Laminar Spray Research Articles

A laminar spray group combustion experiment (LSGCE) with in situ image diagnostic methods and image analysis algorithms.

Spray combustion is important for different engines. The understanding of spray combustion should be further promoted especially in the non-dilute region, and there is lack of well-defined spray experiments. In this study, an experimental platform was developed. Using this platform, a cylindrical quasi-laminar spray can be formed and ignited by a thin and straight hot wire, making it a simple configuration. Two image diagnostic methods were also developed to capture in situ microscopic droplet images and macroscopic droplet-flame images synchronously. Different image analysis algorithms were developed to obtain droplet statistics (diameter, velocity, and number density) and flame information (size, location, and flame propagation speed) from the raw images. The design, diagnostic methods, and image analysis methods are detailedly presented. This experimental platform can cover a wide range of operating conditions, with Gig in a range of 0.01-0.06 and temperature in a range from room temperature to 1400K. In addition, this platform is small in size and is capable of further implanting into a ground-based microgravity facilitaty. The whole experimental system can be applied in spray ignition and combustion studies and can provide legitimate data for further model development.

Numerical simulation of TTIP/[formula omitted]-xylene precursor solution spray flames in the counterflow configuration

Sprays of precursor solutions are used in flame spray pyrolysis (FSP) to produce functional nanoparticles with specific properties. The present study concerns the precursor titanium(IV) isopropoxide (TTIP) in p-xylene. A dilute monodisperse spray carried by an air stream is directed against an air stream to establish a counterflowing laminar spray flame. The complex thermophysical properties of the bi-component droplets are incorporated into the code as well as the detailed chemical reactions of the system with 213 chemical reactions among 52 species; the chemical reaction mechanism includes thermal decomposition of the TTIP and the combustion of p-xylene. The droplets are modeled using the multicomponent evaporation model. Drag and gravity forces are considered in the equation of droplet motion. Monodisperse spray flame structures are presented depending on the gas strain rate, the initial droplet size, the precursor mass loading as well as the equivalence ratio. The preferential evaporation of the more volatile p-xylene provides the gaseous fuel for combustion, and the thermal decomposition of the precursor TTIP considerably reduces the flame temperature.

A priori and a posteriori studies of a novel spray flamelet tabulation methodology considering evaporation effects

Turbulent spray flames are complicated combustion configurations involving many coupled processes including atomization, evaporation, mixing, chemical reactions, etc. Different flamelet models with intrinsically high computational efficiency have been proposed and extended to model spray combustion. One popular approach for flamelet modeling is based on the gaseous flamelet library, but the accuracy is not guaranteed for combustion with gas–liquid two-phase flows. The other approach is based on the spray flamelet library that takes into account the effects of evaporation on the flamelet structure, but problems related to the non-monotonicity of mixture fraction occur. We develop a novel tabulated spray flamelet/progress variable (TSFPV) model in the present work. An additional mixture fraction Zcarr (named carrier mixture fraction) is defined to describe the mixing process, and it proves to avoid the problem of non-monotonicity of the spray flame structure. The empirical formulas of mass and energy evaporation source terms are obtained and then applied to one-dimensional flames in order to create the flamelet library considering evaporation effects. The developed TSFPV model is validated via simulations of laminar spray counterflow flame and turbulent spray reactive jet in cross-flow, where strong interactions between spray droplets and flame occur and the traditional Flamelet/Progress Variable (FPV) model fails. The a priori and a posteriori studies of TSFPV model are conducted, and the model shows to achieve good agreement with the detailed chemistry solutions. The double-reaction spray flame structure can be captured correctly. The TSFPV model can also give sound quantitative predictions for temperature and minor intermediate products (such as OH, C2H2, and C2H4).

A Study on the Heat Transfer Characteristics of Steel Plate in the Matrix Laminar Cooling Process.

Accurate prediction and control of the steel plate temperature in the laminar cooling process are very challenging. In this research, an experimental platform was built to measure the heat transfer characteristics of the steel plate in the process of matrix laminar spray cooling when the steel plate is one millimeter away from the upper surface. The “buried couple method” was used, including the cooling temperature and cooling rate. Then, the temperature and the integrated heat transfer coefficient at the steel plate surface were calculated by the time-sequential function method (TSFM). The obtained results show that the fast cooling stage under the water cooling condition occurred in the first 1.5 s, and the measuring point temperature decreased by 8%. The “re-reddening” phenomenon of the steel plate appeared with time, and the measuring point temperature increased by 37%. Second, the maximum calculated difference between the surface temperature and the measuring point temperature was 0.75 °C, and the integrated heat transfer coefficient conformed to the periodic boundary features. The comprehensive convective heat transfer coefficient on the surface was in agreement with the periodic boundary characteristics, and its value exhibited oscillatory attenuation with the cooling process, and the oscillatory peak period was about 6 seconds. Two methods, sequential function method (SFM) and finite difference method (FDM), were used to verify the correctness of TSFM.

