# Parametric investigation of surface texturing on performance characteristics of water lubricated journal bearing using FSI approach

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## Abstract

In this paper, parametric analysis of various operating and surface texturing parameters on the performance characteristics of water lubricated journal bearing was carried out using Fluid Structure Interaction approach. Initially, a simulation model was validated with experimental results from the literature. An effect of journal speed and eccentricity ratio along with elastic deformation on performance characteristics viz. lubricant pressure, load carrying capacity, and coefficient of friction with different groove location was studied. Further investigation was carried out to examine the influence of the number of grooves on the performance characteristics. From the analysis, it was observed that 0–180° grooved journal bearing with 5 number of grooves has shown improved performance as compared to journal bearing with 0–90° grooved, 90–180° grooved and plain surface.

## Keywords

Fluid Structure Interaction Water lubricant Surface texturing Fluid film pressure Coefficient of friction Load carrying capacity## 1 Introduction

Working of machines with high efficiency, more energy saving and decreasing the environmental pollution is become important criteria in machine design [1]. Hydrodynamic journal bearings are broadly used in applications such as electric generators, gas turbines, hydro turbines, IC Engines, marine propellers, turbo generators and hard disk drives due to their simplicity and superior damping characteristics [2]. Use of water as lubricant is become popular because of its safe, green, energy saving tendency and convenience. Because of this, water lubricated journal bearings are used in industrial machinery, shipbuilding, food industry, transportation industry and pharmaceutical industry [3]. Wang et al. [4] studied load carrying capacity and friction using sliding surfaces of the polymer in water. They used Polytetrafluoroethylene (PTFE) material for bearing with the benefit of very small friction coefficient and good corrosion-resistant property, excellent chemical stability and water absorption. They concluded that for the lubrication performance of water-lubricated bearing, the deformation effect cannot be ignored. Su et al. [5] studied deformation of textured (dimple) surface in the soft elastohydrodynamic lubrication (EHL) contacts and showed that an uneven deformation emerges around a dimple. They also concluded that the sliding velocity and applied load have noteworthy influence on the deformation of soft surface, particularly on deformation area. Habchi et al. [6] carried out an investigation on the elastohydrodynamic line or point contact lubrication problem by pairing Reynolds equation and elastic deformation using a full-system approach. Gertzoset al. [7] investigated the performance characteristics of the journal bearing lubricated with the Bingham fluid using Computational Fluid Dynamic approach. The cavitation phenomenon is studies by implementing half-Sommerfeld boundary condition. Hartinger et al. [8] obtained a solution for thermal and shear thinning elastohydrodynamic line contact problem by employing a homogenous equilibrium cavitation model. Shenoy et al. [9] analysed an elastohydrodynamic lubrication of a hydrodynamic journal bearing employing the sequential application of CFD and computational structural dynamics and the cavitation was modeled by keeping all the calculated negative pressure and their gradients as zero. Liu et al. [10] investigated pressure distribution, cavitation and center orbit of journal of the elastohydrodynamic lubrication problem by employing using CFD and FSI methods. The bearing model in considered in their study was simple cylindrical journal bearing without any groove and thermal effects were not taken in account. Shinde et al. [11, 12, 13, 14, 15] studied an effect of partial groove shape texturing on performance characteristics of cylindrical shape journal bearing without considering the elastic deformation phenomenon. The performance characteristics were considered viz. fluid pressure, frictional torque, load carrying capacity (LCC) and power loss and concluded that groove shape texturing present in the positive pressure region enhances the performance of journal bearing. Further study extended for conical shape journal bearing along with ellipsoidal shape dimples texturing on bearing surface and enhanced the performance of bearing system. Shi et al. [16] investigated the influence of microdimples and microgrooves on the load-carrying performance of mechanical gas seals. They proved that both microgrooves and microdimples improve the load-carrying performance under a small clearance condition. Bouyer and Fillon [17] analyzed a single-groove plain journal bearing with thermoelastohydrodynamic (TEHD) effect under steady load in which elastic deformation of bearing surface and expansion of the journal were considered. Lin e al. [18] investigated the effect of surface texturing on the performance characteristics of hydrodynamic bearing working under the time dependant condition using fluid–structure interaction (FSI) method. In their analysis, displacements of journal and eccentricity ratio were considered as main parameters for actual operation of journal bearing. They showed that, location of texturing can increase or decrease the performance of a hydrodynamic bearing in terms of the developing the LCC. Profito and Zachariadis [19] implemented three partitioned fluid structure coupling methods to evaluate performance characteristics of steady state hydrodynamic journal bearings working in the elastohydrodynamic lubrication zone. Molka et al. [20] analyzed the influence of the bearing surface deformation on the performance of a cylindrical shape journal bearing. They employed the FEM with an iteration method for solving both the Reynolds equation and the three-dimensional elasticity expressions representing the displacement area in the bearing domain. They showed that elastic deformations extend the pressure area in the bearing and increase the minimum film thickness which reduces the LCC. Meng et al. [21] studied the effect of the compound dimple on the performance behavior of a journal bearing using a fluid structure interaction (FSI) method. They showed that the compound dimple can supply the larger load-carrying capacity and lower friction coefficient due to its twice hydrodynamic action in comparison with the simple dimple. Tala-Ighil et al. [22] examined the full and partial textured journal bearing for different working conditions. They concluded that, appropriate texture distribution iproves the film thickness, frictional torque and fluid pressure. Jadhav et al. [23] studied the influence of micro-texturing on the performance of hydrodynamic bearing considering the elastic deformation behavior of bearing surface. They observed 38.28% reduction in coefficient of friction in textured bearing system. Tauviqirrahmam et al. [24] investigated the effect of surface texturing and slippage on the friction coefficient and load carrying capacity without considering the effect surface deformation of bearing. They showed 50% improvement in the load carrying capacity in case of textured bearing as compared to smooth surface bearing system. Tala-Ighil et al. [25] conducted numerical analysis of spherical shape dimple texturing on bearing surface and improved the performance of hydrodynamic bearing. They suggested that film thickness, fluid pressure, frictional torque and oil flow can be improved by introducing texturing in positive pressure zone. Gu et al. [26] suggested that, well designed texturing can reduce friction and positive influence on bearing performance. Cupillard et al. [27] studied the influence of texturing on load carrying capacity and friction.They showed that, texture of suitable geometry can reduce friction and increase load carrying capacity. Yu et al. [28] suggested that texturing introduced in rising phase of fluid pressure increases the load carrying capacity and reduces it in falling phase.

