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CHAPTER 1

INTRODUCTION

CHAPTER 1                                                          INTRODUCTION

1.1 HISTORICAL REVIEW

An Earth tube heat exchanger is way of dissipates heat or capture heat from the underground of the earth. They use the earth near constant subterranean temperature though out the year to cool or warm air or other fluids for agricultural, residential or industrial uses. When air from buildings is blow through the exchanger for heat recovery ventilation and they are called earth tubes (also known as earth warming tubes or earth cooling tubes) in Europe or earth–tube heat exchangers in North America. ETHE are often a viable and economical alternative or supplement to air conditioning systems or conventional central heating. Since there are no chemicals, compressors or burners and mostly blowers are required to blow the air. These are used for either full or partial heating and /or cooling of facility ventilation air. The idea of using earth energy as a heat sink was known in ancient times. In past about 3000 B.C., Iranian architects used underground air tunnels and wind towers for passive cooling. Earth tube heat exchangers have been used in horticultural facilities (green houses) and agricultural facilities (animal buildings) in the United States before several year decades and probably beginning in the Persian Empire it have been used in industry with solar chimneys in hot a rid areas for more than thousands year.

In 1853 lord Kelvin had told about heat pump. In 1855 Peter Rittervon Rittinger developed the heat pump. Robert C Webber built the first direct exchange ground – source heat pump in 1940. In 1948 the first successful commercial project was installed in Portland, Oregon for the Common wealth Building, and it has been designated a National Historic Mechanical Engineering Landmark by ASME. In 1970, the technology became famous in Sweden and has been increasing slowly in world wide acceptance since then.  Open loop systems cover the market until the development of Polybutylene pipe in 1979 made closed loop systems economically viable. There are over a million units installed world wide providing 12 GW of thermal capacity as of 2004. Each year, about 80,000 units are installed in the US (all 50 states used geothermal energy today, with great potential for market growth and savings) and in Sweden it is 27,000. A geothermal heat pump used in Finland was the most common heating system choice for new houses between 2006 and 2014 with market share exceeding 40%.

Underground air tunnel (UAT) systems, now a day’s also known as earth tube heat exchangers (ETHEs), have been in use for many years in developed countries due to their higher energy consumption efficiencies compared to the conventional cooling and heating systems. Implementation of these systems in Germany, Austria, India and Denmark, has become common since the mid-1990, and North America being adopted is slowly. Earth tube heat exchangers are one of the rapid growing applications in world for renewable energy, with an annual the number of installations increase with 10% in about 30 countries over the last 10 years. With the exception of Switzerland and Sweden, the market penetration is still modest through out Europe but for further improvements in the technology it likely to grow and the increasing need for energy savings. From the middle of the 20th century, many investigators have studied the cooling and heating potential of buried pipes. Since at that time, a number of analytical studies and experimental of this technique have appeared in the literature. Till 2014, about 4000 passive house units have been built in Germany and this amount sensibly increasing doubles every year. In Europe, already more than 8000 passive house units have been successfully built and completed

1.2 INDIAN SCINARIO

The usage of the earth energy as a heat sink or heat source is not a new invention. In fact, from thousands of years it has been used in example of Persian architecture. At the last of the 1970 and the beginning of 1980.As an alternative gained attention of air to earth heat exchangers to air conditioning. A few systems were installed, but did not receive much attention on the market, the investment cost was high, and as the efficiency was low. Moreover, the air quality yielded from ETHE was satisfactory.

Earth tubes have gained renewed attentionin the recent years, mainly due to the increasingly higher requirements for energy consumption. Earth tubes utilize the fact that the ground temperature is relativity constant during the annual year. The air intake gets pre-conditioned which travel through an earth tube before reaching the house ventilation, by rejecting heat to the soil in the summer and  by acquiring heat from the soil in the winter. There are few models are adapted and studies to a warm climate like Southern Europe and India. Few studies have been also made for a Nordic climate.

Several publications have treated an experiment on earth tubes. However, in many of them simplifying assumptions are made such as a constant temperature or that they only consider one duct or that no latent heat will exchange takes place in the earth.

Bansal and Sodha (1986), Trombe et.al. (1991), Girja Saran and Rattan Jadhav (2000), Thanu et.al. (2001), Sharan and Jadhav (2003), Jalaluddin (2011), Vikas and Misra (2012), Ashish Chaturvedi (2014) have all used an experiment to investigate the performance of earth tube heat exchanger. No one of them investigations treated a Nordic climate.

Tzaferis et.al. (1992), Bojic et.al. (1997), Stahl (2002), Lee and Strand (2006), Cucumo et.al. (2008), Ascione et.al. (2011), Su et.al.(2012), Ashish Chaturvedi (2014) and more have all published numerical investigations. None of them, except Stahl (2002), treated a Nordic Climate, and the effect of latent heat exchange in not investigated by Stahl.

