CFD Analysis of Geothermal Heat Exchanger at Different Orientation

: The main objective of present work to investigate the arrangements of piping system of earth tube heat exchanger for better thermal comfort. For these work CFD analysis on three different designs of earth tube heat exchanger for summer and winter session for Bhopal location have been performed. computational fluid dynamics analysis have been performed on earth tube heat exchanger using horizontal pipe at various air velocity such as 0.5m/sec, 1 m/sec, 2m/sec, 3m/sec, 4m/sec & 5m/sec for summer session, to get temperature distribution inside the earth tube heat exchanger. Results show that there are drop of temperature in summer session range from 318K to 296K and rise of temperature in winter session range from 288K to 296.7K. It has been observed from the results of computational fluid dynamic analysis that the earth tube heat exchanger using horizontal pipe gives better result as compared with vertical and inclined piping arrangement. So it is recommended that the earth tube heat exchanger using horizontal pipe arrangement may be used for better thermal comfort.


I. INTRODUCTION
A grounding pipe is a set of heat exchanger in which a pipe or pipe is buried to facilitate the transfer of geothermal heat with air. This system uses an almost constant soil temperature to dissipate or dissipate the thermal energy from or to the air inflow into the ground. The ambient air that circulates in the grounding pipes is heated or cooled before entering the building's air system. In summer, the earth is colder than the outside air temperature and the air is cooled as it passes through the pipes. The opposite occurs in winter. As materials for pipes, plastics, metal or concrete can be used. Each type of material has its advantages and disadvantages. This is considered a "passive" renewable energy system because no external mechanical energy is used to produce the heat transfer effect, but only a fan to move the air through the pipe. In general, the temperature variation of this type of system is insufficient to completely condition the air passages, but is a simple and efficient means of energy for preheating or precooling.

Figure -1 Schematic diagram of Earth Tube Heat Exchanger
In recent years, the cooling of outdoor air by underground pipes has been recognized using a terrestrial heat exchanger (EAHE) to increase building comfort and reduce energy requirements. This is due to one of the most important thermal properties of the earth: at a depth of 1.5-2 m, the soil temperature remains practically the same throughout the year. The internal temperature of the Earth remains higher in winter than the surface temperature of the Earth and vice versa in the summer.

1) Ground Heat Transfer Mechanisms
The temperature field in the ground is influenced by different dimensions. While the influence of runoff water or geothermal heat is likely to be neglected, it is important to consider incident solar radiation and the exchange of long-wave radiation between the ground and the sky. The absorption of solar radiation depends on the ground cover and color, while the loss of radiation from long waves depends on the surface temperature of the soil. The radioactive balance between solar gain and loss of long waves is generally positive in summer and negative in winter. [ Carmody, 1985] II. LITERATURE REVIEW S.H. Hammadi [1] In climates, outdoor hot water supplies are exposed to direct sunlight, in addition to the warm summer breeze. The storage water temperature exceeds 50 ° C. To solve this problem, a time-based analysis of a cylindrical tank of hot water exposed to intense sunlight is performed. The analysis includes an energy balance of the water tank and a groundwater heat exchanger that lowers the water temperature during the summer months. The predominant equations are solved numerically with the fourth order Runge-Kutta method. It was found that the use of underground heat exchangers lowered the water temperature by about 16 ° C. A comparison of the numerical results with the experimental data showed a good agreement. Namrata Bordoloi at el. [2] Energy saving is an integral part of the current scenario. EAHE heat exchangers are a technique that promotes energy savings. EAHE is an unconventional technique that uses geothermal energy from the earth for heating and cooling. This article gives an overview of the different EAHE combinations. The overview provides a complete summary of previous EAHE activities. In the overview, the analytical and experimental studies of the various EAHE combinations are described in detail and the results of thermal performance are analyzed. It also takes into consideration the environmental aspect of energy saving. It was concluded from the abstract that the design parameters have a direct or indirect influence on the outlet temperature. The result also shows that the pipe material does not have much influence on the outlet temperature. When energy is saved, EAHE technology saves more energy than conventional air conditioning systems. As a result, this technology can effectively reduce greenhouse gases and improve the environment. Sayeh Menhoudj at el [3] this paper presents a study on the energy efficiency of an air-to-ground heat exchanger (EAHE) for buildings located in the Maghreb climate context (Oran, Béchar and Aurar in Algeria). To verify the influence of the material, two air ducts (one in galvanized plate and the other in PVC) are considered in the same geometric conditions (20 m cable with 120 mm diameter and 2 m mounting depth). They are used separately to ventilate two adjacent rooms that form a test cell located on the IGCMO-USTOMB campus (Oran, Algeria). An experimental unit has been set up to measure the temperature at different points (air inlet / outlet). The measurements were made in August 2015.