Hydrodynamic lubrication difficulties are mainly solved using the modified or classical Reynolds equation, which is derived from the fundamental Navier–Stokes equations. Although numbers of important CFD and experimental based investigations have been reported in which the effect of surface texture on the performance of oil lubricated journal bearings were studied without considering the effect of deformation of bearing surface. In all the studied investigations, either the bearing was treated as rigid or the lubricant was oil. It is very important to consider the effect of fluid pressure on the solid domain which significantly affects the performance behavior of bearing systems and there are very less reports about analysis of water lubricant based journal bearings with surface texturing using fluid–structure interaction approach.

Therefore, in this research, fluid structure interaction-based investigation of water lubricated surface textured journal bearing for various performance characteristics was carried out. The investigation of performance characteristics like development of fluid pressure, LCC, coefficient of friction and elastic deformation was performed at various operating and texturing conditions viz. speed of journal, eccentricity ratio and number of grooves. In this study, fluid structure interaction was implemented in which the effect of lubricant pressure on bearing surface was considered where journal domain was considered as rigid domain. The interest of work was consideration of bearing surface deformation due to fluid pressure and study the behavior of water lubricated surface textured journal bearings. This FSI approach provides realistic operating conditions of journal bearing along with surface texturing.

## 2 Fluid structure interaction (FSI) approach

In hydrodynamic journal bearing system, as journal rotates a continuous film of lubricant was developed between rotating journal and stationary bearing. The hydrodynamic positive pressure develop in lubricant in convergent region while terminates in divergent region. The pressure developed in lubricant film supports the external applied load. The FSI approach considers the actual operating condition of e journal bearing system. In this approach one way coupling between lubricant and bearing domain was examined. The governing equations used for this approach was described in this section.

### 2.1 Lubricant domain

^{3}), h is lubricant thickness (m), µ is viscosity (Pa.s), p is pressure (Pa), a is location (m) of the channel base, v

_{a}is tangential velocity (m/s) of the chanl base, b is location (m) of the solid wall, and v

_{b}represents the tangential velocity (m/s) of the solid wall. The rotating journal is considered to be a solid wall. Because the pressure is constant through the lubricant film thickness, COMSOL uses the tangential projection of the gradient operator, ∇

_{T}, to calculate the pressure distribution on the lubricant surface. In this study, the term ρ((∇

_{T}

*b*…

*v*

_{b}) − (∇

_{T}

*a*…

*v*

_{a})) equates to 0, so the governing equation simplifies to

### 2.2 Cavitation model

A full film region where the pressure varies but is limited from below by the cavitation pressure.

- A cavitation region where only part of the volume is occupied by the fluid. Because of the presence of the gas in the void fraction, the pressure in this region is assumed as constant and equal to the cavitation pressure.Elrod and Adams modified a general form of Reynolds equation by introducing a switch function, g, equal to 1 in the full fluid film zone (θ ≥ 1) and 0 in the cavitation zone (θ < 1). This switch function allows for solving single equation for the full film and the cavitation region and leads to a modified version of the average velocity used in the Reynolds equation:where first and second term on right hand side correspond to the average Couette and average Poiseuille velocities, respectively. This switch function sets the average Poiseuille velocity is to zero in the cavitation region. Because the average Poiseuille velocity is set to zero in the cavitation region, the density needs to be a function of the pressure variable and it is defined as$$V_{av} = V_{av,c} - gV_{av,p} \nabla_{t} p_{f}$$(3)$$\rho = \rho_{c} e^{{\frac{{p - p_{c} }}{\beta }}}$$(4)

### 2.3 The bearing domain

*h*film thickness,

*C*radial clearance, \(\Delta h\) groove height, \(d_{b}\) total elastic deformation of bearing surface.

