Minimum Equivalence Ratio Research Articles

Effects of key parameters on performance of a pre-cooled core engine based on the closed helium cycle

A pre-cooled combined engine based on a closed helium (He) cycle offers high specific thrust and high specific impulse. Therefore, evaluation of the performance of such an engine is crucial for engine applications and key technology research. This study employs an analytical approach to investigate the effects of key parameters on the performance of a pre-cooled core engine, assuming a perfect gas model. The findings revealed that the specific thrust and specific impulse of the pre-cooled core engine are related to the pressurized coefficient of the airflow passage and equivalence ratio (ER). An increase in the pressurized coefficient leads to an increase in both specific thrust and specific impulse. However, within a certain range, although the specific thrust is positively correlated with the ER, the specific impulse is greatly reduced. With specific component parameters and a fixed thermodynamic cycle, a minimum ER exists, which satisfies the cycle-matching requirement. Moreover, the value of the minimum ER is related to the closed-cycle efficiency. For a pre-cooled core engine with a simple closed He cycle, the minimum ER is approximately 2.5–3.5.

The impact of residence time on ignitability and time to ignition in a toroidal jet-stirred reactor

Understanding of ignition processes is central to design for reliable and safe aerospace combustor systems. Ignition is influenced by many factors including combustor geometry, flow conditions, fuel composition, turbulence intensity, ignition source, and energy deposition method. A toroidal jet-stirred reactor (TJSR) utilizes bulk fluid motion, presence of recirculation zones, a bulk residence time, and turbulence intensities which emulate characteristics relevant to cavity stabilized and swirl stabilized combustors. In this work, a TJSR was used to quantify ignitability and time-to-ignition of premixed ethylene and air. The effects of inlet temperature, residence time, and reactivity were studied on forced ignition processes. Experimental conditions ranged from residence times of 15–35 ms, mixture temperatures of 340–450 K, and equivalence ratios of 0.5–1 using capacitive spark-discharge ignition. The minimum equivalence ratio for ignition (MER), or the equivalence ratio at 50% probability, shows an inverse relationship with mixture temperature and residence time. Prior theory of real engine combustor performance for lean light off, proposed by Ballal and Lefebvre, was compared to the MER and displayed similar trends to the model. Spatially integrated OH* chemiluminescence was used to measure time to ignition within the reactor. Reduction in ignitibility was experienced as the time-to-ignition approached the residence time stressing the importance of device flow time scales in relation to kernel growth dynamics and ignition probability.

Improvement of the turn-down ratio of gas turbines by autothermal on board syngas generation

Abstract The low reactivity of natural gas leads to a sudden increase of carbon monoxide (CO) and unburned hydrocarbons (UHC) emissions below a certain load level, which limits the part load operation range of current utility gas turbines in combined cycle power plants (CCPP). The feasibility of catalytic autothermal syngas generation directly upstream of gas turbine burners to improve burn-out at low flame temperatures is studied in this paper. The adiabatic reformer is supplied with a mixture of natural gas, air and water and generates syngas with high reactivity, which results in better low-temperature combustion performance. Substitution of part of the natural gas by syngas provides the opportunity of lowering overall equivalence ratio in the combustion chamber and of extending the operation range towards lower minimum power output without violating emission limits. A generic gas turbine with a syngas generator is modelled by analytic equations to identify the possible operating window of a fuel processor constrained by pressure loss, low and high temperature limits and carbon formation. A kinetic study shows good conversion of methane to syngas with a high hydrogen share. A calculation of the one-dimensional laminar burning velocity of mixtures of syngas and methane and the assessment of the corresponding Damköhler number show the potential for lowering the minimum equivalence ratio with full burn-out by fuel processing. The study shows that such a fuel processor has a possible operating range despite the before mentioned constraints and it has potential to reduce the lowest possible load of gas turbines in terms of thermal power by 20%.

