Experimental investigation of the limits of ethanol combustion in the boundary layer behind an obstacle

Experimental data on the flow structure and mass transfer near the boundaries of the region existence of the laminar and turbulent boundary layers with combustion are considered. These data include the results of in-vestigation on reacting flow stability at mixed convection, mass transfer during ethanol evaporation “on the floor” and “on the ceiling”, when the flame surface curves to form the large-scale cellular structures. It is shown with the help of the PIV equipment that when Rayleigh–Taylor instability manifests, the mushroom-like structures are formed, where the motion from the flame front to the wall and back alternates. The cellular flame exists in a narrow range of velocities from 0.55 to 0.65 m/s, and mass transfer is three times higher than its level in the standard laminar boundary layer.     

Concept and combustion characteristics of ultra-micro combustors with premixed flame

Under micro-scale combustion influenced by quenching distance, high heat loss, shortened diffusion characteristic time, and flow laminarization, we clarified the most important issues for the combustor of ultra-micro gas turbines (UMGT), such as high space heating rate, low pressure loss, and premixed combustion. The stability behavior of single flames stabilized on top of micro tubes was examined using premixtures of air with hydrogen, methane, and propane to understand the basic combustion behavior of micro premixed flames. When micro tube inner diameters were smaller than 0.4 mm, all of the fuels exhibited critical equivalence ratios in fuel-rich regions, below which no flame formed, and above which the two stability limits of blow-off and extinction appeared at a certain equivalence ratio. The extinction limit for very fuel-rich premixtures was due to heat loss to the surrounding air and the tube. The extinction limit for more diluted fuel-rich premixtures was due to leakage of unburned fuel under the flame base. This clarification and the results of micro flame analysis led to a flat-flame burning method. For hydrogen, a prototype of a flat-flame ultra-micro combustor with a volume of 0.067 cm³ was made and tested. The flame stability region satisfied the optimum operation region of the UMGT with a 16 W output. The temperatures in the combustion chamber were sufficiently high, and the combustion efficiency achieved was more than 99.2%. For methane, the effects on flame stability of an upper wall in the combustion chamber were examined. The results can be explained by the heat loss and flame stretch.     

Stability and emissions control using air injection and H2 addition in premixed combustion

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H₂ pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO_x emission benefits from the air jet, while, with H₂ addition, NO_x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO_x. The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO_x reduction.     

An experimental study of partially premixed flame sound

The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in the causes, mechanisms, suppression, and control of combustion noise. Noise generated by enclosed flames is of greater practical interest but is more complicated than that by open flames, which itself is not clearly understood. Studies have shown that different modes of combustion, premixed and non-premixed, differ in their sound generation characteristics. However, there is lack of understanding of the region bridging these two combustion modes. This study investigates sound generation by partially premixed flames. Starting from a non-premixed flame, air was gradually added to achieve partial premixing while maintaining the fuel flow rate constant. Methane, ethylene, and ethane partially premixed flames were studied with hydrogen added for flame stabilization. The sound pressure generated by methane partially premixed flames scales with M⁵ compared to M³ for turbulent non-premixed methane flames. Also, the sound pressure generated by partially premixed flames of ethane and ethylene scales as M^4.5. With progressive partial premixing, spectra level increases at all frequencies with a greater increase in the high-frequency region compared to the low-frequency region; flames develop a peak and later a constant level plateau in the low frequency region. The partially premixed flames of methane, ethylene, and ethane generate a similar SPL as a function of equivalence ratio when the fuel volume flow rate is matched. However, when fuel mass flow rate is matched, the ethane and ethylene flames produce a similar SPL, which is lower than that produced by the methane flame.     

