In UE5 Niagara fluid simulations, the red box you see is the “Grid Bounds Overlay,” indicating the fluid’s simulation space. To achieve your desired gas explosion effect, you can:
1- Hide the box: Uncheck “Grid Bounds Overlay” in the Emitter Summary’s Scalability tab. This removes the visual box in-game but the simulation still functions within its boundaries.
2- Scale the box: You can’t directly adjust the bounding box size. However, manipulating the Emitter’s “Spawn Parameters” like Initial Speed and Lifetime can expand the fluid’s influence beyond the box, effectively creating a larger explosion within its limitations.
Remember, scaling beyond the box might impact performance. If needed, consider creating multiple smaller emitters strategically placed to achieve a larger, seamless explosion effect.