Dizel Motorun Hesaplamalı Akışkanlar Dinamiği İle Modellenmesi Ve Yanma Analizi

Abstract

Tez çalışmasında bir dizel motorun hesaplamalı akışkanlar dinamiği ile yanma analizi yapılmıştır. Çalışma kapsamında FORTE Reaction Design programı tercih edilmiştir. Analiz sonucu elde edilen veriler deneysel sonuçlar ile karşılaştırılmış olup hata payları incelenmiştir. Kimya modelinde Chemkin'den yararlanılmış olup RNG k-epsilon türbülans modeli tercih edilmiştir. Egzoz gaz geri dönüşüm (EGR) sistemi ele alınarak is partikülü ve NOx üzerindeki etkileri incelenmiş olup EGR yüzdesi için gerekli optimizasyonlar yapılmıştır. Son olarak ise dizel motorun tam yanma odası ve Phi-T konturları incelenmiştir.

In this thesis, there has been made combustion analysis of a diesel engine as using Computational Fluid Dynamics (CFD) method. FORTE Reaction Design software which includes Chemkin Pro-Solver has been chosen in order to take the best solution. In diesel engine models, researcher usually prefer sector mesh for acqusition much time. If the geometry is symmetrical then sector mesh can be used so sector mesh which is usually prefered diesel engine is used. There are two mesh size are researched as sector mesh and thin mesh size is chosen closer to the experimental data than thick mesh size. Time step is chosen 1,28×〖10〗^(-5) second during Start of injection, combustion duration and expansion stroke for at least time decomposition. The combustion analysis has been examined between intake valve closing (IVC) and exhaust valve opening period (EVO). In cylinder pressure values as a result of combustion analysis using FORTE software have compared experimental datas. As a consequences of comparing, it can be seen that there is a four percent defect between CFD analysis and experimental datas. Furthermore, There are two turbulence models in FORTE called RNG k- ε and Standart k- ε turbulence models. RNG k- ε and Standart k- ε turbulence models are generally preferred if the reynolds number is high. However, RNG k- ε turbulence model has more advantageous than Standart k- ε turbulence model with regard to addition less small scale in the calculation. RNG k- ε turbulence and Standart k- ε turbulence models are used in order to compare combustion results and it can be seen that the result of CFD analysis with RNG k- ε turbulence model is closer than Standart k- ε turbulence model to the experimental datas because of more calibration on RNG k- ε turbulence model. Moreover, G-Equation model is prefered as Flame model. In addition, there are a few spray models in FORTE. Spray model is chosen as Radius of Influence Collision Model (ROI) from Droplet Collision and Coalescence Model because of high Weber number and there is seen to appear catastrophic break up of fuel particles. Radius of Influence Collision Model is generally used for combustion analysis. Chemkin is used as a chemical model in order to see combustion reactions. This is tremendously important using Chemkin as a chemical model because there are lots of chemical reactions during combustion processes and Chemkin is the best software to observe these reactions. There are made use of 173 species to take better predictions and FORTE provide opportunity in order to use approximately 428 species. However, it is necessary additional license to use 428 species. In FORTE, there is only one soot model called two-step soot model. mean soot diameter is accepted as 25 nm in two step model. When NOx and soot emissions may provide barrier for the application of diesel engines, the difficulty for dealing with the problem comes also from the trade-off feature between NOx and soot emissions. It is always very difficult to reduce both kinds of emissions simultaneously since factors that tend to decrease one usually increase the another. In this case, The effects of Exhaust Gas Recirculation system are researched. Exhaust Gas Recirculation (EGR) which is a kind of Exhaust After-Treatment System has been researched effects on NOx and Soot emissions and proper EGR rate has determined as making optimizations. The specific heat of Exhaust gases more than the specific heat of the gases in the inlet manifold. According to increasing of the total specific heat in cylinder gases, maximum temperature which is as a result of combustion diminish. If the rate of EGR increases, then the amount of NOx which is released after combustion decreases because of decreasing the rate of maximum temperature in cylinder. However, the percentage of soot rises crucially due to falling the percentage of burnt soot particles. Therefore, it is momentous to chose the correct EGR rate in order to provide NOx and soot balance. Moreover, if the correct EGR rate can not chosen then the rate of power decrease crucially because of falling maximum temperature values. In addition, NOx and soot emission rate changes have been tried to determine depending on equivalence ratio, maximum temperature values and EGR rate in Phi-T contours. Ultimately, all combustion chamber is modelled. It is also showed fuel particles and temperature plots during injection and combustion. Furthermore, we can see that swirl ratio completely affects combustion.