Counterflow Configuration Research Articles

Improving round-trip energy efficiency of a unitized regenerative fuel cell by adopting staircase flow channel and counter flow configuration

The flow field design is an effective measure to improve hydrothermal management within a fuel cell. This paper proposes a staircase flow channel used in the oxygen-side flow channel of a unitized regenerative fuel cell. Based on a two-dimensional two-phase non-isothermal model, the hydrothermal distribution variation induced by staircase channels with different step numbers is investigated in detail. Results show that oxygen transport is boosted and liquid water accumulation is reduced in the fuel cell mode with the use of staircase structures, thus local oxygen starvation under high current density is alleviated. In addition, the temperature uniformity of electrodes is effectively improved, and the fuel cell net power is enhanced by up to 14.24% at 0.2 V with a five-step flow channel. In electrolytic cell mode, however, the advantage of downward staircase structures is not prominent. Therefore, a counter-flow method is suggested during the mode switching stage, which can improve average current density by up to 10.83%. Besides, round-trip energy efficiency is introduced to evaluate the comprehensive performance. The staircase flow channel coupled with counter-flow method improves the efficiency of unitized regenerative fuel cell by up to 49.63% under high current density, compared with conventional flow channels with co-flow configuration.

On the understanding of a cryogenic two-phase LOX/GH2 flame: Parametric sensitivity, characteristic scaling and phase instability

This study concerns the cryogenic hydrogen combustion in subcritical pressure conditions, with practical relevance to rocket engine applications. A cryogenic two-phase flame in the counterflow configuration is calculated and analyzed with the consideration of real-fluid effects and heat/mass transfer across the liquid-gas interface. The effects of pressure, strain rate, fuel inlet temperature, and heat loss on the flame characteristic behaviors are examined. It is found that the vaporization rate of liquid oxidizer scales with the square-root of pressure (p) times strain rate (ast), and is not a limiting factor for combustion. The total heat release scales with past at lower strain rates while with p4/5ast1/3 at higher strain rates. The significant results on the phase-stability of cryogenic flame are also established. It is found that the unstable phase, in terms of vapor-liquid equilibrium, arises in the vicinity of the liquid-gas interface; however, all unstable-phase states still stay in the metastable region of the phase diagram. For all the flame solutions considered in this study, no thermochemical state enters the spinodal region.

Numerical analysis of heat transfer characteristics of a novel heat exchanger for Lorenz-Meutzner cycle with zeotropic mixtures

This study evaluates the performance enhancement of the Lorenz–Meutzner (LM) cycle with zeotropic mixtures of multiple refrigerants using a numerical simulation model. The computational simulation includes a heat exchanger model developed for predicting the air outlet temperature accurately. The predicted results are validated against experimental gliding temperature with varying mass fractions of different zeotropic mixture combinations. The R600/R290 mixture demonstrates the best performance characteristics. Various design changes of the heat exchanger are adopted to enhance the energy efficiency of the LM cycle. Counterflow configuration is found to be superior to the parallel configuration, particularly in the LM cycle, owing to the gliding temperature difference of zeotropic mixtures. Multiple refrigerant flow paths decrease the air outlet temperature further and prove to be more effective than the single flow path design. Based on careful investigation of the geometrical effect of heat exchangers on the LM cycle, a novel design of the heat exchanger is proposed, and its overall performance in the LM cycle is evaluated. The novel design of the heat exchanger indicates the largest air temperature difference, leading to the highest coefficient of performance (system energy efficiency) of the LM cycle compared with the baseline cycle with R600a.

