Procedia Engineering 127 ( 2015 ) 1211 – 1218

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1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICCHMT – 2015doi: 10.1016/j.proeng.2015.11.466

ScienceDirect

International Conference on Computational Heat and Mass Transfer-2015

CFD Study on Pressure Drop and Uniformity Index of Three Cylinder LCV Exhaust System

Om Ariara Guhan C Pa, Arthanareeswaren G a,*, Varadarajan K N b

aDepartment of Chemical Engineering, National Institute of Technology, Tiruchirappalli -620015, India.bTech Mahindra Ltd., Electronic city, Bangalore-560100 ,India

Abstract

The current scenario of high growth rate of automobile usage, the automobile industry is forced to adopt the government emission norms to keep the environment green. Latest technologies have been developed in the automotive exhaust system to acknowledge the emission norms. Diesel oxidation catalyst and Muffler both are playing major roles in reducing emission and noise level as well. Diesel oxidation catalyst reduces CO and unburned HC emissions. Muffler reduces noise level of exhaust gases. Nowadays automobile industry is using CFD software extensively to analyze the flow properties inside the diesel oxidation catalyst and Muffler. Flow analysis helps to optimize the geometric design of Diesel oxidation catalyst to oxidize the CO and unburned HC of exhaust gases. In this present work we studied pressure drop and uniformity index of an existing exhaust system which consists of close couple catalytic convertor, under body catalytic convertor and muffler. Exhaust system has been modelled by using CATIA V5 which is advanced CAD software. The substrate has been modelled as porous medium for analysis purpose. These models have been imported in CFD tool for analysis. After importing the CAD data inside the CFD software, with proper boundary conditions, the CFD analysis has been carried out. Based on the study, individual system contributions to the total pressure drop and flow uniformity have been analyzed and improvement areas of the existing system for better flow uniformity have been suggested.© 2015 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of ICCHMT – 2015.

Keywords: Exhaust system; Diesel oxidation catalyst; Catalytic convertor; Computational fluid dynamics; Pressure drop; CFX; CATIA V5

* Corresponding author. Tel.:+91431-2503118; Fax: +91431-2503103E-mail address: arthanaree10@yahoo.com (G.Arthanareeswaran).

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICCHMT – 2015

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1. Introduction

In an automobile exhaust system catalytic convertor and muffler are important components to reduce emission. Diesel oxidation catalyst has been proved that it is playing an important role in converting harmful gas of automotive emission [1]. Catalytic converter contains honey comb structure substrate, the surface is coated with wash coat. Precious metal loading, Wash coat thickness, substrate material (ceramic or metal), geometric configuration and position of catalytic convertor are playing major roles in designing of diesel oxidation catalyst. Flow uniformity, pressure drop and light-off performance are the most importance factors that are to be considered [2, 3]. Julia Windmann et al. [4] analysed how flow uniformity affects the engine light off performance. Flow uniformity inside the catalytic convertor is dependent on exhaust manifold [5], inlet, out let cone design, substrate dimensions and configuration. The design of catalytic converters is still dependent more on empirical practice than a science. But the situation may need to be shifted towards science as the emission standards are becoming more stringent. Optimising the catalytic convertor by experimenting is time consuming and expensive. For each and every experimental set up, different type of geometry has to be designed and the prototype has to be developed. Nowadays computer simulation software packages are used in automotive industry to optimise which had significantly helped in cost reduction with increase in performance and saving time ultimately. S.F. Benjamin [6] predicted and reported uniform conversion efficiency across the monolith of an automotive catalytic converter by using computational fluid dynamics (CFD) technique. Tsinoglou D.N. et al. [7] analyzed the flow field inside the catalytic converter and radial velocity profiles by using commercial CFD code. Wu Guojiang and Tan Song [8] analyzed the temperature and concentration in the monolith during the cold-start period by using numerical simulation and they reported the effects of the flow distributions in the monolith on the performance of a catalytic convertor. In IC engine working cycle, there is power loss due to gas exchange as a result of pumping gas from inlet to higher exchange pressure , Which affects engine volumetric efficiency and performance directly proportional to the back pressure [5]. Soo-Jin Jeong and Woo-Seung Kim [9] studied and reported that the optimal design of an auto-catalyst need a good compromise between the pressure drop and flow uniformity in the monolith. Lun et al. [10] used AVL CFD software and performed CFD analysis. Kumar A and Mazumder S [11] conducted simulation of full-scale monolithic catalytic converters with complex heterogeneous chemistry. Hayes R.E et al. [12] conducted a computational study of a full size catalytic converter. They validated simulations against experimental data obtained from the literature.

