LES–ODT study of turbulent premixed interacting flames

The LES–ODT model is implemented for the study of twin turbulent premixed flames in decaying isotropic turbulence. The approach is based on the coupling of large-eddy simulation (LES) for mass and momentum with a fixed 3D lattice of 1D fine-grained solutions based on the one-dimensional turbulence (ODT) model. The ODT solutions for momentum and reactive scalars are designed to capture subgrid scale physics that is not captured by LES. The LES–ODT formulation is capable of capturing important fine-scale processes, such as flame–flame interactions, which play an important role in flame shortening in turbulent premixed flames, and the role of preferential diffusion on curved flames’ structures.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

A comparison of the heat transfer behaviors of biogas–H2 diffusion and premixed flames

An experimental study was conducted to investigate the influence of hydrogen addition on the heat transfer characteristics of a biogas (60%CH₄–40%CO₂) flame. Results show improved flame stability and higher flame temperature in the premixed flame upon hydrogen addition. Both temperature and burning speed are increased in 1.0 ≤ Ф ≤ 1.5. Comparison of the premixed and diffusion flames reveals that the former yields higher heat transfer than the latter, due to higher flame temperature and larger volume of hot gas in the premixed flame. The total heat transfer rates of the two flames show opposite trends with increasing level of hydrogen addition, which is explained by the different structures. In the premixed flame, the contact of large cool core with target plate configures the high-temperature flame zone to a radial location with larger distance from the stagnation point than that of the diffusion flame, contributing to its higher heat transfer rate.     

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

An experimental and numerical study on laminar burning characteristics of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.     

Numerical and experimental analysis of falling-film exchangers used in a LiBr–H2O interseasonal heat storage system

This paper investigates heat and mass transfer occurring in an interseasonal absorption heat storage system using LiBr/H₂O as the sorption couple. It focuses on the poor performances of the falling film exchangers with vertical tubes, which are characterized by low flow rate compared to conventional absorption machines. A numerical model was developed for the study and validated with specific experimental results. Comparison of the numerical model to experimental results from the heat storage prototype shows the presence of abnormally high thermal resistance between the falling films and the exchanger surfaces. The deterioration in performance appears to originate in the low wetting rate of the surfaces. A new design of the exchangers is proposed to solve this problem and thus attain the desired performance.     

An experimental study of forced convective heat transfer for fully developed Al2O3–water nanofluid in an annulus tube

Nanofluids have been known as practical materials to ameliorate heat transfer within diverse industrial systems. The current work presents an empirical study on forced convection effects of Al₂O₃–water nanofluid within an annulus tube. A laminar flow regime has been considered to perform the experiment in high Reynolds number range using several concentrations of nanofluid. Also, the boundary conditions include a constant uniform heat flux applied on the outer shell and an adiabatic condition to the inner tube. Nanofluid particle is visualized with transmission electron microscopy to figure out the nanofluid particles. Additionally, the pressure drop is obtained by measuring the inlet and outlet pressure with respect to the ambient condition. The experimental results showed that adding nanoparticles to the base fluid will increase the heat transfer coefficient (HTC) and average Nusselt number. In addition, by increasing viscosity effects at maximum Reynolds number of 1140 and increasing nanofluid concentration from 1% to 4% (maximum performance at 4%), HTC increases by 18%.     

Numerical and experimental study on granular flow and heat transfer characteristics of directly-irradiated fluidized bed reactor for solar gasification

Solar gasification is one of the promising techniques to convert the carbonaceous materials to clean chemical fuels, which offers the advantages of being transportable as well as storable for extended period of time. In this study, thermal performance of a recently developed 5 kW_th fluidized bed reactor for solar gasification has been investigated and reported. Discrete element method (DEM) has been used for modeling the granular flow, and computational fluid dynamics (CFD) method has been used for modeling the multiphase flow. To validate the developed model, experiments were preformed and compared with modeling results. Discrete ordinate radiation model has been used to solve the radiative transfer equation. The thermal performance of the reactor and particulate flow behavior have been predicted and the effect of particle size, particle size distribution and gas flow rate are analyzed. The results indicate that the performance of the bed increases when fluidizing the annulus region particles as the high porosity increases the diffusion rate of radiation throughout the bed.     

