Isochoric Combustion Research Articles

Effects of the turbocharged Miller cycle strategy on the performance improvement and emission characteristics of diesel engines

The turbocharged Miller cycle strategy is studied to improve the power density of diesel engines and reduce emissions. A thermodynamic model and a 1D simulation model of turbocharged diesel engine are established. Results show that the introduction of the Miller cycle reduces the thermal efficiency under naturally aspirated conditions because of the low effective compression ratio, whereas it increases the thermal efficiency under a turbocharged condition owing to the energy recovered by the turbocharger. Under restricted combustion pressure and fixed intake mass, the thermal efficiency first increases and then decreases with increasing Miller cycle ratio, and the peaks occur at approximately 30%–50%. The gain of isochoric combustion ratio overlaps the loss of effective compression ratio due to the Miller cycle on the lower side, whereas it reverses on the higher side. With maximum and equal intake mass, the maximum power initially increases and subsequently decreases with increasing Miller cycle ratio, reaching a peak at 40%. Under a fixed isochoric combustion ratio, the thermal efficiency first increases and then decreases with increasing intake mass, and the optimum intake mass corresponding to the highest thermal efficiency decreases with increasing Miller cycle ratio. The lower the restricted combustion pressure is, the higher the gain in power and thermal efficiency by the Miller cycle strategy. Based on the calculation of the 1D model validated using a practical engine, the power can be increased from 41.6 kW/L to 100 kW/L while the brake thermal efficiency can be increased from 34.98% into 38.55% by increasing the Miller cycle ratio from 19% to 30% and the combustion pressure from 17.7 MPa to 35 MPa. With the application of the supercharged Miller cycle, when the Miller cycle ratio is 30% and the power intensity is increased from 60 kW/L to 100 kW/L, NOx decreases by 32.4%, CO decreases by 28%, showing a tendency to decrease and then stabilize, and HC increases by 5.3%. When the power is 80 kW/L and the Miller cycle ratio is increased from 10% to 30%, NOx decreases by 8.6%, CO decreases by 2%, and HC increases by 0.04%.

Laminar flame speed evaluation for CH4/O2 mixtures at high pressure and temperature for rocket engine applications

Pure methane-oxygen mixtures in liquid rocket engines lead to extreme pressure and temperature conditions that are prohibitive for most of the experimental setups. Hence, there is very little data on such flames in the literature, especially concerning the laminar flame speed Su, often limited at atmospheric pressure. The recent development of methalox rocket engines, which design process often requires CFD calculations, brings this lack of data to the forefront. Indeed, the CFD simulations require valid chemical schemes in the real operating conditions. To address this problem, flame measurements have been performed in a special isochoric combustion chamber with full optical access (OPTIPRIME) developed at ICARE. An extensive database in conditions never tested before is generated for several equivalence ratios, temperature and pressure ranges. Multiple chemical mechanisms were then compared to those results, showing various levels of agreement. Hence, the best mechanism from the literature on OPTIPRIME results and other literature experimental data was selected. A sensitivity analysis was performed to identify key chemical reactions controlling the flame speed. These key reactions could later be tuned by an optimization process to perfectly match the experimental results. Finally, additional measurements were performed in order to develop a Su=f(P,T) correlation to build a future flame speed database under rocket engines relevant conditions.

Elucidating the challenges in extracting ultra-slow flame speeds in a closed vessel—A CH[formula omitted]F[formula omitted] microgravity case study using optical and pressure-rise data

Refrigerants with a low global warming potential (GWP) possess mild flammability. Hence, a fundamental understanding of their combustion characteristics is required to assess their fire-hazardous potential. The laminar burning velocity is one fundamental property to describe these under safety evaluation aspects. However, measuring laminar burning velocities for slow-burning components, such as low-GWP alternatives, is challenging. This is due to influencing effects such as buoyant flame deformation, stretch, radiation, and confinement, especially at the flammability limits. In the present study, we investigated near-limit flames of the representative refrigerant difluoromethane (CH2F2) with nitrogen-enriched oxidizer mixtures to elucidate the potentials and limitations at ultra-slow flame speeds of two widely used flame speed measurement methods in the closed vessel configuration: the optical flame speed measurement in the early quasi-isobaric regime and the flame speed determination from the pressure-rise during the later phase of the isochoric combustion process. Experiments were performed under terrestrial- and microgravity, conducted in a specially designed setup suitable for drop tower applications, using both methods in parallel. Recommendations for the extraction of flame speed data are derived from the non-buoyant reference investigations under microgravity and transferred to ultra-slow flames at terrestrial gravity. Near-limit flames under microgravity were found to be significantly affected by stretch effects for the optical method so that flame speed extrapolations to unstretched conditions are prone to errors. Stretch also affects the pressure method’s lower evaluation limit, and errors can be estimated by combining flame speed data from the pressure method and Markstein lengths from optical experiments.

