Planar laser-induced fluorescence diagnostics of water droplets heating and evaporation at high-temperature

• Published in: Applied thermal engineering Vol. 127; pp. 141 - 156
• Main Authors: Volkov, Roman S., Strizhak, Pavel A.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 25.12.2017; Elsevier BV
• Subjects: Air flow; Air temperature; Automobile industry; Automotive engineering; Automotive parts; Cleaning; Droplets; Emulsions; Evaporation; Evaporation rate; Evaporators; Fire fighting; Fluorescence; Gas heating; Gasification; Heat exchangers; Heat flux; Heating; High temperature; High-temperature gases; Laser beam heating; Mathematical models; Non-homogeneous and non-steady temperature field; Planar laser induced fluorescence; Slurries; Steam electric power generation; Temperature distribution; Water droplet; Non-homogeneous and non-steady temperature field; Water droplet; Planar laser-induced fluorescence; Evaporation rate; High-temperature gases
• ISSN: 1359-4311; 1873-5606
• DOI: 10.1016/j.applthermaleng.2017.08.040

[Display omitted] •Temperature field in the water droplet is highly non-homogeneous during the heating.•PLIF allows observing temperature fields in droplets even at high air temperatures.•Maximum temperature differential from the droplet surface to centre can achieve 30–40°C.•Nonlinear evaporation rate dependences are plotted for the main parameters.•Water evaporation rate increases dozens of times during the droplet heating. Gas-steam-droplet technologies are widely used in systems operating at high temperatures ranging between 400 and 2000°C, namely: fire-fighting systems, thermal fluid cleaning, fuel compounding, industrial waste gasification and evaporation systems for advanced fuel components, cleaning of thermally loaded surfaces of power equipment, etc. System parameters are usually selected empirically via multiple tests and continuous trial operation of appropriate units, aggregates, assemblies, and installations. This situation is caused by insufficient basic knowledge of conditions and parameters of high-temperature (over 500°C) heating and evaporation of water and water-based emulsions, solutions, and slurries. Limited information is available regarding the evaporation rates dependent on the temperature of gaseous medium. Consequently, the up-to-date evaporation models allow the researchers to achieve adequate values (in good agreement with the experiment) of evaporation rates at air temperatures not exceeding 300–400°C. The paper presents a set of experiments on water droplets with the size ranging from 1 to 2mm, which is used to create the information database on high-temperature evaporation parameters. The approach to measuring the evaporation rate involves observation of the droplet size or more exactly its mean radius, and recording the time of its existence. A high-speed video camera and Tema Automotive software with different tracking algorithms are used for experimental observations. During gas heating, the distribution of highly non-homogeneous and non-steady temperature field in evaporating water droplets is detected by the hardware and software cross-correlation system and Planar Laser-induced Fluorescence optical diagnostics. Instantaneous and medium evaporation rates are computed for the whole period of the droplet lifetime. Highly nonlinear evaporation rate dependences are suggested for gas temperatures and the water droplet surface, size, and time of gas heating. Approximate relationships are described for the prediction of evaporation rates depending on the basic parameters. The evaporation rate is shown to increase several-fold during the heating of a water droplet in a high-temperature gaseous medium. The experimental data are described most accurately by exponential dependences of water evaporation rate versus the airflow temperature and velocity. This result makes it possible to predict the corresponding highly nonlinear dependences of evaporation rate versus the density of convective heat flux and the overall thermal conditions in the industrial chambers, evaporators, heat exchangers, etc. The mathematical expressions obtained can be used to identify the effective conditions of water droplet heating in a large group of high-temperatures applications as well as for developing high-temperature liquid evaporation models.