 # Electro-magneto-hydrodynamic lubrication

• and Andrzej Miszczak
From the journal Open Physics

## Abstract

The topic of the presented paper aims to demonstrate a new principle of hydrodynamic lubrication in mechanical, thermal and electro-magnetic fields. Up till now, when dealing with the hydrodynamic theory lubrication, many authors of scientific papers have assumed the constant oil dynamic viscosity value without variations caused by temperature crosswise the film thickness. Simultaneously, due to the numerous AFM measurements, it appears that oil temperature gradients and oil viscosity changes in the bearing gap height directions cannot be omitted. Therefore, in this paper, the problem of the viscosity changes across the lubricant thin layer was resolved as the main novelty in principles of mechanical thermal lubrication. The method of solving the mentioned problem was manifested by a general model of semi-analytical solutions of isothermal electro-magneto-elastohydro-dynamic and non-Newtonian, lubrication problem formulated for two deformable rotational surfaces in curvilinear, co-ordinates.

## 1 Introduction and governing equations

This paper presents a semi analytical method of solution of the asymmetrical, laminar, steady, non-Newtonian lubrication flow problem between two rotational and deformable, curvilinear orthogonal movable surfaces in conjugated elasto-hydro-electro-magnetic fields. The fluid flow between the two above mentioned solid surfaces in the electromagnetic field will be described by the 3 momentum equations of equilibrium in a vector form, a fluid continuity equation, and by equation of a conservation of energy equation in a scalar form. Hence, we obtain the following system [1, 2]:

DivS+μo(N)H+12μorotN×H+J×B=ρdvdt(1)
ρt+divρv=0,(2)

The above-mentioned system of equations is completed by magneto-thermo-elasticity equations describing problem in stresses for two solid surfaces restricting the thin fluid layer. This system consists of the three partial differential equations (4) in a vector form. Heat conductivity equation in a solid body (5) is added to this set of equations and we obtain a system in the following form [2, 3, 4]:

DivS+J×B+μoNH=ρ2ut2(4)

Moreover, we add the Maxwell and Ohm equations as well for the two surfaces as for the thin boundary liquid layer between two surfaces. Thus we get such equations :

B=0,B=0,×H=J+Dt,×H=J+Dt(6)
×E=Bt,×E=Bt,J=σE,J=σ(E+v×B)(7)

We assume the following notations: μo – magnetic permeability in vacuum H/m, T – fluid temperature in K, T* – solid body temperature in K, B – magnetic induction vector in T, N - magnetization vector A/m, E – electric intensity vector V/m, H – magnetic intensity vector A/m, (– first derivative of magnetization vector with respect to temperature A/mK, σ - electrical conductivity coefficient S/m, J - electric current density in A/m2, D - electric induction vector As/m, ρ - fluid density kg/m3, κ - thermal conductivity coefficient W/mK, v – fluid velocity vector in m/s, φF – dissipation of energy in W/m3, S - stress tensor in the fluid in Pa, u - displacement vector of the solid body in m, t - time in s, cv - specific heat in J/kgK. The symbols with an asterisk are related to the solid body.

The relationship between stress tensor S and strain rate tensor 2Td = A1i.e. constitutive equations are assumed for the lubricant in the following form :

S=pδ+ηpA1(8)

whereas unit tensor δ, strain rate tensor A1 have the following components: δij, Θij in s–1. For power law of compressible fluid the apparent viscosity ηp has the form [2, 3, 4]:

ηpηpr=2n1m(n)12I12I2n12,φF=4ηp12I12I2pI1,(9a)
I1=divv,I2=Θ11Θ22Θ122+Θ11Θ33Θ132+Θ22Θ33Θ232,(9b)

We assume: n – dimensionless flow index (0.5, 1.2), m = m(n) fluid consistency coefficient in Pasn.

The Duhamel Neumann relations between the components τij of the stress tensor S* of the elastic body on the sleeve and the strain tensor components ϵij take the following form:

τij=2Gεij+Λεkk3αTKΔTδij(10)

for i,j = 1,2,3, where: δij – Kronecker unit tensor component (δij = 1 for i = j and δij = 0 for ij), K = Λ + 2G/3, G = EY/[2(1+ ϑ)], Λ = EYϑ/(1+ϑ)(1–2ϑ), whereas EY - Young‘s Modulus in Pa, αT – thermal coefficient of linear expansionin K–1, ϑ – Poisson ratio.

