Numerical simulation analysis of heat and mass transfer of saline wastewater in a falling film evaporation tube based on different inlet modes Open Access

In recent years, oil, gas, and coal chemical industries have been widely used in human production and life (Lefebvre & Moletta 2006). These industries not only require a large amount of domestic water in the process of utilization and development, but also discharge many industrial wastewater that cannot be directly used. With the continuous increase of people's water consumption, the problem of how to realize the development and utilization of wastewater has plagued the further development of the world economy (Zhang et al. 2002). Salt-containing wastewater has been widely studied by its high discharge, difficult treatment, and complex composition. It is important to carry out basic research on the resourceful and efficient utilization of saline wastewater to effectively enhance the utilization value of wastewater and achieve ‘zero discharge’ of wastewater (Dolatabadi et al. 2021).

At present, there are many treatment methods for saline wastewater, including biochemical method (Zhao et al. 2017), gas fluidized bed method (Shi et al. 2020), and thermal treatment method (Ng et al. 2015; Lappalainen et al. 2017; Zhou et al. 2019). Among these methods, the thermal treatment method can take advantage of the difference in solubility of the components of the mixture in the same solvent or the significant difference in solubility between hot and cold conditions, while the crystallization method is used for separation (Liu et al. 2020; Bing et al. 2021). It is also widely used in the treatment of salt-containing wastewater because of its obvious advantages such as small footprint, high heat transfer coefficient, and more stable performance (Al-Sahali & Ettouney 2007). The falling film evaporator has been widely studied as the core heating device of the thermal treatment method.

To improve the evaporation effect and evaporation efficiency, the role of various factors on the mass and heat transfer is generally studied by simulating in a falling film evaporation tube. For this reason, many scholars have conducted more systematic research on vertical falling film evaporation tubes. The process of studying evaporation needs to be supported by adequate theory. Zhang et al. (2015) and Tian et al. (2018) conducted a numerical simulation and experimental study of the heat transfer mechanism of pure water in a saturated pool using a computational fluid dynamics (CFD) model. Gommed et al. (2001) developed a prediction tool that can predict the heat and mass transfer during the absorption of ammonia vapor into the liquid layer inside a vertical tube. Also, it is necessary to study the microscopic flow in the falling film evaporation tube. Oubella et al. (2015) performed a numerical study of mixed convective heat and mass transfer during evaporation, which can play an important role in improving the evaporation efficiency of liquids in evaporation tubes. These scholars’ studies on vertical tubes are based on the heat and mass transfer of evaporation tubes under the vertical inlet method, and the heat transfer efficiency does not change much. To obtain higher heat transfer efficiency, changing the inlet method is one of the ways to improve efficiency.

Wang et al. (2020) used the volume of fluid (VOF) model to numerically simulate the flow characteristics of pure water in vapor–liquid two-phase counterflow under different structural conditions, and obtained the effects of tube length, tube diameter, and annular cloth spacing on membrane distribution and membrane fluctuations. Li et al. (2017) investigated the evaporative mass transfer characteristics of a falling water film under counterflow laminar conditions, reviewed previous studies on heat transfer in water film flow and expressed these factors in a comprehensive manner, and then conducted an experimental study to reveal the characteristics of the local Sherwood number along the flow direction and determined the correlation between evaporative mass transfer. Compared to pure water, the physical parameters in brine have a greater impact on the performance of brine in evaporation (Cao et al. 2019). Therefore, there is a lack of research on the flow of brine in the evaporation tube.

To investigate the differences in heat and mass transfer in different inlet methods of falling film evaporation tubes. This paper studied the difference of various parameters between the simulated working conditions of wastewater entering the evaporation tube with the new tangential inlet method and the traditional inlet method in a vertical falling film evaporation tube. Also, on this basis, the differences in the physical properties of pure water and simulated working wastewater under the new tangential inlet method and the process of heat and mass transfer in the falling film evaporation tube were studied, providing a new direction for achieving high efficiency evaporation.

The turbulent viscosity is correlated with the turbulent energy k and the dissipation rate of turbulence ɛ.

The settings of the boundary and initial conditions are shown in Table 1. In addition, heat walls are defined as constant temperature, no-slip, no penetration, and zero thickness, while other walls are defined as no-slip, no penetration, and insulation.

This paper simulates a falling film outside the tube and vapor inside the tube. The results of the study help to better understand the heat flow process of the falling film evaporator. The following are the hypotheses of this paper:

• The fluid is continuous, incompressible, and has constant physical properties.

• The vertical tube wall is completely wetted by the fluid, so the contact angle is 0 degrees.

• The operating conditions are adiabatic.

• There is no pressure difference in the simulation domain.

• The effect of vapor velocity on the descending film is negligible.

