Combustion behavior in a dual-staging vortex rice husk combustor with snail entry

The combustion characteristics of rice husk fuel in a dual-staging vortex-combustor (DSVC) are experimentally investigated. In the present work, the vortex flow is created by using a snail entrance mounted at the bottom of the combustor. The temperature distributions at selected locations inside the combustor, the flue gas emissions (CO, CO₂, O₂, NO_x), and the combustion/thermal efficiency are monitored. Measurements are made at a constant rice husk feed rate of 0.25 kg/min with various excess airs (37%, 56%, 74% and 92%) and different secondary air injection fractions (λ = 0.0, 0.15 and 0.2), respectively. The combustion chamber is 1800 mm high and 300 mm in diameter (D) with a centered exhausted pipe while the middle chamber of the combustor is set to 0.5D. The smaller section at the middle chamber is introduced to split the chamber to be dual-staging chamber where a large central toroidal recirculation zone induced by swirl flow through the small section is generated in the top chamber. The experimental results reveal that the highest temperature inside the combustor is about 1000 °C whereas both the thermal and the combustion efficiency are 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). In addition, the emissions are CO₂ = 8.1%, O₂ = 9.3%, CO = 352 ppm, NO_x = 294 ppm and small amount of fly ash. Therefore, the DSVC shows an excellent performance, low emissions, high stabilization and ease of operation in firing the rice husk.     

Flame stability and emission characteristics of propane-fueled flat-flame miniature combustor for ultra-micro gas turbines

An engineering model of a propane-fueled miniature combustor was developed for ultra-micro gas turbines. The combustion chamber had a diameter of 20 mm, height of 4 mm, and volume of 1.26 cm³. The flat-flame burning method was applied for lean-premixed propane–air combustion. To create the stagnation flow field for a specific flat-flame formation, a flat plate was set over the porous plate in the combustion chamber. A burning experiment was performed to evaluate the combustion characteristics. The flame stability limit was sufficiently wide to include the design operation conditions of an equivalence ratio of 0.55 and air mass flow rate of 0.15 g/s, and the dominant factors affecting the limit were clarified as the heat loss and velocity balance between the burning velocity and the premixture flow velocity at the porous plate. CO, total hydrocarbons (THC), and NO_x emission characteristics were established based on the burned gas temperatures in the combustion chamber and the temperature distribution in the combustor. At an air mass flow rate of less than 0.10 g/s, CO and THC emissions were more than 1000 ppm due to large heat loss. As the air mass flow rate increased, the heat loss decreased, but CO emissions remained large due to the short residence time in the combustion chamber. NO_x emission depended mainly on the burned gas temperature in the combustion chamber as well as on the residence time. To reduce emissions despite the short residence time, a platinum mesh was placed after the combustion chamber, which drastically decreased the CO emissions. The combustor performance was compared with that of other miniature combustors, and the results verified that the present combustor has suitable combustion characteristics for a UMGT, although the overall combustor size and heat loss need to be reduced.     

Computational and experimental investigations of turbulent asymmetric vortex flames

In the present article we present computational and experimental investigations of a turbulent asymmetric vortex flame. Such flame was created in a novel asymmetric combustor, which is described for the first time in this article. The three dimensional isothermal and reacting flow fields have been described using a computational methodology that impalements the R_ε/k − ε and the eddy dissipation turbulence and combustion models, respectively. The computational model is validated for both isothermal and reacting flows. Additionally, the visible flame structure was captured by direct photography at a wide range of equivalence ratios in order to emphasize the exceptional stability of such flame. The mechanism of flame stability and interaction with the forced vortex field is preliminarily discussed. Finally, the basic characteristics of the asymmetric vortex flames are concluded.     

