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Directly using liquid ammonia in combustion devices can simplify the fuel supply system and reduce equipment costs. However, the detailed ignition and propagation mechanisms of liquid ammonia flames have not been fully understood. This study aims to investigate the ignition and propagation of liquid ammonia flames numerically under various conditions. Spherically expanding liquid ammonia flames are studied using detailed chemistry. The effects of initial temperatures, droplet diameters, equivalence ratios, and heat fraction are investigated. The results indicate that the rapid flash boiling of liquid ammonia droplets leads to a strong local heat loss, hindering the ignition and propagation of the flame. Preheating can significantly increase the reaction rate, which compensates for the heat loss caused by phase change, allowing the flame kernel to ignite and propagate outwards. While droplet-flame interaction is present but not significant due to the rapid evaporation of liquid ammonia. The effect of droplet on the flame surface curvature is only distinguishable at conditions of large diameter. Equivalence ratio and ammonia heat fraction ratio change the amount of liquid ammonia. The more liquid ammonia, the more heat is absorbed by the phase change process, making the flame more difficult to be ignited.

The direct use of detailed chemical kinetics in combustion simulations is limited by the extremely high computational costs. Recently, Owoyele and Pal (Energy and AI, 2022), proposed the neural ordinary differential equations (NODE) method to accelerate calculations of chemical kinetics and proved its effectiveness in zero-dimensional calculations of hydrogen combustion considering 9 species and 21 reactions. However, its performance for more realistic high-dimensional calculations and more complex kinetic systems remains unexplored. Therefore, this study further applies the method for more complex chemical kinetics of ammonia combustion, especially with optimizations in the data sampling, model training strategies, and model application methods that remedy the problems of versatility and application to more practical simulations. The newly developed NODE models are comprehensively validated in the zero-dimensional calculations of ammonia auto-ignition, one-dimensional calculations of laminar freely-propagating ammonia-premixed flames, and two-dimensional direct numerical simulation (DNS) of ammonia-premixed flames in a temporally evolving jet. Present NODE models focus on seven chemical species, namely NH₃, O₂, H₂, OH, H₂O. N₂, and NO, and the results show that, compared with the results obtained by using detailed chemical kinetics, this method is able to reduce the computational costs of the zero-dimensional auto-ignition reaction to 1/24 while reproducing the ignition delay time for a wide range of initial temperatures and equivalence ratios with relatively good accuracy. Additionally, the method is able to reduce the computational costs of the one-dimensional freely propagating flame and two-dimensional jet flame to 1/4 and 1/38 respectively, while acceptable reproduction of the laminar flame speed and temporal evolution of the gas temperature and mass fractions of the interested species can be achieved.

Ammonia is regarded as one of the carbon-free alternative fuels for energy systems. The direct utilization of liquid ammonia in gas turbines can reduce the complexity and start-up time of the system. Liquid ammonia is susceptible to flash boiling because of its low boiling point, especially under standard conditions. The boiling point is 239.7 K under 1 atm, which is different from those of traditional hydrocarbon fuels and brings challenges for liquid ammonia spray and combustion simulation. However, the validity of phase-change models for liquid ammonia remains unclear and needs further exploration. Therefore, the present work aims to numerically evaluate the existing evaporation and boiling models for liquid ammonia flash spray. Large eddy simulations were conducted in the Euler-Lagrangian framework, considering droplet kinetics, evaporation, boiling, radiation, and gas-particle interactions. Specifically, five phase-change models were evaluated by comparing the simulation results with recently published experimental data for the vaporization of the ammonia flash spray in terms of the penetration length, spray morphology, and temperature-diameter distribution law. The results indicated that the combined model (the Zuo model combined with the Langmuir-Knudsen model) was more suitable under superheated conditions and agreed reasonably with the experimental results obtained under various temperature and pressure conditions. The evaporation rate tended to increase with decreasing pressure, leading to a significant reduction in the droplets and gas field temperatures. The interaction between the droplet and gas phases was significantly affected by pressure. The ammonia vapor mass fraction first decreased and then increased with an increase in temperature under low pressure, whereby it exhibited only a positive correlation at high ambient pressure. Finally, the effect of the particle diameter distribution was explored and discussed.

