Characterization of the Effects of Internal Channel Roughness on Fluid Flow and Heat Transfer in Additively Manufactured Microchannel Heat Sinks

As the power density of computing devices increases, advanced liquid cooling thermal solutions offer an attractive thermal management approach. In particular, the low thermal resistance offered by microchannel heat sinks used in liquid cooling systems may enable increased total heat dissipation within fixed component temperature limits. There has been extensive work on the design of microchannel heat sinks, with many recent efforts to explore novel geometries and emerging manufacturing techniques. Of particular interest is additive manufacturing to allow for designs having complex, non-traditional internal geometries and package structures that cannot be made through conventional means. Despite the potential benefits for design and construction, additive manufacturing introduces new geometric uncertainties that could affect device performance. Direct metal laser sintering methods suitable for printing metal heat sinks typically produce a high internal roughness and other shape deviations in the flow paths of the final part. This extreme relative roughness and potential tortuosity in fluid flow through additively manufactured microchannels could lead to significant deviations in pressure drop and heat transfer predicted with traditional correlations and models. This work seeks to characterize the effects of high relative roughness on the friction factor and Nusselt number in additively manufactured microchannels having a rectangular cross section. Straight microchannel samples of 500 µm, 750 µm, and 1000 µm channel heights, and aspect ratios from 1 to 10 were manufactured to identify the design dimensions that resulted in visibly open channels, albeit with deviations in cross-sectional shape for submillimeter channel sizes and high internal roughness. Heat sink test samples were then printed with an array of these microchannels connected in parallel by inlet and outlet headers. Using water as the working fluid, the pressure drop and heat transfer performance of these sample heat sinks were characterized to explore how their behavior deviated from conventional predictions assuming smooth-walled channels. Flow through these additively manufactured microchannels displayed higher pressure drops than predicted, as well as a flow rate dependence of the hydrodynamic and thermal performance. These observed deviations are explored as effects of the physical conditions inside the channel as a result of additive manufacturing. Severe constriction of the channel would account for the difference in magnitude between the experimental and predicted results, while the introduction of flow redevelopment could lead to a flow rate dependence.  By further understanding the impact of these artifacts and deviations, these factors can be accounted for in the design and modelling of more complex additively manufactured heat sinks.