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Description
When storing hydrogen in pressure vessels, there is a possibility that the stored hydrogen will be released into the atmosphere via exhaust units if the pressure rises above a maximum value. If the released hydrogen mixes with air, a potentially explosive atmosphere (Ex-zone) can result. This contribution examines the minimization of Ex-zone dimensions above vertical exhaust units on hydrogen systems.
Based on the geometry of the standard Lambda exhaust unit commonly used in the industry, optimizations and modifications of this geometry were carried out. A total of six additional exhaust units were developed. The dimensions of the geometries correspond to the nominal diameter DN125 of the Lambda exhaust unit. The various exhaust units were investigated using numerical simulations with the ANSYS CFX 2022 R2 software.
The simulated system consists of two components: the exhaust unit and the spreading area. These simulation steps were carried out sequentially, with the results of the first step serving as input parameters for the second step. In the first simulation step, the seven exhaust units were simulated with mass flow rates ranging from 0.001\,kg/s to 0.01\,kg/s. In the evaluation, the flow velocities and the hydrogen mass fraction were analyzed, as these represent the input parameters for the spreading area in the second simulation step.
The investigation of the Ex-zones revealed, in a parameter study, that the outlet velocity has a stronger influence on the height of the Ex-zone than the hydrogen mass fraction at the outlet of the exhaust unit. The influence of the hydrogen mass fraction increases as the outlet velocity decreases. To determine the dimensions of the Ex-zone resulting from the investigated exhaust units, the maximum values at the outlet of each exhaust unit were used for the worst-case estimation.
It was observed that the exhaust unit with the lowest maximum values of outlet velocity and hydrogen mass fraction over the entire mass flow range has the smallest Ex-zone dimensions. This exhaust unit is a straight unit that has been supplemented by a diffuser tube with a diameter extension in the lower section. This extension forms a larger mixing chamber. The Ex-zone dimensions of this exhaust unit are the only ones among all variants examined to be smaller than those of the standard Lambda exhaust unit.
