Heat transfer enhancement of water flow over a heating flat plate using 20 kHz ultrasonic waves irradiated from submerged horn-type transducer

Abstract

The enhancement of heat transfer of water flow over a flat plate under 20 kHz ultrasound irradiated from a submerged horn-type transducer was investigated experimentally in a water tunnel. To examine the ultrasonic effects, the transducer emitted waves perpendicular to the mainstream direction, and the freestream velocity varied between 0.12 and 0.17 m/s. The transducer position was fixed at 0.45 m from the plate leading edge in a streamwise direction. This position was altered in a heightwise direction as dimensionless heightwise distances (Z) that were divided by the diameter of the horn-tip transducer (12 mm) at 1.5, 3.0, and 4.5 from the plate surface. A hot film sensor and thermochromic liquid crystals coated on the test surface were used to demonstrate the hydrodynamic and thermal characteristics, respectively. The maximum temperature reduction of 3.5 °C was observed in the case of Z = 1.5. By employing a horn-type transducer, the acoustic jet and cavitation phenomena played a key role in destabilizing the thermal and velocity boundary layers, which resulted in a decrease in surface temperature. The incident wave shape was triangular, with an enlarging lateral zone along the flow stream. When the waves were released at Z = 1.5, the local Nusselt number was increased up to 2.55 compared to without waves. Under ultrasonic effects, the near-wall flow velocity was detected to be decreasing upstream prior to the incident zone while increasing in the incident region. When waves existed, the highest turbulent intensity was up to 29 times more intense than it was in the absence of waves. Furthermore, a predictive formula for the Nusselt number with ultrasound was established depending on the local Reynolds number and the distance between the ultrasonic source and test surface.