1. Introduction
Mathematical modeling of the heat conduction is an important stage in the design of systems subjected to thermal loads, because it allows the appropriate selection of materials to avoid adverse phenomena related to the occurrence of thermal stresses. For example, the temperature distribution in the tested system may lead to the formation of thermal stresses, which may cause micro-cracks in the components of this system [
1,
2,
3], and cyclical changes in the thermal load of the device may cause undesirable vibrations of the components of this device [
4,
5,
6]. On the other hand, the heat source causing high temperature of biological tissue may destroy diseased tissue, but it may also damage healthy tissue [
7,
8]. Different mathematical models are used to accurately describe the heat conduction in the considered bodies.
The classical mathematical model of heat conduction is derived from Fourier’s law of the heat conduction. The Fourier law establishes a proportionality between the heat flux vector and the temperature gradient [
9]
where
is the heat flux vector,
is the thermal conductivity of the material,
is the point in the considered region,
is the time,
is the gradient operator and
is the temperature. Although the Fourier law quite accurately describes the heat conduction in most practical macroscopic problems, the relationship (
1) implies an unrealistic infinite speed of the heat propagation; this means that the sudden temperature change at some point in the domain will be felt everywhere and instantaneously at distant points in the domain—hence, Fourier’s law can be treated as having the unphysical property [
10]. To eliminate this drawback of the mathematical model, the Fourier law (
1) is replaced by the following relationship [
11]
where
and
are the phase lags. Relationship (
2) for
and
is the dual-phase-lag constitutive equation for the heat conduction. The introduction of these phase lags into this model is interpreted as the relaxation times accounting for the effects of thermal inertia. The left and right sides of Equation (
2) are expanded into the Taylor series. Depending on the assumed number of the terms of this series, the different forms of the dual-phase-lag model can be obtained. In Ref. [
10], the authors discussed different forms of the dual-phase-lag model as well as presented their characteristics. In this work, the first-order approximation of the functions occurring on the left- and right-hand side of Equation (
2) is considered, which leads to the constitutive equation for the heat conduction in the form
In addition, the energy equation is introduced [
12]
where
is the volumetric rate of the heat generation,
is the density of the material and
is the specific heat capacity. Eliminating the heat flux from Equations (
3) and (
4), we obtain the heat conduction equation in the form
where
is the Laplace operator and
is the thermal diffusivity.
The dual-phase-lag heat shown in Equation (
5) has been applied in mathematical modeling of the heat transfer in functionally graded materials [
13,
14,
15], ultrafast pulse-laser heating problems [
16,
17], porous media [
18,
19,
20], nanocomposites [
21,
22], and living tissue [
23,
24,
25]. If
in Equation (
5), then the classical parabolic heat conduction equation is obtained. For
and
, one obtains the single-phase-lag equation of hyperbolic type. The wave character of this heat conduction equation was used in the investigations of propagations of heat waves in papers [
26,
27].
A generalization of the dual-phase-lag model of the heat conduction is obtained by replacement of the derivatives in the constitutive equation and energy equation by the derivatives of non-integer order. In the present paper, the Caputo derivative of non-integer order
is used and is defined by [
28]
The properties of the derivative can be found in many books devoted to fractional calculus, for instance in the books [
28,
29,
30,
31].
Replacing the time derivatives in the constitutive Equation (
3) and energy Equation (
4) by the Caputo time fractional derivatives, one obtains
where the coefficient
is introduced to keep the accordance of dimensions. Combining Equations (
7) and (
8), one obtains the heat conduction equation with the Caputo fractional derivatives in the form
We use this equation for modeling the heat conduction in a multi-layered body. According to the authors’ knowledge, this approach to the problem of two-dimensional time-fractional heat conduction in a multi-layered medium was not used in the literature. Due to the definition of the Caputo derivative, we can note that the differential Equation (
9) describes the temperature distribution in a body, taking into account the history of changes of the temperature.
