Mild Regime Research Articles

Experimental Analysis on the Material Properties of A356.0 Aluminum Alloy Surface Nanostructured by Severe Shot Peening

The effects of severe shot-peening process and formation of a nanostructured surface layer on mechanical properties of A356.0 alloy were investigated in this paper. X-ray diffraction analyses revealed that the average size of near-surface grains in severe shot-peened specimens is 75.8 nm. Three types of disk-shaped specimens, non-treated, conventionally shot-peened, and severely shot-peened were subjected to pin-on-disk wear test in the dry condition, in different loading and sliding speeds. Shot-peening process increases both hardness and roughness of the surface, and these two factors have, respectively, positive and negative effects on wear resistance. However, because of high-density grain boundaries in severe shot-peened specimens, the effect of increased hardness overcomes the effect of increased roughness. As a result, the wear resistance of severely shot-peened specimens is higher than the others. Wear tracks were characterized by scanning electron microscopy. By changing the normal load from 5 to 10 N, the wear regime alters from mild to severe. Morphology of the specimens illustrated that the predominant wear mechanism in the mild regime and the severe regime are, respectively, oxidation and abrasive. Experimental results also showed that severe shot-peening procedure improves hardness, yield strength, and fracture toughness up to 52, 16, and 49%, respectively.

The role of dilution level and canonical configuration in the modeling of MILD combustion systems with internal recirculation

MILD combustion regime emerged as one of the most promising technology to ensure low pollutant emissions and large fuel flexibility.Large-scale use of such technology requires important developments of its numerical modelling. The difficulty of this task lies in the recirculation of mass and heat that results in coupling chemistry and turbulence phenomena.In this work, the effect of the internal exhaust gas recirculation on the performances of a tabulated chemistry model was investigated by carrying out an experimental and numerical study on a cyclonic burner. FGM (Flamelet Generated Manifold) model was adopted. It makes two main assumptions: a turbulent flame can be considered as an ensemble of 1D laminar flames (Flamelets), and the space of compositions can be represented by a lower-dimensional manifold by defining few controlling variables. In this work, two different 1D flame configurations and two controlling variables (a mixture fraction and a progress variable) were used. The comparisons between model and experiments were reported for different dilution conditions at inlet and two diluent species.Results showed that the role of the internal EGR is crucial in the establishment of MILD regimes. In fact, for air conditions the model is not able to predict the low and more distributed reactivity of the system. On the other hand, inlet diluted conditions show a good agreement with the experiments due to burned product effects. CO2-dilution instead of N2, improves the model results due to the lower reaction rates. Finally, the Flamelet configuration does not strongly affect the simulation results.

Dislocation dynamics during cyclic loading in copper single crystal

Crystalline plasticity can take place through numerous, small, uncorrelated dislocation motions (mild plasticity) or through collaborative events: dislocation avalanches (wild plasticity). Here, we study the correlation between dislocation patterning under cyclic loading and the nature of collective dislocation dynamics. The dislocation motion of a [110] oriented pure copper single crystal was dynamically followed using Acoustic Emission (AE) for different imposed stress amplitudes. The dislocation structure between each cyclic stress step was investigated using Electron BackScattered Diffraction (EBSD) and Rotational-Electron Channeling Contrast Imaging (R-ECCI) in a Scanning Electron Microscope (SEM). At low imposed stress, when the structure consists of dislocation cells, few dislocation avalanches are observed, while for a wall structure, at higher imposed stress, the contribution of avalanches is increased during the first cycles. For a given stress amplitude, the evolution of mild plasticity is synchronous with the plastic strain-rate, and rapidly vanishes after few cycles due to work hardening. The mean free path of the dislocations in this mild plasticity regime corresponds to the characteristic size of the dislocation structure (cell size, distance between walls). From one stress level to another, brutal rearrangements of the dislocation structure occur within a few numbers of cycles. Those rearrangements take place, at least partly, through dislocation avalanches. Upon reloading at a larger stress amplitude, dislocation avalanches can travel over distances much larger than the former dislocation mean free path. As the dislocation avalanches spread within the crystal, the memory of the previous dislocation structure is lost and a new dislocation structure emerges.

