# Enhancement of Convection Heat Transfer in Air Using Ultrasound
## 1 Introduction  
Almost every food product is dried during its processing. Drying extends storage life and facilitates handling as well as transportation. Drying also influences the quality of a final product. It changes the colour, flavour, and nutritional value of food. Due to the quality of the product, low-temperature drying is recommended. Unfortunately, low-temperature convective drying is energetically inefficient. Therefore, new methods to improve the process without raising the temperature are sought. One of these methods is the use of high-intensity ultrasound (US) [1]. Reviews of the use of ultrasound to enhance drying could be found in [2; 3; 4]. According to the literature [5; 6; 7; 8; 9; 10], ultrasound enhances the capacity for moisture transport inside the material to be dried in addition to enhancing heat and moisture transfer between the material surface and the surrounding drying medium. [PERSON] et al. [5] considered sonic and ultrasonic dewatering. They presented the possible effects of acoustic waves during the process. The use of ultrasounds improved the kinetics of preslimmon [6], apples [7; 8; 10], carrots [8], eggplant [8], and grape stalk [9] drying. An increase in the mass transfer coefficient due to the application of ultrasonic in drying is widely reported [6; 11; 12; 13; 14; 15; 16; 17]. [PERSON] et al. reported an increase of between 12% and 34% during perslimmon drying [6], 10% and 47% during carrot drying [11], 47% during strawberries drying [16], 79% during green pepper drying [17], 96% during raspberries drying [15], 107% during orange peel drying [12], and even 229% during eggplant drying [13]. [PERSON] et al. [14] studied the influence of drying temperature on the ultrasound enhancement of thyme drying. They did not obtain an increase in the moisture transfercoefficient in high temperature drying (80 \({}^{\circ}\)C); they did obtain a 65% increase in low temperature (40 \({}^{\circ}\)C), however. Only a few authors determined the heat transfer coefficient based on drying kinetics. [PERSON] et al. [14] obtained an increase of between 12% and 47% of the heat transfer coefficient during thyme drying. [PERSON] et al. [18] reported an increase of between 12% and 44% during carrot drying. [PERSON] et al. [19] received a 31% increase in the case of apple drying and 83-94% in the case of carrot drying. Taking into account the literature devoted to drying, only [PERSON] and [PERSON] [20] carried out the experiments in which they directly studied the influence of ultrasound on the heat transfer coefficient. They performed heating and cooling of an aluminum plate under the same conditions as used in drying experiments. They found out that ultrasound caused an increase in this coefficient (27.9%) in the case of heating, and a small decrease during cooling. It is also obvious that ultrasound energy could be absorbed by the dried material. [PERSON] [21] described the phenomenon on the basis of irreversible thermodynamics; however, he did not present any numerical results in his work. The increase in material temperature due to ultrasound absorption in the range of 5 K is reported in [22] for the drying of apples. Similar values of the temperature increase were obtained by [PERSON] et al. [19]. The authors modelled the drying kinetics and they obtained ultrasound energy absorption in the range of 1-4.5 per mille. This thermal effect is minor or even not observed, and is usually neglected [11; 23; 24; 25; 26]. On the other hand, the phenomenon of heat transfer augmentation through the use of ultrasound is widely reported [27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. In most cases, the heat transfer in water [27; 28; 32; 33; 36; 37; 38; 39] and other liquids (methanol [27; 28], liquid FC-72 [30], ethylene glycol-water mixture [34], and LiBr solution [40]) was studied. In most cases, free convection in liquids and boiling was considered [28; 30; 35; 39]. Only a few works were dedicated to forced convection [32; 33; 40], which is typical of the drying processes. [PERSON] et al.