Experimental study on the intrinsic instabilities of spherically expanding CH4/H2/CO2/O2 flames

To study the instabilities of spherically expanding CH4/H2/CO2/O2 flames with an equivalence ratio of 0.80, isochoric combustion experiments were carried out in a 36 L constant volume bomb at an initial temperature (298±2 K) and atmospheric pressure (100 kPa). The crack length Lcrack, average cell area Scell, critical flame radius Rc and critical Peclet number Pec of CH4/H2/CO2/O2 flames were evaluated. The results show that the evolution of Lcrack at low hydrogen fractions is slightly affected by oxygen enrichment but significantly affected at high hydrogen fractions. Thermodiffusive instability dominates the evolution of Scell at low hydrogen fractions, but hydrodynamic instability dominates at high hydrogen fractions. At lower oxygen fractions, hydrogen addition results in more significant enhancement of the thermodiffusive and hydrodynamic instabilities. As the oxygen fraction increases, the maximum size of the thermodiffusive cells decreases, and the thermodiffusive instability weakens due to the increasing effective Lewis number, but the flame thickness decreases. This indicates that the weak thermodiffusive instability is amplified by the small flame thickness. With increasing oxygen fraction, the unstable region of the peninsula by the linear stability theory narrows in the Pe–n plane but broadens in the R–n plane. Therefore, the unstable peninsula in the R–n plane is consistent with the trend of crack growth observed in the experiments. Additionally, the experimental critical flame radius Rc,1 is closer to Rc calculated by the linear stability theory than Rc,2, where Rc,1 and Rc,2 represent the cross-cracking of the thermodiffusive cells and the onset of the hydrodynamic cells, respectively. Pecl exhibits a linear dependence on the Markstein number Ma. Moreover, a power law correlation of the experimental Rc with respect to oxygen enrichment and hydrogen addition is proposed.

Computational study of the multi-injector isobaric combustion concept in a heavy-duty compression ignition engine

Compared to the conventional diesel combustion engine that is equipped with a single centrally installed injector, the multi-injector spray-combustion concept has the potential to achieve higher thermal efficiency. In this work, a numerical study to explore the combustion characteristics of the multi-injector concept on an isobaric combustion compression ignition engine was conducted. The simulation results reveal that, compared to the single-injector case, a significantly lower heat transfer loss was induced using two side injectors owing to the less intense flame-piston interaction. In addition, good fuel flexibility was achieved using multiple injections, which was attributed to the initially generated hot regions downstream of the injection trajectory. To further optimize the engine combustion performance, the effects of several major design parameters such as the injector location and rotation angle, hole number and gap angle, spray angle, and piston geometry were investigated. The results showed that the spray/flame-wall interaction had a significant effect on the development of combustion heat release. Various design parameters showed a significant impact on the combustion heat release during the late combustion period when an intense flame-wall interaction was occurring. To attain a better engine performance, the injector location and nozzle design should be adequately adjusted according to the inner piston radius and depth. The reduction of intake temperature and introduction of an appropriate amount of isochoric combustion process were found to further promote the engine work, making it possible to achieve a thermal efficiency of over 50%.

Effect of wall heat transfer on the dynamics of premixed spherical expanding flames

Premixed laminar flame speed determination is a crucial point since its values are used for the sizing of every combustion systems. In order to measure this parameter, an isochoric combustion method can be used. It consists in measuring the pressure (and possibly other parameters) for a spherical expanding flame. This method allows to get flame speed data for a large scope of important pressures and temperatures supposing an isentropic compression. However, the entirety of the flame propagation process cannot be used to compute the flame speed as heat losses will start to appear as the flame come close the wall, making the isentropic compression assumption invalid. In order to precisely determine when significative heat losses occur, a criterion based on the evolution of the flame preheat zone thickness is described in this paper. The evaluation of this new method is performed using Direct Numerical Simulations for different mixtures (methane with various diluents) at different equivalence ratios and thermodynamic conditions. Finally, the criterion is compared to already existing methods, showing a relatively good accuracy to describe wall heat losses effect on the flame dynamics at high pressure.

