In the equation of motion for a particle, Equation 4 in ANSYS CFX-Solver Theory Guide | Particle Transport Theory of the CFX 11.0 documentation, the Left Hand Side (LHS) represents the time rate of change of the particle momentum, the Right Hand Side (RHS) represents the relevant forces modeled. For an evaporating particle, is a (dm/dt)*v term added to the LHS? Does a corresponding source term for the particle mass transfer appear in the continuous phase momentum equation?
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In the equation for the particle velocity, the LHS of the momentum equation can be written as d(m*v)/dt; however, there is an extra term v*d(m)/dt that should be added to the RHS (the rate of loss of momentum due to evaporation). Expanding the LHS derivative will result in two terms; m*d(v)/dt + v*d(m)/dt. Therefore, the v*d(m)/dt terms cancel out. All this means is that a particle does not accelerate just because it is evaporating. In the equation for the gas phase, however, there is an extra term due to the v*d(m)/dt term. This term says that evaporation moves the gas velocity towards the particle velocity. In the case of condensation, the opposite should apply: that is the gas velocity should be unaffected by condensation, and the particle velocity should move towards the gas velocity. However, the current implementation of particle transport in CFX 11.0 does not include this effect. (The Eulerian-Eulerian multiphase code does have the effect of condensation implemented properly.) |
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