**Abstract**

In order to combat climate change, the new rigorous standards for pollutant reduction have shone a light on the use of exhaust gas recirculation system in order to minimize the NOx emissions of vehicles. For this reason, the fouling problem that appears on the exhaust gas recirculation line, caused by the deposition of soot particles and hydrocarbons that are part of the exhaust gas, has become particularly relevant in the last few years. In this field, researches have proposed numerical models in order to estimate and predict the deposit formation and growth. Using various numerical techniques, they intend to determine and reproduce the fouling layer buildup considering the different mechanisms that are involved in the deposit formation. This chapter provides a detailed and comprehensive account of the numerical approaches that have been proposed to analyze the fouling phenomenon that occurs inside the exhaust gas system. The main characteristics of each numerical model, as well as their main strengths and weaknesses, are exposed and evaluated, and their simulation capabilities are examined in detail.

**Keywords:** EGR, fouling, soot agglomerates, thermophoresis, hydrocarbon condensation, erosion, CFD, numerical simulation

#### **1. Introduction**

The Sustainable Development Goals (SDG), known as the 2030 Agenda for Sustainable Development, have been adopted by 193 countries since 2015 [1]. Reducing air pollution, development of sustainable cities, and combating climate change are some of the main goals of this plan of action, and, within that context, the reduction of pollutant emissions from vehicles is an important activity to be faced.

In order to minimize greenhouse gas emissions, vehicle emissions for passenger cars have been regulated worldwide by means of several standards, such as the Euro emission standards in Europe or the Tier standards in the USA [2–4]. These successive standards, which define more stringent acceptable limits for polluting emission and fuel economy, push car manufacturers to use the best technology available for vehicle emission control, and this is one of the biggest technical challenges that the automotive industry faces.

The public concern about diseases derived from air pollution and recent emissions scandals, like *dieselgate*, have shone a light on vehicle emissions, particularly in terms of nitrogen oxides (NOx) and particulate matter emissions [5, 6].

In this context, since 2014, the EURO 6 emission standard set the emissions limit for nitrogen oxides (NOx) in 60 and 80 mg/km for gasoline and diesel light-duty vehicles, respectively [7]. This fact has extended the use of techniques like the exhaust gas recirculation (EGR) system, which have proven to be an effective way of reducing NOx formation. Nowadays, the EGR system is used together with other systems, such as diesel oxidation catalyst (DOC), lean NOx trap (LNT), or selective catalytic reduction (SCR), to fulfill the NOx emissions in internal combustion engines [8, 9].

The EGR system, whose main components are the EGR pipe, the EGR valve, and the EGR cooler, is a technique in which a portion of exhaust gas is returned to the intake manifold, reducing the oxygen content inside the cylinder—oxygen-poor environment [10]. Since the NOx formation is increased in an exponential function with a temperature increase, lower oxygen content of the diluted fresh charge leads to a cooler combustion process that drastically reduces the NOx formation [11]. To increase its effectiveness, the EGR cooler—a compact heat exchanger that uses engine coolant—is in charge of reducing the exhaust gas temperature prior to entering the combustion chamber [12]. The quantity of EGR is regulated by controlling the EGR valve, which manages the EGR rate required under the different work conditions of the engine.

studies fall into two broad categories: one group intends to determine and analyze the deposit growth using in situ measurements, i.e., employing experimental procedures to quantify the morphology and characteristics of the fouling layer [17, 18], whereas the second intends to reproduce and recreate the fouling formation employing numerical approaches. The studies of this second category encompass the analysis of the EGR deposit using different numerical models like zerodimensional (0-D) models, one-dimensional (1-D) models, or advanced computational fluid dynamics (CFD) simulations, which have been created to simulate and reproduce the behavior of the fouling layer that appears inside the EGR technology. In the following sections, specific features of the different types of numerical approaches used to study the fouling in the EGR system are presented in detail. The functions offered by the several numerical models are examined, and their implementation and results are thoroughly analyzed. In this context, both the composition and characteristics of the particulate matter and the fouling mechanisms

*Photographs of different fouling layers: (a) and (b) show deposits generated by diesel particulate matter inside shell-and-tube heat exchangers, (c) depicts the fouling layer generated on a cylindrical probe which is positioned transverse to the diesel exhaust, and (d) shows the deposit formed by dry soot particles on a tube-and-fin heat*

