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Chapter 5: Problem 13

Explain how the finite difference form of a heat conduction problem is obtained by the energy balance method.

Short Answer

Expert verified
The heat conduction problem involves modeling the transfer of thermal energy in a solid medium due to a temperature gradient, which can be described using the heat equation. Energy balance method consists of dividing the spatial domain into discrete segments or grid points and formulating an equation for each point by equating the energy inflow and outflow within the control volume. By applying this approach to a one-dimensional, steady-state heat conduction problem, we can derive a finite difference form of the equation as \( T_{i-1} - 2T_i + T_{i+1} = 0 \) where T_i is the temperature at each grid point. This set of linear equations can then be solved numerically to find the temperature distribution within the spatial domain.

Step by step solution

01

Understanding the Heat Conduction Problem

Heat conduction is a phenomenon where heat or thermal energy is transferred through a solid medium due to a temperature gradient. The conduction process can be mathematically described using the heat equation, which is a partial differential equation (PDE) that relates the rate of change of temperature with respect to spatial dimensions and time. For a one-dimensional, steady-state heat conduction problem, the heat equation is given by: $$ \frac{\partial^2 T}{\partial x^2} = 0 $$ where T is the temperature and x is the spatial coordinate.
02

Energy Balance Method

The energy balance method is an approach to convert a differential equation into a numerical discretization or finite difference form by considering the balance of energy in a given control volume. It involves dividing the spatial domain into discrete segments or grid points and formulating an equation for each point by equating the energy inflow and outflow through the control volume. These equations are then solved simultaneously to obtain the temperature distribution along the spatial domain.
03

Deriving the Finite Difference Form

By using the energy balance method, we can derive the finite difference form of the heat conduction problem. Let's consider a one-dimensional, steady-state heat conduction problem with a uniform grid spacing, 螖x, and a control volume having boundaries at x - 螖x/2 to x + 螖x/2. 1. For each point x, identify inflow and outflow of heat: - Inflow: Heat Conducted In (left boundary) \(q_{in} = -k\frac{\partial T}{\partial x}|_{x - \Delta x/2}\) - Outflow: Heat Conducted Out (right boundary) \(q_{out} = -k\frac{\partial T}{\partial x}|_{x + \Delta x/2}\) where k is the thermal conductivity. 2. Apply the energy balance equation (inflow = outflow): $$ -k\frac{\partial T}{\partial x}|_{x - \Delta x/2} = -k\frac{\partial T}{\partial x}|_{x + \Delta x/2} $$ 3. Use central difference approximation to rewrite the derivatives in terms of temperature at grid points: $$ -k\left(\frac{T_{i-1} - T_i}{\Delta x}\right) = -k\left(\frac{T_i - T_{i+1}}{\Delta x}\right) $$ 4. Rearrange the equation and simplify: $$ T_{i-1} - 2T_i + T_{i+1} = 0 $$ This equation gives the finite difference form of the one-dimensional, steady-state heat conduction problem at each grid point, where T_i is the temperature at the ith grid point. By applying the energy balance method, we obtain a system of linear equations that can be solved numerically to find the temperature distribution within the spatial domain.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Energy Balance Method
The energy balance method is a key approach to studying heat conduction problems. It's like a bookkeeper's ledger but for energy instead of money: it ensures that what goes into a system must come out or be stored within. In heat conduction, this means that the heat energy entering one side of a material should exit the other side unless it's absorbed or released inside the material.

Let's imagine slicing the material into thin slices, or 'control volumes', each balanced on its own. Energy enters on the left, heat is possibly stored or released, and the remaining energy exits on the right. By applying the first law of thermodynamics, which is essentially the rule of energy conservation, we can express this balance mathematically. Through such an energy ledger, we translate the continuous heat conduction process into a set of equations for each slice, where we can quantify the temperature changes and understand the energy dynamics within the control volumes. This sets the stage for transforming a continuous mathematical expression into a form that can be handled by computers, which is where numerical discretization comes into play.
Heat Conduction Problem
When we talk about a 'heat conduction problem', we are focusing on finding out how heat, or thermal energy, moves through a material. It's similar to figuring out how a crowd moves through a narrow hallway - there's a flow from high concentration to low concentration.

