Use of Ultrasound Thermometry to Study Natural Convection in Opaque Nanofluids

Nanofluids are being evaluated as alternative heat transfer fluids, and thus their behavior during storage or low velocity applications in which natural convection can be significant has to be known and documented. Buoyancy induced flows in rectangular enclosures using nanofluids can be studied experimentally using thermocouples, thermistors, surface heat flux sensors, and ultrasound thermometry. The effects of the mass fraction concentration of nanoparticles, the enclosure aspect ratio, and the inclination have been studied experimentally, but more could be done. The opacity of nanofluids does not permit the use of particle image velocimetry, laser induced fluorescence, or any other means of flow visualization or visual temperature measurement of the local fluid temperature. However, the temperature field can be observed using a non-invasive method such as ultrasound thermometry. The experimental enclosure here was validated using water as the initial fluid; measured values of the local fluid temperature were compared with numerical simulations utilizing commercial software. Nanofluid mass fractions of 10% and 25% were used for comparative purposes to study the effects of concentration on the temperature field. Buoyancy force reversal effects were witnessed in both 10% and 25% concentrations. The nanofluid also prolonged the multicellular effects that occur in buoyancy inversion flows. A Rayleigh number inversion was observed with the 25% mass fraction nanofluid. The multicellular regime transitions to a boundary layer regime at about Ra = 1 × 107 when the aspect ratio is 2.625 and at about Ra = 2 × 108 when the aspect ratio is 1.000 for different concentrations of nanofluid. The observations could be physically explained. The current work confirms that temperature measurements of the flow field can be made to assess convective regimes and flow phenomena that induce significant temperature variations. The use of ultrasound thermometry is successfully demonstrated for opaque nanofluid.