## 1 Introduction
The small cross-section of the channels together with their large number results in a large heat exchange surface. This allows a high value of heat flux density to be handled. In addition, the method allows for good temperature uniformity in the cooling system. Water was usually used as the coolant. However, the better thermal properties of metal suspensions resulted in a growing interest in their use for heat exchange in microchannels. The disadvantage of this type of suspension is the possibility of channel clogging, as well as the lack of flow stability [14]. Therefore, it was proposed to use nanofluids, i.e., suspensions of solid particles with dimensions of the order of nanometers [15]. Although nanofluids used in the microchannels can improve cooling capacity, after a period of operation, the deposition of nanoparticles leads to clogging of the microchannels, making them ineffective and even damaging electronic devices [1]. A significant part of the research on the use of microchannels for refrigeration is devoted to the shape of the channels. Most of the work concerns rectilinear micro-channels. The simplest solutions are channels with a constant rectangular [13; 16], trapezoidal [16; 17], triangular [16], or circular cross-section [18]. A comparison between these cross-sections shows better thermal performance of rectangular and circular channels than trapezoidal and triangular ones [16; 18]. The second group consists of rectilinear microchannels with a variable cross-section. Various structures are proposed, e.g., alternately converging and diverging microchannels [19] with different ribs [20; 21] or fans inside the microchannel [22]. A comprehensive overview of these types of flow structures is provided in [14]. The third group of works is devoted to the study of microchannels with a constant cross-section but non-rectilinear course of the channel. [PERSON] and [PERSON] [23] experimentally investigated heat transfer in straight and zigzag-shaped microchannels with water as the cooling fluid. They observed that the heat transfer in zigzag microchannelsincreased from 58 to 88% compared to straight channels. At the same time, they observed an increase in pressure drop from 22 to 35% compared to simple microchannels. [PERSON] and coworkers experimentally [24; 25] and numerically [26] investigated flow friction and heat transfer in sinusoidal microchannels with a rectangular cross-section. They used water as a coolant. The test results showed that because of efficient mixing in sinusoidal channels, the heat transfer efficiency is much better than in straight channels with the same cross-sections. At the same time, the increase in pressure drop was much smaller than the improvement in heat transfer. Wavy microchannels with a rectangular cross-sections were examined by [PERSON] et al. [27]. They analyzed water and three nanofluids as a coolant. In the case of water as coolant, the wavy shape was clearly better. The higher the amplitude and shorter the wavelength of the wave channel, the lower the thermal resistance, especially for higher Reynolds number values. However, when nanofluids were used as coolant, the effect of the wavy channels was unnoticeable and the wavy and straight runs gave very similar cooling efficiency. This means that the use of nanofluids in curved microchannels is not appropriate. [PERSON] et al. [28] studied numerically the flow of water in similar wavy microchannels with a rectangular cross-section. By optimizing the shape of the microchannel, they achieved a heat transfer improvement of 2.8 times compared to the flow in a straight-line channel for flow with a high Reynolds number. [PERSON] et al. [29; 30] examined numerically the influence of the microchannel shape on the hydraulic and thermal performance. They analyzed straight, sinusoidal, zigzag, curvy, and step-shaped microchannels. All microchannels had a rectangular cross-section, and water was used as the coolant. The best thermal performance shows a zigzag microchannel.