Internal diffusion of molecules in porous materials plays an important role in many chemical processes.However, the pore diffusion capacity of porous materials cannot be measured by conventional catalyst characterizat...Internal diffusion of molecules in porous materials plays an important role in many chemical processes.However, the pore diffusion capacity of porous materials cannot be measured by conventional catalyst characterization methods. In the present paper, a pore diffusion factor, the ratio of the diffusionconstriction factor to the pore tortuosity of the porous materials, was proposed to measure the diffusion ability of pores inside solid materials, and a method was proposed for measuring the diffusion factor using a well-defined and uniform pore size material as a reference. The diffusion factor was calculated based on the effective diffusion coefficients and the diffusion-constriction factor and pore tortuosity of the reference porous materials. The pore diffusion factor measurement can be performed at room temperature and atmospheric pressure. The pore diffusion factor of conventional porous materials was found to be much smaller than 1, indicating that there is a lot of room for improving the diffusion ability of the conventional catalysts and adsorbents, and could be significantly increased through adding small number of fibers into the conventional porous materials as template.展开更多
For the incompressible Navier Stokes equations, a new artificial diffusion factor is put forward in the Streamline Upwind/Petrov Galerkin formulation. The corresponding formulae of finite element methods are derived ...For the incompressible Navier Stokes equations, a new artificial diffusion factor is put forward in the Streamline Upwind/Petrov Galerkin formulation. The corresponding formulae of finite element methods are derived in Newton Raphson form, in which velocity and pressure are iterated synchronously. An element with nine nodes satisfying inf sup condition is established, which has a parabolic velocity interpolation and linear pressure distribution. Four numerical examples are presented, and solutions obtained demonstrate the effectivity of the method proposed.展开更多
The diffusion and reaction phenomenon in a Fe-based catalyst pellet for Fischer-Tropsch synthesis was studied. It was considered that the pores of catalyst pellets were full of liquid wax under Fischer-Tropsch synthes...The diffusion and reaction phenomenon in a Fe-based catalyst pellet for Fischer-Tropsch synthesis was studied. It was considered that the pores of catalyst pellets were full of liquid wax under Fischer-Tropsch synthesis conditions. The re- actants diffused from the bulk gas phase to the external surface of the pellet, and then the reactants diffused through the wax inside the pellet and reacted on the internal surface formed along the pore passages of the pellet. On the basis of reaction kinetics and double a-ASF product distribution model, a diffusion and reaction model of catalyst pellet was established. The effects of diffusion and reaction interaction in a catalyst pellet, the bulk temperature, the reaction pressure and the pellet size on the reactivity were further investigated. The relationship between the internal diffusion effectiveness factor of spherical catalyst pellet and the Thiele modulus were also discussed. The bulk temperature and pellet size have significant effects on the reactivity, while the pressure shows only a slight influence on the reactivity. The internal diffusion effectiveness factor decreases with an increasing Thiele modulus.展开更多
For most aircraft engines,inflow distortion is inevitable.Inflow distortion is known to degrade the aerodynamic performance and stable operating limits of a compressor.Tandem rotor configuration is an arrangement that...For most aircraft engines,inflow distortion is inevitable.Inflow distortion is known to degrade the aerodynamic performance and stable operating limits of a compressor.Tandem rotor configuration is an arrangement that effectively controls the growth of the boundary layer over the suction surface of the blade.Therefore,a higher total pressure rise can be achieved through this unconventional design approach involving the splitting of the blade into forward and aft sections.It is expected that the effect of inlet flow distortion would be more severe for a tandem-rotor design due to the greater flow turning inherent in such designs.However,this aspect needs to be thoroughly examined.The present study discusses the effect of circumferential distortion on the tandem-rotor at different rotational speeds.Full-annulus RANS simulations using ANSYS CFX are used in the present study.The performance of the rotor at a particular flow coefficient and different rotational speeds is compared.The total pressure and efficiency are observed to drop at lower mass flow rates under the influence of circumferential distortion.The loss region in each blade passage is mainly associated with the blade wake,tip leakage vortex,secondary flow,and boundary layer.However,their contribution varies from passage to passage,particularly in the distorted sector.At the lower span,the wake width is found to be higher than that at a higher span.Due to the redistribution of the mass flow,the circumferential extent reduces at a higher span.In the undistorted sector,the strength of the tip leakage vortex is significantly higher at the design rotational speed than at lower speeds.The distortion near the tip region promotes an early vortex breakdown even at the design operating condition.This adversely affects the total pressure,efficiency,and stall margin.Under clean flow conditions,this phenomenon is only observed near the stall point.At the design operating condition,the breakdown of the forward rotor tip leakage vortex is detected in four blade passages.The axial velocity deficit and adverse pressure gradient play a significant role in the behaviour of tip leakage vortex at lower rotational speeds in the distorted sector.A twin vortex breakdown is also observed at lower speeds.展开更多