超临界压力CO2在水平圆管内流动传热数值分析

采用SST k-w低雷诺数湍流模型对加热条件下超临界压力CO2在内径di=22.14 mm,加热长度Lh=2440 mm水平圆管内三维稳态流动与传热特性进行了数值计算.通过超临界CO2在水平圆管内的流动传热实验数据验证了数值模型的可靠性和准确性.首先,研究了超临界压力CO2在水平圆管内的流动传热特点,基于超临界CO2在类临界温度Tpc处发生类液-类气“相变”的假设,揭示了水平圆管顶母线和底母线区域不同的流动传热行为.然后,分析了热流密度qw和质量流速G对水平圆管内超临界压力CO2流动换热的影响,通过获取流体域内的物性分布、速度分布和湍流分布等详细信息,重点解释了不同热流密度qw和质量流速G下顶母线内壁温度Tw,i分布产生差异的传热机理,分析结果确定了类气膜厚度d、类气膜性质、轴向速度u和湍动能k是影响顶母线壁温分布差异的主要因素.研究结果可以为超临界压力CO2换热装置的优化设计和安全运行提供理论指导.     

Turbulence/flame/wall interactions in non-premixed inclined slot-jet flames impinging at a wall using direct numerical simulation

In the present work, three-dimensional turbulent non-premixed oblique slot-jet flames impinging at a wall were investigated using direct numerical simulation (DNS). Two cases are considered with the Damköhler number (Da) of case A being twice that of case B. A 17 species and 73-step mechanism for methane combustion was employed in the simulations. It was found that flame extinction in case B is more prominent compared to case A. Reignition in the lower branch of combustion for case A occurs when the scalar dissipation rate relaxes, while no reignition occurs in the lower branch for case B due to excessive scalar dissipation rate. A method was proposed to identify the flame quenching edges of turbulent non-premixed flames in wall-bounded flows based on the intersections of mixture fraction and OH mass fraction iso-surfaces. The flame/wall interactions were examined in terms of the quenching distance and the wall heat flux along the quenching edges. There is essentially no flame/wall interaction in case B due to the extinction caused by excessive turbulent mixing. In contrast, significant interactions between flames and the wall are observed in case A. The quenching distance is found to be negatively correlated with wall heat flux as previously reported in turbulent premixed flames. The influence of chemical reactions and wall on flow topologies was identified. The FS/U and FC/U topologies are found near flame edges, and the NNN/U topology appears when reignition occurs. The vortex-dominant topologies, FC/U and FS/S, play an increasingly important role as the jet turbulence develops.     

Flame/flow dynamics at the piston surface of an IC engine measured by high-speed PLIF and PTV

Resolving fluid transport at engine surfaces is required to predict transient heat loss, which is becoming increasingly important for the development of high-efficiency internal combustion engines (ICE). The limited number of available investigations have focused on non-reacting flows near engine surfaces, while this work focuses on the near-wall flow-field dynamics in response to a propagating flame front. Flow-field and flame distributions were measured simultaneously at kHz repetition rates using particle tracking velocimetry (PTV) and planar laser induced fluorescence (PLIF) of sulfur dioxide (SO₂). Measurements were performed near the piston surface of an optically accessible engine operating at 800?rpm with homogeneous, stoichiometric isooctane-air mixtures. High-speed measurements reveal a strong interdependency between near-wall flow and flame development which also influences subsequent combustion. A conditional analysis is performed to analyze flame/flow dynamics at the piston surface for cycles with ‘weak’ and ‘strong’ flow velocities parallel to the surface. Faster flame propagation associated with higher velocities before ignition demonstrates a stronger flow acceleration ahead of the flame. Flow acceleration associated with an advancing flame front is a transient feature that strongly influences boundary layer development. The distance from the wall to 75% maximum velocity (δ₇₅) is analyzed to compare boundary layer development between fired and motored datasets. Decreases in δ₇₅ are strongly related to flow acceleration produced by an approaching flame front. Measurements reveal strong deviations of the boundary layer flow between fired and motored datasets, emphasizing the need to consider transient flow behavior when modeling boundary layer physics for reacting flows.     

