#### Abstract

The solutions of nonlinear heat equation with temperature dependent diffusivity are investigated using the modified Adomian decomposition method. Analysis of the method and examples are given to show that the Adomian series solution gives an excellent approximation to the exact solution. This accuracy can be increased by increasing the number of terms in the series expansion. The Adomian solutions are presented in some situations of interest.

#### 1. Introduction

In the classical model of the heat equation, the thermal diffusivity and thermal conductivity of the medium are assumed to be constant. In some media such as gases, these parameters are proportional to the temperature of the medium giving rise to a nonlinear heat equation of the following form [1]:

where is the conductivity, is diffusivity, and is a constant.

However, in some situations the diffusivity is proportional to , which gives rise to a more general nonlinear heat equation

In this paper we investigate the nonlinear heat equation

#### 2. Method of Solution

Introducing the operator , (1.3) takes the form

We solve (2.1) subject to the initial condition

Applying inverse operator to both sides of (2.1) yields

The desired series solution by Adomian decomposition method is given by (cf. [26] for details)

and are calculated from recursive relation

where are the Adomian polynomials for the nonlinear operator

The formulas that can be used to generate Adomian polynomials are discussed by Adomian in [6]. Here we employ the algorithm of Wazwaz [10] to calculate Adomian polynomials, which seems quite natural and suited for implementation by software.

#### 3. Applications and Results

We consider the nonlinear heat equation

with power nonlinearity . We are interested in investigating the case of power nonlinearity due to the fact that this assumption is made in most of the applied nonlinear problems of heat transfer and flows in porous media. For instance, corresponds to fast diffusion processes of plasma diffusion and thermal expulsion of liquid Helium [1618]. The diffusivity is used to model process of melting and evaporation of metals [1719]. For the initial temperature profile, we consider typical cases like a quadratic function or or which corresponds to soliton like initial profile.

Case A (). The Adomian solution for general a, b, c, and can be obtained from authors as Mathematica file. Some particular cases for a, b, c, andare considered as follows.
(i)
The Mathematica code to obtain Adomina solution in this case consists of the following commands:
maximum number of polynomials and solution terms: Finding Adomian polynomials: Making vector of admian polynomials: Finding solution u(x,t): The Adomian solution obtained is The solutions in Figure 1 increase algebraically as is expected from algebraic behavior of initial condition and the form of .
(ii) (Figure 2).
As the diffusivity in this case is decreasing function of , the solution exhibits the change in the quadratically increasing initial temperature.
(iii) (Figure 3).

Case B (). The Adomian solution for general can be obtained from authors as Mathematica file. Some particular cases are considered as follows.
(i) and
Figure 4 displays how the bell-shaped initial temperature interacts with quadratic dependence of diffusivity.
(ii) and (Figure 5).

(iii) and (Figure 6).

Case C (). The Adomian solution for general can be obtained from authors as Mathematica file. Some particular cases are considered as follows.
(i)
Here the initial condition is soliton like. This is reflected in the Figure 7 as the diffusivity varies quadratically.
(ii) (Figure 8).