CFD-13-Backpressure Study in Exhaust Muffler of Single Cylinder Diesel Engine using ... · · 2015-07-28Engine using CFD Analysis Puneetha C G ... Exhaust gas emitted from vehicles

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Backpressure Study in Exhaust Muffler of Single Cylinder Diesel

Engine using CFD Analysis

Puneetha C G Project trainee

Altair Engineering India Pvt. Ltd.

Bangalore 560 103 [email protected]

Manjunath H Assistant professor

Siddaganga institute of technology

B H road Tumkur [email protected]

Shashidhar M.R Manager –CAE

Faurecia emission control technology Bangalore Tech center

Bangalore. [email protected]

Abbreviations: CFD-computational fluid dynamics. Keywords: muffler, backpressure,

Abstract

The essential function of a muffler is to route the exhaust gases from the engine exhaust manifold while reducing the noise and back-pressure. Noise reduction is an emerging concern in the automotive industry, and reduction in back-pressure enhances the fuel economy of the engine. One important characteristic of mufflers is how much backpressure they produce. Because of all of the turns and holes the exhaust has to go through, mufflers like those in the previous section produce a fairly high backpressure. This subtracts a little from the power of the engine In this Study comprehensively analyzes four different models of exhaust muffler and concludes the best possible design for least pressure drop. Back pressure was obtained based on the flow field analysis and was also compared with all muffler design. Virtual simulation for back-pressure testing is performed by Computational Fluid Dynamic (CFD) analysis using Acusolve CFD. Finite Element (FE) model generation of the muffler structure is performed using Hyper Mesh as the preprocessor. The structural mesh is modeled using 2D shell elements, wherein the internal tubes with fine perforated holes are considered. The CFD fluid meshing is done with tetra elements using AcuConsole as the CFD preprocessor. The back pressure generated across the muffler is determined by measuring and inputting the mass flow rate of the exhaust gases entering the muffler inlet pipe. Field View is used for post processing the CFD results and reviewing the airflow streamlines. The exhaust gas velocity and pressure plots are studied across the internal tubes and perforated holes, and the back pressure is measured.

INTRODUCTION

Using diesel engines mostly as main power element has increased the importance of the technical specification of the diesel engine itself and its other during-and-after design belongings. The muffler is defined as a device for reducing the amount of noise emitted by a Engine. The muffler is engineered as an acoustic soundproofing device designed to reduce the loudness of the sound pressure created by the engine by way of Acoustic quieting. Due to increased environmental concerns requiring less noise emissions combined with reduced emission of harmful gases, it is becoming very crucial to carefully design the exhaust system mufflers for road transport applications. Exhaust gas emitted from vehicles contains many components that contribute to air pollution, namely carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx).


Figure 1: the Exhaust muffler model

When the exhaust gases from inlet pipe pass through the perforations inside the shell, the gases get scattered in different directions. After reflection from the inside surface of the shell, the sound cancellation of waves occurs. The gases pass through the perforations multiple times and even get reflected from the shell surface. Due to the combined effect of these, the level of sound at the muffler outlet is reduced significantly. The flow through the muffler and variation of various parameters such as velocity and pressure along the length of the model can be accurately demonstrated with the help of CFD analysis which display accurate results within a short span of time. Perforated pipe

Perforated tubes in the muffler as used to escape are deliver the hot exhaust gases through the perforation holes in the tube. The hole diameter and number of perforations are calculated as per the muffler volume and requirement. The perforation need have to proper spacing between the each hole for better performance. Here perforations are closed with the creation of surfaces and it should be in separate collector. After geometry simplification, perforation surfaces are to be meshed with shell elements with minimum element length to have more number of nodes on the surface to achieve proper flow of gases and minimize noise level.

Baffle plate

Baffle plates have many design uses in general in different applications. However, If baffles are created to the pressurized conditions required within our systems design for proper distribution, and flow rate throughout our system. Many baffles are adjustable and some are fixed. Air noise is something that baffles are used to help reduces the backpressure during the hot gas passing through the muffler. Perforated baffles are used to expand the exhaust gases from one room to another room through the use of their perforations. Perforated and non-perforated baffle plates are merged with muffler shell with the use of trim operation. Unavoidable side effect of the muffler is pressure drop or back pressure,

Back pressure Backpressure usually refers to the pressure exerted on a moving fluid by obstructions against its direction

of flow. The word back may suggest a pressure that is exerted on a fluid against its direction of flow indeed, but there are two reasons to object. First, pressure is a scalar quantity, not a vector quantity, and has no direction. Second, the flow of gas is driven by pressure gradient with the only possible direction of flow being that from a

Shell

Inlet

pipe

Perforated pipe

Outlet

pipe


higher to a lower pressure. Gas cannot flow against increasing pressure .It is the engine that pumps the gas by compressing it to a sufficiently high pressure to overcome the flow obstructions in the exhaust system.

