International Communications in Heat and Mass Transfer 137 (2022) 106162 Available online 8 July 2022 0735-1933/© 2022 Elsevier Ltd. All rights reserved. Capillary boosting for enhanced heat pipe performance through bifurcation of grooves: Numerical assessment and experimental validation Samet Saygan a , Yigit Akkus a, b , Zafer Dursunkaya c , Barbaros Cetin d, * a ASELSAN Inc., 06200 Yenimahalle, Ankara, Turkey b Ericsson AB, 164 40 Kista, Sweden c Department of Mechanical Engineering, Middle East Technical University, 06800 Cankaya, Ankara, Turkey d Mechanical Engineering Department, ˙ I.D. Bilkent University, 06800 Cankaya, Ankara, Turkey A R T I C L E INFO Keywords: Grooved heat pipe Groove bifurcation Hierarchical wick Tree-like fractal architecture H-PAT Capillary boosting ABSTRACT In this study, an enhanced heat pipe performance for grooved heat pipes has been demonstrated through capillary boosting with the introduction of the bifurcation of grooves. Wider grooves regularly branch to nar- rower grooves such that the total cross-sectional liquid fow area remains approximately the same. Following the computational framework drawn by a recently developed heat pipe analysis toolbox (H-PAT), we develop a numerical model for the heat pipes with tree-like groove architecture. Then we utilize the model to design a fat- grooved heat pipe with one step groove bifurcation at the evaporator. To verify our numerical fndings, two heat pipes with and without groove bifurcation are manufactured and experimented under the same conditions. Experimental results show that the numerical model can predict the thermal performance quite accurately. The results reveal that groove bifurcation can be a viable option for a better thermal performance than that of heat pipes with standard grooved heat pipes with straight grooves which leads to at least 25% higher maximum heat transport capacity. The effect of number of branching on the temperature fattening across the heat pipe is also demonstrated for different evaporator lengths. 1. Introduction Continuous developments in semiconductor fabrication techniques have resulted in the integration of exponentially increasing numbers of transistors on integrated circuits (ICs) [1]. Moreover, Dennard scaling [2], which suggested the constant power density of transistors regardless of the decrease in their dimension, broke down because of the current leakage at small sizes [3]. The overall consequence is the formation of enormous heat fux values, which makes thermal management a chronic issue. Even in the post-Moore era, this issue still worsens due to the use of vertical direction for transistor integration in three-dimensional ICs. The key strategy of dealing with the heating issue is to increase the heat spreading ability of the thermal architecture. Apart from the geometrical optimizations, the upper bound of heat spreading is dictated by the thermal conductivity for solids. In the case of cooling methods adopting single-phase coolants, achievable pumping power restricts the cooling performance. Cooling methods adopting two-phase coolants, on the other hand, utilize the advantage of liquid-vapor phase change, which enables the transport of a substantial amount of energy at near-constant temperatures. Two-phase passive heat spreaders, namely heat pipes and vapor chambers, merge the advantages of latent heat of vaporization and passive fuid pumping. Consequently, they are positioned as the core elements of thermal management in the design of electronics in both terrestrial and aerospace applications. While the vapor transport is driven by the concentration gradient, two-phase passive heat spreaders utilize the capillarity of the wick structures for the liquid pumping. In addition, wick structures provide the sites for evaporation and condensation, thereby playing crucial roles for phase change dynamics. Eventually, the wick structure determines the performance of two-phase heat spreaders. Mesh, sintered, and grooved wicks are traditional wick types each of which has the certain advantage over others commonly utilized in two- phase heat spreaders. In fact, the wicking performance is governed by the interplay of capillarity and permeability of the wick [4]. When the feature size of the wick decreases, capillarity enhances due to the for- mation of smaller-sized menisci inside the capillaries. Consequently, enhanced Laplace pressure provides higher liquid pumping. Sintered wicks composed of micrometer-scale grains are examples of the wicks * Corresponding author. E-mail address: barbaros.cetin@bilkent.edu.tr (B. Cetin). Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt https://doi.org/10.1016/j.icheatmasstransfer.2022.106162 Received 15 March 2022; Received in revised form 19 May 2022; Accepted 28 May 2022