The present work involves CFD study of the exhaust system of Light commercial Vehicle on pressure drop and uniformity index. Based on this study we can find out the pressure drop in the catalytic convertor and flow uniformity. We used CATIA V5 software for design purpose and system was computationally analysed the pressure drop and uniformity Index by using ANSYS CFX software.

2. Theory and Formula

In this present work, ANSYS CFX software is used for 3D CFD simulations. A standard k-e turbulence model is used with standard wall function treatment. Substrate is assumed as porous media. Fully developed laminar flow is assumed inside the substrate [13]. Pressure drop across the substrate is specified with reference to the cells per square inch, Wall thickness and coating thickness are as follows [14] :

(1)

(2) Automotive catalytic convertor industry is using flow uniformity index to explain the degree of flow

distribution in the front of substrate which is defined as:

(3)

(4)

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3. Experimental work

3.1. Design

Design of exhaust system has been completed by using CATIA V5 software for CFD study. Exhaust system contains close coupled catalytic convertor, under floor catalytic convertor and Muffler. The catalytic convertor (close coupled and under body) contains inlet cone, monolith and outlet cone. The straight section of the system between the inlet cone and the outlet cone contains the monolith (catalyst) substrate. A typical catalytic converter consists of a catalyst substrate, mat-insulation material, and an outer metallic shell. The monolith substrate consists of a large number of small channels. Mufflers are used to attenuate exhaust noise by using acoustic elements. In Reflective type muffler the acoustic energy is reflected back toward the source and this is working based on destructive Interference / wave cancellation method. This type muffler is good for low frequencies. The 3D view of exhaust system is illustrated in Fig. 1. Parametric modeling methodology is used to design the exhaust system components. Basic sketches have been fully constrained to control the degree of freedom. Various design models have been developed and these models have been associated with assembly. This 3D software helps us to save time for updating models whenever we are changing the design.

Fig. 1. Exhaust system of three cylinder light commercial vehicle.

3.2. Computational modelling and grid generation

CATIA V5 3-D model was imported in ICEM CFD pre-processing tool and meshed. In this present work we used tetrahedral mesh elements to get accurate results and to reduce computation time. The mesh quality had been checked for 0.3, The aspect ratio was kept as 0.3 and above. Meshed models are shown in Fig. 2. Numbers of elements were around 3.6 million

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3.3. Assumptions on the model

In this present work following assumptions are made on the model for the analysis. Analysis executed for steady state condition. Catcon substrate assumed as porous domain. Since the scope is study for design optimisation the chemical reaction has not been considered and heat transfer also has not been considered. Surface roughness has been considered through co efficient of porous media.

Fig. 2. Meshed fluid domains of closed coupled catcon , under body catcon and muffler.

3.4. Boundary conditions of the model

We used k-epsilon as turbulence model, exhaust gas (density=0.5508 kg/m3, Viscosity = 3.814e-5 Pa.s) as working fluid, mass flow rate = 320kg/hr and temperature = 520 deg Celsius as inlet conditions. Boundary conditions are explained for different fluid domains as follows.

Close coupled CATCON: The outlet pressure = 0.045 bar, Volume porosity = 0.65, Inertial coefficient = 20.015 1/m and Viscous coefficient = 3.236E07 1/m²

Under body CATCON: The outlet pressure = 0.069 bar, Volume porosity = 0.65, Inertial coefficient = 20.015 1/m, and Viscous coefficient = 3.236E07 1/m²

Muffler : The outlet pressure = 0 bar, Fluid Medium = Air (Ideal gas) Ensure the text area is not blank except for the last page.

