D.R. Oliver

Load enhancement effects due to polymer thickening in a short model journal bearing

Abstract A model bearing is described which is 20.0 mm in diameter and 2.5 mm in length; a short bearing of diameter to length ratio eight. The clearance is large (500 μm) and the rotor may be run true or eccentric on its own shaft; in each case the mean load and frictional (tangential) force is measured as the centreline eccentricity is varied. Comparison is made between the lubricating performance of Newtonian and highly elastic liquids; the latter give load enhancement ratios of up to 300 and reductions in coefficient of friction by factors of the order 30. These effects are greatly in excess of those obtained when dealing with bearing of diameter to length ratio close to unity; possible reasons for this are discussed. A Newtonian oil and a polymer-thickened oil are tested in the same way, the latter oil is found to give load enhancement ratios of 1.4 (true rotor) and 3.5 (eccentric rotor) with corresponding reductions of coefficients of friction by factors of 1.5 (true rotor) and 3.0 (eccentric rotor). Such effects had not previously been observed when using oils in the internal cylinder geometry (journal bearing type) although somewhat similar effects have been found in the external cylinder and squeeze film geometries. The rheological properties of the polymer-thickened solutions are measured and the relevance of the results to friction and load bearing discussed.

The flow of polymer thickened motor oils in convergent jet thrust nozzles

Abstract Jet nozzles are described in which liquids are subjected to uniaxial and biaxial stretching motions, shear also being present near the nozzle walls. Jet thrust measurements show that the behaviour of polymer-thickened oils can best be studied by comparison with the behaviour of Newtonian base oils at equivalent Reynolds numbers. These data compare well with measurements previously made in long, straight tubes. Jet thrusts for polymer-thickened oils are lower than those of base oils and show evidence of a threshold point at which the two forms of behaviour deviate. The axial stress in the oil is almost independent of the strain rate, at high strain rates, but is related to axial strain in a simple manner. For 3.5 per cent polymer dissolved in Iranian light base oil, the axial stress is shown to be τ11 = 2000 γG dyn cm−2 for uniaxial strain and τ11 = 2800 γ1.30G dyn cm−2 for biaxial strain, where γG is the measure of strain defined by Green. For a standard Shell Multigrade 20W/50 motor oil the equivalent equation for biaxial strain is τ11 = 3200 γ1.02G dyn cm−2. The stresses developed during biaxial extension are higher than those developed during uniaxial extension. The presence of fully elastic behaviour in polymer-thickened oils at extensional strain rates of the order 500 s−1 and principal extension ratios of only 8 is of potential significance to lubrication technology. The stress rises with the square of the extension ratio and should become large as lubricated surfaces approach. Polymer degradation will commence as the stress rises; it is suggested that further tests be carried out at higher stresses and temperatures.

Load enhancement effects by polymer-thickened oils in strip squeeze film flow

Abstract A modification to existing equipment is described which permits continuous squeeze-film flow to be obtained between parallel-sided strips of material rather than between disc-shaped surface. Squeeze film flow is simulated by having liquid move through one of the surfaces via an array of equispaced holes. The Squeeze-film behavior of a Newtonian base oil is first tested at temperatures of 24°C and 55°C. It is shwon that loads are in reasonable agreement with theretical predictions and that end effects (corrected by means of a guard ring) and fluid maldistribution effects are of small proporitons. At the very highest liquid flowrates, the rapid liquid flow through the holes may influence the measured load. The Polymer-thickened oils, representinhg 10 W/30 and 10 W/50 motor oils, are tested at temperatures of 24°C and 55°C. Both oils five marked load enhancement, compared with Newtonian oil under similar flow conditions, at the higher flowrates used. The 10 W/50 oil gives load enhancement of 76 per cent at 55°C, increasing rapidly with the simulated approach velocity. Fluid inertia effects in the squeeze film flow aslo increase the load significantly. The results confirm earlier data using disc-shaped surfaces; load enhancement is greater in the present work on strip squeeze films because the fluid deformation rates are greater (2000 s t-1 in planar extention and 2 x 10 5 s t-1 in shear). It is suggested that the fuel consumption of cars could be improved by the development of elastic, shear stable oils of lower viscosity than those currently in use.

The triple jet: A new method for measurement of extensional viscosity

SummaryApparatus for the measurement of extensional viscosity of polymer solutions is described. The range of extensional strain rates is 100–800 sec−1.Two converging liquid jets of high velocity impinge on a central low-velocity jet, causing the liquid in the central jet to be rapidly stretched. The force causing extension of this jet is measured from the change in reaction on the central capillary nozzle, whilst the initial and final velocities are obtained by jet thrust and photographic methods respectively. The Instron system provides accurately-controlled liquid flowrates and continuous recording of the various forces involved.Tests are reported on polyacrylamide solutions of concentration 0.10% and 0.07%, and on poly (ethylene oxide) solutions of concentration 0.10%. The extensional viscosity of the more concentrated polyacrylamide solution is over 30 poise at an extensional strain rate of 600 sec−1, exceeding the shear viscosity by a factor of 400 (the “Trouton” ratio). For the poly (ethylene oxide) solution, theTrouton ratio reaches 1400 at an extensional strain rate of only 200 sec−1.The results are shown to be consistent with orifice jet thrust data for similar solutions at higher strain rates. The axial stress rises rapidly with increasing extensional strain rate, but there is no evidence of an abrupt increase of stress in the range of strain rates between 102 and 104 sec−1. The triple jet method is of value in giving data for intermediate extensional strain rates and being dependent on fewer assumptions than the orifice jet thrust experiment.

