  Scalar transport (e.g. heat or chemical species) in laminar flows is key to
many industrial activities and fluid stirring by flow reorientation is a common
way to enhance this process. However, "How best to stir?" remains a major
challenge. This study aims to contribute to existing solutions by the
development of a dedicated flow-control strategy for the fast heating of a cold
fluid via a hot boundary in a representative case study. Fluid deformation acts
as the "thermal actuator" via which the flow affects heat transfer yet this may
both enhance and diminish heat exchange between fluid parcels and thus
restricts the beneficial impact of flow. Moreover, the impact of fluid
deformation on the global fluid heating is primarily confined to the proximity
of the moving boundary that drives the flow. These insights make incorporation
of thermal behaviour essential for effective flow-based enhancement strategies
and expose fluid mixing, the conventional approach in industry, as potentially
sub-optimal. The notion that global heating encompasses two concurrent
processes, i.e. increasing energy content and thermal homogenisation, yields
the relevant metrics for the formulation of the control problem as the
minimisation of a dedicated cost function. This facilitates step-wise
determination of the "best" flow reorientation from predicted future evolutions
of actual intermediate states and thereby paves the way to (real-time)
regulation of scalar transport by flow control. Performance analyses reveal
that this "adaptive flow reorientation" significantly accelerates fluid heating
and thus is superior to conventional periodic schemes (designed for efficient
fluid mixing) both in consistency and effectiveness. The controller in fact
never selects these periodic schemes and achieves the same superior performance
for all flow conditions irrespective of whether said mixing occurs.
