Nanofluid as the working fluid of an ultrasonic-assisted double-pipe counter-flow heat exchanger

In this paper, the simultaneous impacts of using nanofluid and ultrasonic vibrations in a double-pipe heat exchanger are experimentally investigated. The vibrating heat exchanger is designed so that the ultrasonic waves with the power of 60 watts and frequency of 40 kHz are applied to its body at equal length distances in a uniform and effective manner. Water-based Al₂O₃ nanofluid is used in this research. The available empirical correlation has been used to confirm the accuracy of the measurements and validate the results. The effective thermal parameters have been tested in three cases using water, nanofluids, and ultrasonic-excited nanofluids as the working flow of the double-pipe heat exchanger. These tests have been performed in a relatively wide range of flow rate (113–257 lh⁻¹), Reynolds number (3230–7431), inlet hot fluid temperature (40–60 °C), and nanoparticle volume fraction (0.4–0.8%). The results indicate the positive effect of adding nanoparticles and applying ultrasonic vibrations, especially at higher inlet hot fluid temperatures and higher nanofluids concentrations. The nanoparticles are more effective at high-flow rates, whereas the ultrasonic vibration is highlighted at low-flow rates. Also, the effectiveness-NTU analysis carried out for the current heat exchanger shows that using nanofluid and ultrasonic-excited nanofluid instead of water can increase the efficiency of the thermal system up to 18.3% and 42.3%, respectively.