A mass rate-of-rise model for additively manufactured wick structures

Abstract

The hydraulic performance of heat pipes is typically characterized by the ratio of the wick's permeability to effective pore radius, K/reff. To experimentally quantify these values, mass rate-of-rise (mROR) testing is performed by dipping the tip of the wick into a liquid reservoir to track liquid uptake; the experimental data is then fitted to an ROR model. For conventional wicks, the gravity-based m-t model is free of user decisions and the most reliable. However, here we show that this model poorly fits ROR data for additively manufactured (AM) wicks, resulting in underprediction of K/reff. Experimental tests were conducted using 2 AM wicks—grooved and simple cubic—fabricated by laser powder bed fusion. Visual observations revealed that AM wicks exhibited a superposition of two wicking behaviors: a primary flow and microgroove/corner flow. The primary flow quickly fills the innermost area of the wicking structure. In contrast, the secondary flow gradually creeps up the 3D-printed strut surface through the printed fine scale features. A third term was introduced to improve the data fitting, based on previous research on capillary rise in corners. The revised model measured the AM grooved and simple cubic wicks' K/reff values as 1.23 μm and 1.02 μm, respectively.