Trombe et.al. (1994), Mihalakakou et.al. (1994,1996), Kumar et.al.(2003), Ghosal et.al. (2005), Lachal and Hollmuller (2005), Wu et.al. (2007), Vikas Bansal et.al. (2009), Nayak and Tiwari (2009), Vikas Bansal,  Rohi (2012), Ashish Chaturvedi (2014) and many more.

1.3 DISCRIPTION OF ETHE(EAHE)

Earth tube heat exchangers, also called ground coupled heat exchangers are an interesting technique to reduce consumption of energy in a building. They can heat or cool the ventilation air, using heat or cold accumulated in the soil. An Earth tube heat exchanger is way of dissipates heat or capture heat from the underground of the earth. They use the earth underground constant temperature to warm or cool air or other fluids for residential, industrial uses. They are also called earth tubes or ground tube heat exchanger or earth-air heat exchangers. Earth tubes are often a viable and cheap alternative or supplement to conventional heating or air conditioning systems.

In the case of cooling a building, the building to be cooled acts as heat source and the underground is the heat sink, In the case of heating, these processes are reversed-the building heat sink and the underground becomes the heat source. With the help of buried pipe heat is extracted from or rejected to the ground, through which a fluid flows. The buried pipe is mostly called ground loop heat exchanger. They can make better contributions to reduce energy consumption  but in this` system, the actual heat transfer to and from the ground loop heat exchanger it  varies continuously according due to changing building energy requirements. Despite the changing boundary conditions, adjustment of net fall temperature with the flow of the air so as to give conditions in the room. The result variations affect the coefficient of performance (COP) of the system and thus change in influence the overall system performance.

1.4 DESIGN PERAMETER FOR ETHE

SUFACE AREA: It requires a place where the system is to be installed as permanently area either it is for cooling or heating. Intakes of air should away from source of pollutants.

PIPE DEPTH: The temperature of underground earth remain constant through out the year for the depth of pipe between 1.5 m to 3 m.

HEAT  TRANFER: The process of conducting heat to and from soil of the earth. It required which type soil preferred.

TUBE METARIAL: For this we include plastics concrete, metal. With good heat transfer rate, most conductive material for the low cost, least air flow resistance and offer against corrosion.  .

CONDITION: According to application engineers, boundary condition is considered such as velocity, friction, Temperatures, flow, diameter, length, friction losses, layout drainage and velocities is as experimental condition which is considered for analysis.

AIR INTAKE: For the analysis condition is considered as intake air supply through the pipe with temperature. It varies with respect to time, climate, and temperature.                  .                      

ENERGY ANALYSIS: To evaluate the capital and operating costs of the system including all the electricity to run the blower should be lower than the cooling or heating power offered by the system.

SCIENCE ISSUSE: Depending on the type of system there could be issues with short circuiting and infiltration of the ground exchanger. It is necessary that both duct and building are sealed tightly to avoid differential pressures across

1.5 OBJECTIVES

The main objectives of the thesis are as follows:

1. To perform an extensive literature review to identify the research and development status of this technique and the current guidelines for designing earth to air heat exchanger systems.

2. To develop a transient thermal network model for finding the transient temperature profile around an earth tube in order to determine the velocity of intake air in  heat exchanger systems and the change of temperature of the soil due to prolonged usage.

3. To develop a new methodology for CFD simulation of the earth tube system taking into account both condensation and heat transfer by different it velocity.

4. To study this technology we determine the velocity of blower according to change in climate temperature.

CHAPTER 2

LITERATURE REVIEW

CHAPTER 2                                              LITERATURE REVIEW

A literature has been reviewed on the earth tube heat exchanger technology. It has been study that the research in this field mainly took place in the following area:

• Working

• Design of earth tube heat exchanger

• Energy Saving

• CFD on material

2.1 LITERATURE REVIEW

Girja Saran et.al. [1] have worked on Performance of single pass, earth tube heat exchanger by using geothermal energy of the earth for cooling and heating. Specific aim of those investigations is to determine the operating characteristics of ETHE in heating and cooling mode and collect data of years. ETHE is made parameter of 50 m long MS pipe, 10 cm nominal diameter and 3 mm wall thickness. ETHE is buried 3 m deep underground surface. By the use of 400 W blowers ambient air is pumped. Outlet air velocity in the pipe is 11 m/s is measured air temperature in the inlet, middle (25 m), and at the outlet (50 m), by thermistors placed inside the pipe. They observed ETHE was able to reduce and increase the temperature of hot/cold ambient air by as much as 14oC in May and January. The basic soil temperature in May was 26.6oC and in January was 24.2oC. Finally they concluded the coefficient of performance (COP) in heating mode averaged to 3.8. heating tests were of 14 hour continuous duration during the day. In cooling mode it averaged to 3.3. Cooling tests were of 7 hour continuous duration through the night.

