Natural convection within enclosures for thermal management in low-pressure environments

To address the need for a comprehensive understanding of thermal dynamics in electronic systems aboard airborne payloads, especially within the unique thermal environment characterized by low air pressure (P-air), this study investigates the impact of low air pressure (1 kPa < P-air < 100 kPa) on natural convection within vertical enclosures of varying aspect ratios (0.1 < Ar < 10). As electronic systems continue to rapidly miniaturize, the Rayleigh number (Ra-w) associated with natural convection in such systems falls into a previously unexplored range. The results reveal that even at small air pressures, with Ra-w as low as 1000, the value of Nu(av) remains significant. This underscores the importance of accurately predicting heat transfer through natural convection, as it can contribute to the reduction of overall weight and size of the thermal management system and payload. As Ra-w increases, the fluid flow within enclosures undergoes a transition from a horizontal boundary layer regime to a vertical layer regime, encompassing shallow (Ar < 1), square (Ar = 1), and rectangular (Ar > 1) enclosures. The study provides valuable insights into this transition across different aspect ratios, shedding light on the behaviour of natural convection in airborne payloads subjected to varying air pressures. The study proposes numerical correlations that predict the onset of natural convection and capture the dependence of average Nusselt number (Nu(av)) on Ra-w and Ar. These correlations offer practical applications for designing efficient thermal management systems tailored to the unique challenges posed by the thermal environment in airborne payloads, ultimately enhancing the performance and reliability of electronic systems in such settings.