Notes on Enthalpy
Evolution Equations
The compressible and low Mach number formulations of the governing
equations both share the unapproximated continuity and momentum equations
shown here (where
In the compressible formulation we complete the system with an energy
equation as well as an equation of state; in the low Mach number
formulation we can derive a constraint on the velocity by setting
In low Mach number combustion,
Derivation of Velocity Constraint
Differentiating the equation of state, written in the form,
Then, by rearranging the terms, we get
with
Using the Enthalpy Equation
Now writing
we can express
where
We could then write,
When we derived this expression we explicitly retained the dependence
of
Then, replacing
where we define
Using the Energy Equation
Now writing
we can express
where
which leads to
Note that we can replace
Comparison of Constraints
If we set
and the constraint derived using
We note that if we evaluate both constraints for
Enthalpy vs Energy Equation
The full enthalpy equation, with no approximations, appears as:
Here,
The mismatch between the pressure implicit in the definition of
However, if we solve the evolution equation for
but if we solve the evolution equation for
The second equation subtracted from the first gives:
but this equation is only true, in general, if
Suppose we solve the current enthalpy equation, but when we call the
EOS, we subtract
which is identical to solving the energy equation with
Constant Gas
Going back to the constant
Similarly, we can derive a pressure evolution equation from the energy equation
Now, if we further make the assumption that
Plugging this back into either of Eq.414 or Eq.415 gives
If div constraint for
constant gamma
, the difference between the enthalpy equation and the
energy equation, Eq.412, can be
rewritten as
where the equality holds from Eq.417. In other words, for the constant
Outstanding Questions
Why do we want to start with enthalpy instead of internal energy?
We believe that the original desire stems from our experience with smallscale combustion. There, stratification is not important and
, so the enthalpy equation becomes a conservation equation for .Should we call the EOS with
as is, or call the EOS with ?When we stay on the constraint, i.e.
, then the equations for and for are identical. However, once we are off the constraint, do the terms in the current evolution equation for serve to drive us back to the constraint? Recall our current “volume discrepancy factor” acts as a source term which modifies the divergence constraint, which effectively modifies both and (or or ). The term in the enthalpy equation only modifies Is this relevant and/or useful?? Recall that the current “volume discrepancy factor” takes the form of adding to the r.h.s. of the constraint:(419)Suppose we corrected the
(or equation) by using the full instead of ? Would this be more or less consistent (one could imagine doing this as a correction after solving for earlier in the timestep).In computing the thermodynamic coefficients in
for the projection, don’t we need these to be in terms of instead of ?