Sprays and Droplets Dynamics in Fuel Injector Combustion Applications - Case Study Example

INTRODUCTION. A combustor is the complex device in which a coupled range of interacting chemical and physical phenomena exists. The liquid fuel that is used to provide energy has to be atomized to smaller droplets. This helps in increasing the surface fuel exposed to the hot gases, as well as in facilitating faster mixing and gasification with oxygen rich ambient. Combustion emissions and performances are largely influenced by liquid fuel atomization, fuel droplet evaporation and motion, as well as, mixing of air and fuel. Spray dynamics, as well as, combustion studies are used in determining flames stability behavior at varying loads. This helps in ensuring safety and efficient energy utilization and in understanding the mechanics behind pollutant destruction and formation. Combustion dynamics is the area that has recently received fresh emphasis. This is because of advancements in efficiency of energy, in low emissions combustion system for the aerospace power plant and the ground base. The existence of instability in thermal acoustic commonly referred to as combustion instability has been hindrances in the developing combustors for jet engines, domestic heaters, rockets, and for the power generating gas turbine. It is important to understand how to control the combustion instability. In recent times, a reduction of the combustion noise has been prioritized, in an effort to reduce noise pollution emanating from power plant. The low emission combustors are susceptible to flame blow out, and combustion instability. Faster developments in the area of pulse denotations engine have fuelled research in this area. Pulse combustors are currently applicable in energy in improving energy intensive efficiency processes, taking advantage of the increase in energy transport, mass, and momentum. Liquid fuels are used in combustors. This implies that combustion dynamics, droplet dynamics, spray dynamics and the atomization are closely related. This paper explores the application of spray and droplets dynamics in fuel injector combustion in real life situation, as well as, their application in industrial processes. APPLICATION Sprays are used in many places. They are used to facilitate accurate fuel injection. In this respect, droplet and spray dynamic within a fuel injector has vast applications. For instance, in the optimization of spray it is applied in agricultural spraying, paint sprays, fuel injector nozzles, spray design nozzle, cosmetic sprays, pharmaceutical sprays, spray coating, atomization process, spray drying, ink jets, electro sprays, icing studies, Numerical model validating and in liquid metal spraying. In agricultural spraying, droplet and spray dynamic is used in the techniques used to measure velocity, and size of the droplets that are used in agricultural sprays. The phase Doppler droplet analyzer and the probe measuring instruments are used in measuring speed. The two instruments use the principles of droplet and spray dynamics. The instruments operate in single planes and by use of a nozzle that is computer-controlled so as to enable the sampling of the whole spray. If well configured, the instruments allow, the measuring of velocity and size distribution, droplet trajectories, spatial structure and entrained air speed of sprays. The two instruments are particularly useful in measuring characteristics of the agricultural sprays. Another application is in atomization processes. Atomization is the breaking down of a large amount of liquid into droplets. There are many atomization processes that employ the droplet and spray dynamics. To start with, some examples of atomizers that are commonly used in homes include perfume sprays, hair sprays, garden sprays, garden hoses and shower heads. Liquid coming from a pitcher is one such example of an atomization process that occurs naturally. If the pitcher is gradually lifted, the liquid steams out in an elongated manner. As this happens, at some point, the liquid breaks down and forms droplets. In airless atomization, the fluid is forced through a smaller nozzle by high pressure. The fluid would then emerge as a solid sheet with high speed. The stream is disrupted by the friction that develops between the air and the fluid. This friction breaks the steam into fragments in the first place and eventually into droplets. The source of energy for this type of atomization is from fluid pressure. The fluid pressure is converted into momentum during the movement of the fluid from the nozzle. There are some factors that affect airless spray. These factors include the atmosphere, the relative speed between the air and fluid and the orifice diameter. Considering the orifice diameter, the law that governs the orifice diameter states that as the size of an atomizer orifice increase, the average size of the droplets in a spray also increases and as the