彭升
【Abstract】In order to study the combustion characteristics of hybrid fuel after butanol has been mixed with different ratio of hydrogen, AVL FIRE software is used to simulate the process of the combustion of hybrid fuel in constant volume combustion bomb. This study uses the software of CATIA to complete the creation of three dimensional model of the constant volume combustion bomb and the software of AVL FIRE to complete grid drawing and numerical simulation of combustion. According to the ratio of 5%,10%,15% and 20% which hydrogen mixed with butanol, then study on the characteristics that mean pressure, rate of heat release and so on of combustion of the four kinds of mixed fuels with different initial conditions.The results shows that the parameters of combustion characteristics of butanol mixed hydrogen: pressure, rate of pressure rise, heat release and rate of heat release are all increased with increasing of ratio of hydrogen-doped; with increasing of ratio of hydrogen-doped, the burning duration is shortened and the peak of combustion parameters is advanced.
【Key words】Butanol; Hydrogen; Combustion; Numerical Simulation; Ratio of hydrogen-doped; Burning duration
0 Introduction
China is a country which produces and consumes a lot of energy and the quantity of energy consumption has leapt to the second of the world[1]. With the rapid development of China´s economy, the automotive and engine industry are developing rapidly and the demand for energy is increasing. At present, China has become a country that relies on importing oil and the quantity of oil that depend on importation has exceeded 50%, while the oil for engine is 2/3 times of the total oil consumption[2]. Therefore, the search for new energy to replace oil is particularly important. In recent years, researchers in domestic and abroad are all turning to the study of the new fuels.
In the alternative fuels, butanol has a longer carbon chain and a higher calorific value and boiling point compared with ethanol. And the calorific value of butanol is very close to gasoline, the calorific value of butanol is higher than methanol and ethanol.In addition butanol has a high calories, so butanol has a better fuel economy. As we know, fuel of pure hydrogen is carbon-free and zero-emission. So if engine switches to hydrogen fuel, it will greatly broaden the scope of lean mixture combustion of engine, improve the fuel economy and reduce emissions of nitrogen oxides[3]. After analyses, the fuel of butanol mixed hydrogen has the following advantages:
1）The reaction of combustion of hydrogen is by chain reaction mechanism, which means that hydrogen-fueled engine closer to the ideal isochoric heat cycle.It can improve the efficiency of the engine, so after butanol mixes with hydrogen it can increase the efficiency of the mixed fuel. The flame propagation velocity of hydrogen (2.91m/s) is 7.72 times of gasoline, the quicker flame propagation speed speeds up the combustion of mixed fuel and reduce the circulation changes[4].
2）The burning limit of hydrogen is 4~75,so there is a large range of air/fuel mixture.The burning limit of butanol is 1.45~11.25. Compared with butanol, hydrogen can burn under greater air/fuel ratio, so the hydrogen internal combustion engine is easy to implement lean burn. After butanol mixes with hydrogen, it can increase the ratio of air/fuel mixture and improve the engine´s economy.
3）Although the autoignition temperature of hydrogen is higher than butanol, but the requiring of ignition energy is low and the minimum can be as low as 0.02mJ. The hydrogen-fueled engine almost never not be caught fire, so it is easy to start[5]. After butanol mixes with hydrogen,The startup performance of engines can be improved.
4）The diffusion coefficient of hydrogen in air is larger than butanol and the diffusion coefficient of hydrogen is 12 times than gasoline. Hydrogen mixes with air more easily than butanol to form a uniform mixture. So after butanol mixes with hydrogen, it is more likely to form a uniform mixture.
5）Due to hydrogen combustion chain reaction produces a lot of activation, so the chemical reaction of CO and HC in the cylinder is more fully. After butanol mixes with hydrogen it can reduce the emission of CO and HC.
Research results shows that, when mix too much hydrogen in gasoline, the amount of air in the engine is reduced and the power output is downed, then prone to tempering[3]. The study of gas mixes hydrogen by Masahiro[6] has shown that when mixes with the ratio of 20% of hydrogen in the gas, the performance of engine is improved obviously[7]. WANG Jin-hua, HUANG Zuo-hua and someothers persons in Xi`an Jiaotong University in China, who studied different ratio of 0%, 9%, 19%, and 37% of hydrogen of the HCNG on combustion properties of GDI engine. The results shows that, the ratio of hydrogen is about 20% is the best choice after considering the dynamic performance and emission performance[8].
Given a lot of advantages of butanol mixes hydrogen. This paper will refer to the previous research of the fuel that mixed with hydrogen. Select the ratio of hydrogen-doped as 5%, 10%, 15%and 20% and use AVL FIRE software to make the model of combustion of butanol mixes hydrogen in the constant volume combustion bomb. After the simulation, analyses the combustion characteristics of burning of mixture under the different excess air ratio and the initial pressure. Provide the valuable reference for the experiment of butanol mixes hydrogen.
