Layer Combustion Research Articles

Activation of the process of layer combustion of bitter coal with iron nitrate and waste of metal rolling production

The influence of metal-rolling production waste and iron nitrate on the characteristics of the process of layer combustion of coal is investigated. As a solid fuel, coal of the T grade was used. Metal scale and iron nitrate were introduced into the fuel by mechanical mixing. According to XRD data, phases of iron oxides Fe3O4 and manganese Mn3O4 were identified in the metal scale. The characteristics of the combustion process of the studied samples were studied using high-speed video with the use of a combustion chamber at a heating medium temperature of 700°C. Combustion Also, the process of activated layer combustion coal was scaled using a solid fuel boiler unit. It has been experimentally established that the use of metal scale and iron nitrate leads to an increase in the reactivity of the fuel, as evidenced by a decrease in the ignition delay time. Due to the intensification of the combustion process, the fuel underburning and the concentration of CO formed in the composition of gas-phase combustion products were reduced.

Distinguishing the vertical air-staging low-NOx function of secondary air for suspension combustion within an industrial-scale coal-fired reversal grate furnace

The numerous coal-fired grate furnaces in China, which were designed to partition primary air on the grate and arrange secondary air in the freeboard for air-staging combustion, suffered from poor burnout and high NOx. However, secondary air was conventionally closed or set at marginal openings in real operations and no investigation was reported to explain why. Upon these off-design secondary-air circumstances, industrial-scale tests and modeling investigations were performed for a 75 t/h coal-fired reversal grate furnace at secondary-air ratios of 0 %, 3 %, 6 %, 10 %, and 16 %, respectively. The primary aims were to (i) uncover its combustion and NOx formation characteristics with varying secondary air and (ii) confirm whether secondary air acted on a real air-staging function in reducing NOx. Flow-field deflection and asymmetric combustion were found in the furnace, with the upward gas entirely deflecting towards the rear side. The front-half furnace was filled with a large but weak recirculation zone without effective combustion, acting on a huge dead zone to waste the furnace-space utilization. With increasing the secondary-air ratio, the upward gas deflection deteriorated continuously. Layer combustion worsened while suspension combustion first strengthened and then weakened. NOx emissions displayed an ascent-to-descent trend. Carbon in slag raised while carbon in fly ash decreased. Among the five case setups, the 0 % setting (i.e., fully closing secondary air) exhibited the lowest NOx emissions and burnout loss. Compared with the habitual 6 % setup, fully closing secondary air not only reduced NOx emissions by 22 % but also improved a little burnout. These trends meant that opening secondary air only strengthened suspension combustion for burning well fly ash while raised both NOx and total burnout loss. Secondary air thus failed to role as an air-staging function in reducing NOx.

Effect of the coal-spreading air on airflow and low-NOx combustion within a 75 t/h coal-fired grate furnace

Coal-fired grate furnaces equipped with coal throwers, which conventionally own a low-NOx combustion framework consisting of (i) the partitioned primary air for the horizontal air staging on the grate and (ii) the coal-spreading air and secondary air for the vertical air staging in the freeboard, actually suffer from poor burnout and apparently high NOx emissions at habitual operations. While comprehensive investigations on the in-furnace airflow, coal combustion, and NOx formation, which are devoted to first explaining and then improving these problems, are absent up to now. In order to unveil the airflow, combustion, and NOx emission characteristics within a 75 t/h coal-fired reverse grate furnace, evaluate its coal-spreading air impact on the low-NOx combustion performance, and determine an appropriate coal-spreading air ratio for improving the furnace performance, both industrial-scale tests and numerical simulations were performed at the coal-spreading air ratios of 2 %, 4 %, 6 %, and 8 %, respectively. The flow-field deflection and asymmetric combustion appeared in the upper furnace, with the upward gas deflecting towards the rear-wall side and the front-half furnace part dominated by a weak recirculation zone without effective combustion. With increasing the coal-spreading air, problems of the flow-field deflection and asymmetric combustion aggravated. The combustion intensity weakened in the layer combustion zone while displayed an increase-to-decrease trend in the suspension combustion zone. In terms of indexes related to the furnace performance, burnout rate decreased continuously, and NOx emissions initially raised but then decreased. A comprehensive consideration of the coal-spreading requirement via air, flow-field symmetry, low-NOx combustion, and burnout suggests that a 2 % coal-spreading air ratio should be preferred, where NOx emissions and burnout loss were gained at 249 mg/m3 (6 % O2) and 5.75 %, respectively. It reduced NOx emissions by 49 % while raised slightly burnout loss, as compared with the corresponding levels of 486 mg/m3 and 5.4 % at the habitual case. These results not only negate the air-staging function of the coal-spreading air but also highlight the novelty of this work.

