ABSTRACT Managing thermal loads in medium‐ to large‐scale electronic systems remains a key engineering challenge, particularly under space‐constrained and passive‐cooling conditions. Heat sinks used in such systems are often subject to high heat fluxes, and their geometry, especially their height relative to the cavity, plays a critical role in determining cooling efficiency. Yet, the influence of heat‐sink height has received limited attention in jet‐assisted mixed‐convection configurations operating in strongly unsteady transitional regimes, where buoyancy and forced flows interact nonlinearly. This study investigates the effect of heat‐sink height, defined as a normalized ratio () of the total cavity height, on thermal performance within a confined enclosure subject to side air injection. Two identical heated blocks are placed at the bottom of a rectangular cavity with periodic lateral boundaries, lateral inlets ( each), and a central ceiling outlet. Simulations are performed for Rayleigh numbers of and , corresponding to strongly unsteady transitional mixed‐convection regimes relevant to compact electronic enclosures. The Cascaded Lattice Boltzmann Method (CLBM) is used as a high‐resolution numerical solver due to its numerical robustness and ability to capture the dominant unsteady flow structures associated with buoyancy‐forced flow interaction. Results demonstrate that the heat‐sink height substantially affects the convective flow structures, thermal stratification, and overall heat removal. Quantitatively, increasing the height ratio from the lowest to the highest configuration leads to an enhancement of the global mean Nusselt number of about 40% for and exceeding 70% for , highlighting the amplified role of buoyancy at higher Rayleigh numbers. Intermediate configurations () consistently yield the most balanced performance, minimizing thermal stagnation while maintaining regular and coherent flow structures. Both local and global Nusselt numbers are analyzed, and surface‐specific contributions are detailed. Although the results are subject to numerical uncertainties associated with unsteady regimes and temporal averaging, the observed trends remain physically robust and reproducible. A correlation between heat‐sink height, Rayleigh number, and global Nusselt number is proposed, providing a predictive tool for thermal optimization. This work extends prior CLBM studies by systematically assessing a geometry‐driven parameter under transitional mixed‐convection conditions, directly tied to real‐world thermal design. The findings offer physically grounded insights for developing more reliable and energy‐efficient passive cooling strategies, with relevance to compact enclosures, power electronics, and embedded thermal systems.
Amine et al. (Fri,) studied this question.