Inlet Mixture Temperature Research Articles

Modeling catalytic oxidation of lean mixtures of methane–air in a packed-bed reactor

Abstract A mathematical model developed for simulation of combustion of preheated lean homogeneous methane–air mixtures in an adiabatic catalytic packed bed is described. The unsteady one-dimensional model includes inter-phase heat and mass transfers, longitudinal mass dispersion in the gas phase and longitudinal conduction and radiation heat transfer modes. The solid and gas phases are assumed not to be in local thermal equilibrium. Multi-step reaction mechanisms are employed for both the surface and gas phase chemical reactions. The surface reaction mechanism for the oxidation of methane on Pt includes 36 elementary surface reactions and 11 surface adsorbed species. The gas phase reaction mechanism includes 28 elementary reactions and 21 species. The governing equations are the unsteady equations of conservation of mass, chemical species and separate energy equations for solid and gas phases. These equations were solved using the commercial CFD code ‘Fluent’ and a number of modifying subroutines especially are developed to include the equations of surface reactions in the computations. The model has been used to investigate the effects of operational conditions such as the mixture inlet temperature (600–1250 K), approach velocity (0.5–15 m/s) and equivalence ratio (0.15–0.5) on the oxidation of methane within both the catalytic and non-catalytic packed-bed reactors under adiabatic conditions. The calculated values of methane conversion showed good agreement with the corresponding available experimental data. Moreover the amount of heat release at different inlet velocities was calculated and the optimum value discussed.

FLOW PATTERNS OF THE ESTER OIL-REFRIGERANT R134A MIXTURE FLASHING FLOW THROUGH A SMALL DIAMETER TUBE

This work presents an experimental investigation of the ester oil ISO VG10-refrigerant R134a mixture flashing flow with foam formation through a straight horizontal 3.22 mm-diameter-6.0 m- long tube. An experimental apparatus was designed to allow the measurement of both pressure and temperature profiles along the tube as well as the visualization of the flow patterns. Tests were performed at different mass flow rates, several refrigerant mass fractions at the inlet of the flow, and inlet mixture temperatures around 28 and 39 °C. A liquid mixture flow with constant temperature and pressure gradient could be noticed at the inlet of the tube. As the flow proceeded towards the exit of the tube the pressure drop produced a reduction of the refrigerant solubility in the oil yielding to formation of the first bubbles. Initially, small and few bubbles could be noticed and the flow behaved as a conventional two-phase flow. Eventually, the bubble population increased and foam flow was observed at the exit of the tube. Due to the great formation of bubbles, both the temperature and pressure gradient of the mixture were greatly reduced in this region of the flow.

On the stability of ultra-lean H 2/CO combustion in inert porous burners

The stability of ultra-lean H 2 / CO mixtures in a porous media burner is numerically investigated. The one-dimensional model considers separated energy equations for the solid and gas phases and accounts for radiative heat transfer in the solid porous foam. A simplified reaction kinetics (two-step mechanism) was employed in order to perform a linear stability analysis. This results are used to investigate the stability of the steady solutions as a function of burning velocity and flame location. The present analysis includes the effects of H 2 / CO volumetric ratio, inlet mixture temperature and equivalence ratio on the flame stability. From the linear stability results it can be concluded that the insertion of inert porous media for premixed combustion promotes the ultra-lean burning of H 2 / CO mixtures.

Performance Enhancement of The I.C. Engine By Using Thermal Storage Technique

The project of this work is to control the temperature of mixture which enters to the I.C.E. and keeping it within a certain range which enhance its performance. Four stroke, Engine working with benzene as a fuel was used. Experimental data were taken included the exhaust temperature and the flow rate of both air and fuel. These data were formulated by using suitable theoretical relations for heat exchanger. The dimensions of the exchanger were assumed to control the inlet mixture temperature. The results proved that, it is possible to control the entering mixture temperature which enters to the I.C.E. by controlling to the speed of rotary exchanger. The results were that, at the temperatures of the environment 5oC and 10 oC it was prove that the RPM of the exchanger should be 10 RPM, also at the environment temperatures 20,30, and 40 oC ,the rotation speed should be within the ranges (3-10),(2-5.5) and (1-2) RPM respectively. In order to guarantee staying the entering mixture temperature within, the required range.