In our example, the concentration we're dealing with is temperature; heat flows from hot to cold, obeying the temperature gradient鈥攋ust like people move from crowded to less crowded areas. Mathematically, we capture this using the heat equation, a partial differential equation that tells us how temperature changes not only in space but also in time. In simple terms, the heat equation is the rulebook that defines how temperature evolves within a material under given conditions. In real-life applications, solving this equation can help engineers predict how much insulation they need for a house or how to keep electronic devices from overheating.
Numerical Discretization
Numerical discretization is a technique that approximates a continuous problem, like heat flow, with a set of discrete points. Think about it like turning a smooth ramp into a staircase. While the ramp is a continuous slope, you can only stand on the discrete steps of the stair鈥攏umerical discretization works in a similar way with functions and equations.

For example, to discretize the heat equation, we divide the material into a grid and then approximate the temperature at each grid point rather than needing an infinite number of points in a continuous space. This way, we transform the problem into something that can be computed using numerical methods. It's a blend of mathematical techniques and the practical necessity of solving complex problems with computers. By discretizing, we move from the realm of calculus, with its derivatives and integrals, to the realm of algebra, making use of familiar addition, subtraction, and multiplication to crack the code of the heat conduction problem.
Partial Differential Equations
Partial Differential Equations (PDEs) are to mathematicians what recipes are to chefs: they provide the steps needed to mix ingredients鈥攊n this case, functions and their derivatives鈥攖o produce a desired result. PDEs are equations that involve rates of change with respect to more than one variable, like time and space.

In the context of the heat conduction problem, PDEs describe how the temperature changes in a material when it's affected by heat flow. The 'partial' part means we're looking at how a change along a single dimension, say 'space', affects the overall temperature, without forgetting that things could be simultaneously changing with time. These equations are super powerful, allowing us to model a myriad of natural phenomena, from the flow of water in a river to the deformation of a bridge under load. Due to their complexity, solving PDEs often requires simplifications, approximations, or numerical methods鈥攍ike the finite difference method we use for tackling heat conduction problems.

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Most popular questions from this chapter

Consider transient one-dimensional heat conduction in a pin fin of constant diameter \(D\) with constant thermal conductivity. The fin is losing heat by convection to the ambient air at \(T_{\infty}\) with a heat transfer coefficient of \(h\) and by radiation to the surrounding surfaces at an average temperature of \(T_{\text {surr }}\). The nodal network of the fin consists of nodes 0 (at the base), 1 (in the middle), and 2 (at the fin tip) with a uniform nodal spacing of \(\Delta x\). Using the energy balance approach, obtain the explicit finite difference formulation of this problem for the case of a specified temperature at the fin base and negligible heat transfer at the fin tip.

Consider steady one-dimensional heat conduction in a plane wall with variable heat generation and constant thermal conductivity. The nodal network of the medium consists of nodes \(0,1,2,3\), and 4 with a uniform nodal spacing of \(\Delta x\). Using the finite difference form of the first derivative (not the energy balance approach), obtain the finite difference formulation of the boundary nodes for the case of uniform heat flux \(q_{0}\) at the left boundary (node 0 ) and convection at the right boundary (node 4) with a convection coefficient of \(h\) and an ambient temperature of \(T_{\infty}\).

What are the limitations of the analytical solution methods?

Consider a large plane wall of thickness \(L=0.4 \mathrm{~m}\), thermal conductivity \(k=2.3 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\), and surface area \(A=20 \mathrm{~m}^{2}\). The left side of the wall is maintained at a constant temperature of \(95^{\circ} \mathrm{C}\), while the right side loses heat by convection to the surrounding air at \(T_{\infty}=15^{\circ} \mathrm{C}\) with a heat transfer coefficient of \(h=18 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Assuming steady one-dimensional heat transfer and taking the nodal spacing to be \(10 \mathrm{~cm},(a)\) obtain the finite difference formulation for all nodes, \((b)\) determine the nodal temperatures by solving those equations, and (c) evaluate the rate of heat transfer through the wall.

Consider steady heat conduction in a plane wall with variable heat generation and constant thermal conductivity. The nodal network of the medium consists of nodes \(0,1,2,3\), and 4 with a uniform nodal spacing of \(\Delta x\). Using the energy balance approach, obtain the finite difference formulation of the boundary nodes for the case of uniform heat flux \(\dot{q}_{0}\) at the left boundary (node 0 ) and convection at the right boundary (node 4) with a convection coefficient of \(h\) and an ambient temperature of \(T_{\infty}\).
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