In recent years, particle methods, which are good for moving boundary problems, have become an effective approach to understand and predict flows in complex geometry, such as lubrication behaviors in rolling bearings. This study adopted a physically consistent particle method, i.e., the moving particle hydrodynamics for incompressible flows (MPH-I) method. For capturing the free surface flows in lubrication, a surface tension model was included. In order to maintain the physical consistency in the MPH-I method, the surface tension model expressed with the two density potentials, which are cohesive pressure potential (CPP) and density gradient potential (DGP), was adopted. The MPH-I method with the two-potential-based surface tension model enabled to handle negative pressure and nearly incompressible flow with very large bulk modulus. In fact, the MPH-I method could successfully reproduce fundamental pressure generation effects in the fluid film lubrication, i.e., the wedge film and squeeze film effects. Furthermore, the computed lubrication pressure agreed well with the experimental results and the classic prediction with Reynolds equation. This implies that the present numerical method was validated under the fluid film lubrication problems.

Alcohol-to-Jet Synthetic Paraffinic Kerosene (AtJ-SPK), an approved sustainable aviation fuel (SAF) by blending with conventional Jet A fuel, has recently been experimentally studied, and detailed mechanisms have been developed to describe its combustion behavior. The present study aims to develop reduced mechanisms of AtJ-SPK and its blends with Jet A for high-fidelity and computationally affordable computational fluid dynamics. Specifically, two reduced mechanisms were developed for pure AtJ-SPK and its blends with Jet A from LLNL-AtJ-SPK [Richter et al. Combust. Flame 2022, 240, 111994] and POLIMI detailed mechanisms [Ranzi et al. Prog. Energy Combust. Sci. 2012, 38 (4), 468-501], respectively, using a combined reduction method. The reduced mechanisms can achieve 80%/92.4% and 92%/90% reductions of the species/reaction numbers, respectively, compared with those of the master detailed mechanisms. The developed reduced mechanisms were further validated under various conditions by comparing the predicted ignition delay times, laminar flame speeds, and temporal/spatial profiles with those predicted using the master detailed mechanisms as well as experimental data. Finally, the computational cost comparison in the two-dimensional direct numerical simulation demonstrated that the developed reduced mechanisms can impressively accelerate the calculation speed by more than 5000 and 529 times, respectively.

The Bingham fluid simulation model was constructed and validated using a physically consistent particle method, i.e., the Moving Particle Hydrodynamics (MPH) method. When a discrete particle system satisfies the fundamental laws of physics, the method is asserted as physically consistent. Since Bingham fluids sometimes show solid-like behaviors, linear and angular momentum conservation is especially important. These features are naturally satisfied in the MPH method. To model the Bingham feature, the viscosity of the fluid was varied to express the stress-strain rate relation. Since the solid-like part, where the stress does not exceed the yield stress, was modeled with very large viscosity, the implicit velocity calculation was introduced so as to avoid the restriction of the time step width with respect to the diffusion number. As a result, the present model could express the stopping and solid-like behaviors, which are characteristics of Bingham fluids. The proposed method was verified and validated, and its capability was demonstrated through calculations of the two-dimensional Poiseuille flow of a Bingham plastic fluid and the three-dimensional dam-break flow of a Bingham pseudoplastic fluid by comparing those computed results to theory and experiment.

Blending hydrogen has proven an efficient method to enhance the combustion stability of gaseous ammonia flames. Heat release rate (HRR), as an important parameter to indicate combustion process, is hard to be directly measured and highly dependent on the fuel components, equivalence ratios, and operation conditions. This paper presents a comprehensive study on developing a general HRR surrogate (HRRS) for ammonia–hydrogen premixed flames under various conditions. Firstly, reaction mechanisms for ammonia/hydrogen premixed combustion are evaluated under various conditions, by comparing the predicted laminar flame speeds with the experimental data collected from published literature. The reaction mechanism developed by Shrestha et al., (2021) performs the best under various conditions. Then, series of one-dimensional freely propagating premixed flames under various conditions are calculated using this reaction mechanism and analyzed to explore the effects of blending ratios, pre-heat temperatures, equivalence ratios, and pressures on the HRR reconstruction for ammonia–hydrogen premixed flames from the viewpoint of chemical kinetics, and also serve as a database for the identification of HRRS. It is found that equivalence ratio and blending ratio have significant effects on the HRR reconstruction, while the effects of pressure and initial temperature are relatively limited. Subsequently, the general HRRS, [NH₂]^{1.53} [OH]^{0.28}, is identified for ammonia–hydrogen premixed flames under various conditions using a grid-research optimization method. Finally, the general HRRS is further comprehensively validated on several 2D and 3D turbulent premixed flames of ammonia–hydrogen under various conditions. The comparisons with the two previous HRRSs also demonstrate that the present developed HRRS is superior and more stable temporally.

