Scenario dependence of hydrogen–helium similarity: Multi-scenario comparison and applicability boundaries

When energy systems shift toward cleaner fuels, hydrogen has emerged as a leading candidate because it carries a great deal of energy for its weight and leaves no carbon emissions when used. However, its extreme lightness and wide range of flammability make it dangerous if it leaks, as it can spread quickly and ignite. To design safe storage and transport systems, engineers must understand exactly how a hydrogen cloud moves and mixes with the air after a leak. Testing this directly with hydrogen is difficult and risky, often requiring explosion-proof facilities and strict safety controls. Because of these dangers, researchers frequently use helium as a stand-in. Helium is also much lighter than air and behaves similarly in many ways, but it is not flammable, making it far safer to handle. The central challenge, however, is that helium is not identical to hydrogen. They have different densities and physical properties, meaning a simple rule that converts a helium measurement into a hydrogen prediction does not always work. The question is not just whether helium can be used, but under exactly what conditions a helium test will accurately tell us what would happen with hydrogen.

A team of researchers at Tongji University set out to map the boundaries of this similarity. They wanted to know if a single, fixed conversion factor could ever be trusted, or if the answer depends entirely on the specific situation of the leak. To find out, they built a controlled laboratory setup to release jets of both gases and measure how their concentrations changed as they traveled through the air. They tested three distinct scenarios that represent real-world risks: a jet shooting out horizontally into open space, a jet shooting straight up, and a jet released inside a semi-confined room with vents. In each case, they varied the size of the hole the gas came out of and the speed at which it was released, creating eight different conditions to see how the gases behaved.

The researchers discovered that the idea of a single, universal conversion rule is flawed. In an open space where a jet shoots out horizontally, the behavior of the gas is a tug-of-war between two forces: the initial push of the gas leaving the hole, which we can call exit momentum, and the natural tendency of the light gas to float upward, known as buoyancy. When the gas leaves the hole with low momentum, the buoyancy takes over quickly. Because hydrogen is lighter than helium, it rises and drifts away from the horizontal path faster than the helium does. This causes the two gases to separate, and the helium measurement no longer matches what the hydrogen would have done. The researchers found that at low flow rates, this separation happened within less than a meter, rendering the helium data unreliable for predicting hydrogen behavior further down the line.

However, the story changes when the researchers increased the force of the release. When they pushed the gas out faster or through a larger opening, the initial momentum was strong enough to keep the jet moving straight for a longer distance before the buoyancy could pull it apart. In these high-momentum cases, the helium and hydrogen stayed closer together for longer, extending the distance over which a helium test could safely predict hydrogen behavior. The researchers also found that the size of the nozzle mattered significantly. A larger nozzle at the same flow rate produced a slower, weaker jet that lost its similarity to hydrogen much faster than a smaller, faster jet. This means that simply matching the volume of gas released is not enough; the speed and force of the exit are critical factors that a simple conversion rule ignores.

The direction of the leak also played a decisive role. When the researchers pointed the jet straight up, the buoyancy force worked with the momentum instead of against it. Both gases rose together, and their concentration profiles remained much closer for a longer distance compared to the horizontal jets. In this vertical orientation, the helium test remained a good predictor of hydrogen behavior even at lower flow rates. This showed that the orientation of the leak is just as important as the speed of the gas. A fixed rule that works for a vertical leak might fail completely for a horizontal one, and vice versa.

The final scenario involved a semi-confined space, like a garage or a room with limited ventilation. Here, the walls of the room changed the game entirely. The walls prevented the gas from spreading out and rising freely, effectively trapping the buoyancy forces and keeping the gases mixed together. In this environment, the hydrogen and helium stayed remarkably similar, with their concentration ratio remaining steady and close to the standard conversion values used in safety guidelines. The confinement acted as a stabilizer, preventing the gases from separating even when the initial momentum was low. This suggests that in enclosed spaces, the simple conversion rules used by engineers are much more reliable than they are in open air.

The study concludes that there is no single magic number to convert helium results into hydrogen predictions. Instead, the accuracy of a helium test depends on a specific balance between the initial push of the gas, the direction it is traveling, and the space it is moving through. The researchers developed a framework based on these physical factors to help engineers decide when a helium test is valid and when it is not. They found that in open horizontal jets, similarity is fragile and easily lost, while in vertical jets or confined rooms, it is much more robust. This work provides a clear, physics-based guide for safety engineers, showing that to predict a hydrogen leak accurately, one must first understand the specific conditions of the release, rather than relying on a one-size-fits-all formula.