Numerical investigation on NOX formation in oxygen-CH4 MILD combustion with air leakage

In this paper, we focus on formation of NOX under oxy-fuel MILD combustion using JHC (jet-in hot co-flow) burner. The effect of the different oxygen dilution ratio in the co-flow is analyzed for MILD combustion formation and also for characterizing NOX emission. RANS (Reynolds averaged Navier Stokes) with the modified k-e equation and EDC (eddy dissipation concept) model were applied for 2D-axisymmetric computational domain using the Ansys Fluent. It is found that the effect of the oxygen concentration rather than the composition of dilution gas is more significant for the formation of the MILD combustion and as the oxygen concentration of the dilution gas increases under MILD combustion conditions, the region with Da < 1 expressed as MILD regime tends to be decreased. The effect of a diluted oxygen concentration in the oxy-fuel combustion on NOx emission under MILD condition is found to be much greater than the amount of air leakage into co-flow, and NO emissions are almost unaffected by the increase of N2 concentration in case of MILD combustion condition within practical air leak ranges.

Effect of Water Stress on Photosynthesis, Chlorophyll Fluorescence Parameters and Water Use Efficiency of Common Reed in the Hexi Corridor

Water stress is the major environmental stress that affect agricultural production worldwide, especially in arid and semi-arid regions. This research investigated the effect of water stress on common reed (Phragmites australis (Cav.) Trin. ex Steud.) grown in the West Lake Wetland of Zhangye City, China. We designed with four water treatments (100, 95, 85 and 75% of water field capacity, i.e. CK (100%), mild (95%), moderate (85%) and severe (75%) water deficit regimes). The effect of water stress on photosynthesis and chlorophyll fluorescence parameters was investigated. There was a midday depression in net photosynthetic rate (Pn) for CK, mild and moderate drought, but not for severe drought. Stomatal limitation was dominant for mild treatment. But under severe drought stomatal limitation to photosynthesis was dominating in the morning and nonstomatal limitation was dominating in the afternoon. Compared with CK treatment, Pn, stomatal conductance (gs), intercellular CO2 concentration (Ci), transpiration rate (E) and water use efficiency (WUE) were always lower at drought stress. This result suggests that water stress caused the photosynthesis of common reed to be completely blocked and the common reed was a drought-sensitive plant. We found 75% is the moisture threshold for common reed. The decrease in Fm, Fv, Fv/Fm and increase F0 suggested that Rubisco activity reduced and PSII partially inactivated during the day under drought. However, part of this inactivation of PSII might be alleviated under mild or moderate drought, but severe drought cannot. Drought stress also affected the photosynthesis Pn-PAR response curve. Drought stress increased LCP, RD and decreased LSP, Pmax and AQY. This indicates that when common reed suffers from drought, the utilization range of light intensity become narrowed and photosynthetic ability or adaptability to light reduced.

Complex influence of temperature on oxalic acid anodizing of aluminium

Two-step aluminium anodizing is widely used to prepare oxide films with a highly-ordered porous structure through the whole thickness. It can be realized solely at relatively low values of current density in the kinetic anodizing regime that makes this process extremely time-consuming. Temperature is an important thermodynamic parameter which is able to accelerate chemical reactions and mass-transport and, thus, could be suggested as an effective tool to control the growth rate of anodic alumina. Here we report a systematic study of the porous anodic alumina films formation in 0.3 M oxalic acid electrolyte in a wide temperature range. The influence of electrolyte temperature on the kinetics of aluminium anodizing and on the morphology of the obtained anodic alumina films is addressed quantitatively. Current transients registered at various temperatures are used for the calculation of the apparent activation energy of the anodization process in both mild and hard anodizing regimes. In the case of thick oxide films and high electrolyte temperature, the increase in diffusion current contribution leads to switching the kinetically controlled anodizing to the mixed regime which results in destroying the hexagonal pore ordering. Based on the experimental findings, rational electrolyte temperatures and porous layer thicknesses for the first and second anodizing steps are proposed as a guide for express preparation of alumina films with well-ordered porous structure.