Large Eddy Simulation of a Turbulent Spray Burner Using Thickened Flame Model and Adaptive Mesh Refinement

Abstract The accurate simulation of two-phase flow combustion is crucial for the design of aeronautical combustion chambers. In order to gain insight into complex interactions between a flame, a flow, and a liquid phase, the present work addresses the combustion modeling for the large eddy simulation (LES) of a turbulent spray jet flame. The Eulerian–Lagrangian framework is selected to represent the gaseous and liquid phases, respectively. Chemical processes are described by a reduced mechanism, and turbulent combustion is modeled by the thickened flame model (TFM) coupled to the adaptive mesh refinement (AMR). The TFM-AMR extension on the dispersed phase is successfully validated on a laminar spray flame configuration. Then, the modeling approach is evaluated on the academic turbulent spray burner, providing a good agreement with the experimental data.

Influence of a Standing Wave Flow-Field on the Dynamics of a Spray Diffusion Flame

A theoretical investigation of the influence of a standing wave flow-field on the dynamics of a laminar two-dimensional spray diffusion flame is presented for the first time. The mathematical analysis permits mild slip between the droplets and their host surroundings. For the liquid phase, the use of a small Stokes number as the perturbation parameater enables a solution of the governing equations to be developed. Influence of the standing wave flow-field on droplet grouping is described by a specially constructed modification of the vaporization Damkohler number. Instantaneous flame front shapes are found via a solution for the usual Schwab–Zeldovitch parameter. Numerical results obtained from the analytical solution uncover the strong bearing that droplet grouping, induced by the standing wave flow-field, can have on flame height, shape, and type (over- or under-ventilated) and on the existence of multiple flame fronts.

Waterdrop Removal From Hot-Rolled Steel Strip Surfaces Based on Progressive Recurrent Generative Adversarial Networks

Automated visual inspection (AVI) instrument of surface defects for hot-rolled steel strips is conventionally installed closely before the terminal crimping machine, where the adjacent upstream process is laminar spray cooling. Waterdrops, spreading more or less over the steel strip surface, often trigger false alarms, which is a quite common problem in AVI. Stimulated by the idea of image rain removal in visual enhancement field, this article considers the surface waterdrops, pseudodefects in essence, as a conceptual “rain-like layer.” A targeted method, namely progressive recurrent generative adversarial network (PReGAN), is designed for active waterdrop tracking and fine-grained image inpainting. Meanwhile, a steel surface database (2400 raw images with the resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1000\times1000$ </tex-math></inline-formula> ) captured from actual hot-rolling line is constructed for the first time for open evaluation of waterdrop removal. The experimental results indicate that images enhanced by the PReGAN perform the most informative and spotless, with 52.2073 peak signal-to-noise ratio (PSNR) and 0.9502 structural similarity (SSIM) index, when compared with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">four</i> prestigious networks. Assisted by the PReGAN, the false alarms are proved to be reduced at least a half during the application tests using <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">four</i> traditional simple detection methods.

Numerical study of multicomponent spray flame propagation

A computational study of one dimensional multicomponent laminar Jet-A/air spray flames is presented. The objective is to understand the effect of various spray parameters (diameter, droplet velocity, liquid loading) on the spray flame structure and propagation. Simulation of the Eulerian gas phase is coupled with a Lagrangian tracking of the dispersed liquid phase. Jet-A surrogate of n-dodecane, methyl-cyclohexane and xylene is considered. A discrete multicomponent model for spray vapourisation is used along with an analytically reduced chemistry for computing the gas phase reactions. Both overall lean and rich cases are examined and compared with existing literature for single component spray flames. The preferential evaporation effect, unique to multicomponent fuels causes a variation of fuel vapour composition on both sides of the flamefront and this has a direct impact on the spray flame structure and propagation speed. In the rich cases, multiple flame structures exist due to the staged release of vapours across the reactive zone. Spray flame speed correlations proposed for single component fuels are extended to the multicomponent case, for both zero and high relative velocity between the liquid and the gas. The correlations are able to accurately predict the effective equivalence ratio at which the flame burns and hence the laminar spray flame speeds of multicomponent fuels for all cases studied in this work.