Lazaros Aresti, Paul Christodoulidesb & Georgios
Florides [4] Advances in technology and renewable energy systems have evolved considerably over the years. Geothermal energy was introduced in Italy in 1904 and since then has greatly increased its efficiency. Geothermal heat pumps (GSHP), one of the main types of RES, are used in combination with geothermal heat exchangers (GHE) to heat and cool a room. Geothermal energy extracts heat from the earth through a network of pipes. The closed circuit system (vertical or horizontal) is the most common configuration. Pipes can also use natural underground wells in an open circuit. GHEs offer much better performance than traditional air-to-air heat exchanger systems. Reducing their costs and improving their overall efficiency through their design is essential for research.

3)
Flow of air is uniform along the length of the pipes. 4) It is assumed that the soil properties are isotropic and there is perfect contact between the soil and the pipe.

1) Mathematical Analysis:
Reynolds number and prandtl number inside the pipe: Heat transfer along the pipe can be calculated as Where Convective thermal resistance between the internal surface of the pipe and air in the pipe  Where x is the axial coordinate, r is the radial coordinate, is the axial velocity, and is the radial velocity.

7) Momentum Conservation Equations
Conservation of momentum in an inertial reference frame is described by Where p= static pressure ̿ = stress tensor, ⃗ = gravitational body force and ⃗ = external body forces The stress tensor ̿ is given by where = molecular viscosity I = unit tensor, For 2D axisymmetric geometries, the axial and radial momentum conservation equations are given by

Where
= ℎ for an incompressible phase and ℎ = sensible enthalpy for phase k −∈ The turbulence kinetic energy, k, and its rate of dissipation, ∈, are obtained from the following transport equations: ∈ ] + 1∈ ∈ ( + 3∈ ) − 2∈ ∈ 2 + ∈ is the generation of turbulence kinetic energy due to buoyancy, represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate, 1∈ , 2∈ , and 3∈ are constant. and ∈ are turbulent Prandtl numbers for k and∈, And ∈ are user-defined source terms. 9) CFD analysis of Earth tube heat exchanger using horizontal pipe CAD Modeling: In this paper, a two-dimensional CAD model of a horizontal tube heat exchanger with horizontal tubes was created using the ANSYS Workbench modular design. the outer and inner diameters of the tube are 80 mm and 78 mm respectively and its total length is 50 m.
A two-dimensional view of the ground tube heat exchanger with horizontal pipe is shown in figure no. Meshing: Once the CAD geometry is complete, the horizontal tube geothermal heat exchanger is imported into the ANSYS work bench for further fluid flow analysis. The next step is a mesh. Networking is a critical process in analyzing finite elements in this process. The geometry of the CAD is divided into a large number of small parts called mesh. The total number of nodes generated in this job is 104832 and the total number of elements is 84195. The types of elements used are Hex8, which have a hexagonal shape with eight nodes on each element.

10) Factor that affect the mesh quality:
• Rate of convergence: if the mesh quality is good the rate of convergence will be grater which means the correct solution can be achieved faster. • Solution precision: A better mesh quality provides a more precise solution. • Computational processing time required: for the highly refined mesh the computational time will be relatively large. • Grid Independence result: Once the computations are done and the desired property of fluid does not vary with respect to different mesh elements then it represents that further change in elements doesn't vary the results this term known as Independent Grid. 11) Defining Material Properties: For any kind of analysis material property are the main things which must be defined before moving further analysis. There are thousands of materials available in the ANSYS environment or library, if required materials are not available in ANSYS material directory, new material directory also can be created as per requirement. The working fluid is taken as air flowing inside earth tube heat exchanger having density of 1.22 kg/m3, specific heat = 1006.43 J/kg-k, thermal conductivity = 0.24 w/m-k and The pipe material of earth tube heat exchanger is taken as steel having density of 8030 kg/m3, specific heat = 502.48 J/kg-k and thermal conductivity = 16.27 w/m-k. www.ijoscience.com 5

12) Boundary condition:
To determine the temperature distribution inside the earth tube heat exchanger need to on energy equation.