Effects of Increased Back Pressure

• At increased back pressure levels, the engine has to compress the exhaust gases to a higher pressure which involves additional mechanical work and/or less energy extracted by the exhaust turbine which can affect intake manifold boost pressure. This can lead to an increase in fuel consumption, PM and CO emissions and exhaust temperature. The increased exhaust temperature can result in overheating of exhaust valves and the turbine. An increase in NOx emissions is also possible due to the increase in engine load.

• Increased backpressure may affect the performance of the turbocharger, causing changes in the air-to-fuel ratio-usually enrichment—which may be a source of emissions and engine performance problems. The magnitude of the effect depends on the type of the charge air systems. Increased exhaust pressure may also prevent some exhaust gases from leaving the cylinder (especially in naturally aspirated engines), creating an internal exhaust gas recirculation (EGR) responsible for some NOx reduction. Slight NOx reductions reported with some DPFsystem, usually limited to 2-3% percent, are possibly explained by this effect.

• Excessive exhaust pressures can increase the likelihood of failure of turbocharger seals, resulting in oil leak agent the exhaust system. In systems with catalytic DPFs or other catalysts, such oil leak can also result in the catalyst deactivation by phosphorus and/or other catalyst poisons present in the oil.

• All engines have a maximum allowable engine back pressure specified by the engine manufacturer. Operating the engine at excessive backpressure might invalidate the engine warranty.

• It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that's Mercury – inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design etc

Table 1: VERT Maximum Recommended Exhaust Back Pressure

From back pressure point of view most important parameters are exhaust velocity.

Exhaust Velocity

Exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4-cylinder motor will have 4 distinct pulses per complete engine cycle a 6 cylinder has 6 pulses and so on. More the pulses produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

It is a general misconception that wider exhaust gives helps in better scavenging. But actually main factor behind good scavenging is exhaust velocity. The astute exhaust designer knows that flow capacity must be balanced with velocity. The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principle of exhaust pulse scavenging is that a fast moving pulse creates a low-pressure area behind it. This low-pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area

Engine Size Back pressure Limit

Less than 50kW 40kPa

50-500kW 20kPa

500kW and Above 10kPa


immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. Process Methodology Present work has been carried out in following steps:

• CAD Modeling. • Meshing the CAD model. • CFD analyses.

The details of the above steps are as follows:

CAD MODELING: Base model under study has been modeled using CATIA and some modifications required are done in Hyper Mesh. Four different types of muffler have been designed, while designing the mufflers are explained in detail along with their pictures.

Figure 2 the base model.

Figure 2 shows the design 1 of the study muffler. “Inlet” is the region from which the exhaust gas will enter, muffler pipe allows the exhaust gas to split and pass to another chamber, and outlet carries the gas from muffler chamber to atmosphere.

MESHING:

Meshing of the base model is done using Hyper Mesh. Following are the steps carried out on all the four mufflers cases to get the final meshed models:

• Importing and Repairing CAD: In this, the base model modeled using CATIA Will be imported to Hyper Mesh. The model file will be imported in .stp or .iges Format. After importing into Hyper Mesh, check for the free edges, duplicate Surfaces will be carried out. Surface will then be trimmed with proper tolerance so that mesh can be easily put on the surface.

• Meshing the model: The cleaned up model will be meshed with specific Element size. The meshing will be carried out with criteria as shown in below table:

Table: 2: Meshing criteria

Element size 2 mm

Element type Trias

Minimum angle > 20 �

Maximum angle < 120 �


Minimum and maximum angle refers to the minimum and maximum angle in the Triangular element.

For CFD analysis, preferred mesh type is “Trias”.

Figure 3 Surface mesh

CFD MESHING

Pre-processing methodology using CFD tool Pre-processing for CFD analysis involves the creation of surface mesh, volume mesh and setup of boundary conditions for backpressure study. Meshing is a key step to creating accurate model, correct mesh continuity and mesh density are needed to efficiently compute results and capture the boundary layer effects. The quality of CFD solution is depending on the quality of the underlying volume mesh.