And by virtue of known linear geometrical relations, the strain tensor components Θij(vi) in the oil, and strain components ϵij(ui) in the sleeve are mutually connected with the oil velocity components vi and displacement vector of solid body components ui, respectively:

Θij=0.5(vij+vji),εij=0.5(uij+uji).(11)

## 2 Electro-magneto-hydrodynamic equations

We assume: unsymmetrical, incompressible (for invariant I1 = 0), steady, pseudo-plastic lubrication in electro-magnetic field for power law model without viscous-elastic properties. Oil apparent viscosity ηp varies in length, width and gap-height directions and depends on pressure, temperature flow shear ratio, and electro-magnetic field [1, 2, 3, 4, 5]. Inertia forces and terms of energy convections are neglected, pressure is constant in gap height direction. The total distance ϵT between the two surfaces is signifucantly smaller than other dimensions of the considered surfaces. Taking into account the layer boundary simplification i.e. neglecting in (1)-(7) the negligibly small terms of order Ψ = 0.001 (radial clearance) presenting the quotient of characteristic gap height (0 to the radius R of journal, the problem is made in local curvilinear and orthogonal coordinates (α1, α2, α3) connected with one of the movable surfaces, where α2 denotes the direction of gap height. The parallel and longitudinal intersections of the cooperating deformed, rotational surfaces have curvilinear and non-monotone generating lines. Hence, Lame coefficients have the form: h1 = h1(α3), h2 = 1, h3 = h3(α3).

We put physical and geometrical dependencies (9), (10), (11) into expanded boundary simplified equations (1)-(7). Hence, we obtain the system of equations of conservation of momentum, continuity, energy, thermo-elasticity, heat transfer, Maxwell simplified results in the following form of curvilinear orthogonal co-ordinates (α1, α2, α3):

0=1hipαi+α2ηpα1,α2,α3viα2+Mi,pα2=0,(12)
0=1h1v1α1+v2α2+1h1h3α3h1v3(13)
α2κTα2+[ηpα1,α2,α3]Vv1,v3=MT.(14)
α2κTα2=0,α22G+Λδ2iuiα2==δ2iα23KαTT+J×Biμ0NHi(15)
ηpα1,α2,α3mn,T,B,EVg(v1,v3),Vv1α22+v3α22,(16)
gnn12,MTα1,α3μ0TΞvH+J2/σ,Miα1,α3μ0NHi+12μorotN×Hi+J×Bi,(17)

where the length, width and gap- height directions, are limited respectively: 0 < α1 ≤ 2π, –bmα3bs, 0 ≤ α2ϵT and i = 1,2,3. The system of equations (12)((17) contains the following unknown dimensional values: pressure p(α1,α3), temperature in oil T(α1,α2,α3) and sleeve T*, three oil velocity components vi (α1,α2,α3) and three layer displacement components ui (α1,α2,α3), for i = 1,2,3 in three curvilinear, orthogonal dimensional directions: α1,α2,α3.

## 3 Boundary conditions

The lubricant flow in the bearing gap is generated by the rotation of a rotational, curvilinear journal. Hence the boundary conditions for lubricant velocity components have the form:

v1=ωh1forα2=0,v1=0(18)

for α2 = ϵT, v2 = 0 for α2 = 0,

and for α2 = ϵT, v3 = 0 for α2 = 0, v3 = 0 for α2 = ϵT,

where ω denotes the angular velocity of rotational journal, in the circumferential direction.

Decrements or increments that are above the characteristic environmental temperature T0 have constant value fc on the journal surface and variable unknown values fp (α1, α3) on the sleeve surface. Heat flux is transferred from the rotational surface of bearing journal into lubricant. Hence the boundary temperature values are as follows:

T(α1,α2,α3)=T0+fcforα2=0,(19a)
T(α1,α2,α3)=T0+fp(α1,α3)forα2=ϵT(19b)
κTα2=qcυΔfforα2=0,(19c)

where Δf – temperature difference across the film thickness, υ – heat transfer coefficient in W/m2K, qc - heat flux density in oil on journal surface in W/m2.

