The Study on Dual Fuel Spray and Characteristics of Combustion of Diesel , Natural Gas and Dual Fuel

Combustion of fossil fuels provides around 88% of total energy supply for modern society, and meanwhile causes many environ mental problems and social problems such as air pollution and energy crisis. Therefore, both at home and abroad are focusing on the research and development of natural gas engine recently. However, the mass production and application of this kind of engine are restricted by some unsolved technical difficulties. This paper explored the injection, combustion and emission processes of diesel/ natural gas and dual fuel engine based on Chemkin II, Fire software and Schlieren method. The experimental study on the mixing process of dual fuel jet was carried out by using high-speed Schlieren method in a constant volume bomb. Based on the Fire software, and then applied to Chemkin II software to analyze the effect of the initial temperature and pressure on the net heat production and combustion emissions. The results show that Dual fuel mechanism is capable of producing different heat production behaviors when varying the initial pressure and temperature CO2 emissions are minimized by changing the initial pressure above or below the atmospheric value. However, CO emissions are peaked when diesel fuel is used. The dual fuel engine minimizes the CO emissions amount caused by diesel fuel. Increasing the initial pressure eliminates dual fuel CO2 combustion emissions. The research of this paper is important to optimize the in cylinder combustion processes of natural gas engine, and have a certain important meaning to guide the development of diesel ignition dual fuel engine. DOI : COMING SOON Corresponding author: Qian Wang, School of Energy and Power Engineering, Jiangsu University, 301 Xuefu Rd., 212013 Zhenjiang, Jiangsu, China., Tel: +8613912802056,Email address: qwang@ujs.edu.cn


Introduction
Although tremendous studies on alternative energy solutions such as combined fuels and hybrid-engines have been under focus for many years, yet there's still a lack of a complete substitution of the conventional internal engines [1][2][3][4][5]. Therefore, conventional engines are employed through a lot of applications to utilize the engine's optimum conditions [6].
Internal combustion engines generally are nominated to reduce the emissions amount below the emissions regulations. Commonly, conventional diesel engines are modified to work as dual fuel engines in which methane fuel provides most of the chemical energy while diesel fuel plays a main role in a pilot ignited one with the methaneair mixture [7][8][9][10][11]. However, adding the natural gas fuel option as a supplement for conventional diesel fuel proved that it's not only a very clean choice but also its more economically viable [12][13][14][15]. Combining these two types of fuel will elevate the conventional diesel engine's heating value, lower the fuel cost, and reduce the engine's adiabatic flame temperature [10,16,17]. Most of the existed conventional diesel engines can be modified to serve as dual fuel engines maintaining its compression ratio with less emissions of NOx and particles [13,14].
However, adding methane fuel to the engine makes it more competitive in terms of fuel air mixing and knocking resistance [18].
Recently, the natural gas/diesel combined fuel engine's implementations have been under focus both theoretically and experimentally to specifically understand the reaction nature [15,19,20]. The natural gas/diesel fuel ratio, the natural gas initial percentage and the natural gas initial composition with their initial temperatures and pressures conditions were studied and evaluated taking in consideration the intake air temperature [15, 21,22].The most significant drawback of the dual fuel engines compared with the conventional diesel engine is represented in efficiency redundancy in addition to higher CO emissions in case of low fuel loads [23,24]. At high and intermediate loads both of the engine's output and CO emissions are better than those in the conventional diesel engines [20,23,24]. Changing the engine parameters, increasing the pilot fuel amount and changing the injection time might have a better effect on the engine's output along with CO and HC emissions.   Where ρ denotes the force on the fluid, μ is the dynamic viscosity, Su, Sv and Sw are the source term.

Mass Conservation Equation
The increase of fluid mass is equal to the mass of the inflow into the fluid in unit time：

Eq.(4)
In which, t is the time, ρ is the density, u, v and w are the velocity vector components of u in the x, y and z directions.

Energy Conservation Equation
The increase of the energy in the fluid is equal to the sum of the net heat flow and the external force on the fluid in unit time. The energy conservation equation is: In which T is the temperature, Cp is the Specific heat capacity, k is Heat transfer coefficient, S T is the source term.

