nozzle exit velocity. However, results have suggested that the curved nozzle can achieve much higher velocities by optimizing the nozzle length.

High Speed X-Ray Imaging for Nozzle Exit Velocity and Density Distribution Measurements of GDI Nozzles

Investigation of the primary breakup region of gasoline sprays is important for future nozzle development. It improvesthe principal understanding of inner nozzle flow and spray breakup. It also allows validating and developingCFD models. Due to the high optical density common measurement techniques like Phase Doppler Anemometryreach their limit in optical dense sprays as in the primary breakup region. High Speed X-Ray Imaging is capable tomeasure 2D velocity distributions directly at the spray hole exit. For generating the intense X-Ray beam the synchrotronAdvanced Photon Source at Argonne National Laboratory is used. Passing through the spray the X-Raybeam is changed by two different physical principles: absorption and phase contrast. Absorption can be applied tomeasure the density of the spray. Phase contrast is used to visualize the borders of droplets and ligaments withhigh contrast. The accelerated electron bunches inside the synchrotron have a constant period length to each other.This leads to an accurate pulsed X-Ray beam (periodicity: 68 ns). The use of multi exposure with very short X-Raypulses (17 ns) shows the traveled distance of the spray droplets and ligaments. The spray speed (150-250 m/s) iscalculated by dividing these distances with the time gap between two X-Ray pulses. The X-Ray measured densitydistributions and velocity distributions are combined to calculate the spray force rate. The so gained force rate isvalidated with a spray force measurement performed at the Spray Momentum Test Bench (SMTB) at ContinentalAutomotive GmbH. The study is focusing on the measurement setup of High Speed X-Ray Imaging at ArgonneNational Laboratory and the evaluation algorithms.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4598

Flame Hysteresis Effects in Methane Jet Flames in Air-Coflow

Presented are the results of experiments designed to investigate flame lift-off behavior in the hysteresis regime for low Reynolds number turbulent flows. The hysteresis regime refers to the situation where the jet flame has dual positions favorable to flame stabilization: attached and lifted. Typically, a jet flame is lifted off of a burner and stabilized at some downstream location at a pair of fuel and coflow velocities that is unique to a flame at that position. Since the direction from which that condition is arrived at is important, there is an inherent hysteretic behavior. To supplement previous research on hysteretic behavior in the presence of no coflow and low coflow velocities, the current research focuses on flames that are lifted and reattached at higher coflow velocities, where the flame behavior includes an unexpected downstream recession at low fuel velocities. Observations on the flame behavior related to nozzle exit velocity and coflow velocity are made using video imaging of flame sequences. The results show that a flame can stabilize at a location downstream despite a decrease in the local excess jet velocity and assist in determining the effect of coflow velocity magnitude on hysteretic behavior. These observations are of utility in designing maximum turndown burners in air coflow, especially for determining stability criteria in low fuel-flow applications.

Study of Mixing Characteristics of Rectangular Turbulent Jets

This paper reports a numerical study to investigate the effect of various parameters on the mixing characteristics of two-dimensional turbulent co-flowing jets. The paper is a part of an ongoing research to simulate and investigate the mixing and combustion processes of subsonic jets. Results are obtained with a finite volume CFD code. Turbulence is treated with a two equation k-ε model. The effects of nozzle width (H/W), and nozzle exit velocity (U) on the merging and the combined points are also studied. Different initial jet velocities have been examined corresponding to Reynolds number of 2,000 to 35,000 moreover, three values of the nozzle width ratio H/W = 1.5, 2.0, and 2.5 are used. The results provide acceptable agreement in comparison with the experimental results in prior literature. Results show that the initial velocity of the inner jet has a significant effect on the location of the merging and combining points from the jet exit plane. Also, the increase of the initial jet velocity of the outer jets increases the mixing rate of the jets.