Experimental and numerical investigation of temperature fluctuations in the near-wall region of an optical reciprocating engine

Accurate prediction of in-cylinder heat transfer processes within internal combustion engines (ICEs) requires a comprehensive understanding of the boundary layer effects in the near-wall region (NWR). This study investigates near-wall temperature fluctuations of an optical reciprocating engine using a combined approach of planar laser-induced fluorescence (PLIF) thermometry and numerical conjugate heat transfer modeling. Single-line excitation of toluene and subsequent one-color emission detection is employed for PLIF thermometry, while large-eddy simulations (LES) using commercial CFD software (CONVERGE v2.4.18) is utilized for modeling. The PLIF signal is calibrated to predicted in-cylinder temperatures from a GT-POWER simulation, and precision uncertainty of temperature is found to be ±1.5 K within the calibration region. Near-wall temperature fluctuations are determined about the multi-cycle mean, and the development of thermal stratification is captured in the NWR under motored and fired conditions during the compression stroke. Regions of the largest cycle-to-cycle temperature fluctuations are identified closer to the in-cylinder head surface indicating the unsteadiness of the thermal boundary layer. Analysis includes an assessment of cyclic variability of near-wall temperature fluctuation, and the effects of compression on temperature fluctuations. Additionally, thermal stratification is found to be similar under motored and fired conditions before ignition timing. Lastly, spatial correlation analysis of temperature fluctuations is performed in the wall-normal direction, and it reveals higher correlations under fired conditions. Spatial correlations experience an initial drop outside of the buffer layer in the NWR, and the location of the drop is well captured in the simulations. Analysis of fluctuating temperatures needs to be extended to fluctuations about the spatial average temperature which directly affects the spatial thermal gradients relevant to engine heat transfer.     

A numerical modeling of the acceleration of a flame by an additional energy input ahead of its front

The acceleration of a flame after an additional energy input ahead of its front was simulated using numerical methods. The combustion of a hydrogen-air mixture in a semiopen channel was considered. The calculations were performed within the framework of a two-dimensional hydrodynamic model of premixed flames, with consideration given to heat transfer, multicomponent diffusion, and chemical kinetics. It was demonstrated that, when the interaction of the flame front with the near-wall boundary layer is taken into account, even a moderate energy input could substantially promote the development of the Landau-Darrieus instability and, possibly, deflagration to detonation transition.     

Control of boundary layer on a flat plate by means of cavity flow

This paper describes the control of boundary layer on a flat plate by means of cavity flow. In this study, classifying the shapes of cavities into circular arc, rectangle and triangle makes the discussion, and the depths of the cavities are changed systematically. It is made clear, by numerical calculations and experiments, what states of flow are shown in the internal parts of cavities and what kinds of influence are exerted to the boundary layers in the upstream and downstream flows. As a result, the following facts are made clear. By taking up properly the depth ratios (k/c) of cavities complying with the individual cavity shapes, the boundary layer thickness in the flow upstream and downstream of the cavities can be controlled. Meanwhile with any of the cavities, the existence of the minimum boundary layer thickness (δ/δ _m ) _min min is seen at a depth ratio.     

Propagation of ion shock in solid DT target with nonlinear force-driven plasma blocks

The plasma block (piston) with pressure P ₁ is generated as a result of the nonlinear (ponderomotive) force in laser–plasma interaction. The plasma block can be used for the ignition of a fusion flame front in a solid density deuterium–tritium (DT) target by compressing the fuel that creates an ion shock propagating with velocity u _{ion? shock} in the inside of a solid DT target. The ignition is achieved by creating an ion shock during the final stages of the implosion. We estimated the effect of an ion shock in solid DT target at an early stage with no compression and at the last stage with compression, where density increases by a factor of solid-state density. According to the theoretical model, a large target with a very thin layer of fuel (high-aspect ratio target) would be ideal to obtain the very strong shocks. Results indicate that the maximum compression even by an infinitely strong single shock can never produce more than four times the initial density of DT fuel. The results reported that the threshold ignition energy in a solid DT target is reduced by a factor of 4.     

