Cross-flowing Air Stream Research Articles

Experimental and CFD analyses of pollutant dispersion around an isolated cylindrical building

An investigation of the dispersion of pollutants ejected from a chimney around a three-dimensional cylindrical obstacle within a crossflow air stream was conducted in this paper. The dynamic evolution of air seeded with glycerin particles ejected from an elevated jet around the isolated obstacle in a wind tunnel was experimentally studied using the particle image velocimetry technique and under different velocity ratios between the chimney ejection and the wind source. The dispersion of CO2 pollutant around the cylindrical building was numerically predicted based on measured experimental data using the finite volume method and the Reynolds stress model. A good agreement between experimental and computational results was found. Velocity, temperature, and concentration results revealed that the velocity of the wind together with the presence of the isolated building obstacle influenced the flow structure.

Experimental investigation of crossflow air effect on spray characteristics of gelled Jet-A1 fuel using an internally impinging air-blast atomizer

The present study aims to experimentally analyse the influence of crossflow aerodynamic parameters on the spray characteristics of gelled Jet-A1 fuel. A Jet-A1 gel fuel is prepared by combining 85 percentage by weight (wt. %) of Jet-A1 liquid as base fuel with an organo-gel solution having Thixatrol ST (7.5 wt. %) as gelling agent and Rectified xylene (7.5 wt. %) as solvent. Shear flow test is performed on the gel fuel using a rotational rheometer, to understand its non-Newtonian rheological properties. An internally impinging air blast atomizer is used to study the gel spray characteristics by injecting the gel fuel transversely into the subsonic cross flowing airstream environment. Gel spray images are captured by generating a laser sheet along the midplane of the test section across the crossflow air stream direction using the Laser Sheet Photography (LSP) technique. The captured gel spray images are used to visualize the gel spray structure in both spray axis as well as the crossflow air stream direction. These images are then investigated by developing an algorithm in MATLAB using an image processing tool. By incorporating the canny edge detection method into the MATLAB code, gel spray characteristics such as droplet size distribution, Sauter-mean-diameter, Arithmetic-mean-diameter, windward trajectories, and maximum penetration height are determined. Further, the effect of aerodynamic parameters such as crossflow Reynolds’ number and aerodynamic Weber number on the gel spray characteristics are investigated. Droplet size distribution and maximum penetration height of gelled Jet-A1 spray revealed its dependence on crossflow Reynolds’ number. Sauter-Mean-Diameter of gel droplets and windward trajectories of the gel spray decreases significantly as the aerodynamic Weber number increases. Thus, it can be concluded that the fineness of the spray is also a function of the crossflow aerodynamic parameters other than the atomizer.

Effect of Cross-Flow Swirl on the Trajectory of Spray in an Annular Passage

Abstract This paper presents the experimental analysis of the impact of swirl number of cross-flowing air stream on liquid jet spray trajectory at a fixed air flow velocity of 42 m/s with the corresponding Mach number of 0.12. The experiments were conducted for 4 different swirl numbers (0, 0.2, 0.42, and 0.73) using swirl vanes at air inlet having angles of 0 deg, 15 deg, 30 deg, and 45 deg, respectively. Liquid to air momentum flux ratio (q) was varied from 5 to 25. High-speed (at 500 fps) images of the spray were captured, and those images were processed using matlab to obtain the path of the spray at various momentum flux ratios. The results show interesting trends for the spray trajectory in swirling air flow. High swirling flows not only lead to spray with lower radial penetration due to sharp bending and disintegration of liquid jet but also result in spray with large spread and spray area. Based on the results, correlations for the spray path have been proposed, which incorporates the effects of the swirl number of the air flow.

Experimental Analysis and Phenomenological Model for Liquid Jet Breakup in Swirling Flow of Air

This paper presents detailed analysis of an experimental investigation of the impact of swirl number of subsonic cross-flowing air stream on liquid jet breakup at an airflow Mach number of 0.12, which is typical in gas turbine conditions. Experiments are performed for four different swirl numbers (0, 0.2, 0.42, and 0.73) using swirl vanes at air inlet having angles of 0 deg, 15 deg, 30 deg, and 45 deg, respectively. Liquid to air momentum flux ratios (q) have been varied from 1 to 25. High-speed images of the interaction of liquid and air streams are captured and processed to estimate the jet penetration height as well as the breakup location for various flow conditions. The results show unique behavior for each swirl number, which departs from the straight flow correlations available in the literature. Based on the results, an attempt has been made to understand the physics of the phenomena and come up with a simplified physical model for prediction of jet penetration. Furthermore, the high-speed images show a dominant influence of liquid column fluttering on fracture mechanism (column or shear breakup mechanism).