size of an atomizer decreases, the average size of the droplet also decreases. The resistance provided by the atmosphere breaks down the fluid. This resistance will appear to overcome the properties of the fluid such as viscosity, density and surface tension. Apart from this, temperature of the air may also have some effects on atomization process. The relative fluid and air speed also have an effect to the droplet size. Increase in the pressure of the fluid, increases the velocity and decreases the size of the droplet and decreasing the pressure of the fluid, decreases the velocity and increases the droplet size. Considering the air spray atomization, the fluid comes at a relatively low velocity from the nozzle. A high speed from a stream of air surrounds this speed. The friction is formed in between the air and liquid. This friction speeds up the steam of fluid leading to its disruption. This disruption of the fluid causes atomization. Atomization obtains its energy from the air pressure. The fluid flow rate can be regulated by the operator even without relying on the energy source. Centrifugal atomization is also one of the atomization processes. In this process, fluid is introduced by a nozzle at the spinning cup’s centre. The fluid is carried by centrifugal force up to the disk edge, and the fluid is then thrown off. The fluid develops ligaments that go through a breakdown to form satisfactory droplets. The source of energy for rotary or centrifugal atomization is mainly centrifugal force. Droplets develop closely at the edge of the disk having rotational speed that moves at a low rate. This is a contrast with the rotational fluid speed moving at a high rate. The pattern of the spray appears to move away radically from the disk. This occurs in all directions. Having rotary atomization, the disk speed and the flow rate can be controlled independently by the operators. Many rotary applications of spray coating apply electrostatic charge to the spray, so as to attract droplets towards an object in the ground. In other atomizers air is added to push the spray ahead towards an axial direction. Another application is in electrostatic atomization where a fluid is exposed to a strong electric field that is between grounded piece of work and charged atomization. The charge is transferred towards the fluid. Repulsive forces established between the fluid and the atomizer breaks down the droplet. The broken droplet is then sent to the surface of the work. Energy used in electrostatic atomization is due to electric charge that is received by the fluid. With this atomization, particle size is a function of fluid flow rate, electric strength of the field and fluid properties. However, electrostatic atomization in coatings of high viscosity can never be successful. Another atomization process, though rare in the industrial spray coating, is the ultrasonic atomization. This is a process that relies on electromechanical devices with high speed vibration frequency. The fluid passes across a vibration surface. This vibration makes the fluid be broken down into droplets. This technology is applied in areas such as; drying of liquids such as powdered milk surface coating used in electronic industries, and medical nebulizers that are used in inhalation therapy. However, this technology is devilishly effective if used under low viscous fluids. INDUSTRIAL APPLICATION. Apart from its application in the optimization of sprays, droplet and spray dynamic is also used in industries. In this respect, droplet and spray dynamic is used in a direct injecting diesel engine. In the diesel engine, droplet and spray dynamic is used to determine the amount of the gas that needs to be injected. It is also used in the geometry of the combustion chamber, in regulation of the motion of air inside the cylinder. The diagram below shows how fuel combustion occurs (fig 1) Since it has a numerous holes of different sizes, it can also be used in Sac Nozzles and VCO nozzle holes respectively. In a diesel engine, the fuel is injected into the combustion chambers as can be demonstrated in the diagram below (fig 2) Figure 2: A part of a jet engine demonstrating the combustion nozzle. The amount of diesel that is discharged spreads out for a certain period of time. The diesel spray gets into the hot chamber of combustion without igniting. The fuel spray then breaks down into tiny droplets. Once the liquid droplets are developed, the outer surface will start evaporation immediately. A liquid core that is surrounded by layers of water vapor is formed. The hydrocarbon fuel burning at this point is due to oxidation. Hence, heat generated out of the fuel vapor oxidation would be minimum compared to the conduction and convention heat extraction rate. However, a temperature is established the oxidation generated heat rate is more than the radiation and convectional heat rate. Due to this, the temperature increases hence, speeding up the