Simultaneous stereo‐PIV and OH×CH2O PLIF measurements in turbulent ultra lean CH4/H2 swirling wall‐impinging flames

We present experimental results from turbulent low-swirl lean H₂/CH₄ flames impinging on an inclined, cooled iso-thermal wall, based on simultaneous stereo-PIV and OH×CH₂O PLIF measurements. By increasing the H₂ fraction in the fuel while keeping Karlovitz number (Ka) fixed in a first series of flames, a fuel dependent near-wall flame structure is identified. Although Ka is constant, flames with high H₂ fraction exhibit significantly more broken reaction zones. In addition, these high H₂ fraction flames interact significantly more with the wall, stabilizing through the inner shear layer and well inside the near-wall swirling flow due to a higher resistance to mean strain rate. This flame-wall interaction is argued to increase the effective local Ka due to heat loss to the wall, as similar flames with a (near adiabatic) ceramic wall instead of a cooled wall exhibit significantly less flame brokenness. A second series of leaner flames were investigated near blow-off limit and showed complete quenching in the inner shear layer, where the mean strain rate matches the extinction strain rate extracted from 1D flames. For pure CH₄ flames (Ka ≈ 30), the reaction zone remains thin up to the quenching point, while conversely for the 70% H₂ flames (Ka ≈ 1100), the reaction zone is highly fragmented. Remarkably, in all near blow-off cases with CH₄ in the fuel, a large cloud of CH₂O persists downstream the quenching point, suggesting incomplete combustion. Finally, ultra lean pure hydrogen flames were also studied for equivalence ratios as low as 0.22, and through OH imaging, exhibit a clear transition from a cellular flame structure to a highly fragmented flame structure near blow-off.     

Two-dimensional cycle-resolved exhaust valve temperature measurements in an optically accessible internal combustion engine using thermographic phosphors

Phosphor thermometry was employed to measure the temperature distribution of the exhaust valves in an optically accessible direct injection internal combustion engine. A CMOS high-speed camera was used to two-dimensionally resolve the temperature dependent luminescence decay of the phosphor Gd₃Ga₅O₁₂:Cr. Measurements were performed under motored and fired conditions for several degrees crank angle to determine the temperature distributions within cycles. Additionally, several binders have been tested in terms of survivability and signal strength to guarantee ideal phosphor coating.     

Numerical Investigation of Heat Transfer Characteristics of scCO2 Flowing in a Vertically-Upward Tube with High Mass Flux

In this work, the heat transfer characteristics of supercritical pressure CO₂ in vertical heating tube with 10 mm inner diameter under high mass flux were investigated by using an SST k-ω turbulent model. The influences of inlet temperature, heat flux, mass flux, buoyancy and flow acceleration on the heat transfer of supercritical pressure CO₂ were discussed. Our results show that the buoyancy and flow acceleration effect based on single phase fluid assumption fail to explain the current simulation results. Here, supercritical pseudo-boiling theory is introduced to deal with heat transfer of scCO₂. scCO₂ is treated to have a heterogeneous structure consisting of vapor-like fluid and liquid-like fluid. A physical model of scCO₂ heat transfer in vertical heating tube was established containing a gas-like layer near the wall and a liquid-like fluid layer. Detailed distribution of thermophysical properties and turbulence in radial direction show that scCO₂ heat transfer is greatly affected by the thickness of gas-like film, thermal properties of gas-like film and turbulent kinetic energy in the near-wall region. Buoyancy parameters Bu < 10⁻⁵, Bu* < 5.6 × 10⁻⁷ and flow acceleration parameter K_v < 3 × 10⁻⁶ in this paper, which indicate that buoyancy effect and flow acceleration effect has no influence on heat transfer of scCO₂ under high mass fluxes. This work successfully explains the heat transfer mechanism of supercritical fluid under high mass flux.     