Foam flow of oil-refrigerant R12 mixture in a small diameter tube

This work presents an experimental investigation of the ester oil ISO VG10-refrigerant R134a mixture flow with foam formation through a straight horizontal 3.22 mm ID diameter, 6.0 m length tube. An experimental apparatus was designed to permit the measurement of both pressure and temperature profiles along the tube as well as the visualization of the flow patterns of the two-phase flow. Tests were performed at different mass flow rates, several refrigerant mass fractions at the inlet of the flow, and inlet mixture temperatures around 28 and 39 °C. A liquid mixture flow with constant temperature and pressure gradient could be seen at the inlet of the tube. As the flow proceeded towards the exit of the tube the pressure drop produced a reduction of the refrigerant solubility in the oil yielding to formation of the first bubbles. Initially, small and few bubbles could be noticed and the flow behaved as a conventional two-phase flow. Eventually, the bubble population increased and foam flow was observed at the exit of the flow. Due to the great formation of bubbles, both the temperature and pressure gradient of the mixture were greatly reduced in this region of the flow.

Modeling of Deposition from “Waxy” Mixtures in a Pipeline under Laminar Flow Conditions via Moving Boundary Formulation

A mathematical model, based on the moving boundary approach, is developed for the growth of the deposit from paraffinic mixtures due to heat transfer in a pipeline. The model extends a recent study on the deposition from “waxy” mixtures by including unsteady-state energy balance and heat transfer in both radial and axial directions for hydrodynamically established laminar flow. Numerical solutions were obtained for the growth of deposit with time, both radially and axially, from a binary eutectic mixture of n-C16H34 and n-C28H58. Two different scenarios for the initiation of the deposition process were investigated. Even though the transient results for the temperature profile and deposit thickness differed significantly, identical steady-state predictions were obtained for the two scenarios. For a constant pipe-wall temperature, the steady-state deposit thickness was predicted to increase with the pipe length. The predicted deposit thickness was lower for a higher inlet mixture temperature, pipe-wall temperature, and Reynolds number. The trends in model predictions were compared with the published results from deposition experiments performed on similar waxy mixtures.

Effect of fuel and engine operational characteristics on the heat loss from combustion chamber surfaces of SI engines

Understanding of engine heat transfer is important because of its influence on engine efficiency, exhaust emissions and component thermal stresses. In this paper, the effect of various parameters such as compression ratio, equivalence ratio, spark timing, engine speed, inlet mixture temperature and swirl ratio as well as fuel type on the heat transfer through the chamber walls of a spark ignition (SI) engine is studied. For this purpose, a proper tool is developed which uses a KIVA multidimensional combustion modeling program and a finite-element heat conduction (FEHC) code iteratively. Also, an improved temperature wall function is used for the KIVA program. It was found that this iterative scheme and the new wall function can improve the predictions considerably. A parametric study shows that this methodology is efficient in predicting the engine heat transfer and the effects of changes in the fuel type and engine operational parameters on the engine thermal behavior.

Bifurcation of flame structure in a lean-premixed swirl-stabilized combustor: transition from stable to unstable flame

The present work addresses unsteady flame dynamics in a lean-premixed swirl-stabilized combustor, with attention focused on the transition of flame structure from a stable to an unstable state. It was found that the inlet temperature and equivalence ratio are the two most important variables determining the stability characteristics of the combustor. A slight increase in the inlet mixture temperature across the stability boundary leads to a sudden increase in acoustic flow oscillation. One major factor contributing to this phenomenon is that as the inlet mixture temperature increases, the flame, which is originally anchored in the center recirculation zone, penetrates into the corner recirculation zone and flashes back, due to the increased flame speed. As a consequence, the flame is stabilized by both the corner- and the center-recirculating flows and exhibits a compact enveloped configuration. The flame flaps dynamically and drives flow oscillations through its influence on unsteady heat release. This problem has not previously been studied mechanistically. The results improve our understanding of the mechanisms of initiation and sustenance of combustion instabilities in gas-turbine engines with lean-premixed combustion.

The extinction limits and near-limits behaviors of premixed ethanol/air flame

Experimental studies on the extinction limits and near-limits flame behaviors of premixed ethanol/air flames in a stagnation point flow were made by using an axisymmetric counter flow, twin-flame configuration, with adjustable nozzles separation distance. The extinction limits were obtained under various conditions included equivalence ratios, flame stretch rates, inlet mixture temperatures, with and without heat loss; and the flammability of ethanol was then obtained by extrapolating the extinction limits to a stretchless condition. Finally, various near-limits flame extinction phenomena were discussed.