The fractional Equation (
9) describes the heat conduction in a region which is specified by the considered medium. In this paper, we consider the fractional heat conduction in a spherical body. The Laplace operator which occurs in Equation (
9) in the spherical co-ordinates is defined as follows
where
is the radial coordinate,
is the polar and
is the azimuthal angle coordinate. Assuming that the temperature distribution in the body is azimuthally invariant (azimuthal symmetry), the last term in the bracket in Equation (
10) can be dropped. A simplified form of the Laplace operator can be obtained by introducing a new variable
which is related to the polar angle
by the relationship
Taking into account the azimuthal symmetry, the Laplace operator in the new coordinates can be written in the form
In this paper, we present a dual-phase-lag fractional heat conduction model for a multi-layered spherical body with azimuthal symmetry. An analytical solution of the problem is derived in the form of the double series of spherical Bessel functions and Legendre functions. Numerical computations of the temperature distribution in the body include a composite solid sphere, hemisphere and spherical cone. Effects of the fractional order of time-derivatives and phase lagging occurring in the heat conduction model on the temperature distribution in the considered bodies are investigated numerically.
The problem considered in this paper is a continuation of the research that has been presented in papers [
32,
33,
34,
35,
36]. A solution of the fractional heat conduction problem in the solid sphere was presented in [
32]. The authors studied a fractional single-phase lag heat conduction problem in the whole-space 1D domain [
34], in the slab [
35] and in the hollow cylinder [
33]. A fractional dual-phase-lagging heat conduction in the whole-space 1D domain was presented in [
36].
2. Formulation of the Problem
Let us consider the fractional heat conduction in a spherical body (with coordinates
consisting of
concentric spherical layers which are defined by:
(
),
, where
. The body is a full solid sphere when
tends to
, for
, the body is a hemisphere, and for
, the body is a spherical cone (
Figure 1).
The heat conduction in the
-th spherical layer is governed by the following time-fractional differential equation
where
is the temperature,
is the thermal conductivity,
is the thermal diffusivity,
is the volumetric heat source in the
-th layer and
denotes the fractional order of the Caputo derivative,
and
.
The fractional differential Equation (
13) for
is complemented by boundary and initial conditions and the conditions providing the perfect thermal contact of the neighboring layers. The conditions are as follows:
where
and
are the outer heat transfer coefficient and ambient temperature, respectively. It should be noted that in general, the boundary conditions (
17) and (
18) should be formulated using the dual-phase-lag model [
37,
38]. However, if values of the relaxation times are identical in two neighboring layers, than we can assume the simplification in the above boundary conditions. The initial conditions are assumed in the form
3. Solution of the Problem
An analytical solution to the initial-boundary problem (
13)–(
20) can be presented in the form of a sum
where the function
satisfies the fractional differential Equation (
13) and homogenous boundary conditions (transient problem) and
is a solution of a steady-state problem. Substituting the function
in the form (
21) into Equation (
13), we receive differential equations for the functions
and
. For the function
, one obtains an equation with fractional time derivative
and for the function
, one obtains the Laplace equation
Suppose that
is a function of the form
Taking into account Equation (
24) in the homogenous differential equation obtained by omitting the last term in Equation (
22) and separating the space and time variables, we receive the Helmholtz equation for the function
where
, whereas
is the separation constant.
In turn, introducing functions
,
,
, we write the functions
and
as
Substituting the functions
and
into Equations (
23) and (
25), separating the variables and assuming the separation constant as
where
is a real number, one obtains the three homogenous differential equations: Lagrange equation, Euler equation and spherical Bessel equation:
The functions
,
, and
satisfy boundary conditions which are obtained by taking the functions (
21), (
24) and (
26) in conditions (
14)–(
18). The function
satisfies the conditions:
The functions
,
satisfy the same conditions at
and at interfaces
,
(the superscripts are omitted):
where
. Moreover, the function
satisfies the homogenous condition at
and the function
, as the radial part of the function
, satisfies the condition
The solutions of Equations (
27)–(
29) with appropriate conditions among (
30)–(
35) are presented in
Section 3.1.
3.1. Solution of the Lagrange Equation
The solution to Equation (
27), which satisfies the condition (
30), is the function
where
is the Legendre function of the first kind and
is a constant. Using the derivative of the Legendre function [
39]
and the boundary condition (
31), one obtains the following equation
The roots of this equation for
(solid sphere) are
,
and for
(hemisphere) are
,
. In these cases, the eigenfunctions
and
where
is a positive integer number are the Legendre polynomials. The roots to Equation (
38) for
are numerically determined.