Optimized resistivity of p-type Si substrate for HIT solar cell with Al back surface field by computer simulation

For HIT (heterojunction with intrinsic thin-layer) solar cell with Al back surface field on p-type Si substrate, the impacts of substrate resistivity on the solar cell performance were investigated by utilizing AFORS-HET software as a numerical computer simulation tool. The results show that the optimized substrate resistivity (R_op) to obtain the maximal solar cell efficiency is relative to the bulk defect density, such as oxygen defect density (D_od), in the substrate and the interface defect density (D_it) on the interface of amorphous/crystalline Si heterojunction. The larger D_od or D_it is, the higher R_op is. The effect of D_it is more obvious. R_op is about 0.5 Ω cm for D_it = 1.0 × 10¹¹/cm², but is higher than 1.0 Ω cm for D_it = 1.0 × 10¹²/cm². In order to obtain very excellent solar cell performance, Si substrate, with the resistivity of 0.5 Ω cm, D_od lower than 1.0 × 10¹⁰/cm³, and D_it lower than 1.0 × 10¹¹/cm², is preferred, which is different to the traditional opinion that 1.0 Ω cm resistivity is the best.     

Combustion characteristics of a lean premixed LPG–air combustor

Fuel/air mixing effects in a premixer have been examined to investigate the combustion characteristics, such as the emission of NO_x and CO, under simulated lean premixed gas turbine combustor conditions at normal and elevated pressures of up to 3.5 bar with air preheat temperature of 450 K. The results obtained have been compared with a diffusion flame type gas turbine combustor for emission characteristics. The results show that the NO_x emission is profoundly affected by the mixing between fuel and air in the combustor. NO_x emission is lowered by supplying uniform fuel/air gas mixture to the combustor and the NO_x emission reduces with decrease in residence time of the hot gases in the combustor. The NO_x emission level of the lean premixed combustor is a strong function of equivalence ratio and the dependency is smaller for a traditional diffusion flame combustor under the examined experimental conditions. Furthermore, the recirculation flow, affected by dome angle of combustor, reduces the high temperature reaction zone or hot spot in the combustor, thus reducing the NO_x emission levels.     

Effect of H2S in methane/air flames on sulfur chemistry and products speciation

Reaction behavior of H₂S/O₂ under different equivalence ratios in methane/air flames is examined. Three equivalence ratios extending from fuel-lean (Φ = 0.5), stoichiometric (Φ = 1.0), to fuel-rich (Claus condition, Φ = 3.0) are examined. The results revealed that the presence of H₂S prevents hydrogen oxidation in the primary reaction zone, while in the secondary reaction zone oxidation competition occurs between H₂ and H₂S. In presence of oxygen, oxidation of hydrogen sulfide forms sulfur dioxide. However, under Claus conditions, the depletion of oxidant causes the direction of hydrogen sulfide reaction to shifts towards the formation of elemental sulfur. Higher hydrocarbons are formed in trace amounts under Claus conditions wherein sulfur dioxide acts as a coupling catalyst which enhances the dimerization of CH₃ radical to form higher series of hydrocarbons. Under Claus conditions, sulfur deposits are formed in low temperature regions of the reactor including the sampling line. The deposits are analyzed using X-ray powder diffractometer and were found to be cyclo-S₈ (α-sulfur) with orthorhombic crystal structure. The formation of α-sulfur is mainly due to the agglomeration of elemental sulfur (S₂) during its condensation at low temperatures.     

The micro-tube heat transfer and fluid flow of water based Al2O3 nanofluid with viscous dissipation

The numerical modeling of the conjugate heat transfer and fluid flow of Al₂O₃/water nanofluid through the micro-tube was presented in the paper. The laminar flow regime was considered along with viscous dissipation effect. The diameter ratio of the micro-tube was D_i/D_o = 0.1/0.3 mm with a tube length L = 100 mm. The heat transfer rate was fixed to Q = 0.5 W with three different Br = 0.1, 0.5 and 1. The water based Al₂O₃ nanofluid was considered with various volume concentrations of Al₂O₃ particles ? = 1, 4, 6, 9% and two diameters of the particles D_p = 10 nm and 47 nm. The analysis was performed on the results for local heat transfer coefficient.     

