3.1. Feasible Methods
Convective heat transfer coefficient enhancement is normally realized by improving the uniformity of fields, such as the velocity field and temperature field, or reducing the angle between the velocity vector and the heat flux vector. In engineering practice, there are two methods to enhance the convective heat transfer coefficient: enhancement with no extra power and enhancement with extra power, as shown in
Table 3 [
17]. The feasibility of these methods needs to be considered from two aspects: whether they affect the conductor performance of the pyro-breaker or the arc extinguishing medium.
Suitable methods to enhance the convective heat transfer coefficient for the cavity in the presented pyro-breaker include surface expansion, surface roughening and jet strengthening. The surface expansion method, however, involves an increase in explosive dosage and other problems caused by the enlargement of the barrel conductor. Therefore, the surface expansion method is not considered in this paper.
3.2. Numerical Calculation
Jet strengthening is an important method to improve the convective heat coefficient. For the cylindrical cavity discussed in this paper, jet strengthening can be realized by increasing the flow rate of cooling water and changing the structure of the inlet water channels. Compared with the integrated type pyro-breaker, the separated type pyro-breaker has a more flexible structure, which is easier to optimize. In a previous study, the influence of changing the inlet angle of the water on the convective heat transfer coefficient was analysed [
16]. In this paper, the structure with a 50 degree inlet angle has been selected for further analysis.
In the integrated type pyro-breaker, the structures of the upstream and downstream conductors are different. This will lead to an asymmetry of the inlet and outlet water channels. The CS in the separated type pyro-breaker is designed with symmetrical inlet and outlet water channels, which are able to improve the uniformity of the velocity field. Numerical models are built in CFX [
18] at the full scale of the cavity to calculate, compare and analyse the optimizations based on the suitable methods for the enhancement of convective heat transfer of the cavity in the presented pyro-breaker.
The boundary conditions of the fluid domain are shown in
Table 4. Unlike the other steady-state conditions, the inlet speed and the wall roughness are set in order to compare the results.
The
k-ε and
k-ω two-equation models are used as turbulence models in the simulation. They offer a good compromise between numerical effort and computational accuracy. One of the advantages of the
k-ω formulation is the near-wall treatment for low-Reynolds-number computations. The model does not involve the complex nonlinear damping functions and is therefore more accurate and more robust [
19].
Numerical calculations of the heat transfer are performed using the ANSYS-CFX code. The continuity Equation (1), the momentum Equation (2), and the total energy Equation (4) are applied as governing equations [
19].
where the stress tensor
τ is related to the strain rate by Equation (3):
where
htot is the total enthalpy, related to the static enthalpy
h (
T,
p) by:
represents the work due to viscous stresses and is called the viscous work term. This models the internal heating by viscosity in the fluid and is negligible in most flows.
represents the work due to external momentum sources and is currently redundant. More details about these equations can be found in [
19].
Different sizes are selected for mesh generation to study the grid independence.
Table 5 shows the grids for the symmetrical structure under an inlet speed of 1 m/s and a smooth wall roughness. The average calculated convective heat transfer coefficients are compared under different body sizes. It was observed that the deviation between the size of 1 mm and 2 mm is only 0.75%. Thus, for further calculations, a structural and hexagonal mesh (
Figure 5) with body size of 2 mm is chosen.
As shown in
Figure 6, the velocity stream and the convective heat transfer coefficients of asymmetric and symmetric structures under different inlet water velocities are simulated.
Figure 6a and
Figure 6b illustrate the velocity stream and the convective heat transfer coefficients with an inlet velocity of 1 m/s.
Figure 6c and
Figure 6d illustrate the velocity stream and the convective heat transfer coefficients with an inlet velocity of 4 m/s. The result shows that the maximum velocity in the asymmetric structure is higher than in the symmetric structure, while the symmetric structure has a more uniform velocity stream. The convective heat transfer coefficient of the symmetric structure is significantly higher than the asymmetric structure. The difference is more pronounced with the increase of inlet water velocity.
The thermal load generated by the barrel conductor is mainly consumed through the inner surface. The contact surfaces of the barrel conductor with the upstream and downstream conductors in the integrated type are located on both the outer surface and the inner surface. The surface roughness of the entire barrel conductor is R30. The barrel conductor in the separated type pyro-breaker is designed to only contact other conductors on the outer surface. Therefore, changing the roughness of its inner surface will not affect the contact resistance. The influence of surface roughening on the convective heat transfer coefficient can also be simulated in CFX by changing the parameters of the surface conditions. Convective heat transfer coefficients with a surface roughness of R30 and R100 are simulated. As shown in
Figure 7, although an increase in the surface roughness reduces the fluid velocity, the convective heat transfer coefficient is obviously improved.
Table 6 shows the average calculated convective heat transfer coefficient of the cavity under different structural optimizations. The convective heat transfer coefficient of the symmetrical structure with a surface roughness of R100 and an inlet water velocity of 4 m/s can meet the demanding requirements of a 100 kA pyro-breaker, as calculated by ANSYS/Workbench.
3.4. Results and Discussion
The results of the experiments are shown in
Figure 8. The rising speed and the steady value of the average measured temperature on the barrel conductor of the symmetrical structure are significantly lower than the asymmetrical structure. The temperature difference between a surface roughness of R30 (76.64 °C) and R100 is relatively smaller but has an obvious effect on the heat transfer enhancement. This indicates that the selected methods are feasible to enhance the convective heat transfer coefficient.
As the average convective heat transfer coefficient of the cavity is unable to be measured, the calculated convective heat transfer coefficient of the cavity is verified by the comparison between the simulation and the experiment of the barrel conductor, as illustrated in
Table 7. The temperature rise for different prototypes has been simulated with the calculated convective heat transfer coefficient of the cavity in ANSYS/Workbench. The measured temperature rises have good consistency with the simulated temperature rises in all three prototypes. For the asymmetrical structure, the difference in the temperature rise between the simulation and the experiments is larger than that in the symmetrical structure. This may be due to the uniformity of the velocity field becoming even more severe under non-ideal conditions, which, on the other hand, proves the advantage of a symmetrical structure in the water channels.