Effect Of Frictional Heating Research Articles

The thermofrictional oscillations under sliding

We have studied the stability of sliding, taking into account the frictional heating effect. The new - thermofrictional - sliding instability is found to exist; instability threshold and dynamics differ qualitatively from the analogous characteristics of the “stickslip” phenomenon. It is shown that stable thermofrictional oscillations can occur on the nonlinear stage of the sliding instability development.

Effect of Frictional Heating and Compressive Work in Rotating Axisymmetric Flow

Effect of Frictional Heating and Compressive Work in Rotating Axisymmetric Flow

Layered convection induced by phase transitions

We report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection. Two‐dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the “extended Boussinesq approximation,” i.e., considering the effects of adiabatic gradient, latent heat, and frictional heating in the energy equation. We find that the critical value of the negative Clapeyron slope, which must be surpassed in order to induce layered convection, decreases in magnitude with increasing Rayleigh number Ra in the range 104 ≤ Ra ≤ 2×106. Near the critical Clapeyron slope, vacillations between double‐ and single‐layer convection or strongly leaking double‐layer convection are possible. The breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences. The value of the critical Clapeyron slope also seems little affected by the width of the transition zone or by its depth. A possible superplastic rheology within the transition zone would tend to favor layered convection. Scaling the model results to the 670‐km discontinuity in the earth's mantle as a possible endothermic phase boundary, we estimate the critical Clapeyron slope to be in the range of −4 to −8 MPa/K (−40 to −80 bar/K). The possibility that the spinel → perovskite + periclase transition is within this range appears to be remote but certainly cannot be neglected.

Millstone Hill incoherent scatter observations of exospheric temperature over 25 to 60 degrees north latitude

Elevation scan observations made by the Millstone Hill incoherent scatter radar are analyzed to produce local‐time/latitude maps of exospheric temperature. By lowering the antenna elevation to 4 degrees, observations in the thermosphere are possible over the latitude range 25 to 60 degrees north. These radar maps of exospheric temperature provide a new capability for the simultaneous monitoring of the detailed temporal and spatial morphology of the thermosphere. The combination of such maps from different radars promises to provide a powerful new tool for global thermospheric studies. The results for two multi‐day experimental periods in March and November 1982 are presented and described. During magnetically quiet periods the latitudinal variation of exospheric temperature is observed to be small in comparison with the diurnal variation. During disturbed periods large ion frictional heating events are observed at high latitudes and may extend even to mid latitudes during the more energetic events. These frictional heating effects upon the ions severely contaminate and exaggerate the deduced neutral temperatures. The second of the two November days showed substantially higher temperatures than the first even at mid and low latitudes. It is suggested that the high level of magnetic activity commencing on the prior evening and persisting for more than 24 hours served to generate an aurorally driven circulation cell that deposited auroral energy throughout the radar field of view.

The Effect of Periodic Loading on the Shear Strength Properties of Thin Organic Polymeric Films

The paper commences with a description of a technique used to obtain the shear strength properties of thin (ca 100nm) solid polymeric films. The effect of velocity in this sliding experiment is rationalized in terms of the contact time in which a load is applied to an element of material and the shear strain rate in the film. The velocity range that may be employed is small in order to minimize the effects of frictional heating which somewhat restricts the investigation of the relative influences of these two parameters. A new experimental technique is described that may be used in conjunction with the conventional sliding system so as to elucidate uniquely the contact time effect. The method involves modulating the normal load with a small sinusoidal component, and while the experiment has not been analyzed rigorously, the accumulated data suggest that the response of the polymer film is dependent on the period in which it is loaded and that retardation in compression occurs with all polymer films to var...

Temperature distribution in a laminar circular jet

The effect of frictional heat on the temperature distribution in a laminar circular jet has been studied. It is found from the analysis and the graphs that as the Prandtl number decreases from unity the overall temperature difference near the axis of the jet increases but as we move away from the axis it goes on decreasing. The reverse phenomenon happens in the case of increasing Prandtl number.

Thermal Model for Subduction with Dehydration in the Downgoing Slab

A finite-difference numerical program has been constructed to model the thermal structure of a subduction zone, including the combined effects of frictional heating and the heat sink created by dehydration of the subducted oceanic crust. Frictional heating is assumed to begin at the surface and to decrease linearly to zero at the depth at which dehydration begins. Dewatering of the oceanic crust occurs in the 80-125 km depth range and removes approximately 50 cal/gm of oceanic crust dehydrated. In young (<60 m.y. old) island arcs, isotherms are sufficiently elevated by frictional heating to melt the slab, perhaps accounting for the early tholeiitic volcanism recognized in some arcs. In old subduction zones, however, the frictional heating term must be increased to the equivalent of > 10 Kb shear stress (at the surface) before isotherms below the dehydration zone are elevated above the level predicted by conduction models. For lower stresses, the slab at $$\leq 150$$ km depth never becomes hotter than 600°C and thus does not melt. Alternative heat sources for arc volcanism are convection of asthenosphere above the slab and lowering of the melting point of overlying peridotite by release of water from dehydration of the subducted oceanic crust. In either case, the released water is probably a major transporter of Si, Na, K, Rb, Sr, Ba, and REE from the oceanic crust to the overlying wedge.

