Crossflow Interaction Research Articles

Investigating crossflow interactions in multi-perforation cooling systems: Experimental and numerical insights

This study investigates the effects of employing a multi-perforation cooling system on crossflow interactions, utilizing both numerical simulations and experimental methods. A 30° inclined multi-perforation cooling system is applied on a heated flat plate as part of the experimental setup. Large Eddy Simulation (LES) is utilized for Computational Fluid Dynamics (CFD) calculations to analyze the impact of multi-perforation on heat transfer dynamics. Validation is conducted by comparing the results with experimental investigations. The study encompasses both aerodynamic and thermal considerations, employing Particle Image Velocimetry (PIV) and infrared techniques for analysis, while wall heat flux is measured using electrical discharge. Experimental data on film cooling are collected through these methods, focusing on the impact of blowing ratio (M) and the number of open jet rows (R). The LES-based CFD simulations accurately predict the trajectory of the cooling film. Notably, the influence of flow through the multi-perforation on the primary flow is observable across both configurations of open jet rows (4R and 8R near the wall). The outcomes substantiate the formation of a cooling film emanating from the jet’s fourth row (4R configuration), thereby validating the LES simulations. Visualization of wall temperatures illustrates the formation of a cooling film, discernible through the visible heat distribution from row 4R. However, discrepancies arise in predicting the adiabatic cooling efficiency towards the exit of the 4R region and the commencement of the 8R region, potentially attributed to the alterations induced by the additional multi-perforated injection. The novelty and the main objective of this study is to evaluate the impact of the variation of geometric parameters between the jets and the main flow on the dispersion of the jets themselves. A novel function, tailored to the temperature evolution of the cooling gas, is employed for data analysis. Two geometry configurations, featuring varying open hole areas, are evaluated. The film-cooled heat transfer coefficient, depicted by the Nusselt number, and the adiabatic film cooling efficiency results, serve to appraise the film cooling efficacy of the configurations.

Heat Transfer Enhancement by Mitigating the Adverse Effects of Crossflow in a Multi-Jet Impingement Cooling System in Hexagonal Configuration by Coaxial Cylindrical Protrusion—Guide Vane Pairs

A novel compound multi-jet impingement system for enhanced cooling of a flat surface by augmenting its area with cylindrical protrusions (CPs) equipped with coaxial guide vanes (CGVs) and reducing deflection of jets by crossflow has been developed for high-heat removal applications. The cooling performance of coaxial circular jets impinging on the top faces of CPs placed in hexagonal configuration on a flat plate is evaluated by three-dimensional (3D) computational fluid dynamics (CFD) simulations. Jets impinging on the top faces of the protrusions are directed to their lateral faces and then to the base plate by the CGVs around the protrusions, resulting in up to 62.8% improvement in heat transfer rate with a minor increase in pressure drop. Effects of protrusion height and diameter on the pressure drop and cooling performance are studied for jet Reynolds (Re number range of 5000–20,000. Due to both shortened jet impingement lengths as the height of protrusions is increased and directing the expended fluid away from the impinging jets by CGVs, adverse effects of jet–crossflow interactions on cooling performance and fluid pumping power are significantly reduced. Performance evaluation criterion (PEC) of the novel compound multi-jet impingement cooling system (CMJICS) can be as high as 1.52.

Flow structure comparison of film cooling versus hybrid cooling: a CFD study

Abstract Film and jet impingement cooling are widely used techniques in gas turbine vane and blade cooling. The present work investigates and compares the flow structure of a film-cooled flat plate with a hybrid cooling scheme. The hybrid cooling scheme combines both impingement hole and film holes and is named combined impingement-film (IFC) cooling. Experimental validation and computational analyses are carried out on a flat plate with film holes. Different flow parameters, such as velocity pattern, Turbulent kinetic energy, and streamline flow structure, are compared for the two cases in different regions of the flat plate. It is observed that the hybrid scheme shows advantages over film cooling. The jet-to-jet interaction, jet crossflow interaction, and vortex formation are the main factors affecting film cooling performance. There is a 52 % drop in turbulent kinetic energy for the hybrid cooling compared to the film cooling at the film hole exit. More mixing in the coolant and mainstream interaction is observed for the FC case than in the IFC.

