Numerical Simulations Of Low Density Ratio Particle-Fluid Two-Phase Fluidization Characteristics

Energy consumption is the main sources of carbon emission,in order to reduce carbon dioxide emissions and relieve the pressure of environmental protection,the utilization of coal and biomass resources with high-efficient,clean and low-carbon emission can be realized through gasification technology using sub-and super-critical fluids and high temperature molten salt fluid.Compared with the conventional gas-solid fluidized bed,the densities of fluid and solid phases in the high-temperature and high-pressure fluidized bed are close to each other,and the density ratio is low(ρ_s/ρ_f<100).Recently,research results indicated that the simulation results using classical kinetic theory of granular flow(KTGF)shown good agreement with direct numerical simulation results in high density ratio fluidization condition(ρ_s/ρ_f>1000),while large discrepancies were appeared under the fluidization condition of low-density-ratio.The reason is that the classical-KTGF is based on gas molecular dynamics,and the effects of fluid turbulence on particle motion are ignored.It is obvious that the classical-KTGF can’t accurately predict the fluidization characteristics of sub-and super-critical and high temperature molten salt fluidized beds.Moreover,the density ratios between fluid and solid phases will change greatly with the change of fluid temperatures and pressures in low density ratio fluidization system.Therefore,the classical-KTGF should be improved to meet the simulation requirements of the low-density-ratio fluidization system.Based on the integral results of solid phase stress tensor,fluctuating energy diffusion term in classical-KTGF.Using the second-order moments of solid phase fluctuation stress and the third-order moments of solid phase fluctuating kinetic energy conduction flux,which were obtained by considering the fluid phase turbulent effect and the inelastic collision effect between particles,the expressions of solid phase stress tensor and fluctuating energy flux were got.Compared with the conservation relationship,the transport coefficients of solid phase,including solid phase pressure,shear viscosity,bulk viscosity and conductivity coefficient,can be obtained under the low-density-ratio fluidization condition.Therefore,the low-density-ratio-KTGF was established.and the transport coefficients were expressed as a functions of Stokes number.The transport coefficients can be degraded into the forms of classical-KTGF as Stokes number trends to infinity.Hence,the low-density-ratio-KTGF has a wide range of application and can meet the simulation requirements of low-density-ratio fluidization system.The fluidization characteristics of ambient,sub-and super-critical carbon dioxide fluidized beds were simulated,and it was found that both the ambient and supercritical carbon dioxide fluidized beds exhibited aggregative fluidization,but the fluidization characteristics were quite different,the former was expressed bubble-like flow,while the latter was formed a particle aggregative-like flow.The coexistence of wave-like and chunk-like flows occur in the subcritical carbon dioxide fluidized bed.The fluidization state coincided with prediction results of stability function and different criteria to distinguish from particulate and aggregative fluidizations.The effects of fluid temperatures,pressures,inlet fluid superficial velocities and particle densities on the transitional fluidization state and particle cluster parameters were analyzed in the subcritical carbon dioxide fluidized bed.Hydrodynamics between ambient,subcritical,supercritical water and particles mixture were investigated in fluidized beds under constant inlet velocity condition.The fluidization state changed gradually from particulate fluidization to aggregative fluidization.In addition,fluidization characteristics of subcritical water fluid-particle mixtures is modelled by means of a sinusoidal pulsation inlet velocity condition.Simulated results show that the frequency of solid particle concentration is the same as the inlet frequency when the inlet velocity pulsation frequency f_a is less than the main frequency f_m of particle concentration under the constant inlet velocity condition.On the other hand,the main frequency of bed tended to f_m when the inlet frequency exceeds f_m.At low pulsation frequency,the waves are characterized as less and coarser modes at the bottom part of fluidized bed.Whereas they are characterized as more and finer zones at high pulsation frequency.When changing the inlet velocity pulsation amplitudes,the wave-like flow is replaced by the plug-like flow.The length of the plug-like flow structure increased with the increase of pulsation amplitude,while the rising speed shown a decreasing trend.The statistical results of time-averaged data show that the pulsating velocity inlet condition mainly affected the instantaneous fluidization characteristics,and has less effects on time-averaged characteristics.The mixing/segregation behaviors of binary particles were studied in supercritical water fluidized bed.According to the distribution of time-averaged solid volume fraction along bed height direction,the bed was divided into a subsidence region,segregation region,transition region and freeboard region.The segregation angle parameter was proposed to quantify the segregation degree of binary particle.The influences of temperature,pressure and inlet velocity of supercritical water fluid on mixing of binary particles were analyzed.Qualitatively and quantitatively comparison between the simulation results obtained from the low-density-ratio-KTGF based on TFM approach and the linear spring-damping theory based on CFD-discrete element method(DEM)in methanol fluidized beds.The predicted bed expansion height using TFM approach was smaller,and the main frequency of local instantaneous solid volume fraction was larger.The difference between the turbulence models led to one order of magnitude difference in fluid turbulence energy k and energy dissipation rateε.The distributions of normal Reynolds stresses of fluid and solid phases obtained from two simulation methods were consistent with each other.However,the TFM simulation results were smaller than that using CFD-DEM method under subcritical conditions,while the opposite trend was observed under supercritical conditions.In addition,the influences of fluid temperatures and pressures on the fluidization characteristics were investigated using TFM method with low-density-ratio-KTGF in supercritical ethanol fluidized bed.Under the background of biomass particle gasification,computational simulations of high temperature molten salt fluidized bed were performed using low-density-ratio-KTGF.The results shown that the number of waves near the inlet region decreased,and the space between adjacent waves increased with the increasing of molten salt fluid temperature.The results of power spectrum densities analysis shown that the main frequency of solid volume fraction decreased with the increasing of fluid temperatures,meaning that the generation rate of waves decreased.The increase of fluid inlet velocity and particle diameter and the decrease of particle density made fluidization state tend to aggregative fluidization.Moreover,the increase of particle density and diameter led to the decrease of the main frequency of local instantaneous solid volume fraction in molten salt fluidized beds.