Large-eddy simulation (LES) of a lean-premixed hydrogen turbulent jet flame with combustion instability (CI) in a low-swirl combustor (LSC) is performed by employing a dynamically thickened flame model with a detailed chemical reaction model with 9 chemical species and 20 reactions, and the LES validity and the CI characteristics are investigated in detail. The results show that the present LES can accurately reproduce the experimentally observed characteristics of the CI such as intensity, frequency, sporadic decay of pressure oscillations, and a flame–flow interaction inducing the periodic transitions of an inverted conical flame structure and a flat flame structure in the LSC. The sporadic decay of pressure oscillations and the flame–flow interaction are caused by the temporal decoupling of pressure and heat release rate and the periodic outward and inward deflections of the inflow, which is associated with the flow behavior in the upstream injector channel, respectively.

An open lean-premixed hydrogen/air low-swirl (LPHALS) turbulent flame exhibiting a pronounced peak in its combustion noise spectra, is investigated numerically using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. Under this framework, the reacting flow-field of the flame is computed via Large-Eddy Simulation (LES), while the direct combustion noise it produces is captured by solving the Acoustic Perturbation Equations for Reacting Flows (APE-RF). Flame configuration and simulation conditions correspond to those of an experimental study on an open lean-premixed H₂/air flame stabilized using a Low-Swirl Burner (LSB). LES results are validated against experimental data. The CAA simulation is able to predict a pronounced sharp peak in the computed combustion noise spectra, similar to one of the two characteristic peaks observed in the measured combustion noise spectra. Frequency of this spectral peak predicted by the CAA simulation is 840 Hz, which is close to that of the higher frequency secondary spectral peak at 940 Hz measured in the experiment. Upon examining the hybrid LES/APE-RF results, the noise generation mechanism at 840 Hz is found to be the intense local heat release rate fluctuations, caused by strong interaction between the flame and the periodically generated vortical flow structures in the shear layers, downstream of the LSB exit. Additionally, analysis of the spectral content and directivity of the noise generated by different acoustic source terms is performed, in order to investigate their impact on the radiated acoustic field, and hence the characteristics of direct combustion noise produced by the open LPHALS flame.

A numerical framework used to model dense spray flames is proposed. In this framework, the liquid fuel (acetone) atomization is solved by a detailed high-resolution VOF simulation, and the Eulerian components of liquid droplets are transformed into Lagrangian droplets, which are stored in a database at a certain downstream cross-section. Then, the combustion process is solved by a LES/FPV (flamelet progress variable) adopting the pre-stored database of Lagrangian droplets (i.e., the position, size, and velocity of each droplet) as the inlet boundary conditions. This framework is a one-way coupling between a VOF simulation and a combustion simulation. The validity of this approach is investigated by comparing the computations with the experiments of the Sydney Piloted Needle Spray Burner. The VOF simulation shows that the volume flux of the droplets at the nozzle exit fluctuates both temporally and spatially and the larger droplets tend to be located away from the center axis compared to the small droplets. The computed breakup length is in good agreement with the empirical correlation. In the database of the Lagrangian droplets for the LES/FPV of spray flames, the location of the sampling cross-section, the sampling time, and the threshold value for Eulerian–Lagrangian (E-L) transformation strongly affect the properties of the Lagrangian droplets, and are critical for the successful use of the LES/FPV. Two spray flames with different recess distances are computed using their optimal pre-stored droplets databases and both show generally good agreement with the experiments in terms of the gas temperature and droplet size distributions. The spray flame with a longer recess distance, which is more representative of a dilute spray, is considered to have a longer and wider premixed core than that with a shorter recess distance representing a dense spray. The discrepancy in the prediction of denser spray flames becomes more evident leading to over-predictions of gas temper

Design of Compression Ignition (CI) engines with improved thermal efficiencies needs better understanding of the heat transfer mechanism from spray flame to the combustion chamber wall. In this regard, heat transfer occurring during the interaction between impinging spray flame and wall, under CI engine-like conditions, is investigated in this study using 3-Dimensional numerical simulations based on an Eulerian–Lagrangian framework. Simulations are performed for different fuel spray injection velocities (which are representative of different fuel injection pressures in CI engines), to examine their influence on the heat transfer between impinging spray flame and wall. To couple the convective and radiative heat transfer at the wall surface with the conduction heat transfer occurring within the finite thickness wall, Conjugate Heat Transfer (CHT) is incorporated in the simulations. A Non-Adiabatic Flamelet/Progress Variable (NA-FPV) approach is employed as the combustion model of n-dodecane, which is considered to be the fuel for liquid spray. Dynamics of the liquid film formed on the wall surface by impinging fuel droplets are captured using a particle-based approach. Contribution of radiative heat flux is taken into consideration using the Discrete Ordinates (DO) method. Results indicate that the total heat flux (sum of convective and radiative heat fluxes) at the wall surface increases with the fuel injection velocity. It is observed that the total wall heat flux is largest in the stagnation zone where the spray flame impinges directly on the wall surface, while the radiative heat flux at the wall surface becomes larger as the distance from this stagnation zone increases. Additionally, it is found that the influence of fuel injection velocity on the radiative heat flow rate at the wall surface is rather small. This radiative heat flow rate when expressed as a percentage of the total wall heat flow rate, ranges from ≈ 18% to 30% (depending on the 3 cases investigat

