# Heat Transfer Model

The heat transfer model is a specific implementation of the `Model`

interface dedicated to handle the dynamic heat equation.

## Theory

The strong form of the dynamic heat equation can be expressed as

with \(T\) the scalar temperature field, \(c_v\) the specific heat capacity, \(\rho\) the mass density, \(\mat{\kappa}\) the conductivity tensor, and \(b\) the heat generation per unit of volume. The discretized weak form with a finite number of elements is

with \(i\) and \(j\) the node indices, \(\vec{n}\) the normal field to the surface \(\Gamma = \partial \Omega\). To simplify, we can define the capacity and the conductivity matrices as

and the system to solve can be written

with \(\vec{Q}^{\text{ext}}\) the consistent heat generated.

## Using the Heat Transfer Model

A material file name has to be provided during initialization.
Currently, the `HeatTransferModel`

object uses dynamic analysis
with an explicit time integration scheme. It can simply be created
like this

```
HeatTransferModel model(mesh, spatial_dimension);
```

while an existing mesh has been used (see ref{sect:common:mesh}). Then the model object can be initialized with:

```
model.initFull()
```

This function will load the material properties, and allocate / initialize the nodes and element `Arrays`

More precisely, the heat transfer model contains 4 `Arrays`

:

**temperature**contains the nodal temperature \(T\) (zero by default after the initialization).**temperature_rate**contains the variations of temperature \(\dot{T}\) (zero by default after the initialization).**blocked_dofs**contains a Boolean value for each degree of freedom specifying whether the degree is blocked or not. A Dirichlet boundary condition (\(T_d\)) can be prescribed by setting the**blocked_dofs**value of a degree of freedom to`true`

. The**temperature**and the**temperature_rate**are computed for all degrees of freedom where the**blocked_dofs**value is set to`false`

. For the remaining degrees of freedom, the imposed values (zero by default after initialization) are kept.**external_heat_rate**contains the external heat generations. \(\vec{Q^{ext}}\) on the nodes.**internal_heat_rate**contains the internal heat generations. \(\vec{Q^{int}} = -\mat{K} \cdot \vec{T}\) on the nodes.

Only a single material can be specified on the domain. A material text file (*e.g.* material.dat) provides the material properties as follows:

```
model heat_transfer_model [
capacity = %\emph{XXX}%
density = %\emph{XXX}%
conductivity = [%\emph{XXX}% ... %\emph{XXX}%]
]
```

where the `capacity`

and `density`

are scalars, and the `conductivity`

is specified as a \(3\times 3\) tensor.

## Explicit Dynamic

The explicit time integration scheme in `Akantu`

uses a lumped capacity
matrix \(\mat{C}\) (reducing the computational cost, see Chapter Solid Mechanics Model).
This matrix is assembled by distributing the capacity of each element onto its nodes. Therefore, the resulting \(\mat{C}\) is a diagonal matrix stored in the `capacity`

`Array`

of the model.

```
model.assembleCapacityLumped();
```

Note

Currently, only the explicit time integration with lumped capacity
matrix is implemented within `Akantu`

.

The explicit integration scheme is *Forward Euler* [Cur92].

Predictor: \(\vec{T}_{n+1} = \vec{T}_{n} + \Delta t \dot{\vec{T}}_{n}\)

Update residual: \(\vec{R}_{n+1} = \left( \vec{Q^{ext}_{n+1}} - \vec{K}\vec{T}_{n+1} \right)\)

Corrector : \(\dot{\vec{T}}_{n+1} = \mat{C}^{-1} \vec{R}_{n+1}\)

The explicit integration scheme is conditionally stable. The time step has to be
smaller than the stable time step, and it can be obtained in `Akantu`

as
follows:

```
time_step = model.getStableTimeStep();
```

The stable time step is defined as:

where \(\Delta x\) is the characteristic length (*e.g* the in-radius in the
case of linear triangle element), \(\rho\) is the density,
\(\mat{\kappa}\) is the conductivity tensor, and \(c_v\) is the specific
heat capacity. It is necessary to impose a time step which is smaller than the
stable time step, for instance, by multiplying the stable time step by a safety
factor smaller than one.

```
const Real safety_time_factor = 0.1;
Real applied_time_step = time_step * safety_time_factor;
model.setTimeStep(applied_time_step);
```

The following loop allows, for each time step, to update the `temperature`

,
`residual`

and `temperature_rate`

fields following the previously described
integration scheme.

```
for (UInt s = 1; (s-1)*applied_time_step < total_time; ++s) {
model.solveStep();
}
```

An example of explicit dynamic heat propagation is presented in
`examples/heat_transfer/explicit_heat_transfer.cc`

. This example consists
of a square 2D plate of \(1 \text{m}^2\) having an initial temperature of
\(100 \text{K}\) everywhere but a none centered hot point maintained at
\(300 \text{K}\). Fig. 24 presents the geometry
of this case. The material used is a linear fictitious elastic material with a
density of \(8940 \text{kg}/\text{m}^3\), a conductivity of
\(401 \text{W}/\text{m}/\text{K}\) and a specific heat capacity of
\(385 \text{J}/\text{K}/\text{kg}\). The time step used is
\(0.12 \text{s}\).