A comprehensive experimental analysis for optimal flow configurations in a triple tube heat exchanger (TTHXr)

In this research, an effort has been made to design and construct a heat exchanger that enables the transfer of heat across three different fluids. This heat exchanger is made up of three tubes: an aluminum tube (innermost tube), a copper tube (intermediate tube), and a GI tube (Outer most tube), which are arranged at the innermost, intermediate, and outermost positions, respectively. The study's aim is to find the optimal fluid flow arrangement of NHN, CHN, and NHC in the TTHXr that will provide the highest performance to heat exchangers. The heat exchanger's performance is assessed for three distinct mass flow conditions of fluid streams; (i) ṁAl = const, ṁCu=ṁGI (variation) (ii) ṁGI=ṁCu = const, ṁAl (variation) (iii) ṁAl≠ṁCu≠ṁGI (variation). Experiments were carried out using counter-flow fluid stream configurations with turbulence flow conditions of hot fluid to evaluate the effectiveness of the heat exchanger for maximum heat extraction from hot fluid streams. Variations in the flow of the outermost fluid stream were discovered to have a substantial impact on heat exchanger performance. The results revealed that the NHC flow configuration of fluid streams had the best performance when compared to other fluid flow arrangements.

The effect of fuel droplets on the burning velocity of strained laminar acetone/air flames

The effect of fuel droplets on the burning velocity of strained laminar premixed flames is investigated via experimentation and simulation. The twin counterflow configuration was used to obtain reference flame speeds as a function of strain rate for a prevaporized flame and a dilute spray flame simultaneously, both composed of acetone and air. The mixtures were varied with respect to nominal equivalence ratio (0.8–1.4) and strain rate (200–600 s−1). Gas velocities were measured using particle image velocimetry. Droplet size, velocity, and concentration were measured using phase Doppler anemometry: non-reacting flow measurements were taken at a position upstream of the stagnation plane; reacting flow measurements were taken along the stagnation streamline. The droplet Sauter mean diameter ranged between 65–75 μm, and the estimated fuel liquid fraction varied between 6–22%, increasing with nominal equivalence ratio. The results show that the reference flame speed of the spray flame decreases slightly relative to that of the prevaporized flame in the case of lean mixtures, but appears unchanged in the case of stoichiometric and rich mixtures. Gas velocity profiles and droplet measurements along the stagnation streamline suggest that the spray flame vaporization is incomplete, such that the reference flame speed corresponds to that of a lower equivalence ratio. Conversely, the opposing flame is affected by the fraction of droplets that do not vaporize in the spray flame and either penetrate the flame front, causing fuel enrichment, or evaporate near the stagnation plane, reducing the adiabatic flame temperature. The effect appears primarily for rich mixtures with higher liquid fraction, as evidenced by the lower reference flame speed and lower axial velocity rise across the flame. This study provides a systematic framework to examine the influence of fuel droplets on laminar flame propagation using a single-component fuel in the counterflow configuration.

Experimental and numerical study on the thermal and hydraulic characteristics of porous-media heat exchangers in cryogenic conditions

The thermal performance of a metal-foam heat exchanger in cryogenic environments was examined through an experiment and a computational fluid dynamics (CFD) simulation. A plate-type heat exchanger with metal foam inserted in the channels was designed and manufactured. The metal foam was 20-pore-per-inch nickel metal foam and brazed to stainless-steel channel plates. Nitrogen gas in cryogenic conditions was supplied to both hot and cold-side channels with a counter-flow configuration. The pressure drop was measured in a wide range of mass-flow rates of 8–48 kg/h. Heat transfer experiments were conducted while adjusting the gas temperature using heating wires in a range of mass-flow rate of 10–20 kg/h. The data obtained through the experiment were compared with those of CFD simulations and previous correlations obtained at room-temperature conditions. The CFD prediction of the pressure drop agreed well with the experimental data with less than 10 % discrepancy. Furthermore, the pressure-drop correlation models at room-temperature conditions agreed well with the experimental results in cryogenic conditions. Among the pressure-drop models, the Dietrich model showed the best agreement. CFD predicts the heat transfer well within 10 % discrepancy from the experiment. However, the heat transfer in cryogenic conditions is 15–20 % higher than that of the previous correlations obtained at room-temperature conditions. This can be explained by the hydraulic performance not being affected much by the temperature difference, while the Nusselt number of the metal-foam-filled channel has higher value due to lower thermal conductivity at a cryogenic environment.

