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Calculation and Analysis of the Thermophysical Parameters of
Micro-emulsified Ethanol—diesel Blended Fuel
Key words：ethanol—diesel；micro—emulsified fuel；thermophysical parameters
Abstract :The micro-emulsified ethanol-diesel blended fuel is an ideal alternative oxygenous fuel for diesel engines due to its physical and chemical properties and excellent combustion characteristics Fueled with this fuel, a diesel engine can give off remarkably lower harmful emissions，especially particulate. It is necessary to take into account instantaneous variations of the thermophysical parameters of blended fuels with temperature and pressure. For these reasons, this puper use the empirical formulas calculates the thermophysical parameters of this blended fuel, including the specific heat capacity, viscosity, thermal conductivity and diffusion coefficient. And the effects of the temperature, pressure and ethanol content on the initial blended fuel's condition were investigated. The results show that the addition of some ethanol to the diesel oil can contribute to the evaporation of the micro-emulsified ethanol-diesel blended fuel, and these empirical formulas were to be employed to calculate the thermophysical parameters of this blended fuel.
Introduction
The increasing petroleum price and environmental concern due to global warning has developed the firust in search of renewable fuels for diesel engines. Recently, much attention has been paid to the development of alternative fuels in order to meet the emission standards and to reduce the dependency on fossil fuel[1-3]. Especially, ethanol has been considered as one of major alternative fuels, as it can be derived from renewable sources. This fuel is well-oxygenated, therefore the adicition of some ethanol to the diesel oil have a great potential to reduce emissions [4-6]. When micro-emulsified diesel researched for a researched for a number of applications in diesel engines and other sarfimary combustion appliances, it is necessary to take into account instantaneous variations of the thermophysical parameters of blended fuels with temperature and pressure. However, the relations are acquired from design handbooks difficultly, especially for the surfactant such as span80.therefoce,the relationships among temperature, pressure and the thermophysical parameters need to be established. For these reasons, this paper takes the micro-emulsified ethanol-diesel for eacmple, and using the empirical formulas calculates the thermophysical parameters of this blended fuel. And the effects of the ambient temperature, pressure and ethanol content on the initial blended fuel’s condition are investigated.
preparation of the micro-emulsified ethanol-diesel
In the course of the experiments, the reagents contain diesel, ethanol and span80, and essential equipment contains JJ-1 fixed time electrically operated mixer and JY 10001 electronic balance.
The preparation process of the micro-emulsified ethanol-diesel blended fuel involves the following steps. At first, 15g of diesel and 3g of span80 are mixed well in a beaker. Then, 0.lg of ethucd each time is added. Until these mixtures became turbid, the mass of ethanol dissolved completely is 2.8g. There are three types of the fuel ratio: system 1-78.9% diesel, 5.3% methanol diesel and 15.8% span80; system 2-75.0% diesel, 10.0%methanol and 15.0% span80; system 2-73.2% diesel, 12.2% methanol and 14.6% span80 by mass.
The calculation of the thermophysical parameters
(1)The calculation of the enthalpy of vaporization
At first, using the Riedel method calculates the enthalpy of vaporization of the ith species at normal boiling point [7]. Then, using the relation between 4H and T put forward by Watson calculates the enthalpy of vaporization of the ith species at any temperature. Its mathematical expression is given by the equation (1).
where Tc is the critical temperature, Tbr is the ratio of the normal boiling temperature to the critical temperature, and the unit of the enthalpy of vaporization is kJ/mol.
According to Eq.(1), for at any temperature, their mathematical expressions are
When the enthalpy of vaporization of the ith species is calculated by Eqs.(2)the enthalpy of vaporization of blended fuel is calculated based on the following equation.
Where △Hvi is the enthalpy of vaporization of the ith species, xi is the corresponding molar fraction Tc and Pc are the critical temperature and critical pressure, respectively, Tbr is the ratio of the normal boiling temperature to the critical temperature.
Fig.1 represents the relation of between the enthalpy of vaporization and ambient temperature There are three types of the fuel ratio by mass at an ambient pressure of 1.OMPa. According to Fig.l, the enthalpy of vaporization of the micro-emulsified ethanol-diesel blended fuel decreases with the increase of both the temperature and ethanol content. After the addition of some ethanol to the diesel oil the enthalpy of vaporization of mixed fuel decreases and the evaporation rate increases significantly. That is to say, the droplet radius becomes smaller, which can improve the fuel mixing with air and in-cylinder combustion.
(2) The calculation of the heat of formation
The heat of formation of various compounds is calculated conveniently and the relationship between the molecular structure and its energy is analyzed by the bond en additive method . This method is capable of calculating the heat of formation of unusual specimens, such as the surfactant, and the calculation precision can satisfy the engineering demand. This paper calculates the heat of formation of span80 by the bond energy summation method, and results are showed in the table 1. The heat of formation of diesel (C12H26) and ethanol is calculated by bond energy summation method, or their values are acquired from design handbooks.
