## 1 Introduction
For example, this can cause flow turbulence and a corresponding intensification of heat transfer by up to two times compared to smooth surfaces [12; 13]. [PERSON] and colleagues found that a significant increase in pipe roughness also caused a rise in the flow turbulence intensity (TI) and an intensification of heat transfer by 4-13% compared to a smooth pipe [14]. There are also other ways to turbulize the flow of liquid and gas in order to intensify heat exchange, for example, by installing an aerodynamic spoiler or creating special fins on the channel surface [15; 16]. This can lead to an increase in the HTC by 170% compared to a smooth surface. It is also known that the cross-sectional shape of a pipeline can have a significant impact on the gas-dynamic conditions of heat exchange of stationary and pulsating gas flows [17; 18; 19; 20]. Thus, [PERSON] and colleagues studied in detail the formation and development of secondary flows in a pulsating flow in the corners of a square pipe [17; 18]. The authors described the physical mechanism of eddy currents in the corners of a square and proposed a mathematical model for their prediction. [PERSON] et al. reviewed the studies on the features of gas dynamics and heat transfer in triangular tubes [19]. It was shown that strong secondary flows were also formed in triangular tubes. These secondary flows significantly transformed the gas-dynamic characteristics and also caused a modification of the heat exchange level in the gas-dynamic system under consideration. Many researchers have paid attention to the comparison of the heat transfer intensity of stationary and pulsating flows in square and triangular channels [19; 21; 22; 23]. Thus, [PERSON] et al. found that there was a slight decrease in the HTC in a square with a pulsating movement of the medium compared to a stationary flow [21]. Similar data were obtained by [PERSON]: gas-dynamic non-stationarity in a square pipe caused suppression of the HTC by up to 15% [22]. [PERSON] et al. compared the level of heat exchange of stationary flows in round and square pipes [23]. It was shown that changing the shape of the cross section has a weak effect on the heat transfer intensity. At the same time, [PERSON] and colleagues found a slight intensification of heat exchange in a triangular pipe (TrP) compared to a round one [19]. Accordingly, it can be concluded that the gas dynamics and heat exchange of flows in pipes with different cross-sectional shapes have not been studied in sufficient detail: different configuration and regime conditions in the formulation of the research problem lead to the opposite effects. There have also been developments and studies on the use of specific tube designs for heat exchange intensification [24; 25; 26]. For example, [PERSON] and colleagues investigated the features of gas dynamics and heat exchange in wave and arc tubes [24]. The results showed a noticeable intensification of heat exchange in almost all studied modes (for all Reynolds numbers). [PERSON] and [PERSON] obtained similar results for a sinusoidal tube [25]. It should be noted that the studies were conducted for stationary and pulsating flows. In this case, gas-dynamic non-stationarity led to an additional increase in the HTC. [PERSON] et al. used diamond-shaped tubes in a heat exchanger to improve heat transfer [26]. Accordingly, the study of the influence of configurational (various cross-sectional shapes of channels) and regime (stationary and pulsating flows) factors on the gas dynamics and heat exchange of flows in pipelines remains a popular and important task in the development of science and technology (for example, heat pipes [27], power machines [28], thermoelectric generators for exhaust gases [29], etc.).