Natural convection in vertical enclosures with conjugate boundary conditions

A numerical investigation and reduced-order modelling of natural convection in a cavity with differentially heated sidewalls is discussed. The effect of conjugate boundary conditions on the cavity's heat transfer and natural flow circulation with varying aspect ratios and Rayleigh numbers is examined. Validation of the canonical differentially heated cavity reveals that a modification to the definition of the Rayleigh number (Ra) is required to reconcile the $Nu\sim Ra^{1/4}$ scaling (where Nu is the Nusselt number) and validity of previously proposed correlations for the heat transfer in vertical enclosures. Dynamic mode decomposition is used to uncover the underlying time-dependent flow structures and the results are compared with stability bifurcation studies in the literature. A flow mode previously associated with smaller aspect ratio cavities is identified as the unstable mode for a larger aspect ratio of 4. The effect of conjugate boundary conditions is scaled based on the ratio of the internal and external boundary layers, wherein higher external Reynolds numbers aid in the heat transfer as the Nusselt number approaches the isothermal limit. A reduced-order theoretical model is proposed to predict the Nusselt number for the conjugate boundary conditions. The performance of conjugate boundary conditions is connected to the flow stability, with the parallel-flow configuration acting in a destabilizing manner while the counterflow configuration has a stabilizing effect and results in the highest heat transfer. We also found that relatively large heat transfer can be achieved with substantially lower external actuation with intermediate aspect ratios, translating to less power in practice.

Study of Diffusion Cool Flames of Dimethyl Ether in a Counterflow Burner under a Wide Range of Pressures.

Cool flames have been studied for more than three centuries since the first observation. However, there are few achievements on the effects of the pressure on cool flames. In this work, a diffusion cool flame has been for the first time established using a counterflow configuration under a wide range of pressures. Dimethyl ether is used as the fuel because its low-temperature chemistry has been well tested. The pressure range of the experiments is from 0.05 to 0.15 MPa. The extinction limits, flame temperatures, and combustion products have been measured and simulated. In general, the reactivity of cool flames is stronger with increasing pressure. Specifically, at a fixed fuel mass fraction, the cool flame has a higher extinction strain rate, temperature, and concentration of products under higher pressure. However, the enhancement effect decreases with the increase of pressure. Interestingly, it was observed that the flame became thicker when the pressure increased. Moreover, the cool flame would deflagrate and transform to a hot flame when the pressure exceeds a certain value. The model captures the trends well but underpredicts the extinction limits and overpredicts the flame temperatures and product concentration. Path flux analyses and heat release rate analyses were carried out. It was found that the main heat release reactions are the reactions with CH3OCH2 radicals under low pressure, and CO prefers to form CO2 indirectly through HOCHO radicals. The study advances the understanding of cool flames in a wide range of pressures and provides experimental data for the improvement of the models.

Performance Investigation of a Three Fluid Heat Exchanger Used in Domestic Heating Applications

Recent work analytically investigates the heat transfer characteristics of a three fluid heat exchanger used for domestic heating applications with respect to different design parameters, i.e. flow rate, inlet temperature, tube diameter, coil diameter, and coil pitch. The present and previous results are compared with the literature. Overall agreement among these results are observed with little variation. Afterwards, the present temperature data was verified with prior experimental data and little deviation observed in these results vary from -4.28 % to +6.68 % and -6.17% to +5.92% in parallel and counter flow configuration, respectively. It is ensued that the coil side Nusselt number increases with the rise in coil side fluid flow rate and inlet temperature, coil outside fluid inlet temperature and coil diameter respectively. The increment in coil side flow rate and inlet temperature are identified as the major contributors, with 297% and 39.5% contributions. Similarly, growth in coil outside Nusselt number is observed with the rise in coil side fluid inlet temperature and flow rate, coil outside fluid flow rate and inlet temperature, and coil pitch respectively. The coil pitch and flow rate at the coil outside are identified as major contributors with 36% and 28.5% contribution repsectively. Distinct correlations for heat transfer in the present HEx are proposed for coil inside and outside fluid flow in a turbulent flow regime. The developed correlations results are compared with the present result, and reasonable agreement is observed within the data range of +13% to -14% and +10% to -11% for coil inside and outside Nusselt number, respectively.