oxidation leading to a further increase in heat production until an ignition is established. The oxidation process is also referred to as fuel combustion. This temperature where ignition takes place is referred to as self-ignition temperature. Heat that would be required for fuel drop to evaporate is normally provided from combustion. A combustion nozzle is highly effective in maintenance of the work efficiencies of an engine. It is attached directly to the combustion chamber as shown (in figure 2) above. WORKED EXAMPLE. Example 1 Calculate the temperature of the compressor at the exit given the inlet compressor temperature as 100oc and the ration of the compressor pressure as 32. Solution: The changes of the air’s temperature within the compressor are related closely to changes in the pressure by a relation referred to as an isentropic relation. TB/TA = (PB/PA) (γ-1)/γ TA=273.15 + 100 = 373.15K But PB/PA = 32 Hence, TB = 1004 K Example 2. Given a jet engine with an engine cycle, that takes in the air at 0.3 atmospheres and has its highest pressure as10 atmospheres. Calculate the ideal efficiency, for such an engine. Solution Representation of an engine process as a Brayton cycle is an ideal way of determining the efficiency of the jet engine. Efficiency of the cycle relates to PB (ration of highest pressure within the jet engine) to PA (lowest pressure). If the cycle, of an engine takes in the air at 0.3 atmospheres, and the engine‘s highest pressure is 10 atmospheres, The ideal efficiency for the cycle is; η = 1- (PB/PA) (1-γ) γ. Where g represents the ratio of the specific heat that is at a constant volume (=1.4) and pressure. η = 1- (10/0.3) -0.2857 = 0.6328 DIFFERENT THEORIES. Advanced large memory computers with high speed processors boosted the theoreticians. The development of the computers enabled theoreticians in formulating and numerically solving comprehensive models with detailed consideration of chemical and physical processes being involved in spray. It is evident that simulation flow field within a spray combustor is quite challenging and complex. The spray combustion takes place in a 3-dimensional, time-dependent system having two-phase flow of turbulent and in the incompletely well understood chemical reaction. Spray characteristics immensely affect the combustion characteristics. The NOx formation and the formation of other pollutants closely relate to the spray combustion process. Other factors that come into play include the mixing of fuel-air, radiation heat transfers, and the turbulent. All the factors are interrelated to each other. Computational fluid dynamics models commonly known as the CFD have increasingly become famous as far as gaining insights of the processes is concerned. This helps in improving combustion performances and a reduction in emissions with no compromise in fuel economy. Numerical sprays combustion models were proposed two decades ago. The spray and combustion model comprehensive reviews are found in the literature law. Generally, the models are classified into 3 types. These are two-fluid models, separate flow Lagrangian Trajectory model, and the direct numerical simulations model. The two fluid models treat particle as the continuum, while the separate flow Lagrangian tracks individual particle in the field gas. Although there is much advancements in CFD model for spray combustion, more work still need be done for the development of efficient and more accurate physical sub-models. Sub-models needed in spray combustion include the atomization, breakup, turbulent, vaporization, dispersion, spry injection, coalescence, ignition, wall heat transfer, combustion, and emissions. Modeling of the spray combustion demands that one knows the interactive transport processes occurring during combustion and gasification. Given that the typical combustion dimensions are of the order ten to about one hundred centimeters and that drops are order of few micros, a cluster of droplets having many representatives of drops can be considered depending on where it is located in the combustor. DISCUSSION. The process of spray combustion is divided into five groups. These are transport, combustion, atomization, vaporization and unmixedness. Generally, the liquid fuel is first injected into the combustion chamber through a nozzle system. It is then atomized forming a spray of droplets. A spray plume structure is shown in the figure 1 above. The liquid fuel gets integrated forming droplet and ligaments. This happens in the atomization region. The dense spray region contains high value s of volume fractions and involves the secondary ligament and droplet break-up, and drop-drop interactions, as well. The drop-drop interaction involves the collisions and the coalescence of the droplets. In the region, of dilute spray, sprays have very strong interactions with turbulent airflows and the drops in this region are also well formed. Generally, spray structures depend