Measurement of flow velocity during turbulent natural convection in nanofluids

Abstract

Increased cooling performance is eagerly required for many cutting edge engineering and industrial technologies. Nanofluids have attracted considerable interest due to their potential to enhance the thermal performance of conventional heat transfer fluids. However, heat transfer in nanofluids is a controversial research theme, since there is yet no conclusive answer to explain the underlying heat transfer mechanisms. This study investigates the physics behind the heat transfer behavior of Al2O3–H2O nanofluids under natural convection. A high spatial resolution flow velocimetry method – Particle Image Velocimetry – is employed in dilute nanofluids inside a Rayleigh-Benard configuration with appropriate optical access. The resulting mean velocity and flow structures of pure water and nanofluids are reported and their overall heat transfer performances are compared for Rayleigh numbers, Ra, of the order of 109. This paper aims to identify the contribution of the suspended nanoparticles on the heat and mass transfer mechanisms in low flow velocity applications, as those occurring during natural convection. The outcome of this work is a first step towards the evaluation of the applicability of nanofluids in applications where more complex heat transfer modes, namely boiling and Critical Heat Flux, are involved that are of great importance for the cooling of Fusion reactors

Assessing the flow characteristics of nanofluids during turbulent natural convection

“…It is noted that the reported data are mean values out of 12 experiments (four experiments per plane), while the methodology for the heat transfer calculations is similar with that reported in previous work by the authors [2]. By comparing the general heat transfer, as expressed with the Nu, no change is observed within the experimental uncertainty between water and nanofluids, thus verifying earlier PIV experiments conducted in a single plane close to the centre of the cubic RB cell under three different Ra [44].…”

“…Following earlier findings [44], it is reconfirmed that the maximum temporally averaged absolute velocity V j j avgÀmax , the spatially and temporally averaged absolute velocity V j j avg and the temporally and spatially averaged standard deviation of the turbulent velocity fluctuations Stdev are consistently higher for nanofluids than for DI water in the field of view. Additionally, it is demonstrated that this trend is consistent in all three planes, with the maximum difference between the velocity characteristics for DI water and nanofluid taking place at an absolute distance z of 4 ± 1 mm (front plane) from the central plane.…”

“…The nanofluid results were compared with those of DI water that was used as a benchmark. Following earlier PIV experiments [44], it was confirmed that the addition of a small amount of Al 2 O 3 nanoparticles to DI water, u v = 0.00026 vol.%, modifies the mass transfer behaviour of the base fluid in the field of view, under hard turbulence-Rayleigh number of the order of 10 9 . It was demonstrated that the maximum temporally averaged absolute velocity V j j avgÀmax and the spatially and temporally averaged absolute velocity V j j avg are higher for nanofluids compared to DI water at all three planes inside the RB cell.…”

Low-Intensity Pulsed Ultrasound Effect on MIO-M1 Cell Viability: Setup Validation and Standing Waves Analysis

“…Interior boundaries were set with a continuity condition. Natural convective heat transfer was assumed small with a velocity component of 0.1 mm/s in the z-direction [27]. Mesh was kept the same as in the acoustic models.…”

“…In this model, it was omitted the effect of agitation produced by the ultrasound radiation force that could act as an external force convection and should permit a more distributed heat. Although nature convection was included based on [27], the effect of acoustic agitation should be more significant. Therefore, real-temperature increments in the dish produced by ultrasound absorption should be smaller and more uniform than those shown in Figure 4 and can be comparable to the measured increments of 0.3 • C reported by [13].…”