# Enhancement of Convection Heat Transfer in Air Using Ultrasound
## 4 Results and Discussion

Table 4: Results of heating in temperature 60 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Applied US Power**} & \multicolumn{2}{c}{**0 W**} & \multicolumn{2}{c}{**50 W**} \\ \cline{2-5}  & \(h\) **(W/K-m2)** & \(\dot{Q}_{abs}\) **(W)** & \(h\) **(W/K-m2)** & \(\dot{Q}_{abs}\) **(W)** \\ \hline Small cylinder & 19.68 \(\pm\) 0.10 & - & 21.93 \(\pm\) 0.08 & 0.14 \(\pm\) 0.09 \\ \hline Cylinder & 18.81 \(\pm\) 0.17 & - & 22.43 \(\pm\) 0.10 & 0.07 \(\pm\) 0.04 \\ \hline Cube & 17.72 \(\pm\) 0.09 & - & 24.97 \(\pm\) 0.68 & 0.17 \(\pm\) 0.03 \\ \hline Cuboid & 14.35 \(\pm\) 0.08 & - & 15.43 \(\pm\) 0.08 & 0.06 \(\pm\) 0.02 \\ \hline \multirow{2}{*}{**Applied US Power**} & \multicolumn{2}{c}{**100 W**} & \multicolumn{2}{c}{**200 W**} \\ \cline{2-5}  & \(h\) **(W/K-m2)** & \(\dot{Q}_{abs}\) **(W)** & \(h\) **(W/K-m2)** & \(\dot{Q}_{abs}\) **(W)** \\ \hline Small cylinder & 24.60 \(\pm\) 0.15 & 0.26 \(\pm\) 0.12 & 28.65 \(\pm\) 0.14 & 0.46 \(\pm\) 0.02 \\ \hline Cylinder & 30.40 \(\pm\) 0.16  
The results confirmed that the greater the power of the ultrasound, the greater the improvement in heat transfer. In the investigated range of applied ultrasound power, the  
Figure 5: Heat transfer coefficients \(h\) versus applied ultrasound power \(P\): (**a**) small cylindrical sample; (**b**) cylindrical sample; (**c**) cube; and (**d**) cuboid. Figure 6: Heat transfer enhancement ratios \(\eta\) (%). Air temperature: (**a**) 40 \({}^{\circ}\)C; (**b**) 60 \({}^{\circ}\)C. Presented data are mean \(\pm\) standard deviation. Different letters above the bar of the same color indicate statistically significant difference at \(p\leq 0.05\) in accordance to Tukey posthoc mean comparison. heat transfer coefficient is a linear function of the applied power (see Figure 5). It can be observed that the heat exchange enhancement strongly depends on the size of the samples. Due to the use of ultrasound, the convective heat exchange coefficient increased from 45% to almost 250%. There are few results in the literature describing the increase in the heat transfer coefficient due to the use of ultrasound. [PERSON] et al. [18] found a 12-44% increase in this coefficient and [PERSON] et al. [43] obtained a 34% increase, both in the case of carrot drying, while [PERSON] et al. [9] obtained a 65% increase in grape stalk drying. Heat transfer enhancement for both small (small cylinder) and large (cuboid) samples was smaller than the improvement in heat transfer for medium samples (cylinder and cube). In the latter case, an increase in growth of almost 250% of the heat transfer coefficients was achieved. The obtained results show that, in the studied temperature range, a higher air temperature is associated with a greater heat exchange improvement. This is contrary to the results presented by [PERSON] et al. [9] and [PERSON] et al. [14], who ascertained that an increase in temperature causes a decrease in the enhancement of heat transfer. This inconsistency may result from the difference in the construction of the dryers, particularly in the way the ultrasound is brought. The enhancement of heat transfer can be caused by various mechanisms. For instance, pressure pulsations could increase the turbulence inside the boundary layer and reduce the layer thickness. It is worth noticing that the heat transfer through a boundary layer is done by the slowest mechanism, i.e., conduction. If the thickness of the boundary layer is reduced, the heat transfer proceeds more effectively. Moreover, alternating compression and rarefaction could improve heat transfer through the boundary layer. Fluctuations of pressure close to the material surface could also promote circulation flows of air and enhance the heat transfer. A number of additional mechanisms may be considered, e.g., acoustic streaming. The thermal effect was also observed in our experiments. The maximum temperature increase due to absorption of the ultrasound energy by the samples was in the range of 5-8 degrees.