With increasing emphasis on renewable sources of energy,the gas turbine engine faces several challenges in evolving its design,to remain relevant.Compressor is one of the main components,which accounts for one-third o...With increasing emphasis on renewable sources of energy,the gas turbine engine faces several challenges in evolving its design,to remain relevant.Compressor is one of the main components,which accounts for one-third of the engine length.Compressor designers have been exploring different ideas to achieve maximum pressure rise with minimum number of stages required.Tandem blading is one such novel design that has demonstrated higher diffusion capability than a single rotor blade.A single blade,with a higher diffusion factor,carries the risk of flow separation against the adverse pressure gradient of the compressor flow.In the tandem blading concept,a single blade is split into forward and aft blade.The gap that is created between the forward and aft blade,serves as a mechanism to energize the sluggish flow over the aft blade suction surface,which in turn helps in mitigating the flow separation.The present experimental work is aimed at exploring the feasibility of a tandem rotor in an axial flow compressor under the clean and radially distorted inflows.Steady and unsteady experimental results of the tandem rotor are included in this paper.The stage performance characteristics,variation of total pressure,flow coefficient,and exit flow angle along the blade span for clean and distorted flow is included in this paper.Some results of a steady Reynolds-averaged Navier-Stokes simulation are also included to give some insight into the complex flow field of the tandem rotor.Wavelet transform,fast Fourier transform analysis,and visual inspection of casing pressure traces are used to analyze the unsteady result of the tandem rotor in clean and distorted flow.The tandem rotor is able to achieve its design pressure ratio and has a stall margin of 9%under the clean flow condition.Initially,stall appears as a low-intensity spike for all the cases,which turns into a long length-scale disturbance within three rotor revolutions.A modal wave of low frequency is also observed under clean and distorted inflows.展开更多
Based on effective media theory, a predictive model, relating chloride diffusivity to the capillary pores, gel pores, tortuosity factor, and pore size distribution of hardened cement, is proposed. To verify the propos...Based on effective media theory, a predictive model, relating chloride diffusivity to the capillary pores, gel pores, tortuosity factor, and pore size distribution of hardened cement, is proposed. To verify the proposed model, the diffusion coefficient of chloride ions, the degree of hydration, and peak radius of capillary pores of cement paste specimens were measured. The predicted results for chloride diffusivity were compared with published data. The results showed that the predicted chloride diffusivity of hardened cement paste was in good agreement with the experimental results. The effect of the evolution of pore structures in cement paste on chloride diffusivity could be deduced simultaneously using the proposed model.展开更多
基金supported by the National Nature Science Foundation of China (Grant No:91534120)。
文摘Internal diffusion of molecules in porous materials plays an important role in many chemical processes.However, the pore diffusion capacity of porous materials cannot be measured by conventional catalyst characterization methods. In the present paper, a pore diffusion factor, the ratio of the diffusionconstriction factor to the pore tortuosity of the porous materials, was proposed to measure the diffusion ability of pores inside solid materials, and a method was proposed for measuring the diffusion factor using a well-defined and uniform pore size material as a reference. The diffusion factor was calculated based on the effective diffusion coefficients and the diffusion-constriction factor and pore tortuosity of the reference porous materials. The pore diffusion factor measurement can be performed at room temperature and atmospheric pressure. The pore diffusion factor of conventional porous materials was found to be much smaller than 1, indicating that there is a lot of room for improving the diffusion ability of the conventional catalysts and adsorbents, and could be significantly increased through adding small number of fibers into the conventional porous materials as template.
文摘For the incompressible Navier Stokes equations, a new artificial diffusion factor is put forward in the Streamline Upwind/Petrov Galerkin formulation. The corresponding formulae of finite element methods are derived in Newton Raphson form, in which velocity and pressure are iterated synchronously. An element with nine nodes satisfying inf sup condition is established, which has a parabolic velocity interpolation and linear pressure distribution. Four numerical examples are presented, and solutions obtained demonstrate the effectivity of the method proposed.
基金Financial support from the National Basic Research Program of China(973 Program,2010CB736203)
文摘The diffusion and reaction phenomenon in a Fe-based catalyst pellet for Fischer-Tropsch synthesis was studied. It was considered that the pores of catalyst pellets were full of liquid wax under Fischer-Tropsch synthesis conditions. The re- actants diffused from the bulk gas phase to the external surface of the pellet, and then the reactants diffused through the wax inside the pellet and reacted on the internal surface formed along the pore passages of the pellet. On the basis of reaction kinetics and double a-ASF product distribution model, a diffusion and reaction model of catalyst pellet was established. The effects of diffusion and reaction interaction in a catalyst pellet, the bulk temperature, the reaction pressure and the pellet size on the reactivity were further investigated. The relationship between the internal diffusion effectiveness factor of spherical catalyst pellet and the Thiele modulus were also discussed. The bulk temperature and pellet size have significant effects on the reactivity, while the pressure shows only a slight influence on the reactivity. The internal diffusion effectiveness factor decreases with an increasing Thiele modulus.