To avoid the calculation error caused by the grid accuracy problem, grid-independent detection was performed. Seven groups of meshes with total mesh numbers from 94,986 to 1,628,483 were used and the film thicknesses at the middle of the evaporation tube were compared, and the simulation results are shown in Figure 2(b), where the calculated results increase with the number of mesh divisions. However, when exceeding 500,000, the results show only a slight difference. To save time and computational resources, a total of 505,216 mesh numbers were selected.

The fluid flow velocity, turbulence intensity, and fluid temperature of the saline wastewater in the evaporation tube are changed for both vertical and tangential inlet methods (Fang et al. 2019). In this section, the vertical inlet method and tangential inlet method of the vertical pipe are investigated using salt-containing wastewater (2% NaCl). The variation of thermodynamic parameters of saline wastewater in vertical tubes at 30, 100, 150, and 200 mm from the inlet was highlighted for comparison.

The conditions for the numerical simulation in this section are as follows: inlet temperature of 313 K, heating temperature of 383 K, and spray density of 0.49 kg/(m·s).

The results show that the fluid velocity in the vertical tube with tangential inlet is larger. Under the same conditions of inlet flow rate, the higher the velocity in the liquid phase region, the higher the turbulence intensity and the better the heat transfer effect. In the vapor phase region, especially at the vapor–liquid interface, larger vapor velocities are also beneficial for convective heat transfer and liquid film evaporation processes here.

As shown in Figure 3(a), the magnitude of fluid velocity in the vertical tube fluctuates widely in the tangential inlet method due to different designs. The fluid velocity of the vertical tube with tangential inlet is large, especially in the liquid film area, where the maximum difference in liquid film velocity is around 0.16 m/s.

In addition, as shown in Figure 3(b), the overall fluid velocity of the vertical tube with a tangential inlet is greater than that of the vertical inlet vertical tube here due to the difference in the initial velocity into the evaporation tube. The maximum velocity difference in the liquid film area is around 0.08 m/s, and the maximum difference in the velocity size in the vapor phase area is around 0.21 m/s.

Compared to Figure 3(b) in Figure 3(c), the difference in fluid velocity magnitude decreases in the liquid film region near the tube wall, while the difference is larger in the vapor phase region at the center of the tube. In the liquid film area, the difference of the inlet method has a small effect on the liquid film velocity, the magnitude of which ranges from 0.84 to 0.9 m/s. While in the vapor phase region, the maximum difference in fluid velocity magnitude is around 0.2 m/s, with the maximum fluid velocity in the center of the tube.

Combined with Figure 7(c), it can be obtained that in Figure 3(d), the fluid velocity variation in the vertical tube by gravity and other influences basically tends to be stable (Mura & Gourdon 2016). The velocity in the liquid film region of the vertical tube with tangential inlet is about 0.5 m/s higher than that of the vertical inlet; the velocity of the fluid in the vapor phase region is basically constant.

As shown in Figure 4(a), the fluid turbulence intensity in the vertical tube with tangential inlet, especially at the vapor–liquid interface, is about 5% higher than that in the vertical inlet vertical tube. The turbulent intensity of the fluid in the vertical tube in this region is relatively small overall, and in the liquid phase region, along the radial direction, the turbulent intensity of the liquid film increases relative to the wall distance and reaches a maximum at the vapor–liquid intersection. In the vapor phase region, the vapor phase turbulence intensity is more stable in the range of −15 to 15 mm from the tube mid-axis. In the radial direction, the turbulence intensity of the vapor phase increases with increasing distance from the center of the tube in the range of 15 mm from the center of the tube to the vapor–liquid interface.

From Figure 4(b), it can be seen that the turbulence intensity maximum is at the vapor–liquid interface and decreases along the radial direction from the vapor–liquid interface to both sides. In the liquid phase region, the reduction of the turbulence intensity of the liquid film is larger along the radial direction, while the reduction is smaller in the vapor phase region. The difference in the turbulence intensity of the liquid film in the liquid phase region is small when the inlet mode is different. While at the vapor–liquid interface, the turbulence intensity of the fluid in the vertical tube with a tangential inlet is larger with a maximum difference of about 3%.

The magnitude of turbulence intensity in these two cross-sections in Figure 4(c) and 4(d) is similar to the turbulence intensity distribution pattern in Figure 4(b). It is worth noting that the maximum values of fluid turbulence intensity in these two cross-sections due to the flow velocity do not appear at the vapor–liquid interface, and the overall fluid turbulence intensity increases (Karimi & Kawaji 1999). The maximum value of turbulence intensity of the fluid reaches about 24% at Figure 4(c). While in Figure 4(d), the maximum turbulence intensity of the fluid in the vertical tube with tangential inlet reaches about 27%.

From Figure 5(a) and 5(b), it can be seen that the temperature of the liquid film increases faster in this range and the temperature gradient of the radial section decreases. The main reason is that the initial temperature of the liquid film is low, the temperature of the tube wall is high, and the liquid film is fully heated after entering the evaporation tube. Since the liquid film is better buffered in the tangential inlet method, the temperature of the liquid film in the vertical tube with tangential inlet is higher and the temperature rise is large. The maximum difference in temperature between the two compared to the vertical inlet method is about 9 K.