The functions
,
, create an orthogonal set of functions. The orthogonality condition of the functions can be derived using an indefinite integral of the product of functions
and
where
. Utilizing Equation (
27), this integral can be expressed as follows
where
is an arbitrary constant. Taking into account the antiderivative given by Equation (
39) and the boundary condition (
31), one obtains
In order to find the antiderivative of the square of the Legendre function, the integral occurring in Equation (
39), employing Equation (
37), is rewritten in the form
Evaluating limits of the expressions occurring on the left and right-hand sides of Equation (
41) as
tends to
, the square of the norm
of the Legendre function for
, one obtains in the form
where
are the roots of the Equation (
38). For
, we have
. Finally, the orthogonality condition of the functions
,
, can be written as
where
is the Kronecker delta.
3.2. Solution of the Euler Equation in the Multi-Layered Spherical Region
The general solution to the Euler Equation (
28) for
,
, is given by
where
,
are arbitrary constants. Using boundary conditions (
32), one obtains a set of
equations with unknowns:
,
,
. These equations are as follows
where
. Due to the condition (
32), we assume
in Equations (
44)–(
46).
The system of Equations (
45) and (
46) is complemented by an equation which is obtained on the basis of the boundary condition (
35) for the function
. The functions
are given by Equation (
26) as a product of two functions. To fulfil the condition (
35), we assume that
Substituting the function
into the condition (
35), multiplying the received equation by
, integrating with respect to
in the interval
and using the orthogonality condition (
43), one obtains the condition for the function
Employing Equation (
44) for
in Equation (
48), the following condition is obtained
where
. For the function
defined by
the integral
may be expressed in an analytical form as
Equation (
49) together with Equations (
45) and (
46) form a system of
equations which is solved for
,
with respect to
,
,
,
,
,
. Hence, the functions
,
are now fully defined by Equation (
47), whereas the functions
are given by Equation (
44).
3.3. Solution of the Spherical Bessel Equation in the Multi-Layered Region
The general solution to Equation (
29) can be written as follows
where
,
,
are constants, and
and
are spherical Bessel functions of the first and second kind, respectively. The spherical Bessel functions are defined by (see Ref. [
9])
where
and
are the Bessel functions of the first and second kind, respectively. The functions
are defined for
, wherein
. As the function
tends to infinity when
tends to zero, i.e.,
, then taking into account the condition (
14), we assume
in Equation (
51) for
.
Substituting the function
into the continuity conditions (
33) and the boundary condition (
34), we obtain a system of
homogenous equations with unknown
,
,
,
,
,
. This system of equations in the matrix form can be written as
where
is the column matrix of the unknowns and
is the coefficients matrix of the equations system. The non-trivial solution of Equation (
54) exists if the characteristic equation is satisfied
Next, this equation is solved numerically with respect to
for
,
. Next, for the roots
,
of Equation (
55), the coefficients
(index
is omitted) are successfully determined as a solution of Equation (
54) with
. Taking into account the eigenvalues
and the coefficients
in Equation (
51), we obtain the functions
. These functions satisfy the orthogonality condition which can be derived using the differential Equation (
29), the conditions at the interfaces (
33) and the boundary condition (
34). The orthogonality condition has the form
where
The functions
, defined by Equation (
26), can be now written as
3.4. Solution of the Time-Fractional Differential Equation
The functions
are used to express the functions
in the double series form
where the functions
satisfy the nonhomogeneous Equation (
22). Substituting the functions (
59) into Equation (
22) and using the orthogonality conditions (
43) and (
56), we receive the fractional differential equation for the functions
in the form
This differential equation is complemented by initial conditions which are derived using Equations (
19), (
20) and (
59) and the orthogonality conditions (
43) and (
56). The initial conditions for the function
have the form
In order to determine a solution of the initial value problem (
60) and (
61), we apply the Laplace transform technique. The Laplace transform
of the function
is defined as
The Laplace transform of the Caputo derivative of order
is given by
The application of the Laplace transform to Equation (
60) allows us to express the transform
in the form
where
For the purpose of determining the inverse Laplace transform
, we find firstly four inverse transforms:
,
,
and
. Introducing notation:
with
and
, we rewrite
as
where
. Employing the Laplace transform pair given by
where
is a three parameter Mittag–Leffler function, which is also known as the Prabhakar function [
28], and we obtain the inverse transform
for
in the form
If
, then on the basis of (
67), we have
Using the property of the Laplace transforms, we can find the inverse Laplace transform
as an integral of the function
. Utilizing the formula for integration of the Mittag–Leffler and Prabhakar functions
in Equations (
69) and (
70), we obtain
The inverse Laplace transform
can be obtained using Formula (
68)
Employing the functions
,
,
and the Laplace transform
given by Equation (
65), we can write the functions
as
If the functions
are given by
where
is a constant, then the functions
can be written in the form
Taking the function
into account in Equation (
59), we obtain the function
. This function and the function
given by Equation (
47) are used in Equation (
21), which determines the temperature
in the
-th layer of the body under consideration.