1.
Defining of material property, set working fluid as air flowing inside the earth tube heat exchanger and wall of pipe steel with thermal conductivity of 0.6 W/m -K & 16.27 W/m -K respectively. 2. For the outlet boundary condition the gauge pressure needs to be set as zero because the flow of air inside the earth tube heat exchanger is atmospheric. 3. Different velocity of air is used for performing CFD analysis ranging from 0.5 to 5 m/sec. 4. The Fluent solver is used for computational fluid dynamic analysis. 13) Computational fluid dynamics analysis for of earth tube heat exchanger using horizontal pipe at velocity of 0.5m/sec for summer session: The temperature drop from 318K to 296K has been recorded and the temperature difference of 22 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 0.94 Pa has been recorded. 14) Computational fluid dynamics analysis for of earth tube heat exchanger using horizontal pipe at velocity of 0.5m/sec for winter session: Figure 12: Temperature distribution inside the earth tube heat exchanger using horizontal pipe at velocity of 0.5m/sec for winter session The temperature increase from 288K to 296K has been recorded and the temperature difference of 08 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 0.931 Pa has been recorded.

15) Computational fluid dynamics analysis for of earth tube heat exchanger using horizontal pipe at velocity of 1 m/sec for summer session:
The temperature drop from 318K to 297.1K has been recorded and the temperature difference of 20.9 degree has been observed.  The temperature increase from 288K to 295.8K has been recorded and the temperature difference of 7.1 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 2.12 Pa has been recorded.

17) Computational fluid dynamics analysis for of earth tube heat exchanger using horizontal pipe at velocity of 2 m/sec for summer session:
The temperature drop from 318K to 299.7K has been recorded and the temperature difference of 18.3 degree has been observed.  The temperature increase from 288K to 294.8K has been recorded and the temperature difference of 6.8 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 4.75 Pa has been recorded.

19) Computational fluid dynamics analysis for of earth tube heat exchanger using horizontal pipe at velocity of 3 m/sec for summer session:
The temperature drop from 318K to 302.1 K has been recorded and the temperature difference of 15.9 degree has been observed.  The temperature increase from 288K to 294.6 K has been recorded and the temperature difference of 6.6 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 8.03 Pa has been recorded.

21) Computational fluid dynamics analysis for of earth tube heat exchanger using horizontal pipe at velocity of 4 m/sec for summer session:
The temperature drop from 318K to 302.5 K has been recorded and the temperature difference of 15.5 degree has been observed.  The temperature increase from 288K to 293.2 K has been recorded and the temperature difference of 5.2 degree has been observed.  The temperature drop from 318K to 305 K has been recorded and the temperature difference of 13 degree has been observed.  The temperature increase from 288K to 293 K has been recorded and the temperature difference of 5 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 17 Pa has been recorded.

25) CFD analysis of Earth tube heat exchanger using vertical pipe CAD Modeling:
In which the outer and inner diameters of the pipe are 80 mm and 78mm respectively, and the total length of pipe is 50 m. A two dimensional view of Earth tube heat exchanger using vertical pipe is shown in figure No. The temperature drop from 318K to 296.7 K has been recorded and the temperature difference of 21.3 degree has been observed.  The temperature increase from 288K to 296.7 K has been recorded and the temperature difference of 8.7 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 86332 Pa has been recorded.

29) Computational fluid dynamics analysis for of earth tube heat exchanger using Vertical pipe at velocity of 1 m/sec for summer session:
The temperature drop from 318K to 297.9 K has been recorded and the temperature difference of 20.1 degree has been observed.  The temperature increase from 288K to 295.3 K has been recorded and the temperature difference of 7.3 degree has been observed. The computational fluid dynamics analysis has been performed on earth tube heat exchanger using vertical pipe at velocity of 1 m/sec for winter session, to get velocity distribution inside the earth tube heat exchanger. The maximum velocity of 1.043 m/sec has been recorded. The maximum pressure inside the earth tube heat exchanger for summer session is 2E+5 Pa has been recorded.

31) Computational fluid dynamics analysis for of earth tube heat exchanger using Vertical pipe at velocity of 2 m/sec for summer session:
The computational fluid dynamics analysis has been performed on earth tube heat exchanger using vertical pipe at velocity of 2 m/sec for summer session, to get temperature distribution inside the earth tube heat exchanger. The temperature drop from 318K to 299.7 K has been recorded and the temperature difference of 18.3 degree has been observed.  The temperature increase from 288K to 294.7 K has been recorded and the temperature difference of 6.7 degree has been observed. The maximum pressure inside the earth tube heat exchanger for summer session is 3E+5 Pa has been recorded.

33) Computational fluid dynamics analysis for of earth tube heat exchanger using Vertical pipe at velocity of 3 m/sec for summer session:
The temperature drop from 318K to 301.1 K has been recorded and the temperature difference of 16.9 degree has been observed. The temperature increase from 288K to 294.2 K has been recorded and the temperature difference of 6.2 degree has been observed. The maximum velocity of 3.14 m/sec has been recorded. The maximum pressure inside the earth tube heat exchanger for summer session is 2E+5 Pa has been recorded.

