Experimental and numerical investigation on combustion characteristics of premixed hydrogen/air flame in a micro-combustor with a bluff body

Combustion characteristics of lean hydrogen/air mixture in a planar micro-channel with a bluff body were investigated experimentally and numerically. Effects of the inlet velocity and equivalence ratio on the blow-off limit, combustion efficiency and exhaust gas temperature were examined. The results show that the blow-off limit is greatly extended as compared with that of the micro-combustor without a bluff body. Moreover, the blow-off limit increases as the equivalence ratio is increased from 0.4 to 0.6. Furthermore, with the increase of inlet velocity, the flame front is prolonged and becomes narrower, and the high temperature segment of outer wall shifts downstream. In addition, the combustion efficiency and exhaust gas temperature increase first and then decrease with the increase of the inlet velocity. Finally, comparatively high combustion efficiency can be maintained over the whole combustible velocity range at a moderate equivalence ratio.     

钝体高度变化对煤粉工业锅炉燃烧器流场特性影响的冷态实验研究

针对某新型适用于燃煤粉工业锅炉的燃烧器,冷态实验研究了其一次风出口处加装的钝体高度改变对燃烧器流场特性的影响。研究结果表明:钝体高度越高并不能使得回流效果越好,适当高度的钝体可以起到增加气流回流的效果。     

Cold flow characteristics of a novel high-hydrogen Micromix model burner based on multiple confluent turbulent round jets

As a promising commercial hydrogen-rich gas turbine combustion technology, micro-mixing combustion has been characterized for its excellent performance with low NOx emissions. New flame stabilization mechanism of micro-mixing flames may produce new design criteria. In order to explore that, cold flow characteristics of a novel Micromix model burner based on multiple confluent round jets has been studied experimentally and numerically, which is considered to be the basis for the exploration. A three-dimensional laser Doppler velocimetry system (3D-LDV) was used to measure the flow field of the model burner. It was found that the cold flow characteristics of the burner were different from the twin plane jets, the twin round jets, and the low Reynolds number confluent round jets. Compared to which, the interior of the micro-mixing nozzle is at a very high turbulence intensity level, and the jets merging point of the burner moved upstream; however, the position of the combined point of the burner was close to the confluent round jets. There is no recirculation region between jets near the burner outlet when the nozzle spacing was equal to 3 times the nozzle diameter and the Reynolds number was less than 16,702. The steady computational Reynolds averaged equations (RANS) model results were used to compare with the experimental results. It was found that the RANS results can match the experimental results well, and the three RANS models predict the spatial mixing deficiency less than 1% at the outlet, indicating that the fuel and air were almost completely premixed uniformly.     

Temperature measurements of the bluff body surface of a Swirl Burner using phosphor thermometry

Flames are often stabilised on bluff-bodies, yet their surface temperatures are rarely measured. This paper presents temperature measurements for the bluff body surface of the Cambridge/Sandia Stratified Swirl Burner. The flame is stabilized by a bluff body, designed to provide a series of turbulent premixed and stratified methane/air flames with a variable degree of swirl and stratification. Recently, modellers have raised concerns about the role of surface temperature on the resulting gas temperatures and the overall heat loss of the burner. Laser-induced phosphorescence is used to measure surface temperatures, with Mg₄GeO₆F:Mn as the excitation phosphor, creating a spatially resolved temperature map. Results show that the temperature of the bluff body is in the range 550–900 K for different operating conditions. The temperature distribution is strongly correlated with the degree of swirl and local equivalence ratio, reflecting the temperature distribution obtained in the gas phase. The overall heat loss represents only a small fraction (<0.5%) of the total heat load, yet the local surface temperature may affect the local heat transfer and gas temperatures.     

Optimization of producer gas fired premixed burner

Much work is reported in the literature pertaining to premixed burners using hydrocarbon fuels. However, very little work is available on similar burners using producer gas as a fuel. The present work aims at testing and optimization of a premixed burner with producer gas as a fuel.A burner of 150 kW capacity is used in the experimental investigations. The burner is of concentric tube type fully premixed in which air is supplied through central pipe and gas is supplied through annular passage. Swirl vane is provided to air and gas for thorough mixing. The bluff body is provided for flame stabilization. The premixed burner was tested on open core throat-less down draft gasifier for flame quality. A stable and uniform flame was observed with this premixed burner. Thereafter, an instrumented test set up to evaluate burner performance was installed on an open core gasifier. The burner was experimentally optimized for size and location of bluff body and flammability limits. The burner was optimized by using bluff bodies of 46, 61, 73, 80, 85, 98, 110 and 122 mm diameters. The burner was operated in batch operation of 6–8 h for optimization of various parameters. The experiment reveled that the uniform and high-temperature premixed flame was observed at conventional bluff body having blockage ratio of 0.65. The flammability limits for producer gas fired burner was established in the range of 40–55.     