The liquid film temperature in the vertical tube with different inlet methods in Figure 5(c) is elevated to more than 350 K. The temperature of the liquid film in the vertical tube with tangential inlet is higher, and the difference between the liquid film temperature and that in the vertical inlet vertical tube is about 5 K.

As shown in Figure 5(d), the liquid film increases to a more stable value after a full development in the upper part of the vertical tube. Thereby this results in a smaller difference in fluid temperature in the vertical tubes of the two different inlet methods.

To more accurately understand the flow and heat transfer characteristics of highly saline wastewater in a vertical tube, this section compares the physical parameters of pure water and saline wastewater. On this basis, the temperature distribution, liquid volume fraction distribution, and tangential velocity distribution of the two liquid films in the vertical tube were compared and analyzed.

First, numerical simulations of pure water and saline wastewater were carried out under the conditions of inlet water temperature T₀ = 303 K, isothermal wall temperature T_w = 383 K, and inlet water velocity V₀ = 0.4 m/s. In simulating the evaporation of saline wastewater by falling film, in order to avoid the problem of crystallization due to high salt concentration at the late stage of evaporation, 2% saline wastewater (the wastewater is mainly NaCl for example), this paper only considers the concentration process of saline wastewater in a vertical tube and focuses on the heat and mass transfer mechanism in this process. It is worth mentioning that the diameter of the evaporation tube for this simulation is 40 mm and the length is 1,000 mm.

Figure 6(a) depicts the variation of the density of the two liquids with temperature. It can be seen that the density of the liquid decreases with the increase of temperature, and the density of the saline wastewater is slightly greater than that of pure water. Figure 6(b) shows the variation of specific heat capacity of the two liquids with temperature. From the figure, it can be seen that the specific heat capacity of saline wastewater increases with the increase of temperature, while the specific heat capacity of pure water decreases with the increase of temperature between 273 and 320 K and increases with the increase of temperature between 320 and 383 K. What's more, the specific heat capacity of pure water is slightly larger than that of saline wastewater. As can be seen in Figure 6(c), the thermal conductivity of the two liquids increases with the increase of temperature, and the thermal conductivity of saline wastewater is slightly larger than that of pure water. As shown in Figure 6(d), the viscosity of the two liquids decreased with the increase of temperature, but the viscosity of saline wastewater was slightly larger than that of pure water.

Figure 7 shows the temperature variation along the axial direction of the liquid film formed in the vertical tube for saline wastewater and pure water. It can be seen from the figure that the difference between the temperature of the saline-containing wastewater liquid film and the pure water liquid film temperature is small within the range of 0–193 mm from the inlet, while the temperature of the salt-containing wastewater liquid film within the range of 193–1,000 mm from the inlet is slightly different which is higher than the temperature of the pure water film.

The reason for this situation is related to the specific heat capacity and thermal conductivity of both. Referring to Figure 6(b) and 6(c), it can be seen that the specific heat capacity of saline wastewater is smaller than that of pure water, and the thermal conductivity is greater.

In this paper, the VOF turbulence model and the RNG k–ɛ model are utilized to numerically simulate the flow in a vertical falling film evaporation tube with saline wastewater under certain structural parameters and different operating parameters. In addition, the differences in heat and mass transfer between saline wastewater and pure water in a falling film evaporation tube were compared. The main conclusions are as follows:

• The fluid velocity in the tangential inlet vertical tube is greater than that in the vertical inlet vertical tube, and the difference is 0.1m/s at the vapor–liquid intersection, which makes its heat transfer effect better.

• Compared with the vertical inlet vertical pipe, the tangential inlet vertical pipe fluid turbulence intensity is greater, especially at the vapor–liquid interface, the average difference is about 5%, and the larger turbulence intensity difference is more conducive to the evaporation of liquid film in the pipe.

• The fluid temperature in the vertical tube under the tangential inlet is higher, with a difference of 10K or less, which leads to higher evaporation efficiency.

• Since the specific heat capacity of saline wastewater is smaller and the thermal conductivity is greater, the liquid film temperature is higher than that of pure water in the range of 193–1,000mm from the inlet, which results in better heat transfer.

• The liquid volume fraction of saline wastewater is about 0.05 smaller than the volume fraction of pure water liquid membrane, so that the evaporation effect of saline wastewater is better than that of pure water by the falling membrane.

This study was financially supported by the scientific research fund project of the National Natural Science Foundation of China (Grant No. 52070123), Natural Science Foundation of Shandong Province (Grant No. ZR2020ME224), Key Scientific and Technological Innovation Project in Shandong Province (Grant No. 2019JZZY020808), and Project of Shandong Province Higher Educational Young Innovative Talent Introduction and Cultivation Team [Wastewater Treatment and Resource Innovation team].

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.