Effects of Preheating and CO2 Dilution on Oxy-MILD Combustion of Natural Gas

Oxy-moderate or intense low-oxygen dilution (MILD) combustion, which is a novel combination of oxy-fuel technology and MILD regime, is numerically studied in the present work. The effects of external preheating and CO2 dilution level on the combustion field, emission, and CO formation mechanisms are investigated in a recuperative laboratory-scale furnace with a recirculating cross-flow. Reynolds-averaged Navier–Stokes (RANS) equations with eddy dissipation concept (EDC) model are employed to perform a 3-D simulation of the combustion field and the turbulence–chemistry interactions. In addition, a well-stirred reactor (WSR) analysis is conducted to further examine the chemical kinetics of this combination when varying the target parameters. The simulations used the skeletal USC-Mech II, which has been shown to perform well in the oxy-fuel combustion modeling. Results show that with more preheating, the uniformity of temperature distribution is noticeably enhanced at the cost of higher CO emission. Also as inlet temperature increases, the concentration of minor species rises and CO formation through the main path (CH4→CH3→CH2O→HCO→CO→CO2) is strengthened, while heavier hydrocarbons path (C2H2→CO) is suppressed. Meanwhile, greater CO2 addition notably closes the gap between maximum and exhaust temperatures. In a highly CO2-diluted mixture, chain-branching reactions releasing CH2O are strengthened, while chain-terminating reactions are weakened. CH2O production through CH3O is accelerated compared with the straight conversion of methyl to formaldehyde. When diluting the oxidant, methylene CH2(s) plays a more influential role in CO formation than when pure oxygen is used, contributing to higher CO emission.

On the reciprocating sliding wear of polypropylene against polyamide 6 in dry and aqueous environments

Polymer-polymer tribological components with sliding contacts usually are lighter, cheaper and more corrosion resistant in aqueous environment, presenting some advantages over the classic metal-polymer equivalents under certain conditions, such as in wear resistant external coatings for pipes in industry. Reciprocating sliding wear of polypropylene (PP) against polyamide 6 (PA6) in dry and aqueous environments (distilled water and synthetic sea water) was mapped as a function of contact pressure (2.4–7.2 MPa) and sliding speed (36–108 mm/s), aiming the identification of favorable operating conditions (low wear). Mild wear regime (k = 1.3–5.3 × 10−6 mm3/N m) was found for PP in all conditions, except for the highest sliding speed in dry environment, where there was a severe regime (k = 0.5–0.8 × 10−3 mm3/N m). Meanwhile, the PA6 counter-body always operated in the mild wear regime for all studied conditions. Wear mechanisms in the mild regime were: surface fatigue microcracks, film transfer and abrasion for both materials and, for PA6 only, also wear by roll formation. At higher speed, frictional heating (0.14 < μ < 0.25) and low thermal diffusivity of the materials resulted in high contact temperatures (up to 120 °C), which is enough to promote the removal of molten or softened material in the PP, typical of severe wear regime. The mild-to-severe wear regime transition observed for PP-PA6 tribopair occurs at a much lower sliding speed than in metal-polymer pairs, which may limit the applicability of these tribosystems. For PA6, the severity of surface damage as a function of the environment increased from dry to distilled water and sea water condition, as this material is susceptible to attack by polar solutions, which degrades its mechanical properties.

