ORGANIZATION OF LABORATORY WORK ON «NUMERICAL METHODS IN THERMAL PHYSICS» IN THE CONTEXT OF ONLINE EDUCATION

ABSTRACT

This study is devoted to the topical issue of efficient combustion of liquid fuels and the organization of laboratory work on the topic "Numerical Methods of Thermal Physics" as part of online training. The effect of Weber numbers on the processes of atomization and combustion of tetradecane was studied using the KIVA software package. The obtained results can be used to reduce costs in the development of fuels with certain characteristics that solve problems in the design of various technical devices, such as internal combustion engines, increase the efficiency of fuel combustion and reduce emissions.

АННОТАЦИЯ

Данное исследование посвящено актуальному вопросу эффективного сжигания жидких топлив и организации лабораторных работ по теме «Численные методы теплофизики» в рамках онлайн-обучения. Влияние чисел Вебера на процессы распыления и горения тетрадекана исследовали с помощью пакета программ КИВА. Полученные результаты могут быть использованы для снижения затрат при разработке топлив с определенными характеристиками, решающих задачи при конструировании различных технических устройств, например двигателей внутреннего сгорания, повышения полноты сгорания топлива и снижения выбросов.

Keywords: numerical methods, computer simulation, KIVA-II, liquid fuel combustion, tetradecane, Weber number.

Ключевые слова: численные методы, компьютерное моделирование, KIVA -II, горение жидких топлив, тетрадекан, число Вебера.

Introduction

To date, KIVA-II is one of the computer programs that make it possible to study the flow of atomized liquid by numerical calculations, taking into account a number of phenomena that affect the combustion process in the combustion chamber [1, p. 265].

In addition to fossil fuels, energy sources include solar power, hydropower, wind power, and nuclear power. Energy produced on a large scale around the world is obtained by burning fossil fuels.

Prolonged and intensive use of such limited resources results in: 1) the gradual depletion of these resources; 2) an increase in the concentration of harmful substances in the atmosphere. Therefore, in our time it is necessary to look for ways to optimize the use of resources [2, p. 25].

The use of virtual laboratories solves many problems in teaching students when it is difficult to obtain the necessary equipment for research and when it is impossible to work offline [3, p. 29].

The numerical simulation method also takes into account other phenomena, such as molecular transitions, non-small chain chemical reactions, radiation, and heat transfer.

In this work, computational experiments on the combustion of tetradecane were carried out and the influence of high Weber numbers on its atomization and combustion in the combustion chamber was studied. Weber's numbers ranged from 4 to 8, through 0.5.

At a temperature of 300 K, liquid fuel is injected into the combustion chamber through a round nozzle located in the middle of the lower part of the chamber. The combustion chamber is a cylinder of 15 cm high and 2 cm in radius, filled with air at a temperature of 900 K and with the pressure of 32 bar.

The chemical kinetics of the combustion process of tetradecane is given below.

.

Tetradecane is an organic compound of the alkane class. Tetradecane is found in petroleum products and is one of the components of diesel fuel. Under normal conditions, the substance is a colorless, flammable liquid, insoluble in water, but soluble in non-polar solvents.

Research results.

Experiments on the study of the processes of injection and combustion of liquid fuels at various values of the Weber number correspond to the following results.

In Figure 1, tetradecane droplet sizes range from 1.02 to 7.65 µm. In this picture, one can see that the droplets are mainly collected at the bottom of the combustion chamber.

Figure 1. Radial distribution of tetradecane droplets at time t = 1.5· 10^-4 s. at We = 4.5

Figure 2. Radial distribution of tetradecane droplets at time t = 1.5· 10^-4 s. at We = 6

If the Weber number value is 6, the liquid fuel atomization is more efficient than the value shown in Figure 1, resulting in a higher and more intense combustion temperature, as shown below. The combustion process takes place in the combustion chamber, the radii of fuel droplets range from 0.645 to 7.095 microns.

Figure 3. Radial distribution of tetradecane droplets at time t = 1.5· 10^-4 s. at We = 8

Figure 4. Dependence of the radius on the Weber number

Analysis of fig. 3 shows that the tetradecane droplet size ranges from 0.442 to 6.2 µm. For Weber numbers from 4 to 8, the drop sizes decrease. It can be seen in Figure 4.