Fuel flexibility potential for isobaric combustion in a compression ignition engine: A computational study

This work numerically explored the potential of using various kinds of fuels for the isobaric combustion concept in a compression ignition engine. Primary reference fuels (PRFs) including PRF0, PRF20, PRF40, PRF60, PRF80, and PRF100 were employed at the same middle engine load. Different injection strategies ranging from single to four injections were studied. The results demonstrated that the tested PRFs showed significant differences when using a single injection method, due to the different fuel auto-ignition characteristics: the lower fuel reactivity led to the longer ignition delay and thus more premixed combustion heat release. Fuel flexibility was achieved by utilizing at least two injection events, under which condition various fuels shared similar engine combustion performance and emissions. Predicted in-cylinder distribution of temperature revealed that the first injection event generated initial high-temperature pockets downstream of the nozzle, which helped to ignite the upstream air–fuel mixture from the following injection events and resulted in a relatively stable spray-controlled heat release process. To reduce soot emissions, various amounts of three shorter-chain alcohols (methanol, ethanol, and n-butanol) were blended with the baseline fuel (n-heptane). The methanol-blended fuels yielded the lowest soot emissions, but the fuel economy suffered due to the highest heat transfer losses. By increasing the nozzle number and introducing an adequate amount of isochoric combustion, the fuel economy for pure methanol combustion was effectively improved.

Numerical investigation of the effect of injection strategy on a high-pressure isobaric combustion engine

High-pressure isobaric combustion adopted in the double compression expansion engine (DCEE) has the prospect to achieve higher thermal efficiency compared to conventional diesel combustion. This work numerically explored the effects of various injection strategies on the combustion and emission characteristics of isobaric combustion. The study developed a mathematical model to predict the injection rate profile. After validations, extensive simulations were conducted with a peak pressure of up to 300 bar – mimicking the high-pressure unit of DCEE. Several major engine design parameters such as the exhaust recirculation gas (EGR) rate, engine speed, injection strategy, and intake pressure were varied and evaluated. The results demonstrated that a higher EGR rate resulted in a higher exhaust loss but a lower heat transfer loss owing to the lower combustion temperature, so the thermal efficiency exhibited a firstly growing and then declining trend. Besides, a higher engine speed generated a higher thermal efficiency due to the shorter combustion duration and thus lower heat transfer loss. Consequently, a peak thermal efficiency of 47.5% was achieved at EGR = 50% and 1800 rpm. The high-pressure cylinder performance can also be improved with an appropriate introduction of the isochoric combustion, but its impact on the whole DCEE setup needs further investigation.

A derivation of temperature-based energy equation for LES of isochoric turbulent combustion with FDSGS model

In this study, with the aim of deriving a temperature-based energy equation and evaluating the fractal dynamic SGS (FDSGS) combustion model under an isochoric condition, modelling of the filtered energy conservation equation in the temperature form is considered and is tested in the contexts of a priori and a posteriori, comparing with a corresponding DNS of turbulent combustion in a constant volume vessel. Conditional averages of various terms in the filtered energy conservation equation in the temperature form show that three unclosed terms, the SGS scalar flux, the SGS pressure-dilatation and the heat release rate, are leading terms. New SGS models for the SGS pressure-dilatation and the heat release rate are proposed and are tested in a priori manner. The zeroth-order models are also compared. The heat release rate models are proposed for reaction zones and post-flame zones separately, since the present analysis revealed a substantial contribution of heat generated in the post-flame zones. In addition to the tests of the SGS models for the energy conservation equation, isochoric treatments for various flame quantities under varying mean pressure and unburnt temperature due to the isochoric condition are also proposed and are tested. LES using the FDSGS combustion model with the new SGS models and isochoric treatments is performed considering the same combustion conditions and configuration as the DNS. The LES with several other SGS combustion models are also performed and are compared. The performed LES show reasonable prediction of flame propagation for any of the SGS combustion models tested, although the prediction by the FDSGS combustion model shows closest to the DNS. Regardless of the SGS combustion models, the peak values of the mean pressure are close to the DNS value, which suggests that the proposed SGS models and isochoric treatments for the energy conservation equation work well.

