On modeling the diffusion to kinetically controlled burning limits of micron-sized aluminum particles

Aluminum particle burn rates are known to be a strong function of particle size as the mode of burning transitions from diffusion to kinetically controlled. To better understand the rate dependent diffusion and kinetic processes, a fully compressible, one-dimensional, spherically symmetric particle burn model is developed. Several cases are studied to explore the burning of aluminum particles in air, carbon-dioxide and steam environments. Predictions of burn rates versus particle size reveal significant deviations from a diffusion controlled burning limit – highlighting the importance of accounting for finite-rate chemistry in modeling the burning of sub-micron aluminum particles. While overall agreement to data is satisfactory, the detailed model cannot be directly used in system level tools due to computational cost. Two reduced modeling strategies are therefore explored to account for finite-rate chemistry effects in simpler models for use in system level CFD analysis. The first is an augmented D²-law$D^2-\mathit{law}$ where the finite-rate chemistry is treated as a perturbation to flame sheet approximation via augmented burn rate “constants”. Predictions using this approach of deflagration speeds in dusty aluminum-air gases agree well with experiments and show evidence of a maximum flame speed for a given mass loading. The second modeling approach uses a reduced numerical model and kinetics mechanism resulting in computationally efficient solutions. Results using this approach show up to two orders of magnitude reduction in computational effort while maintaining reasonable accuracy for predictions of flame structure, burn rates and burn times.     

Thermal-economic multi-objective optimization of plate fin heat exchanger using genetic algorithm

Thermal modeling and optimal design of compact heat exchangers are presented in this paper. ε–NTU$\epsilon ''–''\mathit{NTU}$ method was applied to estimate the heat exchanger pressure drop and effectiveness. Fin pitch, fin height, fin offset length, cold stream flow length, no-flow length and hot stream flow length were considered as six design parameters. Fast and elitist non-dominated sorting genetic-algorithm (NSGA-II) was applied to obtain the maximum effectiveness and the minimum total annual cost (sum of investment and operation costs) as two objective functions. The results of optimal designs were a set of multiple optimum solutions, called ‘Pareto optimal solutions’. The sensitivity analysis of change in optimum effectiveness and total annual cost with change in design parameters of the plate fin heat exchanger was also performed and the results are reported. As a short cut for choosing the system optimal design parameters the correlations between two objectives and six decision variables with acceptable precision were presented using artificial neural network analysis.     

Effects of bluff-body shape on LPG–H2 jet diffusion flame

The effects of bluff-body lip thickness on the several physical parameters like flame length, radiant fraction, gas temperature and _NOx${\mathrm{NO}}_x$ emissions in liquefied petroleum gas (LPG)–H₂ jet diffusion flame are investigated experimentally. Results indicate that the flame length reduces with the addition of hydrogen in the bluff-body stabilized flame, which can be attributed to the enhanced reactivity and residence time of the mixture gases. Moreover, with increasing lip thickness of the bluff body, the flame length also gets reduced. The soot free length fraction (SFLF) is observed to be enhanced with H₂ addition to the fuel stream. In contrast, the SFLF gets reduced with increasing lip thickness repetition, which is due to the reduced induction period of soot formation. The emission index of _NOx${\mathrm{NO}}_x$ (_EINOx${\mathrm{EINO}}_x$) is found to be attenuated in coaxial burner with hydrogen addition. In contrast it is observed to be enhanced in bluff-body stabilized flame. The former is due to the reduction in residence time of gas mixture, whereas the latter can be explained on the basis of increased flame temperature. Besides this, _NOx${\mathrm{NO}}_x$ emission level is also found to be enhanced with increasing lip thickness due to enhanced residence time.     