The elastic layer of bearing alloy is laying on the rigid ring in place α2 = c3, and therefore the contact surface of these bodies is not deformed by the pressure. The contact surface α2 = c3 is yet deformed by variable temperature, which implies the shape and the total volume change of the rigid ring. These deformations are not considered now.

The temperatures and surfaces localization is presented in Figure 1. Figure 1

Motionless solid A2 and moving body A1 loaded by means of pressure and temperature and magnetic field; N, S - magnetic poles; a) cylindrical bearing, b) other bearing

The boundary conditions in the elastic layer have the following form:

τij(α1,α2=c2,α3)=p(α1,α3)δi2δij(20)
T(α1,α2=c3,α3)=T0+e2(α1,α3)(21)
T(α1,α2=c2,α3)=T0+f2(α1,α3)(22)
κTα2=qp,(23)

where i = 1,2,3. Moreover: T0 - average ambient temperature, qp - heat flux density across the elastic layer located on the sleeve surface, f2(α1, α3) - temperature increases above the temperature T0 on the external surface of the elastic layer for α2 = c2, additionally e2(α1,α3) temperature increases in the excess of temperature T0 on the bottom surface of the elastic layer in the place of α2 = c3.

## 4 Oil consistency and viscosity variations

Temperature decreases and magnetic induction, electric intensity increases the oil consistency m and dynamic viscosity coefficient η. This phenomenon is described in the following form:

m(α1,α2,α3)=m(n,T,B,E)=η0η1(T,B,E)v0/ε01n,η1(T,B,E)=expδBB+δEEδTT,(24)

with coefficient δB, δE of magnetic, electric influence on the oil viscosity and coefficient δT of temperature influence on the oil viscosity Mentioned phenomena are confirmed by virtue of authors own measurements.

## 5 Sketch of semi-analytical solutions

After double integration of two equations (15) in α2 direction we apply the low heat flux continuity (qp = qc) in the contact region between the elastic alloy layer and lubricant, and finally we obtain the external displacement of the sleeve surface in the following dimensional form:

u02=u2α1,α2=c2,α3==εs1+ϑ1ϑ12ϑEpfp12εsqpκεsαT1+ϑ1ϑ++1212ϑE1+ϑ1ϑεs2NB2+J×B2(25)

where ϵs = c3c2, 0 < α1 < 2π, –bm < α3 < +bm. Solution (25) valid if and only if EY - Young modulus and solid body coefficients αT, κ*, ϑ - Poisson ratio, are independent of α2, The hydrodynamic pressure p, the load carrying capacity and temperature T distributions with oil velocity components v1, v2, v3 in machine slide journal bearing are solved by the means of the quasi half numerical Frobenius small parameter method implemented by the finite difference method using Mathcad 15 Professional Program. A system of partial differential equations (12)-(14) defined in thin space (α1, α2, α3, t) between two movable rotational surfaces and for continuous m(n), η == m(n = 1), has an analytical solution in the form of the following infinite uniform convergent power function series in following form:

vi(α1,α2,α3)=vi(0)+gvi(1)++gkvi(k)+T(α1,α2,α3)=T(0)+gT(1)++gkT(k)+(26a)
p(α1,α3)=p(0)+gp(1)++gkp(k)+qc=qc(0)+gqc(1)++gkqc(k)+(26b)

where qc(0)=qc(1)==qc(k)q, with qc = q/(1 – g); for i = 1,2,3; k = 0,1,2,…, g = g(n)(n-1)/2 where for 0 < n ≤ 3/2 the small parameter g is less than +1/4 and greater than –1/2. In order to achieve the linearization of apparent viscosity (16), we put functions (26a) into Eq. (16) and expand function in closed interval [1, n] or [n, 1] for 0 < n ≤ 3/2 using Taylor series in the neighborhood of point n = 1 with respect to the small parameter in the following form:

ηpα1,α2,α3=η0η1T,B,E1+gηpr1++gkηprk+(27)

where η1(T, B, E) = η1(α1, α2, α3) and functions ηprkηprk(v1, v3) for k = 1, 2, … – are the dimensionless expansion coefficients of dynamic viscosity dependent on α1, α2, α3, whereas for k = 0 ηpr0 = 1. Such functions after involved calculations are determined in analytical form. Putting the infinite series (26), (27) into the system (12)-(14), multiplying the series by Cauchy methods and equating the terms by the same powers k = 0,1,2,… of the small parameter g, and then using the L. Kronecker symbol i.e. δk0 = 1 for k = 0, δk0 = 0 for k = 1,2,…, we obtain the linear differential systems of equations for k = 0,1,2,… i = 1,3 in following form:

1hip(k)α1=α2η0η1B,E,T(k)vi(k)α2++α2η0η1B,E,T(k)Si(k)+Miδk0,p(k)α2=0,(28)
1h1α1v1(k)+α2v2(k)+1h1h3α3h1v3(k)=0,(29)
α2κT(k)α2=η0η1B,E,T(k)VT(k)+MTδk0.(30)

with

η1B,E,T(k)=η1BEB,EeδTT(k)α1,α2,α3,η1BEB,Eη1BEα1,α3,Si(0)=0,VT(0)Vv1(0),v3(0),Si(1)vi(0)α2ε0v02VT(0),(31a)
VT(1)VT(0)lnε0v02VT(0)+2v1(0)α2v1(1)α2+2v3(0)α2v3(1)α2,(31b)

For k = 0 the abovementioned system of equations (28)-(31) determines functions: vi(0),Ti(0),p(0) for Newtonian oil taking into account the boundary conditions in steady motion (18), (19). For k = 1,2,… and i = 1,2,3 the system of equations (28)-(31) determines the first, second,… corrections: vi(k),T(k), p(k) caused by the non-Newtonian oil. The boundary conditions for k = 0 are as follows:

v1(0)=ωh1,v2(0)=0,v3(0)=0,T(0)=T0+fc(32a)

for α2 = 0,

v1(0)=0,v2(0)=0,v3(0)=0,T(0)=T0+fp(0)

for α2 = εT;

and for k = 1,2,… we have:

vi(k)=0,T(k)=sfpk,fors=α2/εT,α2=0,α2=εT;(32b)

The total gap height ϵT includes surface displacement (25). Integrating twice (28) and (29) once with respect to the variable α2, then the oil velocity components for k = 0,1,2,… by virtue of boundary conditions (32a, b) have the following form:

vi(k)=1η0η1BE1hip(k)αiKi(k)(α1,α2,α3)++K2(k)(α1,α2,α3)0εTSi(k)dα20α2Si(k)dα2++ωh1[1K2(k)(α1,α2,α3)]δi1δk0+δk0ΔMi,(33)
v2(k)=1h10α2v1(k)α1dα21h1h30α2α3h1v3(k)dα2,(34)

for i = 1, 3; 0 < α21 < α2 < ϵT, 0 < α1 < 2π, –bm < α3 < +bm, whereas we denote:

ΔMi=1η0η1BE0α20α21Midα21dα2+1η0η1BEs0εT0α21Midα21dα2=εT22η0η1BEMi(ss2),(35a)
K1(k)(α1,α2,α3)detΩ(k)Ω1(k)(α1,α2=εT,α3),K2(k)(α1,α2,α3)Ω1(k)(α1,α2,α3)Ω1(k)(α1,α2=εT,α3),(35b)
Ω(k)Ω2(k)(α1,α2,α3)Ω2(k)(α1,α2=εT,α3)Ω1(k)(α1,α2,α3)Ω1(k)(α1,α2=εT,α3),Ω1(k)(α1,α2,α3)0α2eδTT(k)dα2,Ω2(k)(α1,α2,α3)0α2α2eδTT(k)dα2.(35c)

It is easy to see, that:

K1(k)(α1,α2=0,α3)=0,K1(k)(α1,α2=εT,α3)=0,K2(k)(α1,α2=0,α3)=0,K2(k)(α1,α2=εT,α3)=1.(35d)

Imposing the boundary condition (32ab) in point α2 = ϵT on the solution (34) and taking into account the law of integral differentiation with variable limits of integration, we obtain the following modified Reynolds equation, which determines the unknown function p(k)(α1, α3):

1h1α11η0η1BE(α1,α3)p(k)(α1,α3)α10εTK1(k)(α1,α2,α3)dα2+1h3α31η0η1BE(α1,α3)h1h3p(k)(α1,α3)α30εTK1(k)(α1,α2,α3)dα2++α1ΠNNi(k)δk0εT3M1η0η1BE(α1,α3)+h1h3α3ΠNN3(k)δk0εT3M3η0η1BE(α1,α3)==ωh1α10εTK2(k)(α1,α2,α3)dα2εT,(36)

with

ΠNNi(k)(α1,α3)0εTSi(k)dα20εTK2(k)(α1,α2,α3)dα20εT0α2Si(k)dα2dα2,(37)

for i = 1,3; k = 1,2,3.