An experimental study of syngas combustion in a bidirectional swirling flow

The paper reports on the results of an experimental study of methane and syngas combustion as well as their co-firing in a bidirectional swirling flow. The results confirmed that the bidirectional flow structure provides a significant decrease in the lean blow-off equivalence ratio as well as that of emissions of main pollutants. The combustion intensification becomes more evident when using syngas is as fuel. The composition of the used syngas is as follows (by volume): H₂ - 29.42%; CO - 14.32%; CH₄ - 3.8%; N₂ - 49.11%; H₂O - 3.35%. In this case, the lean blow-off is achieved at ? < 0.1, NOx emission is halved, while CxHy and CO emissions become 20 times less compared to pure methane combustion. However, according to experimental results, the co-combustion of syngas (volume fraction V_syn = 15%) and methane is the most appropriate fuel utilization mode. It provides blow-off and emission properties similar to those for combustion of pure syngas, whereas energy consumption for its production is much lower. Moreover, unlike hydrocarbon fuel combustion, that of syngas in a bidirectional swirling flow is characterized by the presence of density stratification. This is accompanied by the flame formation at significantly different locations in the combustion chamber at lean and “ultra-lean” modes of operation. Hydrogen combustion most likely to occur in the core region at near-blow-off modes ? < 0.1, whereas normal ‘operating modes in the range 0.2 = ? ≤ 0.4 result in the formation of a conical flame surface where CH₄ and CO combustion occurs. These new results with respect to the flame structure as well as blow-off and emission properties make it possible to consider bidirectional vortex combustors for application in modern gas turbine power plants in order to meet the strict environmental and energy requirements.     

Heat transfers and pressure drops for porous-ring turbulators in a circular pipe

A numerical heat-transfer and pressure-drop analysis is presented for porous rings inserted in a pipe at a distance L apart. A constant heat-flux is applied to the outer surface of the pipe. Numerical calculations are conducted with the Fluent 6.1.22 code, using the shear-stress transport (SST) k–ω model. Air is the fluid. The heat-transfer increase is analyzed for Reynolds numbers from 3 × 10³ to 45 × 10³. The porous-ring height is taken as H = 1 or 2 mm. The distance between two porous rings is 0.5D, D or 2D where D is the inside diameter of the pipe. An increase in L/D caused a decrease in heat-transfer. High Nusselt numbers were obtained when H/D and L/D ratios were 0.4 and 0.5, respectively, for a Reynold number of 45,000. The maximum Nusselt number occurred when L/D = 1 if H/D is selected as 0.2.     

Effect of variation of hydrogen injection pressure and inlet air temperature on the flow-field of a typical double cavity scramjet combustor

In the present research work, computational simulation of the double cavity scramjet combustor have been performed by using the two-dimensional compressible Reynolds-Averaged Navier–Stokes (RANS) equations coupled with two equation standard k–ɛ turbulence model as well as the finite-rate/eddy-dissipation reaction model. All the simulations are carried out using ANSYS 14-FLUENT code. Additionally, the computational results of the present double cavity scramjet combustor have been compared with experimental results for validation purpose which is taken from the literature. The computational outcomes are in satisfactory agreement with the experimentally obtained shadowgraph image and pressure variation curve. However, due to numerical calculation, the pressure variation curve obtained computationally is under-predicted in 5 locations. Further, analyses have been carried out to investigate the effect of variation of hydrogen injection pressure as well as the variation of air inlet temperature on the flow-field characteristics of scramjet engine keeping the Mach number constant. The obtained results show that the increase in hydrogen injection pressure is followed by the generation of larger vortex structure near the cavity regions which in turn helps to carry the injectant and also enhance the air/fuel mixing whereas the increase in the inlet temperature of air is characterised by the shifting of incident oblique shock in the downstream of the H₂ injection location. Again for T₀ = 1500 K, the combustion phenomena remains limited to the cavity region and spreads very little towards the downstream of the combustor.     

