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Chapter 3: Problem 112

Consider two finned surfaces that are identical except that the fins on the first surface are formed by casting or extrusion, whereas they are attached to the second surface afterwards by welding or tight fitting. For which case do you think the fins will provide greater enhancement in heat transfer? Explain.

Short Answer

Expert verified
Answer: The fins formed by casting or extrusion will provide greater enhancement in heat transfer due to the lower contact resistance between the fins and the base surface, allowing for more effective heat transfer from the surface to the fins.

Step by step solution

01

Identify key factors for heat transfer enhancement

In order to determine which surface will have greater heat transfer enhancement, we need to consider factors that could affect heat transfer. These factors include the fin material, the fin geometry, the contact resistance between the fins and the surface, and the surface temperature.
02

Case 1: Fins formed by casting or extrusion

In this case, the fins are formed as part of the surface itself through casting or extrusion. This results in a very good bond between the fins and the surface, minimizing contact resistance between the fins and the base surface. The fin material, geometry, and surface temperature will be the same for both cases.
03

Case 2: Fins attached by welding or tight fitting

In this case, the fins are attached to the surface through welding or tight fitting. While these attachment methods can provide a strong bond, they typically result in higher contact resistance between the fins and the surface compared to casting or extrusion. This additional resistance can act as a barrier to heat transfer from the surface to the fins.
04

Compare the two cases

When comparing the two cases, we see that the key difference between them is the contact resistance between the fins and the base surface. In the first case (formed by casting or extrusion), there is minimal contact resistance, which allows for better heat transfer from the surface to the fins. In the second case (attached by welding or tight fitting), the contact resistance is higher, which can hinder the overall heat transfer.
05

Conclusion

Based on the analysis, it can be concluded that the fins formed by casting or extrusion will provide greater enhancement in heat transfer than the fins attached by welding or tight fitting. This is due to the lower contact resistance between the fins and the base surface in the first case, which allows for more effective heat transfer from the surface to the fins.

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Most popular questions from this chapter

Steam in a heating system flows through tubes whose outer diameter is \(5 \mathrm{~cm}\) and whose walls are maintained at a temperature of \(180^{\circ} \mathrm{C}\). Circular aluminum alloy 2024-T6 fins \((k=186 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\) of outer diameter \(6 \mathrm{~cm}\) and constant thickness \(1 \mathrm{~mm}\) are attached to the tube. The space between the fins is \(3 \mathrm{~mm}\), and thus there are 250 fins per meter length of the tube. Heat is transferred to the surrounding air at \(T_{\infty}=25^{\circ} \mathrm{C}\), with a heat transfer coefficient of \(40 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Determine the increase in heat transfer from the tube per meter of its length as a result of adding fins.

Two plate fins of constant rectangular cross section are identical, except that the thickness of one of them is twice the thickness of the other. For which fin is the \((a)\) fin effectiveness and \((b)\) fin efficiency higher? Explain.

Consider a house whose attic space is ventilated effectively so that the air temperature in the attic is the same as the ambient air temperature at all times. Will the roof still have any effect on heat transfer through the ceiling? Explain.

What is the difference between the fin effectiveness and the fin efficiency?

Superheated steam at an average temperature \(200^{\circ} \mathrm{C}\) is transported through a steel pipe \(\left(k=50 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, D_{o}=8.0 \mathrm{~cm}\right.\), \(D_{i}=6.0 \mathrm{~cm}\), and \(L=20.0 \mathrm{~m}\) ). The pipe is insulated with a 4-cm thick layer of gypsum plaster \((k=0.5 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\). The insulated pipe is placed horizontally inside a warehouse where the average air temperature is \(10^{\circ} \mathrm{C}\). The steam and the air heat transfer coefficients are estimated to be 800 and \(200 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), respectively. Calculate \((a)\) the daily rate of heat transfer from the superheated steam, and \((b)\) the temperature on the outside surface of the gypsum plaster insulation.
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