Thermal performance of a ground U-shaped tube with twisted tapes in sand/graphite backfill materials

ABSTRACT

Thermal characteristics of water in a ground U-tube with twisted tape inserts were addressed. At inlet temperatures of 60°C to 75°C, and Reynolds numbers of 3000 to 9000, the surface heat transfer coefficients for water with twisted tapes ranged from 442 to 897 W m⁻² K⁻¹, which were higher than that in smooth tube, 384–810 W m⁻² K⁻¹. Average convective heat transfer coefficient for the smooth tube in sand/graphite blend ameliorated by 36% compared to that in sand. Average apparent thermal diffusivities of sand/graphite at hot end temperature of 70°C rose by about 17% compared to sand.

KEYWORDS:

• Ground heat exchanger
• twisted tape
• sand
• graphite
• heat transfer

Nomenclature

A	=	heat transfer surface area (m2)
c	=	specific heat capacity (J kg−1 K−1)
d	=	diameter (m)
f	=	friction factor
h	=	heat transfer coefficient (W m−2 K−1)
l	=	length (m)
Nu	=	Nusselt number
Pr	=	Prandtl number
p	=	pressure drop (Pa)
qv	=	volumetric flow rate (m3 s−1)
Q	=	heat flow rate (W)
Re	=	Reynolds number
t	=	time (s)
T	=	temperature (°C)

Greek letters

U	=	flow velocity (m s−1)
λ	=	thermal conductivity (W m−1 K−1)
ν	=	kinematic viscosity (m2 s−1)
ρ	=	density (kg m−3)
ω	=	water content

Subscripts and Superscripts

h	=	hot end
i	=	inner/initial
in	=	inlet
l	=	liquid
o	=	outer
out	=	outlet
w	=	wall

Supplementary material

Supplemental data for this article can be accessed publisher’s website.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China under No. 51376017.