Parametric Study on Design of a Heat Exchanger for an Exhaust Gas Recirculation System

Transcription

1 Parametric Study on Design of a Heat Exchanger for an Exhaust Gas Recirculation System P. Sai Chaitanya, K. Vijaya Kumar Asst.Professor, Department of Mechanical Engineering, B.I.E.T, Hyderabad, India Abstract Direct injection (DI) diesel engine is well designed today as a main power train solution for transportation due to its several advantages. However, at the same time emission legislation, mainly for oxides of nitrogen s (NOx) and particulate matter (PM) becomes more obvious, reducing their limit to extremely low usage as transport vehicles. One efficient method to control NOx in order to achieve the future emission limit is rather exhaust gas recirculation (EGR). Heat exchanger is used in EGR system to cool the exhaust gas before it is being reintroduced into the engine. For effective working of EGR an optimum design of heat exchanger is required. So, a parametric study was adopted to design a heat exchanger for EGR system. The design opted should be economical and simple in construction. Index Terms Direct injection, EGR, Legislation. I. INTRODUCTION The goal of Exhaust Gas Recirculation (EGR) is to reduce the amount of NOx produced. The EGR valve recirculates gases into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are recirculated. These gases displace some of the normal intake charge. This chemically slows and cools the combustion process by several hundred degrees thus reducing NOx formation. Exhaust consists of CO2, N2 and water vapors mainly. When a part of this exhaust gas is re-circulated to the cylinder, it acts as diluents to the combusting mixture. This also reduces the oxygen concentration in the combustion chamber. The specific heat of exhaust gas is much higher than that of fresh air. Hence EGR increases the specific heat of the intake charge, thus decreasing the temperature rise for the same heat release in the combustion chamber [1-2]. Ming Zheng et.al. briefly reviewed the paths and limits to reduce NOx emissions from diesel engines and highlighted the inevitable uses of EGR. The impact of EGR on diesel operations was analyzed and a variety of ways to implement EGR were outlined. Also new concepts regarding EGR stream treatment and EGR hydrogen reforming were proposed [3].Experimental investigations by N. Ladommatos, et.al. analyzed and quantified the principle constituents of EGR, viz. carbon dioxide and water vapour. The effect of increased inlet temperature and thermal throttling of inlet charge, both arising from the use of the hot EGR, were also investigated. Finally tests were carried out during which the CO2 added to the engine air flow increased the charge mass flow rate to the engine, rather than displaying some of the oxygen in the inlet air. It was found that when CO2 and H2O displayed in the inlet charge, both the chemical and thermal effects on the exhaust emission were small. However, the dilution effect was substantial, and resulted in very large in exhaust NOx at the expense of the higher particulates and un-burnt hydrocarbon emission. Higher inlet charge temperature increased exhaust NOx and particulate emissions, but reduced un-burnt hydrocarbon emission. Reduction in inlet charge due to thermal throttling reduces NOx emission but raises all other particulates. Finally when CO2 was additional to the inlet air charge (rather than displaying O2), large reduction in NOx were recorded with little increase in particulate emission [4]. Mohamed Y.E. Selim studied the effects of EGR ratio, engine speeds, loads, temperature of recycled exhaust

2 gases, intake charge pressure and engine compression ratio on combustion noise and thermal efficiency and observed that Exhaust gas recirculation at an EGR ratio of 5 % has a positive effect on increasing the thermal efficiency. The use of a low EGR ratio of 5% is also favorable for reduced combustion noise and reduced NOx emission. However, increasing the EGR reduces the thermal efficiency. The hot EGR increases the pressure rise rate at all loads and at all EGR ratios used as compared with cooled EGR [5]. Cold EGR, i.e. cooling the exhaust gases before recalculating them to the cylinder, was found to be more effective than hot EGR in NOx reductions. This helps in attaining a lower intake air temperature, as compared to hot EGR. Cold EGR results in poor combustion in the engine cylinder and hence reduced peak temperature. Due to low temperature and less oxygen available in the engine cylinder during combustion, the NOx emissions will reduce. Nidal H. Abu-Hamdeh carried out a study on spiral fin exhaust pipes, to determine the effect of cold EGR on the chemical composition of the exhaust gases and the reduction in the percentages of pollutant emissions in diesel engines. The gases examined in this study were oxides of nitrogen (NOx), carbon dioxide (CO2) and carbon monoxide (CO). In addition, O2 concentration in the exhaust was measured. The two designs adopted in this study were exhaust pipes with solid and hollow fins around them. The first type uses air flow around the fins to cool the exhaust gases. The second type consists of hollow fins around the exhaust pipe to allow cooling water to flow in the hollow passage. Different combinations and arrangements of the solid and hollow fins exhaust pipes were used. It was found that decreasing the temperature of the EGR resulted in reductions in the oxides of nitrogen (NOx) and carbon dioxide (CO2) but increased the carbon monoxide (CO) in the exhaust gases. In addition, the oxygen (O2) concentration in the exhaust was decreased. As a general trend, the percentages of reduction in the NOx gas concentrations were lower than the percentages of increase in the CO emissions as a result of cooling the EGR of a diesel engine by a heat exchanger. Using water as a cooling medium decreased the exhaust gas temperature and pollutants more, than did air as a cooling medium. In a separate series of tests it was observed that, increasing the cold EGR ratios decreased the exhaust NOx significally [6]. So a heat exchanger plays a crucial role in EGR system. So high precision design of a heat exchanger or a EGR cooler is advantageous. 2. EXHAUST GAS RECIRCULATION SYSTEM EGR system introduces a portion of the engine exhaust gas back into the cylinder, mixing the exhaust gas with fresh intake air. This process leads to reduction in concentration and thus modified combustion and reduced combustion temperature, which results in reduced [NOx].At 2500 degrees Fahrenheit or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons (HCs) and the presence of sunlight, produce an ugly haze in our skies known commonly as smog. There are several types of EGR systems being used and explored and while most effectively reduce emissions, it is often done at the cost of reduced engine efficiency and higher fuel consumption. Recently, Hino Motors of Japan has announced the development of an EGR system that is intended to reduce engine emissions without a significant loss of engine efficiency.

