Transient conjugate cooling in a confined cylindrical cavity with annular multi-jet impingement

Purpose The purpose of this study is to investigate the start-up (0–3 s) transient conjugate cooling behavior in a confined cylindrical cavity driven by annular-plenum multi-jet impingement. Although jet impingement cooling has been extensively studied under steady-state conditions, existing research has primarily focused on single-jet configurations or simplified open domains, with limited attention paid to the start-up thermal response of confined multi-jet systems. In practical high heat-flux applications, however, the transient cooling capacity during the initial operating stage is often critical for preventing local overheating and thermal failure. To address this gap, a normalized cooling index (NCI) is induced to transient cooling for evaluating both mean cooling and hot-spot suppression. Design/methodology/approach A three-dimensional transient conjugate heat-transfer model is developed using unsteady Reynolds-averaged Navier–Stokes (URANS) simulation with the SSTk-ω turbulence model. An initial-temperature treatment is introduced for the solid domain to effectively capture the start-up thermal evolution of the coupled fluid–solid system. Grid independence and model reliability are validated against a benchmark case of a circular impinging jet. The influence of Reynolds number (Re),), inlet temperature (Tin) and geometric height ratio (H/D) are systematically investigated. The proposed NCI is defined by combining the average solid temperature T¯s and the maximum local temperature (Tm), allowing simultaneous evaluation of mean cooling performance and local thermal non-uniformity. Findings The start-up cooling process exhibits a two-stage behavior. The early stage is convection-dominated and characterized by stagnation impingement, wall-jet development and cavity recirculation, whereas the later stage is increasingly limited by solid thermal capacitance and internal conduction. Increasing Re improves both mean cooling and hot-spot suppression; however, the enhancement becomes progressively weaker due to confinement effects, jet interaction and crossflow interference. Variations in Tin mainly affect the absolute temperature level and show little influence on NCI within the range of 283–303 K. A moderate geometric height ratio (H/D≈ 0.5) provides the best overall cooling performance by balancing jet impingement intensity and flow recirculation. The optimal operating condition is identified as Uin = 40 m/s, Tin = 283 K and H/D = 0.5. Originality/value This study extends conventional impingement-cooling research from steady-state analysis to start-up transient conjugate cooling in a confined cylindrical cavity with annular multi-jet impingement. It clarifies the stage-dependent cooling mechanisms governing transient thermal evolution and introduces NCI as a unified dimensionless metric for assessing overall cooling performance and hot-spot mitigation. The results provide theoretical support and design guidance for thermal management in confined high heat-flux systems requiring rapid and reliable start-up cooling.