Numerical Simulation of Pressure Loss and Heat Transfer Characteristics for Additively Manufactured Channels

This study provides best practice guidance for near wall mesh resolution while implementing surface roughness in a numerical simulation of an internal cooling passage.  Increased use of additive manufacturing or 3D printing techniques for turbomachinery components enable the manufacture of complex features to achieve higher operational performance. Accurate modeling of flow losses and heat transfer effects are critical to designing parts which achieve optimal efficiency paired with durability. Surface finish is rougher compared to more traditional manufacturing techniques; therefore enhancing the pressure loss and heat transfer effects. Proper implementation of surface roughness within the computational model and correct modeling of the near wall boundary mesh must be maintained to produce accurate results. This study focuses on the comparison of near wall mesh treatment coupled with surface roughness to determine a practice for obtaining accurate pressure loss and heat transfer within a cooling passage as compared to measurements.

With Fluent as the flow solver, a steady-state computational fluid dynamics (CFD) model consisting of a wind tunnel inlet nozzle and outlet diffuser, along with internal cooling passages represented using small channels, will have varying near wall mesh treatment for a range of Reynolds numbers. The near wall mesh treatment varies from wall integration formulation, recommended when modeling heat transfer, and wall function formulation, which is recommended when surface roughness is included in Fluent. Analysis of a baseline configuration with aerodynamically smooth walls is first compared to the measured data to verify the assumption of aerodynamically smooth walls. Surface roughness is then added to the channel walls, from published test coupons, and compared to published measured data for the range of Reynolds numbers with different near wall treatments. Due to the additive manufacturing process the surface roughness of the coupons varies between the vertical and horizontal faces, which is included in the CFD model. The metal surrounding the passages is also included as a conjugate heat transfer model providing heat addition to the fluid. Pressure loss and heat transfer is compared to the measured data as a friction factor and Nusselt number for the range of Reynolds numbers. Since surface roughness units vary an effect of surface roughness values on pressure loss and heat transfer will also be investigated to determine the importance of using and converting to the correct units. This will generate a guideline that will help when both heat transfer and surface roughness are included in a CFD model.