Experimental study on thermal and friction characteristics of liquid flow across a heating cylinders under low-frequency ultrasound

Abstract

• The heat transfer of water flow across a cylinder under a 25 kHz ultrasound was studied. • Visualization by TLCs on shaded surface was achieved using unique image processing. • Heat transfer area on cylindrical surface could be identified by TLCs. • The waves emitted downstream could affect the cylindrical surface at the upstream. • A prediction formula for Nu of the flow across a cylinder under ultrasound was proposed. The heat transfer capability was investigated for water flow across a cylinder under the influence of 25 kHz ultrasound in a rectangular duct with a Reynolds number ( Re ) in the range of 110–2,140. A heating cylinder with constant heat flux was set at the center of the duct. The transducer was mounted on the top wall, and its location was set at -2, -1, 0, +1, and +2 diameters from the cylinder in the streamwise direction. Thermocouples and thermochromic liquid crystals (TLCs) were used to characterize the heat transfer mechanism on the heating surface. In particular, a visualization technique using TLCs was developed to obtain temperature results on a cylindrical surface. The temperature of the heating surface was reduced by the acoustic cavitation process with ultrasonic involvement. The local Nusselt number was increased by 2.37-fold by the waves at a cylindrical angle of 90° and Re value of 110 when the transducer was also located in the middle of the duct. Furthermore, the results indicated that ultrasonic effects could be transported along with the flow when the transducer was located upstream. In addition, when the waves were released downstream, they affected the heat transfer of the cylindrical surface at low Re values. The friction loss in the system was investigated by comparing the pressure drops with and without ultrasound. Depending on the position of the ultrasonic transducer, the pressure drop ratio was in the range 0.88–1.27. Furthermore, the areas most affected by ultrasound were identified using the TLC measurement method, and regions of augmented heat transfer were detected locally on the shaded and unshaded sides of the cylinder. A predictive formula was proposed for the local Nusselt number ratio with and without ultrasound as a function of the Re value, spanwise direction, and cylindrical angle.