Direct simulation of liquid metal nucleate boiling in the presence of magnetic fields

The evolution of momentum, magnetic flux density, current and internal energy in liquid metals in the presence of applied, or internally generated, magnetic fields is an incredibly complex process that involves highly coupled physics. Developing mathematical frameworks, and the associated numerical implementations, that can be used to predict these flows is very challenging, and until recently such frameworks were only developed for homogeneous liquid metals (where hydrodynamic and electromagnetic properties are constant).

Recent developments at the University of Manchester [1] mean that these systems can be described mathematically and numerically for heterogeneous systems with rapidly changing magnetic fields. This permits the investigation of multi-phase flows where the properties may have large spatial contrast.

Nucleate boiling occurs when a solid surface is heated above an adjacent liquid's saturation temperature. In this scenario, vapour forms at preferential nucleation sites on the heated surface. While there has been much research into the computational modelling of nucleate boiling [2]. there have been comparatively fewer studies into the boiling of liquid metals, especially in the presence of magnetic fields.

This is despite the relevance to nuclear fusion blanket design. Fusion blankets are multi-purpose chambers that surround plasma in a tokamak reactor. A critical role of the blanket is to transfer nuclear heat (arising from high-kinetic energy bombarding neutrons) away from the first wall and into a steam generator where the steam is subsequently used to drive a turbine in a standard thermodynamic steam cycle.

In the context of fusion, a Lithium-Lead eutectic is often used as a working fluid. This metal flows in the presence of strong magnetic fields, inducing electric currents. Magnetohydrodynamic (MHD) body forces act on the fluid (the Lorentz force). This MHD effect opposes the motion of the metal and lowers the flow rate for a given pump head; typically, flow rates are too low to transfer sufficient heat from the first-wall to prevent thermal damage.

A proposed solution to this is to allow the coolant to boil. The advantage of this is twofold: 1) nucleate boiling is a highly effective method of heat transfer due to the high latent heat of vaporisation of typical working fluids. 2) In the vapour phase, the Lorentz force is much less severe. This allows for greater flow rates (and therefore enhanced heat transfer) for the same pressure drop along a given duct length.

The study of such flows is currently predominantly undertaken experimentally (with limited insights and high cost due to the fact metals are not transparent to visible wavelengths of light). The magnetic field alters bubble nucleation dynamics (bubble shape, departure diameter and frequency, nucleation site density, etc.).

The aim of this PhD is to extend the recent developments at The University of Manchester to incorporate state transitions, such as the aforementioned nucleate boiling and subsequent condensation, in the presence of applied and internally generated magnetic fields. Once developed and validated against existing experimental data in the literature [3], the tool will be used to perform fundamental flow physics studies in both pool and flow boiling configurations to better understand the complex flow physics involved.

The objectives for this PhD are to develop a numerical framework that extends current frameworks to incorporate the additional physics of interest. This will involve a significant amount of C++ programming and running the developed numerical tool on the high-performance computing resource at The University of Manchester.

You will have opportunities to collaborate with researchers at UKAEA. [1] Magneto-hydrodynamics of multi-phase flows in heterogeneous systems with large property gradients TF Flint, MC Smith, P Shanthraj - Scientific Reports, 2021 [2] De Rosis, A. and Skillen, A., 2022. Vorte