On the effects of different regimes of plasma pulses affecting the material due to their succession

The tungsten (>99.97% W) samples were irradiated on plasma focus device (PF-12), with deuterium (D) as a working gas, in Tallinn University in Estonia. Two of the samples serve as references, being irradiated in only one regime (either in harsh regime with heat flux factor ∼1000 MW s1/2 m2 for plasma and ∼1.5•105 MW s1/2 m2 for fast ions, or in the mild regime with heat flux factor ∼40–45 MW s1/2 m2 for plasma and ∼2000 MW s1/2 m2 for fast ions). Three of the samples received 27 pulses combining the two aforementioned regimes. The combined effects are obtained by using two different plasma flux values and also by varying the timing when the harsh regime is implemented during the irradiation cycle of each sample. The research is conducted by analyzing the SEM images of the damaged surfaces, measuring the electrical conductivity of the material in the bulk, studying the SEM images and micro-hardness of sample's cross-sections. This will enable to draw conclusions on the range of damages due to the varied succession of used regimes. The results are then combined to estimate the applicability of conductivity measurements instead of using destructive invasive measuring methods (i.e., measurement of micro-hardness and SEM imaging of cross sections).The aims of our research is to shed some light on (1) whether and how the succession of different series of irradiations with varying power flux densities affects the formation of damages on and within the samples and (2) estimating the three-dimensional damages that are created within the bulk of material due to high-temperature plasma and ion shockwaves. Our study indicates that the damage in bulk is related to the timing of harsh regime plasma pulses. The use of measurements of conductivity enables to estimate the future development of the damages in bulk of the material.

Computational analysis on oxygen MILD combustion using synthesis gas

In this paper, a possibility of the oxygen-syngas MILD combustion was assessed using the CFD analysis by applying the JHC (Jet-in Hot Co-flow) combustion system. The effect of the various syngas composition ratios was analyzed, and heating value of synthesis gas was controlled by ratio of CO and H2. The Ansys Fluent, commercial CFD code with RANS (Reynolds Averaged Navier-Stokes), modified k-ε equations and EDC (Eddy Dissipation Concept) model using 2D-axisymmetric computational domain was used in order to analyse oxygen-syngas MILD combustion. It was confirmed that the oxygen-syngas combustion can be formed in the MILD combustion under conditions of the oxygen concentration of 3 ~ 9 %, which is similar condition to the air-fuel MILD combustion region in the Adelaide and Delft flame. As the proportion of H2 in the composition of the synthesis gas is decreased, the region of the MILD combustion with Da < 1 is found to become wider due to the decrease in the Arrhenius reaction rate. At higher oxygen concentration in the co-flow, the region with Da <1 known as MILD regime tends to be decreased.

Stability and emission characteristics of nonpremixed MILD combustion from a parallel-jet burner in a cylindrical furnace

This experimental study investigates the stability and emission characteristics of the nonpremixed MILD combustion from a parallel jet burner firing propane (C3H8) in a cylindrical furnace without preheating and dilution. Forty cases are examined by varying the fuel-air nozzle separation S (= 8 mm–105 mm), global equivalence ratio Φ (= 0.6–1.0) and thermal input Pin (= 10 and 15 kW) to highlight the impact of S. The investigation is carried out based on the measurements of in-furnace temperature and exhaust species concentrations, as well as the visual inspection into the furnace. It is demonstrated that the NO emission (ENO) decreases generally with increasing either S (at S > 8 mm) or Φ (at least for Φ ≤ 1.0). Interestingly, the combustion for S = 8 mm behaves like a premixed counterpart and so is distinct from the other cases: e.g., this combustion is fully flameless or stable in the MILD regime over a wider range of Φ and its ENO has a weaker dependence on the furnace temperature. By contrast, for S = 45 mm, the flameless mode takes place only at Φ = 1.0 due to the small mixing zone under this air-fuel arrangement, which is revealed by aerodynamic analysis. Hence, the corresponding experimental ENO-S relationship appears to have a turning point around S = 45 mm. For the present study, all the measured NO emissions for S = 8–105 mm fall as the overall furnace temperature rises, which seemingly differs from the existing knowledge. To explain this and other results, the numerical analysis from computational fluid dynamics (CFD) is performed. It is found that the variations of NO emissions depends on the maximum temperature within the furnace rather than the average furnace temperature. Finally, the present study approves that the fuel-air jet separation should be taken as a critical operational parameter when designing the parallel multiple jet burner for nonpremixed MILD combustion.