Heat transfer enhancement in a square channel with a set of triangular prisms: an experimental study

ABSTRACT In this study, heat transfer behavior in a channel adapted with a set of equilateral triangular prisms was investigated experimentally in a turbulent flow regime, with a range of Reynolds numbers corresponding to 1x104<Re<8x104. The cross-section of the channel is square, and the triangular prisms were installed in the middle of the channel. In the test section of the channel, only the bottom wall was heated with constant heat flux. The square channel geometry and the triangular obstacle dimensions were assumed as fixed. Multiple equilateral triangular prisms were used with three different tandem arrangements and four different attack angles in the cross-flow of air. The heated wall temperatures, and channel inlet and outlet temperatures were measured using thermocouples on the test section. The triangular prisms changes flow structure in the flow field by cross-flow interaction, and increases the heat transfer from the heated surface. It was observed that compared to the smooth channel, the heat transfer was enhanced by about 22% for a single triangular prism. The obtained results are given as dimensionless parameters and discussed in this study. Results are compared with the existing literature and show strong similarities. Overall, in our study, triangular prisms showed a more noticeable increase in heat transfer for channel flows.

Numerical study on the transverse jet flow and mixing characteristics of hydrogen/metal powder fuel in powder fuel scramjet

The powder fuel scramjet is one of the most promising propulsion systems for future hypersonic vehicles. To study the flow mechanism and mixing characteristics of gas-particle two-phase fuel in a supersonic crossflow. In this study, the porosity was introduced into the Navier-Stokes equation, and a program suitable for solving gas-particle two-phase flow in the supersonic flow field was developed. The evolution process of vortex structure, hydrogen/metal powder fuel distribution, jet penetration depth, and mixing efficiency in the single-hole and the double-hole jet flow fields were analyzed. The results show that the particle disturbance reduces the center position of the Mach disk, the height of the barrel shock and jet shear layer, intensifies the Kelvin-Helmholtz instability in the flow field and forms a large number of large-scale coherent structures downstream of the jet. The jet and supersonic crossflow interaction form a large-scale CVP structure for hydrogen fuel. Among them, three pairs of CVP structures (including CVP-B, CVP-C, and CVP-D) appear before and after the downstream of the double-hole jet. CVP-C is the main factor affecting the shape and area of the hydrogen fuel reaction zone. For powder fuel, the penetration depth of fuel is mainly affected by the height of the Mach disk and the shear layer. The higher Mach disk and shear layer position, the greater the velocaity obtained by particles and the higher the penetration kinetic energy. The double-hole injection system has a higher fuel penetration depth and higher mixing efficiency of hydrogen and powder fuel. The mixing efficiency of hydrogen fuel at the outlet of the double-hole jet is 0.728, which is 10% higher than that of the single-hole jet; The mixing efficiency of powder fuel is 0.342, which is 2% higher than that of the single-hole jet. However, the complex flow field structure of the double-hole jet leads to a large total pressure loss, about 0.74% higher than that of the single-hole jet.

Large Eddy Simulation of Scalar Mixing in Jet in a Cross-Flow

Jet in a cross-flow (JICF) is a flow arrangement found in many engineering applications, especially in gas turbine air–fuel mixing. Understanding of scalar mixing in JICF is important for low NOx burner design and operation, and numerical simulation techniques can be used to understand both spatial and temporal variation of air–fuel mixing quality in such applications. In this paper, mixing of the jet stream with the cross-flow is simulated by approximating the jet flow as a passive scalar and using the large eddy simulation (LES) technique to simulate the turbulent velocity field. A posteriori test is conducted to assess three dynamic subgrid scale models in modeling jet and cross-flow interaction with the boundary layer flow field. Simulated mean and Reynolds stress component values for velocity field and concentration fields are compared against experimental data to assess the capability of the LES technique, which showed good agreement between numerical and experimental results. Similarly, time mean and standard deviation values of passive scalar concentration also showed good agreement with experimental data. In addition, LES results are further used to discuss the scalar mixing field in the downstream mixing region.

