Numerical parametric study of a new earth-air heat exchanger configuration designed for hot and arid climates

ABSTRACT

Thermal potential for cooling and heating can be achieved by new configuration of earth–air heat exchanger (EAHE). This paper presents a numerical investigation of thermal performance of a spiral-shaped configuration of EAHE intended for the summer cooling in hot and arid regions of Algeria. A commercial finite volume software (ANSYS FLUENT) has been used to carry out the transient three-dimensional simulations and the obtained results have been validated using the experimental and numerical data obtained from the literature. The agreement between our simulation results and those from literature is very satisfactory. A parametric analysis of the new geometry of (EAHE) has been performed to investigate the effect of pitch, depth, pipe length and of the flow velocity on the outlet air temperature and the EAHE’s mean efficiency as well as its coefficient of performance (COP). It has been shown that when the pitch space varies between 0.2 and 2 m the difference of outlet air temperature increases by 6 ^°C. When the air velocity increases from 2 to 5 m/s the mean efficiency decreases from 60 % to 33 % and the COP of the EAHE decreases from 2.84 to 0.46.

KEYWORDS:

• Geothermal Energy
• ground temperature
• Earth-Air Heat Exchanger (EAHE)
• CFD simulation
• spiral pipe

Nomenclature

Cp	=	thermal capacity, J/kg. °C
C1, C${ }_2$, C${ }_{\mu }$	=	coefficients of k-$\epsilon$ turbulence model
d	=	Pipe Diameter, m
E	=	Energy, W
g	=	Gravite, m/s${ }^2$
H	=	Heigith of the soil domain, m
i,j	=	number of segment
L	=	Length of the soil domain, m
m	=	Mass flow rate, kg/s
p	=	Pressure, Pa
Pr	=	Prandtl number
R${ }_1$	=	Minimum radius of spiral, m
S${ }_h$	=	Source term, W/m${ }^3$
T	=	temperature, ${ }^{\circ }$C
T${ }_A$	=	is the inlet air temperature, ${ }^{\circ }$C
Tamp	=	is the amplitude of surface temperature,${ }^{\circ }$C
T${ }_{mean}$	=	is soil’s average temperature, ${ }^{\circ }$C
T${ }_{max}$	=	is the maximal temperature, ${ }^{\circ }$C
T${ }_{min}$	=	is the minimal temperature, ${ }^{\circ }$C
T${ }_{in}$	=	is the temperature at the inlet of the EAHE, ${ }^{\circ }$C
T${ }_{out}$	=	is the temperature at the outlet of the EAHE, ${ }^{\circ }$C
t	=	time, h
t${ }_0$	=	is the hour of the year with the minimum temperature value of the surface, h
u	=	Velocity, m/s
V	=	Volume flow rate, m${ }^3$/s
V${ }_{in}$	=	Velocity inlet, m/s
W	=	Width of the soil domain, m
x	=	Cartesian coordinate
z	=	Depth, m

Greek symbols

$\alpha$s	=	Thermal diffusivity of the soil, m${ }^2$/h
${\delta }_{ij}$	=	Kronecker delta function
$\epsilon$	=	mean efficiency Or Specific dissipation Rate
${\eta }_{Fan}$	=	refers to the output of fan electro-mechanical conversion.
$\mu ,{\mu }_t$	=	Dynamic and turbulent viscosity, Pa s
$\rho$	=	Density, kg/m${ }^3$
${\sigma }_k,{\sigma }_{\epsilon }$	=	Turbulent Prandtl numbers for diffusion of k and $\epsilon$

Subscripts

A	=	air
in	=	inlet
max	=	maximum
min	=	minimum
out	=	outlet
soil	=	Soil

Abbreviations

CFD	=	Computational Fluid Dynamics
COP	=	Coefficient Of Performance
EAHE	=	Earth-Air Heat Exchanger
Low	=	Lower
N	=	number of meshes
Pt	=	Pitch (is the lateral distances between the turns of the spiral pipe)
SIMPLE	=	Semi- Implicit Method for Pressure Linked Equation
Up	=	Upper

Acknowledgments

The author would like to acknowledge the financial support of the Ministry of Higher Education and Scientific Research of Algeria under (Exceptional National Program, P.N.E 2016–2017). The meteorological data used in this paper were supplied by the Algerian National Office of Meteorology.