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Hydrothermal analysis of conventional and baffled geothermal heat exchangers in transient mode

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Abstract

In this paper, the idea of using the baffles to improve the performance of the coaxial geothermal heat exchanger (CGHE) has been proposed. Thermal and hydrodynamic analysis of two types of CGHE during heat injection and thermal recovery processes was investigated. A transient numerical model and turbulent model have been developed to study the CGHE. In this study, for both cases, the active borehole length was 165 m and the flow rate and the injected heat were 0.58 L s−1 and 6.4 kW, respectively. Augmentation of the heat transfer from the ground to the annular flow and reduction in the heat transfer rate between the annular flow and central pipe flow were the primary goals of this study. For this purpose, 40 baffles were considered inside the annular portion (over the central pipe). Comparisons between the obtained results and available data showed that the simulations were valid. The outlet (and inlet) temperatures of the baffled CGHE were 13.2% lower than those for the conventional CGHE. Also, the baffled CGHE had better thermal performance than the conventional CGHE and caused a quicker recovery of temperature, which is useful for improving the performances of the CGHE. Also, the pressure drop of the annular flow for the baffled CGHE was higher than the conventional CGHE, while the pumping powers for two CGHEs were not significantly different.

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Abbreviations

c p :

Specific heat/Jkg−1 K−1

f :

Friction factor

D h :

Hydraulic diameter/m

D ω :

Cross-diffusion term/kgm−3 s−2

G k :

Turbulent kinetic energy production rate/kgm−1 s−3

G ω :

Production rate of the specific dissipation/kgm−3 s−2

H :

Convection heat transfer coefficient/Wm−2 K−1

K :

Kinetic energy of turbulence/m2s−2

L :

Length of the borehole/m

\(\dot{m}\) :

Mass flow rate/kgs−1

Nu:

Nusselt number

P :

Pressure/Pa

Pr:

Prandtl number

Q :

Heat injection rate/W

r :

Radius/m

R :

Thermal resistance/mKW−1

t :

Time/s

T :

Temperature/K

\({\text{u}}_{\rm i}\) :

Velocity in ith direction/ms−1

X :

Loss coefficient of the baffles

Y k :

Dissipation rate of k/kgm−1s−3

Y ω :

Dissipation rate of ω/kgm−3s−2

z :

Depth direction/m

An:

Annular region

ave:

Average

b:

Borehole

e:

Effective

f:

Fluid

g:

Ground

h:

Hydraulic

in:

Inlet

out:

Outlet

p:

Pipe

s:

Solid

t:

Turbulent

*:

Dimensionless values

ρ:

Density/kgm−3

λ :

Thermal conductivity/Wm−1K−1

μ :

Dynamic viscosity/kgm−1s−1

ν :

Kinematic viscosity/m2s−1

Г :

Diffusion Coefficient/kgm−1s−1

τ :

Dimensionless time

ω :

Specific dissipation rate/s−1

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Authors and Affiliations

  1. Faculty of Mechanical Engineering, Semnan University, P.O. Box: 35131-19111, Semnan, Iran

    Soleiman Iry & Roohollah Rafee

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  1. Soleiman Iry
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  2. Roohollah Rafee
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Correspondence to Roohollah Rafee.

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Iry, S., Rafee, R. Hydrothermal analysis of conventional and baffled geothermal heat exchangers in transient mode. J Therm Anal Calorim 143, 2149–2161 (2021). https://doi.org/10.1007/s10973-020-09582-2

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