In Eq. (10), *u* is the bulk fluid velocity. The fluid dynamic viscosity, *ν*, cancels and does not appear in Gz*. In the case of channel flow, the hydraulic diameter, *D*_{H}, would be four times the ratio of channel cross-sectional area over perimeter (4*A*/*P*). However here, *D*_{H} cancels and leaves *L* in Gz*, where *L* is the length parallel to the flow direction. In order to retain the effect of the finite thickness of the storage medium, a characteristic length *L* is defined as the ratio of storage medium volume over the heat transfer area inside the channel (effectively the channel length, and reducing to that exactly for small storage medium volumes, analogous to a thin-walled pipe). The thermal diffusivity, *α* Eq. (9), contains the material properties relevant to heat transport.