Thermal Performance Analysis of a Rectangular Finned Heat Sink under Dust Deposition

This study investigated the effect of dust deposition on the fin surfaces of a rectangular finned heat sink for natural convection. Therefore, experimental analysis was carried out on two cases, i.e., under fine dust and fiber dust conditions. The experiment was conducted inside a closed duct on the heat sink consisting of 17 fins. After analyzing the data and variation of temperature values at various positions of the fin surfaces, it was observed that the fine dust accumulation had a negligible effect on the thermal performance. However, fiber dust accumulation had a significant effect on the thermal performance. Moreover, the obtained values of Nusselt number were 26.68, 26.21, and 23.75 for natural convection under no dust, fine dust, and fiber dust conditions, respectively. Hence, the amount of heat dissipated by natural convection was minimal under fiber dust conditions.


Introduction
Heat removal has become a very important concern nowadays for the optimal functioning of equipment that generates significant amounts of heat during their operation, especially in the case of electronic devices (i.e.: motherboard, relays, transistor) to prevent malfunctioning as well as to prolong the working life of these equipment.So, design modification and thermal analysis are needed so that the generated heat can be carried away from the device and dissipated safely.Mounting the heat sink on the heated surface of the electric devices is an impactful mechanism to increase their effective surface area and thereby enhance the heat dissipation rate.Fins of different sizes and shapes are being used as a heat sink to reduce the temperature of the device.Dust is one of the major obstacles that reduces heat dissipation on any surface.Dust is the fine particles of solid matter.Dust in the open place normally consists of the fine particles in the atmosphere that appear from miscellaneous sources such as soil elevated by wind (an Aeolian process), pollution, volcanic eruptions, textile fibers, paper fibers, plant pollen, animal fur, minerals from outdoor soil, burnt meteorite particles, and many other materials which may be found in the local environment.Dust found in homes contains dead skin cells of an amount of about 20-50% [1].As stated earlier, heat sinks are used to dissipate heat from any heat source.However, the expected heat convection is not gained after some days of usage.The main reason is dust layer deposition on the fin surface of the heat sink.As the dust increases, the convection of heat will decrease further.The dust layer creates a fouling effect on the heat convection surface.As a result, the fins cannot provide the required performance due to the effect of dust.This study aims to analyze the performance degradation of heat sinks due to dust accumulation.A study by Aharon Nabi, Peter Rodgers, and Avram Bar-Cohen investigated the heat sink fouling mechanisms by both analytical and experimental analysis for the first time.This investigation assessed the impact of two heat sink fouling modes on heat sink thermal resistance, (a) accumulation of thermally insulative fouling material on the fins, and (b) blockage of leading-edge entrance [2].David A. Moore depicted the characteristics of fiber accumulation fouling for fine-pitched heat sinks.He characterized fiber and fine dust from several sources and showed significant resemblance in the types of fiber and fine particles found [3].A study by Yang and Peng (2008) examined the effects of the fin shape of the heat sink on the thermal performance.The study showed that the junction temperature was inversely proportional to the fin height which was nearer to the center of the heat sink.They experimented with four heat sink models having non-uniform fin height.Their findings also highlighted a potential scope for optimizing the non-uniform fin height design [4].Apart from this study, [5][6][7][8][9] illustrated heat transfer aspects of rectangular finned heat sinks for different applications.From the reviewed literature, numerous studies on optimal fin design for maximum heat extraction have been identified.However, there appears to be a lack of research on the effects of dust deposition on the thermal performance of rectangular finned heat sinks.Therefore, this study experimentally investigated the thermal performance of a rectangular finned heat sink under dust deposition including fine dust and fiber dust.

Experimental setup
In this experiment, 17 rectangular profile straight fins with a spacing of 5 mm were used (shown in Fig. 1).The fin height was 6.6 cm and the base area was 11.6 x 11.6 cm 2 .The fin length was 11.6 cm.Five holes were drilled on the fin surface (first 8 fins from left/right side) namely H1, H2, H2u, H2d, and H3.The hole diameter was 6 mm.

Figure 1. Heat sink
A plate heater of 100W capacity was utilized to generate heat and provide the heat to the heat sink.A steel sheet was used for making the box.The grade of the steel sheet was 18 gauge.The fabrication process is exhibited in Fig. 2.

Figure 2. Heater box fabrication
After fabricating the heater box, all necessary equipment was assembled inside the box to conduct the experiment (shown in Fig. 3).At first, the heat sink was installed with the heater box with the help of nuts and bolts.After the heat sink was attached, a mica sheet was attached to the base of the heat sink.The heater was placed over the mica sheet.The heater circuit was completed at this stage.Then the whole box was filled up with glass wool.Glass wool was compressed as far as possible.

