Multiscale Modeling Of Momentum And Heat Transfer Characteristics Of Non-Spherical Particles In Supercritical Water

Hydrogen production based on supercritical water(SCW)has significant advantages such as high efficiency,pollution-free emission and strong material adaptability.The technique is of great significance for alleviating energy shortages and mitigating environmental pollutions.The most common reactor used for the SCW-based hydrogen production process is the fluidized bed.However,due to the extreme environmental constraints such as high temperature and pressure,traditional experimental measurements of SCW have many difficulties,and only limited data can be obtained.This leads to the fact that the research on the mechanism of fluidsolid interaction in the SCW fluidized bed is far from meeting the actual demand.The research on the momentum and heat transfer characteristics of non-spherical particles in SCW is just in the infancy.As a result,numerous difficulties are encountered in the design,operation or optimization of the SCW fluidized bed.In this thesis,particle-resolved direct numerical simulations are carried out to study the momentum and heat transfer characteristics of nonspherical particles in SCW,and a multi-scale model is established.The numerical simulation results can provide a theoretical basis for the design of the SCW fluidized bed reactors.The influences of the physical parameters of SCW on the drag coefficient(Cd)and average Nusselt number(Nu)are revealed,and the key parameters affecting Cd and Nu are determined.Cd and Nu of an ellipsoidal particle in SCW under different working conditions are calculated.The influences of Reynolds number,particle shape and orientation on Cd and Nu are explored,and the predictive correlations of Cd and Nu for ellipsoidal particles in SCW are established.The forced convection heat transfer model of Platonic particles in SCW is constructed.The momentum and heat transfer characteristics of five Plato particles in SCW and conventional fluid environment are compared.Cd and Nu of the Plato particles in the two fluids are calculated,respectively.The influences of sphericity and number of sides of regular polyhedron on Cd and Nu are discussed.Similarly,the predictive correlations of Cd and Nu for Plato particles in SCW are also constructed.The effects of wall constraints on the momentum and heat transfer characteristics of nonspherical particles in SCW are investigated.For two types of particle motion problems,namely the fluidized particle problem and the sedimentation particle problem,the influences of wall factors on Cd and Nu are revealed.The relationship between Cd and Nu and λ and Re is constructed with two different boundary conditions,respectively.