# Construction of a Numerical Model for Flow Flash Evaporation with Non-Condensable Gas
## 1 Introduction  
Low-pressure flash evaporation is a unique evaporation technique that is different from evaporation from an overheated surface. Steam is primarily produced at the liquid surface because ambient atmospheric pressure is lower than the saturation pressure of the temperature of the liquid [1]. This technique offers a high evaporation rate without requiring excessively high liquid temperatures. Flash evaporation is widely used in many industrial fields, such as seawater desalination, chemical engineering, food processing, ocean thermal energy conversion, and waste heat recovery [2; 3]. Many factors have a significant effect on the flash evaporation rate, such as the liquid temperature, atmospheric pressure, saturation pressure, initial liquid height, and liquid flow rate [4]. Several experimental studies have been conducted on this topic. [PERSON] et al. [5] experimentally studied flash evaporation under low-pressure conditions, with the water temperature ranging from 50.0 to 90.5 \({}^{\circ}\)C, initial water height of 400 mm to 550 mm, and superheals ranging from 1.5 K to 17.8 K; the results showed that an increase in water temperature and a decrease in the initial liquid level height can enhance the extent of flash evaporation. [PERSON] et al. [6] studied water flash evaporation with different values of flow rate (400-1200 L h\({}^{-1}\)), water film concentration (0-10%), and pressure (7.4-47.4 kPa). [PERSON] et al. [7] experimentally investigated the effect of different sealing air flows on the flash evaporation rate. The valve opening ratio varied from 0.125 to 1. [PERSON] et al. [8] experimentally studied the thermal characteristics of water flash evaporation from a thin liquid film. The experimental operating conditions were an initial temperature in the range of 60-85 \({}^{\circ}\)C, a liquid film height of 3-20 mm, and a superheat in the range of 2-22 K. [PERSON] and [PERSON] [9] experimentally investigated the use of electrolytically generated hydrogen bubbles to improve flash evaporation performance. They found thathydrogen bubbles promoted ebullition in the regions of the flash stage, and thus reduced the non-equilibrium temperature difference by up to 15%. [PERSON] et al. [10] investigated water flash evaporation under a microwave. The temperature ranged from 19 to 50 \({}^{\circ}\)C, pressure from 4-12 kPa, flow rate from 10-40 L h\({}^{-1}\), and the microwave power ranged from 0.81 to 1.35 kW. [PERSON] et al. [11] conducted experimental and theoretical studies on water flash evaporation and developed a one-dimensional (1D) mathematical model. The model considered four main physical processes: gas outflow due to depressurization, mild evaporation, flash evaporation, and the updated flashing chamber pressure and gas components. [PERSON] et al. [12] experimentally studied the static flash evaporation of aqueous NaCl solutions at flash speeds in the range of \(4.8\times 10^{-4}\)-2.18 s\({}^{-1}\). While many experimental studies have explored some physical regulations, there still remain gaps in the understanding. However, owing to the limitations of experimental test methods, our understanding of the internal heat and mass transfer mechanisms remains unclear. In addition, experimental tests are time consuming and costly, and it is difficult to reproduce the experimental results in other working conditions. Similarly, there are problems of errors and assumptions in the calculations and numerical model, making the simulation not always completely accurate, but with the development of computers, numerical methods have attracted increasing attention. [PERSON] et al. [13] used the volume of fluid (VOF) and Lee models to simulate the flash point of ethylene glycol. The results showed that the flash point varied at different measurement points, and a lower flash point could be obtained at the measurement point with a higher vapor volume fraction.