An experimental and kinetic modeling study of the autoignition of α-methylnaphthalene/air and α-methylnaphthalene/n-decane/air mixtures at elevated pressures

The autoignition of α-methylnaphthalene (AMN), the bicyclic aromatic reference compound for the cetane number (CN), and AMN/n-decane blends, potential diesel surrogate mixtures, was studied at elevated pressures for fuel/air mixtures in a heated high-pressure shock tube. Additionally, a comprehensive kinetic mechanism was developed to describe the oxidation of AMN and AMN/n-decane blends. Ignition delay times were measured in reflected shock experiments for Φ = 0.5, 1.0, and 1.5 AMN/air mixtures (CN = 0) for 1032-1445 K and 8-45 bar and for Φ = 1.0 30%-molar AMN/70%-molar n-decane/air (CN = 58) and 70%-molar AMN/30%-molar n-decane/air mixtures (CN = 28) for 848-1349 K and 14-62 bar. Kinetic simulations, based on the comprehensive AMN/n-decane mechanism, are in good agreement with measured ignition times, illustrating the emerging capability of comprehensive mechanisms for describing high molecular weight transportation fuels. Sensitivity and reaction flux analysis indicate the importance of reactions involving resonance stabilized phenylbenzyl radicals, the formation of which by H-atom abstractions with OH radicals has an important inhibiting effect on ignition.     

Effects of operating conditions and fuel properties on emission performance and combustion efficiency of a swirling fluidized-bed combustor fired with a biomass fuel

This work reports an experimental study on firing 80 kg/h rice husk in a swirling fluidized-bed combustor (SFBC) using an annular air distributor as the swirl generator. Two NO_x emission control techniques were investigated in this work: (1) air staging of the combustion process, and (2) firing rice husk as moisturized fuel. In the first test series for the air-staged combustion, CO, NO and C_xH_y emissions and combustion efficiency were determined for burning “as-received” rice husk at fixed excess air of 40%, while secondary-to-primary air ratio (SA/PA) was ranged from 0.26 to 0.75. The effects of SA/PA on CO and NO emissions from the combustor were found to be quite weak, whereas C_xH_y emissions exhibited an apparent influence of air staging. In the second test series, rice husks with the fuel-moisture content of 8.4% to 35% were fired at excess air varied from 20% to 80%, while the flow rate of secondary air was fixed. Radial and axial temperature and gas concentration (O₂, CO, NO) profiles in the reactor, as well as CO and NO emissions, are discussed for the selected operating conditions. The temperature and gas concentration profiles for variable fuel quality exhibited significant effects of both fuel-moisture and excess air. As revealed by experimental results, the emission of NO from this SFBC can be substantially reduced through moisturizing rice husk, while CO is effectively mitigated by injection of secondary air into the bed splash zone, resulting in a rather low emission of CO and high (over 99%) combustion efficiency of the combustor for the ranges of operating conditions and fuel properties.     

Performance characteristics of methanol and kerosene fuelled meso-scale heat-recirculating combustors

A meso-scale heat recirculating combustor has been developed for the combustion of methanol and kerosene fuels with oxygen enriched superheated steam as an oxidizer. The steam oxygen mixture is a surrogate for the decomposition products of hydrogen peroxide, and as such the combustor development is toward meso-scale bi-propellant propulsion. Both the extinction behavior and thermal performances have been examined under partially-premixed and non-premixed configurations of a unique design incorporating heat recirculation. Stable combustion with thermal efficiencies of ∼90% has been demonstrated with both methanol and kerosene. Global flame behavior is investigated through direct image photography of the flame that revealed different flame modes at various equivalence ratios (Φ), including “flameless” combustion of kerosene. Density impulse values calculated based on exhaust temperatures and simulated equilibrium gas properties and assuming 1 atm chamber pressure and expansion to vacuum show that the maximum density impulse of kerosene/steam/oxygen combustion to be within 6% of the adiabatic density impulse of hydrazine/nitrogen tetroxide.     