### 2.4 Performance characteristics

*FF*) is calculated by [11, 30]

*COF*) is derived by using ratio of friction force (FF) and LCC [21].

## 3 FSI model

Input parameters for FSI analysis

Parameter | Value | Parameter | Value |
---|---|---|---|

Journal diameter (D | 80 (mm) | Groove numbers in Axial direction | 1, |

Bearing length (H) | 80 (mm) | Groove width (w) | 2 (mm) |

Bearing thickness (t) | 10 (mm) | Groove height (\(\Delta h\)) | 20 (μm) |

Eccentricity ratio (ε) | 0.4, 0.5, | Circumferential groove region(θ | 0–90, 90–180, 0–180 |

Radial clearance (C) | 40 (μm) | Spacing between grooves (SP) | 3 (mm) |

Viscosity of water (μ) | 0.001 (Pa s) | Elastic modulus of PTFE (E) | 1400 (MPa) |

Viscosity of water vapour (μ) | 1.34E−5 (Pa s) | Density of PTFE (\(\rho_{b}\)) | 2200 (kg/m |

Density of water (ρ) | 998.2 (kg/m | Poisson’s ratio (ɳ) | 0.36 |

Density of water vapour | 0.5542(kg/m | ||

Journal speed [J |

Results for various sizes of mesh and precisions for 0–180 grooved bearing configuration

Case | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|

Max. size of element in mm | 2.25 | 2.00 | 1.75 | 1.5 | 1.25 |

Mini. size of element in mm | 0.008 | 0.008 | 0.004 | 0.004 | 0.004 |

Total elements | 346,387 | 477,932 | 697,863 | 1,106,881 | 1,937,185 |

Lubricant pressure in MPa | 9.012 | 9.198 | 9.325 | 9.325 | 9.325 |

## 4 Results and discussion

In this section validation of plain journal bearing, the influence of speed of journal and eccentricity ratio on lubricant pressure development, LCC, elastic deformation and COF is examined with different configurations [0–90°, 90–180°, 0–180°] of journal bearing system.

### 4.1 Validation

### 4.2 Lubricant pressure field

Further, investigation was carried out at 1, 3 and 5 number of axial grooves with eccentricity ratio of 0.6 as depicted in Fig. 8b. From Fig. 8b it was seen that, as speed of journal increased, the development of lubricant pressure was increased linearly. The maximum pressure is observed for bearing with 5 grooves. It was also seen that, as number of grooves increased from 1 to 5, the value of lubricant film pressure increased. At journal speed of 3500RPM, the maximum pressure value of 2.07 MPa, 1.96 MPa, 1.69 MPa and 1.22 MPa is observed in journal bearing with 5 grooves, 3 grooves, 1 groove and plain surface. The bearing surface with 5 grooves increased lubricant film pressure by 69.34% as compared with plain journal bearing.

The effect of number of groove on the development of lubricant pressure at different eccentricity ratio was carried out at journal speed of 1500RPM. Bearing surface with 1 groove, 3 grooves and 5 grooves were considered for investigation. From Fig. 9b it was seen that maximum pressure was developed in case of bearing surface with 5 grooves as compared with other configurations. At eccentricity ratio of 0.7, the highest pressure value of 1.28 MPa, 1.15 MPa, 1.07 MPa and 0.889 MPa was seen in journal bearing with 5 grooves, 3 grooves, 1 groove and plain surface.

### 4.3 Load carrying capacity (LCC)

### 4.4 Effect on coefficient of friction

### 4.5 Effect on elastic deformation

## 5 Conclusions

- 1.
From FSI analysis of non rigid bearing surface, it is observed that, elastic deformation significantly affects the bearing performance characteristics. It reduces the lubricant film pressure, LCC and COF.

- 2.
From the analysis, it is observed that 0°–180° grooved journal bearing with 5 grooves shows maximum lubricant pressure, LCC, and low COF as compared with other configurations.

- 3.
It is also seen that, highest elastic deformation is seen in plain journal bearing system in cavitation region as compared with other configurations.

- 4.
A 0°–180° grooved journal bearing with 5 grooves shows 69.34%, 72.41%, and 49.30% improved performance in lubricant pressure, LCC and COF at journal speed of 3500RPM and eccentricity ratio of 0.6.

## Notes

### Compliance with ethical standards

### Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

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