## 3 Experimental Research Results, Analysis and Discussion
At the same time, an increase in the pulsation frequency \(f\) actually does not affect TI (the differences are within the limits of experimental uncertainty). Other patterns of change in the flow TI occur in a triangular pipe (Figure 9). TI values grow with increasing flow velocity for stationary air flow in a Trp (Figure 9). However, the opposite trend is observed for pulsating flows: the TI values decrease with increasing velocity in a Trp. At the same time, the TI values are 1.36-2.2 times higher for pulsating flows compared to steady air movement at velocities \(w<40\) m/s. The TI values are approximately the same (within the experimental uncertainty) for steady and pulsating flows for \(w>40\) m/s. The flow pulsation frequency \(f\) also has little effect on turbulence  
Figure 8: Dependences of turbulence intensity TI in a round straight pipe without a turbulator on the average flow velocity \(w\) for different conditions: 1—steady flow; 2—pulsating flow with \(f=4.6\) Hz; 3—pulsating flow with \(f=15.3\) Hz. Figure 7: Dependences of the local air flow velocity \(w_{x}\) (1) and the local HTC \(\alpha_{x}\) (2) on time \(\tau\) in a triangular tube with a turbulator (\(\Pi\approx 0.10\)) under the following conditions: \(w=17.5\) m/s, \(f=16.6\) Hz. intensity. The observed features of TI change in a triangular pipe can be explained by longitudinal vortex structures formed in the channel corners. They probably stabilize the flow and prevent gas-dynamic deformation under the influence of external factors. Accordingly, a TrP should be used to suppress turbulence (alignment) of the flow in technical devices of cyclic action. The stabilization effect is clearly expressed for velocities \(w>50\) m/s. Juxtaposition of Figures 8 and 9 shows that:  
* There is a significant influence of gas-dynamic non-stationarity on the flow structure and pulsating components of the air flow velocity in a TrP;
* There are opposite tendencies in the change of TI in stationary and pulsating flows in a TrP;
* It is necessary to use optical methods or an entropy generation method [36] for a detailed study of the gas dynamics of pulsating flows (the thermal anemometry method does not provide a complete physical picture of the processes taking place). A significant change in the gas-dynamic conditions in the triangular tube should lead to a modification of the heat exchange characteristics of stationary and pulsating air flows. Therefore, the average HTC \(\alpha\) was additionally calculated in both tubes for different boundary conditions. The coefficient \(\alpha\) was defined as the mathematical expectation of the function \(\alpha_{x}=f\left(\tau\right)\) for one pulsation period. The influence of gas-dynamic non-stationarity of the flow and the shape of the pipeline cross-section on the heat transfer level in straight pipes can be traced based on Figures 10 and 11. Figure 10: Dependences of the average HTC \(\alpha\) in a round straight pipe without a turbulator on the air flow velocity \(w\) for different conditions: 1—steady flow with TI = 0.04; 2—pulsating flow with \(f=4.6\) Hz and TI = 0.04; 3—pulsating flow with \(f=15.3\) Hz and TI = 0.04. Figure 9: Dependences of the turbulence intensity TI in a triangular straight pipe without a turbulator on the average flow velocity \(w\) for different conditions: 1—steady flow; 2—pulsating flow with \(f=3.0\) Hz; 3—pulsating flow with \(f=15.8\) Hz. It can be seen from Figure 10 that gas-dynamic non-stationarity causes an intensification of heat transfer in a round straight pipe by 22-30%. At the same time, a rise in the flow pulsation frequency \(f\) does not cause a noticeable further increase in the HTC (the increase in \(\alpha\) is 4-5.5% with an increase in \(f\) from 4.6 Hz to 15.3 Hz, which is within the uncertainty limits of the experiment). Similar data on the influence of air flow pulsations on the level of heat exchange in various gas-dynamic systems can be found in the work of other authors [37; 38].