The exhaust gas flow emitted from internal combustion engines has been categorized as dilute flow, where the low concentration of particulate matter (PM) makes negligible the effect of particles on gas flow [19]. Several factors, such as the air-fuel ratio, the EGR rate, the engine load, or the cylinder temperature, can alter the particulate loading in the exhaust flow, and, in the same way, they can influence

According to the size of the particulate matter, the nanoparticles emitted from internal combustion engines can be classified into three modes: nucleation, accumulation, and coarse. Nucleation mode is formed by particles that are less than 50 nm in diameter, and, according to the number distribution, most of the particles reside in this mode, as **Figure 3** reports. In the accumulation mode, the agglomerates consist of a collection of much smaller particles, and the size of these aggregates ranges from 50 nm to 1 μm, and particle mass distribution highlights that accumulation mode accounts the largest portion. The biggest particles—diameters between 1 μm and 10 μm—represent only a small fraction of the number of particles, and

involved in this process are briefly presented in advance.

*Numerical Modelling of Fouling Process in EGR System: A Review*

*DOI: http://dx.doi.org/10.5772/intechopen.93062*

the formation, agglomeration, and growth of the particles [20].

**2. Particulate matter involved in EGR fouling**

**Figure 2.**

*exchanger.*

they belong to the coarse mode [21–23].

**329**

One of the problems encountered in EGR systems is the fouling of the heat exchanger walls. The carbonaceous soot particles and condensable hydrocarbons derived from the combustion process lead to the formation of a highly porous deposit with low thermal conductivity that can cause the degradation in heat transfer performance in the range of 20–30% [13], as **Figure 1** shows. The accumulation of this unwanted material also causes the increase of the pressure drop along the heat exchanger, adversely affecting the control of the EGR rate and decreasing the fuel efficiency due to the increased pumping work [14]. Under significant fouling conditions, the massive increase of the thickness of the deposit can clog some tubes of the heat exchanger, as **Figure 2** shows, hampering the full normal functioning of the device [15].

In the last few decades, numerous investigations have been focused on the study of the fouling process that takes place on the heat exchanger walls of the EGR system. Numerous attempts in analysis, measurement, and prediction of the deposit have contributed to increase the knowledge of the deposit formation, and many of them have pointed out the complexity of the dynamics of this phenomenon. These

**Figure 1.** *Thermal efficiency evolution of an EGR cooler [16].*

*Numerical Modelling of Fouling Process in EGR System: A Review DOI: http://dx.doi.org/10.5772/intechopen.93062*

#### **Figure 2.**

In this context, since 2014, the EURO 6 emission standard set the emissions limit for nitrogen oxides (NOx) in 60 and 80 mg/km for gasoline and diesel light-duty vehicles, respectively [7]. This fact has extended the use of techniques like the exhaust gas recirculation (EGR) system, which have proven to be an effective way of reducing NOx formation. Nowadays, the EGR system is used together with other systems, such as diesel oxidation catalyst (DOC), lean NOx trap (LNT), or selective catalytic reduction (SCR), to fulfill the NOx emissions in internal combustion

The EGR system, whose main components are the EGR pipe, the EGR valve, and the EGR cooler, is a technique in which a portion of exhaust gas is returned to the intake manifold, reducing the oxygen content inside the cylinder—oxygen-poor environment [10]. Since the NOx formation is increased in an exponential function with a temperature increase, lower oxygen content of the diluted fresh charge leads to a cooler combustion process that drastically reduces the NOx formation [11]. To increase its effectiveness, the EGR cooler—a compact heat exchanger that uses engine coolant—is in charge of reducing the exhaust gas temperature prior to entering the combustion chamber [12]. The quantity of EGR is regulated by controlling the EGR valve, which manages the EGR rate required under the different

One of the problems encountered in EGR systems is the fouling of the heat exchanger walls. The carbonaceous soot particles and condensable hydrocarbons derived from the combustion process lead to the formation of a highly porous deposit with low thermal conductivity that can cause the degradation in heat transfer performance in the range of 20–30% [13], as **Figure 1** shows. The accumulation of this unwanted material also causes the increase of the pressure drop along the heat exchanger, adversely affecting the control of the EGR rate and decreasing the fuel efficiency due to the increased pumping work [14]. Under significant fouling conditions, the massive increase of the thickness of the deposit can clog some tubes of the heat exchanger, as **Figure 2** shows, hampering the full normal functioning of

In the last few decades, numerous investigations have been focused on the study

of the fouling process that takes place on the heat exchanger walls of the EGR system. Numerous attempts in analysis, measurement, and prediction of the deposit have contributed to increase the knowledge of the deposit formation, and many of them have pointed out the complexity of the dynamics of this phenomenon. These

engines [8, 9].

the device [15].