文摘For most aircraft engines,inflow distortion is inevitable.Inflow distortion is known to degrade the aerodynamic performance and stable operating limits of a compressor.Tandem rotor configuration is an arrangement that effectively controls the growth of the boundary layer over the suction surface of the blade.Therefore,a higher total pressure rise can be achieved through this unconventional design approach involving the splitting of the blade into forward and aft sections.It is expected that the effect of inlet flow distortion would be more severe for a tandem-rotor design due to the greater flow turning inherent in such designs.However,this aspect needs to be thoroughly examined.The present study discusses the effect of circumferential distortion on the tandem-rotor at different rotational speeds.Full-annulus RANS simulations using ANSYS CFX are used in the present study.The performance of the rotor at a particular flow coefficient and different rotational speeds is compared.The total pressure and efficiency are observed to drop at lower mass flow rates under the influence of circumferential distortion.The loss region in each blade passage is mainly associated with the blade wake,tip leakage vortex,secondary flow,and boundary layer.However,their contribution varies from passage to passage,particularly in the distorted sector.At the lower span,the wake width is found to be higher than that at a higher span.Due to the redistribution of the mass flow,the circumferential extent reduces at a higher span.In the undistorted sector,the strength of the tip leakage vortex is significantly higher at the design rotational speed than at lower speeds.The distortion near the tip region promotes an early vortex breakdown even at the design operating condition.This adversely affects the total pressure,efficiency,and stall margin.Under clean flow conditions,this phenomenon is only observed near the stall point.At the design operating condition,the breakdown of the forward rotor tip leakage vortex is detected in four blade passages.The axial velocity deficit and adverse pressure gradient play a significant role in the behaviour of tip leakage vortex at lower rotational speeds in the distorted sector.A twin vortex breakdown is also observed at lower speeds.
文摘With increasing emphasis on renewable sources of energy,the gas turbine engine faces several challenges in evolving its design,to remain relevant.Compressor is one of the main components,which accounts for one-third of the engine length.Compressor designers have been exploring different ideas to achieve maximum pressure rise with minimum number of stages required.Tandem blading is one such novel design that has demonstrated higher diffusion capability than a single rotor blade.A single blade,with a higher diffusion factor,carries the risk of flow separation against the adverse pressure gradient of the compressor flow.In the tandem blading concept,a single blade is split into forward and aft blade.The gap that is created between the forward and aft blade,serves as a mechanism to energize the sluggish flow over the aft blade suction surface,which in turn helps in mitigating the flow separation.The present experimental work is aimed at exploring the feasibility of a tandem rotor in an axial flow compressor under the clean and radially distorted inflows.Steady and unsteady experimental results of the tandem rotor are included in this paper.The stage performance characteristics,variation of total pressure,flow coefficient,and exit flow angle along the blade span for clean and distorted flow is included in this paper.Some results of a steady Reynolds-averaged Navier-Stokes simulation are also included to give some insight into the complex flow field of the tandem rotor.Wavelet transform,fast Fourier transform analysis,and visual inspection of casing pressure traces are used to analyze the unsteady result of the tandem rotor in clean and distorted flow.The tandem rotor is able to achieve its design pressure ratio and has a stall margin of 9%under the clean flow condition.Initially,stall appears as a low-intensity spike for all the cases,which turns into a long length-scale disturbance within three rotor revolutions.A modal wave of low frequency is also observed under clean and distorted inflows.
基金Project supported by the National Basic Research Program (973) of China (No. 2009CB623200)the National High-Tech R&D Program (863) of China (No. 2008AA030794)the Postgraduates Research Innovation in University of Jiangsu Province, China (No. CX10B-064Z)
文摘Based on effective media theory, a predictive model, relating chloride diffusivity to the capillary pores, gel pores, tortuosity factor, and pore size distribution of hardened cement, is proposed. To verify the proposed model, the diffusion coefficient of chloride ions, the degree of hydration, and peak radius of capillary pores of cement paste specimens were measured. The predicted results for chloride diffusivity were compared with published data. The results showed that the predicted chloride diffusivity of hardened cement paste was in good agreement with the experimental results. The effect of the evolution of pore structures in cement paste on chloride diffusivity could be deduced simultaneously using the proposed model.