Figure 4 solid meshing

Results & Discussions

Muffler design 1 and muffler design 2

• Muffler design-1 • Muffler design-2


Figure 5 the inside components of muffler design 1 and muffler design 2

Figure 5 shows the inside components of design 1 muffler. In this design muffler pipe doesn’t has perforation and baffles have three holes for the purpose of circulating or reflecting the gas. The figure also shows the inside components of muffler design 2. In this design muffler pipe and baffles have perforations. Perforations have ability to reflect the flow and split the flow. Diameter of perforation is 5 mm as shown above.

In this study, inlet condition is defined as a “velocity boundary condition” where velocity of the inlet gas and its density will be specified and outlet is defined as a “Pressure boundary condition” where outlet pressure boundary conditions are specified.

INLET BOUNDARY CONDITON

Table 3: Inlet boundary conditions

Inlet gas velocity 10.24 m/s

Gas temperature 633 K

Mass flow rate of gas 18.5 kg/hr

OUTLET BOUNDARY CONDITION

Table 4: Outlet boundary conditions

Outlet pressure Atmospheric pressure

Inlet and outlet condition are kept same for all muffler designs analyzed.

Middle

Baffles Pipe

With

Perfor

ation

Dia of

Perfor

ation

is

5mm


ENGINE SPECIFICATION

Following engine parameters were considered for calculation of mass flow rate at 15 kg loading condition.

Table 5: Engine specifications for calculating the mass flow rate.

Parameter Value

engine Single cylinder diesel engine

Calorific value of the fuel 45500kj/kg

Density of the fuel 850kg/m3

Bore & stroke 80mm,& 110mm

Swept volume 553mm or 0.55 liter

Compression ratio 16:1

ANALYSES OF MUFFLER DESIGN-1AND MUFFLER DESIGN- 2

Muffler design1 Muffler design2

• Volume mesh

Number of elements 17 million

• Volume mesh

Number of elements is 19 million

• Velocity distribution • Velocity distribution


• Flow representation by streamlines


• Absolute total pressure distribution

• Absolute total pressure distribution

Figure 6 comparison between muffler design-1 and muffler design-2.

A

B

E F

C

A

B

C

D EF


Muffler design 3 and muffler design 4

• Muffler Design-3

• Muffler Design-4

Figure 7 shows the inside components of muffler design 3 and muffler design 4

Figure 7 shows the inside components of muffler design 3. In this design muffler pipe and baffles have perforations. The chamber is split into three separate zones. The end chamber zones have perforation pipe and the middle chamber is empty. Perforations have the ability to reflect the flow and split the flow. Diameter of perforation is 5 mm as shown above.

The same figure 7 also shows the inside components of design 4. This design has an oval shaped chamber. In this design muffler pipe are having perforations and baffles have perforation. The chamber is split into three separate zones. The end chamber zones have perforation pipe and the middle chamber is empty. Perforations have the ability to reflect the flow and split the flow.

ANALYSES OF MUFFLER DESIGN-3AND MUFFLER DESIGN- 4

Muffler design3 Muffler design4

Middle

Baffles Pipe

With

Perfor

ation

Dia of

Perfor

ation

is

5mm

Middle

Baffles Pipe

With

Perfora

tion

Dia of

Perforat

ion is

5mm


• Volume mesh

Number of elements is 17.7 million

• Volume mesh

Number of elements is 18 million

• Velocity distribution

• Velocity distribution




• Absolute Total pressure distribution

• Absolute Total pressure distribution

Figure 8 comparison between muffler design-3 and muffler design-4.

Table - 6: Absolute Total pressure drop (kPa) at different Section of muffler designs

o From figure 6, it can be seen (design-1) that the gas flows through the pipe and baffle holes and passes from one chamber to next and finally through the outlet pipe to the atmosphere. There is

Section Desi

gn1

Desi

gn2

Desi

gn3

Desi

gn4

Section

(A-B) 0.96

0.95

0.9 0.9

Section

(B-C) 0.57

1.2

0.65 0.65

Section

(C-D) 1.08 0.5 1.09 1.11

Section

(D-E) 1.08 1.25 1.25 1.54

Section

(E-F) 0.20 0.74 0.75 0.42

Section

(A-F) 4.55 4.64 4.58 4.62

A

B

C

E F

A

B

C

D E F


flow circulation happening inside the muffler shell and here velocity will decrease.

o From figure 6, it can be seen that the gas flow through the pipe with perforation and baffles with perforation passes from one chamber to next. The flow then hits the wall and circulates in this chamber before finally flowing through the outlet pipe perforation to the atmosphere,

o There the flow is circulated inside the muffler shell and here the flow velocity will decrease, because of the flow having more time to circulate inside the muffler. As a result back pressure will occur.

o The total pressure distribution as shown in figure 6. Total pressure drop across muffler and the details of this are tabulated in table 6.

