Advanced Numerical Modeling for Nuclear Thermohydraulics: Applications in Supercritical Fluid Heat Transfer and Fuel-Coolant Interaction


Student thesis: Doctoral Thesis

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Award date22 Aug 2019


In recent years, numerical modeling plays a more and more important role in the design and analysis of nuclear thermohydraulics. In the present thesis, two advanced numerical models, namely finite volume method (FVM) and lattice Boltzmann method (LBM), are used to study characteristics of supercritical fluid heat transfer and fuel-coolant interaction, respectively.

For the applications of FVM, firstly, heat transfer deterioration (HTD) is numerically studied for supercritical fluids flowing upward in circular tubes at high heat fluxes and low mass fluxes. The simulations are conducted with Shear Stress Transport (SST) k-ω turbulence model in commercial software Fluent 15.0. Both water and CO2 are simulated and the results are consistent well with the experiments. It is found that there are two wall temperature peaks when the heat transfer deterioration occurs, the first peak is narrow and sharp while the second peak is lower and broader. The mechanism of two wall temperature peaks is analyzed in detail based on radial distributions of velocity and turbulent kinetic energy at different axial positions. It is found that the mechanism of the first peak is quite different from the second peak. The first peak is caused by buoyancy effect, which flattens the velocity distribution in the near wall region and leads to the reduction of turbulent kinetic energy and the impairment of heat transfer in the near wall region. For the second peak, it is the shear stress that flattens the velocity distribution in the main flow region and leads to the reduction of turbulent kinetic energy, which again impairs the heat transfer and causes the second peak. Then Diameter effect on HTD is numerically studied for supercritical water flowing upward in circular tubes and annular channels at high heat flux and low mass flux. When the same boundary conditions are applied, i.e. heat flux, mass flux, and inlet temperature, it is found that in circular tubes the first wall temperature peak moves upstream and the magnitude of the peak increases first and then decreases with the increase of tube diameter, the second peak moves downstream with the increase of tube diameter. Whereas in annular channels with a fixed inner diameter, HTD is suppressed when the outer diameter is small and HTD occurs gradually with the increase of outer diameter. These phenomena are consistent with previous experimental results. The mechanism is analyzed based on fully developed turbulent velocity profile at the inlet of the heated sections. Increasing inner diameter for circular tubes or outer diameter for annular channels will result in the decrease of maximum velocity and velocity gradient in the near wall region, which makes velocity profile in this region much easier to be flattened by the buoyancy. Then an M-shaped velocity profile is formed, which will significantly decrease the Reynolds shear stress and turbulent kinetic energy and hence impair the heat transfer and cause HTD. For the same flow conditions, HTD is much easier to occur in circular tubes than in annular channels with the same hydraulic diameters.

The LBM is used to study fuel-coolant interaction. Firstly, an experimental apparatus is set up to investigate the characteristics and mechanism of melt jet breakup in water in the premixing phase of fuel-coolant interaction during nuclear reactor severe accident. Wood’s metal is used as the simulant material. The melt jet breakup experiments are conducted under different jet diameters, melt temperatures, water temperatures and penetration velocities. The breakup process is recorded by a high speed video camera. The characteristics of the breakup of the leading edge and the jet column are analyzed in detail. For the breakup of the leading edge, drop-shaped leading edges are formed in the air for all jets before penetration into water, which will later form the mushroom-like leading edge in the water due to the drag force. For the breakup of the jet column, undulations in the form of sinusoid due to Rayleigh-Plateau instability appear on the jet column and even pinch the jet into a stream of pupa-shaped segments when the free fall distance of the jet in the air is large enough. It can be concluded from the experiments that Rayleigh-Plateau instability has the dominant effect on the melt jet column breakup in water. Then nonorthogonal central-moment multiple-relaxation-time color-gradient lattice Boltzmann method is used to study the hydrodynamic melt jet breakup process in three-dimensional space. Firstly, the accuracy of the current model to reproduce the melt jet breakup process is validated by comparing the simulation results with two demo Wood’s metal jet breakup experiments. The mechanisms of melt jet breakup in water are analyzed in detail. Then the model is used to study the characteristics of molten corium jet breakup in water under actual reactor conditions. The corium jet penetration depth, breakup length, fragment size and the corium droplet film boiling heat transfer are thoroughly studied.

    Research areas

  • supercritical fluids, heat transfer deterioration, finite volume method, two wall temperature peaks, diameter effect, fuel-coolant interaction, melt jet breakup, experiments, lattice Boltzmann method