Fuel droplets consisting of multiple components are known to exhibit complex evaporation behaviour. The Eulerian framework employed in Computational Fluid Dynamics (CFD) which treats the droplets as a continuum, is effective for simulating the complex evaporation behaviour of such multicomponent droplets. In this study, the evaporation of binary-component or bi-component fuel droplets (which consist of two chemical species) in air, is simulated using the Eulerian framework with an extended evaporation model and its validity is examined. Spatio-temporal evolution of the gas-liquid interface is captured using the Level Set method, and a Ghost Fluid method is incorporated to consider the jump conditions at the interface. The computed evaporation speed of a fuel droplet consisting of a mixture of n-heptane and n-decane is compared with that of a previous experiment. Results show that n-heptane evaporates preferentially and the total evaporation speed is in an overall good agreement with that in the experiment. This confirms that the numerical framework used in the present simulation is capable of capturing the evaporation phenomenon of a bi-component fuel droplet. Additionally, several parametric simulations with different initial compositions (i.e., mass ratio of the constituent chemical species) of the bi-component fuel droplet and different initial ambient temperatures are performed, to investigate their influences on the evaporation characteristics of bi-component fuel droplet. The applicability of the proposed extended evaporation model and the numerical framework under these various simulation conditions are verified.

In this study, a three-dimensional Direct Numerical Simulation of an open turbulent jet spray flame has been used to investigate the statistical behaviour of displacement speed Sd and its components to provide physical explanations for the observed behaviours at different axial locations downstream of the jet exit. The open turbulent jet spray flame exhibits fuel-lean conditions close to the jet exit but fuel-rich conditions have been observed further downstream due to the evaporation of fuel droplets. For the axial locations considered, combustion takes place under low Damköhler number conditions. The displacement speed of reaction progress variable isosurfaces shows qualitatively similar behaviour for all axial locations considered – predominantly positive across the major part of the flame but with small, potentially negative, values towards the burned-gas-side. The components of displacement speed arising from chemical reaction rate and flame normal molecular diffusion remain leading order contributors and the competition between these determines the mean behaviour of displacement speed. These observations are consistent with studies of turbulent spray flames in canonical configurations and low Damköhler number turbulent premixed and stratified flames. This suggests that flow geometry in the absence of mean curvature might not be important in determining the mean behaviour of displacement speed and its components. Therefore, the modelling methodologies employed for turbulent stratified flames can potentially be extended for turbulent spray flames. However, the modelling methodologies, which implicitly assume equality between the surface-weighted values of density-weighted displacement speed and local laminar burning velocity, might be rendered invalid for turbulent spray flames.

In this study, the applicability of the flamelet approach to numerical simulations of hydrazine (N₂H₄)/nitrogen tetroxide (NTO, NO) combustion, in which hypergolic ignition and thermal decomposition occur, is investigated in terms of two-dimensional numerical simulations of two types of N₂H₄/NTO jet flames, namely, the gaseous N₂H₄/NTO jet flame and the N₂H₄ spray jet flame in the gaseous NTO stream. In case of the gaseous jet flame, the numerical simulation is performed employing the unsteady flamelet/progress variable (UFPV) approach. In case of the spray jet flame, on the other hand, a non-adiabatic unsteady flamelet/progress variable (NAUFPV) approach, which combines the UFPV approach and the non-adiabatic flamelet/progress variable (NAFPV) approach, is proposed. The validity of these and some other existing flamelet approaches is investigated by comparison with the results obtained using the exact approach, in which detailed chemical reactions are directly solved. The results show that the UFPV and NAUFPV approaches drastically improve predictions of the N₂H₄/NTO gaseous and spray combustion behavior, respectively, including the ignition process, the flame lift-off height, and the distributions of temperature and chemical species concentrations. This indicates that self-decomposition flame of fuel is successfully captured by the UFPV approach, and that the NAUFPV approach can additionally take into account the heat loss effect due to evaporation of droplets.