Following [8], the corrected values of functional group and are -24.7kJ/mol and -12.6kJ/mol, respectively. Hence, the heat of formation of span80 is 1335.0kJ/mol.
(3)The calculation of liquid specific heat capacity
At first，the gas specific heat capacity of the ith species can be calculated by the group contribution method put forward by Rihani and Doraiswamy[9]. This method can be used to calculate the gas specific heat capacity in various types of organic compounds. Its mathematical expression is
where ni is the functional group number of the ith type. The gas specific heat capacity of span80 by Rshari-Doraiswamy group contribution method put forward is showed in the table 2. The gas specific heat at capacities of diesel(C12H26) and ethanol is calculated by Rihani-Doraiswamy group contribution method, or their values is acquired from design handbooks. Their mathematical expression are
Then using Sternling-Brown equation proposed Sternling and Brownby calculates the liquid specific heat capacity of the ith species. They are calculated based on the following equations.
Where ω is the acentric factor, Tc is the critical temperature, Tbr is the ratio of the normal boiling temperature to the critical temperature, and the unit of the specific heat capacity is cal/(mol ?k).
When the liquid specific heat capacity of the ith species is calculated by Eq.(6), the liquid specific heat capacity of liquid mixtures is calculated based on the following equation.
where Cpls liquid specific heat capacity of the ith species, and x; is corresponding molar fraction.
The combination of Eqs.(4), (6)and (7), we can calculated liquid specific heat capacity of the micro-emulsified ethanol-diesel blended fuel, and the effects of different ambient temperatures on liquid specific heat capacity of the micro-emulsified ethanol-diesel have been presented for in Fig.2.
As shown in Fig.2, the liquid specific heat capacity of the micro-emulsified ethanol-diesel blended fuel increases with the ambient temperature rising. After the 400K, the liquid specific heat capacity significantly increases with the ambient temperature. Besides, the liquid specific heat capacity of liquid mixtures increases with the increase of the ethanol content, and the difference among three types of the fuel ratio is reduced. The addition of some ethanol to the diesel oil increases the liquid specific heat capacity. That is to say, more heat is absorbed when the same quality fuel is evaporatived , which is beneficial to reduce the cylinder temperature and can depress the formation of NOx.
(4)The calculation of liquid thermal conductivity
At first, the liquid thermal conductivity of the ith species at any temperature is given by the equation (8). Then, liquid thermal conductivity of blended fuel can be expressed as equation (8).
where r is a constant, and equals to 1 in engineering calculation, yi is the mass concentration of the ith species. Effects of ethanol content and temperature on the liquid thermal conductivity are showed in Fig.3.
As shown in Fig.3, the liquid thermal conductivity of the micro-emulsified ethanol-diesel blended fuel decreases with the liquid's temperature rising, and it increases with the increase of the ethanol content. The addition of some ethanol to the diesel oil is beneficial to raise temperature and can contribute to the evaporation.
(5)The calculation of diffusion coefficient
Using the method proposed by Fuller，Schettler and Giddings calculates the gas diffusion coefficient of the ith species. The expression is [10]:
where T is the absolute temperature, p is the pressure and its unit is bar, and v is the molecular diffusion volume and it is obtained by the sum of the atomic diffusion volume of all types of functional groups, and the unit of DA-B is cm2/s. According to Eq. (15), mathematical expressions the diffusion coefficient of diesel, ethanol and span80 are
After the diffusion coefficient of the ith species in the air calculated with Eq.(15), the mean diffusion coefficient of the vapor is calculated by Eq.(17). Effects of temperature and pressure on coefficient of the micro-emulsified ethanol-diesel have been presented in Fig. 4.
(13)
According to Fig.4, the mean diffusion coefficient of the vapour in the vicinity of the droplet increases with the ambient temperature rising, which is also good for the droplet evaporation. Meanwhile, Fig.2 shows that the mean diffusion coefficient of the vapor decreases with the ambient pressure rising. Since the diffusion coefficient is inversely proportional to the ambient pressure, the diffusion coefficient decreases with the increase of environmental pressure, which reduces the rate of droplet evaporation
Summary
The thermophysical parameters, including the enthalpy of vaporization, heat of formation, liquid specific heat capacity, liquid thermal conductivity and diffusion coefficient, are caculated using the engirical formulas. The effects of the ambient temperature, pressure and ethanol content at the initial blended fuel's condition are investigated. The results shows that the addition of some ethanol to the diesel oil can contribute to the evaporation of the micro-emulsified ethanol-diesel blended fuel and these empirical formulas are employed to calculate the thermophysical parameters of the micro-emulsified ethanol-diesel blended fuel.
Acknowledgement
This study was financially supported by the National Natural Science Foundation of China under the contract No.50976051 and by the Priority Academic Program Development of Jiangsu Higher Enduction Institutions (PAPD)and by Scientific Research Innovation Foundation for Graduate student of Jiangsu Province(CXZZ 12 0675) and by Automobile Engineering Key Laboratory Opening Fund of Jiangsu Province(No. QC201102).
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