Hi y'all, thanks in advance to anyone who knows a bit more about compressible/isentropic flow than me for the help, I have an interesting problem which I've been working on for a bit. Basically, I would like to measure species coming out of a reactor using molecular beam mass spectrometry. The reactor should be at medium (1000-1200 K) temperatures and atmospheric or higher pressure. Because I want to measure the chemistry, I would like to transport molecules "instantly" from the sampling point to the mass spec. However, the issue is that because of the steep pressure and temperature gradients as we leave the reactor, there is about an inch between the exit of my reactor and the stage 1 skimmer of the MBMS (see MS paint masterpiece below). So what I have there is a converging mach nozzle, which goes into a low pressure region, so the gas expands/accelerates and forms a beam at the skimmer. The problem with this setup is there is a lot of cooling as the reactor goes through the unheated wall, so we are not getting a good picture of the species exactly as the reaction ends. Now I am not an isentropic flow expert, and know nothing about nozzle geometries. My question is: what happens if we change the nozzle geometry to be more like this? (Brace yourself for more MS masterpiece....) So, is the flow close to M=1 through the whole thin section of the nozzle? Or does that geometry cause shocks or something which will kill the isentropic-ness of it? Is there a geometry that help me to "stretch" the nozzle so that the time that the gas spends at low temperature is very short, but that it still expands very close to the stage 1 skimmer? The key here is that the time that the gas spends in the area with uncertain temperature must be very low, but at the same time, the nozzle exit/expanding part must be within about 5 mm of the skimmer, due to the location of the mach disk. Would something like this be much better? Thanks for any help! Science!