Experiments and modeling of the autoignition of methylcyclohexane at high pressure

New experimental data are collected for methyl-cyclohexane (MCH) autoignition in a heated rapid compression machine (RCM). Three mixtures of MCH/O₂/N₂/Ar at equivalence ratios of ? = 0.5, 1.0, and 1.5 are studied and the ignition delays are measured at compressed pressure of 50 bar and for compressed temperatures in the range of 690–900 K. By keeping the fuel mole fraction in the mixture constant, the order of reactivity, in terms of inverse ignition delay, is measured to be ? = 0.5 > ? = 1.0 > ? = 1.5, demonstrating the dependence of the ignition delay on oxygen concentration. In addition, an existing model for the combustion of MCH is updated with new reaction rates and pathways, including substantial updates to the low-temperature chemistry. The new model shows good agreement with the overall ignition delays measured in this study, as well as the ignition delays measured previously in the literature using RCMs and shock tubes. This model therefore represents a strong improvement compared to the previous version, which uniformly over-predicted the ignition delays. Chemical kinetic analyses of the updated mechanism are also conducted to help understand the fuel decomposition pathways and the reactions controlling the ignition. Combined, these results and analyses suggest that further investigation of several of the low-temperature fuel decomposition pathways is required.     

Swirling distributed combustion for clean energy conversion in gas turbine applications

Distributed combustion provides significant performance improvement of gas turbine combustors. Key features of distributed combustion includes uniform thermal field in the entire combustion chamber, thus avoiding hot-spot regions that promote NO_x emissions (from thermal NO_x) and significantly improved pattern factor. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of hot and active species present in the flame and their rapid turbulent mixing with the reactants. Distributed combustion with swirl is investigated here for our quest to explore the beneficial aspects of such flows on clean combustion in simulated gas turbine combustion conditions. The goal is to develop high intensity combustor with ultra low emissions of NO and CO, and much improved pattern factor. Experimental results are reported from a cylindrical geometry combustor with different modes of fuel injection and gas exit stream location in the combustor. In all the configurations, air was injected tangentially to impart swirl to the flow inside the combustor. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes for the geometries investigated here. Swirling flow configuration, wherein the product gas exits axially resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from traditional diffusion mode to distributed combustion regime. Results showed very low levels of NO (∼3 PPM) and CO (∼70 PPM) emissions even at rather high equivalence ratio of 0.7 at a high heat release intensity of 36 MW/m³-atm with non-premixed mode of combustion. Results are also reported on lean stability limit and OH^* chemiluminescence under both premixed and non-premixed conditions for determining the extent of distribution combustion conditions.     

Experimental investigation of energy separation in a counter-flow Ranque–Hilsch vortex tube with multiple inlet snail entries

The energy/temperature separation phenomenon and cooling efficiency characteristics in a counter-flow Ranque–Hilsch vortex tube (RHVT) are experimentally studied. The ascertainment focuses on the effects of the multiple inlet snail entries (N = 1 to 4 nozzles), cold orifice diameter ratios (d/D = 0.3 to 0.7) and inlet pressures (P_i = 2.0 and 3.0 bar). The experiments using the conventional tangential nozzles (N = 4), are also performed for comparison. The experimental results reveal that the RHVT with the snail entry provides greater cold air temperature reduction and cooling efficiency than those offered by the RHVT with the conventional tangential inlet nozzle under the same cold mass fraction and supply inlet pressure. The increase in the nozzle number and the supply pressure leads to the rise of the swirl/vortex intensity and thus the energy separation in the tube.     

Design and performance of a pressurized cyclone combustor (PCC) for high and low heating value gas combustion

The combustion difficulties for low heating value (LHV) gases derived from biomass fuels via a gasification process have led to more investigations into LHV gas combustors. Cyclone combustors provide good air/fuel mixing with long residence times. In this study, a small-scale pressurized cyclone combustor (PCC) was designed and optimized using computational fluid dynamics (CFD) simulation. The PCC, along with a turbocharger-based, two-stage microturbine engine, was first characterized experimentally with liquefied petroleum gas (LPG) fuel and then with both LPG and LHV gas derived from biomass in dual-fuel mode. The combustor achieved ultra-low CO and NO_x emissions of about 5 and 7 ppm, respectively, for LPG fuel and of about 55 and 12 ppm, respectively, in dual-fuel mode at the maximum second-stage turbine speed of 26,000 rpm with stable turbine operation.     