Film cooling by oblique slot injection in high-speed laminar flow

Wall-cooling effectiveness is investigated for oblique injection of coolant through single or multiple wall slots into a high-speed laminar compressible boundary layer by numerical solutions of the boundary-layer equations. A grid control procedure which maintains a constant flow rate between grid lines is found to be well suited to the present injection calculations wherein the boundary-layer growth in the slot is as much as a hundred-fold and the logitudinal component of the injection velocity is in some cases as large as the freestream velocity. Film-cooling effectiveness is reported for a variety of injection configurations so that the effects of coolant mass flow rate, injection angle, upstream boundary-layer thickness, slot width, and the presence of upstream cooling slots can be investigated. For the coolant mass flow rates considered, normal injection provides better cooling than tangential injection, particularly when frictional heating effects caused by tangential injection become a dominant consideration. However, the excessive boundary-layer growth which accompanies normal injection may reduce aerodynamic performance, thus making inclined injection a desirable compromise.

Free convection flow of an elastico-viscous liquid through a vertical circular cylinder

The problem of free convection flow of an elastico-viscous liquid through a vertical circular cylinder at constant temperature including the effects of frictional heating and distributed heat sources or sinks has been considered. The equations of motion and energy have been solved by an iteration technique. The effects of the elasticity of the liquid, buoyancy forces and heat sources or sinks on the velocity and temperature fields, skin-friction and Nusselt number have been discussed.

Effect of frictional heating on brake materials

An exploratory investigation was conducted concerning materials and their properties for use in aircraft brakes. The overall objective of the program is to develop improved brake materials primarily from a safety point of view. In this investigation a study was made in order to determine what improvements could be made in present brake materials. Primary consideration was given to the heat dissipation. Used brake pads and rotor disks, taken from aircraft at overhaul were analyzed to provide additional information on material behavior. A simplified analysis was conducted in order to determine the most significant factors which affect surface temperature. Where there are size and weight restrictions the specific heat and maintaining the contact area appear to be the most important factors. A criterion is suggested for determining the number and thickness of brake disks, within the limited space available in a wheel, to provide an optimal compromise between effective surface temperature of the disk and weight. This criterion is that the quantity ( αt max l 2 ) 1 2 should be approximately equal to 1 where α is the thermal diffusivity, t max is the time at the end of the braking cycle and l is half the disk thickness. For lower surface temperatures maximum benefit can be obtained by using materials of high specific heat (and density) and by maximizing the contact area.

The effect of forced flow on heat transfer from a disc rotating near a stator

This paper describes the application of the Spalding-Patankar numerical integration procedure to the case of heat transfer from an air-cooled disc rotating close to a stationary casing. Results are calculated to show the effect of frictional heating, arbitrary disc temperature distributions and non-unity Prandtl numbers on heat transfer, and these effects are shown to be qualitatively similar to those predicted by Dorfman for a free disc. Mean Nusselt numbers, calculated for a range of flow parameters, tend to the empirical correlation of Kapinos at high Reynolds numbers but diverge at lower Reynolds numbers. The latter divergence is consistent with the weak dependence of Nusselt number on rotational speed noted by Mitchell and Kreith at low Reynolds numbers.

Ionospheric warming by neutral winds

The effect of frictional heating by means of neutral winds on the ion and electron temperature in the undisturbed ionosphere is studied theoretically by solving a system of basic ionospheric and atmospheric equations. The study shows that both the electron and ion temperatures are increased in the night-time ionosphere through friction. In the region between 150 and 200 km T i may exceed T 6 by as much as 130°. The increase of T i due to friction amounts to about 100–200°, depending on the atmospheric model employed in calculating the neutral wind velocity. It is illustrated that frictional heating may be very important for the determination of the neutral temperature from measured ion temperature values.