**Figure 1.**

**328**

*Thermal efficiency evolution of an EGR cooler [16].*

work conditions of the engine.

*Environmental Issues and Sustainable Development*

*Photographs of different fouling layers: (a) and (b) show deposits generated by diesel particulate matter inside shell-and-tube heat exchangers, (c) depicts the fouling layer generated on a cylindrical probe which is positioned transverse to the diesel exhaust, and (d) shows the deposit formed by dry soot particles on a tube-and-fin heat exchanger.*

studies fall into two broad categories: one group intends to determine and analyze the deposit growth using in situ measurements, i.e., employing experimental procedures to quantify the morphology and characteristics of the fouling layer [17, 18], whereas the second intends to reproduce and recreate the fouling formation employing numerical approaches. The studies of this second category encompass the analysis of the EGR deposit using different numerical models like zerodimensional (0-D) models, one-dimensional (1-D) models, or advanced computational fluid dynamics (CFD) simulations, which have been created to simulate and reproduce the behavior of the fouling layer that appears inside the EGR technology.

In the following sections, specific features of the different types of numerical approaches used to study the fouling in the EGR system are presented in detail. The functions offered by the several numerical models are examined, and their implementation and results are thoroughly analyzed. In this context, both the composition and characteristics of the particulate matter and the fouling mechanisms involved in this process are briefly presented in advance.

### **2. Particulate matter involved in EGR fouling**

The exhaust gas flow emitted from internal combustion engines has been categorized as dilute flow, where the low concentration of particulate matter (PM) makes negligible the effect of particles on gas flow [19]. Several factors, such as the air-fuel ratio, the EGR rate, the engine load, or the cylinder temperature, can alter the particulate loading in the exhaust flow, and, in the same way, they can influence the formation, agglomeration, and growth of the particles [20].

According to the size of the particulate matter, the nanoparticles emitted from internal combustion engines can be classified into three modes: nucleation, accumulation, and coarse. Nucleation mode is formed by particles that are less than 50 nm in diameter, and, according to the number distribution, most of the particles reside in this mode, as **Figure 3** reports. In the accumulation mode, the agglomerates consist of a collection of much smaller particles, and the size of these aggregates ranges from 50 nm to 1 μm, and particle mass distribution highlights that accumulation mode accounts the largest portion. The biggest particles—diameters between 1 μm and 10 μm—represent only a small fraction of the number of particles, and they belong to the coarse mode [21–23].

the condensation of hydrocarbons and water or the spallation of the deposit, can collapse the nanostructure of the fouling layer, slightly modifying its thermal properties [20, 29–33]. It is no easy task to determine and quantify the deposit's chemical and physical characteristics due to the fragile nature of the structure, but it is an

The gas-particle multiphase flow and the formation of fouling layer inside the EGR system are complex phenomena in which several mechanisms are involved. Thermophoresis, diffusion, inertial impact, hydrocarbon condensation, gravitational settling, removal due to shear force, water vapor condensation, or turbulent

Excluding the thermal effects, other parameters, such as the particle diffusion, the gravitational settling, the inertial impact of the turbophoresis, play an important role in the EGR fouling formation. The particle diffusion is the dominant mechanism for the small particles, particles with dimensionless relaxation times (*t*

than 0.1, while the transport of large particles, particles with dimensionless relaxa-

Inside the EGR cooler, thermophoresis—induced by the temperature gradient drives the nanoparticles from the bulk gas flow to the near cool walls, causing the deposition of the soot particles over the heat exchanger surfaces. It has been reported by several authors that under non-isothermal conditions, thermophoresis is the primary mechanism of soot deposition in the particle size typically encountered in exhaust gas, 10 nm to 1 μm, and some correlations from literature, such as

It has been extensively reported in literature that the formation of the fouling deposits depends on two simultaneous phenomena: the deposition and the removal of particles [13, 44–48]. Such categorization usually selects thermophoresis, particle diffusion, gravitational drift, inertial impact, or hydrocarbon condensation as deposition mechanisms. On the contrary, water vapor condensation, the shear force, or the turbulent burst are usually classified as removal mechanisms.

