## 3 Results
### Effect of the Cooling Water Flow Rate
Temperature distributions at Re\({}_{\text{in}}\approx\) 21,000 are presented in Figure 5. As the Figure shows, that the characteristics are not the same as in Figure 2 (case of lower Re\({}_{\text{in}}\)). Moreover, in Figure 5, the dew point temperature (curve 4) is higher than the wall temperature (curve 2). As depicted in Figure 5a, the flue gas temperature (curve 1) is almost constantly decreasing along the tube with some small fluctuations at the end. Temperatures of the tube wall and cooling water (Figure 5a, curves 2 and 3) change very similarly along the length of the model. An intensive increase in the water temperature is observed from x/d \(\approx\) 0 until x/d \(\approx\) 30. Farther, the increase becomes less significant. Until the outlet, the cooling water temperature increases by 43 \({}^{\circ}\)C, i.e., from 10 \({}^{\circ}\)C to 53 \({}^{\circ}\)C (Figure 5a, curve 3). This temperature variation is almost opposite in comparison to the results presented in Figure 2a. The characteristics of the wall temperature (Figure 5a, curve 2) are similar to the water temperature. Although, the wall temperature is lower compared to the dew point temperature, the difference between them at the beginning of the tube is small, i.e., 2-3 \({}^{\circ}\)C (cf. Figure 5a, curves 2 and 4). That is why the condensation is not intensive at that point (Figure 6a). As the mentioned temperature difference increases with the increase of x/d (Figure 5a), the Nu\({}_{\text{t}}\) also increases until x/d \(\approx\) 100 (Figure 6a). Farther along the tube, it remains almost constant with some fluctuations and after that starts to decrease. The decrease in heat transfer happens most likely because of the decreased water vapor mass fraction in the flue gas, despite the fact that the temperature difference between the flue gas and the tube wall remains almost constant until the end of the test section. When the cooling water flow rate was increased, the characteristics of temperatures changed (Figure 5b) in comparison to those presented in the case of a lower water flow rate (Figure 5a). When the water flow rate was higher (Figure 5b), two zones of flue gas temperature decrease could be also distinguished: one from x/d = 0 until x/d \(\approx\) 110 where a sharp temperature decrease was observed, and the other from x/d \(\approx\) 110 until the end of the tube, where a much less decrease was observed. Figure 4: Comparison of the Nu\({}_{\text{t}}\) distribution along the heat exchanger at Re\({}_{\text{in}}\approx\) 9500, at different inlet temperatures, as well as cooling water flow rates. Figure 5: Temperature distribution in the heat exchanger at \(\rm{Re_{in}}\approx\) 21,000, different blue gas inlet temperatures and the cooling water flow rates of 60 kg/h (**a**,**c**) and 120 kg/h (**b**,**d**): (1) centre of the tube, (2) the inner wall of the tube, (3) cooling water, (4) the dew point temperature. Figure 6: Distribution of the local \(\rm{Nu_{t}}\) in the heat exchanger at \(\rm{Re_{in}}\approx\) 21,000, at different blue gas inlet temperatures (smaller (**a**), higher (**b**)) as well as cooling water flow rates. \(\approx\)12 \({}^{\circ}\)C to \(\approx\)35 \({}^{\circ}\)C. This suggests that water vapor condensation should result in higher heat transfer in the range x/d = 0-120, than in the range x/d \(\approx\) 120-170; indeed, the distribution of the Nu\({}_{\rm t}\) presented in Figure 6a confirms that, although it is not very clearly pronounced. As the temperature difference between the dew point and the tube wall temperatures at the beginning of the tube (Figure 5b, curves 2 and 4) is much larger in comparison with the results presented in Figure 5a, the heat transfer in the case of a larger cooling water flow rate is also higher (Figure 6a). However, at the water flow rate of 120 kg/h, the heat transfer is nearly constant (Nu\({}_{\rm t}\approx\) 275) until x/d \(\approx\) 120. Farther on, till the end of the tube, there is a slight decrease in Nu\({}_{\rm t}\).