Numerical investigation for turbulent heat transfer of TiO2–SiO2 nanofluids with wire coil inserts

A numerical model has been developed for turbulent flow of hybrid nanofluids in a tube with wire coil inserts. The model was developed from van Driest eddy diffusivity equation. The model can be implemented with the consideration of new variables in eddy diffusivity of momentum and heat by using the coefficient, K and Prandtl index, ζ, respectively. The numerical analysis are undertaken for wide range of Reynolds number, different volume concentration, ? and various pitch ratio, P/D of wire coil. The numerical results were validated with the experimental data of TiO₂–SiO₂ nanofluids undertaken for wide range of Reynolds number and volume concentration. The final regression models of coefficient K and Prandtl index ζ were developed as a function of Reynolds number, Re or dimensionless radius, R⁺, volume concentration, ? and pitch ratio, P/D. A good agreement between the experimental data and numerical model indicating the validity of the numerical model for hybrid nanofluids with wire coil inserts. The numerical analysis was proved that the hybrid nanofluids contributes to higher Nusselt number and thus have better heat transfer performance compared to single nanofluids.     

Effects of hydrogen addition on the premixed laminar-flames of ethanol–air gaseous mixtures: An experimental study

The effects of hydrogen addition on the laminar premixed-flame characteristics of ethanol–air gaseous mixtures were investigated experimentally by using outwardly propagating spherical flames. The experiments were conducted in a constant-volume combustion vessel with a central ignition at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 100%, and an equivalence ratio from 0.6 to 1.6, and the flame images were obtained by a high-speed schlieren camera system. The results show that the unstretched flame propagation speeds and burning velocities increase exponentially with the increase in hydrogen fraction for a constant equivalence ratio. When the hydrogen fraction is equal to or less than 60%, the burned gas Markstein length reduces with the increase of equivalence ratio, indicating a positive correlation between the flame instability and hydrogen fraction, while the opposite effect is observed when the hydrogen fraction is greater than 60%. At an equivalence ratio below 1.4, the Markstein length decreases with increased hydrogen fraction, indicating that the flame instability is exacerbated with hydrogen addition, while the reverse holds in the case of equivalence ratio above 1.4. Finally, an empirical formula is developed to estimate the laminar burning velocity of ethanol–hydrogen–air flames on the basis of present experimental data.     

Numerical study of syngas production via CH4–H2S mixture partial oxidation

The detailed kinetic mechanism of pyrolysis and oxidation of the H₂S–CH₄ mixture was developed. The mechanism was validated on experimental data on the ignition delay, laminar flame velocity, conversion degree of H₂S and CH₄, as well as on the yield of main conversion products – H₂ and CO. The developed mechanism was used for a numerical study of the conversion degree of H₂S and CH₄, the syngas yield and syngas composition during partial oxidation of the H₂S–CH₄ mixture with H₂S/CH₄ = 1/9, 1/4 and 3/1 in a plug flow reactor of 1 m in length in the wide range of initial temperature (T₀ = 600–1400 K) and fuel-to-air equivalence ratio (ϕ = 1–30). It is shown that the maximum relative yield of syngas can be obtained at ϕ = 3–5 depending on T₀ and the H₂S/CH₄ ratio in the fuel. The mole fraction of H₂ in syngas is higher than that of CO. For the mixture with H₂S/CH₄ = 3/1, the mole fraction of H₂ can be greater than the equilibrium value in a certain range of ϕ∼6–10. The reasons for this effect are analyzed. The mole fraction of CO in conversion products rapidly decreases with increasing ϕ. As a result, the ratio γ_H2/γ_CO increases fast with the growth of ϕ. Besides H₂ and CO, the conversion products can contain S₂ and NO (at ϕ∼2), CS (at ϕ∼3), CS₂ (at 3 < ϕ < 10), unburned hydrocarbons (at ϕ > 3) and other species. The least amount of conversion byproducts is observed at ϕ = 3–3.5 when there is the maximum syngas yield. Syngas selectivity turned out maximal at ϕ = 2.5–3. Therefore, ϕ∼3 seems to be the most optimal value for carrying out the conversion of H₂S–CH₄ mixtures.     

Mechanistic study of Ce–La–Fe/γ-Al2O3 catalyst for selective catalytic reduction of NO with NH3

A number of Ce–La–Fe/γ-Al₂O₃ catalysts were prepared by impregnation and microwave hydrothermal heating based on the ratio of Ce, La, and Fe in natural bastnäsite to examine the synergistic relationship between Ce, La, and Fe in natural bastnäsite and its effects on ammonia selective catalytic reduction (NH₃-SCR). The results demonstrated that an increase in Fe species is beneficial to the denitration performance of catalysts with a Ce:La:Fe mole ratio of 1:0.6:0.12, thus providing high NO conversion (>95%) at 350 °C. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy results confirmed the formation of a solid solution between Ce, La, and Fe in the catalysts. The BET isotherms and NH₃-TPD demonstrated that the addition of Fe increased the surface area and strengthened the surface acidity of Lewis acid sites. XPS and in situ DRIFTS indicated that electron transfer occurred between Fe³⁺/Fe²⁺ and Ce⁴⁺/Ce³⁺ redox cycles in the catalyst, thereby improving the adsorption and activation ability of NO and NH₃. NO species were adsorbed on the catalysts in the form of monodentate nitrate, and then monodentate nitrate was oxidized to bidentate nitrate. The SCR reaction route was the coexistence of Eley–Rideal and Langmuir–Hinshelwood mechanisms.     