Vikas Bansal et.al. [2] have worked on Performance evaluation & economic analysis of integrated earth air tunnel heat exchanger evaporative cooling system by applying implicit model based on CFD. For use of ETHE system integrated with evaporative cooling to be determine for evaluating the energy saving obtained. Four base cases of existing systems, i.e. electric heater and air-conditioner. Moreover, three different types of blower (i.e., standard blower,energy efficient blower and inefficient blower) are considered for evaluating the financial viability and energy saving of the proposed system.

Fabrizio Ascione et.al. [3] have worked on Earth-to-air heat exchangers for Italian climates by using an ETHE for an air conditioned  have been evaluated for both summer and winter. It is observed in three Italian climates taken (cities of Milan, Rome, Naples). The energy required for the system is evaluated as a function of the boundary conditions (such as tube material, the typology of soil, velocity of the air crossing the tube, tube length and depth, control modes, ventilation air flow rates). In an experiment, earth soil to be homogeneous with constant temperature, the pipe has uniform diameter, surface temperature of the pipe is uniform and convective flow inside the pipe. During this experiment the concluded that it is save 44% of energy, about the length of pipe, depth of earth and velocity of air is calculated, the influence of the tube material such as PVC, metal, concrete.

Akio Miyara et.al. [4] worked on  the experiment to Analysis of Short Time Period of Operation of Horizontal Ground Heat Exchangers by using the simulation process. They discussed numerical analysis of thermal performance for horizontal GHE loops in different operation and orientations modes. It was found that the loop orientation is not such important due to the low effect on its thermal performance.

Fuxin Niu et.al. [5] study a Heat and mass transfer performance analysis & cooling capacity prediction of earth to air heat exchanger. They used the method by comparing against the experimental data from an existing testing facility of renewable energy. After the calibration performance were analyzed with six factors, namely, air relative humidity, air temperature, tube surface temperature, tube length, air velocity at inlet of EAHE and diameter. For predicting the cooling capacities including total, latent and sensible cooling capacity with high accuracy were obtained.

Haorong Li et.al. [6] presented the Performance of a coupled cooling system with earth-to-air heat exchanger and solar chimney. They investigate an innovative passive air conditioning system coupling earth-to-air heat exchangers (EAHEs) with solar collector. By equally utilizing solar energy and geothermal, the system save large amount of energy within the building sector and reduce electrical demand in the summer season. It is observed that the increase in the outdoor temperature of air and solar radiation increases the natural draft of solar chimney and the amount of air flow to the building by which the amount of cooling capacity get increase in provided to the building.

Clara Peretti et.al. [7] have worked on the design and environmental evaluation of earth-to-air heat exchangers (EAHE). They make a literature review by using the following key terms: earth-to-air heat exchanger (ETAHE, EAHX, ATEHE, EAHE), pre-conditioning of air, buried-pipe system, and ground-coupled heat exchanger. They observed a research performed in order to analyze the design, characteristics of HVAC and EAHE system coupling is get coupled. And they calculated that EAHE can be installed in various types of climate, such as hot desert, humid subtropical, Mediterranean, and oceanic climates. Both designed can be done for cool climates. EAHE may eliminate the need for an air –conditioning system. More over, Incombination with other good thermal building design and low- energy cooling techniques.

Ashish Ku Chaturvedi et.al. [8] have worked on Performance of Earth Tube Heat Exchanger Cooling of Air using the ground temperature below a certain depth remains relatively constant during the year. For checking the performance they make an experimental setup on in Bhopal, India (2014). They used the 5 cm diameter buried below the ground at a depth of 3 m, the pipe is spread horizontally for a length of 3 m. The total length of the experimental set-up is 9 m with the flow velocities 11 m/s by blower. During experiment at Cooling Model Test: The ETHE was operated for seven hours in 3 days (28, 29 & 30 May-2014) for May month. System was turned on at 10.00 AM and turns off at 5 PM. heating mode test tests were carried out for three Day of Jan-2015 (6, 7 & 8th) the system was turned on at 10am and operated for 8 hours continuously, turn off 5 pm. The conclusion is temperature rise of 3.23oC-6.1oC has been observed; COP obtained in summer climate is 2.817 at time 14:00 and in winter climate is 1.321 at time 22:00

A.K. Athienitis et.al. [9] have worked on Design and simulation of a hybrid ventilation system with EAHE by using numerical model for the two underground ducts and a CFD study for the HVAC system. It is observed to determine the maximum temperature and air velocity in the seated area for good comfort conditions. The result analysis of energy efficiency of the hybrid HVAC system it will be evaluated temperature distribution and the velocity in the theatre will be taken under the various conditions such as with the chimney operating and part load conditions

