Chapter 2: Problem 21
Write down the one-dimensional transient heat conduction equation for a plane wall with constant thermal conductivity and heat generation in its simplest form, and indicate what each variable represents.
Chapter 2: Problem 21
Write down the one-dimensional transient heat conduction equation for a plane wall with constant thermal conductivity and heat generation in its simplest form, and indicate what each variable represents.
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Get started for freeHot water flows through a PVC $(k=0.092 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\( pipe whose inner diameter is \)2 \mathrm{~cm}$ and whose outer diameter is \(2.5 \mathrm{~cm}\). The temperature of the interior surface of this pipe is \(35^{\circ} \mathrm{C}\), and the temperature of the exterior surface is \(20^{\circ} \mathrm{C}\). The rate of heat transfer per unit of pipe length is (a) \(22.8 \mathrm{~W} / \mathrm{m}\) (b) \(38.9 \mathrm{~W} / \mathrm{m}\) (c) \(48.7 \mathrm{~W} / \mathrm{m}\) (d) \(63.6 \mathrm{~W} / \mathrm{m}\) (e) \(72.6 \mathrm{~W} / \mathrm{m}\)
Consider a large plane wall of thickness \(L=0.8 \mathrm{ft}\) and thermal conductivity $k=1.2 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft} \cdot{ }^{\circ} \mathrm{F}$. The wall is covered with a material that has an emissivity of \(\varepsilon=0.80\) and a solar absorptivity of \(\alpha=0.60\). The inner surface of the wall is maintained at \(T_{1}=520 \mathrm{R}\) at all times, while the outer surface is exposed to solar radiation that is incident at a rate of $\dot{q}_{\text {solar }}=300 \mathrm{Btu} / \mathrm{h} \cdot \mathrm{ft}^{2}$. The outer surface is also losing heat by radiation to deep space at \(0 \mathrm{~K}\). Determine the temperature of the outer surface of the wall and the rate of heat transfer through the wall when steady operating conditions are reached. Answers: $554 \mathrm{R}, 50.9 \mathrm{Btu} / \mathrm{h}^{\mathrm{ft}}{ }^{2}$
In subsea oil and natural gas production, hydrocarbon fluids may leave the reservoir with a temperature of \(70^{\circ} \mathrm{C}\) and flow in subsea surroundings of \(5^{\circ} \mathrm{C}\). Because of the temperature difference between the reservoir and the subsea environment, the knowledge of heat transfer is critical to prevent gas hydrate and wax deposition blockages. Consider a subsea pipeline with inner diameter of \(0.5 \mathrm{~m}\) and wall thickness of \(8 \mathrm{~mm}\) is used for transporting liquid hydrocarbon at an average temperature of \(70^{\circ} \mathrm{C}\), and the average convection heat transfer coefficient on the inner pipeline surface is estimated to be \(250 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). The subsea has a temperature of \(5^{\circ} \mathrm{C}\), and the average convection heat transfer coefficient on the outer pipeline surface is estimated to be $150 \mathrm{~W} / \mathrm{m}^{2}\(. \)\mathrm{K}$. If the pipeline is made of material with thermal conductivity of $60 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\(, use the heat conduction equation to \)(a)$ obtain the temperature variation in the pipeline wall, \((b)\) determine the inner surface temperature of the pipeline, \((c)\) obtain the mathematical expression for the rate of heat loss from the liquid hydrocarbon in the pipeline, and \((d)\) determine the heat flux through the outer pipeline surface.
Consider a medium in which the heat conduction equation is given in its simplest form as $$ \frac{1}{r} \frac{d}{d r}\left(r k \frac{d T}{d r}\right)+\dot{e}_{\text {gen }}=0 $$ (a) Is heat transfer steady or transient? (b) Is heat transfer one-, two-, or three-dimensional? (c) Is there heat generation in the medium? (d) Is the thermal conductivity of the medium constant or variable?
Consider a large plane wall of thickness \(L=0.05 \mathrm{~m}\). The wall surface at \(x=0\) is insulated, while the surface at \(x=L\) is maintained at a temperature of \(30^{\circ} \mathrm{C}\). The thermal conductivity of the wall is \(k=30 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\), and heat is generated in the wall at a rate of $\dot{e}_{\mathrm{gen}}=\dot{e}_{0} \dot{e}^{-0.5 \mathrm{l} / \mathrm{W}} \mathrm{W} / \mathrm{m}^{3}$ where \(\dot{e}_{\mathrm{gen}}=8 \times 10^{6} \mathrm{~W} / \mathrm{m}^{3}\). Assuming steady one-dimensional heat transfer, \((a)\) express the differential equation and the boundary conditions for heat conduction through the wall, (b) obtain a relation for the variation of temperature in the wall by solving the differential equation, and (c) determine the temperature of the insulated surface of the wall.
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