# Construction of a Numerical Model for Flow Flash Evaporation with Non-Condensable Gas
## 1 Introduction
The vapor volume fraction and temperature showed strong fluctuations, and the measurement deviations of the flash points were closely related to the fluctuations of the vapor temperature. A mathematical model was developed by [PERSON] et al. [14] to numerically simulate the droplet and film flash evaporation. The results showed that droplet flash evaporation significantly affected the cooling heat transfer of vacuum flash evaporation. [PERSON] et al. [15] proposed a mathematical model to predict the flash evaporation of LiCl droplets released into a vacuum. They found that the pressure is the core factor affecting the evaporation rate, i.e., the lower the pressure, the more intense the flash. Droplets with a higher initial temperature only strengthened the intensity of flash evaporation at the beginning. The model can accurately predict droplet flash evaporation without radiation heat. [PERSON] et al. [16] conducted a three-dimensional (3D) simulation of the flash evaporation of a non-uniform spray in a saturated vapor environment using the Eulerian-Lagrangian framework in the software Fluent. The spray initially underwent violent evaporation and a significant temperature reduction within the vicinity of the nozzle exit. As the spray develops downstream, evaporation becomes moderate. Smaller droplets were distributed in the central spray region, with a comparatively higher evaporation rate and larger velocity variation. [PERSON] and [PERSON] [17] used a single-phase model to study the flow-field distribution in a flash evaporation chamber. [PERSON] et al. [18] established a 1D droplet flash evaporation model by introducing a simple heat balance correlation and a phase-transition model to [PERSON]'s law. The effects of ambient pressure, temperature, droplet size, and the fraction of vapor concentration on the flash evaporation and spray cooling rate were presented in detail. The presence of non-condensable gases in the vapor can have a serious effect on the heat transfer process in the condenser, as it reduces the partial pressure of the vapor around the cooling surface and increases the transfer resistance of the vapor to the cooling surface, which ultimately leads to a reduction in the amount of heat transferred and an increase in the vacuum load of the condensing system. Many scholars have studied the effects of non-condensable gas. [PERSON] [19] studied the significant influence of non-condensable gases on the steam condensation process; in particular, the presence of non-condensable gases increases the pressure in the equipment and affects the condensation efficiency. [PERSON] et al. [20] investigated the effect of non-condensable gases in a low-temperature thermal desalination system and showed that each unit mass of non-condensable gas flowing into the condensation chamber resulted in 1.6 times the unit vapor loss from the vacuum pump outlet. [PERSON] et al. [21] studied the effect of non-condensable gases in multiple-effect distillation systems and showed that the presence of non-condensable gases increased the heat transfer area of a horizontal tube condenser by about 10-30%, while each unit of non-condensable gases led to a loss of 20-30 units of vapor under severe operating conditions. The aforementioned numerical studies all treated liquid as a single phase. However, in the industry, liquids often include and dissolve a significant amount of non-condensing gases. Therefore, we propose a new coupled numerical model that includes multiphase, multicomponent, phase-change, and turbulent-flow modules that can accurately predict the liquid flash evaporation process with a non-condensing gas. The non-condensing gas was considered to be dissolved and entrained into two components in the liquid. The effects of the two types of non-condensing gases on the flash evaporation mechanism and steam generation rates are discussed in detail.