# Experimental and Numerical Evaluation of Toxic Pool Evaporation
## 1 Introduction
### Brief Overview  
Before describing the experimental set-up and corresponding results, it is important to give some background about the expected phenomena, since these physical phenomena govern the experimental design.  
Evaporation models are commonly based on the boundary layer theory of [PERSON] [1], which considers the advection-diffusion equation [2]. This approach, combined with experiments, has allowed several evaporation models to be developed, each on the basis of a specific situation [3; 4; 5; 6; 7; 8; 9]. Several comparisons between those correlations have been published in the literature, for example, in [10].  
The Mackay and Matsugu correlation formula, although now quite old, is still widely used in the context of risk management and land use planning, and therefore it was used as a basis for our experimentation. While simplified in some cases [11], the Mackay and Matsugu correlation was originally written as follows:  
\[q_{ev}=k_{m}(P_{v}-P_{\infty})\ M/\!RT_{p} \tag{1}\]  
\(q_{ev}\)evaporation rate (kg/s);  
\(P_{v}\)vapor pressure at the pool temperature and concentration (Pa);  
\(P_{\infty}\)vapor pressure at the boundary layer limit (Pa);  
\(M\) molar weight (kg/mol);  
\(R\)Universal gas constant (8.31 J/mol\(\cdot\)K);  
\(T_{p}\)pool temperature (K).  
The mass transfer coefficient, \(k_{m}\), is written as  
\[k_{m}=C_{m\delta m}\ U^{0.78}\ (2\pi r_{p})^{-0.11}\ S_{c}^{-0.67} \tag{2}\]  
\(C_{m\delta m}\)Mackay and Matsugu coefficient (4.786\(\cdot\)10\({}^{-3}\));  
\(U\)air velocity 10 m above the pool surface (m/s);  
\(r_{p}\)pool radius (m)-2\(\
u_{p}\) represents the pool diameter or, for non-circular pools, to the wind-exposed pool length;  
\(S_{c}\)Schmidt number.  
One of the key criteria in Equation (1) is the product's vapor pressure. This quantity is a function of both the pool's temperature and concentration (Figure 1) but is considered as a constant in the equation.  
Figure 1: Evolution of the vapor pressure for ammonia as a function of temperature and pool concentration.  
Regarding toxic pools, most evaporation models used in the industry literature are based on a constant vapor pressure during the evaporation process. Very few papers have dealt with the unsteady process of evaporation [12]. On the basis of experiments with hydrogen chloride, the authors of [12] claim that a concentration gradient occurs over time and leads to an unsteady phenomenon. Such a concentration gradient may strongly influence the evaporation rate of ammonia, and thus it was specifically investigated in this paper. This evolution over time was also studied for hydrocarbons, taking temperature evolution into consideration [13; 14; 15]. Some authors specifically studied the evolution of evaporation rate of concentrated ammonia solutions in small-scale pools without wind [16] or with assessing the influence of wind [17; 18].  
This paper presents a new series of experimental tests for ammonia evaporation, taking into consideration various parameters such as pool concentration and its temperature and the ambient humidity. Two different scales were used to evaluate the influence of the scale factor by modifying the pool size.