Physical Model and Research Program
The physical model is a conceptual model modeled by small or large species, which allow visualization, from examining the model, of information about the thing the model represents. The physical model investigates the geometry of the model and the mathematical correlations applied in this model:

Geometric Model of Constant Volume Bomb
A cylindrical constant volume bomb is used in the experiment, the structure is simple, and the location of the nozzles, size, direction and other parameters in the simulation process were directly defined. Therefore, geometric model of constant volume bomb and meshing is directly built in the Fire software, as shown in Fig.2.The volume of the constant volume bomb is larger, The mixing process of diesel spray, natural gas jet and diesel jet mainly occurs in the center area of the constant volume bomb. In order to shorten the calculation time and ensure the accuracy of the calculation results, the center of the constant volume bomb center is refined. The surrounding area grid size is about 3.5 times than the volume of the encrypted location grid, and the total number of meshes is 120393, which belongs to the structured grid.

Mathematical Model and Boundary Condition
For natural gas jets, there will be an incomplete expansion phenomenon under very high ejection pressure conditions; the jet process is very complex therefore; in this paper, the Pure Gas Jet model is selected as the gas jet model and KH-RT is the broken model. Because the injection mixing process of natural gas/diesel dual fuel in the fixed The Influence of Ambient pressure on Natural Gas / Diesel and Dual Fuel Jet Development The pilot ignited diesel fuel is introduced inside the combustion burning chamber as illustrated in Table.1.
High digital CCD in-cylinder camera FASTCAM was    ms in all of these traces, the pressure begins to fall because of the interaction of the driver wave with the reflected shock region [27].
If the shock tube reactor were an ideal constant volume reactor, then even with the energy release process occurring during ignition, the modeled pressure along with the measured pressure would achieve the same levels

Results and Discussion
The results of the numerical simulation using Chemkin software investigating the mechanism of natural gas fuel, diesel fuel and dual fuel combustion as investigating the net heat production of combustion paying great attention to the effect of both initial temperature and pressure along with their related pollutant emissions behavior are reported as follow: The Net Heat Produced Applying the Three Types of Fuels: Unlike the in-cylinder pressure estimation based on diesel mechanism and Zheng and Yao mechanism, diesel mechanism equivalent ratio variety range might not be sufficient to model the diffusion combustion [28]. From previous studies it is stated that the diesel mechanism is the only applicable mechanism at the standard equivalent ratio 1.0 [29], however Zheng and Yao provided a modified mechanism that is applicable even when the equivalent ratio is 1.5 which gives more flexibility in modeling the diffusion combustion [29].
In the continuous flow device; at relatively high temperature, the liquid fuel is injected directly to the air flow, the combined stream ignites as it flows through the combustion chamber [30]. After the ignition; a massive temperature increase occurs under constant pressure which produces the heat of production. The net production heat is thus defined as the net heat produced from burning the fuel inside the internal combustion engine [8].
The heat transferred from higher temperature regions to lower temperature regions is converted to mechanical work [8,30,31].
The predicted behavior of methane, diesel and dual fuel is verified in Fig.4. Using each fuel mechanism applied individually in Chemkin II software [32], with estimated experimental end time at 1 second. The associated operating conditions of the dual fuel are given in Table.3.     increasing the initial temperature doesn't clearly affect the behavior of the dual fuel but it destroys the trend of diesel fuel to become heat consumer at relatively high initial temperature. In each case 10% of combustion energy is obtained from diesel when applying the methane/diesel dual fuel mode.
As the engine works as dual fuel, more premixed fuel and oxidizers exist [8].The model results generally show some similarities when changing the fuel type.  Modifying the conventional diesel engine to serve as dual fuel engine increases its efficiency at relatively higher initial pressure. However, changing the initial temperature doesn't really affect the performance of the dual fuel engine.

Conclusions
After obtaining the model results, the main conclusions are as follow; • Diesel is injected first inside the combustion chamber then followed by Natural gas through separate injectors. The fuel mixes inside the chamber and ignites.
• Dual fuel mechanism is capable of producing different heat production behaviors when either increasing or decreasing the initial pressure and temperature.
• The scientific values added of this field include;