Temperature profiles for polymer melts in tube flow. Part II. Conduction and shear heating corrections

AbstractA temperature probe system to measure radial temperature profile of polymer flowing in a rod die and a method to systematically correct the conduction and the frictional shear effects were developed. Experimental data obtained on a 1‐1/2‐inch extruder using a blow molding compound show that both conduction and frictional shear heating effects are significant in melt temperature measurement and that the radial temperature profiles of the melt in the rod die are influenced by the RPM of the screw and the die‐wall temperature.The reliability of the temperature data obtained was compared with the solution obtained from the equations of motion and energy. A good agreement between the predicted versus experimental temperature profile exists. For this polymer system, the relationship between local Nusselt number and the velocity parameter could be adequately described with the theory of Van LeeuWen.

Boundary layer models of ocean floor spreading

The equations of conservations of momentum and energy scaled with the characteristic values of the mantle indicate the presence of the upper boundary layer to produce the estimated rate of the ocean floor spreading by convection and the importance of the frictional heating. The depth of the upper boundary layer can be estimated from the balance of the viscous force with the horizontal pressure gradient at the sea floor. It is of the orders of 100 km and becomes deeper for the Pacific than for the Atlantic Ocean and also with frictional heating than without it. The frictional heating increases the surface heat flow of the heat conduction by ten to twenty percent for the Pacific Ocean but only by a few percent for the Atlantic Ocean. The similarity solutions are determined for the temperature and horizontal velocity in the upper boundary layer. These solutions are expressed in power series of the variabley x−n, wherex, y, andn are horizontal and vertical coordinates and numerical constant, respectively. Both temperature and horizontal velocity within the boundary layer are higher for the Pacific than for the Atlantic Ocean. When a larger viscosity is applied, it causes the increase of horizontal velocity below the surface because of the surface boundary conditions of the finite velocity and of vanishment of the velocity shear. The higher horizontal velocity generates higher temperature because it advects hotter material from the mid-ocean ridge site. The direct effect of frictional heating on the temperature distribution of the similarity solution is almost negligible, since the shear zone is deep and near the lower boundary of the upper boundary layer. In the similarity solution, the surface heat flow which is increased by the frictional heating is given as the boundary value. The effect of the frictional heating is important below the mid-ocean ridge.

Thermal behavior of elastomers at high rates of tensile straining

AbstractThe results of high strain‐rate tensile straining of a variety of elastomers at a number of temperatures are reported. Temperature increases, ΔT, occurring within the sample were measured during the deformation; i.e., the technique of adiabatic stretching was applied. The use of well characterized rubbers of different tendencies to crystallize upon elongation gave information on the changes in molecular packing, of the contribution from entanglements to the effective crosslink density, and of the effect of chemical and structural factors on mechanical behavior during rapid extension. An analysis of the effect of frictional heating, of deformational energy conversion, and of the anticipated enthalpy of crystallization was performed. The experimentally obtained temperature increases, the magnitude of the above three main factors which contribute to ΔT, and the amount of strain‐induced crystallization found are in agreement with theoretical considerations.

The effect of humidity on the friction and life of unbonded molybdenum disulphide films

The friction characteristics of unbonded molybdenum disulphide surface layers have been studied as a function of ambient humidity, specimen temperature, speed and load. Measurements have been made of static as well as kinetic friction in order to distinguish the influence of frictional heating. The static friction coefficient increases with increasing humidity; this effect is much more pronounced at low specimen temperatures. For given specimens the static friction is a unique function of the relative humidity at the bearing interface. The results provide evidence that the process governing the changes in static friction is one of physical adsorption of water. A similar effect governs the kinetic friction; in this case the adsorbed water is partially desorbed by the friction heating and this results in a marked decrease in friction coefficient. This effect also explains the “transient” high friction coefficients reported previously. These occur at the beginning of sliding in humid air before frictional heating has any effect. No transients are found in dry-air conditions. The friction decreases with increase of load independently of the effect of frictional heating or of humidity. High humidity is also found to increase the wear rate of the unbonded lubricant layer.

Structures of Well Preserved Australite Buttons from Port Campbell, Victoria, Australia

Abstract. Complete and nearly complete australite buttons in good states of preservation from Port Campbell, Victoria, show excellent structural details and are of great scientific importance. Some of the features on their posterior surfaces are doubtfully assigned a primary origin in an extraterrestrial birthplace but have been modified by terrestrial solution‐etching. Secondary features on their anterior surfaces are due to the effects of aerodynamic frictional heating during transit with stable orientation at supersonic velocity through the earth's atmosphere. Tertiary processes such as subaerial weathering have played some part in slightly modifying their shape and sculpture patterns. They contrast strongly with the many thousands of australites collected from the arid and sub‐arid regions of Australia, and with a considerable number that were abraded by stream or gravity transportation in the more temperate zones of the strewnfield. The majority of such specimens have been more severely weathered with the resultant loss of much or all of their primary and secondary features.

