# Enhancement of Convection Heat Transfer in Air Using Ultrasound
## 2 Materials and Methods  
The aim of the presented work is to examine only the thermal effects. Mass transfer is not studied. That is why all tested samples are made of carbon steel (density \(\rho\) = 7820 kg/m\({}^{3}\), specific heat \(c\) = 473.3 J/kg\(\cdot\)K, and thermal conductivity \(\lambda\) = 42.9 W/m\(\cdot\)K). Four samples were used: a small cylinder, a cylinder, a cube, and a cuboid (Figure 1). Sample dimensions are given in Table 1. In order to measure the temperature, holes with a diameter of 2.5 mm to a depth of 10 mm were drilled into the geometric centre of the samples. During the test, the measuring probe of the temperature recorder was placed in these holes.  
The samples were heated convectively inside a hybrid dryer (Figure 2) equipped with the Airborne Ultrasound System--AUS (Pusonics, Madrid, Spain), which allows generation of a low-frequency (26 kHz), high-power ultrasound (maximal power 200 W). The AUS works with a focalized acoustic field, which means that the intensity of the waves converges with increasing distance from the transducer, and at about 420 mm from the radiator it attains its maximum at 160-170 dB. The distance between the emitter and the sample was fixed at 415 mm to ensure that the sample stayed within the focusing area during the drying process.  
For each sample, eight measurement series were made. The measurements were performed for two different air temperatures and four ultrasound powers. The process parameters are shown in Table 2. The temperature and flow velocity of the air were measured continuously during the experiment with the HD26371 TC1.5 transmitter (DeltaOHM, Caselle di Selvazzano, Italy). According to the device spec sheet, the accuracy for velocity is \(\pm\)0.1 + 3% and for temperature 0.3 \({}^{\circ}\)C. Experimental conditions ensured a small Biot number regime of heat transfer (the conduction resistance is negligible).  
During the experiments, the temperature inside the test samples was measured every 30 s, with the use of a type-K thermocouple and recorded with a CENTER 309 datalogger (New Taipei City, Taiwan) with an accuracy of \(\pm\)1 \({}^{\circ}\)C + 0.3%. Each experiment was conducted until three equal temperature readings were obtained.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Diameter** & **Height** & \multirow{2}{*}{**Sample**} & **Length** & **Width** & **Height** \\  & **(mm)** & **(mm)** & & **(mm)** & **(mm)** & **(mm)** \\ \hline Small cylinder & 20 & 20 & Cube & 30 & 30 & 30 \\ \hline Cylinder & 30 & 30 & Cuboid & 120 & 20 & 30 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sample dimensions.  
Figure 1: Used samples.  
Figure 2: The scheme of the hybrid dryer: (1) air heater and fan, (2) dryer chamber, (3) ultrasound transducer, (4) microwave generator, (5) balance, (6) control system (PC), and (7) ultrasound generator.  
Obtained experimental data were processed with the use of OriginPro 2021b (Origin-Lab Corporation, Northampton, MA, USA). Data presented are mean \(\pm\) standard deviation. Additionally, a one-way ANOVA and Tukey posthoc mean comparison were performed. Statistically significant differences at the level of \(p\leq\) 0.05 are marked with different letters. All calculations were performed using Statistica ver. 12 software produced by StatSoft (StatSoft, Tulsa, OK, USA).