Owing to the increasing worldwide demand for natural gas, the development of a large submerged combustion vaporizer is required. Its burner is equipped with a water spray nozzle to reduce nitrogen oxides, and a practical simulation method is required for the optimal design. The non-adiabatic flamelet approach can predict the combustion emissions and is useful for reducing simulation costs. However, as the number of control variables increases, the database requires larger memory and cannot be dealt with by general computers. In this study, an artificial neural network (ANN) model based on a five-dimensional flamelet database, which includes the effects of heat loss and vapor concentration by sprayed water evaporation, is developed. Furthermore, large eddy simulations (LESs) for turbulent combustion fields with and without water spray are conducted employing flamelet generated manifold (FGM) approach with this ANN model, and the validity is investigated. For comparison, a lab-scale burner equipped with a water spray nozzle is manufactured, and combustion experiments with and without water spray are conducted. The results show that CO, NO, temperature, and reaction rate of progress variable predicted by the present ANN model are in good agreement with those of a five-dimensional flamelet database. In the condition without water spray, the flame behavior predicted by the LES employing the FGM/ANN approach is in good agreement with that employing the conventional FGM approach, while indicating much lower memory, although there appeared some quantitative discrepancies in the temperature against the experiment probably partially because of the insufficiency of the FGM approach for the present complex flame structure. In the condition with water spray, the LES employing the FGM/ANN approach is able to capture the effect of the water spray on the flame behavior in the experiment, such that the water spray decreases the temperature, which causes the decrease in NO but increa

Combustion instabilities occurring in spray combustion fields inside a backward facing step combustor have been investigated by performing large-eddy simulations (LES). In this study, the influence of fluctuations in the incoming oxidizer air velocity (caused by drastic pressure oscillations in the combustor during combustion instability) on the droplet diameter distribution (due to atomization) of the injected liquid fuel spray, as well as the influence of pressure oscillations on the fuel flow rate have been taken into consideration using appropriate models. For the temporal fluctuations in fuel droplet diameter distribution, a model for the Sauter Mean Diamter (SMD) of atomized droplets, obtained as a function of spray injection parameters and gas/liquid properties, is incorporated in the LES. Additionally, to consider the temporal fluctuations in fuel flow rate along with its phase difference with the pressure oscillations, a model derived from Bernoullis principle is proposed and employed in the LES. The objective is to examine in detail, the impacts of the fluctuations in fuel droplet diameter distribution and the fluctuations in fuel injection rate individually, as well as the impact of the mutual interaction of these two fluctuations, on the spray combustion instability characteristics. Results of the LES reveal that the temporal fluctuations in fuel droplet diameter distribution resulting from combustion instability, lead to a reduction in the intensity of pressure oscillations and hence the combustion instability’s strength. Additionally, the temporal fluctuations in liquid fuel flow rate strongly influence the intensity of spray combustion instability, and it is observed that the combustion instability intensity increases with the increase in phase difference between the fuel flow rate fluctuations and pressure oscillations. Furthermore, the effect of the temporal fluctuations in fuel droplet diameter distribution resulting in the reduction of combustion

Combustion generated noise of a turbulent spray flame has been investigated using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. In this two-step framework, the reacting flow-field of a flame is simulated using Direct Numerical Simulations (DNS) in the first step, while the acoustic wave propagation in a non-uniform mean flow is captured by solving the Acoustic Perturbation Equations extended to Reacting Flows (APE-RF) in the second step. The numerical approach used in this study is therefore, termed as the hybrid DNS/APE-RF approach. First, this hybrid DNS/APE-RF approach is used to simulate a benchmark experimental open turbulent non-premixed flame, and the results obtained for the non-premixed flame’s flow-field statistical quantities, as well as radiated sound intensities are extensively validated against experimental data. Next, the hybrid DNS/APE-RF approach is applied to an experimental open turbulent spray flame, for simulating its two-phase reacting flow-field along with its combustion generated acoustic field, to predict and analyze the noise radiation behavior. The DNS results of the spray flame are compared with measurements, in terms of droplet velocity statistics and gas-phase temperatures, and favorable agreement is observed. The computed acoustic power spectra of the spray flame exhibit a power law dependence of the form f⁻²˙⁴ (where f is the frequency) in the frequency range 300 Hz < f < 1000 Hz. And, the computed noise spectra of the spray flame contain a nearly constant sound pressure level plateau, for frequencies greater than 1000 Hz and up to 3000 Hz. Acoustic refraction effects induced by sound speed variations in the flame, resulting in the attenuation of high-frequency noise radiated in the flame downstream direction, are also captured in the simulation. Noise radiation characteristics of the turbulent spray flame are found to resemble those of turbulent premixed and non-premixed flames.

