Experimental and Numerical Study of Swirling ... - Solid Mechanics

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Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines Chapter 1 for the scavenging process. Moreover, it not only serves to scavenge the cylinder exhaust gases but also as to fill the cylinder with fresh air charge for the next cycle which, in case of four stroke engine, is also accomplished in a separate piston stroke. This makes scavenging process very important for engine performance and efficiency both in terms of fuel and emissions. This is because scavenging process not only removes exhaust gases and provides fresh air to the engine but also provides the necessary and initial swirl to the fresh air in which the diesel fuel is to be injected. As diesel combustion is a mixing enhanced combustion process, a proper and good mixing of fuel with air ensures better/ complete combustion of the fuel injected. Further, the resulting heat release defines the production of different pollutants like NOx and Soot etc. The heat release during combustion also defines the temperature distribution in the cylinder fluid mixture (injected fuel, fresh air, combustion products) and heat transfer through cylinder walls, cylinder head and piston. This in turn defines the distribution thermal stresses generated in the aforementioned solid materials and the requirement of their cooling. From the point of view of engine research and development, for example in computational modeling of in-cylinder diesel combustion, an initial flow field is to be defined in the compressed air pocket in which the diesel fuel is to be injected. This flow field then governs the simulation of fuel evaporation and mixing, consequent combustion and pollutant formation and heat generation and transfer through the cylinder enclosure walls. An improper definition of this flow field gives erroneous simulation results and hinders the efforts for understanding the overall process and identifying the key aspects required to be improved in order to develop the so called future engines. The knowledge about the initial flow field can only be understood by understanding the scavenging process. During the compression stroke, when the piston closes the scavenging ports, the in-cylinder flow field at that moment is the initial condition for the final piston-compressed flow field in which the fuel is to be injected. Practically, in the current marine diesel engines, the amount of air supplied during the scavenging process is more than what is required. This makes some amount of fresh air to enter and leave the cylinder through exhaust valve. This is called as Short Circuiting (Pulkrabek, 2003). In addition to having not well understanding of the in-cylinder fluid dynamics during scavenging process (flow field and interaction of fresh air with exhaust gases), another aspect of supplying more air is cooling the exhaust valve. Efficient scavenging improves the combustion performance of the 2-stroke diesel engines. Thus understanding, optimizing and developing an efficient scavenging process is one of the key aspects of developing efficient and environmentally friendlier marine diesel engines. 4 Introduction
Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Figure 1.3: Schematic c Pictures of types of Sc cavenging (a) Cross Scavenging S (b) Loop Scavenging S (c) Uniflow w Scavenging. (Heywood d, 1988). 1.3 There are a mainly thre ee types of scav avenging systemms used in two stroke marinne diesel engines (Figure e 1.3) (Heywoood, 1988). 1.3.1 In this type of scaven nging, the scavvenge/ air intake port is at one side of thhe cylinder near the bot ttom dead cenntre (BDC) andd the cylinderr exhaust port is on the other side of the cylinder ( (Figure 1.3a). Both the intaake and exhauust ports ar re opened and d closed by piiston during iits motion. Thhe exhaust poort opens before b the int take port andd some portioon of exhaust gases, being at higher pressure than atmospheric, leaves the cylinder. Then ppiston uncoveers the inta ake port. The piston p is givenn a special shaape which direects the fresh aair up towards the cylin nder head andd then air flowws down to exxhaust port annd remove es the exhaust gases. g 1.3.2 Types of f Scavengging Systeems Cross Sca avenging Loop Scav venging Chapter 1 In loop scavenging, both b the exhauust and intake ports are on tthe same side of the cyli inder. The pis ston does nott have a speciial profile as iin case of crooss scaveng ging. Similar to t cross scaveenging, the piiston opens thhe exhaust poort first an nd then the in ntake ports. TThe incomingg air enters thhe cylinder annd flows in n loop as show wn in figure 1.3b. For botth cross and loop scavenginng types th here exists no exhaust e valve. 5 Introduction
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DTU Mechanical Engineering Section
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