Numerical simulation and experimental results of horizontal tube falling film generator working in a NH3–LiNO3 absorption refrigeration system

This paper describes the work made at the Centro de Investigación en Energía in the development of an absorption refrigeration system for cooling and refrigeration applications with a capacity of 10 kW. The single effect unit utilizes ammonia-lithium nitrate as working pair and it is air cooled. The generator is a falling film type with horizontal tubes where the heating oil flows inside the tube bank and the ammonia-lithium nitrate solution flows as a falling film on the tube outside, where it is heated and ammonia vapor is generated. The generator consists of tree columns and four rows per column of horizontal tubes. The system was tested at controlled conditions with heating oil obtained from an electric resistance heating loop. A numerical model of the horizontal falling film generator was developed that divided the system into three different thermal elements: the flow inside the tube, the heat conduction in the tube wall and the falling film solution flow. The mathematical model was tested and validated with experimental data and a study of the influence of the heat transfer coefficient for ammonia-lithium nitrate solution in the numerical model was carried out. A comparison between experimental and numerical data for the heat flux in the system and the temperature profiles in the oil and solution flows shown a good degree of correlation.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

Numerical study of nanofluid heat transfer for different tube geometries – A comprehensive review on performance

The heat transfer performance of a system can be improved using a combination of passive methods, namely nanofluids and various types of tube geometries. These methods can help enhance the heat transfer coefficient and consequently reduce the weight of the system. In this paper, the effect of tube geometry and nanofluids towards the heat transfer performance in the numerical system is reviewed. The forced convective heat transfer performance, friction factor and wall shear stress are studied for nanofluid flow in different tube geometries. The thermo-physical properties such as density, specific heat, viscosity and thermal conductivity are reviewed for the determination of nanofluid heat transfer numerically. Various researchers had measured and modelled for the determination of thermal conductivity and viscosity of nanofluids. However, the density and specific heat of nanofluids can be estimated with the mixture relations. The different tube geometries in simulation work are analyzed namely circular tube, circular tube with insert, flat tube and horizontal tube. It was observed that the circular tube with insert provides the highest heat transfer coefficient and wall shear stress. Meanwhile, the flat tube has greater heat transfer coefficient with a higher friction factor compared to the circular tube.     

Effects of preferential diffusion on downstream interaction in premixed H2/CO syngas–air flames

Effects of strain rate and preferential diffusion of H₂ on flame extinction are numerically explored in interacting premixed syngas–air flames with the fuel compositions of 50% H₂ + 50% CO and 30% H₂ + 70% CO. Flame stability diagrams mapping lower and upper limit fuel concentrations at flame extinction as a function of strain rate are examined. Increasing strain rate reduces the boundaries of both flammable lean and rich fuel concentrations and produces a flammable island and subsequently even a point, implying that there exists a limit strain rate over which interacting flame cannot be sustained anymore. Even if effective Lewis numbers are slightly larger than unity on the lean extinction boundaries, the shape of the lean extinction boundary is slanted even at low strain rate, i.e. a_g = 30 s⁻¹ and is more slanted in further increase of strain rate, implying that flame interaction on lean extinction boundary is strong and thus hydrogen (as a deficient reactant) Lewis number much less than unity plays an important role of flame interaction. It is also shown that effects of preferential diffusion of H₂ cause flame interaction to be stronger on lean extinction boundaries and weaker on rich extinction boundaries. Detailed analyses are made through the comparison between flame structures with and without the restriction of the diffusivities of H₂ and H in symmetric and asymmetric fuel compositions. The reduction of flammable fuel compositions in increase of strain rate suggests that the mechanism of flame extinction is significant conductive heat loss from the stronger flame to ambience.     

