## 2 Description of the Research Task and Measuring Instruments  
The study of non-stationary gas dynamics and heat exchange of flows in straight pipes was carried out by means of experimental approaches based on the thermal anemometry method. A laboratory setup was developed for these purposes, the scheme of which was shown in Figure 1.  
The laboratory setup consists of the following main elements: (1) a vacuum pump to create air movement in the gas-dynamic system; (2) straight pipelines with circular and triangular cross-sections; (3) static flat air flow turbulators; (4) a rotating damper to create air flow pulsations; (5) an automated measuring system based on a constant-temperature hot-wire anemometer (H-WA). A photograph of the experimental setup is shown in Figure 2 (the measurement system is described below). The flow pulsations in the gas-dynamic system were created by means of a damper (Figure 3a), the rotation frequency of which could be changed by means of a voltage regulator. In this way, air flow pulsations with a frequency \(f\) from 3 to 16.6 Hz were generated. Such pulsating flows are typical for various periodic devices (for example, piston machines) and for pulse devices of various purposes. Stationary flat turbulators were used to change the turbulence intensity TI of stationary and pulsating flows in straight pipes (Figure 3b). Several turbulators with different numbers of holes and different hole diameters were used. Thus, the flow TI varied from 0.03-0.04 (flow without a turbulator) to 0.12 (maximum average TI value). Figure 3: General view of the rotating damper (**a**) and geometric dimensions of the flat turbulator (**b**): 1—pipeline; 2—damper; 3—flat turbulator. Figure 2: Photograph of the experimental setup: 1—inlet section; 2—rotating damper; 3—flow turbulator; 4—test pipe; 5—control section with sensors; 6—damper electric drive; 7—H-WA; 8—ADC; 9—laptop. Two straight pipeline configurations were used in the study. The base pipe had a circular cross-section with an internal diameter of 30 mm. The alternative pipe had a cross-section in the form of an equilateral triangle with a side length of 39 mm. The geometry of the TrP was chosen based on the equality of the cross-sectional area of both pipes. The length of the pipelines was 1 m in both cases. The scientific idea of studying a TrP is based on known data that strong secondary flows are formed in channels with transverse profiling, which significantly affect the gas-dynamic and heat exchange characteristics of the flows [30; 31]. The pipelines were made of a metal alloy. The inner surfaces of the pipes had a technically smooth surface (the roughness of the surface did not affect the intensity of heat exchange between the air flow and the wall). The working medium in the experiments was air with a temperature of 22 \(\pm\) 1 \({}^{\circ}\)C moving at a speed of 5 m/s to 75 m/s (10,000 \(<\) Re \(<\) 150,000), i.e., the flow regime was developed turbulent. The experiments were carried out at an atmospheric pressure of 98.0 \(\pm\) 0.2 kPa. The following parameters were measured in the experiments: (1) instantaneous values of the air flow velocity \(w_{x}\) (using a H-WA with a response time of no more than 3 ms); (2) instantaneous values of the local HTC \(\alpha_{x}\) (using a H-WA with an identical response time); (3) flow temperature (using thermocouples); (4) damper rotation frequency (using an inductive sensor); (5) atmospheric pressure (using a manometer). The thermal anemometry method was the main research method in this work. Special sensors with a thread sensitive element were manufactured for a H-WA to measure \(w_{x}\) and \(\alpha_{x}\)[32; 33]. The thread had a diameter of 5 \(\upmu\)m and a length of about 5 mm. The thread was placed on posts approximately in the center of the pipe to determine \(w_{x}\). The thread was mounted on a substrate and was flush with the inner surface of the pipe to determine \(\alpha_{x}\). A constant temperature H-WA from Irvis (Moscow, Russia) was used; the sensors were manufactured independently.