In the study of the fouling process of the EGR system, both experimental and numerical investigations have been carried out in order to analyze the effects of the

*<sup>p</sup>* ) more than 0.1, is dominated by inertial and gravitational effects [34].

þ *<sup>p</sup>* ) less

essential step to provide accurate inputs to the numerical models.

*Numerical Modelling of Fouling Process in EGR System: A Review*

*DOI: http://dx.doi.org/10.5772/intechopen.93062*

burst are the main mechanisms that engage in the fouling process.

Brock-Talbot or Cha-McCoy-Wood, have been used to determine the thermophoretic velocity as a function of the particle diameter [13, 35–38]. The condensation of HC and acids, which are part of the exhaust flow, is significant on a mass basis compared to soot deposition, and it is an important issue in the deposit formation [39]. As exhaust gas is diluted and cooled, the condensation of hydrocarbons is particularly important inside the EGR system. Condensate, which is mixed with soot particles inside the fouling layer, modifies the microstructure of the soot deposit and changes the characteristics of the deposit, leading to an increase of the density and the thermal conductivity of the fouling layer [40]. The effect of shear force of the gas flow over the deposited particles, the turbulent burst, or the water vapor condensation have been identified as potential mechanisms that cause the removal of particles from the fouling layer [41, 42]. When the drag force over the particle is larger than the adhesion force, removal occurs. In the same way, the condensed water droplets can interact with the deposited particles,

causing a washout of the dry soot deposit [43].

**4. Numerical approaches**

**331**

**3. Fouling mechanisms in the EGR system**

tion times (*τ*<sup>þ</sup>

**Figure 3.**

#### **Figure 4.** *Agglomerate diesel particle.*

Analyzing the composition of the PM of the exhaust gas, the particles are a product of a mix of volatile and nonvolatile species. Volatile faction is composed by sulfates (SO4 <sup>2</sup> + metal sulfate), nitrates (NO3 + metal nitrate), and organic elements (dCH2 + N, O and S). Nonvolatile fraction is composed by carbonaceous particles, commonly referred to as soot, and ash, formed by metals (Fe, Cr, Cu, Zn, Ca) and nonmetals (Si, P, S, Cl) [24]. Several factors, such as fuel and lubricant characteristics or engine work conditions, can influence the composition and proportion of these species, however, in most cases, elemental carbon accounts for around 90% of PM mass [25]. The primary particles—sizes typically between 15 and 30 nm—are composed by carbon and traces of metallic ash, and they aggregate forming complex irregular clusters together with adsorbed and condensed hydrocarbons (HC) [26, 27]. As **Figure 4** shows, the agglomeration of the primary particles causes the formation of clusters with a complex structure with nonuniform shape and compactness [28].

When this particulate matter is deposited on the heat exchanger walls, it forms a fouling layer which coats the heat exchanger surface. The interaction between the particles and the metal surface during the early stages of the deposit formation, and the particle-particle interaction during fouling layer growth, leads to the accumulation of amorphous aggregates on the heat exchanger walls, causing a highly porous deposit (around 98% [18]). This fouling layer, with a complex nanostructure with multiple pores between the deposited aggregates, functions as an insulator between the gas flow and the heat transfer surface. According to the experimental measurements of Lance et al. [18], the fouling layer generated from the deposition of diesel particulate matter has a density around 0.035 g/cm<sup>3</sup> and a low thermal conductivity that is around 0.041 W/mK. However, in some cases, different phenomena, such as the condensation of hydrocarbons and water or the spallation of the deposit, can collapse the nanostructure of the fouling layer, slightly modifying its thermal properties [20, 29–33]. It is no easy task to determine and quantify the deposit's chemical and physical characteristics due to the fragile nature of the structure, but it is an essential step to provide accurate inputs to the numerical models.