Asymptotics of unit Burger number rotating and stratified flows for small aspect ratio

Rotating and stably stratified Boussinesq flow is investigated for Burger number unity in domain aspect ratio (height/horizontal length) δ<1 and δ=1. To achieve Burger number unity, the non-dimensional rotation and stratification frequencies (Rossby and Froude numbers, respectively) are both set equal to a second small parameter ?<1. Non-dimensionalization of potential vorticity distinguishes contributions proportional to (?δ)⁻¹, δ⁻¹ and O(1). The (?δ)⁻¹ terms are the linear terms associated with the pseudo-potential vorticity of the quasi-geostrophic limit. For fixed δ=1/4 and a series of decreasing ?, numerical simulations are used to assess the importance of the δ⁻¹ contribution of potential vorticity to the potential enstrophy. The change in the energy spectral scalings is studied as ? is decreased. For intermediate values of ?, as the flow transitions to the (δ?)⁻¹ regime in potential vorticity, both the wave and vortical components of the energy spectrum undergo changes in their scaling behavior. For sufficiently small ?, the (δ?)⁻¹ contributions dominate the potential vorticity, and the vortical mode spectrum recovers k⁻³ quasi-geostrophic scaling. However, the wave mode spectrum shows scaling that is very different from the well-known k⁻¹ scaling observed for the same asymptotics at δ=1. Visualization of the wave component of the horizontal velocity at δ=1/4 reveals a tendency toward a layered structure while there is no evidence of layering in the δ=1 case. The investigation makes progress toward quantifying the effects of aspect ratio δ on the ?→0 asymptotics for the wave component of unit Burger number flows. At the lowest value of ?=0.002, it is shown that the horizontal kinetic energy spectral scalings are consistent with phenomenology that explains how linear potential vorticity constrains energy in the limit ?→0 for fixed δ.     

Two-stage autoignition and combustion mode evolution in boundary layer flows above a cold flat plate

Boundary layers are omnipresent in fundamental kinetic experimental facilities and practical combustion engines, which can cause ambiguity and misleading results in kinetic target acquisition and even abnormal engine combustion. In this paper, using n-heptane as a representative large hydrocarbon fuel exhibiting pronounced low-temperature chemistry (LTC), two-dimensional numerical simulation is conducted to resolve the transient autoignition phenomena affected by a boundary layer. We focus on the ignition characteristics and the subsequent combustion mode evolution of a hot combustible mixture flowing over a colder flat plate in an isobaric environment. For cases with autoignition occurring within the boundary layer, similarity is observed in the first-stage ignition as manifested by a constant temperature at all locations. The first-stage ignition is found to be rarely affected by heat and radical loss within the boundary layer. While for the main ignition event, an obvious dependence of ignition process on boundary layer thickness is identified, where the thermal-chemical process exhibits similarity at locations with similar boundary layer thickness, and the main ignition tends to first occur within the boundary layer at the domain end and generates a C-shape reaction front. It is found that sequential spontaneous autoignition is the dominant subsequent combustion mode at high-pressure conditions. At low to intermediate pressures, auto-ignition assisted flame propagation is nevertheless the dominant mode for combustion evolution. This research identifies novel features of autoignition and the subsequent combustion mode evolution affected by a cold, fully developed boundary layer, and provides useful guidance to the interpretation of abnormal combustion and combustion mode evolution in boundary layer flows.     

Numerical evaluation of boundary layer assumptions for laminar diffusion flames in microgravity

A systematic numerical study of a low Reynolds number laminar diffusion flame is presented. The configuration used is that of a boundary layer flow established over a flat plate burner. The importance of this configuration relies on its potential use for the assessment of the flammability of materials to be used in space vehicles. The study focuses on the validity of boundary layer formulations to the study of these flames. The characteristic velocities are representative of microgravity environments [O(100 mm/s)]. Parietal injection results eventually in flow separation establishing two characteristic regimes: non-separated and separated flows. Non-separated flows show an increased local acceleration but allow the use of two-dimensional assumptions at the plane of symmetry. It was demonstrated that classical boundary layer assumptions can be used if the flow is non-separated. Three-dimensional flow fields at the trailing edge of the injection zone characterize separated flows. Energy release enhances the positive pressure perturbations and leads to flow acceleration that cannot be damped by viscous shear. Acceleration appears at the vicinity of the flame, thus it is dramatically amplified by the decreased density in this region. Significant errors are generated if boundary layer assumptions are used to describe diffusion flames established in separated flows.     

