Numerical Modelling of Local Climate Inside Electronics Enclosures

Condensation and moisture related problems are the cause of failures in many cases and consequently serious concerns for reliability in electronics industry. Therefore, it is important to control the moisture content and the relative humidity inside electronics enclosures. In many applications electronics enclosures are exposed to uncontrolled environmental conditions. Understanding the effects of harsh ambient conditions on the relative humidity management and local climate inside the enclosures and applying this knowledge during the design phase is crucial for reducing the chance of failure and controlling maintenance costs. For decades thermal management has been extensively studied; however, the RH (relative humidity) of the operating environment is commonly disregarded during the design of electronics enclosures.
Considering the fact that experiments for monitoring the local climate inside enclosures are often tedious, and are typically lasting for a couple of days, there is a need for a reliable predictive model which is computationally fast enough and therefore capable of long term predictions.
In this work, the effect of ambient conditions on local climate inside electronics enclosure is numerically investigated. First, a finite element based CFD (computational fluid dynamic) model is developed to estimate the time constant of the moisture transfer into a typical enclosure exposed to constant ambient conditions. Comparing the CFD simulation results with experimental data from the literature, a good agreement is found. In order to study the effects of initial moisture content, temperature and size of the opening in the enclosure, an analysis of variance is applied on both experiments and CFD simulation results in factorial designed set of points. Thereafter, several cyclic ambient conditions are applied on a typical enclosure and the relative humidity evolution on the PCB is monitored. Since the CFD simulations are time consuming and CPU-intensive, a Cauer ladder RC circuit model is developed for the long term prediction of the local climate inside the enclosure. The conventional RC (resistance-capacitor) modelling is not very clear in describing the resistance and capacity values for natural convection. In this work, using the CFD simulation data, a correlation is developed for estimating these values.
The presence of the components inside the enclosure especially the heatsink causes a delay to the ambient thermal cycles and also shortens the amplitude of the cyclic changes. Thus, it helps to reduce RH on the PCB (printed circuit board). Considering this effect, unlike the conventional configuration of heatsinks, where they are attached to the walls of the enclosure in order to facilitate the transfer of the generated heat to the ambient, a new configuration where the heatsink is placed inside the enclosure with no contacts with the walls is studied in this work. With this arrangement the heat sink can store the heat generated by the electronics to be used for lowering the RH later when the electronics are not working. In this part of the study, the electronics enclosure is exposed to Copenhagen outdoor conditions. According to the results, a well-designed thermal mass can maintain RH low enough to avoid condensation. Due to the dynamic nature of the heat transfer in an electronics enclosure, an excessively large thermal mass does not necessarily provide the most desirable conditions for the PCB. The size of the thermal mass should be optimized based on the enclosure’s boundary and operational conditions, including the working cycle of the electronics, the amount of the generated heat, and the ambient conditions.
The buoyant flow inside the enclosure reduces the mass transfer resistance inside the enclosure. The cyclic internal heat load also helps this effect. The simulation results demonstrate that the temperature profile and consequently the RH profile are mainly controlled by the buoyant air flow (natural convection) rather than heat conduction inside the enclosure. On the other hand, the moisture transfer into the enclosure happens through the bottom hole. Despite the fact that a temperature increase accelerates the diffusion, the mass transfer resistance which is affected by the geometrical sizes beside the diffusion coefficient is not significantly affected. Thus, the moisture transfer is mostly controlled by the opening size rather than the temperature.
According to the simulations for the Cauer ladder RC model, it is demonstrated that in an enclosure with fixed geometry the dependence of Nu (representing natural convection heat transfer coefficient) on Ra and ∆T inside the enclosure follows a simple power function. Furthermore, a linear relation is observed between Ra and ∆T. A comparison between the magnitudes of the thermal energy and kinetic energy that are stored in the trapped air inside the enclosure reveals that the kinetic energy can be neglected. Although, the Cauer ladder RC model slightly underestimates the temperature, it predicts the trends of temperature and RH changes inside the enclosure successfully. The technique is useful for long-term predictions.