3[7]Gu Yian, O Neal Dennis L.Development of an equivalent diameter expression for vertical U- tube used in ground - coupled heat pumps [J]. ASHARE Trans. ,1998, 104: 347-355.
4[8]Gu Yian, O Neal Dennis L. Modeling the effect of backfills on U-tube ground coil performance [J]. ASHRAE Trans., 1998, 104: 356-365.