Variables Affecting NOx Formation in Lean-Premixed Combustion

The formation of NOx in lean-premixed, high-intensity combustion is examined as a function of several of the relevant variables. The variables are the combustion temperature and pressure, fuel type, combustion zone residence time, mixture inlet temperature, reactor surface-to-volume ratio, and inlet jet size. The effects of these variables are examined by using jet-stirred reactors and chemical reactor modeling. The atmospheric pressure experiments have been completed and are fully reported. The results cover the combustion temperature range (measured) of 1500 to 1850 K, and include the following four fuels: methane, ethylene, propane, and carbon monoxide/hydrogen mixtures. The reactor residence time is varied from 1.7 to 7.4 ms, with most of the work done at 3.5 ms. The mixture inlet temperature is taken as 300 and 600 K, and two inlet jet sizes are used. Elevated pressure experiments are reported for pressures up to 7.1 atm for methane combustion at 4.0 ms with a mixture inlet temperature of 300 K. Experimental results are compared to chemical reactor modeling. This is accomplished by using a detailed chemical kinetic mechanism in a chemical reactor model, consisting of a perfectly stirred reactor (PSR) followed by a plug flow reactor (PFR). The methane results are also compared to several laboratory-scale and industrial-scale burners operated at simulated gas turbine engine conditions.

Forced convection in-tube steam condensation in the presence of noncondensable gases

An experimental and analytical investigation has been conducted to determine the effects of the presence of noncondensable gases on in-tube steam condensation under forced convection conditions. The local mixture Nusselt numbers were correlated in terms of the local mixture Reynolds number, mixture Jakob number, and the gas mass fractions or the mixture Schmidt number. Correlations including the mixture Schmidt number did better in representing the condensation process in the presence of one noncondensable gas. The experiments covered steam/gas mixture inlet Reynolds numbers from about 6000 to 26000, inlet helium mass fraction range from 0.022 up to 0.20, inlet air mass fraction range from 0.045 up to 0.20, and mixture inlet temperature of 100°C and 130°C (corresponding to a pressure range from 114 to 603 kPa). The ratios of the condensate film thermal resistance to the thermal resistance of the steam/noncondensable gas mixture at the same bulk temperature were calculated. In general, the condensate film thermal resistance was found to be significant for turbulent gas mixture conditions (Re > 6000) and relatively low gas mass fraction (W < 0.2). A diffusion-based, simplified boundary layer model has been developed. The steady-state radial diffusion equations of the mixture components were solved in order to obtain the steam mass flux at the interface between the condensate film and the mixture. The effect of the axial flow was included in terms of an effective boundary layer thickness which decreases with mixture Reynolds number. The model included the effect of more than one noncondensable gas on steam condensation. The model-predicted mixture heat transfer coefficients were favorably compared to the experimental results.

Laboratory testing of a gas turbine vaporizer

A laboratory test apparatus was constructed for assessing the metal temperature and the state of different airfuel mixtures at the outlet of a gas turbine vaporizer fuel injector. The performance of the apparatus was according to the imposed requirements for idle engine operation. Fuels tested were JP5, JP7, and Diesel Fuel Marine. Data consisted of vaporizer metal temperatures, inlet and exit mixture temperatures, as well as power input to the vaporizer. Photographic observations of the exit mixture showed that the liquid phase consisted of droplets, ligaments, as well as irregularly shaped particles originating as pendant droplets around the vaporizer exit. There was no observable difference between the results for JP5 and JP7. The liquid exit temperature, however, for these two fuels appeared to be lower than that for the diesel fuel. The computed extent of vaporization was in qualitative agreement with the experimental observations.

Factors affecting NO X formation in turbulent premixed confined flames

Formation of NO X was investigated experimentally in premixed turbulent flames stabilized by a perforated plate. Gasoline, kerosene and light diesel fuel as well as their blends were investigated. During experiments, burner pressure was atmospheric and the fuel/air mixture inlet temperature was kept constant at 468 K, while both inlet mixture velocity and equivalence ratio were varied. In some experiments argon was added to combustion air to control the flame temperature. The combustion gases were sampled along the burner axis with an isokinetic probe and analysed for NO X. Experimental results showed a rapid increase in NO X in the vicinity of the reaction zone, while the rate of its formation dropped in the post flame zone. The NO X levels were affected by the equivalence ratio, the fuel type and Reynolds number. Increasing Reynolds number increased NO X formation rate, even if both the equivalence ratio and flame temperature were kept constant. This suggests that turbulent transport processes would have a dominant effect.

