Visualisation of heat transfer in laminar flows

Heat transfer in fluid flows traditionally is examined in terms of temperature field and heat-transfer coefficients at non-adiabatic walls. 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. To date this concept has been restricted to 2D steady flows. The present study proposes a generalisation of this concept to 3D unsteady flows by considering the heat transfer as the 3D unsteady motion of a virtual fluid subject to continuity. This ansatz is particularly suited for laminar flows, where flow and thermal paths are well-defined, and has great potential for heat-transfer problems in which insight into the heat fluxes throughout the entire configuration is essential (e.g. conjugate heat transfer, heat exchangers, cooling applications). The heat-transfer visualisation in 3D unsteady flows is provided with a physical framework and demonstrated by way of representative examples.