РАСЧЕТ ПАРАМЕТРОВ ГАЗОВОЙ СРЕДЫ В КОНТЕЙНМЕНТЕ АЭС С УЧЕТОМ ПОЛНОГО АДИАБАТИЧЕСКОГО СГОРАНИЯ ВОДОРОДА КОДОМ КУПОЛ-М

An important factor for the analysis of hydrogen safety is to take into account the effect of possible hydrogen combustion on the parameters of NPP containment atmosphere. In order to evaluate this effect a model of adiabatic isochoric complete combustion (AICC) of hydrogen was developed and implemented in the KUPOL-M containment code. With the model of adiabatic isochoric complete combustion of hydrogen included in the KUPOL-M code, this code will become applicable for the VVER NPP safety analysis performed with the aim to estimate possible thermal mechanical loads on the containment in case of hydrogen burning. This model is simple, conservative and based on the laws of conservation of mass and energy. The following assumptions are made in the model of adiabatic isochoric complete combustion of hydrogen: the volume in which burning takes place is constant; there is no heat transfer; the limiting component (in most cases it is hydrogen) burns down completely; intermediate stages of chemical transformations are not considered. The test results of this model on experimental data are presented in the article. There is a good agreement between the data of calculations and experiments in a wide range of hydrogen concentrations in gas mixtures. Based on the test results it can be concluded that implementation of AICC model in the KUPOL-M code allows conservatively evaluating the parameters in the containment atmosphere without significantly increasing the calculation time.

A comparative study of isobaric combustion and conventional diesel combustion in both metal and optical engines

Isobaric combustion is more efficient than isochoric combustion when the peak cylinder pressure is restricted. Recently a double compression expansion engine concept using isobaric combustion was proposed to improve the engine performance. However, the knowledge about the in-cylinder isobaric combustion process is limited. This study investigated isobaric combustion in both metal and optical engines utilizing a centrally placed direct injector and a multiple injections strategy, and the performance and emissions of isobaric combustion and conventional diesel combustion (CDC) were compared. Mie scattering and fuel-tracer planar laser-induced fluorescence (PLIF) were used to visualize the liquid-phase fuel penetration length and fuel distribution under non-reactive conditions, respectively. The natural flame luminosity and spatial soot distribution were measured with high-speed imaging and laser-induced incandescence (LII), respectively. Results demonstrate an increased soot formation in the isobaric cases compared with the CDC case. The successive fuel injection into reacting zones, which induces a locally fuel-rich mixture and less fuel–air mixing, accounts for the high soot emissions in the isobaric combustion. This study further demonstrates that the late injections of the isobaric cases lead to more combustion in the squish zone and near the cylinder walls, which increases the local temperature gradient and may result in more heat losses. There is still room to reduce the heat losses in the isobaric combustion although the total heat loss of the isobaric combustion is lower than the CDC case due to a lower combustion temperature. Multiple injectors are suggested to reduce the soot emissions of the isobaric combustion by spreading the different injections in space.

Autoignition and combustion wave propagation in a spatial reactivity gradient environment constructed using the shock-converging method

Abstract Studies on autoignition and combustion wave propagation in a spatial reactivity gradient environment constructed using the shock-converging method were conducted to obtain a new understanding of combustion modes and their mutual transition mechanism. Autoignition induced by the converging cylindrical shock wave and weak detonation can be observed by experiment and numerical simulation. Research indicates that for an appropriate spatial reactivity gradient, the velocity of the weak detonation wave can decrease and adjust to the local sound speed. This leads to the transformation of the weak detonation into the self-propagating Chapman-Jouguet (CJ) detonation. The speed of the spontaneous reaction wave characterizing the end of the chemical reaction is never less than the local sound speed; hence, there is no overdriven phenomenon throughout the weak-to-CJ detonation transition. This conforms to the fact that the overdriven phenomenon is not a necessary condition to initiate the detonation, and the combustion has higher thermal efficiency compared with isochoric combustion. Further research indicates that CJ detonation propagating in the preheated reactant can also transform into weak detonation for an appropriate spatial reactivity gradient. The initial CJ detonation degrades into a shock that decays and finally disappears. The wave-eliminating effect of the CJ-to-weak detonation transition can ensure the flow field quality and thermal efficiency of the combustion. In summary, performing a controllable weak detonation between isochoric combustion and CJ detonation in an engine has significant practical importance for improving combustion thermal efficiency and knock suppression.