AN IMPROVED MODEL FOR INVESTIGATING THE EFFECT OF SPRAY POLYDISPERSITY ON LAMINAR PREMIXED SPRAY FLAME IGNITION

AN IMPROVED MODEL FOR INVESTIGATING THE EFFECT OF SPRAY POLYDISPERSITY ON LAMINAR PREMIXED SPRAY FLAME IGNITION

A numerical study on the acoustic forcing of a laminar premixed saturated spray flame

This paper presents a numerical investigation into the interaction between a laminar premixed saturated spray flame and periodic acoustic forcing. A two-phase model is adopted that implements the Euler–Euler governing equations for the liquid and gaseous phases, enabling the flame dynamics to be examined in detail, particularly in terms of the flame transfer function (FTF). Three distinct mechanisms are identified: (1) a direct perturbation of the flame by a velocity-coupled mechanism, (2) a secondary effect acting via perturbations of the advection velocity of the liquid fuel droplets and (iii) a tertiary effect caused by local variations in the droplet concentration due to droplet clustering. The FTF gain at low-frequency approaching is $$ \left| {\text{FTF}} \right| \approx 2 $$. The phase difference between the processes acting in the second and third coupling mechanisms is found to be a key parameter governing the flame response. At low forcing frequencies, the third coupling mechanism is found to dominate over the first two.

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.

Evaluation of different flamelet tabulation methods for laminar spray combustion

In this work, three different flamelet tabulation methods for spray combustion are evaluated. Major differences among these methods lie in the treatment of the temperature boundary conditions of the flamelet equations. Particularly, in the first tabulation method (“M1”), both the fuel and oxidizer temperature boundary conditions are set to be fixed. In the second tabulation method (“M2”), the fuel temperature boundary condition is varied while the oxidizer temperature boundary condition is fixed. In the third tabulation method (“M3”), both the fuel and oxidizer temperature boundary conditions are varied and set to be equal. The focus of this work is to investigate whether the heat transfer between the droplet phase and gas phase can be represented by the studied tabulation methods through a priori analyses. To this end, spray flames stabilized in a three-dimensional counterflow are first simulated with detailed chemistry. Then, the trajectory variables are calculated from the detailed chemistry solutions. Finally, the tabulated thermo-chemical quantities are compared to the corresponding values from the detailed chemistry solutions. The comparisons show that the gas temperature cannot be predicted by “M1” with only a mixture fraction and reaction progress variable being the trajectory variables. The gas temperature can be correctly predicted by both “M2” and “M3,” in which the total enthalpy is introduced as an additional manifold. In “M2,” variations of the oxidizer temperature are considered with a temperature modification technique, which is not required in “M3.” Interestingly, it is found that the mass fractions of the reactants and major products are not sensitive to the representation of the interphase heat transfer in the flamelet chemtables, and they can be correctly predicted by all tabulation methods. By contrast, the intermediate species CO and H2 in the premixed flame reaction zone are over-predicted by all tabulation methods.

An Investigation of the Behavior of Steady-State Laminar Jet Spray Flames

Abstract The theory of steady-state laminar jet spray flames is developed in the limit of small Stokes number. The spray is modeled using the sectional approach. A similarity solution is suggested, and it is shown that for typical operating conditions the leading order small Stokes number solution suffices for determining lift-off and blow-out features of the flames. The latter are demonstrated to be strongly dependent on spray properties, in contrast to the Schmidt number dependence only which was predicted by classical laminar jet gaseous flame theory. The strong influence of the ambient oxygen content and the liquid fuel’s evaporation coefficient on flame characteristics, blow-out and lift-off are established.

Similarity solutions for the evolution of unsteady spray diffusion flames in vortex flows

ABSTRACTA new mathematical analysis for the dynamics of laminar spray diffusion flames in the vicinity of a vortex flow field is presented. The governing equations for a spray evaporating in an unsteady vortex are studied. New similarity solutions are found for the dynamics of the spray and the flame it supports. Analytical solutions for the spray flames are derived using Shvab-Zeldovich parameters, through which the radial evolution of the flames is found. The results based on the solution reveal the significant effects vorticity and droplet evaporation have on the flame dynamics. An extinction analysis is carried out which maps the influence of the evaporation coefficient and vortex intensity on flame extinguishment. A number of competing factors such as vortex intensity and heat loss due to evaporation were found to be responsible for flame extinction. Despite the model’s simplicity, its predictions offer new insights into the driving mechanisms of more complex spray-combustion situations in which droplets are evaporating in vortical environments.