A DNS evaluation of mixing models for transported PDF modelling of turbulent nonpremixed flames

Transported probability density function (TPDF) methods are well suited to modelling turbulent, reacting, variable density flows. One of the main challenges to the successful deployment of TPDF methods is accurately modelling the unclosed molecular mixing term. This study examines three of the most widely used mixing models: the Interaction by Exchange with the Mean (IEM), Modified Curl (MC) and Euclidean Minimum Spanning Tree (EMST) models. Direct numerical simulation (DNS) data-sets were used to provide both initial conditions and inputs needed over the course of the runs, including the mean flow velocities, mixing frequency, and the turbulent diffusion coefficient. The same chemical mechanism and thermodynamic properties were used, allowing the study to focus on the mixing model. The simulation scenario was a one-dimensional, nonpremixed, turbulent jet flame burning either a syngas or ethylene fuel stream that featured extinction and reignition. This test scenario was selected because extinction and reignition phenomena are sensitive to the mixing model. Three DNS cases were considered for both the syngas and ethylene cases with a parametric variation of Reynolds and Damköhler numbers, respectively. Extinction events became more prevalent with increasing Reynolds number in the syngas cases and with decreasing Damköhler number in the ethylene cases. The model was first tested with the mixing frequency defined from the dissipation rate and variance of mixture fraction. With this definition, for the syngas cases this study finds that the TPDF method is successful at predicting flame extinction and reignition using all three mixing models for the relatively lower and intermediate Reynolds number cases, but that all models under-predict reignition in the relatively higher Reynolds number case. In the ethylene fuelled cases, only the EMST mixing model correctly predicts the reignition event for the two higher Damköhler number cases, however, in the lowest Damköhler number case the EMST model over-predicts reignition and the IEM and MC models under-predict it. Mixing frequency was then modelled based on the turbulence frequency and a model constant _C?$C_{?}$, the ratio of scalar to mechanical mixing rates. The DNS cases were reexamined with this definition and the results suggested that the optimal value for _C?$C_{?}$ is mixing model and case dependent. In particular, it was found in the ethylene case considered that reignition could be achieved with the IEM and MC models by adjusting the value of _C?$C_{?}$.     

Total and partial pressure measurements for the sulphur–iodine thermochemical cycle

First results on the measurement of total and partial pressures over the ternary system HI–_I2–_H2O$\mathrm{HI}–{\mathrm{I}}_2–{\mathrm{H}}_2\mathrm{O}$ are reported. Using original optical online measurements, data on the gas phase speciation are obtained which will help to scale and optimize the reactive distillation column we promote for the HI section of the sulphur–iodine cycle.     

Effect of the equivalence ratio,Damköhler number,Lewis number and heat release on the stability of laminar premixed flames in microchannels

The effect of the equivalence ratio on the stability and dynamics of a premixed flame in a planar micro-channel with a step-wise wall temperature profile is numerically investigated using the thermo-diffusive approximation. To characterize the stability behavior of the flame, we construct the stability maps delineating the regions with different flame dynamics in the inlet mass flow rate m   vs. the equivalence ratio ?$?$ parametric space. The flame stability is analyzed for fuels with different diffusivity by changing the Lewis numbers in the range 0.3?_LeF?1.4$0.3?{\mathit{Le}}_F?1.4$. On the other hand, the Lewis number of the oxidizer is kept constant and equal to unity _LeO=1${\mathit{Le}}_O=1$. Our results show that, for very diffusive fuels, the stability of the flame varies significantly with the equivalence ratio, transitioning from stable flames for lean mixtures to highly unstable flames when ?>1$?>1$. As the fuel Lewis number approaches unity, the stability behavior of the flame for lean and rich mixtures becomes more similar to give, in the equidiffusional case _LeF=1${\mathit{Le}}_F=1$, a symmetric stability map around the stoichiometric mixture ?=1$?=1$. In all cases considered, the most stable flames are always found around the stoichiometric mixtures ?=1$?=1$, when the flame instabilities are completely suppressed for very diffusive fuels _LeF<1${\mathit{Le}}_F<1$, or are reduced to a narrow range of inflow velocities for fuel Lewis numbers equal or greater than unity.     