A sensor based assessment of ignition delay for pure diesel and biodiesel blends in an isochoric combustion box

The key to efficient operation of a diesel engine is the smooth combustion of the fuel and controlled detonation. In diesel engine detonation depends on ignition delay (ID) period. An isochoric cylindrical combustion box (CCB) is fabricated for the assessment of ID tendency for simple diesel and vegetable oils-diesel fuel blend (VODFB). The combustion box consist of air heating coils, hot surface plate, fuel injector, pressure gauge, temperature indictor, temperature controller and photo sensor. A two stage compressor with intercooler is used for charging the air in the combustion box. The ignition lag was measured for injection pressures (IP) (100-bar, 200-bar and 300-bar), hot surface temperatures (400°C and 450°C), cylinder pressures (10, 15, 20 and 25-bar) for simple diesel and VODFB by using photo sensor and scope-meter. The results show that ID was decreased with increase in IP, at a specific cylinder pressure and HST for both simple diesel and VODFB. At high IP (300-bar), ID is lower for the blended fuel because at high IP, the fuel covers more physical land and more fuel is vaporized within a given time period and hence ID time decrease.

Assessment of SGS closure for isochoric combustion of hydrogen-air mixture

Direct numerical simulation (DNS) data of freely propagating turbulent premixed flame of stoichiometric hydrogen air mixture inside a closed vessel is analysed to study a sub-grid combustion closure based on unstrained flamelet approach. This modeling framework needs closures for the sub-grid scale (SGS) reaction rate and scalar dissipation rate. The results show that the closure models for these two SGS quantities work quite well. The dissipation rate closure involves a scale dependent parameter, βc, which is related to the flame curvature induced effects. The reactivity of reactant mixture increases with time in isochoric combustion because the mixture temperature and pressure increase with time. This also influences the parameter βc and thus the dynamic evaluation of this parameter is investigated using the DNS data.

STUDIES OF HIGH EXPLOSIVES IMPACT ON REDUCTION OF HCL IN HETEROGENEOUS SOLID ROCKET PROPELLANTS

The paper describes an influence of high explosives: hexogene (RDX), octogene (HMX), and dinitro-diaminoethene (FOX-7) on the properties of heterogeneous solid rocket propellant (HSRP) prepared on the base of Hydroxy Terminated Polybutadiene (HTPB) in which ammonium perchlorate (AP) was partially replaced by sodium nitrate (SN). It reduced the content of HCl in combustion products. Theoretical values of thermochemical and thermodynamic properties such as isochoric combustion heat (Q), specific impulse (Isp) and contents of combustion products in motor combustion chamber and nozzle have been identified by using the ICT-Code program. The rheological properties (virtual viscosity) of the propellant slurry during curing process, the sensitivity to mechanical stimuli (impact, friction), decomposition temperature, calorific value and hardness of propellants containing explosive materials were tested by instruments and ballistic properties were investigated by laboratory rocket motor (LRM).

Propagation indices of methane-nitrous oxide flames in the presence of inert additives

The propagation indices (explosion pressures, rates of pressure rise, severity factors and explosion times) of CH4-N2O flames in the presence of inert gases (He, Ar, N2 and CO2) were determined by experiments performed in a spherical vessel with central ignition. Lean-and stoichiometric mixtures (φ = 0.8 and 1.0) with a variable inert gas concentration between 40 and 60% were studied, at variable initial pressures within 0.50 and 1.75 bar. Inert gas addition to each of the studied CH4−N2O mixtures results in the decrease of both experimental and adiabatic explosion pressure and of the maximum rate of pressure rise, along with the increase of the explosion time. CO2 was found to be the most efficient inert additive, followed by N2, Ar and He. The measured and computed propagation properties are examined as functions of the total initial pressure and the inert gas concentration. The correlation of peak explosion pressures with the initial pressures, derived from the heat balance of the isochoric combustion of a fuel-oxidizer mixture under non-adiabatic conditions, is used to evaluate the heat losses during the closed vessel combustion, dependent on the initial CH4/N2O ratio and on inert concentration of the flammable mixtures.

