Encyclopaedia Index

5) Further Problem Specification Facilities


  1. Reference Pressure and Temperature
  2. Inlet Properties
  3. Inflowing Enthalpy
  4. Inflowing Mass-Flux
  5. Inflowing Gas Speed
  6. Inlet Velocity Profiles
  7. BFC Inlets

5.1 Reference Pressure and Temperature

PHOENICS recognises a reference pressure and temperature (PRESS0 and TEMP0). In the CHEMKIN Interface both of these parameters are fully utilised, and all stored pressures and temperatures are relative to the prevailing values of PRESS0 and TEMP0.

Additional use has been made of PRESS0 and TEMP0 as follows. The value of PRESS0 is set to be a multiple of the standard atmospheric pressure which is obtained from the CHEMKIN data-base. The value of the pressure in atmospheres is set via the variable CHSOC, ie.

By default, the pressure passed to CHEMKIN and TRANLIB subroutines is PRESS0. This is because passing the local instantaneous value of pressure can lead to difficulties computing models.

However, if, in the Q1 file the line

appears between the CHKIBEGIN and CHKIEND lines, the local instantaneous pressure will be used throughout the code.

It is also possible to use the reference temperature, TEMP0, in all CHEMKIN and TRANLIB subroutines so that the temperature variable, TEM1, need be neither SOLVEd nor STOREd. This is not the default, and if required this feature must be activated by setting

in the Q1 file. In pre PHOENICS 2.2, the READ-Q1 facility was used to set CONTEM to T.

5.2 Inlet Properties

It is necessary to specify the properties of gas mixtures flowing into the solution domain in a way that is consistent with the gas properties to be used within the domain.

5.2.1 Inflowing Enthalpy

For the energy (TEM1) equation, the inflowing thermal enthalpy must be specified as a boundary condition. This is implemented using a PATCH with name beginning with NOCPCK, which indicates that the VALue set will not be multiplied by the in-cell mixture effective specific heat, buth rather the inlet thermal enthalpy will be calculated set by the CHEMKIN Interface.

The gas composition is obtained from the COVAL statements for the mass- fractions, and the temperature of the inlet gas may be obtained from the variable TMP1A (as in pre-PHOENICS 2.2 releases), or alternatively by use of the PIL command SPEDAT, e.g:

which defines an inlet temperature of 298K for the PATCH named NOCPCK1. The SPEDAT command is more general in that it allows for a different inlet temperature to be assigned to different inlet PATCHs.

In summary, the user must:

5.2.2 Inflowing Mass-Flux

A facility is provided for the user to specify the inflowing mass-flux from the inlet flow speed and the density calculated from the properties of the inflowing gas mixture using the CHEMKIN Interface. The speed of the inflowing gas is derived from the COVAL data for the velocities. What the user must do is;

The mass-flux is then; FLUX=Rho.SPEED, where the density, Rho, is calculated using a CHEMKIN subroutine.

5.2.3 Inflowing Gas Speed

If the inflowing mass-flux is specified it is possible, given the assumption that only the velocity normal to the inlet PATCH is nonzero, to deduce the inlet velocity from the inlet mass-flux.
The user makes the same settings as above, except that the role of the velocity and pressure COVALs is reversed so that one should;

which will cause the density to be calculated from the inlet gas properties, and for the inlet flow speed to be calculated from the density and mass-flux.

5.2.4 Inlet Velocity Profiles

A limited facility whereby Poiseuille velocity and mass-flux profiles may be calculated for the W1 velocity for LOW and HIGH inlets has been provided for non-BFC cases.

The mean velocity is specified on a PATCH named NOCPCKAx, NOCPCKBx or NOCPCKCx is specified using the variables PROFA, PROFB, and PROFC respectively.

If the case is polar (CARTES=.F.), then if the left-most limit of the PATCH in the Y-direction is zero, the velocity will be at a maximum, otherwise the velocity will be zero on both boundaries of the PATCH.

5.2.5 BFC Inlets

The foregoing practices may also be used when BFC=T, unless it is required to describe a uniform flow across a curvilinear BFC inlet boundary by use of Subroutine GXBFC in the file GXBFGR.FOR. For this case, an example of the required user settings is given below:


where VELX, VELY and VELZ are the Cartesian resolutes of the inlet velocity vector. It should be noted that Subroutine GXBFC is called directly from the CHEMKIN interface to compute the inlet density, mass flux and grid-directed velocity resolutes.