Heat transfer augmentation in a helical-ribbed tube with double twisted tape inserts

Turbulent convective heat transfer characteristics in a helical-ribbed tube fitted with twin twisted tapes have been investigated experimentally. The experiment was carried out in a double tube heat exchanger using the helical-ribbed tube having a single rib-height to tube-diameter ratio, e/D_H = 0.06 and rib-pitch to diameter ratio, P/D_H = 0.27 as the tested section. The insertion of the double twisted tapes with twist ratio, Y, in the range of 2.17 to 9.39 is to create vortex flows inside the tube. The inserted ribbed tube is arranged in similar directions of the helical swirl of the twisted tape and the helical rib motion of the tube (called co-swirl). Effects of the co-swirl motion of the ribbed tube and the double twisted tapes with various twist ratios on heat transfer and friction characteristics are examined. The results obtained from the ribbed tube and the twin twisted tape insert are compared with those from the smooth tube and the ribbed tube acting alone. The experimental results reveal that the co-swirling inserted tube performs much better than the ribbed/smooth tube alone at a similar operating condition. The co-swirl tube at Y ≈ 8 yields the highest thermal performance at lower Reynolds number (Re). In addition, the correlations of Nusselt number and friction factor as functions of Re, Pr and Y are also proposed.     

3D simulation of laminar flow and heat transfer in V-baffled square channel

The article presents a numerical investigation on laminar flow and heat transfer characteristics in a three-dimensional isothermal wall square-channel fitted with inline 45° V-shaped baffles on two opposite walls. The computations based on the finite volume method with the SIMPLE algorithm have been conducted for the airflow in terms of Reynolds numbers ranging from 200 to 2000. The inline V-baffles with its V-tip pointing downstream and the attack angle (or half V-apex angle) of 45° relative to the flow direction are mounted repeatedly on the lower and upper walls. The baffled channel flow shows a fully developed periodic flow and heat transfer profile for BR = 0.2 at x/D≈ 8 downstream of the inlet. Influences of different baffle height ratios (BR) and pitch ratios, (PR) on thermal behaviors for a fully developed periodic condition are investigated. It is apparent that the longitudinal counter-rotating vortex flows created by the V-baffle can induce impingement/attachment flows over the walls resulting in greater increase in heat transfer over the test channel. Apart from speeding up the fully developed periodic flow pattern, the rise of the BR leads to the increase in Nu/Nu₀ and f/f₀ values while that of the PR provides an opposite trend. The V-baffle performs better than the angled baffle at a similar condition. The V-baffle with BR = 0.2 and PR = 1.5 yields the maximum thermal performance of about 3.8 whereas the Nu/Nu₀ is some 14 times above the smooth channel at higher Re.     

The simulation based on CHEMKIN for homogeneous charge compression ignition combustion with on-board fuel reformation in the chamber

In order to control the combustion phase precisely and remarkably extend the operation range of Homogeneous Charge Compression Ignition (HCCI) engine, a method of on-board controllable phase fuel reformation in the reforming chamber is proposed in this paper. HCCI combustion is dominated by chemical kinetics, and H₂, OH, H and O are the key radicals and play an important role in controlling HCCI combustion. The attempt of the proposed method is to try to change the control of chemical kinetics into a manipulation of fuel reforming system. The system includes an independent reformation chamber with an injector and a controllable valve that connects reformation chamber and the main chamber. The reforming fuel is reformed into H₂-rich gas. The reformed gas enters the cylinder to change the combustion phasing at compression stroke. The model of HCCI with reforming process is built with CHEMKIN 4.1 software, and HCCI process with on-board reformation is simulated. The results show that the components of the reformed gas are influenced by initial temperature and reforming mixture concentration. The maximum fraction of H₂ may be obtained by optimizing the trap timing and reforming mixture concentration (optimal value: Φ_T = 31 °CA, λ₃ = 0.4). The optimized reformed gas does have the ability to change the combustion phasing of HCCI engine. With the help of the on-board controllable phase fuel reformation system, HCCI combustion process can be precisely controlled, and the HCCI engine is allowed to operate under lower intake temperature and higher speed condition, and to keep high IMEP and indicated thermal efficiency.