Stability of supersonic boundary layer under the influence of heavy gas injection: experimental study

Experiments were performed on the influence of distributed injection of a heavy gas (elegas SF₆) into the near-wall region of the supersonic (freestream Mach number М_∞ = 2) boundary layer on its stability in relation to natural disturbances. Heavy gas injection for the case of linear development of disturbance field results in boundary layer stabilization. It was experimentally proved that the elegas injection can suppress disturbances at the frequencies higher than 15 kHz for the tested range of the streamwise coordinate.     

Flame acceleration process and detonation transition in a channel with roughness elements on a wall

We experimentally investigated the effect of small roughness elements, which could be regarded as the wall roughness, on flame acceleration and deflagration-to-detonation transition (DDT). Our previous experiments (Maeda et al., 2019) using the sandpaper-like irregular roughness indicated that the flame acceleration and the associated DDT were greatly enhanced by the roughness. In this study, CH* chemiluminescence imaging as well as schlieren imaging was conducted in parallel with pressure measurements using an ethylene-oxygen combustion in the channel (486 mm long, 10 mm square cross-section) with the regular roughness (square pyramid elements with a base length and a height of 1 mm) in order to directly link the interference between the flow-field affected by the roughness and the propagating flame surface resulting the enhancement of chemical reactions, whereas the schlieren imaging alone could not allow to discuss the chemical reaction field in the previous study. After the leading shock wave was formed by the initial finger flame acceleration process, multiple interactions were observed on the flame front with the flow-field and pressure disturbances of the unreacted gas near the roughness elements. The results provided clear evidence that the roughness emphasized the effect of boundary layer, and the region where the disturbance layer and the flame were interacting coincided with the strong chemical reaction in the chemiluminescence image, indicating increase of the flame surface area caused by the turbulence on the flame front, which was also validated by the rough estimation of the burning velocity. The detonation onset was observed at the flame surface near the wall with the roughness elements. The possible factors of the final detonation transition were deduced to be the hot spot formation based on the multiple interactions of pressure waves with the roughness elements and entrainment of the unreacted gas of the highly turbulent flame front.     

Thermal boundary layer for non-Newtonian fluid

Summary Analytical and numerical solutions for the momentum and thermal boundary layer equations of a non-Newtonian power law fluid are presented. The flow is assumed to be under the influence of an external magnetic fieldB (x) applied perpendicular to the surface and an electric fieldE(x) perpendicular toB(x) and the direction of the longitudinal velocity in the boundary layer. For the power law fluid it is assumed that the shear stress is proportional to then-th power of the velocity gradient andn is called the flow index. The variations of the velocity fieldf′, the temperature field θ, the shear stress on the surfaceτ _W , the displacement thicknessδ ₁ and the momentum thicknessδ ₂ with the magnetic-field parameter γ, the flow indexn, the heat transfer indexS and the Prandtl number Pr are studied. It is found that, if the outer flow velocityU(x) (potential flow) is proportional to the arc lengthx raised to a powerm, then the similarity solution for the thermal boundary layer equation is possible only whenm=1/3, which represents flow past a wedge of included angle π/2. It is established that the temperature of the wedge increases with the increase of γ, Pr,S and the decrease ofn. In general the magnetic field can be used as a heater for the surface of the wedge.     

