nnotation：Current knowledge about the evaporation of liquid droplets in the high temperature(more than 500 K)gaseous media are formed based on the concepts by W.E.Ranz and W.R.Marshall obtained in large-scale experimental and theoretical studies and actively developed in the works by M.C.Yuen,L.W.Chen and M.Renksizbulut[Renksizbulut M.,Yuen M.C.Numerical study of droplet evaporation in a high-temperature stream//Journal of Heat Transfer.1983.V.105.No.3.P.389-397;Yuen M.C.,Chen L.W.Heat-transfer measurements of evaporating liquid droplets//International Journal of Heat and Mass Transf.1978.V.21.No.5.P.537-542].However,experimental and theoretical studies of the last 10-20 years show that classical approaches,taking into account only homogeneity and stationarity of a temperature field of the liquid droplet,and the consumption of all the supplied energy for the phase transition,are limited over ranges of temperatures,for which it is possible to obtain the evaporation characteristics satisfying the experimental data.As a rule,up to 600-700 K,it is possible to obtain the deviations from experiments within 10-15%.Such the models include models developed by O.Knake,I.N.Stransky,N.A.Fuchs,D.B.Spalding,M.S.Yuen,L.W.Chen and M.Renksizbulut,S.S.Kutateladze,D.V.Labuntsov,V.I.Terekhov,A.A.Avdeev,V.E.Nakoryakov,S.Ya.Misyura and other well-known researchers.For temperatures of gases more than 1000 K,adequate models of phase transitions have not been yet developed.The adequate models allow the prediction of the rate of phase transformations corresponding to the experimental data with deviations of up to 10-20%.The main reason of the mentioned problem is a deficiency of reliable experimental data.At the same time,there are so many promising applications for these temperature ranges in liquid drop-gaseous environment systems,e.g.contact heat exchange devices,ignition of fuel in combustion chambers,heat carriers based on flue gases,droplets and vapors of water.In addition to the above,solving the problems of the efficiency of both modern and prospective heat and power equipment is the key to the further development of the entire heat and power complex not only in Russia,but all over the world.
In the last 5 years,the author with colleagues has published over ten articles in the top-rated international journals from the first and second quartiles of Web of Science(the main ones are listed in Section 4.8).Results of these papers show that the buffer(vapor)layer around evaporating droplets can essentially influence the evaporation rate of drops.Based on experimental data obtained using high-speed recording software-hardware cross-correlation complexes and systems of tracking interfaces corresponding to the world level,the author's team develops hypothesis on the dominant impact of the droplet thermal protection(i.e.limitations of the surface transformation due to the intensive evaporation).The problem of experimental substantiation of the hypothesis developed by the authors is relevant.Solving this problem will allow creating adequate physical and mathematical models for predicting the rates of evaporation of liquid droplets in a wide temperature range,even up to 2000 K.For the first time,characteristics of the thermal vapor protection(layer thicknesses,temperature gradients,gas composition,etc.)of droplets of liquids,solutions and suspensions will be determined when moving in high-temperature gaseous media.Fundamental results of the project will significantly change concepts on the high-temperature evaporation of liquid droplets.
The authors are going to generalize knowledge of the inhomogeneous and nonstationary character of the formation of a temperature field of an evaporating water drop,to reveal the influence of various impurities and particles(in the case of suspensions,emulsions,solutions)on the parameters of a temperature field.The usage of a group of optical methods(e.g.Particle Image Velocimetry,Particle Tracking Velocimetry,Stereo Particle Image Velocimetry,Interferometric Particle Imagine,Shadow Photography,Planar Laser Induced Fluorescence,Laser Induced Phosphorescence),the high-speed video recording,cross-correlation software-hardware complexes and fluorescent dyes allows obtaining reliable experimental data on the droplet temperature fields,as well as temperatures and concentrations of the vapor-gas mixture around droplets.
It is planned to focus on the use of new knowledge of the thermal vapor protection of droplets of water,water-based suspensions,solutions and emulsions for development of advanced firefighting technologies.In particular,the project’s team plan to develop physical and mathematical models that allow predicting the required conditions for injection of an aerosol into a high temperature gas zone in order to ensure the required mode of water evaporation,minimum entrainment of droplets and other criteria characteristic of the known principles of optimization of heat and mass transfer processes.
