Does Inlet temperature increase have a change in the results obtained for a De-laval Nozzle?

mhmsiyammhmsiyam Member Posts: 3
edited December 2020 in Fluids

Hello all, I am trying to simulate chemically reacting compressible flow with the species model in Fluent. My design was based on the area velocity relationship. The equilibrium combustion product mass fractions were obtained from NASA CEA. I am testing for a range of fuel/air equivalence ratios from 0.8 to 1.1 at 60bar pressure.

For 0.8 and 0.9 the inlet temperature (static) does not exceed 2180K, However when simulating for ratios 1.0 and 1.1, the inlet temperature exceeds 2200K, which does not provide Mach = 1 at throat.

The following are my settings

General: axisymmetric, density based, absolute velocity formation, Transient solver.

Models : Energy equation-on |viscous - Spalart-allmaras 1eq|species - methane air 2step, volumetric, relax to chemical equilibrium and Finite rate reaction settings.

Boundary Conditions : Pressure inlet at 60bar, 2289.18K| Pressure outlet.

Species are defined in-terms of Mass fractions as obtained from NASA CEA.


Residual monitor set to 1e-6

Methods - Transient| formulation -second order implicit|

Courant Number set to 1 and under relaxation factors set to 0.1 for Turbulent Kinetic Energy.

Initialisation from Inlet, Time step size = 1e-6, with 30 iterations per time step and solution runs for 1000time-steps. Convergence is acheived around the 500th iteration.


What could be the problem here, or what may I be doing wrong? Could the rise in inlet temperature be causing this issue? Attached is a screen capture of the MACH plot.

Apologies for the long post, thank you for your time!


Answers

  • RKRK Posts: 436Ansys Employee

    Hello,

    Yes, the increase in inlet temperature could be the cause. The chemical reactions occur much sooner with increase in Mach number. You could reduce the length of the nozzle if you have the liberty to do so. How is your convergence? Are you getting a converged solution for 0.8 and 0.9?

  • mhmsiyammhmsiyam Posts: 5Member

    Thank you for your response, I am able to get a converged solution at phi 0.8 and 0.9, at around 700th time step. However the problem seems to appear only at phi 1.0. Changing the nozzle length seems to have no effect.

    I tried using a Carbon Monoxide air mixture instead to simulate since I am not exactly modelling combustion, only its products, but at Phi 1.0 and 1.1, my exiit mach drops to 2.2. instead of the anticipated 3.1 that the nozzle was designed for.


    Thank you for your time.!

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