we'd use it as the internal part, it would have a cover so the question is how big the porous must be to redirect the gases and slow them down

To determine the appropriate size of the pores for your internal part, there are several factors to consider:

**Gas Properties**: The type of gases you're dealing with (molecular weight, viscosity, etc.) will influence how they flow and how big the pores need to be to redirect and slow them down.

**Desired Flow Rate**: Consider the flow rate of the gases that you need to maintain. Smaller pores can create more resistance to flow, which might help to slow down the gas, but they may also restrict flow too much if they are too small.

**Material**: The material of the cover and the internal part will influence the size of the pores. Some materials can handle smaller pores without becoming clogged, while others may require larger pores.

**Application Requirements**: What is the end goal of redirecting and slowing down the gases? Is it for filtration, mixing, diffusion, or some other purpose? This will affect the design parameters.

**Drag and Flow Dynamics**: The concept of drag in fluid dynamics comes into play. The size of the pores can create turbulent flow, which may help slow down the gas. Computational fluid dynamics (CFD) simulations can be useful here.

**Testing**: If possible, prototyping with varying pore sizes can help find the optimal size for your specific application. 

As a general guideline, the size of the pores typically ranges from micrometers (for gases) to millimeters, but you will need to consider the above factors as you make your decision. Measuring the flow properties with different pore sizes will give better insight into finding the right fit for your application.