Entropy analysis of a flow past a heat‐generated bluff body

Cooling of a bluff body is a topic of interest for many engineers and scientists. Forced convection over the bluff body generates flow separation, which in turn affects the heat transfer characteristics and increases the irreversibilities involved in the system. In the present study, flow over a rectangular solid body with constant heat flux is considered. The governing flow and energy equations are solved in two‐dimensional space numerically using a control volume approach. In order to investigate the effect of the fluid properties on the heating process, three different fluids are taken into account. These are air, ethylene glycol and therminol. To determine the irreversibilities involved in the system, entropy analysis is carried out. It is found that fluid properties have considerable effect on the entropy generation. The entropy generation due to heat transfer well exceeds the entropy generation due to fluid friction. The surface temperature of the solid body highly depends on the cooling fluid employed. Copyright © 1999 John Wiley & Sons, Ltd.     

Efficient study of a coarse structure number on the bluff body during the harvesting of wind energy

In the present study, an energy harvester with coarse passive turbulence control (PTC) structure is represented to harvest piezoelectric wind energy. Wind tunnel experiments are conducted to investigate the influence of the PTC on the vibrational amplitude of the vortex-induced vibration and piezoelectric power output. Parametric studies of the PTC numbers and sizes are presented to determine the optimized PTC structure to enhance the efficiency of piezoelectric energy harvesting. The experimental results show that the specific parameters of θ= 60^° and W = 8 mm are the most efficient PTC group for designing a vortex-induced vibration-based energy harvester with the coarse surface device.     

Mixing and Recirculation Characteristics of A double COncentric Burner with Bluff—Body

The concentric bluff-body jet burner is widely used in industrial combustion systems. This kind of burner often generates a considerably complex recirculation zone behind the bluff body. As a result, the fuel often remains in the recirculation zone, achieving stability of flame. This study investigates, by means of experiments, the variations of the aerodynamics as the fluid is injected into a combustion chamber through a double concentric burner with a bluff-body. The observation and measurement of the aerodynamics in our experiment are conducted under a cold flow. The controlled parameters in our experiment are: variations in the blockage ratio of the center bluff body, the cone angle of the bluff body, and the velocity ratio (U _s/U_p) of the secondary jet and primary jet; the injection of helium bubbles into the primary and secondary jets to observe the recirculation zone behind the bluff body; using Tufts for observing the characteristics of corner recirculation zone in a combustion chamber, measuring the average velocity of each point within the aerodynamics by the 5-hole pitot tube; measuring the distribution of static pressure of the combustion chamber walls with a static pressure tap.     

Large eddy simulation of micro-particles transport with different mass flow rate in turbulent planar jet flow

The motion of micro-particles with different mass flow rate in the planer turbulent jet flow has been simulated,using LES method to obtain the flow vorticity evolution and Lagrangian method to track micro-particles.The results showed that when the flow rate is small,the particles more likely to present in the vortex periphery,the distribution pattern is similar to the flow pattern.When the flow rate is high,some particles will escape from the motion region to the original static region,so that in the jet region,particles are relatively evenly distributed.When the flow field is full developed,the particles average concentration in the y direction affected by the mass flow rate relative slightly,the normalized mean particles concentrations at different flow rate were similar to Gaussian shape.     

Transient supersonic release of hydrogen from a high pressure vessel: A computational analysis

Understanding the characteristics of a hydrogen gas jet exiting from a compressed vessel during vessel rupture or venting is crucial for determining safety requirements for distribution and use of hydrogen. Such jets can undergo several flow regimes during venting, from initial supersonic flow, to transonic, to subsonic flow regimes as the pressure in the vessel decreases. A bow shock wave is a characteristic flow structure during the initial stage of the jet development, and this paper focuses on the development of the bow shock wave and the jet structure behind it. The transient behaviour of an impulsively initiated jet is investigated using unsteady, compressible flow simulations. Both the cases of a hydrogen jet exiting into quiescent hydrogen and of a hydrogen jet exiting into air are presented. The gases are considered to be ideal, and the computational domain is axisymmetric. The jet structure, including the shock wave and flow separation due to an adverse pressure gradient at the nozzle is investigated with a focus on the differences between the single- and multi-component flow scenarios.     