Expected results：-the main expected results
1.The project’s team will manufacture the unique tests benches and develop the experimental techniques based on high-speed cross-correlation video complexes and the combination of optical imaging methods for the simultaneous recording of all key parameters of gas-vapor-droplet flows：temperature,concentration,dispersity,speed,etc.The planned temperature range for the heating of droplets(from 300 K to 1500 K)corresponds to a large group of industrial gas vapor-droplet applications.
2.New knowledge will be obtained about the physics of heating and evaporation of droplets of liquids,solutions and suspensions in combustion products,air,gaseous mixtures,i.e.possible modes,stages,speeds.
3.For the first time,the authors will present temperature distributions(i.e.temperature fields)in evaporating droplets of water,water-based suspensions,emulsions,solutions,as well as in the near-surface vapor-gas layer.Thus,it will be possible to predict the temperature,concentration and aerodynamic traces of evaporating liquid droplets when moving through high-temperature gases.
4.The scientific team will determine the ranges of reliable evaporation rates of widely used liquids under varying sizes of droplets and temperatures in wide ranges：0.1-5 mm,300-1500 K,respectively).
5.It is extremely relevant to develop physical and mathematical models taking into account the most important heat and mass exchange processes：formation of an unsteady temperature field inside droplets and in their near-surface vapor-gas layer,thermal vapor protection of droplets,vapor emission,deceleration of droplets due to evaporation,surface transformation and other new and poorly studied effects.
6.The authors will generalize results,formulate conclusions,present dimensionless criterial expressions and integral dependencies.
7.The authors are going to make methodical materials with research results for use in the educational process at bachelor's,master's and postgraduate degree courses of TPU.
8.Not less than 6 articles will be published in Russian and international publications(in the periodicals indexed by Web of Science or Scopus).
9.The members of the scientific team will evaluate research results at a minimum 4-5 leading Russian and foreign scientific conferences,symposiums,forums,congresses.
-the compliance assessment of the planned results with the world level of research
The review article[Sazhin S.S.Modeling of fuel droplet heating and evaporation：Recent results and unsolved problems//Fuel.2017.V.196.P.69-101]analyzes the achievements and unsolved problems in the field of evaporation of droplets of liquids and fuels,presents the important conclusions on the major challenges of creating adequate evaporation models under the intensive heating.The greatest difficulty,from the point of view of adequate modeling,is the consideration of the process of formation of a substantially unsteady and inhomogeneous temperature field of evaporating droplets,as well as the vapor buffer layer around them and in their wake during evaporation in a flame and combustion products.The current project is also aimed at the solution of this problem.
According to a review of recent publications in peer-reviewed international journals(e.g.,International Journal of Multiphase Flow,International Journal of Heat and Mass Transfer,International Journal of Heat and Fluid Flow,International Journal of Thermal Science,Journal of Heat Transfer,Experimental Thermal and Fluid Science,Thermal Science,Applied Thermal Engineering)the project’s team makes findings of the fact that the planned results of the project are higher the world level of research.
-the possibility of practical use of the project's planned results in economics and social sphere
From the point of view of engineering and technology,the most important direction of application of the obtained project results is increasing in efficiency of thermal power equipment(in particular,contact heat exchangers)due to full utilization of heat of the leaving gases of boiler plants.
From the point of view of science and education,the experimental methods and test benches,the nonstationary and inhomogeneous temperature fields of evaporating droplets,physical and mathematical models,correlations,expressions,dependencies,established effects,conclusions and recommendations will be of great interest for researchers of two-phase and heterogeneous flows.The experimental and theoretical data will substantially expand the modern concepts on physics of the intensive evaporation of liquid droplets.The particular interest involves scientific knowledge of the formation of the buffer vapor layer around the evaporating drops,which,on the one hand,plays the role of thermal protection,and,on the other hand,water vapors by analogy with greenhouse gases intensively emit energy.The reliable measurements of sizes of such layers,component composition and heat fluxes will be of great importance for many gas-vapor-droplet directions of science,engineering and technologies(e.g.firefighting,creation of new heat carriers based on flue gases,vapors and droplets of water,etc.).
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