on injection pressure differences, viscosity of the fuel, density of the fuel, and the injection size. Given the initial injection velocities, the fuel droplets can penetrate into high temperature of the air. Fuel spray can advance over time until hot air and a combustion gas vaporizes the droplets. For the case of spray plume, droplets of fuel get heated by the hot air around it leading to vaporization of combustion gas. This happens at a specific boiling point of the droplets. The vaporization rate wholly depends on boiling point and fuel latent heat, droplet size, as well as, the temperature of the gas. In the spray fume, there is also mixing of fuel air with the entrance air. They start burning whenever the temperature of the gas approaches the ignition point and when the ratio of fuel to air mixture lies within flammability range. The burning process produces both combustion and combustions. The products from the combustion process include carbon dioxide gas, water and some amounts of pollutants such as nitrous gas. In a combustion process, the most critical features are the exchanges of energy, exchanges in momentum, and exchange in mass between the liquid phases and the gas within a spray combustor. Since the spray is made up of droplets, the collective droplet properties influence the bulk spray characteristics which determine the performance of the combustor. The rate of vaporization controls the rate of combustion in the part of the combustor. Droplet trajectories will affect the rate of local vaporization, and the drop trajectory is affected by the droplet drag. The spray combustion processes are major concerns for engineers. Long overdue plans are being implemented by engineers for the achievement of more economical fuel usage and better controlling of pollutants in combustion products. However, the complexity nature of combustion processes and the spray atomization processes necessitated the designing of many practical devices based on the approach of trial-and-error that is somehow very expensive. In the resent times, it has become apparent for one to understand the mechanism that are related to spray combustion. This is the key in developing of high performance devices and the non polluting devices. The best approach is that of using computer models for the fundamental studies, as well as, in the development of practical devices. DROPLET GROUP EFFECTS Sprays involve a considerable large number of droplets. Estimation of the combustion rate considering it as a sum of isolated droops combustion rate can be the simplest analysis. However, the interaction occurring between the droplets alters flow field, changes the drag co efficiency, competes for oxygen and heat, changes the fuel vapor distributed at the drops, and decreases or increases the delay in ignition depending on how dense the spray is. Droplet interaction with the environment and interactions among the droplets themselves is known as group effect. Collective interaction phenomena occur in spatial extension that range from few droplets to droplets of large aggregate. This constitutes majority or fraction pert of the spray system. The phenomena of the group, which include group ignition, group combustion and group evaporation, is a globally collective behavior that is often associated with heat transfer, phase change, aerodynamics and combustion of droplet systems occurring in the effect of long range collective interactions. Such group phenomena are different in the structure from that for the total of each isolated droplets that are in the same hydrodynamic environments. The group phenomena of droplets were observed by numerous experimental researchers. The experiments done by Chgier and Mc Greath (1976) showed that droplets that are in dense sprays do vaporize as a group, with the collection being surrounded by a zone of external flame during vaporization. In 1982, Chiu et al tried to categorize the results of the experiment with group combustion theory application. Through analysis of the inner heterogeneous regions with the homogenous outer gas phase regions for the spherical droplet clouds, a demonstration was done to show that as the density of the spray increases or as spacing of the droplet decreases, the combustion model first exist as a unit droplet combustion, the as external or internal group combustion, the finally external sheath combustion as shown in fig B. The number G, which is a group combustion number, was the proposed number that defines the combustion mode. Both external and internal collection combustion occur, because the droplet group behavior in fuel liquid sprays prevents the air penetration and all forms of non-flammable mixture of high fuel in the central regions of the spherical clouds leading to no droplet burning therein, as shown in figure 2 above. CASE STUDIES. A diesel engine commonly known as a compression-ignition engine is an internal combustion type of engine that make