Contemporary Photoligation Chemistry: The Visible Light Challenge.

Catalyst-free and bond-forming light-induced reactions have seen an unprecedented renaissance in the realm of soft matter materials science due to their efficiency, spatio-temporal controllability and, sometimes, photoreversible nature. However, many of these reactions rely on the application of high energy UV light that can cause photo-degradation and is inapplicable in biological environments. If up-conversion systems or two-photon processes are to be avoided, strategies for red-shifting catalyst-free ligation technology are critically required. This Concept article introduces the reader to recent methods that lead to efficient, catalyst-free visible-light-induced ligation chemistry based on polyaromatic substituted photoreactive compounds-pyrene and anthracene-and, furthermore, emphasizes the broad and facile applicability of these molecules in polymeric material design. Concomitantly, we highlight that a careful action plot analysis of photochemical reactivity can provide deep insights into reactivity patterns, far beyond those suggested by the absorption spectrum. Indeed, we suggest that an action plot analysis is necessary for the evaluation of any photochemical system and its response to structural chemical changes.

Thermal mercury removal from coals: Effect of pyrolysis conditions and kinetic analysis

The fate of mercury (Hg) content of four poor quality coals was investigated under various pyrolysis conditions (temperature, time, pressure), using helium flow. The resulting chars were analyzed for Hg and compared with the parent coal’s content. Hg removal efficiency was found to be affected by both pyrolysis temperature and residence time. Mercury removal continuously increased for pyrolysis temperatures up to 600 °C, and was sharply decelerated for higher ones, resulting in a rather sigmoid shape of the Hg removal curve. Pyrolysis time up to 20 min showed significant impact on Hg removal, negligibly affected Hg release. Thus, pyrolysis at temperatures between 500 and 600 °C for ∼20 min could be considered sufficient enough for “Hg-free” coal production. The role of pyrolysis pressure on Hg removal was found to be limited, and its impact (if any) was observed at short pyrolysis times (≤10 min), being diminished when the residence time is prolonged (up to 20 min). The results revealed three characteristic temperature regimes; a low temperature (200 °C–300 °C) physical desorption mechanism with low apparent activation energy, a mild temperature regime (300 °C–600 °C) with mixed physico-chemical desorption and sublimation mechanism with higher activation energy, and a high temperature regime (700 °C–900 °C) where Hg evolution dramatically decreased. Mercury coal removal during pyrolysis was simulated using a simple first order volume reaction model for the three temperature regimes identified. Τhe apparent activation energy was initially found ∼3.1 kJ/mol at the low temperature regime, increased to ∼6.1 kcal/mol at the mild temperature range, and almost fourfold to ∼22.4 kJ/mol at the high temperature regime. As the apparent activation energy increased, the pre-exponential factor (k0) also increased linearly, tending to compensate the increase of the activation energy. However, the conventional compensation theory does not offer a robust explanation of this behavior and further investigation is required.

Identification of abrasion regimes based on mechanisms of wear on the steel stylus used in the Cerchar abrasiveness test

The wear of tools in rock excavation is a challenging and costly issue. To predict the added costs arising from wear, field and laboratory investigations can be conducted during the early stages of a project. Accordingly, a measure of particle abrasivity is needed. Abrasivity is defined as the capability of different kinds of soils and rocks to cause abrasive wear of metallic surfaces. The Cerchar abrasiveness test is a well-recognized lab test for assessing rock abrasivity. The Cerchar Abrasiveness Index (CAI) is determined by measuring the crater diameter [mm] produced by a conical steel stylus moving under a fixed load and sliding distance. Many authors have been trying to correlate CAI with rock properties, but no consensus has been achieved. The main reason for this difficulty arises from the effects of the microstructural complexity of rocks and their tribological response. This investigation proposes a new approach based on the use of a scanning electron microscope to identify the wear mechanisms acting on the Cerchar stylus (AISI A2 tool steel) during testing, and comparing these observations to the corresponding CAI values for nine different rock types (0.1 < CAI < 6). The observed wear mechanisms were classified into two extremes (mild and severe) with a transition region between them. The mild regime is characterized by the polishing action of rocks (CAI ≤ 1.8). The severe regime is characterized by microcutting with extensive plastic deformation (CAI ≥ 3.1). The reliability of the current findings is discussed in the light of a theoretical conversion of CAI values into their respective wear coefficients. Our proposed approach of using wear coefficients is expected to provide better correlation between the results of the Cerchar laboratory test and real drilling rates.