Numerical study of lean combustibility limits extension in a reciprocal flow porous media burner for ethanol/air mixtures

An analytical study by computational simulation in ANSYS FLUENT of a lean mixture of ethanol/air combustion in a porous media burner was performed. A two dimensional mathematical model for filtration combustion was used to estimate the lean combustibility limits (LCL) in a standard one directional porous burner (SISO) and a reciprocal flow burner (RFB) in order to determine LCL variation. The combustion phenomenon was simulated with the continuum assumption for two different porous media: 80% high porosity 20[ppi] alumina foam and 40% low porosity 5.6[mm] diameter alumina spheres for the two reactor types. In the SISO burner with filtration velocity ug=[0.1;0.7][m/s] minimum equivalence ratio values reach Φmin=[0.05;0.054] and Φmin=[0.044;0.048] for alumina foam and spheres, respectively. The results for the RFB operating in the same ug interval indicate that LCL are substantially extended, allowing to incinerate mixtures as low as Φmin=[0.021;0.027] and Φmin=[0.016;0.022] for alumina foam and spheres as porous media, respectively. The SISO reactor configuration allows incinerating lean ethanol/air mixtures, but with the limitation that the combustion wave eventually reaches the reactor outlet and is extinguished. The RFB reactor scheme provides a permanent and stable combustion, considering that for times longer than 6000[s] alumina foam as a porous medium borders instability as the heated zone approaches the reactor’s end. Finally, comparing LCL in both reactors, in RFB they are reduced by 50–66% relative to SISO.

Ignition and supersonic combustion behavior of liquid ethanol in a scramjet model combustor with cavity flame holder

This study carried out experiments to investigate the combustion behavior of ethanol at Mach 2.0 and stagnation temperatures between 1800 and 2200K using a supersonic combustor with a cavity flame holder. The distribution of droplet diameters in fuel sprays was measured by a Laser Diffraction Spray Analyzer to estimate the vaporization time of fuel in the combustor. CH chemiluminescences and Schlieren photographs were measured by a video camera to identify the combustion mode and to measure the penetration height of fuel in the supersonic flow. Both gaseous and liquid ethanol injections were tested to examine the effect of the fuel vaporization process. The minimum equivalence ratio of injected fuel to air in the main flow and the minimum stagnation temperature for successful combustion were derived based on Weibull distribution statistics. Two combustion modes, intensive and transient, were identified based on the distribution of a rise in pressure and the CH chemiluminescence emissions over the cavity. Supersonic combustion was observed above the stagnation temperature of 2200–2000K for liquid and gaseous ethanol injections. Compared with previous research, the reactivity of ethanol was located between the levels of ethane and ethylene. The ignition limits of the stagnation temperature for gaseous and liquid ethanol injections were also estimated and validated by calculating fuel breakup, vaporization, residence times and chemical reaction time. A fuel penetration height over 10mm resulted in no supersonic combustion for the equivalence ratio of 1.0, although this enhanced fuel and air mixing.

Effects of Fuel Composition on Critical Equivalence Ratio for Autoignition

In a companion study, the critical equivalence ratio (ϕ) for several fuels (defined as the minimum equivalence ratio at which a fuel can autoignite) was examined in a motored engine as a metric to predict the tendency of a fuel to produce incomplete combustion due to an overly lean charge, leading to excessive emissions of total hydrocarbons (THCs) and carbon monoxide (CO). Higher ignition quality fuels (i.e., higher cetane number fuels) were shown to have leaner critical ϕ, but a question remained regarding fuel compositional effects on critical ϕ. In this work, to address that question, blends of n-dodecane/toluene and n-dodecane/isooctane were prepared to have the same derived cetane number (DCN) as n-heptane. Critical ϕ measurements for the two blends and n-heptane revealed that critical ϕ could vary between fuels with the same DCN. The n-dodecane/toluene blend was found to have a leaner critical ϕ than the n-dodecane/isooctane blend under low compression ratios or simulated EGR. It was concluded that the 11% greater n-paraffin content of the n-dodecane/toluene blend compared to the n-dodecane/isooctane resulted in more low-temperature heat release (LTHR) and a leaner critical ϕ. The critical ϕ of the low cetane number FACE (fuels for advanced combustion engines) fuels was also determined. A linear correlation with an R2 coefficient of 0.949 was observed between critical ϕ and n-paraffin mass content of the low cetane number (CN) FACE fuels. This result corroborated the conclusion from the DCN parity blends that the critical ϕ of a fuel is governed by the fraction of more reactive components (n-paraffins), which increases LTHR. The critical ϕ measurements of the low cetane number FACE fuels were compared to CO and THC emissions from a light-duty turbodiesel engine operating in an advanced combustion mode with the low CN FACE fuels. It was concluded that the relationship between low CO and THC emissions and a lean critical ϕ is only present when early SOI timing produces an overly lean fuel–air charge, but not when late SOI produces an overly rich mixture. These results suggest that a fuel can be blended to have a low ignition quality, which is desired for high-efficiency advanced combustion operation and with high n-paraffin content to reduce CO and THC emissions.