To obtain the temperature functions T(k)(α1α2, α3) for k = 0,1,2,…, we put solutions of oil velocity components (33), (34) for i = 1,3; into the temperature equation (30). After double integration with respect to the variable α2i.e. in film thickness direction, using boundary conditions (19ab) in the form: T(k) = fcδk0 for α2 = 0, T(k) = fpδk0 for (α2 = ϵT, and after the term ordering, we finally obtain:

T(k)=η0η1BEs0εT1κ0α2VT(k)eδTT(k)dα2dα20α21κ0α2VT(k)eδTT(k)dα2dα2+0α21κ0α2MTdα2dα2s0εT1κ0α2MTdα2dα2dα2δk0+δk0fc1s+fps,(38)

where sα2/ϵT, 0 ≤ s ≤ 1. Non linear differential equation (38) determines temperature functions T(k) for k = 0,1,2,… Imposing the condition (19c) on the temperature T(k) determined by the (38), we obtain the unknown temperature fp (α1, α3) on the sleeve.

To obtain the oil velocity components (33), hydrodynamic pressure (36) and temperature (38) for Newtonian oil, and taking into account oil viscosity variations crosswise the gap height, we put k = 0 and hence: Si(0)=0,ΠNNi(0)=0 for i = 1.2.3. In this case we obtain oil velocity components vi(0), pressure p(0), temperature T(0). Assuming cylindrical coordinates: α1 = ϕ, α2 = r, α3 = z, h1 = R, h2 = h3 = 1, where R is the radius of the cylindrical journal, we put the temperature T(0), determined from the reduced Eq. (38), into the reduced pressure Equation (36). Finally, the Eq. (36) determines the pressure p(ϕ, z), 0 ≤ ϕ ≤ 2π, –bzb, and tends to the following form:

Rϕp(0)RϕKJ1+zp(0)zKJ1=ωKJ2ϕ,KJ1εT3ηoη1BE+δTεT3ηoη1BE{fcq2κεT15ωR2ηoη1BEκ160ηoη1BEκεT4p(0)Rϕ2+p(0)z2},KJ2=6εT6δTεT{16qκεT+124ωR2ηoη1BEκ+1144ηoη1BEκεT4p(0)Rϕ2+p(0)z2},(39)

0≤ ϕ ≤ 2π, –bzb, p = p(ϕ, z), η1BE = exp(δBB + δEE).

For δT = 0, viscosity is constant in gap height direction and (39) tends to the well known classical form of Reynolds equation.

## 6 Conclusions and results

1. After semi-analytical solutions and initial numerical calculations it appears that the pressure obtained directly for the oil dynamic viscosity variations crosswise the film thickness caused by the temperature gradients in gap height directions are about 5 to 7 percent different in comparison with the pressure values calculated for constant oil viscosity across the film thickness i.e. in the case when the temperature is constant in gap height direction and varies only in bearing length and circumference direction.

2. The mutually direct interactions of influences between hydrodynamic pressure on temperature as well as temperature on pressure, are valid only if the temperature, hence oil dynamic viscosity, varies in the gap height direction.

3. It is worse to notice that constant temperature in the gap height direction does not correspond with 3D temperature field obtained directly from energy equation. The assumptions of constant temperature and viscosity crosswise the bearing gap are in contradiction with the contemporary achievements connected with new devices such as micro-bearing, nano-bearing, magnetic bearings, artificial joints in humanoid robots, micro-motors. Unfortunately, numerous authors in the field of hydrodynamic lubrication, avoid to assume the oil viscosity variations in the gap height direction.

4. The performed numerical calculations show that oil, presented in the calculation, increases the pressure and load capacity in the slide journal bearing for Newtonian and non-Newtonian oil, in the presence of magnetic induction field.

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