Influence of preheating and thermal power on cyclonic burner characteristics under mild combustion

Autoignition and stabilization of distributed combustion regimes have been proved to occur when a sufficient entrainment of hot species in the fresh reactants jets is reached, thus providing simultaneously for the sensible enthalpy to promote the auto-ignition process and the mass to dilute the incoming fresh reactants.The present study investigates the stabilization process along with the performance of the combustion process in a cyclonic burner operated under MILD combustion conditions. The cyclonic flow has been achieved by means of two pairs of oxidant/fuel jets injected using an anti-symmetric configuration in a prismatic combustion chamber thus realizing a centripetal cyclonic flow field directed toward the top-central gas outlet.Propane/air combustion experimental campaigns without external dilution, on a detailed grid of equivalence ratio and preheating temperature values, at different average residence time and nominal thermal power values were made. In each test condition, temperature measurements inside the chamber and gas sampling analyses have been carried out in order to evaluate the operability range of the cyclonic burner and its performances. These tests allowed to demonstrate the feasibility of stable MILD Combustion regimes in a wide range of operating conditions even when feeding the cyclonic burner with undiluted air.The residence time of the streams inside the burner plays an important role for both reactive structure stabilization and combustion performances/emissions. Significantly, fuel-lean conditions correspond, in the considered cases, to simultaneously low CO and NOx emissions.Furthermore, it has been demonstrated that stable combustion can be sustained in absence of any preheating in a considerable thermal load range and that it is possible, in this condition, to achieve a complete fuel conversion, with a remarkably low pollutant emission for thermal loads up to 8 kW.

Floristic distinctiveness and endemic richness of woody plants highlight the biodiversity value of the herriza among all Mediterranean heathlands

Background: Heathlands are relatively abundant in the landscape of the western Mediterranean region, especially in the Strait of Gibraltar region, where it is locally known as herriza. They are associated with a mild Mediterranean climate regime and with acid, nutrient-poor soils. They harbour a high plant diversity, often viewed as a consequence of the transition between European Atlantic heathland and Mediterranean sclerophyllous shrubland floras.Aims: To determine whether species-rich Mediterranean heathlands, including the herriza, constitute distinct heathland formations rather than transitional vegetation units between Atlantic heathlands and Mediterranean garrigue shrublands.Methods: We quantified species richness, endemism and analysed the β-diversity of the woody component of Mediterranean heathland communities throughout its geographic range, with special emphasis on the Strait of Gibraltar region.Results: Mediterranean heathlands, including the herriza, are not transitional communities between Atlantic heathlands and Mediterranean shrublands. Woody species richness and, particularly, endemic richness was the highest in the herriza.Conclusions: The high biodiversity values of the herriza are a likely consequence of the ecological singularity of the Strait of Gibraltar region and its known role as a glacial refugium. Despite its treeless feature, the herriza deserves special recognition and protection from both in its European and North African extension.

