Hydrogen combustion characteristics in a model burner with a coaxial injector

We have developed a burner for a hydrogen combustion turbine and proposed two kinds of burners, in which the combustion region is separated from the mixing region and in the former H₂ and O₂ are burned in a slightly fuel-rich condition or a stoichiometric condition. We manufactured a small-scale, water-cooled model burner, made of oxygen-free copper, and with two pairs of transparent quartz windows for a flame observation. A coaxial type injector was employed, in which an inner tube (for oxygen) was set inside the recess of the outer tube (for hydrogen). The H₂-O₂ combustion characteristics were studied while changing such parameters as injection velocities, equivalence ratios, and swirl numbers adding to a O₂ flow. The stable combustion region ranged from ṁH₂ = 0.3–1.5 g/s and fr φ = 0.9–2.0. Then H₂-O₂ combustion characteristics in steam, N₂ and Ar were studied to get stable combustion conditions. As a result, we obtained the fundamental design materials for a hydrogen combustion turbine.     

Mixing performance of transverse hydrogen/air multi-jet through coaxial injector arrays in supersonic crossflow

Increasing the fuel mixing performance substantially improves the overall performance of the scramjet engine for a long-distance flight. In this paper, the influence of coaxial injector arrays of hydrogen/air multi-jet on the mixing performance of the fuel in supersonic crossflow is fully investigated. Our main goal is to examine the impacts of air and fuel coaxial injector on fuel distribution and penetration downstream of injectors in different operating conditions. In this study, fuel and air are simultaneously injected through coaxial multi-jet at sonic condition while of free-stream Mach number is 4. Computational Fluid Dynamic is applied for simulation of the transverse coaxial jet at supersonic crossflow. The effect of jet diameter with the same mass flow rate of air and hydrogen on fuel mixing is also investigated. The mixing efficiency of different jet spaces and pressures is also examined to obtain an optimum jet arrangement in the combustor chamber. Our study shows that the injection of the coaxial air/hydrogen jet noticeably improves mixing downstream by augmentation of fuel interaction with an air jet. Our results also show that fuel jet space of 7 Dj offers maximum fuel mixing by the formation of multi vortices with uniform strength.     

Breakup Structure of Two-phase Jets with Various Momentum Flux from Porous Injector简

Spray structure and atomization characteristics were investigated through a comparison of a porous and a shear coaxial injector. The porous injector shows better atomization performance than the shear coaxial injector. To in- crease atomization performance and mixing efficiency of two-phase jets, a coaxial porous injector which can be applicable to liquid rocket combustors was designed and tested. The characteristics of atomization and spray from a porous and a shear coaxial injector were characterized by the momentum flux ratio. The breakup mechanism of the porous injector is governed by Taylor-Culick flow and axial shear forces. Momentum of injected gas flow through a porous material which is composed of sintered metal is radically transferred to the center of the liquid column, and then liquid column is effectively broken up. Although the shapes of spray from porous and shear co- axial jets were similar for various momentum ratio, spray structures such as spray angle and droplet sizes were different. As increasing the momentum flux ratio, SMD from the porous injector showed smaller value than the shear coaxial injector     

汽液两相流喷射升压加热器供暖系统特性研究

研究了采用汽液两相流喷射升压加热器的供暖系统的水力特性 ,得到了供回水压力越高、循环水流量越大这一与常规水泵系统完全不同的特性。设计和建立了采用汽液两相流喷射升压加热器的供暖系统并进行了工业性试验。运行试验结果表明该系统运行稳定、可靠 ,可完全满足供暖需要     

无静态泄漏共轨喷油器性能分析及优化

基于AMESim平台建立无静态泄漏共轨喷油器性能分析模型,以提高共轨喷油器喷油速率和降低喷油器高压燃油损失为目标,开展无静态泄漏共轨喷油器的研究,并与静态泄漏共轨喷油器进行对比分析.研究表明:结构参数相同时,无静态泄漏共轨喷油器的平均喷油速率和有效喷油量效率均比静态泄漏有所提高,但无静态泄漏喷油器的最高喷油速率处于整个...     

