Analysis of Enhanced Heat Transfer Characteristics of Coaxial Borehole Heat Exchanger
Abstract
:1. Introduction
2. Geometric Model and Evaluation Index of the Coaxial Borehole Heat Exchanger
2.1. Working Principle of Coaxial Borehole Heat Exchanger
2.2. Geometric Model of Coaxial Borehole Heat Exchanger
2.3. Enhanced Heat Transfer Parameters and Heat Transfer Performance Evaluation Index
2.3.1. Enhanced Heat Transfer Parameters
2.3.2. Heat Transfer Performance Evaluation Index
3. Numerical Analysis Model and Its Validation
3.1. Model Assumptions
- (1)
- The rock and the soil around the geothermal underground coaxial borehole heat exchanger were treated as a homogeneous medium. Moreover, the effect of groundwater seepage is ignored, and the heat transfer in the underground rock and soil is treated as pure heat conduction.
- (2)
- The temperature at the numerical simulation region’s radical boundary is considered constant.
- (3)
- The bottom hole’s heat source and surface temperature are considered constant.
- (4)
- The temperatures of the rock and the wellbore were considered the same because the wellbore has been attached to the rock for a long time.
3.2. Governing Equations
3.3. Boundary Conditions
3.4. Mesh and Model Validation
3.4.1. Meshing and Independence Verification
3.4.2. Numerical Simulation Model Validation
4. Results and Discussions
4.1. Analysis of the Enhanced Heat Transfer Mechanism of Vortex Generator
4.1.1. The Effect of Vortex Generators on the Turbulent Kinetic Energy
4.1.2. Effect of Vortex Generator on the Velocity Field
4.1.3. Impact of Vortex Generator on the Temperature Field
4.2. Influence of Vortex Generator on Enhanced Heat Transfer Parameters
4.3. Heat Transfer Performance Analysis of the Coaxial Borehole Heat Exchanger
4.3.1. Effect of Inlet Flow Rate on Outlet Temperature
4.3.2. Influence of Inlet Flow Rate on Production Power
5. Conclusions
- (1)
- The investigation of the enhanced heat transfer mechanism of the vortex generator revealed that the vortex generator can improve the turbulent kinetic energy of the fluid flow, increase the injected fluid velocity and flow velocity in the boundary layer region of the hot rock wall. Similarly, the fluid’s velocity in the radial direction fluctuates up and down, thereby destroying the high-temperature boundary layer, strengthening the heat exchange inside the fluid and strengthening heat transfer.
- (2)
- From the investigation of the heat transfer performance of four kinds of coaxial borehole heat exchangers, it was found that increasing the inlet flow rate decreases the friction coefficient and increases the Nusselt number. Compared with the ST heat exchanger, when the inlet velocity was increased by 1 m/s, the Nusselt number in BVG, IVG and TVG heat exchangers increased by 4.08%, 9.05% and 20.89%, respectively. The PEC of the TVG heat exchanger reached 1.1, which is favorable for improving the heat transfer performance of the coaxial heat exchanger. The impact of the pressure drop on the heat exchanger performance needs to be reduced.
- (3)
- The analysis of the heat recovery capacity of four kinds of coaxial borehole heat exchangers showed that the heat recovery power increases linearly with the increase in the inlet flow rate. Increasing the inlet flow rate is beneficial in improving the heat recovery capacity of the coaxial heat exchanger; however, it is necessary to comprehensively consider the working performance of the pump to propose the optimal inlet flow rate.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
Nomenclature
Tin | Inlet temperature, K |
Tout | Outlet temperature, K |
Tg | The geothermal gradient, K/km |
Vin | Inlet velocity, m/s |
Vout | Outlet velocity, m/s |
cρ | Specific heat capacity of water, J/(kg·k) |
ρ | Density of water, kg/m3 |
K | Thermal conductivity of water, W·(m·k)−1 |
h | Convective heat transfer coefficient of water |
Tsur | Surface temperature, K |
Tw | Local rock temperature, K |
Tm | Local fluid temperature, K |
H | The distance between the lower end of the inner pipe and the bottom of the well, mm |
z | Well depth, m |
P | Fluid pressure, Pa |
Ac | cross-sectional area, m2 |
Pw | Wet perimeter of cross-section, m |
g | Gravitational acceleration, m/s2 |
μ | Dynamic viscosity of water, kg·(m·s)−1 |
Pout | Outlet pressure, Pa |
D | Radial dimensions of the heat exchanger, mm |
D1 | Heat exchanger’s outer tube diameter, mm |
D2 | Inner pipe’s inner diameter, mm |
D3 | Vortex generator’s diameter, mm |
L1 | Vortex generator’s distance from wellhead, mm |
L2 | Vortex generator’s length, mm |
L3 | The distance from the vortex generator to the bottom of the well, mm |
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Parameter | Value(mm) |
---|---|
L1 | 2850 |
L2 | 300 |
L3 | 7150 |
D1 | 177.8 |
D2 | 100 |
D3 | 130 |
H | 200 |
Parameter/Unit | Water | Inner Pipe and Well Wall |
---|---|---|
Density ρ/kg·m−3 | 998.2 | 8030 |
Specific heat cρ/J·(kg·k) −1 | 4182 | 502.48 |
Thermal conductivity k/W·(m·k)−1 | 0.6 | 16.27 |
Viscosity μ/kg·(m·s)−1 | 0.001003 | 0 |
Geometry | Nodes | Average Skewness | Average Orthogonal Quality | Average Aspect Ratio |
---|---|---|---|---|
ST | 485,750 | 0.078 | 0.985 | 5.309 |
TVG | 538,195 | 0.178 | 0.927 | 2.108 |
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Sun, L.; Fu, B.; Wei, M.; Zhang, S. Analysis of Enhanced Heat Transfer Characteristics of Coaxial Borehole Heat Exchanger. Processes 2022, 10, 2057. https://doi.org/10.3390/pr10102057
Sun L, Fu B, Wei M, Zhang S. Analysis of Enhanced Heat Transfer Characteristics of Coaxial Borehole Heat Exchanger. Processes. 2022; 10(10):2057. https://doi.org/10.3390/pr10102057
Chicago/Turabian StyleSun, Lin, Biwei Fu, Menghui Wei, and Si Zhang. 2022. "Analysis of Enhanced Heat Transfer Characteristics of Coaxial Borehole Heat Exchanger" Processes 10, no. 10: 2057. https://doi.org/10.3390/pr10102057