Oxygen interaction with Cu(100) studied by AES, ELS, LEED and work function changes

MILD/oxy-fuel combustion is proving to be a reliable technology for clean and efficient energy production systems. It is by now established that the related operative conditions drastically change the combustion kinetics and the relative weight of kinetics and fluid-dynamics involved in the process. Thus a fundamental analysis of single sub-processes involved in fuel oxidation under MILD combustion conditions is needed to make easier the tuning and control of real systems in a wider range of working conditions, thus allowing for the spread of such a technology over a broader industrial scale.Such a work is devoted to the characterization of combustion regimes and the evaluation of auto-ignition delay times of methane in highly diluted and pre-heated conditions at atmospheric pressure in a one-dimensional flow reactor.Experimental tests have been carried out changing the mixture inlet temperature from 1100 up to 1400 K, as well as the mixture carbon/oxygen feed ratios from fuel ultra-lean up to rich conditions. Mixtures are diluted in nitrogen from 85% up to 95%. At the same time, numerical simulations have been performed by means of a commercial software and a kinetic mechanism available in literature to value its reliability in predicting experimental results under MILD oxidation conditions.Results clearly show the occurrence of some discrepancies between numerical and experimental auto-ignition delay time data for fuel rich mixtures at high inlet temperatures.In addition, a richness of combustion regimes (slow combustion, pyrolysis, dynamic behavior, transitional combustion, combustion) was experimentally found. These unique behaviors as well as the dependence of ignition process on environmental parameters outline that modest temperature gradients due to high mixture dilution levels typical of MILD oxidation conditions, coupled with heat exchange to the surroundings, slow down the changeover among kinetic routes (namely oxidation and recombination channels) promoted by temperature thus stressing their competition. Such an aspect is generally hidden in traditional systems by a strong heat release that implies high temperature gradients.Finally, the data reported in this paper have an intrinsic value for the identification of stable operative conditions and for either tuning or reducing detailed kinetic schemes because few data are present in literature on methane ignition and oxidation in a simple reactor at atmospheric pressure.

NO x formation in premixed turbulent flames

NOx formation has been studied in lean, premixed, turbulent flames stabilized by perforated plates. The burner pressure was atmospheric, but other parameters were representative of gas turbine operating conditions. Prevaporized Jet A fuel was injected into preheated unvitiated air and thoroughly mixed prior to combustion. Mixture inlet temperature was 750°K and total residence times were approximately 4 msec. NOx, CO, CO2 and hydrocarbon levels were measured along the jet and recirculation zone axes. Results show a very rapid increase in NOx in the reaction zone which cannot be accounted for by O-atom radical overshoot. Post-flame zone NOx formation rates agree with the predictions of equilibrium theory. The relative contribution of “prompt NOx” to the final NOx levels increases with decreasing equivalence ratio. The rate of NOx formation in the reaction zone is found to be higher than in laminar flames. Turbulent transport processes are believed to be the dominant effect. NOx formation via nitrous oxide intermediate does not appear to be important under the present experimental conditions. Large amplitude temperature oscillations observed in turbulent premixed flames were found not to affect NOx formation rates.

Predicted Exhaust Emissions from a Methanol and Jet Fueled Gas Turbine Combustor

A computer model of a gas turbine combustor has been used to predict the kinetic combustion and pollutant formation processes for methanol and simulated jet fuel. Use of the kinetic reaction mechanisms has also allowed a study of ignition delay and flammability limit of these two fuels. The NOX emissions for methanol were predicted to be from 69 to 92% lower than those for jet fuel at the same equivalence ratio which is in agreement with experimentally observed results. The high heat of vaporization of methanol lowers both the combustor inlet mixture temperatures and the final combustion temperatures. The lower combustion temperatures lead to low NOX emissions while the lower inlet mixture temperatures increase methanol's ignition delay. This increase in ignition delay dictates the lean flammability limit of methanol to be 0.8, while jet fuel is shown to combust at 0.4.