同轴套管式换热器周围介质层的传热特性

建立了一套利用同轴套管式换热器模拟周围饱和型多孔介质层传热特性的实验装置，得到了一些实验数据，建立了理论模型并采用实验与理论相结合的方法，为在更广的范围内预测或确定地下土壤层换热速率提供依据。     

Numerical investigations on the performance of a multistrut hydrogen injector in a scramjet combustor

This study aims at investigating the effect of a multistrut-based hydrogen injector in a scramjet combustor underreacting case. The numerical analysis is carried out using two-dimensional Reynolds-averaged Navier–Stokes equations with the Shear Stress Transport k − ω turbulence model in contention to comprehend the flow physics during scramjet combustion. The three major parameters, such as the shock wave pattern, wall pressures, and static temperature across the combustor, are validated with the reported experimental results. The results comply with the range, indicating that the adopted simulation method for single strut injection can be extended for other investigations. It is noticed that with multistrut injectors, as hydrogen jet pressure increases in the supersonic flow field, the jet penetration rate in the lateral direction of the flow and the total pressure loss as compared with the baseline injection pressure conditions has increased. The supersonic flow characteristics are determined based on the flow properties, combustion efficiency, mixing efficiency, and total pressure loss. Compared with the single-strut output of a scramjet combustor, multistruts inclusion increased the combustion efficiency by almost 18%, the mixing efficiency attained the maximum with 16% fewer lengths. The total pressure loss in single-strut is 14% lower than that of multistrut.     

同轴度对引射器性能影响的数值分析

船用燃气轮机需要安装引射器来降低红外特征,受限于机舱尺寸,引射器的主喷管和混合管往往不能同轴线布置。为了研究主喷管和混合管在非同轴线布置下对引射器的影响,本文以某燃气轮机上带有冷却孔的引射器为研究对象,采用数值模拟的方法研究了主喷管相对混合管轴线偏离±30%,±50%,±70%时引射器的性能,并且对比了冷却孔在开闭状态下的性能。结果表明：当冷却孔关闭时,主喷管向负方向偏移时压力损失系数最大提高4.3%,向正方向偏移时压力损失系数最大提高2.5%;当冷却孔开启时,偏移方向对压力损失系数没有明显影响,且引射器内部的燃气总温降低100~150 K。     

Improving the thermal performance of coaxial borehole heat exchangers

Borehole heat exchangers are the fundamental component of ground coupled heat pumps, which are now widely employed for energy saving in building heating and cooling. The improvement of the thermal efficiency of Coaxial Borehole Heat Exchangers (CBHEs) is pursued in this paper by investigating the effects of thermal short-circuiting and of flow rate, as well as of the constituent materials and of the geometrical configuration of the CBHE cross section. The analysis is performed by means of finite-element simulations, implemented through the software package COMSOL Multiphysics. The real 2-D axisymmetric unsteady heat transfer problem is modelled, for both winter and summer working conditions, by considering CBHEs with a length of 100 m placed either in a high conductivity or in a low conductivity ground. The results point out that the effects of flow rate and of thermal short-circuiting are both important, and that the latter can be reduced considerably by employing a low conductivity material, such as PPR80, for the inner tube. Finally, it is shown that the performance of the CBHE could be improved, with respect to the commonly used geometry, by increasing the diameter of the inner tube while leaving the outer tube unchanged.     

Investigation of the effect of pilot burner on lean blow out performance of a staged injector

The staged injector has exhibited great potential to achieve low emissions and is becoming the preferable choice of many civil airplanes. Moreover, it is promising to employ this injector design in military engine, which requires most of the combustion air enters the combustor through injector to reduce smoke emission. However, lean staged injector is prone to combustion instability and extinction in low load operation, so techniques for broadening its stable operation ranges are crucial for its application in real engine.
In this work, the LBO performance of a staged injector is assessed and analyzed on a single sector test section.The experiment was done in atmospheric environment with optical access. Kerosene-PLIF technique was used to visualize the spray distribution and common camera was used to record the flame patterns. Emphasis is put on the influence of pilot burner on LBO performance. The fuel to air ratios at LBO of six injectors with different pilot swirler vane angle were evaluated and the obtained LBO data was converted into data at idle condition. Results show that the increase of pilot swirler vane angle could promote the air assisted atomization, which in turn improves the LBO performance slightly. Flame patterns typical in the process of LBO are analyzed and attempts are made to find out the main factors which govern the extinction process with the assistance of spray distribution and numerical flow field results. It can be learned that the flame patterns are mainly influenced by structure of the flow field just behind the pilot burner when the fuel mass flow rate is high; with the reduction of fuel, atomization quality become more and more important and is the main contributing factor of LBO. In the end of the paper,conclusions are drawn and suggestions are made for the optimization of the present staged injector.     