Application of Boundary Layer Combustion in High-Mach-Number Scramjets

Boundary layer combustion is an approach to the reduction of skin friction in hypersonic propulsion systems based on reducing the Reynolds shear stress in the turbulent boundary layer by combustion heat release. In this study, the Reynolds-averaged Navier–Stokes methodology is adopted to study the drag reduction characteristics of boundary layer combustion in high enthalpy flow. Two novel explanations of the local increase in skin friction caused by ignition delay are given in terms of flow structure and the Kármán momentum integral equation, respectively. In addition, it is found that the inhibitory effect of combustion heat release on Reynolds shear stress is gradually weakened with increasing total enthalpy, which is detrimental to drag reduction. Finally, a combination of radical farming and boundary layer combustion is proposed to improve the drag reduction effect. This approach is verified by using a two-dimensional simplified M12-02 scramjet model, with the results indicating that radical farming can considerably suppress ignition delay and thereby further benefit drag reduction and combustion.

Analysis of the Efficiency of Burning Briquettes from Agricultural and Industrial Residues in a Layer

The combustion of briquettes made from organic and industrial residues in small boilers requires researchers to consider the characteristics of this type of fuel and methods of its combustion. For the efficient combustion of fuel briquettes, a layered combustion method with the ability to regulate the supply of combustion air is better suited. The purpose of this research is to study the thermal technical conditions of briquetted fuel combustion. In order to carry this out, a stand was created, which made it possible to determine the combustion efficiency of this type of fuel. Two types of briquettes were studied: one with 30% sunflower husks and 70% leaves, and one with and 70% sunflower husks and 30% coke breeze. The combustion results of the briquettes show that heat loss from chemical under-burning was no more than 6.25%. To determine the temperature distribution in the fuel layer, a model of unsteady heat transfer in a fixed layer was used. A calculation of the temperature fields in the layer of burned fuel briquettes was carried out, which showed that the most favorable conditions for burning briquettes were created with a layer about 15–20 cm thick for both burned briquette options. The temperature was in the range of 450–750 °C, which on the one hand corresponds to experimental data and on the other hand provides a combustion regime that occurs with a relatively low loss to the environment. This installation and mathematical model will help future studies based on the processes of other types of organic waste combustion with a grate system.

Activation of the Process of Layer Combustion of Coal with Iron Nitrate and Waste of Metal Rolling Production

Activation of the Process of Layer Combustion of Coal with Iron Nitrate and Waste of Metal Rolling Production

Drag reduction study based on boundary layer combustion on single expansion ramp nozzle

In hypersonic flight, frictional drag is an important factor affecting the overall engine performance. The wall frictional drag analysis and drag reduction technology of a unilateral expanding tail nozzle under typical working conditions are carried out by means of two-dimensional Reynolds-averaged N-S equations and SST k − ω turbulence model. The effects of hydrogen injection and combustion with different mass flow rates under different jet positions on the wall friction resistance are analyzed and compared, and the results show that, under the appropriate opening position, a better drag reduction effect can be obtained by injecting hydrogen with a mainstream flow rate of about 2.5%, which can achieve a reduction of 43.7%.

