Flow past orifice

aurel

Hi all,

I wonder if anyone could confirm highly supersonic flow of methane (Mach(M) up to 4) downstream to orifice in pipe with pressure expansion ratio approx. 1:90. The result is from CFD simulation.

Model summary:

Geometry

pipe diameter (upstream orifice) = 0.15 m

pipe diameter (downstream orifice) = 0.3 m (abrupt diam. change)

orifice diameter = 0.037 m

orifice thickness = 0.025 m

BCs:

Pressure Inlet: (absolute pressures)

total pressure = 9 Mpa

initial static pressure = 9 MPa

Total temperature = 300 K

Pressure outlet:

static pressure = 101.325 kPa

Walls: adiabatic

Turbulence: RNG k-e, wall function

Fluid: methane, ideal gas

Result from CFD simulation reveals that the orifice work like overexpanded supersonic nozzle:

- supresonic expansion past orifice M up to 4

- creation of oblique shock wave after the overexpanded region (stat. pressure approx 20 kPa) when expanding fluid particles hit the wall

- normal shock wave as result of interaction of the oblique shock waves near the pipe centerline

- supersonic flow (M cca 2) further downstream the orifice

- low temperatures in supersonic region up to 140 K

- but interestingly creation of temperature boundary layer with nonzero gradient at the wall

- wall temperature cca. 290 K (neighter model with nonadiabatic wall causes (substantial) decrease in wall temperature

Could be all the CFD results considered as correct?

Collegues around me (non CFD spetialists) claim that the the expansion processin orifice should more resamble to throttling porcess.

Perforated plate, instead of orifice with equivalent flow area, shows almost the same results (supersonic expansion, low temperature, weaker oblique shock waves). Is ti correct?

Why is the process in throttling valve more liken to throttling process (isentalpic process, for ideal gas isthermal process)?

Many thanks.