Convolutional neural network analysis of radiography images for rapid water quantification in PEM fuel cell

Polymer electrolyte membrane (PEM) fuel cells produce water as a byproduct, which, if not properly managed, will cause electrode “flooding” and consequently performance loss. Nonintrusive methods, such as neutron or X-ray radiography, have been employed to obtain in-situ water images in PEM fuel cell. This study presents one of the first studies developing a machine learning approach to analyze neutron radiography images using the convolutional neural network (CNN), a deep learning model, to quantify liquid water content in PEM fuel cell. The CNN model is trained using the labeled radiography images constructed from the contour legend, which contains the information of the water areal mass density. Image enhancement is carried out to generate additional data for CNN training. The properly-trained CNN model is subsequently applied to the radiography images to obtain the average water areal mass densities under various current densities, relative humidity (RH), and flow fields. The results show that the water content can be significantly reduced by using low RH inlet flow and the counter-flow configuration renders the fuel cell a higher water content than the co-flow one. In addition, the quad-serpentine flow field increases the water content in the current density range of 0.4–0.8 A/cm2 compared with the single-serpentine one. The CNN model takes less than 0.1 s to analyze an image and its results agree well with the literature data from the conventional image processing method with an accuracy of 91%. The CNN method is applicable to other radiography methods and is suitable for the online/rapid monitoring of liquid water in PEM fuel cell.

Air-to-Air Heat and Moisture Recovery in a Plate-Frame Exchanger Using Composite and Asymmetric Membranes.

The present work studied an air-to-air exchanger comprising a flat plate module with a diagonal channel and a counterflow configuration for the air streams. The objective of this study was to remove moisture and sensible heat from an exhaust air stream by indirect contact with another air stream. The temperature and flow rate of the exhaust air was in the range of 40–80 °C and 1–5 L·min−1, respectively, and the fresh ambient air to exhaust air flow ratio was 1–5. An asymmetric porous membrane (P-MEM), a thin film composite membrane (C-MEM), and a kraft paper were used as the core for the heat exchange module. The most influential parameter was the humid air temperature, with a direct positive effect (50–60%) due to the increase in the kinetic energy of the water molecules. The other effective parameter was the flow rate of the humid gas with a reverse effect on the enthalpy exchanger performance (25–37%). The ratio of “fresh” air to “exhaust” air had the lowest positive effect (8–10%) on the total effectiveness. The sensible effectiveness of different membranes under the studied conditions was relatively the same, showing their similar heat conductivity. However, the kraft paper showed the best performance compared to the synthetic membranes due to having a porous/hydrophile texture. P-MEM with an asymmetric porous texture showed the closest performance to kraft paper. Furthermore, it was found that under limited conditions, such as higher temperatures (70 and 80 °C) and flow rates (5 L·min−1) for the humid air, the performance of P-MEM was a little better than the kraft paper. However, C-MEM with the lowest total effectiveness and overall heat transfer coefficient (150–210 W·m−2·K−1) showed that the hydrophile PEBAX layer could not contribute to moisture recovery due to its high thickness.