Numerical investigation of the multi-pin electrohydrodynamic dryer: Effect of cross-flow air stream

This article presents the results of numerical simulation and experimental study of a multi-pin electrohydrodynamic (EHD) dryer. Combined effect of EHD flow and the external air cross-flow on drying performance was investigated with 3-D numerical model, which accounts for electric field, electric charge transport, external air cross-flow and material-gas moisture transport. Effect of cross-flow air stream on drying was positive in the range of low velocities, changing to negative at high velocities due to counteracting with EHD flow. Numerical simulation predicted previously unknown effect of EHD flow on the cross-flow air stream, which was quantified as an increase of airway resistance. This prediction was fully validated by experiments. Both numerical simulation and experiment proved that for given intensity of EHD flow there is an optimum value of the cross-flow, resulting in maximum drying performance. The numerical model can be applied to determine the optimal operating parameters for multi-pin EHD dryer.

Three-dimensional, numerical investigation of reactant injection variation in a H2/air rotating detonation engine

This study characterizes the mixing performance and non-reacting injection flow field within a rotating detonation engine. Initial reactant mixing is accomplished through a counter-rotating vortex pair developing from a jet-in-a-crossflow situation. Radially injected air wraps around the fuel jet, shearing the hydrogen into the vortex structure. The three-dimensional baseline geometry produces poor hydrogen/air mixing in the injection zone due to low fuel penetration into the wide, cross-flowing air stream and low air entrainment into the vortex structure. Injection parameters, including reactant flow rate, injection area, and fuel injection distribution, are varied to assess the impact on mixing. Decreasing air injection area and fuel injection area improve fuel penetration into the air stream and the vortex mixing mechanism, enhancing the level of mixedness within the annulus. For a given air injection area, an optimized number of fuel injection holes exists that provides the greatest level of mixedness. Variations from the optimized geometry reduce the effectiveness of the vortex mixing mechanism. Staggering fuel injection holes produces a decrease in mixedness when compared to collinear fuel injection.

Numerical investigation of injection within an axisymmetric rotating detonation engine

This investigation characterizes the mixing processes and injection flow field within a rotating detonation engine. The axisymmetric baseline geometry produces poor fuel/air mixing in the immediate injection region due to reduced fuel penetration into the wide, perpendicularly flowing air stream. Injection parameters including reactant flow rate, injection area, and placement of the fuel injection are varied from the baseline geometry to assess the impact on mixing. Decreasing reactant injection areas improves fuel penetration into the cross-flowing air stream, and turbulent diffusion of the fuel is enhanced within the annulus increasing local equivalence ratio. When fuel is injected into the air slot, local equivalence ratio is enhanced due to the increasing mixing length between the recirculation region and fuel injection. Emulating nozzle integration by increasing annulus back-pressure increases local equivalence ratio in the injection region due to increased convection residence time.

Breakup and atomization characteristics of mono-dispersed diesel droplets in a cross-flow air stream

This paper describes the microscopic and macroscopic breakup characteristics, as well as the velocity and size distributions, of mono-dispersed droplets in relation to the breakup regimes. For this experiment, a droplet generator equipped with a piezo stack produced mono-dispersed droplets. The droplet-breakup phenomenon due to the cross-flow was captured in microscopic and macroscopic views by using the following: a spark lamp, a Nd:YAG laser, a long distance microscope and a CCD camera as a function of the Weber number. Along with the analysis of the images, the droplet size and velocity distributions were measured in the near nozzle region by a phase Doppler particle analyzer system at bag, stretching and thinning, and catastrophic breakup regimes. The results of this study showed the size and velocity distributions of disintegrated droplets at the bag, stretching and thinning, and catastrophic breakup regimes. In the bag breakup regime, the droplets separated into small and large droplets during breakup. Alternatively, the droplets disintegrated at a shorter duration and formed a cloud, similar to a fuel spray injected through an injector, in the stretching and thinning and catastrophic breakup regimes.

Mean and fluctuating pressure measurements over a circular cylinder in cross flow using plastic tubing

Not only can mean pressures on a surface over which a fluid flows be accurately measured using a plastic tube which connects the surface tapping points to a remote pressure transducer, but the fluctuating pressures can also, provided that the transfer function, which relates the fluctuating pressures at the opposite ends of the tubing, is known. This technique was used here to measure the mean and fluctuating pressures on the surface of a circular cylinder subject to a cross-flowing airstream in the Reynolds number range from 6.8 × 104 to 9.6 × 104 based on cylinder diameter. Good agreement with published results gave confidence in the technique.

An empirical investigation into the external heat transfer of a U-bend in cross-flow

An experimental investigation into the global external heat transfer coefficients for circular U-bends (180° bends) in a cross-flow air stream was performed using six different curvature ratios (δ) covering a Reynolds number range of 3000–80 000 while imposing a uniform-wall-temperature (UWT) boundary condition. The measurements specifically focused on a zero-degree angle-of-incidence, in which the external flow direction is parallel to the bend plane. A straightforward method to measure the global external heat transfer that is not dependent on geometry but limits the testing to UWT was employed. The external heat transfer coefficients for circular U-bends in cross-flow were found to be consistently higher than those for corresponding straight cylinder flows. Also, a strong dependence on the curvature ratio, δ, was observed. Measurements presented demonstrate a peak enhancement in the external heat transfer for curvature ratios close to an optimum value of δopt ≈ 0.23. A geometrical analysis was performed to explain the experimental finding of a distinct optimum curvature ratio (δopt) as a self-inflicted wake effect. U-bend Nusselt numbers were found to be over four times larger than those for straight tubes of identical diameter and Reynolds number. An engineering correlation for the global external Nusselt number is offered.