LES analysis of flow and turbulent mixing in an unsteady jet

The flow and mixing process of unsteady jets are fundamentally analyzed by large eddy simulations. The effects of nozzle velocity and turbulence intensity on the turbulent eddy structure and mixing process between the nozzle fluid and ambient fluid were investigated. The results show that a toroidal‐shaped vortex, which emerges around the jet tip, primarily accelerates the entraining flow. Also, increasing the turbulence intensity in the nozzle encourages mixing in the jet without changing the jet‐contour. Furthermore, when the rise‐up time of the initial nozzle velocity is elongated, turbulent mixing is suppressed. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 303–313, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20158     

Large eddy simulation of reacting flow in a hydrogen jet into supersonic cross-flow combustor with an inlet compression ramp

Large eddy simulation (LES) has been performed to investigate transverse hydrogen jet mixing and combustion process in a scramjet combustor model with a compression ramp at inlet to generate shock train. Partially Stirred Reactor (PaSR) sub-grid combustion model with a skeleton of 19 reactions and 9 species hydrogen/air reaction mechanism was used. The numerical solver is implemented in an Open Source Field Operation and Manipulation (OpenFOAM) and validated against experimental data in terms of mean wall pressure. Effects of a shock train induced by the inlet compression ramp on the flame stabilization process are then studied. It can be observed that the interaction of the oblique shock and the jet mixing layer enhance the combustion and stabilize the flame. Symmetrical recirculation zone, which contributes to the flame anchoring of the supersonic transverse jet combustion, is observed in the near wall region of 10 < x/D < 20. The hydrogen fuel is transported from the center of jet plume to the near wall region on both sides of the central plane (z/D = 0) and thus intense combustion near the wall is observed due to the enhanced mixing and shock compression heating. Besides, the jet penetration in the reacting field is different from that in non-reacting case with the influence of the interaction between the reflected oblique shock and the jet shear layer on the windward side.     

Large eddy simulation of swirling jet in a bluff-body burner

The large eddy simulation (LES) is applied to an unconfined swirling flow of an air surrounding a bluff-body having a central jet of air, and the complicated flowfield that involves the recirculation and vortex breakdown is investigated. The Smagorinsky model is used as the sub-grid scale model. The results of the present numerical simulation are compared with the experimental data of the mean and stochastic root mean square (RMS) variations of two velocity components. Although the inflow conditions are specified in a simple manner, the obtained numerical results are in reasonable agreement with the experiments, except for a part of RMS variation values near downstream of the bluff body. The present numerical calculations can successfully reproduce the two characteristics of the flow, i.e., an upstream recirculation zone established just downstream of the burner plane and the additional recirculation zone established at the more downstream location.     

Experimental and numerical investigations of hydrogen jet fire in a vented compartment

Hydrogen fires may pose serious safety issues in vented compartments of nuclear reactor containment and fuel cell systems under hypothetical accidents. Experimental studies on vented hydrogen fires have been performed with the HYKA test facility at Karlsruhe Institute of Technology (KIT) within Work Package 4 (WP4) - hydrogen jet fire in a confined space of the European HyIndoor project. It has been observed that heat losses of the combustion products can significantly affect the combustion regimes of hydrogen fire as well as the pressure and thermal loads on the confinement structures. Dynamics of turbulent hydrogen jet fire in a vented enclosure was investigated using the CFD code GASFLOW-MPI. Effects of heat losses, including convective heat transfer, steam condensation and thermal radiation, have been studied. The unsteady characteristics of hydrogen jet fires can be successfully captured when the heat transfer mechanisms are considered. Both initial pressure peak and pressure decay were very well predicted compared to the experimental data. A pulsating process of flame extinction due to the consumption of oxygen and then self-ignition due to the inflow of fresh air was captured as well. However, in the adiabatic case without considering the heat loss effects, the pressure and temperature were considerably over-predicted and the major physical phenomena occurring in the combustion enclosure were not able to be reproduced while showing large discrepancies from the experimental observations. The effect of sustained hydrogen release on the jet fire dynamics was also investigated. It indicates that heat losses can have important implications and should be considered in hydrogen combustion simulations.     

Numerical study of stratified oil-water two-phase turbulent flow in a horizontal tube

Stratified oil-water two-phase turbulent flow in a horizontal tube is numerically simulated using a volume of fluid model. A single momentum equation is solved throughout the domain. The RNG k-ε model combined with a near-wall low-Re turbulence model is applied to each phase, and a continuum surface force approximation is adopted for the calculation of surface tension. The simulation is performed in a time-dependent way and the final solution which corresponds to steady-state flow is analyzed. Results of pressure loss, slip ratio, local phase fraction profile and the axial velocity profile are verified by experimental data in literature. Based on the numerical results of extensive calculations, the flow field characteristics are explored and correlations for pressure loss and hold-up are presented.     

