−δ
0 r 2q/w dζ ≈ −4π R2  ̄ qδ/  ̄w within
the spherical grain volume defined by V = (4/3)π(R3− ˆR 3),
where  ̄ q = −(
−δ
0 q dζ)/δ and  ̄w = −(
−δ
0 (u−D)dζ)/δ =
 ̄ u−D are the spatially averaged heat flux and velocity through
the compaction zone, R is a characteristic grain radius, and
ˆR
< R characterizes the thermal penetration depth into the
grain by conduction. Here, ζ and w are position and velocity
measured relative to a wave attached frame; the head and tail
of the wave are located at ζ = 0 and ζ = −δ, respectively.
We numerically define δ by q ≥ 0.1qmax. The first law of
thermodynamics then gives