Analysis of spray dynamics of urea–water-solution jets in a SCR-DeNOx system: An LES based study

Among various advanced emission control technologies used to reduce NOx emission from diesel engines, the selective catalytic reduction (SCR) technique has proven to be more efficient. However, it is still facing the improvement of low temperature performance and the avoidance of urea deposit formation due to complex spray–wall interaction process.To gain more deep understanding of these processes, a spray module is developed for the first time within a full LES framework based on an Eulerian–Lagrangian approach. It includes especially (1) a primary and a secondary breakup model along with an advanced collision model, both to account for the atomization generated close to the nozzle exit by the multi-hole circular orifice injector used and (2) a multi-component droplet evaporation model along with a two-way coupling between the carrier gaseous phase and the droplet liquid phase throughout the dilute spray region.The suggested model is applied to evaluate the dynamics of the multi-component droplets spray resulting from an adBlue injection into a hot gas cross flow inside a channel mimicking a SCR-DeNOx flow system. First, an assessment of the multi-component droplet evaporation under hot stagnant gas environment is made. Second, the results of the adBlue spray/gas phase cross flow interaction are reported in terms of comparison between experimental data (spray structure, spray angle and penetration depth, droplet size distribution and velocity) with numerical simulations. It turns out that the model designed is able to capture in a satisfactory way the spray properties in a SCR like system.

Characterising the heat and mass transfer coefficients for a crossflow interaction of air and water

An experimental study was performed in order to characterise the heat and mass transfer processes, where an air stream passes through a sheet of falling water in a crossflow configuration. To achieve this, the hydrodynamics of a vertical liquid sheet in a ducted gaseous crossflow were studied. Four distinct flow regimes were identified (a stable sheet, a broken sheet, a flapping sheet and a lifted sheet) and mapped using Reynolds and Weber numbers. Subsequently, the Buckingham π theorem and a least squares analyses were employed leading to the proposal of two new dimensionless numbers referred to as the Prandtl Number of Evaporation and the Schmidt Number of Evaporation. These describe the heat and mass transfer in low temperature evaporation processes with crossflow interaction. Using these dimensionless numbers, empirical correlations for Sherwood and Nusselt numbers for the identified flow regimes were experimentally determined. These correlations were in a good agreement with their corresponding experimental values. It was found that flapping sheets have the strongest heat and mass transfer intensities whereas the weakest intensities were seen for the “stable” sheets.

Structure analysis of adiabatic film cooling effectiveness in the near field of a single inclined jet: Measurement using fast-response pressure-sensitive paint

In the present study, the film cooling effectiveness in the near field (x/D<4) of a single inclined film-cooling jet was measured using fast-response pressure sensitive paint (fast-PSP) and a low-frame-rate CCD camera. Previous experimental data demonstrated considerable variation in this region, and good agreement was established beyond it (x/D>4). The blowing ratios M=0.5 and 1.0 were used. The coolant fluid was nitrogen and the air was the mainstream fluid, and both were kept at the same temperature. A fast-PSP measurement technique was used to determine the variations of the film cooling effectiveness with time. The contours of the time-averaged film cooling effectiveness demonstrated that the coolant spread on the surface throughout the near-hole region at M=0.5, while at M=1.0, the coolant jet detached from the surface immediately behind the hole and reattached downstream around 1.5D behind the hole’s trailing edge. The spatial distribution of the film cooling effectiveness fluctuations and its cross-correlation pattern convincingly reflected the substantial influence of the energetic unsteady flow structures in the jet and cross-flow interaction. Subsequently, the Proper Orthogonal Decomposition (POD) method was used to identify the coherent parts of the film cooling effectiveness, which are regarded as the signatures of the convective large-scale vortical structures above the wall. At M=0.5, the near-hole region was subjected to the dominant influence of the counter-rotating vortex pair (CRVP), characterized by the first two POD modes, which contained up to 40% of the fluctuation energy. Two signatures of large-scale symmetric structures were identified with similar energy levels. The second two POD modes corresponding to the horseshoe vortex near the leading edge of the hole were identified and contained around 10% of the fluctuation energy. Phase-dependent variations of the large-scale convective signatures in relation to the quasi-periodic CRVP and horseshoe vortex were separately detected. At M=1.0, the signatures of the CRVP and the horseshoe vortex were also seen in the POD modes, though they were relatively difficult to distinguish.

