Momentum, energy and scalar transport in polydisperse gas-solid flows using particle-resolved direct numerical simulations

Gas-solid flows are commonly encountered in Nature and in several industrial applications. Emerging carbon-neutral or carbon negative technologies such as chemical looping combustion and CO2 capture are examples of gas-solid flows in power generation industry. Computational fluid dynamics (CFD) simulations are increasingly being seen as a cost-effective tool in the design of technological applications in power generation industry. Device-scale CFD calculations that involve gas-solid flow are based on statistical descriptions that require closure models for the exchange of mass, momentum, energy and heat transfer between the dispersed solid phase and the gas phase. The predictive capability of multiphase flow CFD simulations strongly depends on the accuracy of the models used for the interphase exchange terms. Particle-resolved direct numerical simulation (PR-DNS) is a first-principles approach to develop accurate models for interphase momentum, energy and heat transfer in gas-solid flow. The primary objective of this work is the development of accurate models for the interphase exchange of momentum, kinetic energy and heat transfer in polydisperse gas-solid flows using PR-DNS. A novel computational tool named Particle-resolved Uncontaminated-fluid Reconcilable Immersed Boundary Method (PUReIBM) has been developed as a part of this work to perform PR-DNS of flow past fixed and freely moving spherical particles. We designed the appropriate numerical experiment that can be used to develop closure models for interphase momentum transfer and formally established the connection between PR-DNS and statistical theory of multiphase flow for which the models are intended. Using PUReIBM we developed an improved drag correlation to model interphase momentum transfer in gas-solid flow. The solution fields obtained from PUReIBM PR-DNS have been used to quantify the velocity fluctuations in the gas-phase and a simple eddy viscosity model for the gas-phase pseudo-turbulent kinetic energy has been developed. A novel PR-DNS methodology to study heat transfer in gas-solid flow has been developed. These results provide insight into the role of fluid heating in gas-solid flow and motivate the development of better models for gas-solid flow heat transfer. From PR-DNS of freely evolving gas-solid suspensions we developed a stochastic model for particle acceleration that accounts for the particle velocity distribution. In addition to model development, the implementation of a parallel algorithm that enables PR-DNS of gas-solid flow on petascale supercomputers is also discussed

Enskog kinetic theory for monodisperse gas–solid flows

The Enskog kinetic theory is used as a starting point to model a suspension of solid particles in a viscous gas. Unlike previous efforts for similar suspensions, the gas-phase contribution to the instantaneous particle acceleration appearing in the Enskog equation is modelled using a Langevin equation, which can be applied to a wide parameter space (e.g. high Reynolds number). Attention here is limited to low Reynolds number flow, however, in order to assess the influence of the gas phase on the constitutive relations, which was assumed to be negligible in a previous analytical treatment. The Chapman–Enskog method is used to derive the constitutive relations needed for the conservation of mass, momentum and granular energy. The results indicate that the Langevin model for instantaneous gas–solid force matches the form of the previous analytical treatment, indicating the promise of this method for regions of the parameter space outside of those attainable by analytical methods (e.g. higher Reynolds number). The results also indicate that the effect of the gas phase on the constitutive relations for the solid-phase shear viscosity and Dufour coefficient is non-negligible, particularly in relatively dilute systems. Moreover, unlike their granular (no gas phase) counterparts, the shear viscosity in gas–solid systems is found to be zero in the dilute limit and the Dufour coefficient is found to be non-zero in the elastic limit

Momentum, energy and scalar transport in polydisperse gas-solid flows using particle-resolved direct numerical simulations