Normal stress measurement by means of a jet thrust apparatus

SummaryApparatus is described which enables the normal stress-shear rate behaviour of a viscoelastic liquid to be obtained from observations of the thrust of a jet of the liquid issuing from a capillary tube (or slot). Two types of apparatus are mentioned. In one type the jet thrust is obtained from the impact force on a boat which the jet strikes and in the other the reaction force on the jet nozzle is measured.The assumptions which are made in converting the measured thrusts into normal stresses are discussed. In particular, it is shown that surface tension effects may be allowed for, and that there is no need to assume the validity of theWeissenberg hypothesis when parallel slots are used.A general discussion of the results obtained when using “Newtonian” liquids and polymeric solutions is given. Tests on a series of starch solutions are described in which normal stress data obtained using the jet thrust apparatus are directly compared with those obtained by means of the rheogoniometer.Several applications of the method are briefly given: a description of how the method may be applied to gases; measurement of the elastic properties of very dilute polymer solutions; and an examination of the entrance region by observing the effect of varying tubeL/D ratio on jet thrust.ZusammenfassungEine Apparatur zur Messung der Normalkraft, die bei der Scherung viskoelastischer Flüssigkeiten auftritt, wird beschrieben. Sie beruht auf der Beobachtung des Rückstoßes einer Kapillare (oder eines Spaltes), aus der ein Flüssigkeitsstrahl ausströmt. Zwei experimentelle Anordnungen werden beschrieben. Eine von diesen benutzt die Stoßkraft, die auf ein Schiffchen ausgeübt wird, um das Impulsmoment des Flüssigkeitsstrahles zu bestimmen, während die andere auf der Messung des Rückstoßes der Düse beruht.Die Annahmen, die gemacht werden, um die Stoßkraft in eine Spannung, die rechtwinklig zur Dehnungskraft steht, umwandeln zu können, werden diskutiert. Es ist bemerkenswert, daß Oberflächenspannungseffekte berücksichtigt werden können, so daß es nicht nötig ist, die Gültigkeit derWeissenberg-Hypothese vorauszusetzen, falls parallele Spalte benutzt werden.Eine allgemeine Diskussion der Ergebnisse an einfachen Flüssigkeiten und an Lösungen von Polymeren wird gegeben. Versuche mit Stärkelösungen werden erwähnt, wo mit der Rückstoßanordnung Spannungen in der Normalrichtung gemessen worden sind, die direkt mit denen, die mit einem Rheogoniometer erhalten wurden, verglichen werden.Mehrere Anwendungen der Methode werden kurz gestreift; zum Beispiel die Anwendung der Methode zu Messungen an Gasen, zur Bestimmung der elastischen Eigenschaften sehr verdünnter Lösungen von Polymeren und zur Untersuchung des Einlaufgebiets durch Beobachtung des Effekts, welchen dasL/D-Verhältnis der Düse auf den Rückstoß des Flüssigkeitsstrahls hat.

The triple jet: influence of shear history on the stretching of polymer solutions

Abstract A kerosene-based aircraft safety fuel and aqueous solutions of poly (ethylene oxide) and polyacrylamide are examined using the “triple jet” system. This device allows the solution to be stretched as it flows from a capillary tube and the axial stress, strain and strain rate in the liquid are measured. The shear history of the solution is altered by placing cylindrical inserts in the capillary tube. This is shown to have a large effect on the extensional behaviour of aircraft safety fuel, a moderate effect on the extensional behaviour of poly (ethylene oxide) solution and little effect on the behaviour of polyacrylamide solution. The extensional viscosity of the aircraft fuel is raised by an order of magnitude when a long period of high shear is used; the effects last for periods of up to one second, though traditional methods suggest a relaxation time of the order of 10−3 seconds. A liquid of shear viscosity 4 centipoise may have an extensional viscosity of over 100 poise. Plots of the extensional modulus of the jet as a function of distance along the jet emphasize the importance of shear history for the first two types of solution and suggest that the latter stages of the stretching process are elastic in character. Typical extensional moduli for the solutions tested are in the range 1.3–5.0 × 104 dyn.cm−2. The relevance of the interplay between shearing and stretching flow to the phenomena of lubrication and turbulence suppression is mentioned.

