# Thermal boundary layer

**Numerical simulations **

** Thermal boundary layer**

The thermal study is very important for the further evaporation in the container which represents the main work of this BEI.

We suppose that the incident air flow is at a uniform temperature, and that the surface is maintained in a temperature T_{p} also uniform but different from the air temperature T_{0}.

** a- Temperature field**

*Figure: the thermal boundary layer over the flat plate*

- By exploring the temperature field perpendicularly to the plate, we shall observe a progressive variation of T
_{p}to the air temperature T_{0}; in fact this variation is at first fast then it becomes slower and slower as we penetrate into the incident air flow. - The region in which T varies in a significant way corresponds the the thermal boundary layer. It is noticeable that the thermal boundary layer gets thicker when we go away from the leading edge: it has the same altitude as the dynamic boundary layer.
- At the origin, the temperature profile is uniform, then the influence of T
_{p}appears gradually in the fluid. As a consequence, a temperature gradient emerged as well as a heat flux directed from the plate towards the fluid.

So, as well as the dynamic boundary layer is the expression of momentum diffusion, the thermal boundary layer results from the thermal diffusion in the fluid in movement.

**Temperature profile***Figure: the temperature profile versus the altitude above the plate at x= 0.9 cm*

** b- Boundary layer thickness**

The aim of this part is to estimate the thermal boundary layer thickness values over the flat plate.

The region in which T varies in a significant way corresponds the the thermal boundary layer. However this definition is too vague. The problem was moreover the same with the dynamic boundary layer, and will be approached on the same spirit, namely a conventional definition of the thermal boundary layer thickness.

We define this thickness as following $$\frac{T(x,\delta_T(x)) - T_p (x)}{T_0 - T_p (x)}= 0.99 $$ .

**Blasius model for a flat plate**

Source:http://help.solidworks.com/2013/french/SolidWorks/cworks/c_Convection_Heat_Coefficient.htm?format=P

The Blasius solution estimates the thermal thickness in such case as follows:

$$ \frac {\delta_T}{x} = 4.64 \sqrt {\frac{a}{U_0 x}}$$ where a is the thermal diffusivity.

**c- Comparison**

We compare now the numerical boundary layer thickness with the Blasius solution.

The following figure shows the thermal thickness profile versus Blasius solution:

* Figure: the thermal thickness profiles*

As is the case of dynamic boundary layer, the thermal boundary layer thickness has the same profile as the Blasius solution.

** d- Conclusion:**

The results correspond perfectly to Blasius solution. Therefore, the study of the boundary layer has enabled us to validate our model in order to use it in the evaporation study of the fuel container.