Rheological properties of lubricating fluids

Sammanfattning: Obliteration (gobbing) caused by water based lubricating fluids is studied experimentally by analyzing the flow through a small channel. Flow measurements, energy dispersive X-ray microscopy and photos show, that the rate of gobbing depends on which type of fluid that is being used. The explanation of the obliteration is chemical reactions between the fluids and surrounding material or dust in the fluids. In order to get a better understanding of the rheology of emulsions, the migration of visible spheres, with varying density, flowing downstreams through a vertical, rectangular channel, is studied. The results from the experiments show, that the equilibrium position of a sphere is a function of the density difference between the sphere and surrounding fluid. A density difference of 0.5 per mille is enough to give stable positions. A theory is presented, which gives good agreement with the experimental results. Experiments have also been carried out in order to measure the effective viscosity of emulsions, flowing through a small horizontal channel. The viscosity is lower than the value obtained in a rotational viscosimeter. A theoretical explanation is proposed. An interesting example of two-phase flow is studied by means of a new prototype of a nozzle which lubricates a water jet with air. With this experimental nozzle, the dynamic pressure of the water jet increases 37 per cent. The explanation of this improvement is that some of the air forms a layer between the tube wall and the water, thus reducing friction between wall and water, giving a higher velocity of the water jet. With the results from the experimental nozzle in mind, five different full scale water cannon nozzles are designed, each with a certain method of air injection. The casting length, water flow and power consumption are measured and compared to a conventional nozzle. The performance is strongly dependent on the rate of water flow. For water flows of order 7 dm3/s, an increase in casting length of 12 per cent was achieved compared to a conventional nozzle. To achieve the same casting length, a conventional nozzle needs 39 per cent more power. Unfortunately the water flow with the new nozzle decreased with up to 48 per cent. For water flows of order 3 dm3/s, an increase in casting length of 93 per cent was achieved. To achieve the same casting length, a conventional nozzle needs 90 per cent more power. The water flow with the new nozzle decreased with 36 per cent. The final paper is an investigation of the lubrication between a plate and a rotating ball in normal approach. The normal velocity was varied between 0.1 and 0.5 m/s and the sliding velocity between 0 and 9.2 m/s. The experiments show, that the viscosity is the most important lubricant parameter. The normal velocity, the pressure viscosity coefficient, and the shear strength proportionality constant do not affect the degree of lubrication. An increase of the sliding velocity gave a decrease of the degree of lubrication between 25 and 65 per cent depending on surface roughness and type of lubricant. The surface roughness is also of great importance in getting good lubrication. To avoid wear, one has to increase the viscosity from 8 to 145 mm2/S if the mean surface roughness, Ra, is increased from 0.01 to 0.14.