The flow of polymer-thickened oils in convergent jet-thrust nozzles at a temperature of 84°C

Abstract It is shown that an existing form of jet-thrust device may be modified for satisfactory use at elevated temperatures. Jets are produced from a straight capillary tube and from three nozzles designed to provide different rates of uniaxial extension in the flowing oil, with shear present near the nozzle walls. The behaviour of three simulated multigrade motor oils with additives of different chemical type is compared with that of Newtonian oils at temperature of 84°C. In straight-tube flow, no measurable normal stress is detected in one of the oils (that with an alkylmethacrylate as polymer additive), but the other two oils give stresses which are measurable and, in one case, as high as those obtained at ambient temperature (that with the styrene-butadiene copolymer additive). For these oils the normal stresses measured in tubes are much closer to the axial stresses measured in nozzles than was the case at ambient temperature. In nozzle flow, axial stresses are detected in each oil which are rather lower than those measured at ambient temperature, the deviation increasing with increased jet velocity. The relative importance of axial stress, compared with shear stress, is shown to increase with increasing temperature and shear rate. The ration of axial stress to shear may reach a value of 3 or 4 at a shear rate of 105set−1, the oils with styrene-butadiene and styrene-isoprene copolymer additives being somewhat better performers than that with the alkylmethacrylate copolymer additive. It is suggested that the presence of normal, or axial, stresses might improve lubrication performance in those situations where normal load is applied with little relative movement of the bearing surfaces.

Definitive load-enhancement effects by polymer-thickened oils in a squeeze film experiment

Abstract The continuous-flow squeeze film apparatus has been adapted for use at elevated temperatures. The plates are circular, and the movement of one plate towards the other in the traditional squeeze film case is simulated by having liquid moving through a stationary lower plate, with neither plate moving. The liquid exudes from 1580 holes uniformly distributed over the lower plate surface. Tests using a straight mineral oil at temperatures of 24°C and 75°C are described: these are used to confirm the satisfactory operation of the apparatus and to check the equations governing the load-bearing characteristics of the system. Attention is focussed on possible fluid maldistribution effects and load enhancement effects due to the inertia of the fluid. Three polymer-thickened oils, simulating 10W / 30 and 10W / 50 motor oils, are tested at 24°C and 75°C. One oil, containing an olefin copolymer additive, is markedly elastic at 24°C and also shows load enhancement effects at this temperature despite the flow limitation of the apparatus. All oils show some degree of load enhancement at high approach speeds and a temperature of 75°C. Load increases markedly with increased simulated approach speed, as predicted by theory. It is suggested that low viscosity polymer-thickened oils should be developed which protect the car engine to the same extent as current oils but which significantly reduce the overall fuel consumption.

An experimental test of equations predicting the load bearing capacity of squeeze films of elastico-viscous liquid

Abstract A form of squeeze film apparatus was recently described in which the movement of one plate towards the other was simulated by the continuous volume generation of liquid over the plate area. The liquid exuded from a large number of holes in the lower plate surface and formed a “continous flow” version of squeeze film apparatus with no moving parts [1]. A later paper gave derivations of equations from which squeeze film load bearing capacity could be evaluated, taking into account viscous, inertial and normal stress effects in the liquid film [2]. In order to find the total load in a squeeze film system, it was necessary to obtain the relationship between the first normal stress difference and shear rate for the liquid in use, using an experimental method. At high shear rates, the jet thrust method provided these data [3,4] and from them the load bearing capacity of squeeze films of hot, polymer-thickened oil were predicted [2]. A more complete test of the method is possible with a highly elastic liquid because considerable load enhancement due to extra stress is present at moderate deformation rates in squeeze film systems [1,5,6,7]. Thus a 0.1 per cent aqueous polyacrylamide solution gives well-defined load enhancement and (quite independently) the jet thrust method gives the relationship between normal stress and shear rate from which predictions of load enhancement may be made. Furthermore, convergent nozzles may be used in the jet thrust apparatus [3] to measure the stress development in an elastic liquid which is being simulateneously sheared and stretched, a situation which more closely resembles the squeeze film case than that of steady shear.

The horizontal stretching of test liquid M1

Abstract Tests are described in which a polybutene/kerosene-based liquid (supplied by Dr. T. Sridhar) is stretched horizontally in a simple new apparatus. Liquid issuing from a nozzle is pulled horizontally onto the surface of a cylinder rotating about a vertical axis. Tests are of two types. In the first type, photographs of stretching jets are taken and the stress, strain and strain rate found at equal intervals along the jet. In the second type, with shorter filaments, the speed of rotation of the cylinder is raised progressively and the same quantities are determined over the full length of the jet. The extensional viscosity rises with distance along the jet and, for the two longer jets, reaches a steady level of the order 104 Pa s towards the high-speed end of the jet. This corresponds to a Trouton ratio of 3000; values of up to 6000 are obtained during experiments of the second type. A plot of extensional stress against extensional strain rate shows marked scatter of the experimental points, whilst a plot of extensional stress against total strain gives lower gradients for experiments of the first type than for those of the second type. It is clear from this plot that decreased time of stretching produces increased stress and it is shown that total strain divided by total stretching time controls the extensional stress, under the conditions used. Using this as a measure of extensional strain rate, the extensional viscosity for all tests becomes 110 Pa s, giving a Trouton ratio of 35. The test fluid is stated to combine the properties of a liquid with those of a weak rubberlike solid. The results are discussed and it is suggested that interested workers should contact the author in order to obtain copies of the raw data. Extensional strain rates vary from 2 to 17 s−1 and stretching times from 80 to 700 ms.