HEAT AND MASS TRANSFER AND WALL SHEAR STRESS IN VERTICAL GAS-LIQUID FLOW

Results of an experimental investigation of heat and mass transfer and wall shear stress at gas-liquid flow in a vertical tube are presented. Local wall shear stress and mass transfer coefficients were measured by an electrochemical method. Experiments were performed in the range of Reynolds number variation with respect to liquid Rc_i, = 8.5 × 10³-5.4 × 10⁴, gas Re_g = 3 × 10³-1.4 × 10⁵, pressure 0.1-1 MPa. The relationship between heat and mass transfer and wall shear at gas-liquid flows is shown to exist. The results of measuring heat and mass transfer coefficients are generalized by formulas applied to calculate heat and mass transfer in single-phase turbulent flow.     

Evaporative Heat Transfer Characteristics of R404a and R134a under Varied Heat Flux Conditions

The two-phase heat transfer coefficients of R404A and R134a in a smooth tube of 7.49-mm inner diameter were experimentally investigated at low heat and mass flux conditions. The test section is a 10-m-long counter-flow horizontal double-tube heat exchanger with refrigerant flow inside the tube and hot fluid in the annulus. The heat transfer coefficients along the length of the test section were measured experimentally under varied heat flux conditions between 4 and 18 kW m^?2 and mass flux ranging between 57 and 102 kg m^?2 s^?1 (2.5 to 4.5 g s^?1) for saturation temperatures of ?10°C, ?5°C, and 0°C. The saturation temperatures correspond to pressures of 4.4, 5.2, and 6.1 bar for R404A and 2.0, 2.4, and 3.0 bar for R134a, respectively. The results showed that under the tested conditions, the contribution of the nucleate boiling mechanism is predominant in the heat transfer coefficient throughout the flow boiling process. The Kattan–Thome–Favrat flow pattern maps confirm the occurrence of stratified and stratified-wavy flow patterns for all of the tested conditions. The average heat transfer coefficient of R404A is estimated to be 26 to 30% higher than that of R134a for the same saturation temperature.     

Effect of thermo-hydrodynamic instability on structure and characteristics of filtration combustion wave of solid fuel

Numerical modelling of flame front stability for the inverse wave (with trailing combustion front) of filtration combustion of solid fuel is performed. The problem is treated in terms of dimensionless variables and parameters. It is found that propagation of a plane combustion front becomes unstable under certain conditions. In this case the front spontaneously inclines. The thermo-hydrodynamic mechanism is supposed to be responsible for instability developing. Anisotropic effective mass diffusivity (dispersion) is also taken into account. It turns out that anisotropic diffusivity affects structure and conversion distribution of the inclined combustion front. It is shown that the key parameters determining stability of combustion wave are dimensionless gas flow rate and width of reactor. The range of these parameters corresponding to the stable plane front is determined. It is shown that stability occurs either for small reactor widths (dimensionless values <1), or low gas flow rate (below 0.2). The optimised values of considered dimensionless parameters for maximal productivity are determined.     

W型火焰燃烧的实验研究

本文设计了1 MW“W型”火焰煤粉燃烧实验台,并对焦作无烟煤进行了热态试验,分析了炉膛最高温度(火焰中心)的位置对W火焰的稳定形成的重要性,通过实验测得了其合理的相关位置。实验结果表明: W型火焰燃烧有很强的低负荷稳燃性,特别适合于低挥发份煤的燃烧。当火焰中心位置处于最下层二次风处时,炉内才能形成较好的W火焰;下炉膛中前后墙的壁面热负荷较为均匀,而左右墙的壁面热负荷分布呈“中间温度高两边温度低”的特性;上炉膛左右墙与前后墙的壁面温度分布基本一致。     

