## 3 Results
### Effect of the Cooling Water Flow Rate
From \(\rm x/d\approx 70\) until the outlet (\(\rm x/d=0\)), there is even more increase, i.e., \(\rm-9\)-10 \({}^{\circ}\)C. The heat transfer (Figure 3a) in the distance between \(\rm x/d=0\) and \(\rm x/d\approx 70\) was high (Nu\({}_{\rm t}\) is in the range between 180-160) and confirmed the increased temperature of the cooling water, which was due to vapor condensation, and the release of latent heat. As the most amount of water vapor was condensed in the initial part of the tube, the heat transfer from \(\rm x/d\approx 70\) until \(\rm x/d\approx 170\) was rapidly decreasing (Figure 3a). However, at the end of the tube, it continued to be rather high (Nu\({}_{\rm t}\approx 50\)), yet was still smaller than in the case of the water flow rate of 60 kg/h. For both cases presented in Figure 2a and b, the condensation efficiencies were about 52% and 62%, respectively. Thus, the cooling water flow rate increased by two times, showing no substantial increase neither in heat transfer (Figure 3a), nor in condensation efficiency. At a higher inlet flue gas temperature (Figure 2c), the flue gas temperature decreases abruptly until \(\rm x/d\approx 90\), yet farther along the tube the decrease is more constant (Figure 2c, curve 1). The comparison of this case with the previous one (see Figure 2a) reveals that the distance where more sudden flue gas temperature decrease is observed extends further, i.e., from \(\rm x/d\approx 20\) to \(\rm x/d\approx 90\). The characteristics of the wall and cooling water temperatures are similar (Figure 2c, curves 2 and 3). The water temperature from \(\rm x/d\approx 170\) increases slightly until \(\rm x/d\approx 95\). From \(\rm x/d\approx 95\) until the outflow, the temperature increases more, i.e., from \(\approx\)15 \({}^{\circ}\)C until \(\approx\)35 \({}^{\circ}\)C and is by about 4-5 \({}^{\circ}\)C higher compared to the case of a lower inlet flue gas temperature (Figure 2a). As the dew point temperature is higher than the tube wall temperature (Figure 2c, curves 2 and 4), good conditions for condensation occur from \(\rm x/d=0\) (Figure 3b) until \(\rm x/d\approx 90\). Here Nu\({}_{\rm t}\) varies in the range of \(\approx\) 100-120. Then, Nu\({}_{\rm t}\) decreases slightly but remains between \(\approx\)95 and 110 till the end of the test section. The temperatures of the flue gas, the tube wall, and the cooling water show similar tendencies in the case when the cooling water flow rate was 120 kg/h (Figure 2d). Compared to the case presented in Figure 2c, the temperatures were slightly lower and the increase in the water temperature was not as evident as it was in the case before (cf. Figure 2c). The characteristics of heat transfer at a higher cooling water flow rate are very similar to that of a lower flow rate (Figure 3b). Condensation efficiencies determined for the experiments when the cooling water flow rate was 60 kg/h and 120 kg/h were in the range between 54 and 58%. The results of [13] show that even in an internally finned tube, the maximum condensation efficiency was only about 20%. But in this case, the tube was very short, only \(\rm x/d\approx 9\). The tube used in our experiments was about 20 times longer, and therefore, the surface for vapor condensation was much larger, which resulted in a much higher condensation efficiency. Figure 4 presents a comparison of the Nu\({}_{\text{t}}\) distribution along the model for both the flue gas inlet temperatures and different cooling water flow rates. The results show that if the flue gas inlet temperature is higher, Nu\({}_{\text{t}}\) remains almost constant along the tube irrespective of the cooling water flow rate. For lower flue gas inlet temperatures, Nu\({}_{\text{t}}\) changes similarly along the tube for both water flow rates, but the results indicate that more intense condensation takes place only in a half of the test section, i.e., until x/d \(\approx\) 80-90; farther, Nu\({}_{\text{t}}\) is decreasing.