GENTRA is a software "add-on" for the PHOENICS suite of CFD programs which provides particle-tracking facilities. The name GENTRA stands for GENeral TRAcker.
GENTRA can therefore simulate the motion of particles (the "particulate or disperse phase") through a fluid (the "continuous phase"). GENTRA takes into account the effect of the fluid velocity, temperature, turbulence, etc on the particles; and, conversely, the effect of the presence of particles on the continuous phase is also considered.
The data specification and solution procedure for the continuous phase and the particles are carried out by separate modules:
Of course, the solution procedures for the continuous phase and particles have to interact. Details of such interaction can be found in this manual in Section 1.4.2.
It should be pointed out that PHOENICS has already a "built-in" two-phase capability, which uses Eulerian transport equations to represent the two interacting phases (Spalding, 1980). Similarly to single-phase flows, these equations are solved by discretising the space into computational cells, by integrating the equation over each cell (thus obtaining an algebraic equation) and by solving the resulting system of algebraic equations.
The approach of GENTRA is a different one: while the continuous phase is treated as above, the particles are represented by Lagrangian equations, which are integrated to yield the particle trajectory and the particle properties along this trajectory. This approach is based on the PSI-Cell method of Crowe, Sharma and Stock (1977).
The standard two-phase PHOENICS can therefore simulate particulate flows; however, the GENTRA approach should be preferred by users who:-
The salient features of GENTRA are:-
(a) In respect of pre-processing:-
(b) In respect of modelling features:-
(a) "Tracers" (or "lazy particles"), which simply follow the continuous-phase flow-field;
(b) "Beams" (or "stubborn particles"), which have a constant velocity, regardless of the continuous-phase flow field;
(c) Particles in isothermal flow;
(d) Particles experiencing heat exchange with the continuous phase;
(e) Melting/solidifying particles;
(f) Vaporising droplets.
(a) Particle bouncing, with a user-specified restitution-coefficient;
(b) Particle adhesion to the wall;
(c) Withdrawal of the particle;
(d) Flash vaporisation of the particle (if the particle is a droplet).
(c) In respect of post-processing:
As well as capabilities, GENTRA has also limitations. Appendix A of this guide provides a list of the main limitations known to the GENTRA Development Team at CHAM. The contents of the list changes as known limitations are removed, and new ones found. An updated list of the limitations affecting your version of GENTRA (if different from the one described in this manual) is available through the GENTRA Input Menu (Help and information panel). See Section 2.4 of this Guide for details.
Section 1.1 above classified GENTRA as a PHOENICS "add-on. The present section describes in more detail how GENTRA is related to the rest of PHOENICS.
GENTRA has a pre-processing and a processing part, which are dealt with in the following sub-sections.
The pre-processing part involves the preparation of the GENTRA input, which consists of particle data, solution-control data and output-control data. It can be accomplished in several alternative ways:
(a) By using the GENTRA Input Menu;
(b) By using a set of special PIL commands (the GENTRA PIL);
(c) By loading a case from the GENTRA Input Library.
The pre-processing side of GENTRA uses the general PHOENICS VR environment.
For the benefit of experienced PHOENICS users, it will be pointed out here that all the GENTRA information is sent from the Q1 file to EARTH through the "transfer" arrays RG, IG, LG, and CG.
The processing part of GENTRA takes the form of a collection of FORTRAN subroutines, which are attached to the PHOENICS flow-computing program, EARTH.
GENTRA, which solves the equations for the disperse phase, is called by PHOENICS between the "sweeps" of the computational domain that PHOENICS performs to solve the continuous phase. GENTRA then tracks the particles as they move through the computed flow-field, calculating in the process the interphase interactions (i.e., the transfer of momentum, mass, enthalpy, etc) between the phases. These interaction terms are, after leaving GENTRA, incorporated as sources in the continuous-phase equations for the next PHOENICS sweep.
Since the newly introduced sources are likely to alter the flow field used by GENTRA to track the particles in the first instance, several iterations of the processes PHOENICS-sweep/GENTRA-tracking will normally be needed to obtain a converged solution.
For the benefit of readers with some knowledge of the structure of EARTH, it will be pointed out here that GENTRA is attached to PHOENICS as a Ground-station subroutine that calls, in turn, all of the modules of GENTRA. Most of these modules are delivered in closed (i.e., binary) code; but the Ground station itself (called GENTRA) and a special user-accessible module (GENIUS) are provided in open source.
This Guide has been designed to serve both as a user's guide and reference manual for GENTRA users.
GENTRA, as indeed PHOENICS, has been created to cater for the needs of users with several degrees of knowledge or experience:
A basic knowledge of PHOENICS and its structure is assumed in this Guide.
After this introductory chapter, the reader will find the following chapters and appendices:
The following conventions are used in this Guide:-