Effects of Fuel Ignition Quality on Critical Equivalence Ratio for Autoignition

Many advanced combustion modes utilize a premixed charge, which can vary from partially premixed to fully premixed, to simultaneously reduce emissions of nitrogen oxides (NOX) and particulate matter (PM). However, the partially premixed charge results in excessive emissions of total hydrocarbons (THC) and carbon monoxide (CO) from incomplete combustion. High ignition quality fuels, or fuels with high cetane numbers, have been in shown previous studies to enable reductions of THC and CO emissions during advanced diesel combustion operation. Many factors were suggested which contribute to the lower THC and CO emissions produced by the combustion of a high cetane fuel. Among the proposed factors was that a high cetane number fuel will have a lower combustion lean limit than a low cetane number fuel, thus resisting incomplete combustion. This theory was examined in the present work where the critical equivalence ratio (ϕ), the minimum equivalence ratio at which a fuel can autoignite, was identified for diesel, a high temperature Fischer–Tropsch (HTFT) fuel, and a low temperature Fischer–Tropsch (LTFT) fuel. The fuels were vaporized and premixed with air heated to 260 °C and then fed into a modified cooperative fuels research (CFR) octane rating engine, at compression ratios of 4, 5, 6, and 8. Equivalence ratio (ϕ) was gradually increased until the critical ϕ was determined. At the lowest compression ratio of 4, the diesel fuel did not autoignite while the HTFT and the LTFT fuels did at critical ϕ of 0.76 and 0.35, respectively. At the highest compression ratio of 8, the critical ϕ of the fuels began to converge where diesel, HTFT, and LTFT had critical ϕ values of 0.23, 0.20, and 0.17, respectively. However, in the presence of simulated EGR (O2 10.7 vol %, CO2 8 vol %, and N2 81.3 vol %) at a compression ratio of 8, the critical ϕ diverged dramatically, where the diesel, HTFT, and LTFT had critical ϕ values of 1.00, 0.77, and 0.38, respectively. n-Hexane, n-heptane, and n-dodecane were used as single-component surrogates of similar ignition quality as the diesel, HTFT, and LTFT, respectively. The single-component surrogates had leaner critical ϕ than their multicomponent counterparts did.

Dynamic flammability limits of methane/air premixed flames with mixture composition fluctuations

As a fundamental study in the application to direct-injection spark-ignition engines or gas turbines, in which mixture stratification and partial quenching are of serious concerns, unsteady premixed methane/ air flames subjected to time-varying composition fluctuations are investigated computationally. The code OPUS employs an unsteady opposed-flow combustion configuration, including detailed chemical kinetics, transport, and radiation models, using an adaptive time integration method for a stiff system of differential-algebraic equations with a high index. The primary issue of the study is to establish the concept of the dynamic flammability limit , defined as the minimum equivalence ratio above which the unsteady flame can sustain combustion. For the weak and strong strain rate cases studied, it is observed that the dynamic flammability limit depends on the mean and frequency of the composition fluctuation. The parametric study demonstrated that the flammability limit of an unsteady premixed flame is further extended to a leaner condition as the frequency or mean equivalence ratio fluctuation increases. It is also found that, under all conditions, the mean equivalence ratio and the minimum flame temperature must be higher than those at the steady flammability limit to sustain combustion. It is further shown that the dynamic flammability limit is primarily determined by the instantaneous, branching-termination balance at the reaction zone. The behavior of the flame response attenuation with increasing frequency is found to scale properly using the normalized frequency based on the imposed flow strain rate, which represents the characteristic time scale of the transport process within the flame.