• Designs have been carried out by considering two parameters: � introduced baffles with perforation to flow from one chamber to next. � introduced pipe with perforation for reflect the gas

• Design 4 has been carried out by considering two parameters: � introduced baffles with perforation to flow from one chamber to next. � vary the shape from round to oval and introduced pipe with perforation for reflect the gas.

Total pressure drop comparison for muffler designs

Figure 9 graphical representation of total pressure drop

o Figure 9shows the graphical representation of total pressure drop in different muffler designs.

o The graph shows muffler design 1 has minimum pressure drop.

4.5

4.52

4.54

4.56

4.58

4.6

4.62

4.64

4.66

Design1 Design2 Design3 Design4

Pre

ssu

re d

rop

Muffler Designs

Absolute total pressure


Future Plans

The study presented here has its own advantages and disadvantages. In designs of the mufflers, focus has been put on the parameters like perforation with pipe and baffles; number of baffles and analyses has been carried out with fixed exhaust pipe. Though there is no change expected in the analyses if exhaust pipe which comes from engine is varied, this can be verified in future study. Also in perforation diameter which is maintained as 5 mm throughout the analyses. The effects of varying this diameter can also be studied; a higher diameter is expected to give better results.

In this muffler study, we have only concentrate on pressure drop so we should concentrate on muffler noise level analysis in future study.

Conclusions

In present work, study of four different muffler designs has been carried out to choose the design which can satisfy the flow conditions in an exhaust system. After carrying out analyses and studying the manufacturing feasibility of muffler designs, following conclusion are drawn:

Table 7.total pressure drop in different mufflers design.

• Table 7 shows the total pressure drop occurring in different muffler designs. • From this study it is concluded muffler design 1 gives very low pressure drop. • If results of all muffler design are compared, muffler design 1 is giving good results with low pressure

drop, and also it’s easy to manufacture.

ACKNOWLEDGEMENTS I would like to express my deep and sincere gratitude to my External Guide Mr. Shashikumar M.R, CAE-

Manager, Faurecia Emission Control Technology , Bangalore, & I am deeply grateful to Mr.Abinash Baruah,

Technical Manager-CFD,& Kamleshwar Rajender, CFD Specialist, Altair Engineering India .Pvt, Ltd, Bangalore,

for his valuable advice and friendly help.

Muffler designs Total pressure drop(kPa)

Design1 4.55

Design2 4.85

Design3 4.58

Design4 4.64


REFERENCES

1. Sudarshan Dilip Pangavhane , Amol Bhimrao Ubale , Vikram A Tandon , Dilip R Pangavhane, “Experimental and CFD Analysis

of a Perforated Inner Pipe Muffler for the Prediction of Backpressure “.International Journal of Engineering and Technology

(IJET)13-05-05-163.

2. Prof. Amar Pandhare, Ayush Lal, Pratik Vanarse, Nikhil Jadhav, Kaushik Yemul,“CFD Analysis of Flow through Muffler to Select

Optimum Muffler Model for CI Engine” International Journal of Latest Trends in Engineering and Technology (IJLTET)Vol. 4.

3. Shital Shah, Saisankaranarayana K, Kalyankumar S. Hatti “A Practical Approach towards Muffler Design, Development and

Prototype Validation”

SAE International 2010-032-0021

4. Zeynep Parlar, Şengül Ari, Rıfat Yilmaz, Erdem Özdemir, and Arda Kahraman. “Acoustic and Flow Field Analysis of a Perforated

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7. Ying Li Shao. A Study on Exhaust Muffler Using a Mixture of Counter phase Counteract and Split-gas Rushing. © 2011

Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [CEIS 2011]

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9. Atul A. Patil1,L.G. Navale2,V.S. Patil3 “Simulative Analysis of Single Cylinder Four Stroke C.I. Engine Exhaust System”

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Documents

CFD-13-Backpressure Study in Exhaust Muffler of Single Cylinder Diesel Engine using ... · · 2015-07-28Engine using CFD Analysis Puneetha C G ... Exhaust gas emitted from vehicles