Flamelet modeling of forced ignition and flame propagation in hydrogen-air mixtures

Modeling hydrogen-air flames is challenging for several reasons. Due to its high diffusivity and reactivity, the combustion properties of hydrogen are substantially different from conventional hydrocarbon fuels. Besides differential diffusion and flame stretch, which play an important role in hydrogen combustion, the prediction of ignition events is also relevant for the safety and control of combustors operating with hydrogen. To address these effects with a manifold-based method, a novel flamelet progress variable (FPV) model is presented which consists of a tabulated manifold and a coupling strategy. In contrast to prior work, the manifold is coupled to the CFD simulation by transporting major species and enthalpy instead of transporting the flamelet control variables only. A closure approach is developed to employ a mixture-averaged diffusion model accounting for exact diffusion coefficients. Furthermore, a novel tabulation strategy is presented which is based on a recently published composition space model (CSM). The CSM is used for computing and tabulating flamelets which are consistently blended with constant-pressure homogeneous ignition solutions at elevated temperatures. The FPV model is validated for planar and curved hydrogen-air premixed flames across a range of equivalence ratios (0.7≤ϕ≤1.4). The latter are ignited by an energy deposition followed by an outward propagation as cylindrical or spherical expanding flames. It is shown that the FPV model accurately recovers the characteristics of the forced ignition process in both quiescent mixtures and a counterflow configuration, as well as the flame structures and propagation speed of the outwardly propagating hydrogen-air flames.

Studies on the effect of flow configuration on the temperature distribution and performance in a high current density region of solid oxide fuel cell

• Three different flow configurations of solid oxide fuel cell have been compared. • The temperature distribution at 6000 A/m 2 were analyzed. • In counter-flow, the most uniform temperature distribution was formed. • The power density is 5% higher in counter-flow than the other configurations. • Counter-flow presents the best efficiency of up to 3.35%. The effect of temperature distribution on the performance of solid oxide fuel cells (SOFCs) is analyzed for various channel designs in a high current density region. By considering heat accumulation due to local electrochemical reactions and cooling due to flow patterns, the heat distributions in three flow configurations (co-flow, counter-flow, and cross-flow) are analyzed, with the most appropriate channel in the high power region selected. The average operating temperature of the cell is the highest for the counter-flow configuration and the lowest for co-flow. The temperature of the counter-flow configuration is approximately 10 K higher than that of co-flow. The temperature distribution of counter-flow configuration, however, is the most uniform among different flow patterns. The maximum temperature difference in the counter-flow channel is approximately 8 K, but that in co-flow is approximately 22 K. Furthermore, the performance of the cell using the counter-flow configuration is the best in that it shows 5% higher power density and 3.35% higher system efficiency than the other flow configurations. Therefore, the counter-flow configuration is superior at high power because it has the lowest temperature gradient and the best cell performance. This paper contributes to the commercialization of the fuel cell by presenting appropriate parameters for temperature regulation and suitable flow configuration for operation at high power.

Experimental aerothermal characterization of surface air-cooled oil coolers for turbofan engines

Thermal management is a major challenge for new generation turbofan aero-engines. One of the most promising heat exchangers are the so-called surface air-cooled oil coolers (SACOCs). In this study, an experimental methodology is proposed and implemented in order to characterize SACOCs mounted in turbofan bypass ducts. Three different SACOC geometries have been characterized under the same nominal operating point, while the actual velocity profile in the bypass was reproduced by means of a distortion screen upstream the test section. The heat exchangers were mounted in counterflow configuration and feature the same fin geometry in the oil side. The three prototypes varied only in the air side, being the first a baseline flat plate, the second a SACOC with standard trapezoidal fins and the third featuring optimized fins designed to reduce the pressure drop. Aerothermal results demonstrated that the effect of the SACOC on the bypass flow was confined to a region about the same height and width of the finned area, avoiding the need of reproducing the whole bypass duct. However, for this reduced-height experimental approach to be valid, we show that the velocity profile needs to be rearranged to match the specific section of the whole bypass. We also demonstrate how the optimized fin geometry achieved a 10% lower friction factor than the standard one at nominal flow conditions while increasing the overall heat transfer coefficient by 5.2%.