4. Results and discussion

4.1. Pressure Contours at various locations

CFD results had been plotted by CFD post processer. Pressure contours had been analysed in various locations of the exhaust system. Fig. 3 and Fig. 4 and are explaining about pressure contours in inlet.

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Fig. 3. Pressure contours at intake pipe inlet Fig. 4. Pressure contours at under body inlet

Fig. 5 and Fig. 6 are explaining about the pressure contours in porous inlet and outlet of close coupled catcon.

Fig. 5. Pressure contours at close coupled porous Inlet Fig. 6. Pressure Contours at Close coupled porous Outlet

Fig. 7 and Fig. 8 are explaining about the pressure contours in porous inlet and outlet of under body catcon.

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Fig. 7. Pressure contours at under body porous Inlet Fig. 8. Pressure contours at under body porous outlet

The results were plotted and analysed in inlet pipe ,Close coupled catcon inlet , under body catcon and muffler inlets. Pressure contours were studied in porous inlet and out let of close coupled and under body catcon as well.

4.2. Flow Distribution at various locations

Flow distribution analysed with the help of streamline plots. We observed that there was a reverse flow near to cone area, because of formation of eddies and leads to the loss of head due to sudden enlargement and contraction of the exhaust system. Fig. 9 is explaining the streamline plot of close coupled catcon and Fig. 10 is explaining about under body catcon.

Fig. 9. Flow distribution at across close coupled Fig. 10. Flow distribution at across under body

The results shows that the importance to be given to optimaise the cone to avoid the reverse flow.

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4.3. Velocity Plots at various locations

Velocity plots were analysed in close coupled and under body CATCONs. We had taken middle section to analyse the velocity variation. Fig. 11 shows the velocity plot across close coupled catcon. Fig. 12 shows the velocity plot across under body catcon.

Fig. 11. Velocity plot across close coupled Fig. 12. Velocity plot across under body

4.4. Design Pressure drop

Pressure was calculated in inlet and outlet of close coupled and under body catcon. Based on pressure information the pressure drop was calculated. The pressure drop is the key functional area which gives the optimisation of engine performance. The pressure had been calculated and analysed to find out which system is contributing more pressure drop in the exhaust line. The results are tabulated in Table. 1. The result shows that close coupled catcon is contributing more pressure drop in exhaust line.

Table. 1. Total pressure and Pressure drop

4.5. Uniformity Index

Uniformity index had been calculated in close coupled catcon and under body catcon. Uniformity index gives the information on flow uniformity. The results are tabulated in Table. 2. The results show that flow is more uniform in

S.No Part name Total Pressure Inlet (mbar)

Total Pressure Outlet (mbar)

Pressure drop(mbar)

1 Muffler 94 30 64

2 Underbody catcon 135 89 46

3 Close coupled catcon 212 141 71

4 Exhaust system 212 30 182

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under body catcon compared with close coupled catcon. These results indicate clearly that we need to optimise the close coupled catcon geometry to get more flow uniformity.

Table. 2. Uniformity Index

S.No Location Uniformity Index (g)

1 Close couple catcon 0.822

2 Underbody catcon 0.912

5. Conclusion

We studied in detail about pressure contours , flow distribution , velocity distribution ,pressure drop and uniformity index of the exhaust system of 3 cylinder light commercial vehicle with the help of ANSYS CFX which is commercial CFD software. From the analysis, We are concluding following information with suggestions for better flow uniformity

• The total pressure drop obtained in the entire exhaust system is 182 mbar. • The total pressure drop obtained in the Close coupled catcon is 71 mbar which contributes

more pressure drop in the exhaust system • There is a reverse flow observed in close coupled catcon near to inlet cone enlarging area,

The inlet cone should be optimized to avoid this eddy formation. • Flow is more uniform in under body catcon .The Uniformity index is 0.912 in under body

catcon. • Close coupled catcon geometry to be optimized to obtain more uniformity index.

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