35) Computational fluid dynamics analysis for of earth tube heat exchanger using Vertical pipe at velocity of 4 m/sec for summer session:
The temperature drop from 318K to 302.5 K has been recorded and the temperature difference of 15.5 degree has been observed.  The temperature increase from 288K to 293.7 K has been recorded and the temperature difference of 5.7 degree has been observed. The maximum pressure inside the earth tube heat exchanger for summer session is 7E+5 Pa has been recorded.

37) Computational fluid dynamics analysis for of earth tube heat exchanger using Vertical pipe at velocity of 5 m/sec for summer session:
The temperature drop from 318K to 304 K has been recorded and the temperature difference of 14 degree has been observed. The maximum pressure inside the earth tube heat exchanger for summer session is 9E+5 Pa has been recorded.

38) Computational fluid dynamics analysis for of earth tube heat exchanger using vertical pipe at velocity of 5 m/sec for winter session:
Figure 80: Temperature distribution inside the earth tube heat exchanger using vertical pipe at velocity of 5 m/sec for winter session The temperature increase from 288K to 293 K has been recorded and the temperature difference of 5 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 9E+5 Pa has been recorded.

39) CFD analysis of Earth tube heat exchanger using inclined pipe CAD Modeling:
In which the outer and inner diameters of the pipe are 80 mm and 78mm respectively, and the total length of pipe is 50 m. A two dimensional view of Earth tube heat exchanger using inclined pipe is shown in figure No. The temperature drop from 318K to 299.9 K has been recorded and the temperature difference of 18.1 degree has been observed.  The temperature increase from 288K to 295.9 K has been recorded and the temperature difference of 7.9 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 86332 Pa has been recorded.

43) Computational fluid dynamics analysis for of earth tube heat exchanger using inclined pipe at velocity of 1 m/sec for summer session:
The temperature drop from 318K to 298.1 K has been recorded and the temperature difference of 19.9 degree has been observed.  The temperature increase from 288K to 295.4 K has been recorded and the temperature difference of 7.4 degree has been observed.  The maximum pressure inside the earth tube heat exchanger for summer session is 2E+5 Pa has been recorded.

45) Computational fluid dynamics analysis for of earth tube heat exchanger using inclined pipe at velocity of 2 m/sec for summer session:
The temperature drop from 318K to 299.9 K has been recorded and the temperature difference of 18.1 degree has been observed.  The temperature increase from 288K to 293 K has been recorded and the temperature difference of 5 degree has been observed. The maximum pressure inside the earth tube heat exchanger for summer session is 3E+5 Pa has been recorded.

47) Computational fluid dynamics analysis for of earth tube heat exchanger using inclined pipe at velocity of 3 m/sec for summer session:
The temperature drop from 318K to 301.4 K has been recorded and the temperature difference of 16.6 degree has been observed. The maximum velocity of 3.14 m/sec has been recorded.

Figure105
: pressure distribution inside the earth tube heat exchanger using inclined pipe at velocity of 3 m/sec for summer session The maximum pressure inside the earth tube heat exchanger for summer session is 5E+5 Pa has been recorded.

48) Computational fluid dynamics analysis for of earth tube heat exchanger using inclined pipe at velocity of 3 m/sec for winter session:
Figure106: Temperature distribution inside the earth tube heat exchanger using inclined pipe at velocity of 3 m/sec for winter session The temperature increase from 288K to 294.2 K has been recorded and the temperature difference of 6.2 degree has been observed. The maximum velocity of 3.14 m/sec has been recorded.
Figure108: Pressure distribution inside the earth tube heat exchanger using inclined pipe at velocity of 3 m/sec for winter session The maximum pressure inside the earth tube heat exchanger for summer session is 5E+5 Pa has been recorded.

49) Computational fluid dynamics analysis for of earth tube heat exchanger using inclined pipe at velocity of 4 m/sec for summer session:
The temperature drop from 318K to 302.8 K has been recorded and the temperature difference of 15.2degree has been observed. The temperature increase from 288K to 293.7 K has been recorded and the temperature difference of 5.7 degree has been observed. The maximum pressure inside the earth tube heat exchanger for summer session is 7E+5 Pa has been recorded.

51) Computational fluid dynamics analysis for of earth tube heat exchanger using inclined pipe at velocity of 5 m/sec for summer session:
The temperature drop from 318K to 304 K has been recorded and the temperature difference of 14 degree has been observed. The temperature increase from 288K to 293 K has been recorded and the temperature difference of 5 degree has been observed. The maximum pressure inside the earth tube heat exchanger for summer session is 9E+5 Pa has been recorded.