Nonisothermal desorption of droplets of LiBr salt solution on a heated wall

Nonisothermal desorption and crystallization of droplets of aqueous salts solution of H2O/LiBr were studied experimentally. A droplet was placed on a horizontal heated wall. The initial concentration of salt C0 varied within 0-53 %. The wall temperature Tw varied within 80 235 oC and ambient air pressure was 1 bar. It was shown that the desorption time t1 decreases sharply with increasing Tw and at Tw < 150-160 oC. The desorption time t1 is the time until the onset of crystallization. The desorption time t1 is quasi-constant at Tw > 160 oC and increases with growth in C0.


Introduction
The droplet evaporation was investigated in a considerable number of works [1][2][3][4][5][6]. The evaporation of drops is widely used in modern technologies [1,3]. Gas-droplet flows are used in technical apparatuses [6]. When burning gas hydrate of methane a vapor-droplets flow is formed and a combustion temperature is significantly lowered [7]. The significant decrease in the combustion temperature sharply reduces the efficiency of the combustion technology of this natural raw material. The desorption rate and the heat transfer coefficient depend on wall properties and wettability [3]. The behavior of the desorption depends on a solution convection and a degree of turbulence of a gas flow [8,9]. It is important to investigate the desorption process of aqueous salt solutions for a wide range of technical tasks. The behavior of salt solutions differs from the behavior of one-component liquid [10][11][12][13][14]. The nucleate boiling of an salt solution droplet can be divided into several characteristic regimes [14]. The following typical stages of heat and mass transfer can be distinguished depending on wall temperature T w : 1) evaporation below boiling temperature Т b ; 2) nucleate boiling; 3) transitional stage; 4) boiling at Leidenfrost temperature T L . Heat and mass transfer in the salt solution was investigated in [15]. Crystallization during desorption of a salt solution was considered in [16][17][18]. The onset of crystallization in a droplet is realized at a droplet edges and in a solution layer at a distance from the side heated wall [16,18]. The crystallization kinetics depends on the heat and mass transfer and a degree of a solution supercooling [17]. Aqueous salt solutions can be divided into two kinds depending on the desorption behavior. The structural wall enhances heat transfer and increases the rate of the droplet evaporation [19].

Experimental data
The experiments were carried out at relative air humidity of 40 %, ambient air temperature was 22 °С and ambient air pressure was 1 bar. The droplet of aqueous salt solution of LiBr was placed on the horizontal heated wall. The values of initial mass concentration of solutions C 0 were determined by the standard densimeters. Fig.1 represents the behavior of the time of crystallization onset t 1 for different wall temperature T w and C 0 . With increasing the wall temperature for T w < 150 °C, the heat transfer coefficient increases, and the evaporation time t 1 of the droplet decreases. The desorption time t 1 is the time until the onset of crystallization. The desorption time t 1 is quasi-constant at T w > 160 ºC and increases with growth in C 0 . It is probable that the constant value of t 1 is due to the attainment of T s (T s is the surface temperature of the drop) and the free convection velocity in the liquid of the maximum values (the values of these parameters reach a maximum and then practically do not change). At the boiling crisis of a one-component liquid the growth of the wall temperature leads to an increase in t 1 . However, for a salt solution, the behavior of t 1 is different. With increasing wall temperature (at the boiling crisis), the time t 1 tends to a constant value. There is a stable vapor film under a water droplet at the Leidenfrost temperature T L . For the droplet of salt solution, this stable vapor film is absent. There are three time modes of the droplet evaporation (for T w > 130-160 ºC): 1) In the initial time period, the drop jumps and periodically touches the wall. Touching is accompanied by a spray of liquid. 2) The wall cools down after a while and the drop begins to evaporate like a sessile drop (without detachment from the wall). 3) The evaporation and heat transfer are realized in the presence of a crystalline crust on the droplet surface. Heat and mass transfer in the thermal control system of technical and technological energy equipment Thermal imaging of the droplet of LiBr salt solution is shown in Fig. 2. With increasing salt concentration, the temperature T s on the droplet free surface becomes more uniform due to a suppression of liquid convection. A sharp increase in the concentration of salt leads to a multiple increase in the viscosity of the solution and to the suppression of convection inside the drop.

Conclusions
Experimental studies of droplet of water and aqueous salt solution of H 2 O/LiBr have been carried out.
Experimental data have shown that the behavior of aqueous solutions and water is qualitatively and quantitatively different. The desorption time t 1 decreases sharply with increasing T w for T w < 150-160 ºC. The desorption time t 1 is quasi-constant at T w > 160 ºC and increases with growth in C 0 .
The obtained experimental dependences for t 1 can be used for the calculation of the desorber cycles of a thermal absorption pump.
The study of the evaporation of salt solutions layers was funded by FASO Russia.