Stabilized,flat iron flames on a hot counterflow burner

Metal powder combustion has traditionally been studied to mitigate the risk of industrial accidents and to determine the contributions of metals as additives to the performance of energetic materials. Recently, there has been growing interest in exploring the potential of metal powders as recyclable, zero-carbon energy carriers as an alternative to the hydrocarbons known to contribute to climate change. The present work introduces, for the first time, a stabilized flat iron flame. The counterflow burner used in this work is comprised of an inverted ceramic nozzle which sits above, and is aligned axially with, a lower nozzle producing a laminar flow of particles suspended in an oxidizing gas. A stabilized methane flame sits inside the top nozzle and the hot combustion products impinge upon the two-phase flow from the bottom nozzle, creating a stagnation plane. Spherical iron powder, with 90% of the particles less than 2.5 µm in size, is pre-loaded into a piston and dispersed using mixtures of 30% and 40% oxygen balanced in argon. Flame speeds are measured using particle image velocimetry (PIV), while flame temperatures are determined using multicolour pyrometry. It is found that flame speeds range between 30 cm/s and 45 cm/s for both oxidizing mixtures. Despite having fuel loadings below stoichiometric concentrations, the observed particle combustion temperatures are close to the adiabatic flame temperature of the stoichiometric mixture, indicating combustion in the diffusion-controlled regime for these small particles. Finally, the independence of the flame speeds with respect to oxygen concentration suggests flame propagation in the discrete regime.     

Effect of flow circulation on combustion dynamics of fire whirl

In this paper, the effect of flow circulation on the combustion dynamics of fire whirl is systematically investigated by experiments. New correlations for the burning rate, flame height, radial temperature and mass flow rate are established for fire whirl. It is clarified that flow circulation helps increase both the fuel-flame contact area and the actual fuel surface area, which in turn increases both the heat feedback to the fuel surface and the radial velocity in the ground boundary layer, leading to increase of burning rate. A novel idea for correlation of fire whirl flame height is proposed by assuming that the ratio of the fire whirl flame height to the flame height without circulation solely characterizes the effect of circulation. This idea is fully verified, thereby a new formulation for flame height is established, which successfully decouples the burning rate and the circulation. It is indicated that the fuel-rich core in the flame body of fire whirl significantly affects the radial temperature distribution in the continuous flame region, and the flame body can be described by the combination of a cylinder and a cone. The flow circulation significantly suppresses fire plume radius and thus decreases its increasing rate with vertical distance. It is also demonstrated that the fire whirl flame involves laminarized regions in its lower section, coexisting with turbulent regions in the upper portion. The flow circulation enhances the air entrainment in the ground layer by altering the radial velocity profile and increasing the radial velocity. In the low section of flaming region, the significant decrease of mixture between the combustion products and surrounding air dominates the pure aerodynamic effect of flow circulation on the flame height. Finally, it is clarified that fire whirls maintain higher centerline excess temperature than general pool fires due to the effect of less air entrainment.     

Behaviour of a confined fire located in an unventilated zone

The behaviour of a fire in an enclosure is studied for a configuration where the fuel source is located in the upper hot unventilated zone trapped by a soffit. The experimental study, undertaken in a laboratory-scale compartment with a fuel source above the level of a soffit, included the determination of the parameters (ventilation factor, rate of fuel supply) controlling the combustion or leading to extinction. Measurements (PIV, thermocouples, gas sampling and analysis) were performed to propose a hypothesis on the structure of the flame (flame stabilisation mechanisms, premixed or diffusion types). Video photography is used to determine the area covered by the flames. This information is used as a criterion to identify the combustion regimes. The results show that the gaseous fuel is diluted in the combustion products (CP) in the upper layer and that a recirculatory motion is formed, driven by buoyancy forces, which enhances the mixing of fuel and CP. These then travel horizontally towards the vent along the interface between the lower fresh air and upper zones, and are premixed with the convected air in the enclosure, before entering the reaction zone and being burnt. The flame stabilises at the interface between the upper hot and lower ventilated layers in the compartment. The observed “ghosting flame” is stabilised by a triple flame if the flame speed of the premixed flame is higher than the natural convection velocity induced in the compartment. The flame stability is quantified by a criterion based on the area of the horizontal flame. It has been observed that the combustion is controlled by the available mass fuel flux at the reaction zone if the ventilation is sufficient. This information is essential for the modelling of the phenomena involved in fires with such an underventilated fuel source.     