Numerical modeling of turbulent jet diffusion flames in the atmospheric surface layer

The evolution of turbulent jet diffusion flames of natural gas in air is predicted using a finite-volume procedure for solving the flow equations. The model is three dimensional, elliptic and based on the conserved-scalar approach and the laminar flamelet concept. A laminar flamelet prescription for temperature, which is in agreement with measurements in methane/air flames and accounts for radiative heat losses, has been modified and adapted to natural-gas flames. The k- ϵ- g turbulence model has been used. Different probability-density functions for the conserved scalar and an alternative method which does not require the use of a pdf are employed. The model has been applied to flames in the buoyancy-momentum transition regime, in both cases where the fuel jet is immersed in a co-flowing or in a cross-flow air stream whose properties correspond to the atmospheric surface layer. Experiments have been carried out for a horizontal flame in a wind tunnel with simulated atmospheric boundary layer, and measurements of temperature distributions are compared with the numerical results; a good agreement is found. The influence of wind properties on flame shape has been investigated. For horizontal flames, a correlation is proposed for the stoichiometric flame length as a function of the Froude number and the wind to jet velocity ratio. Flame length predictions have been compared with available experimental data and correlations proposed in the literature.

Experimental study of the flow around a rotating cylinder in crossflow

This paper presents the results for an experimental investigation of the flow pattern around a rotating cylinder placed in a crossflow air stream. The velocity profiles upstream and downstream of the cylinder, skin friction coefficient, stagnation, and separation points are obtained for different rotation speed ratios (α = Ω R/ V ∞) varying from 0.0 to 1.25. Compared to the non-rotating cylinder in crossflow, it was found that the position of the stagnation point for the rotating cylinder is a strong function of the rotational speed; however, the separation points are only slightly dependent on such speed. This investigation indicates also that the skin friction dominates on the upstream moving-wall of the cylinder. Concerning the flow behind the rotating cylinder the measurements indicate that the wake behind the cylinder is asymmetric; for example, it is shifted in the direction of rotation and its axial length and width becomes smaller for the considered rotation speed ratios. For the flow ahead a slight effect of rotation is noticed. Some of the results obtained here are compared with a simple theoretical model [1] and the comparison shows fair agreement.

Total Soot Yield from a Propane Diffusion Flame in Cross-Flow

Abstract In this paper, we examine the soot yield from a propane diffusion flame in a cross-flowing air stream. Soot yield, defined as the net amount of soot per gram of fuel, from a diffusion Hame in cross-flow was compared to that from a diffusion flame in co-flowing air. We observed that the orientation of the fuel jet relative to the air stream has a significant effect on the flame shape and on the soot yield. We varied the air velocity, fuel velocity, and jet diameter to determine The effect of each of these on the soot yield from the flame in cross-flow. At low air velocities, increasing the air velocity decreases the soot yield dramatically but further increases do not have a significant effect unless the fuel velocity is increased also. The jet diameter does not affect the soot yield at a constant velocity. Finally, we propose a correlation of our data to a mixing index defined as the product of the air and fuel velocities.

Effects of Crossflow on Impingement Heat Transfer

An experimental study of the effects of a cross-flowing air stream on the heat transfer characteristics of single lines of circular air jets impinging on plane surfaces is presented. The study models a practically important class of cooling configurations where jets issuing from one wall of a channel and impinging on the opposite wall are deflected by the exhausting flow of upstream jets. The upstream and downstream changes in heat transfer are evaluated separately and compared with heat transfer rates obtained in the absence of crossflow.

Experimental investigation of an electric arc in transverse aerodynamic and magnetic fields.

An experimental study was made of the aerodynamic behavior and heat transfer of an electric arc of 200 to 400 amp which was held stationary in an atmospheric pressure cross-flow airstream of velocities up to 60 fps by a transverse magnetic field up to 50 gauss. An open-jettype facility permitted the use of diagnostic techniques in the arc wake region. The arc's significant dimension transverse and parallel to the flow was measured. This dimension increased perpendicular to the flow and decreased parallel to the flow when the transverse blowing velocity and magnetic field were increased. Velocity profiles, energy flux distributions, turbulence level and frequency measurements were obtained in the arc wake using miniaturized probe techniques. Flow visualization studies were performed. The input and output arc power distribution was obtained. Comparisons between the wake of the arc and that of a heated solid cylinder indicated similarity in vortex shedding frequency and convective heat transfer. The energy convectively transferred to the arc wake was equal to the additional electrical energy input required to maintain the arc in the presence of the transverse blowing. A drag coefficient for the arc, analogous to that of a solid cylinder in cross flow, was defined and measured to have values between 0.7 and 1.2.