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Chapter 9: Problem 73

Why are heat sinks with closely packed fins not suitable for natural convection heat transfer, although they increase the heat transfer surface area more?

Short Answer

Expert verified
Answer: Heat sinks with closely packed fins are not suitable for natural convection heat transfer because they hinder the fluid flow that drives heat transfer in this process. Although they increase the heat transfer surface area, the restricted fluid flow leads to decreased heat transfer efficiency.

Step by step solution

01

Understanding Natural Convection

Natural convection is the process of heat transfer that occurs due to the movement of a fluid, like air or water, as a result of temperature differences. When a fluid comes into contact with a hot surface, it absorbs heat and becomes less dense. This causes the heated fluid to rise, while cooler, denser fluid sinks and replaces it. This circulation of fluid carries heat away from the surface and eventually dissipates it into the surroundings. The effectiveness of natural convection depends on the fluid flow, temperature differences, and the heat transfer surface area. #Step 2: The Role of Fins in Heat Transfer#
02

The Role of Fins in Heat Transfer

Fins are used in heat sinks to increase the surface area available for heat transfer. They effectively extend the surface of the heat source into the cool fluid, creating more contact points for heat exchange. Ideally, as you increase the number of fins, you increase the heat transfer surface area, which in turn increases the overall heat transfer rate. #Step 3: Effects of Closely Packed Fins on Fluid Flow#
03

Effects of Closely Packed Fins on Fluid Flow

While closely packed fins do increase the heat transfer surface area, they also have an adverse effect on the fluid flow, especially in natural convection. When fins are closely packed, the gaps between them become narrower, which restricts the flow of fluid through the heat sink. This restricted flow reduces the effectiveness of natural convection, as the fluid is unable to circulate freely and carry away heat from the heat sink. #Step 4: Decreased Efficiency due to Hindered Convection#
04

Decreased Efficiency due to Hindered Convection

In natural convection heat transfer, the movement of the fluid is the primary driver of heat transfer. Therefore, hindering fluid flow by closely packing fins together can actually result in decreased heat transfer efficiency, despite the increased surface area. For natural convection to be effective, fins should be spaced apart enough to allow for free fluid flow and circulation, which ensures efficient heat exchange. #Step 5: Conclusion#
05

Conclusion

In conclusion, heat sinks with closely packed fins are not suitable for natural convection heat transfer because they hinder the fluid flow that drives heat transfer in natural convection. Although they increase the heat transfer surface area, the restricted fluid flow leads to decreased heat transfer efficiency. For natural convection to be effective, fins should be spaced apart sufficiently to allow for adequate fluid flow and circulation.

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Most popular questions from this chapter

In a plant that manufactures canned aerosol paints, the cans are temperature- tested in water baths at \(55^{\circ} \mathrm{C}\) before they are shipped to ensure that they withstand temperatures up to \(55^{\circ} \mathrm{C}\) during transportation and shelving (as shown in Fig. P9-44 on the next page). The cans, moving on a conveyor, enter the open hot water bath, which is \(0.5 \mathrm{~m}\) deep, \(1 \mathrm{~m}\) wide, and \(3.5 \mathrm{~m}\) long, and move slowly in the hot water toward the other end. Some of the cans fail the test and explode in the water bath. The water container is made of sheet metal, and the entire container is at about the same temperature as the hot water. The emissivity of the outer surface of the container is 0.7. If the temperature of the surrounding air and surfaces is \(20^{\circ} \mathrm{C}\), determine the rate of heat loss from the four side surfaces of the container (disregard the top surface, which is open). The water is heated electrically by resistance heaters, and the cost of electricity is \(\$ 0.085 / \mathrm{kWh}\). If the plant operates \(24 \mathrm{~h}\) a day 365 days a year and thus \(8760 \mathrm{~h}\) a year, determine the annual cost of the heat losses from the container for this facility.

Under what conditions does natural convection enhance forced convection, and under what conditions does it hurt forced convection?

A 4-m-long section of a 5-cm-diameter horizontal pipe in which a refrigerant flows passes through a room at \(20^{\circ} \mathrm{C}\). The pipe is not well insulated and the outer surface temperature of the pipe is observed to be \(-10^{\circ} \mathrm{C}\). The emissivity of the pipe surface is \(0.85\), and the surrounding surfaces are at \(15^{\circ} \mathrm{C}\). The fraction of heat transferred to the pipe by radiation is \(\begin{array}{lllll}\text { (a) } 0.24 & \text { (b) } 0.30 & \text { (c) } 0.37 & \text { (d) } 0.48 & \text { (e) } 0.58\end{array}\) (For air, use \(k=0.02401 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \operatorname{Pr}=0.735, v=\) \(1.382 \times 10^{-5} \mathrm{~m}^{2} / \mathrm{s}\) )

A 3 -mm-diameter and 12-m-long electric wire is tightly wrapped with a \(1.5-\mathrm{mm}\)-thick plastic cover whose thermal conductivity and emissivity are \(k=0.20 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\) and \(\varepsilon=0.9\). Electrical measurements indicate that a current of \(10 \mathrm{~A}\) passes through the wire and there is a voltage drop of \(7 \mathrm{~V}\) along the wire. If the insulated wire is exposed to calm atmospheric air at \(T_{\infty}=30^{\circ} \mathrm{C}\), determine the temperature at the interface of the wire and the plastic cover in steady operation. Take the surrounding surfaces to be at about the same temperature as the air. Evaluate air properties at a film temperature of \(40^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\) pressure. Is this a good assumption?

An average person generates heat at a rate of \(240 \mathrm{Btu} / \mathrm{h}\) while resting in a room at \(70^{\circ} \mathrm{F}\). Assuming onequarter of this heat is lost from the head and taking the emissivity of the skin to be \(0.9\), determine the average surface temperature of the head when it is not covered. The head can be approximated as a 12 -in-diameter sphere, and the interior surfaces of the room can be assumed to be at the room temperature.
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