Heat Transfer in the Acoustic Field of an Ultrasonic Atomizer

In the last few years, molten metals - with high surface tension compared lo other liquids have been disintegrated in acoustic standing-wave fields successfully. Cooling and solidification rates of those materials are very important parameters for the grain structure and therefore the material properties of atomized metal powders. This investigation is focused on the heat transfer as basic knowledge for the calculation of the thermal particle history. The heat transfer has been measured for different flow conditions of the acoustic field without atomization. The acoustic field is generated by two acoustic transmitters, which are oscillating with a frequency of about 20 kHz (10 kHz) and have a frequency difference of about 100 Hz. Cylindrical probes (hot film or hot wire) are used to investigate the heat transfer in the acoustic standing-wave field. There are several papers about heat transfer in oscillating flow fields available but only for lower frequencies and much lower sound levels. Several parameters which can affect the heat transfer more or less directly are investigated. The distance between the two transmitters (10 to 100 mm) and their diameter (35 and 90 mm), the amplitude of the transmitters (40 to 120 μm), the frequency (10 and 20 kHz), the gas pressure (0.1 to 1 MPa) and the gas itself are varied. Due to the high sound pressure it is possible to get about 30-times higher Nu numbers compared to free convection in ambient atmosphere The Nu number increases very strong using higher surrounding gas pressure. Also higher transmitter amplitudes and lower distances between the transmitters lead lo an increase of the heat transfer. The influence of the frequency (in the tested range) is neglectable if the heat transfer is compared at same sound particle velocities. Moreover the local distribution of the Nu number in the acoustic field is shown. Due to the standing wave the heat transfer strongly depends on the measuring location. finally, all results arc summarized in an empirical equation which predicts the heat transfer in the acoustic field.