Lean flammability limits for stable performance with a porous burner

Applications of porous burners are of high interest due to many advantages such as extended lean flammability limit in comparison with free flame structures. In this work, laminar premixed flame propagation of methane/air mixture in a porous medium is numerically investigated. An unsteady one-dimensional physical model of a porous burner is considered, in which the flame location is not predetermined. The computational domain is extended beyond either side of the porous medium to accurately model reactions close to the edges of the solid matrix. After validation of the model and performing a baseline simulation, a parametric study is carried out to investigate the lean flammability limits of the burner and the unstable flash-back/blow-out phenomena. Stable performance diagrams are given for two controlling parameters of turn–down ratio and porous medium porosity. The simulation results indicate that the stable performance range of the burner is extended when the equivalence ratio increases; however, the blow-out region expands with an increase in the firing rate. For constant values of porosity and firing rate, increasing the equivalence ratio can change the operating regime of the burner from blow-out to a stable condition. It is observed that by the variation of porosity in the range of 0.6–0.9, and for the equivalence ratios of more than 0.6, the flame flash-back cannot occur. An equivalence ratio of 0.43 is found to be the lower limit at which the flame stabilizes in the matrix.     

The performance simulation of all-glass vacuum tubes with coaxial fluid conduit

A numerical investigation has been carried out for a solar system, which consists of all-glass (double skin) solar vacuum tubes. Water is heated as it flows through the coaxial fluid conduit inserted in each tube. The space between the exterior of the fluid conduit and the glass tube is filled with antifreeze solution. This is to facilitate heat transfer from the solar heated absorber surface to water and to prevent the functional problems due to freezing in frigid weather conditions. Results show good agreement when compared with other experimental data demonstrating the reliability of the present model. The one-dimensional numerical model could be used efficiently in designing all-glass solar collector tubes with different geometrical parameters other than those considered in the present analysis.     

A three-dimensional performance analysis of all-glass vacuum tubes with coaxial fluid conduit

The thermal performance of a solar system composed of parallel, all-glass (double skin) vacuum tubes has been investigated by using a three-dimensional analytical model. Each vacuum tube is equipped with a coaxial fluid conduit for water to flow through and collect the sun's thermal energy. The space between the exterior of the fluid conduit and the glass tube is filled with antifreeze solution to facilitate heat transfer from the solar heated absorber surface to water and to prevent the functional problems due to freezing in frigid weather conditions. Different from one-dimensional analytical models, the three-dimensional model considered in the present analysis enables the prediction of spatial variation of water temperatures as it flows through the coaxial conduit. This is quite useful in extracting major variables for the operation of the solar system using all-glass vacuum tubes as considered in the present investigation.     

Efficient photo-catalytic hydrogen production performance and stability of a three-dimensional porous CdS NPs-graphene hydrogel

We designed a novel CdS nanoparticles composite graphene hydrogel. Under sunlight, the CdS nanoparticles (NPs) -reduced graphene oxide hydrogel (rGH) had the highest hydrogen production rate of 6.44 mmol/g, which is 1.3 times that of CdS nanoparticles (5.12 mmol/g) and 1.4 times that of CdS (4.6 mmol/g). The enhanced photo-catalytic activity can be attributed to several positive factors such as the formation of composite hydrogels and the quantum size effect of the CdS nanoparticles nanomaterials. The formation of the composite hydrogel improves the specific surface area of the catalyst and increases the active site on the catalyst surface. The quantum size effects of the CdS nanoparticles effectively reduce the recombination probability of electrons and holes. The close contact between the CdS nanoparticles and the graphene gel can effectively separate photo-generated electrons and holes via the unique large π-bond structure of graphene. These positive factors effectively improve the photocatalytic activity of composite materials for water decomposition. In addition, recovery experiments show a composite catalyst recovery rate of up to 95%. The results show that the composite photo-catalyst can effectively avoid secondary pollution during photo-catalytic hydrogen production. This eliminates powder recovery problems. The hydrogen production efficiency of the catalyst remains unchanged after 5 cycles indicating that the formation of the gel system stabilizes the catalyst and inhibits light corrosion of CdS nanoparticles.     