1 The construction of geometric model
The constant volume combustion bomb´s body is a barrel-shaped cylinder. The circle´s diameter which at the end it is 95mm and the high of it is 260mm, then the wall thickness of it is 15mm. It can withstand the pressure of 25MPa. Based on the body model of constant volume combustion bomb, the model of constant volume combustion bomb is maken by the software of CATIA, then save the surface mesh of the constant volume combustion bomb in the form of STL. Import the surface mesh to the sofeware of AVL FIRE and make the grid to calculate. The three-dimension grid as shown in Figure 1.
2 Initial options
At the end of the compression of petrol engine, the mixture´s temperature is about 580K~750K. The selecting of minimum initial temperature should be greater than the normal boiling point of the blends (the boiling point of butanol is 390K and hydrogen is 20K). The point of ignition of hydrogen is higher than butanol (the point of ignition of butanol is 658K, and hydrogen 858K).Therefore, the initial tempera- ture of this paper is identified as 593K.
The ratio of compression in modern petrol engine is about 8.0~12.5. Actually, at the end of the compression of petrol engine, the pressure is about 0.6MPa~1.3MPa. So 1.0MPa and 1.4MPa is selected as the initial pressure boundary conditions of the simulation of burning. The pressure of 1.0MPa as a numerical simulation of combustion of petrol engine and 1.4MPa to show the characteristics of combustion at a high ratio of compression.
The burning limit of hydrogen is 4%~75% and the burning limit of butanol is 1.45%~11.25%. Therefore the limit excess air coefficient of butanol is between 0.31~2.4 and the limit excess air coefficient of hydrogen is between 0.56~10.5. With the increasing of ratio of hydrogen, the mixed fuel´s limit values of the coefficient of excess air can be increased. The maximum ratio of hydrogen of this paper was decided as 20%. In order to study the effects of different stoichiometric ratio on the combustion characteristics of mixed fuels, the excess air coefficient of this paper was defined as 1.0, 1.5, and 2.0.
3 Simulation analysis
3.1 Pressure analysis
Figure2, figure3, figure4 are the compareson charts of the change of pressure and rate of pressure rise in constant volume combustion bomb when the mixed fuel with different proportions of hydrogen (5%,10%,15%,20%)is burning, which under the conditions that: initial temperature is 593K;initial pressure is 1.0MPa; excess air coefficient are 1.0, 1.5, 2.0. From figure2, figure3, figure4 shows that:
1）When the initial temperature, initial pressure and excess air coefficient are unchanged, compare the change of curve of the combustion pressure under the different ratios of hydrogen-doped.The research shows that with the increasing of hydrogen-doped, the pressure and the rate of pressure rise increased.
2）By figure1, figure2 and figure3 we can know that during the time of 0~10ms, the change of pressure and the rate of pressure rise are not obviously.After the time of 10ms, the pressure and the rate of pressure rise increased rapidly. Because of the mixed fuel reactes with oxygen obviously after the time of 10ms.
3）Analysis of figure2, we can konw that when the ratio of hydrogen-doped is 5%, the pressure is no longer rising at the time of 47.4ms; when the ratio of hydrogen-doped is 10%, the time is 44.2ms; when the ratio of hydrogen-doped is 15%, the time is 41.2ms; when the ratio of hydrogen-doped is 20%, the time is 39ms. So the conclusion can be drawn: since the hydrogen has a high flame speed, with the increasing of hydrogen-doped, the time of combustion of the mixture is shorten. It can arrive at the same conclusion from figure3 and figure4.
4）Compare among figure2, figure3 and figure4, we can know that when the excess air ratio λ rose from 1.0 to 2.0, the peak of pressure and rate of pressure rise that with different the ratio of hydrogen-doped are all decreased. When λ=1.0, the value of them are the largest.
3.3 Heat release analysis
Figure5 and figure6 are the comparison chart of the change of the heat release and the rate of heat release under the conditions that: initial temperature is 593K; initial pressure is 1.0MPa; excess air coefficient are 1.0 and 2.0.
Analysis of figure5, we can know that when the λ is unchanged, the ratio of hydrogen-doped is 5%, the peak of heat release is 38.1KJ; the ratio is 10%, the peak of heat release is 42.8KJ, the ratio is 15%,the peak of heat release is 47KJ, the ratio is 20%,the peak of heat release is 52.1KJ. It leads to the conclusion that the increasing of hydrogen- doped could improve the heat release of combustion. The same time, the rate of heat release is increased.
In addition, compare figure5 with figure6. It is similar to the changes of pressure and temperature, with increasing of λ, the quantity of heat release is increased.
4 Conclusion
1）When mixed with 5%~20% hydrogen in butanol, pressure, rate of pressure rise, heat release and rate of heat release of combustion are all increased with the increasing of the ratio of hydrogen-doped.
2）Because of the high speed of the flame propagation of hydrogen, with the increasing of the ratio of hydrogen that in the mixed fuel, the end time of combustion becomes smaller.
3）With the increasing of the excess air ratio, pressure and heat release of combustion reduced. The peak of the rate of pressure rise, rate of heat release of combustion decreased.
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