This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Heat transfer in fluid flows traditionally is
examined in terms of temperature field and heat-transfer coefficients. However, heat transfer may alternatively be considered as the transport of thermal energy by the total
convective-conductive heat flux in a way analogous to the transport of fluid by the flow field. The paths followed by the total heat flux are the thermal counterpart to fluid trajectories and facilitate heat-transfer visualisation in a similar manner as flow visualisation. This has great potential for applications in which insight into the heat fluxes throughout the entire configuration is essential (e.g. cooling systems, heat exchangers). To date this
concept has been restricted to 2D steady flows. The present study proposes its generalisation to 3D unsteady flows by representing heat transfer as the 3D unsteady
motion of a virtual fluid subject to continuity. The heat transfer visualisation is provided with a physical framework and demonstrated by way of representative
examples. Furthermore, a fundamental analogy between fluid motion and heat transfer is addressed that may pave the way to future heat-transfer studies by well-established
geometrical methods from laminar-mixing studies
