Consider a large plane wall of thickness \(I_{\text {, }}\) thermal conductivity \(k\), and surface area \(A\). The left surface of the wall is exposed to the ambient air at \(T_{\infty}\) with a heat transfer coefficient of \(h\) while the right surface is insulated. The variation of temperature in the wall for steady one-dimensional heat conduction with no heat generation is (a) \(T(x)=\frac{h(L-x)}{k} T_{\infty}\) (b) \(T(x)=\frac{k}{h(x+0.5 L)} T_{\infty}\) (c) \(T(x)=\left(1-\frac{x h}{k}\right) T_{\infty}\) (d) \(T(x)=(L-x) T_{\infty}\) (e) \(T(x)=T_{\infty}\)

Short Answer

Expert verified
Answer: The correct equation for the temperature distribution in the wall, under the given conditions, is $$T(x) = T_{\infty}$$

Step by step solution

01

Calculate heat flux through the wall

Since the system is in the steady state, there is no change in temperature with time and no internal heat generation. So, the heat flux \(q''\) through the wall must be constant and determined by the surface heat transfer at the left surface. This can be calculated using convective heat transfer equation at the left surface (x=0): $$ q'' = h(T(0) - T_{\infty}) $$
02

Calculate temperature gradient in the wall

Since the heat flux is constant due to steady-state condition and no heat generation, the temperature gradient in the wall should also be constant. Therefore, the change in temperature with respect to position in the wall should be constant: $$ \frac{dT}{dx}=C $$ Where \(C\) is a constant value.
03

Check the boundary conditions

We should check the given options against the boundary conditions: 1. At x = 0, the left surface should have a temperature \(T_{0}\) which can be calculated from the heat flux equation mentioned in step 1: $$ T(0)=T_{0} = T_{\infty} +\frac{q''}{h} $$ 2. At x = L, the right surface is insulated, so there should be no heat flux through the surface. This means that the derivative of the temperature with respect to x should be zero at the right surface: $$ \frac{dT}{dx}(L) = 0 $$
04

Find the correct option by analyzing the given choices with respect to boundary conditions

- Option (a): At x=0, \(T(0)=0\), which does not meet the boundary condition 1. At x=L, \(\frac{dT}{dx}(L) = -\frac{h}{k}\neq0\), which also does not meet the boundary condition 2. So, this option is not correct. - Option (b): At x=0, \(T(0)=\frac{k}{0.5hL}T_{\infty}\), which does not meet the boundary condition 1. At x=L, \(\frac{dT}{dx}(L) \) is not zero (derivative calculation needed), which does not meet the boundary condition 2. So, this option is also not correct. - Option (c): At x=0, \(T(0)= T_{\infty}\), which meets the boundary condition 1. At x=L, \(\frac{dT}{dx}(L) = -\frac{h}{k}\neq0\), which does not meet the boundary condition 2. So, this option is not correct. - Option (d): At x=0, \(T(0)=(L) T_{\infty}\), which does not meet the boundary condition 1. Derivative calculation is not needed, as it already doesn't follow boundary condition 1. - Option (e): Since we've eliminated the other options, this must be the correct choice. However, let's verify: At x=0, \(T(0)=T_{\infty}\), which meets the boundary condition 1. At x=L, \(\frac{dT}{dx}(L) = 0\), which meets the boundary condition 2. Therefore, the correct equation for the temperature distribution in the wall, under given conditions, is: $$T(x) = T_{\infty}$$

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Most popular questions from this chapter

A spherical vessel is filled with chemicals undergoing an exothermic reaction. The reaction provides a uniform heat flux on the inner surface of the vessel. The inner diameter of the vessel is \(5 \mathrm{~m}\) and its inner surface temperature is at \(120^{\circ} \mathrm{C}\). The wall of the vessel has a variable thermal conductivity given as \(k(T)=k_{0}(1+\beta T)\), where \(k_{0}=1.01 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\), \(\beta=0.0018 \mathrm{~K}^{-1}\), and \(T\) is in \(\mathrm{K}\). The vessel is situated in a surrounding with an ambient temperature of \(15^{\circ} \mathrm{C}\), the vessel's outer surface experiences convection heat transfer with a coefficient of \(80 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). To prevent thermal burn on skin tissues, the outer surface temperature of the vessel should be kept below \(50^{\circ} \mathrm{C}\). Determine the minimum wall thickness of the vessel so that the outer surface temperature is \(50^{\circ} \mathrm{C}\) or lower.

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A \(1200-W\) iron is left on the iron board with its base exposed to ambient air at \(26^{\circ} \mathrm{C}\). The base plate of the iron has a thickness of \(L=0.5 \mathrm{~cm}\), base area of \(A=150 \mathrm{~cm}^{2}\), and thermal conductivity of \(k=18 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\). The inner surface of the base plate is subjected to uniform heat flux generated by the resistance heaters inside. The outer surface of the base plate whose emissivity is \(\varepsilon=0.7\), loses heat by convection to ambient air with an average heat transfer coefficient of \(h=\) \(30 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) as well as by radiation to the surrounding surfaces at an average temperature of \(T_{\text {surr }}=295 \mathrm{~K}\). Disregarding any heat loss through the upper part of the iron, \((a)\) express the differential equation and the boundary conditions for steady one-dimensional heat conduction through the plate, \((b)\) obtain a relation for the temperature of the outer surface of the plate by solving the differential equation, and (c) evaluate the outer surface temperature.

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