Fig. 1. Diagram of straight pipe model, the area enclosed by the thick solid line is the cross section of the model (the grey part denotes the water in the secondary circuit, the blue part denotes the tube support plate) (color online)

Fig. 2. Calculated distribution of the fluid velocities in the secondary circuit model(a) Average velocity over cross sections at various heights, (b) Velocity distributions on three selected cross sections at z = 1.5 m, 2.0 m, 2.5 m, respectively, (c) Velocity distribution on the longitudinal section nearby a support plate at z = 2.0 m, see the dashed line of (b) for the location of the section

Fig. 3. Variation curve of average temperatures of water on different sections along the flow direction in the secondary side model

Fig. 4. Variation curve of the average volume fraction of gas along the flow direction on cross sections at different heights of the secondary circuit model, where the black squares indicate the locations of tube support plates

Fig. 5. Variation curve of the pH values of the water near the heat transfer tube wall along the axis of the secondary circuit model, where the sampling point coordinates are (12.525 mm, 0, z), 0≤z≤3.8 m (see the cross section of (b)). The various molar ratios of the binary alkalizers are also shown in the inset of the figure. (color online)(a) $c_{\mathrm{N}{\mathrm{H}}_{\mathrm{3}}}$= 8.784 mol·m^-3, (b) $c_{\mathrm{N}{\mathrm{H}}_{\mathrm{3}}}$+ c_ETA = 4.56 mol·m^-3, (c) $c_{\mathrm{N}{\mathrm{H}}_{\mathrm{3}}}$+ c_ETA = 1.8 mol·m^-3

Fig. 6. Relation curves of pH value with ETA concentration (c_ETA) change and change rate corresponding to two different alkalizer concentrations (1.8 mol·m^-3, 4.56 mol·m^-3) at z=1.5 m, 2.0 m two points respectively (shown in Fig.5)

Table 1. Physical parameters of relevant materials

Table 2. The dissociation constant and relative volatility of NH₃ and ETA at different temperatures

Table 3. The calculated concentrations of NH₃ and ETA in the secondary circuit model with c₁+c₂ = 4.56 mol·m^{-3 ①}

Table 4. The calculated concentrations of NH₃ and ETA in the secondary circuit model with c₁+c₂ = 1.8 mol·m^-3

Table 5. Thermodynamic parameters of water in its gas and liquid states

Table 6. The boundary values of main parameters