INFLUENCE OF THE WALL ON THE BOILING REGIME

This work is devoted to investigation of wall roughness and its thickness. It is not enough to know the heat flux and the physical properties of the liquid for determining the boiling mode. The wall roughness and its thickness also plays an important role. Changes in the surface roughness from 5 to 7th grade did not affect the behavior of the curves. The width of the transition region for the boiling crisis is much less for the polished surface (roughness grade is 9) compared to the rough wall. The boiling crisis for mirror wall comes at a lower overheating of the wall. The total time of evaporation on the mirror wall is almost 3 times higher than for a rough wall.


Introduction
The droplet boiling in heat exchangers can be implemented in various modes depending on the heat fluxes: nucleate boiling; transition crisis (periodic separation of the liquid from the wall); film boiling under the Leidenfrost temperature (boiling crisis).Evaporation of drops was investigated in [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15].It is not enough to know the heat flux and the physical properties of the liquid for determining the boiling mode.The wall roughness and its thickness also plays an important role.
Research these factors devoted to this work.This work is devoted to investigation of these factors.

Experimental data
Experiments were carried out on a horizontal surface under an air atmosphere (temperature of 22 °C and 1 atm).Description of the setup is given in [1].The wall temperature was maintained constant automatically to within 1 °C.Droplets were obtained using micro dispensers (relative error of less than 2.5% of volume).Effect of the wall roughness on the process of the distilled water evaporation is shown in Fig. 1.Changes in the surface roughness from 5 to 7th grade did not affect the behavior of the curves.The width of the transition region for the boiling crisis is much less for the polished surface (roughness grade is 9) compared to the rough wall.The boiling crisis comes at a lower overheating of the wall.The total time of evaporation on the mirror wall is almost 3 times higher than for a rough wall.Changing modes of evaporation for the working section (stainless steel, wall thickness δ = 1 mm and the wall temperature T w = 214 °C) is shown in Fig. 2. The equivalent droplet diameter was determined from the ratio V 0 = 4/3 • πr 3 (where r is the radius of the spherical droplet).

EPJ Web of Conferences
When d < 1.5 mm the droplet detached from the wall and there is a boiling crisis.With an increase in volume (d = 1.5-2 mm), the material does not compensate the intensive cooling and the wall temperature under the droplet is reduced.The lack of separation results in a more intense nucleate boiling (higher heat transfer).When the droplet diameter of about 2 mm the wall is cooled under the a droplet more stronger and nucleate boiling is stopped, and the evaporation time increases.Thus, a rough wall surface moves heat exchange crisis towards higher overheating.For thin-walled structures modes of boiling can change.This is important for the calculation of the heat exchangers.

Figure 2 .
Figure 2. The total time of evaporation depending on the initial droplet diameter (the thickness of wall (δ) is 1 mm, and T w = 214 °C).