Fast heating of fluids by stirring

May 9, 2023

Ruud Lensvelt defended his PhD thesis at the department of Mechanical Engineering on May 9th.

Image: iStockphoto

In fluid heating, two facets require simultaneous optimization; heat-transfer into a fluid, and heat transport through the fluid. The balance between these competing aspects is crucial for fast heating. For his PhD research, Ruud Lensvelt considers a system that transfers heat into a cold fluid via a hot boundary, with the heat transport influenced by fluid stirring in the system.

For the PhD research of Ruud Lensvelt, the central question is “How is it possible to stir a fluid so as to heat the fluid as quickly as possible?” In his thesis, Lensvelt presents novel active methods to boost fluid heating by stirring.

Flow controller

First, a flow controller, that determines the ‘optimal’ flow field for heating by way of a cost-function prediction, is presented by Lensvelt. The flow controller significantly boosts fluid heating in comparison to the conventional stirring approach.

Moreover, rapid heating of existing systems can be accomplished by either (slightly) altering the conventional stirring sequence or introducing aperiodic transients into conventional stirring sequences. The cost-function captures both fluid heating facets, and an analysis reveals that the flow field acts as the thermal actuator to influence these facets simultaneously.

Cost functions

The analogy of the cost functions with thermal energy makes similar cost functions well suited as Lyapunov functions. This view lies at the foundation of two novel flow controllers that boost fluid heating as well.

The flow controllers developed by Lensvelt rely on the complete temperature field of the fluid. In practice, the field is only partially available through a discrete sensor set. Therefore, an estimator is designed for numerical and experimental temperature field reconstruction.

Title of PhD thesis: Fast heating by feedback flow control. Supervisors: Henk Nijmeijer and Michel Speetjens.