Saddle point of attachment in jet–crossflow interaction

Numerical simulation and theoretical analysis were performed to investigate the upstream topology of a jet–crossflow interaction. The numerical results were validated with mathematical theory as well as a juncture flow structure. The upstream critical point satisfies the condition of occurrence for a saddle point of attachment in the horseshoe vortex system. In addition to the classical topology led by a saddle point of separation, a new topology led by a saddle point of attachment was found for the first time in a jet–crossflow interaction. The degeneration of the critical point from separation to attachment is determined by the velocity ratio of the jet over the crossflow, and the boundary layer thickness of the flat plate. When the boundary layer thickness at the upstream edge of the jet is close to one diameter of the jet, the flow topology is led by a saddle point of attachment. Variation of the velocity ratio does not change the topology but the location of the saddle point. When the boundary layer thickness is less than 0.255 of the jet flow diameter, large velocity ratio can generate a saddle point of attachment without spiral horseshoe vortex; continuously decreasing the velocity ratio will change the flow topology to saddle point of the separation. The degeneration of the critical point from attachment to separation was observed.

Effect of vortex generators on hydrodynamic behavior of an underwater axisymmetric hull at high angles of attack

Underwater vehicles when yawed to free stream flow, like submarines in a turning maneuver, produce large vortical separated regions. This vortical flow affects acoustic, body drag, control effectiveness and maneuverability. A suitable way to reduce the effects of this separated flow is to use vortex generators. The main goal of this study is to investigate the effect of the vortex generator on the flow field around a standard underwater model employing the vortex generators by using the oil flow visualization method in wind tunnel only in yaw direction (0° ≤ β ≤ 30° angles of attack, due to the experimental cost) and the CFD method in the pitch direction (0° ≤ α ≤ 30° angles of attack) and the yaw direction (0° ≤ β ≤ 30° angles of attack). Numerical simulations are performed for the investigation of the DARPA SUBOFF bare hull geometry with the vortex generators placed on it. The cross-flow interaction between the pressure side and the suction side is studied in the presence of angle of attack. For this study, the standard k-e model of turbulence is used for solving the Reynolds averaged Navier–Stokes equations (RANS). The effect of angle of attack on the flow structure, the force coefficients and the wall related flow variables are discussed in detail. The numerical simulations show that the strength of the separation is significantly reduced for the model with the vortex generators. The experimental method used for this study is the application of the oil flow visualizing method which can help one to precisely study the location of the flow separation and the cross-flow vortices around the SUBOFF submarine model. The results show that the vortex generators placed along the submarine do indeed significantly reduce the cross-flow separation on the submarine in a high incident angle.

Large-eddy simulation of a pulsed jet into a supersonic crossflow

Abstract Numerical investigation of a pulsed jet injected into a supersonic crossflow was carried out using large-eddy simulation. The corresponding steady jet injection was also calculated for comparison with the pulsed jet injection and for validation against experimental data. Various fundamental mechanisms dictating the intricate flow characteristics, such as multi-scale vortical structures, complex shock systems, mixing properties and turbulence behaviors, have been studied. As the pulsed jet issuing transversely into the supersonic crossflow, the salient shock systems and vortical structures in terms of the instantaneous and phase-averaged flow field are analyzed. The large-scale jet shear vortices are formed along the interface of jet and crossflow due to the pulsation effect, which can promote the engulfing of the crossflow fluid and the mixing of the jet and crossflow fluids. Further we investigate turbulence properties of the jet and crossflow interaction and scalar statistics of the injectant mass fraction to reveal jet penetration enhancement and mixing property improvement for the pulsed jet. Results obtained in this study provide physical insight into the understanding the mechanisms for the interaction of pulsed jet and supersonic crossflow.

On the correspondence between flow structures and convective heat transfer augmentation for multiple jet impingement

The correspondence between local fluid flow structures and convective heat transfer is a fundamental aspect that is not yet fully understood for multiple jet impingement. Therefore, flow field and heat transfer experiments are separately performed investigating mutual–jet interactions exposed in a self-gained crossflow. The measurements are taken in two narrow impingement channels with different cross-sectional areas and a single exit design. Hence, a gradually increased crossflow momentum is developed from the spent air of the upstream jets. Particle image velocimetry (PIV) and liquid crystal thermography (LCT) are used in order to investigate the aerothermal characteristics of the channel with high spatial resolution. The PIV measurements are taken at planes normal to the target wall and along the centreline of the jets, providing quantitative flow visualisation of jet and crossflow interactions. Spatially resolved heat transfer coefficient distributions on the target plate are evaluated with transient techniques and a multi-layer of thermochromic liquid crystals. The results are analysed aiming to provide a better understanding about the impact of near-wall flow structures on the convective heat transfer augmentation for these complex flow phenomena.