Figure 3. Heater box assembly
For conducting the experiment fine dust (Fig. 4) and fiber dust (Fig. 5) conditions were considered.In this experiment, a square duct, shown in Fig. 6 was used which was installed vertically with the ground.The cross-sectional area of the duct was 12.6 x 12.6 cm 2 .The height of the duct was 0.7 m.There was a slot in the duct where the heat sink could be installed.The size of the slot was 12cm x 12cm.The heater box was installed in the slot with the help of two bolts.At that particular portion of the duct where the heat sink was installed is transparent from the front side.

Methodology
The main idea was to examine the no dust, fine dust, and fiber dust conditions.To examine the effect of dust on electrical equipment, a similar system was constructed where heat was produced by an electric heater.The heater was attached to the heat sink.A closed system was created by the duct to keep the experiment unaffected by the environment.

Recording the temperature
There were some holes in the duct to measure the temperatures using the thermocouple.The thermocouple gave the temperature reading up to two decimal places.Two types of temperature were measured: a) temperature of the fin surface and b) base temperature.Fin temperatures were measured from plate 1 to plate 8.

Results and discussion
To analyze the effect of dust deposition on the thermal performance of the heat sink, the temperature profile at different positions of the plates was plotted in Fig. 9 to Fig. 13.The base area of the heat sink was 11.6 x 11.6 cm 2 .But the heater size was 10x10 cm 2 .As a result, a hot spot was created at the center of the heat sink.In other words, heat flux was less at the sides of the sink.Less heat flux means less thermal energy.So, the temperature of the side fins was lower than the middle fins.
Another reason is, side fins were more open to the air.Airflow took the heat away from the fin surface area and thus the temperature of the side fins was reduced.
If the heat sink temperature readings at no dust condition and after dust accumulation are compared, it is seen that the temperature is higher when dust is accumulated over the fin surface area.The thermal conductivity of the dust particle is much lower than the thermal conductivity of Aluminum.When dust accumulates over the fin surface area, it creates an insulation layer.As a result, heat energy could not be taken away by the airflow.So, the fin temperature as well as the base temperature was increased due to the accumulation of the dust.The effect of fiber dust was more than the effect of fine dust.The temperature under fine dust conditions and the no dust conditions were pretty close.Fine dust increased the base temperature only about 1°C,

Positions of Plates
Fine dust with natural convection Fiber dust with natural convection No dust with natural convection whereas, fiber dust increased the base temperature by 4°C.So, the fiber dust is very much harmful to the heat sink.Fine dust only created an insulating layer over the fin surface area, whereas, the fiber dust created an obstacle to the airflow as well as the fouling effect.Air could not enter into the fin passage easily due to the obstacle created by fiber dust.Fiber dust got clogged at the fin spacing.As a result, airflow was restricted.So, fins could not release heat.Thus, the effect of fiber dust was significant.
Another reason is the variation of components of fine dust and fiber dust.Fine dust is denser and less thermal resistant whereas fiber dust is composed of cotton fiber or cellulose which is very light and highly thermal resistive.Here, as expected, the values of convective heat transfer coefficient (h) reduced under fine dust conditions.The values were further reduced under fiber dust conditions.This indicates that the heat dissipation rate continued to reduce with the transition from no dust to fine dust and further to fiber dust condition.
A similar effect is seen in the case of Nusselt number values which dictates that the convective heat transfer rate decreased compared to the conductive heat transfer rate.
Grashof number was calculated to check if the airflow was laminar or turbulent.After analyzing the values the airflow was found to be laminar which helped to better understand the effect of dust accumulation.

Conclusion
Thermal analysis of a rectangular finned heat sink was experimentally investigated under fine dust and fiber dust conditions in the case of natural convection.After analyzing the data and graphs, the following conclusions can be drawn: • An ascending trend of temperature was found from plate 1 to plate 8 for all the cases i.e., no dust, fine dust, and fiber dust.• Higher values of temperature reading were recorded after dust accumulation on the fin surfaces compared to the no dust condition.This exhibited the performance drop of the heat sink under dusty conditions.• The temperature readings for no dust condition and fine dust condition were moderately close.
However, a significant difference in temperature readings was seen for no dust and fiber dust conditions.This denotes the fact that the accumulation of fiber dust on the fin surfaces has a noteworthy effect on the thermal performance of the heat sinks.• Maximum temperature readings were recorded at the base and the values gradually decreased to minimum temperature at the tips.• The maximum value of the convective heat transfer coefficient (h) was found for no dust condition.This value was then reduced under fine dust conditions and then reached a minimum value under fiber dust conditions.This happened due to the reduced heat dissipation rate with the transition from no dust condition to fine dust condition and further to fiber dust condition.
• Similar results were seen in the case of Nusselt number values.This referred to the fact that the amount of heat dissipated by convection was minimal under fiber dust conditions.

Table 1 .
The obtained numerical values of convective heat transfer coefficient (h), Grashof number (Gr) and Nusselt number (Nu) are summarized in the following table: Summary of calculation