Large eddy simulation of boundary layer combustion of hydrogen and hydrocarbon fuel films in a modeled scramjet combustor

Supersonic fuel film cooling is a promising way to simultaneously reduce the severe wall heat and friction load of the internal passage in a scramjet engine when operating at hypersonic conditions. Large eddy simulations were performed to investigate the cooling and wall friction characteristics of hydrogen and hydrocarbon films under inert and reactive circumstances. The results show that the essential difference of the turbulent state in the mixing layer contributes to the totally different behaviors of the cooling and wall friction reduction performances of the two fuel films. The turbulent transport processes between the hydrogen film and the mainstream are much weaker as compared to the case of hydrocarbon film, making inert hydrogen rather superior in cooling and friction reduction applications. Besides, the increase of wall temperature for hydrogen film under the inert case is mainly driven by the loss of hydrogen with high heat capacity instead of by direct heat addition. However, the film cooling performance severely deteriorates when the hydrogen film burns due to presence of severe heat release sources near the wall. On the other hand, combustion of hydrocarbon film in the boundary layer can remarkably improve its originally barely-satisfactory cooling and friction reduction performance to the level comparable to that of hydrogen film, due to the suppression of turbulent transport processes in the mixing layer and presence of heat absorption sources near the wall. Overall, the hydrogen film is more advantageous in friction reduction, while the hydrocarbon film is more suitable for cooling.

Thermo-elastic Characteristics of Thermal Barrier Coating in a Gas Turbine Combustion Chamber with an Intermediate FGM Layer

This study is focused on the analysis of thermos-elastic characteristics of a gas turbine combustion chamber, where the inner surface experiences a higher temperature while the outer region of the wall should be thermally conductive to dissipate the heat of combustion. Using a single material is not suitable to ensure the above two requirements simultaneously. Instead of using a single material, a layered combustion chamber with high-temperature material at the inner region and thermally conductive material at the outer region can be a solution. This produces the problem of a sharp interface where disbonding occurs due to high thermal stresses. The problem of sharp interface can be eliminated by using a functionally graded material (FGM) layer in between the inner and outer layers of the combustion chamber. The present study considers a gas turbine combustion chamber consisting of three layers: the inner layer is used as a thermal barrier coating, the intermediate layer is a smooth transition from the inner layer to the outer layer, and the outer layer is a thermally conductive material. ANSYS simulation is used for the analysis of thermos-elastic characteristics of the combustion chamber with homogeneous and FGM intermediate layers. The effect of linear and nonlinear material distributions in the FGM layer on the thermos-elastic characteristics is studied. A comparison of results reveals that thermal stress smoothly changes from the inner layer to the outer layer in the case of the FGM intermediate layer compared to the homogeneous intermediate layer. This suggests that a combustion chamber with an FGM intermediate layer is more reliable than a homogeneous intermediate layer.

Revisiting palaeolithic combustion features of Theopetra Cave: A diachronic use of dung and peat as fuel

The chemical diagenesis and palaeoenvironmental reconstruction of the Palaeolithic sequence of Theopetra Cave have been studied extensively, but little information is available regarding the details of its combustion structures. The cave is characterized by extensive beds of multi-sequence combustion layers dated between 140 and 50 ka BP, and thick, often stratigraphically complex bodies of ash and charred remains dated between 16 and 13 ka BP. All combustion features contain large amounts of charred and partially charred fibrous organic matter of non-wood plant material and very little charcoal. The structure, fabric, composition, and chemistry of these remains suggest that a mixture of peat and dung was used as a fuel, occasionally enriched with amounts of wood fuel. The integrity of the sequence of combustion structures precludes the possibility that dung was produced by animals inside the cave as their traffic on its wet substrate would have destroyed the burnt layers and homogenized the sediment. The mixture of peat and dung was most likely collected from peatlands associated with swamps of the former so called Karditsa Lake that probably existed in the area until the beginning of the Holocene. Peat and dung were used as a fuel when wood fuel was not available, during the relatively cold intervals of the glacial periods, but also during the last interglacial when the area close to the cave was wooded. Of great interest is that this same fuel was used during both the Middle and the Upper Palaeolithic, presumably by different human species. Although Theopetra appears to be the first site that peat and dung was used as a fuel during the Middle Palaeolithic, it is suggested that other sites may have used this fuel as well. This has important consequences in understanding the evolution of human pyrotechnology particularly during glacial periods.