Hydrogen generation from liquid phase catalytic reforming of sugar solutions using metal-supported catalysts

Most of the hydrogen production processes are designed for large-scale industrial uses and are not suitable for a compact hydrogen device to be used in systems like solid polymer fuel cells. Integrating the reaction step, the gas purification and the heat supply can lead to small-scale hydrogen production systems. The aim of this research is to study the influence of several reaction parameters on hydrogen production using liquid phase reforming of sugar solution over Pt, Pd, and Ni supported on nanostructured supports. It was found that the desired catalytic pathway for _H2${\mathrm{H}}_2$ production involves cleavage of C–C, C–H and O–H bonds that adsorb on the catalyst surface. Thus a good catalyst for production of _H2${\mathrm{H}}_2$ by liquid-phase reforming must facilitate C–C bond cleavage and promote removal of adsorbed CO species by the water–gas shift reaction, but the catalyst must not facilitate C–O bond cleavage and hydrogenation of CO or _CO2${\mathrm{CO}}_2$. Apart from studying various catalysts, a commercial Pt/γ$\gamma$-alumina catalyst was used to study the effect of temperature at three different temperatures of 458, 473 and 493 K. Some of the spent catalysts were characterised using TGA, SEM and XRD to study coke deposition. The amorphous and organised form of coke was found on the surface of the catalyst.     

Flame balls in mixing layers

We present a study of flame balls in a two-dimensional mixing layer with one objective being to derive an ignition criterion (for triple-flames) in such a non-homogeneous reactive mixture. The problem is formulated within a thermo-diffusive single-reaction model and leads for large values of the Zeldovich number β$\beta$ to a free boundary problem. The free boundary problem is then solved analytically in the asymptotic limit of large values of the Damköhler number, which represents a non-dimensional measure of the (square of the) mixing layer thickness. The explicit solution, which describes a non-spherical flame ball generalising the classical Zeldovich flame balls (ZFB) to a non-uniform mixture, is shown to exist only if centred at a single location. This location is found to be precisely that of the leading-edge of a triple-flame in the mixing layer, and typically differs from the location of the stoichiometric surface by an amount of order β^-1${\beta }^{-1}$ depending only on a normalised stoichiometric coefficient Δ$\Delta$.     

Probability density function approach coupled with detailed chemical kinetics for the prediction of knock in turbocharged direct injection spark ignition engines

In this work a new knock model is derived which accounts for the inherent feature of knocking combustion, namely that it is a stochastic phenomenon. It provides the probability of autoignition and distinct criteria to determine the mean knock onset as well as the relative number of knocking cycles. For modeling purposes an ignition progress variable is proposed to determine the reactive state of the unburnt fuel–air mixture and the occurrence of autoignition. Statistical information of this quantity is introduced by presuming a clipped Gaussian probability density function (PDF). Its shape is defined by the Favre mean and variance of the ignition progress variable for which transport equations are derived. The chemical source terms that appear in these equations are closed by employing a presumed PDF approach to account for turbulence chemistry interaction. A clipped Gaussian PDF distribution for temperature and a β$\beta$-PDF for mixture fraction are employed. Hence, the impact of temperature and mixture fraction fluctuations on the ignition progress variable is accounted for. The chemical source terms are evaluated based on tabulated chemistry incorporating detailed chemical kinetics. For the assessment of the knock model a spark timing sweep was performed on the engine test bench for a full-load operating point at n=2000$n=2000$ rpm. In-cylinder flow simulations including gas exchange, mixture formation, combustion, and knock were carried out and the results are compared with experimental data. It is shown that the knock model is able to predict the mean knock onset with reasonable accuracy and that the impact of a spark timing sweep on the number of knocking cycles is well captured.     

Combustion of micron-sized aluminum particle,liquid water,and hydrogen peroxide mixtures

