Effects of 25 kHz ultrasound from single and double transducers on thermal and friction characteristics of laminar flows in water or water-based Al2O3 nanofluids

The heat transfer and flow resistance were investigated experimentally in a rectangular duct based on compound techniques of heat transfer enhancement using 25 kHz ultrasound and Al2O3-water nanofluid. The experiment was conducted at flow rates of 0.5–2.0 l/min under constant heat flux and laminar flow conditions. Two ultrasonic transducers were mounted on the top wall of the test section to release the waves from single or double transducers in a downward direction, perpendicular to the mainstream flow. Water and Al2O3(nanoparticle volume fraction (φ) = 0.8–6.1%) nanofluid were utilized as different test fluids. The results showed that the surface temperature increased with the volume fraction of Al2O3, and rapidly reduced after applying the ultrasonic waves. At the same flow rate, the peak augmentation of the local Nusselt number using ultrasound from a single transducer was 1.31 in Al2O3(φ = 6.1%), which increased to 1.58 using double transducers in Al2O3(φ = 1.5%). The flow resistance was increased by adding the Al2O3 nanoparticles, with increased resistance with an increased volume fraction. The friction loss increased up to 3 times for φ = 6.1% compared to the base fluid of water, while it slightly increased approximately 1–2% using ultrasound. These results indicated that using these techniques, the thermal performance tended to increase with a decreased Reynolds number and volume fraction. Finally, predictive formulas for the Nusselt number and friction factor were developed for using single or double ultrasonic transducers in water or Al2O3(φ = 0.8–6.1%) nanofluids under a laminar flow regime.