Investigation of moisture transfer mechanism in porous media during a rapid drying process

In this work, the moisture transfer mechanism in wet porous media during a rapid drying process was investigated experimentally and analytically. By means of scanning electron microscopy, the rapid drying processes for potato, carrot, and radish species were observed and recorded. A new displacement model using the pressure gradient in a porous material during rapid drying was suggested. To analyze this displacement flow in a porous material, the variables of this flow in a single capillary tube, such as velocity, flow rate, as well as the displacement time of internal moisture, were calculated. © 2000 Scripta Technica, Heat Trans Asian Res, 30(1): 22–27, 2001     

Investigation of dynamic heat transfer process through coaxial heat exchangers in the ground

Recently, researchers are focussing on using ground coupled heat pump systems as a heat source or sink rather than air source heat pumps for HVAC needs due to the stable temperature and the high thermal inertia of the soil. The investment cost of these systems is too expensive therefore the precise thermal analysis, design and parameter optimization are essential. For an accurate design, the maximum of physical phenomena such as: axial effects, seasonal effects, underground water flow and BHE dynamic behaviour must be accounted for in order to reflect exactly the real physical situation. In the present paper thermal interferences are investigated under seasonal effects and a dynamic heat flux for a vertical coaxial borehole heat exchangers field. This enables to avoid thermal interferences by predicting efficient period of operation corresponding to the beginning of the studied phenomena (interferences) for a given separation distance between two boreholes. To reach this purpose, as a first step, a transient 2D Finite volume method (FVM) for a single borehole heat exchanger was built using MATLAB, which accounts for accurate axial and seasonal effects and a dynamic heat flux that is function of depth and time. This model has been validated against the Finite Line Source (FLS) analytical solution and good agreement between analytical and numerical methods has been obtained. Then the model has been extended to a quasi-3D model in order to investigate thermal interferences between two neighbouring boreholes. After 500 h and at the mid-point of the separating distance (1.5 m) where interferences are the strongest, the temperature is 50% (6.64 °C) lower than the case where there are no interferences.     

Thermal analysis on the cooling performance of a porous evaporative plate for building

In this paper, a wet porous cooling plate has been used for a building wall. Cooling can be achieved due to the evaporation in the porous layer. A mathematical model on the heat and mass transfer in the unsaturated porous media is developed to analyze the influences of ambient conditions and the porous layer thickness on the cooling performance of the porous evaporative plate. With a decrease in ambient relative humidity and an increase in ambient temperature, more cooling of the porous evaporative plate can be supplied for the inside of the room. The heat exchange between the inside surface of the porous plate and the air in the room should be intensified to achieve a higher cooling efficiency of the porous plate. The ambient wind speed and the thickness of the porous plate also have significant influence on the average temperature of the porous plate. All these results should be taken into account for the utilization of the porous evaporative cooling plate. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20284     

Effects of flow direction and thermal short-circuiting on the performance of small coaxial ground heat exchangers

The effects of flow direction and thermal short-circuiting on the performance of small-size coaxial ground heat exchangers, currently used in Northern Italy, are studied by finite-element simulations, performed through the software package COMSOL Multiphysics 3.4 (©Comsol, Inc.). The real 2-D axisymmetric unsteady heat conduction and convection problem is considered, both for winter and for summer working conditions. The flow in the outer annular passage is laminar in winter and turbulent in summer. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the weak form boundary condition available in COMSOL Multiphysics; the forced-convection heat transfer in the outer annular passage is simulated directly. Two Small Coaxial Ground Heat Exchangers (SCGHEs) with the same length (20 m) but different cross-sections are examined; moreover, two values of the ground thermal conductivity, as well as two materials for the inner tube wall are considered. The results point out that the annulus-in flow direction (fluid inlet in the outer annular passage) is more efficient than the center-in flow direction (fluid inlet in the inner circular tube) and that, on account of the small length, the effect of thermal short-circuiting is not important for SCGHEs, especially if the annulus-in flow direction is employed.