Temperatures of Spark Kernels Discharging into Quiescent or Crossflow Conditions

This work determines how the temperature of spark kernels changes for quiescent and crossflow conditions. Previous efforts have primarily considered kernels in quiescent conditions and typically neglected how the temperature changes with time. The latter is significant because the temperature evolution is critical for determining if reactions become self-sustaining. Kernels were generated using a gas turbine engine igniter. Temperatures were determined from radiation intensity measurements (FLIR SC6700) and by solving the radiation transfer equation. Kernels were discharged into a wind tunnel with crossflow velocities between 0 and . Kernels developed into toroidal vortices for all conditions. The average peak temperature in quiescent conditions was near 950 K and nearly 1250 K at the highest crossflow velocity. Higher temperatures were observed at the upstream side of kernels when ejecting into a crossflow. The changes in temperatures observed with crossflow are explained by fluid mechanic literature; vortex crossflow interaction alters entrainment into kernels. The sensible energy of kernels, determined from the temperatures, decreased roughly linearly with time for all flow conditions. The highest crossflow velocity had the lowest sensible energy as a result of reduced apparent volume of the kernel. The trajectory of kernels in a crossflow matches that from pulsed jet literature.

Aerothermal Investigation of a Single Row Divergent Narrow Impingement Channel by Particle Image Velocimetry and Liquid Crystal Thermography

Integrally cast turbine airfoils with wall-integrated cooling cavities are greatly applicable in modern turbines providing enhanced heat exchange capabilities compared to conventional cooling passages. In such arrangements, narrow impingement channels can be formed where the generated crossflow is an important design parameter for the achievement of the desired cooling efficiency. In this study, a regulation of the generated crossflow for a narrow impingement channel consisting of a single row of five inline jets is obtained by varying the width of the channel in the streamwise direction. A divergent impingement channel is therefore investigated and compared to a uniform channel of the same open area ratio. Flow field and wall heat transfer experiments are carried out at engine representative Reynolds numbers using particle image velocimetry (PIV) and liquid crystal thermography (LCT). The PIV measurements are taken at planes normal to the target wall along the centerline for each individual jet, providing quantitative flow visualization of jet and crossflow interactions. The heat transfer distributions on the target plate of the channels are evaluated with transient techniques and a multilayer of liquid crystals (LCs). Effects of channel divergence are investigated combining both the heat transfer and flow field measurements. The applicability of existing heat transfer correlations for uniform jet arrays to divergent geometries is also discussed.

Hydraulics of combining flow in a right-angled compound open channel junction

Although combining flows are common in natural streams, no comprehensive experimental data has been compiled to characterize the three-dimensional flow field within the compound channel confluence. The present study examines the time-averaged flow structure at confluence over a rigid bed. Current knowledge of channel confluence, based on laboratory observation indicates that cross flow interactions exert a significant influence on confluence events. Secondary current and turbulent stresses are reproduced well by the hydraulic model and found greater in the interface region as relative flow ratio decreases. Velocity fields in combining flow region arising from varying discharge ratios are presented. A zone of depression in surface elevation in compound channel junction is observed as well. The flow field in compound channel is seen to be moderately different from that of simple channel junction. This study contributes to a better knowledge of hydraulic key processes into fundamental aspect of combining flow dynamics.

Synthetic jet cross-flow interaction with orifice obstruction

Purpose – The purpose of this paper is to describe the flow structure and the time-resolved and time-mean heat transfer characteristics in the interaction between a synthetic jet and a cross flow, when an obstruction reduces the cross-section of the orifice where the jet is formed. Design/methodology/approach – The microchannel flow interacted by the pulsed jet is modeled using a two-dimensional finite volume simulation with unsteady Reynolds-averaged Navier-Stokes equations while using the Shear-Stress-Transport (SST) k-ω turbulence model to account for fluid turbulence. Findings – The computational results show a good and rapid increase of the synthetic jet influence on heat transfer enhancement when the obstruction of the orifice is superior to 30 per cent and the synthetic jet oscillating amplitudes are below 50 µm. It is found that when the obstruction is close to the exit orifice, the heat transfer enhancement is significant. The obstruction has proved to accelerate the jet and change the formation of large vortical structures. Additional windward vortices appear, which influence the flow field and enhance the heat transfer. Research limitations/implications – The work proposes the use of a compound enhancement technique for electronics cooling. A limited range of operating conditions and geometrical configurations is presented. A further analysis of the performance evaluation, based on the increased energy consumption of the device, would complement the study. Originality/value – The authors provide a compound technique to enhance heat transfer in synthetic-jet electronic cooling devices.