use of the heat from the compression in initiating ignition to help burn the fuel injected into the combustion chamber. This contrasts the petrol engine that uses spark plug in igniting of an air-fuel mixture. The diesel engine was developed in 1893 by Rudolf. Because of having very high compression ratio, the diesel engine possesses the highest thermal efficiency as compared to other regular combustion engines. The low speed engines such as the engines used in ships, and other areas of technology, have thermal efficiency exceeding 50%. Diesel engines are in versions of four strokes and two strokes. The engine systems replaced the steam engines, and they are dominant in heavy equipment and in submarines Majority if not all top Fuel Dragsters use mechanical fuel in injecting fuel. The fuel is injected via nozzles mounted directly into the cylinder heads of the combustion chamber. Some small amounts of fuel are then injected via the supercharger in order to keep it lubed and cooled. The fuel curve always preset making their run and might not compensate for atmospheric changes like how the modern electronic fuel injection systems do. The injection fuel process is what makes a difference between gasoline engines and the diesel engine. Most of the modern cars use port injection method or the carburetors. Fuel is injected before the intake stroke outside the cylinder for the port injection system. A carburetor will the mix fuel and air just before the entry of air into the cylinder. During the intake stroke, all the fuel is loaded onto the cylinder. Fuel/air mixture compression limits the engine compression ratio. In case, it compresses the air too much there is spontaneous ignition of the fuel/ air mixture causing knocking. Excessive heat due to the knocking can cause engine damage. The diesel engine makes use of the direct fuel injection. In this case, the fuel is directly injected directly into the cylinder. In the diesel engine, the injector is a complex component and forms the subject of many experiments. It is located in varied places depending on the engine. It has to withstand pressure, as well as, pressure within the cylinder but still delivering fuel in a fine mist. Circulating the mist within the cylinder, is a problem, therefore, some diesel engines use pre-combustion chambers or special induction valves to swirl the air inside the combustion chamber. Some of the diesel engines have a glow plug. Whenever the engine is cold, the process of compression might not lift the air to a temperature required for ignition of the diesel fuel. There is a glow plug that causes the heating of the combustion chambers raising the air temperature whenever the engine is cold so, the engine starts up. In modern engines all the functions are under the control of the ECM. This ECM links up with the set of sensors to measure everything ranging from R.P.M. to the coolants of the engine, and the temperatures of the oil and the position of the engine. The modern diesel cars have impressed the use of the three dimensional software called the KIVA-3V code. The analysis of the KIVA 3V code helps in calculating turbulent movements, as well as, combustion in internal combustion engines such as the diesel engine. The software has other new features. This are the particle-based liquids wall film model, the linear sorting subroutine characterized by linearity in the node numbers preserving the storage sequence, the mixing-controlled turbulent combustions model, and finally an optional RNGs k-¿ turbulent model. The expanded grid generator, K3PREP, supports the generations of grids with valves, with the shaping of runners and ports valve. The software contains graphic output options that are expanded. Provided that, in a real diesel engine, the temperatures of the walls are not constant, but get varied over the cycles, energy equations in standard K¿VA-code has to be modified due to the effects of wall heat. Because of the shape of piston (4JB1) not being on the symmetry axis, 3600 was modeled thus making it correspond to x-offset 4 mm and y-offset 1 mm. Last but not least, field temperature is compared with data from the experiment. This gives the new theory and methods about how to calculate turbulent movement and combustion exactly in the diesel engine. This software was used to find values of the diesel engine displayed below. Diesel engine specifications. Cylinder diameter 130mm Length of the connecting 290mm Length of the piston movement 145mm Distance that is above the point of death from the cylinder head 1/90 Nozzle injector number 0/350 Number of nozzles 4 Angle of fuel cone 150 0 c Fuel injection time 15oc Length of fuel injection 17oc Injection pressure 190k Fuel temperature of the drop 350k Initial temperature 420k Initial pressure 1/55 bar Results Work cited Weber, J. Optimization methods for mixture formation and combustion process in diesel engines. 2008, New Yolk: Cuvillier Learning. Read More