As a result, as the Weber number increases, the inertial forces acting on the drops increase and deform the drops, leading to their destruction.

Figure 5. Tetradecane fuel concentration distribution at time s.      for We = 4.5

The fuel concentration is shown in fig.5. At the current value of the Weber number, the injection is less intensive, and as shown in the following figures, the fuel burns more efficiently than at higher values of the Weber number. The fuel concentration ranges from 0.05 to 0.7 g/g.

Figure 6. Tetradecane concentration distribution in fuel  at time  s. for  We = 6

The combustion zone in Figure 6 increases, which indicates the intensity of the combustion process. Fuel concentration values vary from 0.05 to 0.06 g/g. This indicates that the fuel is burning faster than in the previous Figure 5.

The fastest combustion in the range of 0.03-0.06 g/g and the corresponding minimum fuel concentration are shown in Figure 7.

Figure 7. Time distribution of tetradecane concentration in fuel at time s. for We = 8

Figure 8. Weber dependence on fuel concentration

Fig. 9 shows the temperature distribution during combustion of tetradecane. Temperature values range from 756 K to 1418 K. Combustion covers a large part of the combustion chamber and occurs at intense and high temperatures, reaching a maximum value of 1418 K.

Figure 9. Tetradecane temperature distribution at time s. for We = 4.5 (T, K).

Figure 10. Tetradecane temperature distribution at time s. for We = 6 (T, K).

Fig. 10 shows the temperature distribution during combustion of tetradecane. The temperature varies from 751 K to 1408 K. Most of the space of combustion chamber is involved in combustion. Combustion is also intense and occurs at high temperatures. The maximum temperature range in this figure 10 is larger than in figure 9 because of increased Weber number to 7 and improvements of the combustion characteristics.

Figure 11. Tetradecane temperature distribution at time s. for We = 8 (T, K)

Fig. 11 shows the temperature distribution during combustion of tetradecane. The temperature value varies from 758 K to 1421 K.

Figure 12. Temperature dependence of the Weber number

The maximum temperature range in figure 11 is the largest compared to figures 9-10, moreover, it can be seen in figure 12. This is due to the improvement in the characteristics of the combustion process with an increase in the Weber number.

Figure 13. Water vapor concentration at time s. for We = 4.5

Analyzing Figure 13, one can see that the concentration of water vapor takes values from 0.0013 to 0.019 g/g.

Figure 14. Water vapor concentration at time s. for We = 6

Figure 14 shows that the concentration of water vapor takes values from 0.0012 to 0.0194 g/g.

Figure 15. Water vapor concentration at time s. for We = 8

Analyzing Figure 15, one can conclude that with an increase in the Weber number, the areas of maximum concentration of water vapor decrease, since the amount of water, which is the product of the reaction, decreases with a more efficient combustion process.

Water vapor concentrations range from 0.0014 to 0.021 g/g. Figure 16 clearly shows that these values are the maximum values.

Figure 16. Dependence of Weber number on water concentration

Conclusion. As a result of studying and analyzing the results of computational experiments, one can draw the following conclusions:

An increase in the value of the Weber number from 1 to 8 leads to: an improvement in the atomization of liquid fuel, a rise in the spread of drops in the space of the combustion chamber, an increase in the area occupied by a temperature flame and temperature in the combustion chamber also rises.

At the optimal value We = 8, the fuel burns completely, while droplets reach a minimum size of 6.6 μm, the combustion chamber heats up to a maximum value of 1422 K, and a small amount of water is formed. With an increase in the Weber number above 8, no significant changes are observed.

The results obtained in this work can be used to reduce the cost of developing fuels with certain characteristics that solve the problems of designing various technical devices, such as internal combustion engines, increasing the efficiency of fuel combustion and reducing emissions.

References:

1. Витман Л. А., Кацнельсон Б.Д., Палеев И.И.  Распыливание жидкости форсунками // пед ред. С.С. Кутателадзе. -М.: Государственное энергетическое издательство. - 1962. -  265 c.
2. Злобин В. Г, Зверев Л.О. Повышение эффективности котельных установок на жидком топливе // Известия высших учебных заведений. Проблемы энергетики. - 2020. - Т. 22., - № 4. -  25 с.
3. Рахматов В.З. Виртуальные лаборатории в системе обучения студентов // Сборник научных трудов ДОНИЖТ 2018. -  №51. - 29 с.