The dissertation is devoted to the actual research and scientific – practical problem of development of friction units for tape –block brakes of drilling draw works.
According to the results of theoretical and experimental studies using the multicriteria method, the design and operational parameters of friction units are optimized and the efficiency of chamber nanofluid cooling of the pulley-belt brake pulley of the drilling draw works with electrothermomechanical frictional interaction in a single dynamic and thermal field is proved. The energy principles of the processes, phenomena and effects in mechanical, electric, thermal, chemical and electromagnetic fields of frictional interaction of friction pairs of brake devices are formulated. Taking into account the results of theoretical research on hierarchical energy levels in tribocouples, a classification is proposed that combines all processes, namely: intensity and distribution of heat fluxes, types of heat transfer, potentials, currents of the washing medium, gradients, rates of processes. Based on the thermal model of the friction pairs of the tape –block brake, a method for estimating the thermal balance of a folded pulley with an air-nanofluid cooling system has been developed. This takes into account the influence of double electric layers that occur at the phase boundaries and are accompanied by adsorption and diffusion processes that significantly enhance the triboeffect.
The nanofluid cooling of the folded pulley in laboratory bench conditions is modeled. According to the research results, the modes of nanofluid flow around the chamber walls have been established, which depend on the energy load of the brake friction pairs and the braking mode. It was found that the intensity and patterns of nanofluid circulation in the chamber are due to the inhomogeneity of the velocity fields and the volume temperature distribution. The forces of internal friction, the speed of convective movement of nanoparticles and the total forces of inertia of the flow, which correspond to its turbulent Reynolds regime, depend on them.
It is proved that in the presence of intensive heat exchange and diffusion there are thermal and diffusion boundary layers. In the thermal boundary layers, the temperature decreases in the direction from the chamber wall to the internal flow. The concentration of the vapor-liquid mixture changes in the diffusion boundary layer. It is established that the dynamic, thermal and diffusion boundary layers extend in the radial direction.
A new approach has been applied in the study of the efficiency of nanofluid cooling of the pulley rim. It was conventionally divided into five sections in the direction from the pinched edge to the free. Studies have shown that in the first section there is a single-phase convective heat exchange of the nanofluid with macroparticles of the inner surface of the rim, which are characterized by thermal and hydrodynamic stabilization. In the second zone, the boiling of the nanofluid begins, in the third there is the beginning of intense nanovaporization, as a result of which the heat transfer efficiency increases rapidly. In the fourth zone, the near-wall two-phase layers close, the mode of motion of the nanovalue mixture is bubble. In the annular flow of the fifth zone, superheated steam moves, there is a consistent change of flow regimes from bubble to disperse-ring, the intensity of heat transfer increases.
When combining the coolants in the chamber of the folded brake pulley, the energy load of its rim is reduced by 21%, as well as surface and depth temperature gradients, which completely eliminates the possibility of microcracks on the friction surface. The choice of nanoparticle type taking into account their physical and chemical properties is substantiated. It is established that in order to achieve the maximum cooling effect, the thermal conductivity of nanoparticles must be much higher than the material of the rim of the pulley and the walls of the chamber. The basic operational parameters of brake friction pairs have been improved: dynamic friction coefficient by 16%, brake torque by 15%. Durability of metal-polymer friction pairs of tape-pad brake increased by 25-30%, the amount of wear of friction pads decreased by 12-14%.
The cracking process in the layers of working surfaces of friction linings at high temperatures that significantly exceed the allowable has been studied. It is established that the intensity of the cracking process is influenced by temperature, specific loads, mutual overlap coefficient, presence of inert gases and chemical properties of the components of the surface layer of the polymer lining.
A triboelectric method for the selection of metals for brake friction pairs based on the valuesof their triboEMF, as well as taking into account the electronic properties and thermophysical characteristics, is proposed. This allows you to create friction pairs with a high triboeffect.