On the mechanism of the deflagration-to-detonation transition in a hydrogen-oxygen mixture

The flame acceleration and the physical mechanism underlying the deflagration-to-detonation transition (DDT) have been studied experimentally, theoretically, and using a two-dimensional gasdynamic model for a hydrogen-oxygen gas mixture by taking into account the chain chemical reaction kinetics for eight components. A flame accelerating in a tube is shown to generate shock waves that are formed directly at the flame front just before DDT occurred, producing a layer of compressed gas adjacent to the flame front. A mixture with a density higher than that of the initial gas enters the flame front, is heated, and enters into reaction. As a result, a high-amplitude pressure peak is formed at the flame front. An increase in pressure and density at the leading edge of the flame front accelerates the chemical reaction, causing amplification of the compression wave and an exponentially rapid growth of the pressure peak, which “drags” the flame behind. A high-amplitude compression wave produces a strong shock immediately ahead of the reaction zone, generating a detonation wave. The theory and numerical simulations of the flame acceleration and the new physical mechanism of DDT are in complete agreement with the experimentally observed flame acceleration, shock formation, and DDT in a hydrogen-oxygen gas mixture.     

Experimental and numerical study of a conical turbulent partially premixed flame

The structure and dynamics of a turbulent partially premixed methane/air flame in a conical burner were investigated using laser diagnostics and large-eddy simulations (LES). The flame structure inside the cone was characterized in detail using LES based on a two-scalar flamelet model, with the mixture fraction for the mixing field and level-set G-function for the partially premixed flame front propagation. In addition, planar laser induced florescence (PLIF) of CH and chemiluminescence imaging with high speed video were performed through a glass cone. CH and CH₂O PLIF were also used to examine the flame structures above the cone. It is shown that in the entire flame the CH layer remains very thin, whereas the CH₂O layer is rather thick. The flame is stabilized inside the cone a short distance above the nozzle. The stabilization of the flame can be simulated by the triple-flame model but not the flamelet-quenching model. The results show that flame stabilization in the cone is a result of premixed flame front propagation and flow reversal near the wall of the cone which is deemed to be dependent on the cone angle. Flamelet based LES is shown to capture the measured CH structures whereas the predicted CH₂O structure is somewhat thinner than the experiments.     

Effects of cryogenic temperature on premixed hydrogen/air flame propagation in a closed channel

As a carbon-free fuel, hydrogen has received significant attention recently since it can help enable low-carbon-economy. Hydrogen has very broad flammability range and very low minimum ignition energy, and thereby there are severe safety concerns for hydrogen transportation and utilization. Cryo-compressed hydrogen is popularly used in practice. Therefore, it is necessary to investigate the combustion properties of hydrogen at extremely low or cryogenic temperatures. This study aims to assess and interpret the effects of cryogenic temperature on premixed hydrogen/air flame propagation and acceleration in a thin closed channel. Different initial temperatures ranging from normal temperature (T₀ = 300 K) to cryogenic temperature (T₀ = 100 K) are considered. Both one- and two-dimensional hydrogen/air flames are investigated through transient simulations considering detailed chemistry and transport. It is found that when the initial temperature decreases from T₀ = 300 K to T₀ = 100 K, the expansion ratio and equilibrium pressure both increase substantially while the laminar flame speeds relative to unburned and burned gasses decrease moderately. The one-dimensional flame propagation is determined by laminar flame speed and thereby the combustion duration increases as the initial temperature decreases. However, the opposite trend is found to happen to two-dimensional flame propagation, which is mainly controlled by the flame surface area increase due to the no-slip side wall constraint and flame instability. Based on the change in flame surface area, three stages including the initial acceleration, steady burning and rapid acceleration are identified and investigated. It is demonstrated that the large expansion ratio and high pressure rise at cryogenic temperatures can significantly increase the flame surface area in early stage and promote both Darrieus-Landau instability (hydrodynamic instability) and Rayleigh-Taylor instability in later stage. These two instabilities can substantially increase the flame surface area and thereby accelerate flame propagation in hydrogen/air mixtures at cryogenic temperatures. The present study provides useful insights into the fundamental physics of hydrogen flames at extremely low temperatures, and is closely related to hydrogen safety.     

