Page 24 - Oil and Gas Simulations

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SHELL GLOBAL SOLUTIONS, UNITED KINGDOM
SHAPE CHANGING For a lubricated line contact, such as the
sliding of a cylinder against a flat surface
LUBRICANTS under steady loading conditions, any
equations used need to take into account
numerous attributes. These include
lubricant oil film thickness at different
Until recently, Shell was using propriety programs to positions in relation to the contact, the
corresponding pressure that determines
model friction in lubricants for the automotive industry.
lubricant density, lubricant dynamic
However, as the complexity of their simulations increase, viscosity, the relative speed of moving
the engineering team at Shell has turned to multiphysics surfaces, the reduced radius of curvature
for the contacting surfaces, the reduced
simulation to develop more in-depth analyses. Young’s modulus of the surfaces, and the
load per unit length.
by JENNIFER HAND “We end up with some rather complex
analytical expressions for the elastic
deformation of a medium due to an
Lubricants that can deliver better fuel causing metal surfaces to deform applied pressure at the surface,” describes
economy or improved energy efficiency elastically. As a consequence of these two Taylor. “It is also worth pointing out that
are in demand and the market is huge. effects, the oil film thickness is greater lubricant viscosity is a strongly varying
Independent consultants Kline & Company than otherwise expected, which is exactly function of pressure. The numerical
estimate that the global requirement for why such high pressure contacts can be solution of these equations is not
lubricants in 2010 was almost 38 million successfully lubricated. The fact that high straightforward since the character of the
tons, approximately 44 billion liters. pressure promotes lubrication rather than partial differential equation changes at
According to Dr. Robert Ian Taylor, hinders it can seem a bit non-intuitive. different points in the contact.”
Technology Manager at Shell Research in The key is to account for these two effects èINCORPORATING ANALYSIS
the UK, a reduction in lubricant viscosity when predicting performance.” TO BETTER UNDERSTAND
is regarded as one of the key approaches This type of lubrication is known as LUBRICATION PROBLEMS
to improving energy efficiency, and the elastohydrodynamic lubrication and
choice of base oil is significant. “There as Taylor notes, it is no easy matter to Finite element analysis, using a
is a move towards synthetics, in which measure lubricant rheology, or flow, multiphysics solver, is an approach for
molecules are highly controlled, often by under such pressures, especially as shear taking into consideration all of the
the further processing of mineral-based rates can also be as high as 108 s-1. “In participating properties. “We wanted
oils. ‘Slippery’ chemical additives, called elastohydrodynamically lubricated to solve the Reynolds’ equation on the
friction modifiers, are also being used.” contacts, we generally have much thinner contact line or surface; find the pressure in
films than one micron, so shear rates are the lubricant; use that calculated pressure
èTHE BENEFITS OF correspondingly higher. Any mathematical to analyze the elastic deformation in the
ELASTOHYDRODYNAMIC models we develop must be capable underlying surface; then use the changed
LUBRICATION of allowing for varying rheological surface shape to recalculate the pressure
descriptions of the lubricant.” distribution (see Figure 1).”
In general, energy efficient lubricants
deliver lower friction because the oil film
thickness in the contact is reduced. Of
course, if oil film thickness is reduced too
much, there is the possibility of higher
wear. It is therefore particularly important
to be able to predict the effect of lubricant
properties on the thickness of the oil film
and the friction of a lubricated contact.
“For lubricated contacts, such as plain
journal bearings or piston rings, and at
pressures below 200 MPa, the Reynolds’
equation can easily be solved to predict
oil film thickness and friction,” comments
Taylor. “However, there are many
important lubricated contacts, such as
gear teeth or rolling element bearings,
where extremely high pressures of up
to 3 or 4 GPa can be generated in the FIGURE 1. Schematic of multiphysics approach to solving lubrication problems. The
lubricant. Reynolds’ equation is solved on the line, or surface. The calculated pressure distribution is
Under such pressures the viscosity then used to calculate elastic deformations and this process is repeated until convergence
of the lubricant increases dramatically, is obtained.
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