A study on the extinction condition in counterflow diffusion flames of methane and LPG under the influence of polydisperse water mist

The extinction condition in methane and LPG (mixture of propane and n-butane) flames under the influence of polydisperse water mist is investigated in a counterflow configuration. LPG is a standard cooking fuel used worldwide. Despite widespread use, there is no existing literature on the behaviour of LPG flames under the influence of water mist. The goal of the present work is to carry out extinction experiments with LPG and compare the behaviour with another fuel studied in greater detail, namely methane. In the present work, we investigate the effect of three different droplet size distributions with Sauter mean diameters (SMD) of 24 µm, 29 µm, and 43 µm, respectively. LPG flames show significantly higher resistance to extinction than methane flames in all the conditions studied. However, the addition of water mist shows similar suppression effectiveness in both flames. Mist with 43 µm SMD shows lesser effectiveness as compared to the other two cases. The experimental investigation is supplemented by numerical modelling in a one-dimensional framework. The numerical modelling predictions are in good agreement with the experimental measurements. The effect of water mist addition on the reaction kinetics is further investigated by Damköhler number analysis. A definition of Damköhler number is proposed to uniquely characterise strain-induced extinction in counterflow diffusion flames under the influence of water mist.

Multiple structures of laminar fuel-rich spray flames in the counterflow configuration

Multiple structures of laminar non-premixed ethanol/air spray flames in the counterflow configuration are presented which exist under fuel-rich conditions and moderate gas strain rates. A mono-disperse liquid fuel spray with carrier gas air is directed against an air stream. Two different spray flame structures are found for identical boundary and initial conditions: one of these flame structures consists of two different chemical reaction zones which reside on either side of the gas stagnation plane. In the second case, the spray-sided flame does not exist and combustion occurs entirely on the gas side of the configuration with largely distinct evaporation and combustion zones. Parametric studies of equivalence ratio between 1.1 and 1.8 and gas strain rates on the spray side of the configuration from 55/s as well as initial droplet size between 10 µm and 50 µm are performed. A regime diagram is presented to display the range of existence of the multiple/single structures as well as the flame extinction. Physical reasons for the existence and breakdown of the multiple structures are explored and discussed. The spray flame structure with a single chemical reaction zone breaks down with increased strain rate when the chemical time scales of OH and fuel vapor are equal at the crossover temperature (Tcross=1104 K) of the reaction rate constants of H + O2⇌ OH + O and H + O2 + M →HO2 + M.

A Tsuji burner in a counterflow

This paper addresses the aerodynamics of a new type of Tsuji burner involving a cylindrical porous fuel injector of radius a placed at the centre of a planar air counterflow configuration with strain rate , with specific attention given to flows with large values of the Reynolds number , where ν represents the air kinematic viscosity. For cases in which the fuel-injection velocity is comparable to the characteristic counterflow velocity , the boundary layer is blown off from the cylinder surface, so that the flame is embedded in the thin twin mixing layers that form about the stream surfaces separating the outer air stream from the fuel stream. Molecular transport effects are confined to these mixing layers, while the flow structure outside is nearly inviscid, with the air-side velocity being potential, while the velocity found on the fuel side is rotational, because fuel injection generates vorticity through the requirement that fuel emerges normal to the cylinder surface. The inviscid flow is computed numerically, with use made of the streamfunction-vorticity formulation for values of the ratio of injection velocity to counterflow velocity , the only relevant parameter of the flow, ranging from small injection velocities to large injection velocities . Asymptotic methods are used to investigate the form of the solution for extreme values of Λ. In the limit of weak injection, the vorticity, scaling with , is confined to a thin near-cylinder boundary layer of thickness Λ that necessarily separates from the cylinder to form a cavity of finite size on both sides of the cylinder. In the opposite limit of strong injection, the vorticity needed to maintain the fuel flow normal to the porous cylinder is found to be small, of order , so that the flow is irrotational in the first approximation. The velocity distribution along the fuel-air interface is seen to determine the evolution of the diffusion flame, including the length of the stretched jet flames that develops along the counterflow centre plane.