Computational fluid dynamics (CFD) investigation of the gas–solid flow and performance of Andersen cascade impactor

The Andersen cascade impactor (ACI) is a device designed to fractionate aerosols by their aerodynamic diameters. This work developed a computational fluid dynamics (CFD) model to investigate flow characteristics and particle deposition behaviour in an ACI. A novel Iterative Simulation (IS) modelling technique was adopted to map the flow information between the source and target zones of individual models, hence enabling the establishment of flow fields across the whole device as a single domain. Particle trajectories and deposition were evaluated using the Lagrangian tracking approach in conjunction with the application of a kinetic energy criterion (KEc). The model was validated by comparing the simulation results with literature and manufacturer's data. The effect of flow rate was evaluated by using the standard and high flow rates of 28.3 and 60Lmin−1. The results showed that operating the ACI at high flow rates reduced the impactor performance by magnifying the cross-flow interactions and recirculation vortices. The heavy wall losses observed at Stages 2 and 3 of the impactor were attributed to more pronounced recirculation vortices at these stages. Increasing particle deposits underneath Stages 0 and 1 around the central ring of jet outlets can be attributed to counter-current flows arising from the central holes in the plates. These conclusions have significant implications for the design and operation of the ACI.

Numerical Simulations of the Near-Field Region of Film Cooling Jets Under High Free Stream Turbulence: Application of RANS and Hybrid URANS/Large Eddy Simulation Models

This paper investigates the flow field and thermal characteristics in the near-field region of film cooling jets through numerical simulations using Reynolds-averaged Navier–Stokes (RANS) and hybrid unsteady RANS (URANS)/large eddy simulation (LES) models. Detailed simulations of flow and thermal fields of a single-row of film cooling cylindrical holes with 30 deg inline injection on a flat plate are obtained for low (M = 0.5) and high (M = 1.5) blowing ratios under high free stream turbulence (FST) (10%). The realizable k‐ε model is used within the RANS framework and a realizable k‐ε-based detached eddy simulation (DES) is used as a hybrid URANS/LES model. Both models are used together with the two-layer zonal model for near-wall simulations. Steady and time-averaged unsteady film cooling effectiveness obtained using these models are compared with available experimental data. It is shown that hybrid URANS/LES models (DES in the present paper) predict more mixing both in the wall-normal and spanwise directions compared to RANS models, while unsteady asymmetric vortical structures of the flow can also be captured. The turbulent heat flux components predicted by the DES model are higher than those obtained by the RANS simulations, resulting in enhanced turbulent heat transfer between the jet and mainstream, and consequently better predictions of the effectiveness. Nevertheless, there still exist some discrepancies between numerical results and experimental data. Furthermore, the unsteady physics of jet and crossflow interactions and the jet lift-off under high FST is studied using the present DES results.

Computational Analysis of Particle Nucleation in Dilution Tunnels: Effects of Flow Configuration and Tunnel Geometry

Measurement of fine particle emission from combustion sources is important to understand their health effects, and to develop emissions regulations. Dilution sampling is the most commonly used technique to measure particle number distribution because it simulates the mixing and cooling of combustion exhaust with atmospheric air, which drives nucleation and condensation of semi volatile material. Experiments suggest that the measured size distribution is dependent on the dilution ratio used and the tunnel design. In the present work, computational analysis using a large-eddy-simulation (LES) based model is performed to investigate the effect of tunnel flow and geometric parameters on H2SO4-H2O binary nucleation inside dilution tunnels. Model predictions suggest that the experimental trends are likely due to differences in the intensity of turbulent mixing inside the tunnels. It is found that the interaction of dilution air and combustion exhaust in the mixing layer greatly impacts the extent of nucleation. In general, a cross-flow interaction with enhanced turbulent mixing leads to greater number of nucleation-mode particles than an axial-flow interaction of combustion sample and dilution air.Copyright 2014 American Association for Aerosol Research