Polydisperse effects in jet spray flames

A laminar jet polydisperse spray diffusion flame is analysed mathematically for the first time using an extension of classical similarity solutions for gaseous jet flames. The analysis enables a comparison to be drawn between conditions for flame stability or flame blow-out for purely gaseous flames and for spray flames. It is found that, in contrast to the Schmidt number criteria relevant to gas flames, droplet size and initial spray polydispersity play a critical role in determining potential flame scenarios. Some qualitative agreement for lift-off height is found when comparing predictions of the theory and sparse independent experimental evidence from the literature.

A consistent Artificially Thickened Flame approach for spray combustion using LES and the FGM chemistry reduction method: Validation in Lean Partially Pre-vaporized flames

A consistent modeling approach in the context of the Artificially Thickened Flame (ATF) method is proposed for the prediction of turbulent Lean Partially Pre-vaporized (LPP) spray flames. Large Eddy Simulations (LES) are carried out in an Eulerian–Lagrangian framework to represent the turbulent two-phase flow. The chemical reactions are described with the Flamelet Generated Manifold (FGM) chemistry reduction method. Various aspects of the thickening of the scalar fields in the frame of reacting two-phase flows are systematically investigated relying on simplified numerical test cases. A correction for the solution of the dispersed phase equations for droplets interacting with the flame is successfully validated for different scenarios. The influences of the presence of evaporating droplets on the reaction development are carefully analyzed along with the adopted FGM database. At this stage a new theoretical expression to predict the laminar spray flame propagation speed is derived. Enhancements on the thickening procedure could be achieved with the inclusion of mixture quality aspects. Finally, the complete designed model is applied to a turbulent LPP burner for three different operating conditions. Results of the droplets velocity, characteristic diameters, liquid volumetric fluxes and reaction progress variable are compared with experimental data. Further findings regarding the mixture stratification, the impact of the SGS wrinkling modeling and the thickening onto the flame structure as well as onto the dispersed phase behavior are also addressed.

A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new insight into the local structure of this complex spray flame.

On Blow-Out and Lift-Off of Laminar Jet Spray Diffusion Flames

ABSTRACTA laminar jet spray diffusion flame is analyzed mathematically for the first time using an extension of classical similarity solutions for gaseous jet flames. The analysis enables a comparison to be drawn between conditions for flame stability or flame blow-out for purely gaseous flames and for spray flames. It is found that, in contrast to the Schmidt number criteria relevant to gas flames, spray-related parameters also play a critical role in determining potential flame scenarios. Excellent qualitative agreement for lift-off height is found when comparing predictions of the theory and independent experimental evidence from the literature.

Transported Joint Probability Density Function Simulation of Turbulent Spray Flames Combined with a Spray Flamelet Model Using a Transported Scalar Dissipation Rate

ABSTRACTNumerical simulations of a turbulent ethanol/air spray flame are performed using a combined formulation of a joint gas mixture fraction-enthalpy probability density function (PDF) and a spray flamelet model, where the scalar dissipation rate of the mixture fraction is described with a transport equation. Spray evaporation is included in the flamelet model through the pre-calculation of counterflowing laminar spray flames with a detailed reaction mechanism of 38 species and 337 elementary reactions for ethanol/air combustion. A transport equation for the mean scalar dissipation rate of the mixture fraction is derived, taking into account the spray evaporation. The computed results are compared with the experimental data provided by A. Masri, University of Sydney, Australia. All major spray flame characteristics, including the Sauter mean diameter, the droplet velocity, and its turbulent fluctuations as well as gas temperature, generally show good agreement with experiments. Different models for the scalar dissipation rate of the mixture fraction are not significantly different for the flame under consideration except for inside the chemical reaction zone, where the new model shows some improvement compared to the experiment.

On Laminar Rich Premixed Polydisperse Spray Flame Propagation with Heat Loss

A mathematical analysis of laminar premixed spray flame propagation with heat loss is presented. The analysis makes use of a distributed approximation of the Arrhenius exponential term in the reaction rate expression and leads to an implicit expression for the laminar burning velocity dependent on the spray-related parameters for the fuel, gas-related parameters and the intensity of the heat losses. It is shown that the initial droplet load, the value of the evaporation coefficient, and the initial size distribution are the spray-related parameters which exert an influence on the onset of extinction. The combination of these parameters governs the manner in which the spray heat loss is distributed spatially and it is this feature that is the main factor, when taken together with volumetric heat loss, which determines the spray’s impact on flame propagation and extinction.