Thermal Division

Heat Transfer - Unsteady Conjugate


Many energy conversion and other thermal-fluid systems exhibit unsteady convective heat exchange. In such systems, generic spatiotemporal variations in the flow give rise to variations in the heat flux for a given fluid–solid temperature difference, which can be interpreted as spatiotemporal fluctuations of the instantaneous heat transfer coefficient.

These variations can lead to unsteady conjugate heat transfer, in which the exchanged heat flux arises from an interaction between the bulk fluid temperature and the temperature in the solid. Further, the nonlinear coupling between the fluctuating temperature differences and the heat transfer coefficient can lead to an effect we refer to as augmentation, which quantitatively describes the ability of a particular arrangement to have a different time-mean heat flux from the product between the mean heat transfer coefficient and the mean temperature difference across the fluid. It is important to be able to understand and to model in a simple framework the effects of the material properties, the geometry and the character of the heat transfer coefficient on the thermal response of the fluid–solid system, and to predict the overall heat transfer performance of these systems.

This work is concerned with the phenomenon of heat transfer augmentation in one-dimensional unsteady and conjugate fluid–solid systems. The overriding purpose is to propose a simple framework for the description of the effect of unsteadiness on the overall heat exchange performance of these systems, leading to the improved understanding and prediction of related processes. The findings can be used in the design of improved heat exchangers or thermal insulation, for example through the novel selection of materials that can exploit these augmentation effects.

IR Measurements on the Front of the Film
IR Measurements on the Front of the Film

 

Thermo Hydraulics of Molten Salt Reactors


This work focuses on the thermo hydraulics of the new, 4th generation, molten salt nuclear power plants. It aims at investigating the transient behaviour of natural circulation loops employed to remove waste heat from the reactor. This study involves the development and of a thermo hydraulic model of the safety system and its software implementation. 3D finite element analysis is used to investigate the behaviour of the molten salt in the pebble beads reactor core. An experimental technique to measure the thermal conductivity of molten salts at high temperature is being developed. This type of measurements presents many challenges due to the aggressiveness of the salt and the high temperatures involved.

Unsteady flow around a sphere at Re = 400 used to validate the physical model
Unsteady flow around a sphere at Re = 400 used to validate the physical model