Flow into a hole in relation to laser drilling: influence of coating thickness

Abstract

Purpose – In laser drilling applications, hole wall remains almost the melting temperature of the substrate material and the thermodynamic pressure developed at high temperature molten surface vicinity influences the heat transfer rates and the skin friction at the surface of the hole wall. This effect becomes complicated for the holes drilled in coated substrates. In this case, melting temperatures of the coating and base materials are different, which in turn modifies the flow field in the hole due to jet impingement. Consequently, investigation of the heat transfer rates from the hole wall surfaces and the skin friction at the hole surface becomes essential. The paper aims to discuss these issues. Design/methodology/approach – Numerical solution for jet impingement onto a hole with high wall temperature is introduced. Heat transfer rates and skin friction from the hole wall is predicted. The numerical model is validated with the experimental data reported in the open literature. Findings – The Nusselt number attains high values across the coating thickness and it drops sharply at the interface between the coating and the base material in the hole. Since fluid temperature in the vicinity of the substrate surface is higher than that of the wall temperature, heat transfer occurs from the fluid to the substrate material while modifying the Nusselt number along the hole wall. This results in discontinuity in the Nusselt variation across the coating-base material interface. The Raighly line effect enhances the flow acceleration toward the hole exit while increasing the rate of fluid strain. Consequently, skin friction increases toward the hole exit. The influence of average jet velocity on the Nusselt number and the skin friction is significant. Research limitations/implications – The findings are very useful to analyze the flow field in the hole at different wall temperature. In the simulations hole diameter is fixed in line with the practical applications. However, it may be changed to examine the influence of hole diameter on the flow field and heat transfer. However, this extension be more toward academic study than the practical significance. Practical implications – The complete modeling of turbulent flow jet flow impinging onto a hole is introduced and boundary conditions are well defined for the numerical solutions. The method of handing the physical problem will be useful for those working in the area of heat transfer and fluid flow. In addition, the importance of heat transfer rates and skin friction at the hole wall is established, which will benefit the practical engineers and the academicians working in the specific area of laser machining. Social implications – The findings are useful for those working to improve the laser technology in the machining area. Originality/value – The work presented is original and never being published anywhere else. The findings are reported in detail such that academicians and engineers are expected to benefit from this original contribution.