Partially premixed spray flames are simulated with flamelet-based tabulated chemistry parameterized by the mixture fraction and progress variable. The transport equation of the reaction progress variable C is reconsidered, and its formulation for the reacting two-phase flows is derived and employed, which allows the inclusion of spray impacts through a new spray source term that is absent in its gaseous form. Both the nonpremixed and premixed flamelets assuming single reaction regime are implemented in LES, and their validities in spray flames and dependence on the evaporation effect when considering two-phase C equation are examined. The effect of spray, reaction and turbulence interaction is then investigated in comparison with experiments of Sydney reacting acetone sprays, covering the rich, lean and stoichiometric cases. The computed results generally follow the experimental data, but a disagreement between two flamelet simulations is observed especially in rich and lean flames. The premixed flamelets tend to capture the downstream jet spreading while overestimating the peak temperature compared to the nonpremixed chemistry. Flame index analysis indicates that in the present spray flames an evaporation-dominated regime exists inside the upstream core jet and it promotes the coexistence of subsequent interacting premixed and nonpremixed reaction zones, which impedes accurate flame prediction by the single regime flamelets. Furthermore, the spray source term appearing in the derived C equation is identified to act as scalar fluxes driven by sprays in flamelet structures. Including this new source term is found to be important to account for the dissipation effect induced by evaporation on the reaction zone in the flamelet simulation of turbulent spray flames.

The mass transfer across a sheared gas-liquid interface strongly depends on the Schmidt number. Here we investigate the relationship between mass transfer coefficient on the liquid side, kL, and Schmidt number, Sc, in the wide range of 0.7 ≤ Sc ≤ 1000. We apply a three-dimensional semi direct numerical simulation (SEMI-DNS), in which the mass transfer is solved based on an approximated deconvolution model (ADM) scheme, to wind-driven turbulence with mass transfer across a sheared wind-driven wavy gas-liquid interface. In order to capture the deforming gas-liquid interface, an arbitrary Lagrangian-Eulerian (ALE) method is employed. Our results show that similar to the case for flat gas-liquid interfaces, kL for the wind-driven wavy gas-liquid interface is generally proportional to Sc−0.5, and can be roughly estimated by the surface divergence model. This trend is endorsed by the fact that the mass transfer across the gas-liquid interface is controlled mainly by streamwise vortices on the liquid side even for the wind-driven turbulence under the conditions of low wind velocities without wave breaking.

Heated moist air from a cooling tower forms a visible plume and needs to be predicted, not only for the performance design of the cooling tower, but also for environmental impact assessments. In this study, a computational fluid dynamics analysis is conducted to predict the scale of a visible plume rising from a cross flow cooling tower with mechanical draft (provided by a rotating fan). The results of computational fluid dynamics analysis are verified by comparing predictions with an actual observed plume. The results show that the predicted visible plume represents the observed plume in an error range of 15%–20%, which is permissible for designing a cooling tower. Additionally, the mixing condition of heated dry air and moist air under dry and wet combined operation is examined, and the condition is thought to affect the scale of the visible plume. It is found that, in the case of a mechanical-draft cooling tower, the fan has a mixing function which performs the complete mixing of wet and dry air, and this suggests that the generation of the plume can be determined by the intersection of the operation line and saturation line. Additionally, the effect of external wind on the scale of the visible plume is large, especially for dry and wet combined operation.

A two-step global reaction scheme for the volatile matter of coal is proposed, and the unsteady coal particle and combustion behaviors in a turbulent pulverized coal jet flame are investigated by performing a direct numerical simulation (DNS) employing the proposed global reaction scheme. The two-step global reaction scheme is constructed to take into account the properties of the volatile matter such as transport coefficients, laminar flame speed and unburned gas temperature and to be applicable to various coal types, and it is validated by comparing the results with those obtained by the detailed reaction mechanism which includes 158 chemical species and 1, 804 reactions. The validity of the DNS is also assessed by comparing the results with those in the previous experiment (Hwang et al., 2005) [16], and the unsteady coal particle motions and combustion characteristics are examined in detail. The results show that the proposed two-step global reaction scheme for the volatile matter of coal can precisely predict the laminar flame speed and burned gas temperature for various coal types from bituminous to low-rank coals over wide ranges of conditions of equivalence ratios, pressures and unburned gas temperatures. In addition, it can correctly take into account the effects of dilutions by H[2]O and CO[2] which compromise the evaporated moisture from coal and products of char reaction. It is also verified that a lab-scale turbulent pulverized coal jet flame is well predicted by the DNS employing the proposed global reaction scheme. That is, the pulverized coal particles' velocity and its fluctuation and the characteristics of particle preferential motions are in general agreement with those observed in the experiment. The DNS reveals that in the turbulent pulverized coal jet flame, there appear premix and diffusion flame layers inside and outside, respectively. In addition, the reaction of the volatile matter and O[2] in coal-carrier air occurs in the inner premixed fl