4. Numerical Examples and Discussion
The presented analytical solution of the fractional heat conduction problem was used to compute the temperature distributions in a layered spherical cone to investigate the effect of the phase-lags and the order of the fractional time-derivative on the heat conduction in the medium. The analysis concerns the heat conduction in the cone of constant initial temperature with an inner or outer heat source. We introduce the non-dimensional time
, the non-dimensional radii
and
where
determines the boundary of the
-th spherical layer,
. Temperature
in the cone is defined as
where
,
and
is the layer number; i.e., the condition
is fulfilled. The spherical cone under consideration consists of five concentric layers wherein
,
. The physical data assumed in the computation are given in
Table 1.
The role of the fractional order occurring in the mathematical model of the fractional heat conduction and its effect on the temperature distribution in the spherical cone was investigated for . Computer simulation data were generated by the Mathematica software.
The first example concerns the heat conduction in a spherical cone without inner and outer heat sources. The cone is specified by the radius
m and the half-angle at the apex of the cone
. The initial temperature, the ambient temperature and the heat transfer coefficient were assumed as:
Fi(
r,
μ) =
T0 = 20 °C,
Ta = 0 °C and
, respectively. The computations were performed for the phase lags
s,
s (artificial values, identical for each layer) and six different values of the order of fractional derivative:
= 0.4; 0.425; 0.45; 0.475; 0.5; 0.525; 0.55; 0.575; 0.6.
Figure 2 presents the dimensionless temperatures in the cone
versus
for the non-dimensional time variable
. The results show the effect of the order of fractional derivative occurring in the heat conduction equation on the temperature distribution in the cone. The temperatures in the points of the cone tend with time to a steady state for each value of the derivative order
. It can be noted that temperature decreases faster for larger values of the order
occurring in the mathematical model of the heat conduction.
In the case of the heat conduction in the spherical cone without an inner heat source, we predict that the function
occurring in Equation (
21) satisfies the condition
for
and all
and
, i.e., the steady state of temperature distribution:
occurs in the cone. For the purpose of the formal proof of this statement, the asymptotic formula may be used for the Mittag–Leffler function
given by
Employing this formula in Equation (
72), we obtain that
. Taking into account the first term on the right-hand side of Equation (
76) for consideration of the heat conduction without an inner source, we have
for
and
Hence, on the basis of Equation (
59), the condition (
78) is fulfilled. Thus, the steady-state temperature distribution in the
-th layer of the solid sphere is given by Equation (
21) in which the first term on the right-hand side can be omitted. In this case, the temperature in the considered region is independent from the phase-lags and the order of fractional derivatives which occur in the heat conduction model. In
Figure 3, the 3D graph and contour plot of the function
illustrating the change of temperature in a hemisphere when its surface is heated by an outer heat source defined by Equation (
50) with
and
= 40 °C shown. The physical data that characterized the hemisphere, which were used in computation, are the same as in the first example.
The Robin condition (
18) on the spherical surface of the cone describes the heat exchange with the environment. If the ambient temperature is higher than the temperature of the spherical surface of the cone, then the spherical surface is heated, and the heat flows in the cone in the radial direction. In
Figure 4, curves are presented that illustrate the changes of temperatures in the cone when the ambient temperature is specified by Equation (
50) with
= 0 °C and
= 20 °C. The calculations were performed assuming that there is no internal heat source.
The next example concerns the spherical cone heated by the internal heat source described by Equation (
76). The curves in
Figure 5 present temperature distributions in the spherical cone for volumetric heat source
and four values of dimensionless time. Decreasing the temperature while increasing the radial distance from the heat source follows as a result of the heat exchange through the spherical surface of the cone with the environment of zero temperature. It should be noted that the temperatures in a part of the cone close to the heat source are higher for the higher fractional orders of the heat conduction models, and the temperatures in a part of the cone close to the spherical surface are higher for the lower orders of the conduction models.