The optimal path of piston motion for Otto cycle with linear phenomenological heat transfer law

An Otto cycle engine with internal and external irreversibilities of friction and heat leakage, in which the heat transfer between the working fluid and the environment obeys linear phenomenological heat transfer law [q ∝△(T -1)], is studied in this paper. The optimal piston motion trajectory for maximizing the work output per cycle is derived for the fixed total cycle time and fuel consumed per cycle. Optimal control theory is applied to determine the optimal piston trajectories for the cases of with and w...     

Analysis of the temporal flame kernel development in an optically accessible IC engine using high-speed OH-PLIF

The investigation of the combustion process of a direct injection spark-ignition internal combustion (IC) engine is crucial in modern engine development. The present study is aimed at inspecting the temporal development of the spark induced flame kernel within single combustion cycles using high-speed planar laser-induced fluorescence (PLIF). The analysis is based upon the excitation of OH radicals, which are an indicator of the transient flame front. To achieve an adequate temporal resolution of the early combustion phase, the image sampling rate was set to 6 kHz, recording one image per crank-angle (CA) degree at 1000 rpm. A further feature of the technique is a large field of view spanning ∼54×53 mm. The performance of the transient combustion process is characterized by temporally tracking subsequential engine cycles individually. Flame front dynamics with different dilution levels of the intake air, simulating exhaust gas recirculation (EGR) are investigated. Resolving flame front dynamics especially with varying EGR is an important step towards an improved understanding of cyclic variations and pollutant formation.     

Extinguishment of propane/air co-flowing diffusion flames by fine water droplets

In the present study, extinguishment of propane/air co-flowing diffusion flame by fine water droplets was investigated experimentally. Water droplets are generated by piezoelectric atomizers with the maximum droplets flow rate of 1500 ml/h. When the fuel injection velocity U_f is low, an attached laminar diffusion flame with a premixed flame at the base is stabilized. At some distance from the burner rim, a transition from laminar to turbulent diffusion flame occurs, and a turbulent diffusion flame is formed in the downstream region. When the fuel injector rim is thin (δ = 0.5 mm), the flame stability deteriorates with increase of the co-flowing air stream velocity U_a and the water droplets flow rate Q_m. The stability mechanism can be explained by the balance of the gas velocity and the burning velocity of premixed flame formed at the base. However, when the injector rim is thick (δ = 5 mm), a recirculation zone is produced downstream of the injector rim. The dependence of the quenching distance H_q on U_f and Q_m is relatively weak, and the stability diagram shows curious features. It was shown that U_a is crucially important since it determines flow residence time; if U_a < 0.4 m/s, water droplets can evaporate when they go by the recirculation zone, and the water vapor can diffuse into the recirculation zone. However, if U_a > 0.4 m/s, the water droplets should pass by the recirculation zone without sufficiently evaporated and are not so effective to extinguish the flame. The supply velocity of droplet-laden air should be low enough so that water droplets can evaporate and water vapor can diffuse into the premixed region at the base to obtain sufficient effectiveness of water droplets for fire suppression.     

Nonlinear hydrodynamic corrections to supersonic F-KPP wave fronts

We study the hydrodynamic corrections to the dynamics and structure of an exothermic chemical wave front of Fisher-Kolmogorov-Petrovskii-Piskunov (F-KPP) type which travels in a one-dimensional gaseous medium. We show in particular that its long time dynamics, cut-off sensitivity and leading edge behavior are almost entirely controlled by the hydrodynamic front speed correction δU_h which characterizes the pushed nature of the front. Reducing the problem to an effective comoving heterogeneous F-KPP equation, we determine two analytical expressions for δU_h: an accurate one, derived from a variational method, and an approximate one, from which one can assess the δU_h sensitivity to the shear viscosity and heat conductivity of the fluid of interest.