Rheological Factors for Attapulgite Suspended in Water

Stress‐strain curves and stress‐relaxation curves were determined at various temperatures (1° to 70°C) and concentrations (16 to 22 wt%) for attapulgite suspended in water. Stress‐relaxation experiments were conducted under different conditions on samples which were relaxed for various durations of time between successive cyclic deformations. The flow properties are given in terms of rheopexy and thixotropy. The flow curve (for shear rates 0 to 700 per second versus shear stress) shows, first, a rheopectic hysteresis loop which changes on repeated shearing to a reversible and reproducible non‐Newtonian flow curve. After resting the system for about 12 hours, the resulting flow curve shows a thixotropic loop. Viscosity as a function of pH shows an inversion at pH ⋍ 6.3. A model for explaining the observed facts is proposed. The script S type of looped‐over flow curves showing both rheopectic and thixotropic characteristics is explained in terms of breakage of “logjammed” and crystalline bundles. A plot of the yield‐point viscosities versus the reciprocal of temperature for the rheopectic and thixotropic curves shows activation energies of about 1 and 2 kcal respectively. An autographic rotational viscom‐eter was designed and utilized to obtain the data reported so that the objectionable effects of frictional heating in the sample were minimized. The calibration response of the viscometer to a glycerol standard shows good agreement with published viscosity data.

Automatic Rotational Viscometer and High-Pressure Apparatus for the Study of the Non-Newtonian Behavior of Materials

The automatic concentric-cylinder rotational viscometer described here records flow curves of shear stress versus rate of shear. It provides an automatic means of increasing and decreasing the applied rates of shear and an automatic control of the time that each shear rate is applied to the sample. In as much as flow properties may change rapidly with time and shear rates, this automatic control is important for materials which are time dependent. Presently, with proper selection of the axle and transmission gears, the cup's rotational speed can be automatically varied from zero to 1830 rpm. A switch permits the operator to select a constant shear rate and record shearing stress changes as a function of time at any point of this range. By simply replacing the gear sets, a speed of approximately 2750 rpm can be achieved. The time required for the speed to change from zero to a preset maximum, at constant acceleration, is controlled by voltage changes through a Variac which affords a continuous range from 11 to 300 sec. Shear rates of up to 4090 sec−1 are obtained. The cup and bob are aligned coaxially and mechanically fixed. Added cup-bob features for bob-flushing effectively control effects of frictional heating within the sample. Other apparatus described here provides a means to measure the consistency of greases, near the ``pseudo solidification'' point induced by high pressures. The flow curves of greases which have a critical solidifying pressure reveal characteristics suggesting stiffer consistencies at increasingly high pressures. A 6-tank ``reservoir'' of hydraulic fluid is used where the fluid under compression and the tensile stresses in the steel of the tanks are used to completely eliminate the antagonistic ``ripples'' in the pressure which are caused by the action of the pump. Pressures of 0–50 000 psi are easily and safely obtainable with this apparatus.

Deformation, heating and melting of solids in high-speed friction

An experimental study has been made of the effects of frictional heating on the deformation of solids rubbing at very high speeds and at reasonably heavy loads. A new method for measuring the friction under these conditions is described. A steel ball, rapidly spinning round its vertical axis, is allowed to fall a short distance and to bounce off an inclined flat solid surface. The friction of steel on various solids in a vacuum of ca . 10 -4 mm Hg, at sliding speeds up to 700 m/s, is determined from the measured direction of the ball’s horizontal velocity after the impact. In addition, separate piezo-electric measurements are made of the load and the friction force. Again the coefficient of friction is found to decrease with increasing sliding speed. The general behaviour is similar to that observed at light loads but there are important differences. With heavy loads the deformation of the solids appears to be primarily plastic. Within a very short time after being brought into contact with a fast-moving surface, solids with a sufficiently low melting point melt on a large scale so that a continuous film of molten material is developed over the area of contact. The resistance to motion is determined primarily by this liquid film so that it may now increase as the speed rises. The heating due to the shearing of this film causes the solid to melt away rapidly, and as a result the wear rate of such solids usually becomes great at high sliding speeds. Certain polymers, however, exhibit a greater wear resistance than metals and other solids which possess a low viscosity in the molten state. Calculations indicate that in these polymers, owing to their high viscosity, the temperature of the sheared film may be considerably higher than the melting temperature. As a consequence, a larger proportion of the heat developed by the shearing may be absorbed in the already molten material, and less heat will be available for further melting. Gas liberated by thermal decomposition may also reduce the friction and wear.