Experimental Study on Cyclic Flow Reversal Combustion in a Porous Medium

An experimental study of combustion in a porous medium, with and without a cyclic flow reversal of a mixture through the porous medium, was performed. The thermal structure, in terms of axial temperature distribution, of two systems (with and without cyclic flow reversal of the mixture) is compared so as to elucidate the performance of the two systems. The effect of dominating parameters, i.e. half-period, equivalence ratio and flow velocity of the gas mixture on the axial temperature distribution is reported. It is shown that the transient behavior of the thermal capacity of the porous medium, induced by a cyclic flow reversal of the mixture (CFRC system), plays a vital role in yielding a higher quality combustion flame than without a cyclic flow reversal system (OWFC system). The half period of the cyclic flow reversal of the mixture has a moderate effect on the maximum flame temperature. Equivalence ratio and flow velocity have a similar effect on the thermal structure with respect to an increase in the maximum temperature and in the time averaged exhaust temperature, leading to an increase in heat losses. The minimum heal content of the mixture that would be able to burn in this study by the CFRC system is 140 kJ/kg at a minimum equivalence ratio of Φ = 0.046, a flow velocity of about 0.2 m/s and a half period of 30 s.

Lean premixture combustion on a coaxial burner

The interaction between inner and external coaxial flames is experimentally investigated in detail. The minimum equivalence ratio t of CH4-air premixtures (test premixtures) for complete combustion is measured as a function of the flow rate by the use of a small pilot flame whose equivalence ratio, p is equal to unity or less. The minimum t is dependent upon p and the flow rate of the test premixture. The effects of radial and axial separations of the two flames are also investigated with the use of three types of burner ports. The existence of diffusive and convective mixing of the two flows on a coaxial burner is proved. The magnitude of the mixing is estimated by a numerical calculation. A small amount of mixing plays an important role in stabilizing the flame in a highspeed flow of the lean premixture. It is concluded that the use of a coaxial burner for the measurements of the flammability limits is invalid. This paper also discusses the merit of simple coaxial burners without any thermal feedback or recirculating flow flow for lean premixture combustions. The overall minimum equivalence ratio attained by the present experiments is 0.6 at a mean flow velocity of 5.3 m/sec.

A comment on the flame angle, double-burner technique of determining flammability limits

The interaction between inner and external coaxial flames is experimentally investigated in detail. The minimum equivalence ratio t of CH4-air premixtures (test premixtures) for complete combustion is measured as a function of the flow rate by the use of a small pilot flame whose equivalence ratio, p is equal to unity or less. The minimum t is dependent upon p and the flow rate of the test premixture. The effects of radial and axial separations of the two flames are also investigated with the use of three types of burner ports. The existence of diffusive and convective mixing of the two flows on a coaxial burner is proved. The magnitude of the mixing is estimated by a numerical calculation. A small amount of mixing plays an important role in stabilizing the flame in a highspeed flow of the lean premixture. It is concluded that the use of a coaxial burner for the measurements of the flammability limits is invalid.This paper also discusses the merit of simple coaxial burners without any thermal feedback or recirculating flow flow for lean premixture combustions. The overall minimum equivalence ratio attained by the present experiments is 0.6 at a mean flow velocity of 5.3 m/sec.

Flame attachment zone of laminar, premixed methane-air flames

The interaction between inner and external coaxial flames is experimentally investigated in detail. The minimum equivalence ratio t of CH4-air premixtures (test premixtures) for complete combustion is measured as a function of the flow rate by the use of a small pilot flame whose equivalence ratio, p is equal to unity or less. The minimum t is dependent upon p and the flow rate of the test premixture. The effects of radial and axial separations of the two flames are also investigated with the use of three types of burner ports. The existence of diffusive and convective mixing of the two flows on a coaxial burner is proved. The magnitude of the mixing is estimated by a numerical calculation. A small amount of mixing plays an important role in stabilizing the flame in a highspeed flow of the lean premixture. It is concluded that the use of a coaxial burner for the measurements of the flammability limits is invalid.This paper also discusses the merit of simple coaxial burners without any thermal feedback or recirculating flow flow for lean premixture combustions. The overall minimum equivalence ratio attained by the present experiments is 0.6 at a mean flow velocity of 5.3 m/sec.