3 Conventional EGR systems take a part of the exhaust gas out of the exhaust manifold then lead it back into the intake manifolds which are produced by combustion at high temperature and at high engine loads. Recirculation of a proportion of the exhaust gases at high loads lowers the combustion temperature and, as a result, reduces the NOx level which mixed then introduced into the cylinder. Parameters such as the mixing ratio of the EGR gas to fresh air are controlled by a microprocessor that adjusts an EGR valve and/or throttle valve. In order to meet future emission standards, EGR must be implemented practically over wider range of engine operation, and heavier EGR rate is to be impinged in a high engine load range since the amount of NOx is larger than the other engine operation conditions. Generally EGR system in diesel engines can be implemented in two ways. One is hot EGR and the other is cold EGR. In hot EGR system the hot exhaust from the engine is cooled in the still air and then it is recirculated back to the inlet of the engine. In contrast cold EGR implements a heat exchanger to cool the hot exhaust from the engine and then it is recirculated back into the inlet. Each of the EGR system has its own advantages. But cold EGR has a more benefits than the hot EGR as in cold EGR the exhaust gas can be cooled quickly and can be cooled to required level. The block diagram of a cold EGR system with a EGR cooler is as shown below, 3. PARAMETRIC STUDY Figure 1: Cold EGR system Parametric study in a design means assessing the impact of changing different parameters on the design. The parameters can include dimensional parameters. Parametric studies allow a designer to nominate parameters for evaluation, define the parameter range, specify the design constraints, and analyze the results of each parameter variation there by resulting in optimum design. Here 18% of exhaust gas is considered to be recirculated back into the engine. For parametric design of the heat exchanger in EGR system the dimensional parameters are varied. Tubular heat exchanger was selected due its simple in design. Tubular heat exchanger consists of two tubes. In one tube exhaust gas is passed and in other tube water is passed. Due to the higher temperature gradient between the two fluids the heats from the exhaust gases are transferred to the water. Theoretical calculations are employed to find the optimum length of the heat exchanger.

4 Engine specifications considered for the calculations are as follows, Table 1:Specifications of the Engine Table 2: Parameters of the Engine Theoretical calculations are performed using the above parameters and specifications. The parameters are varied to get the optimum design of the heat exchanger for EGR system. There are number of parameters absorbed for optimum design of the heat exchanger. The considered parameters for rating a heat exchanger includes: mass flow rate of gas, heat load calculations. Reynolds number calculations include properties of the fluid, dimensions of the pipe, area of the pipe and LMTD. Overall heat transfer coefficient includes convective heat transfer coefficient, fouling factor. Surface area and pressure drop calculations are also included for high precision in design of a heat exchanger. CONCLUSION: The results that are attained from varying parameters is as shown below table,

5 Table 3: Parametric Results The above results represent the parametric study results of the design of heat exchanger for EGR system. From the above results it can be concluded that iteration 4 is the optimum parameters for the design of the heat exchanger. This is because at a smaller length there is more heat transfer in case of iteration 4. Iteration 5 cannot be selected as a optimum design keeping in mind about the economics employed in every design of a component. These results are purely based on the theoretical calculations. Further simulations can be proposed on the analysis softwares for higher precision results. REFERENCES [1] G. Eason, B. Noble, and I. N. Sneddon, On certain integrals of Lipschitz-Hankel type involving products of Bessel functions, Phil. Trans. Roy. Soc. London, vol. A247, pp , April [2] J. Clerk Maxwell, A Treatise on Electricity and Magnetism, 3 rd ed., vol. 2. Oxford: Clarendon, 1892, pp [3] I. S. Jacobs and C. P. Bean, Fine particles, thin films and exchange anisotropy, in Magnetism, vol. III, G. T. Rado and H. Suhl, Eds. New York: Academic, 1963, pp [4] K. Elissa, Title of paper if known, unpublished. [5] R. Nicole, Title of paper with only first word capitalized, J. Name Stand. Abbrev., in press. [6] Y. Yorozu, M. Hirano, K. Oka, and Y. Tagawa, Electron spectroscopy studies on magneto-optical media and plastic substrate interface, IEEE Transl. J. Magn. Japan, vol. 2, pp , August 1987 [Digests 9 th Annual Conf. Magnetics Japan, p. 301, 1982]. [7] M. Young, The Technical Writer's Handbook. Mill Valley, CA: University Science, 1989.