The combustion of aluminum particle, liquid water, and hydrogen peroxide (H₂O₂) mixtures is studied theoretically for a pressure range of 1–20 MPa and particle sizes between 3 and 70 μm. The oxidizer-to-fuel (O/F) weight ratio is varied in the range of 1.00–1.67, and four different H₂O₂ concentrations of 0%, 30%, 60%, and 90% are considered. A multi-zone flame model is developed to determine the burning behaviors and combustion-wave structures by solving the energy equation in each zone and enforcing the temperature and heat-flux continuities at the interfacial boundaries. The entrainment of particles is taken into account. Key parameters that dictate the burning properties of mixtures are found to be the thermal diffusivity, flame temperature, particle burning time, ignition temperature, and entrainment index of particles. When the pressure increases from 1 to 20 MPa, the flame thickness decreases by a factor of two. The ensuing enhancement of conductive heat flux to the unburned mixture thus increases the burning rate, which exhibits a pressure dependence of the form r_b = ap^m. The exponent, m, depends on reaction kinetics and convective motion of particles. Transition from diffusion to kinetically-controlled conditions causes the pressure exponent to increase from 0.35 at 70 μm to 1.04 at 3 μm. The addition of hydrogen peroxide has a positive effect on the burning properties. The burning rate is nearly doubled when the concentration of hydrogen peroxide increases from 0 to 90%. For the conditions encountered in this study, the following correlation for the burning rate is developed: _rb[cm/s]=4.97(p[MPa])^0.37(_dp[μm])^-0.85(O/F)^-0.54exp(0.0066CH₂O₂).$r_b[\text{cm}/\text{s}]=4.97{(p[\text{MPa}])}^{0.37}{(d_p[\mathrm{\mu }\text{m}])}^{-0.85}{(\text{O}/\text{F})}^{-0.54}\exp (0.0066{\text{C}}_{{\text{H}}_2{\text{O}}_2})\text{.}$     

Experimental and modeling study on auto-ignition characteristics of methane/hydrogen blends under engine relevant pressure

Auto-ignition characteristics of methane/hydrogen mixtures with hydrogen mole fraction varying from 0 to 100% were experimentally studied using a shock tube. Test pressure is kept 1.8 MPa and temperatures behind reflected shock waves are in the range of 900–1750 K and equivalence ratios from 0.5 to 2.0. Three ignition regimes are identified according to hydrogen fraction. They are, methane chemistry dominating ignition (XH₂≤40%)$\left(X_{{\text{H}}_2}\le 40\mathrm{\%}\right)$, combined chemistry of methane and hydrogen dominating ignition (XH₂=60%)$\left(X_{{\text{H}}_2}=60\mathrm{\%}\right)$, and hydrogen chemistry dominating ignition (XH₂≥80%)$\left(X_{{\text{H}}_2}\ge 80\mathrm{\%}\right)$. Simulated ignition delays using four models including USC Mech 2.0, GRI Mech 3.0, UBC Mech 2.1 and NUI Galway Mech were compared to the experimental data. Results show that USC Mech 2.0 gives the best prediction on ignition delays and it was selected to conduct sensitivity analysis for three typical methane/hydrogen mixtures at different temperatures. The results suggest that at high temperature, ignition delay mainly is governed by chain branching reaction H + O₂ ⇔ OH + O, and thus increasing equivalence ratio inhibits ignition of methane/hydrogen mixtures. At middle-low temperature, contribution of equivalence ratio on ignition delay of methane/hydrogen mixtures is mainly due to chemistries of HO₂ and H₂O₂ radicals.     

Experimental study of combustion of hydrogen–syngas/methane fuel mixtures in a porous burner

Lean premixed combustion of hydrogen–syngas/methane fuel mixtures was investigated experimentally to demonstrate fuel flexibility of a two-section porous burner. The un-insulated burner was operated at atmospheric pressure. Combustion was stabilized at the interface of silicon-carbide coated carbon foam of 26 pores per centimeter (ppcm) and 4 ppcm. Methane (CH₄) content in the fuel was decreased from 100% to 0% (by volume), with the remaining amount split equally between carbon monoxide (CO) and hydrogen (H₂), the two reactive components of the syngas. Experiments for different fuel mixtures were conducted at a fixed air flow rate, while the fuel flow rate was varied to obtain a range of adiabatic flame temperatures. The CO and nitric oxide (_NOx${\mathrm{NO}}_x$) emissions were measured downstream of the porous burner, in the axial direction to identify the post-combustion zone and in the transverse direction to quantify combustion uniformity. For a given adiabatic flame temperature, increasing H₂/CO content in the fuel mixture decreased both the CO and _NOx${\mathrm{NO}}_x$ emissions. Presence of H₂/CO in the fuel mixture also decreased temperature near the lean blow-off limit, especially for higher percentages of CO and H₂ in the fuel.     