Numerical investigation of the effect of slit-width on the combustion characteristics of a micro-combustor with a centrally slotted bluff body

The effect of slit-width on the combustion characteristics of a micro-combustor with a centrally slotted bluff body is numerically studied. The non-dimensional fractional slit-width (d) is varied in the range of 0.1–0.8. Simulations are performed for inlet velocities ranging from 6 m/s to 35 m/s. The effect of slit-width on the combustion efficiency is observed to be a function of the inlet velocity. At smaller inlet velocities, the combustion efficiency increases upon increasing the slit-width initially (d ≤ 0.4), decreases (d = 0.4 to 0.5) and monotonously increases from thereon. On the contrary, at higher values of inlet velocity, the combustion efficiency monotonously increases with the increasing slit-width. It is observed that the average exhaust gas temperature increases with the increasing slit-width, reaches a maximum at moderate slit-widths (d = 0.6 and 0.7) and then decreases. It is observed that the local exhaust gas temperature at moderate slit-widths for higher values of inlet velocities follows a bi-modal distribution. It is also observed that the operating range of the micro-combustor with moderate slit-width (d = 0.6 and 0.7) is limited by the phenomenon of longitudinal flame splitting.     

几何对称性处理对自由平面湍射流大涡模拟的影响

研究了几何对称性处理自由平面湍射流大涡模拟的影响,以Re数为113000的平面不可压缩湍射流流动为例,采用Chorin的分步投影法求解大尺度涡运动的Navier-Stokes方程,小尺度涡采用标准Smagorinsky亚格子模式模拟。初始条件采用平面射流无粘流动解,出流速度边界使用Sommerfeld辐射开边界条件处理,计算域的横向外边界使用自由裹入边界条件,对计算域的横向对称中心平面分别采用对称性条件和直接求解两种方法。模拟结果显示,采用对称性条件处理,会抑制自由平面湍射流中拟序结构的生长,阻碍大尺度涡从中心平面的穿透、长时间的统计平均不能给出合理的湍流低阶矩的时均结果。相反,对中心平面进行直接求解的做法能真实再现自由平面射流中涡的合并与破碎过程,得到合理的模拟结果。     

Large eddy simulation of flow and scalar transport in a round jet

Large eddy simulation (LES) was performed for a spatially developing round jet and its scalar transport at four steps of Reynolds number set between 1200 and 1,000,000. A simulated domain, which extends 30 times the nozzle diameter, includes initial, transitional, and established stage of jet. A modified version of convection outflow condition was proposed in order to diminish the effect of a downstream boundary. Tested were two kinds of subgrid scale (SOS) models: a Smagorinsky model (SM) and a dynamic Smagorinsky model (DSM). In the former model, parameters are kept at empirically deduced constants, while in the latter, they are calculated using different levels of space filtering. Data analysis based on the decay law of jet clearly presented the performance of SGS models. Simulated results by SM and DSM compared favorably with existing measurements of jet and its scalar transport. However, the quantitative accuracy of DSM was better than that of SM at a transitional stage of flow field. Computed parameters by DSM, coefficient for SGS stresses, C_R and SGS eddy diffusivity ratio, Γ_SGS, were not far from empirical constants of SM. Optimization of the model coefficient was suggested in DSM so that coefficient C_R was nearly equal in the established stage of jet but it was reduced in low turbulence close to the jet nozzle. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 175–188, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20001     

LES modelling of hydrogen release and accumulation within a non-ventilated ambient pressure garage using the ADREA-HF CFD code

Computational Fluid Dynamics (CFD) has already proven to be a powerful tool to study the hydrogen dispersion and help in the hydrogen safety assessment. In this work, the Large Eddy Simulation (LES) recently incorporated into the ADREA-HF CFD code is evaluated against the INERIS-6C experiment of hydrogen leakage in a supposed garage, which provides detailed experimental measurements, visualization of the flow and availability of previous CFD results from various institutions (HySafe SBEP-V3). The short-term evolution of the hydrogen concentrations in this confined space is examined and comparison with experimental data is provided, along with comments about the ability of LES to capture the transient phenomena occurring during hydrogen dispersion. The influence of the value of the Smagorinsky constant on the resolved and on the unresolved turbulence is also presented. Furthermore, the renormalization group (RNG) LES methodology is also tested and its behaviour in both highly-turbulent and less-turbulent parts of the flow is highlighted.     

CFD investigation of the flow and heat transfer characteristics in a tangential inlet cyclone

This work presents a computational fluid dynamics (CFD) calculation to investigate the flow field and the heat transfer characteristics in a tangential inlet cyclone which is mainly used for the separation of the dens phase of a two phase flow. Governing equations for the steady turbulent 3D flow were solved numerically under certain boundary conditions covering an inlet velocity range of 3 to 30 m/s. Finite volume based Fluent software was used and the RNG k −  turbulence model was adopted for the modeling highly swirling turbulent flow. Good agreement was found between computed pressure drop and experimental data available in the literature. The structure of the vortices and variation of local heat transfer were studied under the effects of inlet velocity.