Discussion: “Two-Dimensional Transient Solutions for Crossflow Heat Exchangers With Neither Gas Mixed” (Spiga, G., and Spiga, M., 1987, ASME J. Heat Transfer, 109, pp. 281–286) and “Transient Temperature Fields in Crossflow Heat Exchangers With Finite Wall Capacitance” (Spiga, M., and Spiga, G., 1988, ASME J. Heat Transfer, 110, pp. 49–53)