Purpose. The numerical study of the drill mud flow speeds field at the working face of a borehole. Substantiation of methodology to calculate the convection heat transfer rates on the surface of a diamond core bit during its cooling. Methodology. Computational fluid dynamics methods (CFD modeling) were used in the course of the study. Findings. The physical and mathematical models of fluid mechanics and heat transfer processes at the working face of a borehole during its boring with diamond core bits are suggested herein. The field of circulating drill mud flow speeds around the diamond core bit at the drilling of rock formation was obtained. Regarding the convective heat exchange, the computer simulation demonstrated that a notable change of flow speed field during the boring occurs throughout the height of the bit. In the course of the bit rotation the flow speed field can be considered uniform alongside the azimuthal direction. The ultimate non-uniformity of the speed field is revealed in the water hole passages of the drilling core bit, which results in its heavier wear. Based on the distribution speed calculation results, the convection heat transfer coefficients were defined throughout the height of the drilling core bit. The study revealed that, owing to significant acceleration of the flow, the most intense heat exchange processes took place in the waterhole passages. Originality. The theory of fluid mechanics processes at the working face of a borehole during the boring was further developed. New data were obtained on the distribution of the flow speed field and pressure field in the circulating fluid at the bottom-hole area. The non-uniform nature of the fields was demonstrated herein. It is shown that, due to the locally uniform field of speeds distribution, it is possible to assume that the bit convection heat exchange ratios are constant in the certain areas. Practical value. The outcomes of the fluid mechanics field modeling can be used for determination of the optimum sizes and shapes of the waterhole passages in the course of designing new boring tools and equipment. The obtained heat exchange ratios enable to carry out calculations of temperature fields in the crown bit body, which is necessary to establish the resource and power saving modes of drilling.

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