The effects of turbulent eddies and Langmuir circulations in liquid flow on scalar transfer across a sheared wind-driven gas-liquid interface are investigated by means of a direct numerical simulation of a gas–liquid two-phase turbulent flow with a wind-driven nonbreaking wavy interface. The wind-driven wavy gas-liquid interface is captured using an arbitrary Lagrangian-Eulerian method with boundary-fitted coordinates on moving grids. The results show that Langmuir circulations are generated on the liquid side below the sheared wind-driven gas-liquid interface. The marker particles on the gas-liquid interface, the turbulent eddies in the form of streamwise vortices on the liquid side (i.e., the typical horseshoe vortices associated with bursting motions), and the low scalar flux lines on the gas-liquid interface induced by the turbulent eddies on the liquid side tend to locally concentrate in the regions along the downward flows caused by the Langmuir circulations. It is suggested that the turbulent eddies on the liquid side mainly control the scalar transfer across the sheared wind-driven gas-liquid interface, and the effect of the Langmuir circulations is relatively small.

The effects of difference in fuel components on the droplet evaporation and combustion are numerically investigated. Jet-A is used as liquid fuel, and one (n-decane)-, two (n-decane and 1, 2, 4-trimethyl-benzene)- and three (n-dodecane, iso-octane and toluene)-component fuels are used as the surrogate fuels of Jet-A. The results show that the evaporation of the three-component surrogate fuel becomes faster and slower than those of the one- and two-component surrogate fuels in the initial and subsequent evaporating periods, respectively. The differences in the gas temperature evolution among these three different surrogate fuels are remarkable right after the ignition, but become small with time.

The effects of ambient pressure, initial gas temperature and combustion reaction on the evaporation of a single fuel droplet and multiple fuel droplets are investigated by means of three-dimensional numerical simulation. The ambient pressure, initial gas temperature and droplets’ mass loading ratio, ML, are varied in the ranges of 0.1–2.0 MPa, 1000–2000 K and 0.027–0.36, respectively, under the condition with or without combustion reaction. The results show that both for the conditions with and without combustion reaction, droplet lifetime increases with increasing the ambient pressure at low initial gas temperature of 1000 K, but decreases at high initial gas temperatures of 1500 K and 2000 K, although the droplet lifetime becomes shorter due to combustion reaction. The increase of ML and the inhomogeneity of droplet distribution due to turbulence generally make the droplet lifetime longer, since the high droplets’ mass loading ratio at local locations causes the decrease of gas temperature and the increase of the evaporated fuel mass fraction towards the vapor surface mass fraction.

The effects of equivalence ratio, fuel droplet size, and radiation on jet spray flame are investigated by means of two-dimensional direct numerical simulation (DNS). In addition, the validity of an extended flamelet/progress-variable approach (EFPV), in which heat transfer between droplets and ambient fluid including radiation is exactly taken into account, is examined. n-decane (C[10]H[22]) is used as liquid spray fuel, and the evaporating droplets’ motions are tracked by the Lagrangian method. The radiative heat transfer is calculated using the discrete ordinate method with S8 quadrature approximation. The results show that the behavior of jet spray flame is strongly affected by equivalence ratio and fuel droplet size. The general behavior of the jet spray flames including the heat transfer between droplets and ambient fluid with radiation effect can be captured by EFPV.

The effect of ambient pressure on spray flames is investigated by means of two-dimensional direct numerical simulation (DNS), and the validity of an extended flamelet/progress-variable approach (EFPV) is examined under the high-pressure condition. The DNS is performed not only for a simple jet spray flame with a pilot burner but also for a lifted recirculation spray flame without any pilot burner at ambient pressures of 0.1 and 0.5 MPa. n-decane (C[10]H[22]) is used as liquid spray fuel, and the evaporating droplets’ motions are tracked by the Lagrangian method. The results show that the behaviors of jet and lifted recirculation spray flames are strongly affected by ambient pressure. The effects of the change of the ambient pressure on these spray flame behaviors can be well captured by EFPV and EFPV coupled with G-equation model (EFPV-G), respectively.