Numerical study on the convective heat transfer of nanofluid in a triangular minichannel heat sink using the Eulerian–Eulerian two-phase model

In this paper, the laminar forced convection heat transfer of the water-based nanofluid inside a minichannel heat sink is studied numerically. An Eulerian two-fluid model is considered to simulate the nanofluid flow inside the triangular heat sink and the governing continuity, momentum, and energy equations for both phases are solved using the finite volume method. Comparisons of the Nusselt number predicted by the Eulerian–Eulerian model with the experimental data available in the literature demonstrate that the simulation results are in excellent agreement with the experimental data and the maximum deviation from experimental data is 5%. The results show that the heat sink with nanofluid has a better heat transfer rate in comparison with the water-cooled heat sink. Also, the heat transfer enhancement increases with an increase in Reynolds number and nanoparticle volume concentration. In addition, the friction factor increases slightly for nanofluid-cooled minichannel heat sink.     

Numerical investigation of heat and mass transfer within different configurations of LaNi5–H2 reactor using the unstructured Lattice Boltzmann Method

In this study, the Control-Volume Lattice Boltzmann Method (CVLBM) based on the unstructured grids is proposed as a numerical solver for transient heat and mass transfer in a Metal Hydrogen Reactor (MHR) during the absorption process. To check the validity of the numerical approach, computational results were compared with those of the literature and a good agreement was obtained. The obtained results were also compared with those of the unstructured Control Volume Finite Element Method (CVFEM). We found that the new approach correctly predicts hydrogen absorption phenomena and has less CPU time compared with the CVFEM (about 8 times faster). In addition, various tank geometries were numerically studied and a new geometric configuration is proposed. The dynamic performances of these layouts were compared based on the numerical simulation. We found that the geometrical modification improves the hydriding performance. For the new configuration, we found that the storage time can be reduced by 87% compared to the basic configuration.     

Epoxy-activated acrylic particulate polymer-supported Co–Fe–Ru–B catalyst to produce H2 from hydrolysis of NH3BH3

Epoxy-activated acrylic particulate polymer, namely Eupergit CM, supported Co–Fe–Ru–B catalyst (EP/Co–Fe–Ru–B) for the first time was used to produce H₂ from hydrolysis of NH₃BH₃. The EP/Co–Fe–Ru–B showed very effective performance in the production of H₂ from the hydrolysis of NH₃BH₃. Various techniques such as XRD, SEM-EDS, ICP-OES, and TEM have been used to characterize these catalysts. The parameters on the hydrolysis reaction of NH₃BH₃ such as the effect of metal amount, the effect of Ru percentage, the effect of NH₃BH₃ concentration, the effect of NaOH concentration, the amount of catalyst, temperature, and catalyst durability were investigated in detail. Eupergit CM based polymer support and Ru particles have been found to be highly effective in H₂ production reactions. The hydrogen production rate (HGR) of the EP/Co–Fe–Ru–B catalyst was found to be 36,978 mL/min/g_cat, which was quite good compared to the values reported in the literature. In addition, the activation energy (Ea) of the polymer-supported Co–Fe–Ru–B catalyst was determined as 24.91 kJ/mol.     

Pseudo-stable transitions and instability in chemical heat pumps: the NH3–CoCl2 system

Research in our laboratory has been directed toward the development of a chemical heat pump, based on the NH₃–CoCl₂ system, to provide both refrigeration and heat for the agribusiness industry. In this work the stability of the salt in an inert atmosphere (nitrogen) and in the presence of ammonia was examined experimentally. For a given pressure, the stable composition of the CoCl₂·xNH₃ salt was found to vary with temperature, when the gaseous atmosphere alternated between ammonia and nitrogen. Specifically the salt changed from CoCl₂·6NH₃ to CoCl₂·2NH₃ (140°C, 260 kPa), from CoCl₂·6NH₃ to CoCl₂ (148°C, 260 kPa), and from CoCl₂·2NH₃ to CoCl₂ (170°C, 260 kPa). During the conversion of the salt from one phase to another, pseudo-stable transitions occurred at some processing conditions. In each case they were stable for several minutes, but always less than 1 h. In a nitrogen atmosphere CoCl₂ was found to be unstable above 300°C for pressures from 100 to 600 kPa. In ammonia CoCl₂·2NH₃ stability was found to be a function of temperature and pressure. An explanation for its decomposition, which could lead to the formation of solid NH₄Cl has been suggested. In summary, the CoCl₂·2NH₃ salt was stable at processing conditions close to the phase diagram equilibrium line for 100% decomposition to CoCl₂·2NH₃ and unstable at large departures from it. If commercial chemical heat pumps are to be technically viable, the salt used must be stable for many cycles of synthesis and decomposition. Regions of stability can be defined by plotting experimental results at different processing conditions on phase diagrams of the type developed here.