LAUSR

Abstract Nanofluids are advanced two-phase coolants that exhibit heat transfer augmentation phenomena. Extensive research has been performed since the year 2000 onwards to understand the physical mechanisms of heat transfer in nanofluids when employed inside traditional heat exchanging geometries. The focus of this paper is to understand if and how the geometry of heat exchangers might be potentially affecting the nanofluid coolant flow boundary conditions established and how this might be hence further affecting their thermal characteristics. HyperVapotrons are highly robust and efficient heat exchangers able to transfer high heat fluxes of the order of 10–20 MW/m 2 . They employ a complex two-phase heat transfer mechanism which is strongly linked to the hydrodynamic structures present in the coolant flow inside the devices. A cold isothermal nanofluid flow is established inside two HyperVapotron model replicas. A high spatial resolution (30 μm) visualisation of the nanofluid flow fields inside each replica is measured and compared to those present during the use of a traditional coolant (water). Significant geometry specific changes are evident with the use of dilute nanofluids which is something unexpected and novel. Evidence of a shear thinning mechanism is found inside the momentum boundary layer of the nanofluid flows that might prove beneficial to the coolant pumping power losses when using nanofluids instead of water and is expected to affect their thermal performance from a hydrodynamic point of view.