Mechanisms and characteristics of wall skin friction reduction by boundary layer injection under hypervelocity inflow conditions

In this paper, the influences of boundary layer injection on skin friction reduction are numerically studied using Reynolds-averaged Navier-Stokes equations. The methodology is first validated by comparing the numerical results with the existing experimental data. Studies on the effects of inert gas boundary layer injection reveal that boundary layer injection forms a gas film cooling effect, which reduces skin friction through low viscosity and low temperature effects. However, the enhancement of turbulence in the boundary layer by the jet weakens this effect For fuel boundary layer injection, boundary layer combustion creates a low-density environment in the boundary layer, reducing the momentum transport of the fluid to the wall and significantly enhancing the drag reduction effect. Based on this, the drag reduction effect of boundary layer combustion is found to decrease significantly under mainstream heat release conditions, mainly due to the large-scale heat release in the outer edge of the boundary layer, which puts the fluid in a high-temperature and high-pressure state as a whole, and boundary layer combustion cannot establish a “barrier” for momentum transport in a low-density region.

Neutrino Emission of Neutron-Star Superbursts

Superbursts of neutron stars are rare but powerful events explained by the explosive burningof carbon in the deep layers of the outer envelope of the star. In this paper we perform a simulation ofsuperbursts and propose a simple method for describing the neutrino stage of their cooling, as well as amethod for describing the evolution of the burst energy on a scale of several months. We note a universalrelation for the temperature distribution in the burnt layer at its neutrino cooling stage, as well as theunification of bolometric light curves and neutrino heat loss rates for deep and powerful bursts. We pointout the possibility of long-term retention of the burst energy in the star’s envelope. The results can be usefulfor interpretation of superburst observations.

Large eddy simulation of shock-combustion-film interaction at hydrogen injection in a model scramjet combustor

Supersonic film cooling, which utilizes fuel onboard, is a promising method for protecting the wall of a scramjet combustor from intense heat and friction loads during high-speed flight. Large eddy simulation (LES) is conducted to study the coupled interactions among shock, combustion, and film. First, reference solutions for individual effects of shock and combustion on film performance are computed. Then the coupled effects are analyzed based on the reactive hydrogen film impinged by an oblique shock at a deflection angle of 2° The results show that the incident shock will enhance the transport processes after the impingement point by an increased turbulence production in the separation bubble. On the contrary, boundary layer combustion has the effect of constraining the transport processes in the mixing layer due to the low-density region caused by thermal expansion. This leads to a decrease in the mixing rate and wall friction. Although the heat flux transported to the film is also reduced by combustion, the wall temperature still increases due to the excessive compensation of heat from intense endothermic chemistry. Besides, the boundary layer combustion of hydrogen film exhibits self-limited characteristic since the mixing process is further delayed under combustion circumstance. Therefore, an incident shock will break the mixing bottleneck of the boundary layer combustion, leading to a more intense reaction with a more wrinkled flame front. The current findings indicate that the additional gaining of cooling and friction reduction performance is exclusive, but also point out the direction for simultaneous improvement of the two performances.