Variation of the Flame Propagatin Time in a Spark Ignition Engine

Effects of operating conditions of a CFR engine on variations of flame travel time were studied statistically. Cyclic variations of falme travel time showed some departure form a normal distribution, but those of the "apparent flame propagation velocity", the distance of flame travel divided by the flame travel time, were shown to follow a normal one in general.The mean apparent flame propagation velocity υ^-f was found to have linear correlation with the standard deviation σ. It was shown also that υ^-f/SL (SL : laminar burning velocity) was directly proportional to σ/SL in all the results obtained at different inlet mixture temperatures, and to Harrow's Reynolds number of the mixture before combustion over a wide range.An explanation for actual pattern of cyclic distribution of flame travel was made by incorporating the effects of cyclic variation of mixture stregth and the flame velocity due to the turbulence in the combustion chamber.

Statistical Study on Variations of Flame Travel Time in a Spark Ignition Engine

Flame travel times of some base stocks of commercial gasolines were statistically studied using a CFR engine and a multi-channel flame counter. Two kinds of fuels, catalytically reformed naphtha (RFT) and a 50/50 mixture of it with straight-run light naphtha (RFT/LN) were used throughout the work, and in some cases, isooctane, toluene, xylene and cracked naphtha were used for comparison. Operating conditions of the engine varied were air-fuel ratios from 10:1 to 16:1, inlet mixsture temperatures of 50°, 100° and 150°C, and throttle openings of 90° (full throttle), 26° and 15°. Engine speed was kept constant at 900rpm, and the ignition timing was set at the best power.RFT and RFT/LN seemed to have the minimum flame travel time at a rich air-fuel ratio of 11:1. The flame travel time of RFT was longer than that of RFT/LN and the extent of cyclic disperison of the former was larger than that of the latter.The effect of inlet mixture temperature on the time of flame travel was small for RFT and only slightly noticeable for RFT/LN. The extents of cyclic dispersion at high inlet temperatures of 100° and 150°C for both fuels were not much different from those obtained at 50°C.As the throttle valve was closed, the flame travel time became longer, the extent of cyclic dispersion increased, and their sensitivity to air-fuel ratio become significant.As found in our previous work, where the apparent flame propagation velocity, defined as the distance of flame travel divided by the flame travel time, followed a normal distribution, the results obtained in the present work were analysed using the apparent mean flame propagation velocity Vf and the standard deviation σf and the coefficient of variation σf/Vf.The relationship between Vf/SL (SL=laminar burning velocity) and Reynolds number was discussed and a possibility of similar linear correlation as found for isooctane in the previous work was shown. The correlation of Vf versus σf was also discussed.

The Effects of Operating Conditions of the Engine on Antiknock Properties of Gasolines (5)

The effect of inlet-mixture temperature on antiknock qualities of some stock components of gasoline and 80 ON primary reference fuel, both clear and with antiknock compounds, were studied on the CFR engine.The engine was operated at inlet-mixture temperatures ranging from 40 to 150°C, engine speed of 900rpm, with the best power ignition timing except in the case of straight-run light naphtha, and with air-fuel ratios from 12 to 15.The leaded catalytically reformed naphtha and leaded 80 ON primary reference fuel showed peculiar knocking characteristics different from that of clear fuels. The results were analyzed using the calculated values of compression temperature and compression density, and it was found that tetraethyl lead was most effective in the compression temperature range from 680 to 700°K, which was in favor of the result obtained by B.R. Siegel.It was shown that the knock limited compression density ρ could be expressed with inlet-mixture temperature Ti by the equation; ρ=AeC/Ti. This relationship was explained by assuming that the ignition delay of the end gas would coincide with the time required for consuming the whole mixture by the travel of the normal flame front.

The Effects of Operating Conditions of the Engine on Knocking Characteristics of Gasolines

Knocking characteristics of some stock components of gasoline were studied under varying operating conditions by the CFR engine. The antiknock characteristics of fuels, clear or with antiknock compounds, were studied by the incipient knock limited compression ratio, which was detected from the knock characteristic dp/dt traces of the combustion chamber pressure on the cathode ray oscilloscope. Fuels used were catalytically reformed naphtha, cracked naphtha, straight-run light naphtha and some primary reference fuels.It was shown that the lead responses of reformed naphtha and cracked naphtha were greater at richer mixture strength and at more retarded ignition timing.It was found that the relationship between the incipient knock limited compression ratio and the lead concentration at any operating conditions at the air-fuel ratio of 14.0 could be expressed by the same type of empirical equations for octane number and lead concentration.The effect of inlet-mixture temperature was analysed using the knock limited compression density at the end of compression stroke.Making use of a bled manifold, the effects of volatilities of lead alkyl compounds on its distribution in the front end of fuels were observed.