Study on laminar flame speed and flame structure of syngas with varied compositions using OH-PLIF and spectrograph

Various Bunsen flame information of premixed syngas/air mixtures was systematically collected. A CCD camera was used to capture the flame images. The OH-PLIF technique was applied to obtain the flame OH distribution and overall flame radiation spectra were measured with a spectrograph. Experiments were conducted on a temperature un-controlled burner and syngas over a wide range of H₂/CO ratios (from 0.25 to 4) and equivalence ratios (from 0.5 to 1.2). Results show that increasing hydrogen fraction (XH₂$X_{{\text{H}}_2}$) extends the blow-off limit significantly. The measured laminar flame speed using cone-angle method based on CCD flame imaging and OH-PLIF images increases remarkably with the increase of XH₂$X_{{\text{H}}_2}$, and these measurements agrees well with kinetic modeling predictions through Li's mechanism when the temperature for computation is corrected. Kinetic study shows that as XH₂$X_{{\text{H}}_2}$ increases, the production of H and OH radicals is accelerated. Additionally, the main H radical production reaction (or OH radical consumption reactions) changes from R29 (CO + OH = CO₂ + H) to R3 (H₂ + OH = H₂O + H) as XH₂$X_{{\text{H}}_2}$ increases. Sensitivity analysis was conducted to access the dominant reactions when XH₂$X_{{\text{H}}_2}$ increases. The difference on flame color for different XH₂$X_{{\text{H}}_2}$ mixtures is due to their difference in radiation spectrum of the intermediate radicals produced in combustion.     

A new gaseous and combustible form of water

In this paper we present, apparently for the first time, various measurements on a mixture of hydrogen and oxygen called HHO gas produced via a new electrolyzer (international patents pending by Hydrogen Technologies Applications, Inc. of Clearwater, Florida), which mixture is distinctly different than the Brown and other known gases. The measurements herein reported suggest the existence in the HHO gas of stable clusters composed of H and O atoms, their dimers H–O, and their molecules _H2${\mathrm{H}}_2$, _O2${\mathrm{O}}_2$ and _H2O${\mathrm{H}}_2\mathrm{O}$ whose bond cannot entirely be of valence type. Numerous anomalous experimental measurements on the HHO gas are reported in this paper for the first time. To reach their preliminary, yet plausible interpretation, we introduce the working hypothesis that the clusters constituting the HHO gas constitute another realization of a recently discovered new chemical species called for certain technical reasons magnecules   as well as to distinguish them from the conventional “molecules” [Santilli RM. Foundations of hadronic chemistry with applications to new clean energies and fuels. Boston, Dordrecht, London: Kluwer Academic Publisher; 2001]. It is indicated that the creation of the gaseous and combustible HHO from distilled water at atmospheric temperature and pressure occurs via a process structurally different than evaporation or separation, thus suggesting the existence of a new form of water, apparently introduced in this paper for the first time, with the structure (H×H)$(\mathrm{H}\times \mathrm{H})$–O where “×$\times$” represents the new magnecular bond and “-$-$” the conventional molecular bond. The transition from the conventional H–O–H species to the new (H×H)$(\mathrm{H}\times \mathrm{H})$–O species is predicted by a change of the electric polarization of water caused by the electrolyzer. When H–O–H is liquid, the new species (H×H)$(\mathrm{H}\times \mathrm{H})$–O can only be gaseous, thus explaining the transition of state without evaporation or separation energy. Finally, the new species (H×H)$(\mathrm{H}\times \mathrm{H})$–O is predicted to be unstable and decay into H×H$\mathrm{H}\times \mathrm{H}$ and O, by permitting a plausible interpretation of the anomalous constituents of the HHO gas as well as its anomalous behavior. Samples of the new HHO gas are available at no cost for independent verifications, including guidelines for the detection of the new species.