Gas-solid flows are commonly encountered in Nature and in several industrial applications. Emerging carbon-neutral or carbon negative technologies such as chemical looping combustion and CO2 capture are examples of gas-solid flows in power generation industry. Computational fluid dynamics (CFD) simulations are increasingly being seen as a cost-effective tool in the design of technological applications in power generation industry. Device-scale CFD calculations that involve gas-solid flow are based on statistical descriptions that require closure models for the exchange of mass, momentum, energy and heat transfer between the dispersed solid phase and the gas phase. The predictive capability of multiphase flow CFD simulations strongly depends on the accuracy of the models used for the interphase exchange terms. Particle-resolved direct numerical simulation (PR-DNS) is a first-principles approach to develop accurate models for interphase momentum, energy and heat transfer in gas-solid flow. The primary objective of this work is the development of accurate models for the interphase exchange of momentum, kinetic energy and heat transfer in polydisperse gas-solid flows using PR-DNS. A novel computational tool named Particle-resolved Uncontaminated-fluid Reconcilable Immersed Boundary Method (PUReIBM) has been developed as a part of this work to perform PR-DNS of flow past fixed and freely moving spherical particles. We designed the appropriate numerical experiment that can be used to develop closure models for interphase momentum transfer and formally established the connection between PR-DNS and statistical theory of multiphase flow for which the models are intended. Using PUReIBM we developed an improved drag correlation to model interphase momentum transfer in gas-solid flow. The solution fields obtained from PUReIBM PR-DNS have been used to quantify the velocity fluctuations in the gas-phase and a simple eddy viscosity model for the gas-phase pseudo-turbulent kinetic energy has been developed. A novel PR-DNS methodology to study heat transfer in gas-solid flow has been developed. These results provide insight into the role of fluid heating in gas-solid flow and motivate the development of better models for gas-solid flow heat transfer. From PR-DNS of freely evolving gas-solid suspensions we developed a stochastic model for particle acceleration that accounts for the particle velocity distribution. In addition to model development, the implementation of a parallel algorithm that enables PR-DNS of gas-solid flow on petascale supercomputers is also discussed.</p

Enskog kinetic theory for monodisperse gas–solid flows

The Enskog kinetic theory is used as a starting point to model a suspension of solid particles in a viscous gas. Unlike previous efforts for similar suspensions, the gas-phase contribution to the instantaneous particle acceleration appearing in the Enskog equation is modelled using a Langevin equation, which can be applied to a wide parameter space (e.g. high Reynolds number). Attention here is limited to low Reynolds number flow, however, in order to assess the influence of the gas phase on the constitutive relations, which was assumed to be negligible in a previous analytical treatment. The Chapman–Enskog method is used to derive the constitutive relations needed for the conservation of mass, momentum and granular energy. The results indicate that the Langevin model for instantaneous gas–solid force matches the form of the previous analytical treatment, indicating the promise of this method for regions of the parameter space outside of those attainable by analytical methods (e.g. higher Reynolds number). The results also indicate that the effect of the gas phase on the constitutive relations for the solid-phase shear viscosity and Dufour coefficient is non-negligible, particularly in relatively dilute systems. Moreover, unlike their granular (no gas phase) counterparts, the shear viscosity in gas–solid systems is found to be zero in the dilute limit and the Dufour coefficient is found to be non-zero in the elastic limit.This article is from Journal of Fluid Mechanics 712 (2012): 129–168, doi:10.1017/jfm.2012.404. Posted with permission.</p

Instantaneous particle acceleration model for gas-solid suspensions at moderate Reynolds numbers

Gas-solid flows are encountered in many industrial applications such as fluidized beds and coal gasification. The design and scale-up of such industrial devices required a better understanding of the characteristics of gas-solid suspensions. Device-scale computational fluid dynamics (CFD) simulations that solve for average quantities such as solid volume fraction and phasic mean velocity fields are being extensively used in the industrial design process. The capability of the simulations to accurately predict the characteristics of gas-solid flow depends upon the accuracy of the models for unclosed terms that appear in the equations for mass, momentum and energy conservation. Hrenya and Sinclair (1997) show that the particle granular temperature (particle velocity variance) plays an important role in the prediction of the core annular structure in riser flows. In statistically homogeneous suspensions undergoing elastic collisions, the particle acceleration-velocity covariance alone governs the evolution of granular temperature.This article is from 7th International Conference on Multiphase Flow, ICMF 2010, Tampa, FL, May 30 - June 4, 2010. p.1-7.</p