Modeling the dual-fuel combustion of porous lycopodium particles and diesel using an analytical simulation framework

In this paper, a comprehensive analytical study is performed to assess the lycopodium-diesel dual-fuel combustion system in counter-flow premixed configuration. The system is modeled as multiple zones that are coupled together via proper boundary and jump conditions on interfaces. According to the respective reaction and transport phenomena in these zones, conservation equations of mass and energy are derived, non-dimensionalized, and solved by Matlab and Mathematica in an analytical way. The porosity of lycopodium particles and the thermal radiation from the reaction zone and the post-flame zones into the preheating zone are considered, in order to improve the realism and accuracy of the modeling. Experimental validation of the derived framework is first carried out with good agreement between our results and the data from the literature. Afterwards, the impacts of parameters such as reactants Lewis numbers, equivalence ratio, and critical strain rate on the flame structure, as well as lycopodium particles' combustion behavior in dual-fuel and single-fuel modes are evaluated and compared. It is found that dual-fuel combustion system enhances the vaporization and ignition of lycopodium particles resulting in a 13% increase in flame temperature. In addition, effects of thermal radiation and porosity of particles on both the flame temperature and position are quantified. The presence of biomass particle porosity and thermal radiation reduces the flame temperature and drive the flame front towards the fuel nozzle.

Ignition under strained conditions: Unsteady flamelet progress variable modeling for diesel engine conditions in the transient counterflow configuration

The ignition of non-premixed n-dodecane-air flames is studied in a highly transient counterflow configuration under diesel engine operating conditions. The one-dimensional transient counterflow is configured such that ignition is initially physically inhibited through high strain rates. In a short relaxation period the inflow velocities (thereby the strain rate) are lowered and ignition can occur, a behaviour similarly observed in spray flames. The non-premixed flame, which is igniting in this unsteady flow environment, is computed with direct chemistry and tabulated chemistry based on the unsteady flamelet progress variable approach (UFPV). The flamelet look-up table is constructed from igniting unsteady flamelets considering also the parameter range between stable and unstable branches of the characteristic S-shaped curve which is obtained via a continuation method. The mixture fraction, the progress variable, and the local stoichiometric scalar dissipation rate serve as control variables for the three-dimensional table. Particular attention is put on the inclusion of thermal expansion effects and the treatment of the progress variable source term to capture the onset of ignition. Besides varying strain effects, the igniting flame exhibits low temperature chemistry and multi-stage ignition behaviour which is particular challenging for the UFPV modeling. By direct comparison to the reference solution the predictive capabilities of the UFPV approach are analyzed and assessed in detail. Furthermore, the influence of table resolution on computational efficiency is discussed. The systematic analysis and validation of the UFPV approach in the transient counterflow configuration fills a specific gap in the literature and underlines the fidelity of the approach for flames subject to complex transient processes. Furthermore, the 1D configuration may serve as an environment for adjusting and optimizing UFPV tabulation strategies prior to their application to more complex 3D CFD simulations such as spray LES.

Reaction-Diffusion Manifolds (REDIM) Method for Ignition by Hot Gas and Spark Ignition Processes in Counterflow Flame Configurations

ABSTRACT Ignition processes in laminar counterflow configuration using both detailed and reduced chemistry based on the Reaction-Diffusion Manifolds (REDIM) method are investigated. We choose in this work: (i) ignition by hot gas and (ii) spark ignition. The temporal development of thermo-kinetic quantities (e.g. temperatures and species concentrations) are compared. It is demonstrated that the two-dimensional REDIM reduced chemistry is accurate enough to predict the time evolution of different thermo-kinetic quantities compared with the detailed solutions for flows perturbed by different strain rates. Furthermore, the two-dimensional REDIM model for the spark ignition process can reproduce values of the minimal ignition energy very well. This confirms the concept and quality of the REDIM reduced model, which can be then further applied for turbulent flame simulations.