Wall-impinging laminar premixed n-dodecane flames under autoignitive conditions

Laminar one-dimensional (1D) flames in a stagnation flow stabilised at a wall are used to study flame–wall interaction under diesel engine conditions. The thermochemical conditions correspond to that of the Engine Combustion Network (ECN) Spray A reference case. A range of inflow velocities is considered, where the lowest inflow velocity is chosen such that the flame is detached from the inlet. The presence of a wall is shown to have a significant impact on the flame structure and emission formation. The 1D flame and homogeneous reactor results exhibit two distinct reaction zones due to low- and high-temperature chemistry (LTC and HTC, respectively). The burner-stabilised flames are overall dominated by autoignition for all inflow velocities. For the impinging jet flames, the response of the LTC reaction zone follows closely that of the burner-stabilised flames up to relatively high inflow velocities. The HTC reaction zone, however, deviates strongly from the burner-stabilised flames, already at low inflow velocities and quenches at high inflow velocities. A budget analysis revealed a strong contribution from diffusion in the HTC reaction zone, resulting in an increasing importance of deflagrative combustion as opposed to autoignition. This trend was attributed to enhanced strain rates at higher inlet velocity leading to higher gradients. Wall heat transfer was also investigated. The highest wall heat transfer rates were observed for mixtures between $\Phi =1.0$ and $\Phi =1.5$ and for inlet velocities just below the quenching limit. This was attributed jointly to the higher peak product temperatures for these mixtures and to their enhanced resilience to quenching under strain which leads to higher temperature gradients at the wall just before quenching. NO formation was studied. The highest NO formation was observed near $\Phi =1.0,$ though the response to strain rate was different for stoichiometric and rich mixtures, which was attributed to differing NO formation pathways.     

External combustion of high-speed multicomponent hydrocarbon-air flow under conditions of low-temperature plasma

The stabilization of external combustion (at the plate surface) of high-speed multicomponent (air, alcohol, and propane) flows is realized experimentally. It is shown that heat fluxes during alcohol combustion rise by a factor of about 7 and during propane combustion by a factor of 15, in comparison with the heat flux from a discharge in a high-speed air flow. The electron concentration measured at a distance of 10 cm down-stream from the electrode ends is approximately 10⁹ cm^?3 in the case of the discharge in an air flow, whereas during alcohol combustion it attains 2 × 10¹¹ cm^?3 and during propane combustion it is 3 × 10¹¹ cm^?3. The flame temperature in the area of the discharge existence varies from 2000 up to 2500 K and outside the discharge at the distance of z = 20 cm from electrodes is 1800 K, gradually decreasing downstream. It is shown that the combined discharge in the subsonic flow allows for complete combustion of liquid and gaseous hydrocarbons. The completeness of combustion in supersonic flows attains 95%, depending on the flow velocity.     

Effect of unsteady pressure rise on flame propagation and near-cold-wall ignition

Thermodynamic pressure rise during combustion is a key feature in internal combustion engines. Yet, hardly any studies have been conducted to investigate the effects of transient pressure rise on flame propagation as well as on the ignition of the unburned gas. In this study, the effects of unsteady pressure rise were parametrically studied using a one-dimensional reacting flow model in which the thermodynamic pressure variation is an independent variable and thus its rate of rise can be controlled. It was determined that large rates of pressure rise can significantly increase the mass burning flux of a laminar flame and that this modification becomes more pronounced at higher pressure and temperature conditions. Furthermore, it was shown that the development of ignition near a cold wall, for mixtures that exhibit negative temperature coefficient behavior, is very sensitive to rate of change of pressure. The near-wall ignition behavior was found also to be rather sensitive to the prevailing pressures and temperatures whose values control whether ignition will occur in the main-gas or within the thermal boundary layer.     