Analysis of creation and combustion process of hydrogen-air mixtures by optical method in isochoric chamber

The article considers the analysis of combustion process of hydrogen-air mixture of variable composition. Direct injection of hydrogen into the isochoric combustion chamber was applied and the mixture formation took place during the combustion process. The influence of the dose distribution of the fuel supplied before and after ignition on the formation of the flame front and the course of the pressure in the isochoric combustion chamber was discussed. The filming process and registration of pressure in the isochoric chamber during research of combustion process was applied.

Composite Cycle Engine Concept with Hectopressure Ratio

The investigated concept targets a significant increase in core engine efficiency by raising the overall engine pressure ratio to over 100 (hectopressure ratio) by means of discontinuous cycles allowing for closed volume combustion. To this end, piston engines enable isochoric combustion and augment the conventional Joule/Brayton cycle, thereby producing a composite cycle. An engine concept is chosen based on idealized parametric studies of simplified representations of the cycle as well as qualitative measures embracing weight, size, efficiency, emissions, operational behavior, and the life cycle. The most beneficial mechanical representation of the composite cycle engine in this study features crankshaft-equipped piston engines driving separate piston compressors, a high-pressure turbine driving an axial intermediate pressure turbocompressor, and a low-pressure turbine driving the fan. The powerplant performance calculations show radical improvements in thrust-specific fuel consumption of 17.5% during cruise. Although engine weight increases correspondingly by 31%, a fuel burn reduction of 15.2% can be shown for regional operations at aircraft level relative to year-2025 engine technology. The concept is capable of meeting the emission reduction targets for carbon dioxide and nitrogen oxides aspired to by the Strategic Research and Innovation Agenda targets for carbon dioxide in 2035 and for nitrogen oxides in 2050.

Influence of inert gas addition on propagation indices of methane–air deflagrations

The work examines the characteristic indices of laminar deflagrations propagating in methane–air gaseous mixtures diluted by several inert gases: He, Ar, N2 or CO2. Experiments were performed in two spherical vessels of different volumes with central ignition, at ambient initial conditions. Mixtures with variable methane concentrations (6–12vol%) and variable inert concentrations (5–40vol%) were studied, in order to outline the inert influence on the most important and accessible safety-related parameters: the peak explosion pressure, the maximum rate of pressure rise (or the related property, i.e. the deflagration index) and the explosion time (the time necessary to reach the peak explosion pressure). Among the studied inert additives, CO2 is the most efficient, followed by N2, Ar and He. Inert gas addition to any flammable CH4–air mixture determined the decrease of both experimental and adiabatic explosion pressure and of the maximum rate of pressure rise, along with the increase of the explosion time. Using an equation that describes the heat balance of the isochoric combustion of a fuel–air mixture under non-adiabatic conditions, a correlation between the peak explosion pressure and the mole fraction of inert gas was derived and validated for CH4–air– inert mixtures.

Measurement of the laminar burning velocity using the confined and unconfined spherical flame methods – A comparative analysis

This work investigates the consistency of experimentally determining the laminar burning velocity by using two different approaches: the unconfined flame method, based on Schlieren photography of the flame front evolution, and the confined flame method, based on pressure rise monitoring during isochoric combustion. Radiation corrected laminar burning velocity values were derived by the unconfined flame method using linear as well as non-linear stretch relations. Those values were then used as reference for evaluating the accuracy of the confined flame method. Laminar burning velocity obtained by the latter method were found to depend strongly on the modeled relation between the burned mass fraction x to the pressure P. Thus, we compare several x–P relations and show that the popular linear x–P relation overestimates the burning velocity by up to 20%. However, by “right parametrization” of a more detailed analytical x–P relation, closer results to those obtained by the unconfined flame method may be achieved. The favorable usable pressure data range for the confined flame method was also considered and found to be in the range of (10%<x<55%). Moreover, it was found that stretch effects are not totally negligible during the confined flame propagation, even for low Markstein length-mixtures. In summary, we suggest the analytical confined flame method as we define it in this work, to be a cost-effective and less labor-intense alternative to the unconfined flame method for applications where accuracy can be compromised.