Effects of ammonia combustion on skin friction characteristics for supersonic flow

Boundary layer combustion is an effective method to reduce friction in scramjet combustors. In existing research on drag-reducing fuels, hydrocarbons pose challenges related to carbon deposition and emissions. Meanwhile, hydrogen presents significant difficulties in storage and transportation. For the first time, this paper explores ammonia as a drag-reducing alternative. We employed the Reynolds-averaged Navier-Stokes (RANS) method to investigate the characteristics of drag reduction by ammonia combustion in supersonic flow. Numerical results indicate that the local drag reduction by ammonia combustion is better than hydrocarbon and worse than hydrogen compared to existing research. Ammonia exhibits superior potential for drag reduction. However, the overall drag reduction at room ammonia injection temperature is limited due to the significant distance between the ammonia self-ignition point and the injection port. To address this challenge, we proposed two optimization strategies: increasing the ammonia injection temperature and increasing the inlet ammonia decomposition ratio. As the ammonia injection temperature increases, the self-ignition position moves towards the inlet, and the combustion drag reduction region expands upstream. Interestingly, a higher ammonia inlet decomposition ratio does not necessarily lead to a better drag reduction effect. In fact, an excessively high decomposition ratio can inhibit the combustion drag reduction effect. The most effective drag reduction occurs when the ammonia inlet mass fraction is 50%, resulting in a 42.8% reduction in integrated drag compared to the case without NH3 injection. Overall, this paper offers critical insights into the utilization of ammonia combustion in scramjets to reduce drag.

Combustion of Liquid Fuels in the Presence of CO2 Hydrate Powder

The process of combustion of a liquid fuel layer (diesel, kerosene, gasoline, separated petroleum, and oil) in the presence of CO2 hydrate has been studied. These fuels are widely used in engineering, which explains the great interest in effective methods of extinguishing. Extinguishing liquid fuels is quite a complicated scientific and technical task. It is often necessary to deal with fire extinction during oil spills and at fuel burning in large containers outdoors and in warehouses. Recently, attention to new extinguishing methods has increased. Advances in technology of the production, storage, and transportation of inert gas hydrates enhance the opportunities of using CO2 hydrate for extinguishing liquid fuels. Previous studies have shown a fairly high efficiency of CO2 hydrate (compared to water spray) in the extinction of volumetric fires. To date, there are neither experimental data nor methods for determining the dissociation rate of CO2 hydrate powder at the time of the gas hydrate fall on the burning layer of liquid fuel. The value of the dissociation rate is important to know in order to determine the temperatures of stable combustion and, accordingly, the mass of CO2 hydrate required to extinguish the flame. For the first time, a method jointly accounting for both the combustion of liquid fuel and the dissociation rate of the falling powder of gas hydrate at a negative temperature is proposed. The combustion stability depends on many factors. This paper defines three characteristic modes of evaporation of a liquid fuel layer, depending on the prevalence of vapor diffusion or free gas convection. The influence of the diameter and height of the layer on the nature of fuel evaporation is investigated.

Forests and fields in the pre-pyreneean neolithic and early Bronze Age based on fumier archaeobotanical records

The aim of this article is to present the current state of multidisciplinary archaeobotanical approaches that are being undertaken at Cova Gran de Santa Linya. Information from studies of seeds, charcoal, pollen, and NPP recovered from the Holocene levels of the site contribute to research questions regarding the anthropogenic transformation of the landscape. The signal of human activity in the environment can be detected through the bioarchaeological signatures of deforestation, forest management or agriculture practices. In this sense, the Cova Gran de Santa Linya is a cave deposit located in the northeast of Iberia used as an occupation site, recording mainly domestic activities during the Neolithic period. The settlement was also used as a pen during the Late Neolithic to the Early Bronze Age, preserving burnt and unburned dung layers that formed pen deposits, known as fumiers. The resolution provided by the multidisciplinary nature of this work shows how forests and fields created a mosaic landscape that provided crops, pastures, wood, and fuel and clearly reflects anthropogenic changes over time. The different methodological and analytical scales of this multidisciplinary approach, including taphonomic pollen alteration, provide a better understanding of the dynamics of the cave occupation and, from a broader perspective, the regional diversity related to the availability of plant resources.