Analysis of the influence of nanometric aluminium particle vaporisation on flame propagation in bulk powder media

The combustion of nanometric aluminium (Al) powder with an oxidiser such as molybdenum trioxide (MoO₃) is studied analytically. The analysis was performed to correlate individual Al particle gasification rates to macroscopic flame propagation rates observed in flame tube experiments. Examination of various characteristic times relevant to propagation of a deflagration reveals that particles below about 1.7 nm in diameter evaporate before appreciable chemical reactions occur. Experimental studies used Al particles greater than 1.7 nm in diameter such that a diffusion flame model was developed to better understand the combustion dynamics of multiphase Al particles greater than 1.7 nm diameter relative to experimentally measured macroscopic flame propagation rates. The diffusion flame model predicted orders of magnitude slower propagation rates than experimentally observed. These results imply that (1) another reaction mechanism is responsible for promoting reaction propagation and/or (2) modes other than diffusion play a more dominant role in flame propagation.     

Combustion rates of graphite rods in the forward stagnation field of the high-temperature, humid airflow

Relevant to High-Temperature Air Combustion, an experiment has been conducted to study carbon combustion in the high-temperature oxidizer-flow, by use of a graphite rod set in the forward stagnation field. Effects of humid air and O₂-reduced oxidizer have been examined, after re-confirming that the combustion rate in the high-temperature oxidizer-flow is enhanced, because of elevated transport properties when the mass flow rate of oxidizer is the same, and that it is suppressed, because of reduced mass transfer rates through the thickened boundary layer when the velocity gradient is the same. It is found that high H₂O mass-fraction is favorable for the enhancement of combustion rate at high surface temperatures (>2000 K), because of its participation in the surface C–H₂O reaction, while it is not the case at medium surface temperatures (1400–1700 K), because of the suppressing effect, caused by the establishment of CO flame. As for O₂ and CO₂ concentrations in the high-temperature oxidizer-flow, it is found that O₂ mass-fraction can be reduced without reducing combustion rate in the room-temperature airflow, and that it can further be reduced in existence of enough CO₂ that can be an oxidizer in carbon combustion. Theoretical works have also been conducted for the system with three surface reactions and two global gas-phase reactions. It is found that the Frozen mode without CO flame and the Flame-detached mode with infinitely fast gas-phase reaction can fairly represents combustion behavior before and after the establishment of CO flame, respectively, as far as the trend and the approximate magnitude are concerned. It is also shown that a new mode with suppressing H₂ ejection from the surface can fairly represent the combustion rate, experimentally obtained in humid airflow with relatively low velocity gradient when the surface temperature is high.     

NH3/H2预混旋流火焰稳定性及燃烧极限实验研究

采用叶轮型旋流燃烧器,选取氢气作为燃料添加剂,研究了掺氢比对氨气旋流火焰稳定性的影响,分析了不同旋流数、叶片数、当量比以及预混气总流量条件下,旋流火焰形态变化。测定并分析了不同参数对旋流火焰燃烧极限范围的影响。结果表明,随掺氢比的增大,火焰逐渐由“V”型转化为稳定的“M”型,燃烧反应愈发充分;高旋流数(1.27)或低叶片数(6片)相比低旋流数(0.42)或高叶片数(8片)更有利于旋流火焰的稳定和燃烧的充分进行;相比富燃,贫燃有利于形成稳定的旋流火焰;预混气总流量较大时,火焰高度较高.对于燃烧极限,掺氢比越高,极限范围越大;总流量的变化对贫燃极限影响较小,对富燃极限影响较大;高旋流数(1.27)条件下,燃烧极限范围较大。