In this study, predicting the introduction of a hydrogen fuel cell propulsion line, CFD analysis was performed on the optimal ventilation inside the hydrogen tank storage room in case hydrogen leaks from the hydrogen tank installed on the ship. By 3D modeling the hydrogen tank storage room and comparing it with results of similar experiments, the reliability of the CFD analysis results in the hydrogen tank storage room was also secured.
CFD commercial code ANSYS CFX(v 18.1) was used for numerical analysis. In addition, a standard κ-ε model was used to consider the turbulent flow in which hydrogen diffuses due to hydrogen leakage from the hydrogen tank storage room. In order to improve the stability of convergence and the accuracy of numerical solutions, continuity equations, momentum equations, and turbulent energy were used.
The hydrogen tank storage room was the same size as the LNG tank room of the econuri ship operated by Incheon Port Authority. In the numerical analysis, the hydrogen tank storage room was constructed in a simple form installed inside a 20m³ independent hydrogen tank with a volume(180.2m³) of 5.28m(W) x 8.98m(L) x 3.8m(H).
A stagnation regions of leaked hydrogen in the hydrogen tank storage room was identified; the optimal installation location of the hydrogen sensor was selected; and the ventilation performance of the hydrogen tank storage room was numerically analyzed while changing the supply/exhaust port location, ventilation volume, ventilation method, etc. Through this, the following CFD analysis results were obtained.
Regardless of the apex angle(A) of the ceiling of the hydrogen tank storage room, the leaking hydrogen with a small density stagnated in the corner of the uppermost part of the ceiling just above the leaking part. Due to stratification, the smaller the A, the thicker the upper layer of the leaked hydrogen. The optimal installation positions of the hydrogen sensor capable of quickly detecting leaked hydrogen were S0, S10, and S20.
The location of the supply port is where the air flow does not stagnate in the hydrogen tank storage room. That is, when the leaking part is at the top of the hydrogen tank as in Case I ~ Case III, the air flow direction of the supply air is well formed from the ceiling surface to the exhaust port.
It was confirmed that ventilation performance can be improved by shortening the retention time of leaking hydrogen. As for the location of the exhaust port, the ventilation performance was the best in Case III / Inlet-1 / Vent-1 when A was 177.7° and Case III / Inlet-1 / Vent-2 when A was 120°. It was suggests to be the optimal location for discharging.
The average mole fraction of leaked hydrogen in the hydrogen tank storage room showed the largest decrease by 2~3 times when the ventilation amount was increased from 1Q to 2Q. Although there is a slight difference depending on the location of the supply and exhaust ports, the ventilation performance was improved just by increasing the ventilation volume to 2Q.
However, when A is 120°, compared to 177.7°, the average mole fraction of leaking hydrogen near the ceiling is in the range of 0.04, which is mostly higher than the lower limit of flammability(LFL). Therefore, it was analyzed that as A decreased, increasing ventilation closer to 3Q was a way to improve ventilation performance.
The average velocity distribution in the hydrogen tank storage room was similar according to the inflow velocity regardless of A. For optimal ventilation, if A is 177.7°, it was interpreted that installing the air inlet at the bottom of the side wall and making the ventilation volume at least 2Q were necessary.
When A is 120°, it was analyzed that an increase in the amount of ventilation can further improve the ventilation performance when the exhaust port is installed near the ceiling surface.
As a result of analyzing the ventilation performance according to the ventilation method by selecting the location of the supply and exhaust ports for each case, when A is 177.7°, the results showed the performance order of the Type-II>Type-I>Type-III. The Type-II method showed the best ventilation performance.
When A is 120°, as a result of analyzing the velocity distribution and pressure distribution in the hydrogen tank storage room there was no significant difference according to the ventilation method.
However, in order to apply the negative pressure form mechanical forced ventilation system according to the standard for gas-fueled ships, it was judged that Type-III would be suitable.
The results of CFD numerical analysis in this study are considered to be applied not only to ships but also to onshore storage facilities, and will be used as basic data for system design for optimal ventilation of the enclosed hydrogen tank storage room.