A Study on a Vitiated Air Heater for a Direct-Connect Scramjet Combustor and Preliminary Test on the Scramjet Combustor Ignition

Vitiation air heater (VAH) combustion characteristics for a direct-connect scramjet combustor (DCSC) were experimentally studied. The VAH consists of a head, modular chamber, and circular-to-rectangular shape transition (CRST) nozzle. The CRST nozzle transforms the circular cross-sectioned rocket-type VAH into a rectangular cross-sectioned scramjet combustor. The CRST nozzle exit Mach numbers at the top, middle, and bottom were measured using a tungsten wedge. The oblique shock formed by the wedge was captured using Schlieren visualization and recorded with a high-speed camera. The θ-β-M relation showed that the exit Mach number was 2.04 ± 0.04 with a chamber pressure of 1.685 ± 0.07 MPa. With the VAH design point verified, preliminary scramjet combustor ignition tests were conducted. As the fuel was not auto-ignited by the vitiated air, the forced ignition method, in which VAH ignition flame ignites the scramjet fuel, was used. The Schlieren images showed that a cavity shear layer combustion mode was formed and also showed that the forced ignition method could be used as a reference model for the ignitor-ignition method.

:Ausgrabungen in der frühbronzezeitlichen Siedlung im Heraion von Samos 1966

:<i>Ausgrabungen in der frühbronzezeitlichen Siedlung im Heraion von Samos 1966</i>

Effect of mainstream oxygen concentration on supersonic film cooling and friction reduction performance using hydrocarbon fuel

One of the effective methods to simultaneously achieve the thermal protection and friction reduction requirement of a scramjet engine is the usage of fuel film cooling. As the combustion process inside the combustor proceeds, the oxygen concentration in the burnt gas gradually decreases and will influence the thermal and friction characteristics of the hydrocarbon film. Therefore, the effects of oxygen concentration in the mainstream on the hydrocarbon film are evaluated by numerical simulations based on RANS framework. The results indicate that the cooling performance of the hydrocarbon film is not sensitive to the oxygen concentration, although the fuel film burns more intense as the oxygen concentration increases. While the friction reduction performance is sensitive to the oxygen concentration. Specifically, about 10% friction reduction can be obtained for 10% oxygen in the mainstream compared to that of inert cases, and the wall friction can be further reduced by 30% for 20% oxygen. Further analysis reveals that the mechanism of the above two drag reductions is different. For weak boundary layer combustion, the decrease of the molecular viscosity contributes to the reduction of friction. While for significant boundary layer combustion, a lower near-wall velocity gradient is the main reason.

Effects of combustion on the near-wall turbulence and performance for supersonic hydrogen film cooling using large eddy simulation

Supersonic film cooling using fuel on board is an effective way to simultaneously shield the huge heat and momentum flux transported from the mainstream to the wall in a scramjet engine. The self-ignition and combustion of the injected fuel film will significantly change the turbulent transport behavior in the boundary layer. To reveal the effects of the boundary layer combustion on the near-wall turbulence and wall fluxes, large eddy simulations (LES) of the Burrows–Kurkov supersonic combustion experiment using hydrogen as a film are performed based on the in-house solver scramjetFoam. The solver successfully captures the additional skin friction reduction phenomenon induced by the boundary layer combustion compared to other numerical works using LES in the public literature. The results reveal that further increased anisotropy of turbulence combined with the low-density region contributes to a remarkable suppression of turbulent transport processes in the wall-normal direction. The self-ignition point of the hydrogen film is found to oscillate back and forth in a span of 80 mm, which significantly enhances turbulence in the boundary layer. However, the increased turbulent fluctuating velocity is mainly concentrated in the streamwise direction, while the other two components are suppressed instead. The findings are also essential for improving engineering computations based on the Reynolds averaged simulation method.