The characteristics of flow past a heated/cooled sphere are investigated for particle Reynolds numbers 50≤Rep≤500 in conditions with and without buoyancy by means of three-dimensional numerical simulation in which temperature dependence of fluid properties such as density and viscosity is exactly taken into account. The results show that in the absence of buoyancy, drag coefficients of the heated and cooled spheres are larger and smaller than those of the adiabatic case, respectively, and their Nusselt numbers are smaller and larger than the values estimated by a widely used empirical expression for predicting Nusselt numbers, respectively. In addition, the temperature difference between the sphere and ambient fluid strongly affects the flow separation points, size of vortex ring behind the sphere and Strouhal number for vortex shedding. These changes are attributed to the temperature dependence of fluid properties in the vicinity of the sphere. Even in the presence of buoyancy, the temperature dependence of fluid properties strongly affects the drag coefficient and Nusselt number and therefore the Boussinesq approximation becomes inapplicable as the temperature difference increases, regardless of the magnitude of the Richardson number.

Momentum transfer across the wind-driven breaking air-water interface under strong wind conditions was experimentally investigated using a high-speed wind-wave tank together with field measurements at normal wind speeds. An eddy correlation method was utilized to measure roughness length and drag coefficient from wind velocity components measured by laser Doppler and phase Doppler anemometers. As a result, a new model for the roughness length and drag coefficient was proposed for predicting momentum transfer across the sea surface under both normal and strong wind conditions using the universal relationship between energy and significant frequency of wind waves normalized by the roughness length. The model shows that the roughness length and drag coefficient are uniquely determined at all wind speeds by energy and significant frequency of wind waves, and they can be given againstU10only from the measurements of the wave parameters and one-point mean air velocity in the logarithmic law region.

The effect of turbulent clustering of water droplets on radiative transfer is investigated by means of both a three-dimensional direct numerical simulation of particle-laden homogeneous isotropic turbulence and a radiative transfer simulation based on a Monte Carlo photon tracing method. The results show that turbulent clustering causes the formation of void regions of droplets and hence increases the direct transmittance. This effect decreases as the turbulent Reynolds number increases and is estimated to be negligibly small under the conditions in real clouds.

A three-dimensional direct numerical simulation (DNS) is applied to flows inside and outside an evaporating spherical water droplet in air, and the effect of relative humidity on the heat transfer between the droplet and ambient air is investigated. The initial air temperature is set to be 15 K higher than the initial droplet temperature. The results show that the local evaporation heat loss indicates the maximum on the front of the droplet and decreases on going from the front to the rear, and the reduction on the rear becomes marked for high-droplet Reynolds numbers. This is because the evaporation rate is suppressed on the rear surface by the presence of flow separations behind the droplet. The droplet temperature decreases in the low-humidity condition, whereas it increases in the high-humidity condition. This difference is caused by the heat balance of evaporation heat loss and convective heat gain from ambient air at the droplet surface.

The validity of a steady-flamelet model and a flamelet/progress-variable approach for gaseous and spray combustion is investigated by a two-dimensional direct numerical simulation (DNS) of gaseous and spray jet flames, and the combustion characteristics are analysed. A modified flamelet/progress-variable approach, in which total enthalpy rather than product mass fraction is chosen as a progress variable, is also examined. DNS with an Arrhenius formation, in which the chemical reaction is directly solved in the physical flow field, is performed as a reference to validate the combustion models. The results show that the diffusion flame is dominant in the gaseous diffusion jet flame, whereas diffusion and premixed flames coexist in the spray jet flame. The characteristics of the spray flame change from premixed–diffusion coexistent to diffusion-dominant downstream. Comparisons among the results from DNS with various combustion models show the modified flamelet/progress-variable approach to be superior to the other combustion models, particularly for the spray flame. Where the behaviour of the gaseous total enthalpy is strongly affected by the energy transfer (i.e. heat transfer and mass transfer) from the dispersed droplet, and this effect can be accounted for only by solving the conservation equation of the total enthalpy. However, even the DNS with the modified flamelet/progress-variable approach tends to underestimate the gaseous temperature in the central region of the spray jet flame. To increase the prediction accuracy, a combustion model for the partially premixed flame for the spray flame is necessary.

Two-dimensional numerical simulations are applied to spray flames formed in a laminar counterflow and the effects of radiation on spray flame characteristics and soot formation are studied. N-Decane (C10H22) is used as the liquid fuel, and the droplet motion is calculated by the Lagrangian method. A single-step global reaction is employed for the combustion reaction model. A kinetically based soot model with a flamelet model is used to predict soot formation. Radiation is taken into account using the discrete ordinate method. The results show that radiation strongly affects the spray flame behavior and soot formation. Without the radiation model, flame temperature and soot volume fraction are greatly overestimated. The soot is formed in the diffusion flame regime, and its radiation emission increases with the increase in the equivalence ratio of the droplet fuel. This trend is in good agreement with that of the luminous flame behavior observed in the experiments.