3. Results and discussions :
3.1. Verification of the present model :
The present model is verified by using the experimental study represented in [8]. In the experimental study of C. Baby and R. Balaji [8], a heat sink was modeled with a vertical height of 25 mm, a length of 80 mm, and a width of 62 mm. A plate heater of 2 mm thickness and a slot area of 60*42 mm2 was used. The height of the square pin fins was 20 mm, with an area of 2*2 mm2. N-eicosane was used as a PCM.
The verification test is performed by using the numerical model of a square-pin fin heat sink with a height of 16 mm. A constant power of 3 W is applied to the base of the heat sink for 180 minutes. Figure-3 shows the relationship between the results of the present model and the experimental model proposed by C. Baby and R. Balaji [8]. There is a strong correlation between these two models, where the maximum error is 1.7%. Hence, it can be said that the present model is appropriate for studying the thermal performance of a heat sink with PCMs.
[ insert Figure-3 here]
3.2. The effects of using PCM :
The test is performed to visualize the effects of using PCMs in the micro-pin fin heat sink. A square fin heat sink with a height of 16 mm is used. A constant power of 30 W is applied to the base of the heat sink, and the ambient temperature is kept at 20 °C. Insulation is applied to the boundary walls. Figure-4 shows the temperature vs. time graph of a heat sink with three different types of PCMs as well as without PCM. The graph represents that the temperature of the base plate of a heat sink without PCM is much higher than that of a heat sink with PCM. It takes around 2.5 minutes to reach a temperature of 60 °C for a heat sink without PCM, whereas this time is almost twice as long for a heat sink with PCM in the same conditions. Because after melting, PCMs can absorb and store energy as latent heat, decrease the temperature of the base plate, and increase the cooling process for effective thermal management of electronic equipment.
Another important finding is that polyethylene glycol performs better than the other two PCMs. Polyethylene glycol has a melting temperature of 18 °C, which is much lower than paraffin wax and stearic acid, which have melting temperatures of 46 °C and 69.3 °C, respectively. Therefore, polyethylene melts quickly, and it can be used efficiently in such micro-pin fin heat sinks where the temperature rise is below 70 °C.
[ insert Figure-4 here]
3.3. Effects of geometry of the fins :
Square, triangular, and circular fins with a height of 16 mm are investigated in this study. Polyethylene glycol is used as a phase change material. A constant heat of 30 W is applied to the base of the heat sinks. The convective heat transfer coefficient of the fins is 25 W/m2.K and the ambient temperature is maintained at 20 °C. Figure-5 shows the time-temperature curve of the study. After 10 minutes, the base plate temperature of the square fin heat sink is 71.19 °C, whereas for triangular and circular fin heat sinks, this temperature is 82.97 °C and 80.95 °C, respectively. So, it can be said that square fins have a higher ability to reduce temperature rise than triangular and circular fin heat sinks because square fins have a larger convective heat transfer area.
[insert Figure-5 here]
3.4. Effects of height of the fins :
Figure-6 shows the temperature vs. time graph for three types of fins with heights of 16 mm and 10 mm. From this graph, it is seen that, as the height of the fins increases, it reduces the temperature rise of the base plate of the heat sink. This is because with the increase in the height of the fins, the convective heat transfer area also increases, causing faster dissipation of heat. For square fin heat sink, the temperature rise is 25 °C less for 16 mm height of the fins in comparison to the 10 mm height of the fins. For triangular and circular fins, this temperature difference is almost 27 °C and 29 °C, respectively. However, longer fins also increase the cost of the heat sink because of the higher volume of the heat sink. Hence, the height of the heat sink should be carefully designed.
Figure-7 shows the comparative results of the height of the heat sinks in a column chart when constant heat is applied for 10 minutes.
[insert Figure-6 here]
[insert Figure-7 here]
3.5. Effects of the number of fins with and without PCM :
Three types of heat sinks with fin numbers of 84 and 45 are being studied. At first, polyethylene glycol is used as a phase change material, then air convection without PCM is observed. Figure-8 shows the time-temperature curve for square, triangular, and circular fins with fin numbers of 84 and 45. For an 84-square-foot heat sink, it takes only 6 minutes to reach a temperature of 60 °C, whereas for a 45-square-foot heat sink, it takes at least 8 minutes. So it can be said that the 45-fin heat sink performs better than the 84-fin heat sink with PCM. This is because as the volume of the fins increases, it decreases the volume of the PCM as well. The cooling effect of the PCM is much greater than the natural convection cooling of the fins.
However, the scenario is completely different without PCM. Figure 9 shows the temperature vs. time graph to illustrate the effect of the number of fins without PCM. As the number of fins increases, it increases the volume of air convection heat transfer, which results in a smaller temperature rise. As the effect of PCM does not exist, a higher number of fins is preferable for the effective cooling performance of a heat sink.