Investigations of Phase Change Material (Pcm) Based Heat Sinks with Different Thermal Enhancers

Abstract

Effective thermal management is crucial for ensuring the maximum performance and reliability of electronic devices. As the internal heat generation within a device increases, the risk of failure rises, leading to a decrease in its overall lifespan. A passive cooling method involving the use of Phase Change Materials (PCMs) proves to be a suitable technique for electronic cooling. However, due to the inherently poor thermal conductivity of PCMs, various enhancers, such as fins, metal foams, and nano-particles are employed to mitigate the thermal resistance they pose. Furthermore, the heat sink design must be optimized to establish an efficient storage and retrieval system under the charging and discharging cycles. The current thesis work aims to present both numerical and experimental investigations of PCM-filled heat sinks using different thermal enhancers Numerical simulations are conducted using ANSYS Fluent, employing the enthalpy porosity formulation to model the melting/solidification of PCM. Subsequently, experiments are carried out to further explore the parametric aspects of the investigation. In the first work, both the melting and solidification of n-eicosane filled heat sink are studied numerically. In order to enhance the thermal conductivity, fins and foams are used in this study. Here, a PCM-filled hybrid system of two cases is considered. In case 1, no fin case is compared with rectangular fins and tapered fins. In case 2, different filling heights, such as 10 mm, 15 mm, and 20 mm, with horizontal tapered fins are investigated. Results show that tapered fins are good for the distribution of temperature and uniform melting within the system. During solidification, the fin shape does not influence the process due to the poor natural convection effects. Regarding the foam filling height, during the melting process, filling heights have less significant effect. But it is observed that the solidification rate is faster by increasing the filling heights. For solidification cases, 20 mm filling height foam performs better than all other cases. Following this study, a multi-objective optimization is carried out using a reliable multi-criteria decision making approach for a hybrid heat sink with fins and foams. Different weightage is distributed to the objective functions in this method depending on the choice of the user. The pore size of 0.8-0.95 and pore density of 5-25 pores per inch vary for various filling heights, and 60 cases are considered for both the cycles. When the weightage is biased towards solidification or equally shared between melting and solidification, the 25 PPI, which possesses a more solid structure, has a better performance score. However, when the weightage is biased towards melting, the 0.95 porosity has a higher performance score due to the larger PCM volume. From the results, guidelines for selecting a preferable pore structure are provided based on the filling height and applied weightage. In the next study, Phase change materials (PCM) RT-28HC, RT-35HC, and RT-44HC, with similar thermal properties, are considered, and a combination of PCM acts as an enhancer. The PCM is oriented in increasing order of melting temperature from the left to the right side of the heat sink. Additionally, the fins are attached to the heat sink longitudinally, and its orientation effects are studied. The effect of fins on the charging cycle is assessed by comparing a single and double PCM heat sink. Three initial conditions are investigated by altering the initial temperature to 300 K, 303 K, and 310 K. For high heat input, the negatively angled fins possess a higher melting rate. For different initial conditions, -60o provides higher enhancement, and +60o possesses prolonged melting for almost all cases and applied weightage. In the last study, a modified variable height fin heat sink is compared with the conventional constant height fin heat sink. Experiments are performed for constant loads and also different power surge conditions. The pulsed heat loads are applied for two scenarios: 1. Constant load 4 W - power surge and constant load 4 W - power surge - 1800 s no-load condition, and 2. Power surge (50 s, 100 s, and 150 s) - no-load conditions of 1800 s. During experiments, the proposed variable height fin heat sinks possess better thermal performance for all load scenarios. The variable height fin heat sink accelerates any load's melting rate. The time difference between the wall and the PCM is also less for the variable height fin heat sink. Similar to the melting, a faster discharging rate is noticed during solidification in the variable height fin heat sink.