# Numerical Study of Coolant Flow Phenomena and Heat Transfer at the Cutting-Edge of Twist Drill
## 1 Introduction
The process of developing an effective drilling tool has always been difficult compared to other cutting processes. Numerous thermo-mechanical aspects such as cutting force, heat transfer, wear resistance, torsional and axial stability, and chip evacuation capability must be taken into account when studying drilling operations. Consequently, cutting parameters in drilling operations are often limited to heat dissipation capacity during machining. Furthermore, tool geometry, feed, bore hole quality, lubrication, and cutting materials have significant influence in prolonging tool life and improving the surface quality of the workpiece [16; 19]. [PERSON] and [PERSON] [20] have presented a comprehensive overview of the control factors and mechanisms in both conventional and deep hole drilling processes. Their work emphasizes that the effectiveness of the deep hole drilling depends on torsional rigidity of tool, efficient coolant supply, and proper chip removal. These processes are highly influenced by the cutting speed, application of cutting fluid, and depth of hole. In addressing these challenges, numerical modelling can be useful for investigating fluid dynamics and tribology at the interface of tool and workpiece. Due to the advancement of computational technology, numerical prediction, and optimization of tools, the efficiency of cutting process has been substantially increased. The computational fluid dynamics (CFD) modelling technique has proven to be very effective and convenient for studying real fluid flow and heat transfer in the complex cutting zone, especially when experimental testing and analysis is impractical [21]. Continuum-based CFD simulations can be modelled using two main approaches: Eulerian and Lagrangian. In the Eulerian approach, phases are considered as interpenetrating continua and modelled using different conservation equations and coupled with source terms. Hence, necessary constitutive equations must be applied to describe the interaction between the solid and fluid phases, accounting for solid stress contributions. On the other hand, in the Lagrangian approach, the fluid phase is assumed to be a continuum and solved by using time-averaged Navier-Stokes equations and external forces are taken into account as they act directly on each and every particle by solving the equations of motion. Many researchers have numerically studied the effects of coolants on drills by comparing the results of cutting temperatures, tool wear, bore quality, thrust force, and drilling torque [22; 23]. The most critical aspect of the cooling process is the flow of the cutting fluid, with its primary objectives being heat dissipation and the removal of chips from the cutting zone. In many of these investigations, the finite element method (FEM) approach is commonly applied, which is good for studying solid-state behaviour; however, it is less suitable when analysing coolant flows, particularly when turbulence occurs [2; 24]. In contrast, the application of computational fluid dynamics (CFD) in machining processes remains relatively limited. Only a handful of studies have delved into modelling the cutting process, encompassing the intricate interactions between the coolant and chip formation. This represents a challenging frontier in multiphysics modelling within the domain of machining. [PERSON] and [PERSON] [25] conducted a study on the internal cooling of carbide drills using a CFD simulation. In their predicted results the authors found that heat transfer depends on the cutting fluid flow rate and the exit position of the cooling channels. In a related study, [PERSON] et al. [7] employed a CFD-based simulation to investigate the distribution of coolant lubricant in internally cooled carbide drills. The authors introduced a methodology aimed at enhancing wear resistance and coolant efficiency through a modified flank face geometry. The predicted results demonstrated detailed information of the cooling fluid flow, and consequently, heat dissipation at the tool-workpiece interface subjected to high thermal loads.