Kinetic, volatile release modeling and optimization of torrefaction

Torrefaction is a thermal pretreatment of biomass process carried out in the temperature range of 473–573K under inert atmosphere to enhance fuel properties. In the present study, different biomass are analyzed through numerical simulations and optimized parameters are reported. Kinetic, volatile release and solid composition models are coupled & simulated. Results of the kinetic model are verified by comparing with experimental results. Then, compositional coefficients for volatiles of different biomass are found by volatile release model. Further, volatile release model used to calculate composition of releasing volatiles. By means of mass conservation, composition of remaining solid product is estimated. Effect of torrefaction temperature, residence time and nature of biomass on moisture content of biomass, volatiles release, hydrogen to carbon (H/C) & oxygen to carbon (O/C) molar ratios, energy densification and elemental compositions are given. It is found that during mild torrefaction regime 508–548K; water removal and energy densification increases more rapidly as compared to light 473–508K and severe 548–573K regimes. Further, H/C and O/C molar ratios are found to be decreasing linearly in mild regime. In light and severe torrefaction; moisture removal and energy densification are increasing slowly. So, mild torrefaction was found to be optimum regime of torrefaction for practical applications. In mild torrefaction regime energy densification ratio was 1.05-1.52 for different biomasses studied.

New-Concept Gas Turbine Burner Simulation in Moderate Intense Low-Oxygen Combustion Regime

In a trapped-vortex combustor (TVC) flame stabilization is achieved through intense internal exhaust gases recirculation, which is promoted by the adoption of cavities. Thanks to its peculiar features, a trapped-vortex burner produces low pressure drop and emissions and it is characterized by extended blow-out limits. The strong mixing of fresh reactants with flue gases due to internal recirculation represents the basis for the establishment of a distributed MILD, i.e. "Moderate Intense Low-Oxygen Dilution Combustion" regime, which is characterized by reduced temperature peaks, volumetric distributed reactions, low NOx emissions and no thermo-acoustic instabilities. Aim of the work is to study the possibility to obtain a MILD regime in our available trapped-vortex device, taking the advantage of the combined effect of TVC strong internal exhaust gases recirculation and of oxy-combustion external exhaust recirculation, attaining the benefits of CO2 capture at the same time. To this end a series of computational fluid dynamics simulations were conducted on our TVC device, in order to understand the influence on combustion of the main operating parameters, such as the equivalence ratio, the level of dilution, the injection temperature, the velocity, etc.. A preheating temperature and a range of oxygen concentrations that at the same time complies with a distributed reactions regime and an efficient combustion were identified for the premixed and non-premixed operating modes.

An experimental and numerical study of MILD combustion in a Cyclonic burner

The implementation of MILD combustion systems is limited by a lack of fundamental insight into such combustion regime and therefore novel tools are indispensable compared to traditional combustion systems. In this context CFD simulations for the prediction of the burner behaviour and for design and optimization appears essential for a successful introduction of such concept in some industries. Detailed chemistry has to be included in fluid-dynamics simulations in order to account for the strong turbulence-chemistry interaction in the MILD regime. An effective strategy to overcome this aspect is represented by tabulated chemistry techniques. In particular the implementation of Flamelet Generated Manifold with IML tabulation seems to be a promising tools for MILD systems and therefore high fidelity and comprehensive experimental data are needed for the assessment of such model. The present study is framed in this context and it investigates the characteristics of MILD Combustion in a Cyclonic lab-scale burner that operates with high level of internal recirculation degrees induced by a cyclonic fluid-dynamic pattern obtained by the geometrical configuration of the reactor and of the feeding system. Experimental tests were realized varying the mixture composition. Detailed measurements of local mean temperatures and concentrations of gas species at the stack for several operating conditions were used to validate the FGM model under such unconventional operating conditions. Results suggest that FGM with IML is a promising tool for modeling the complex flame structures of cyclonic MILD burner, with many aspects that need to be further investigated.

Effect of increasing load on the MILD combustion of COG and its blend in a 30 kW furnace using low air preheating temperature

MILD combustion is a high efficiency technique, applied on industrial furnaces to have low polluting emissions and significant energy saving by high air preheating. The main purpose of this work was the investigation of the behavior of two fuels rich in H2 burning in MILD regime at different furnace temperature levels (1200 K ÷ 1300 K), using air preheating temperature which can be guaranteed by low performance heat exchanger too. Both fuels had very good performance but the one with higher H2 content, which reduces the ignition delays, showed higher capability to keep a well-positioned reaction zone at higher heat transfer to the load and consequently wider flue gases temperature range.