Jyotirmay Mathur et.al. [10] is carried out on ‘Derating Factor’ new concept for evaluating thermal performance of earth air tunnel heat exchanger: A transient CFD analysis. By under transient operating conditions in predominantly dry and hot climate using CFD and experimental modeling with FLUENT software. For better thermal performance of EATHE they concluded, the soil situated in the immediate vicinity of the EATHE pipe should have maximum thermal conductivity and at the same time, derating factor should be considered into account so that the EATHE would be able to give a consistent thermal performance for longer term period of operation. The derating factor is also affected by pipe diameter, air flow rate, period of inter mittency, ambient conditions, and type of operation etc.

Vahid Khalajzadeh et.al. [11] performed on Parameters optimization of a vertical ground heat exchanger based on response surface methodology. To study the effect of simultaneous data variation of design parameters; a three dimensional CFD simulation was carried out. They examined the effect of four dimensionless design parameters on two dimensionless response variables. Results conclude that the response variables are strongly affected by the dimensionless inlet fluid temperature and the dimensionless pipe diameter but are weakly affected by dimensionless depth they need condition for analysis.

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2.2 RESEARCH GAP

Many of the research have been done for a long time on earth air heat exchanger in different climate, but due to change in climate there is variation in efficiency. In this experiment different parameter is adopted for experimental set up such as length, diameter and material of the pipe due to which it affect the performance. Since they all are using the heat energy of the earth which is constant all over the year under the depth of 3 m to 8 m. It is getting difficult to make a practical test setup for the experiment because it takes long time and become costly. If during the setup it get failure then a lot investment will be waste. For reducing this problem I am going to do research on ETHE by CFD analysis. In this research I am considering For the pipe of 30 m length and 0.15 m diameter, temperature value considered for inlet is 7 days in January (2017) from 5 to 11  and 7 days in May(2016) from 5 to 11 at hourly to identify our requirement according to our need. CFD analysis is new technology for research in different field. Since CFD analysis become important in the field of ETHE in different climate to save our time and money. Before starting any earth tube heat exchanger project we need CFD analysis for better result according to the different climate.          

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CHAPTER 3

PROBLEM FORMULATION

CHAPTER 3                                      PROBLEM FORMULATION

3.  PROBLEM FORMULATION

In problem formulation a various factors need to be calculating in CFD analysis considered while deciding upon the material of the pipe for this system used. There can be many options while selecting the material of the pipe for CFD to be used with the system. As the pipe has to be taken for CFD analysis & buried underground, it is not easy to replace the pipe often. Hence the long term of the pipe is importance while taking care of the heat transfer. According to the characteristic of the system. There was a wide range of materials available for the selection for use in CFD analysis system.

3.1 MATERIAL SELECTION

3.1.1 Tube Material

 For heat transfer a lot of material is available which we have to select the material for the ETHE which can be easily analysis, effective, suitable and economic for the system. Following are the selected materials    

Copper

For heat transfer copper is usually the best choice for designing materials. The various advantages of using this material for such systems are mainly because of its high thermal conductivity. It is also resistant to corrosion by liquids. Due to high price it is the one key disadvantage of using copper for our system. Moreover, it is generally not suitable for applications where high forces are applied on these pipes as these pipes are prone to bending. So, we avoid for CFD analysis

Aluminum

Aluminum is also good for CFD analysis and it is also known for its high thermal conductivity and thus was actively considered for use in our system. It is found freely in the earth’s crust but never found free. In case of heat and electricity it is a good conductor. But as in comparison of copper, the cost of aluminum is very high and makes it not suitable for our system where we are concerning our selves with cost effectiveness of the overall set-up. So, we avoid due to cost management for CFD analysis

Mild Steel (MS)

Mild steel is untreated and usually cold or hot rolled or in the case of pipe extruded while molten. Due to low carbon content and rusts in humid weather it can be bent easier than other steel. It’s the most affordable type of steel. Mild steel pipe refers to the content of less than 0.25% carbon steel because of its low strength, low hardness and soft. It includes most of the part of high-quality carbon structural steel and ordinary carbon steel, mostly without heat treatment used in engineering structures, some carburizing heat treatment and other mechanical parts required for wear. So, we avoid for CFD analysis

Concrete

Concrete-mix pipes are obtained by mixing cement with concrete in adequate proportions and using reinforcements of steel bars or steel wire. These pipes are of high strength and resistant to corrosion from various environmental factors. But they have an inherent property of porosity that will induce losses for our system. Because of porosity, although they will be better able to transfer heat but a lot of air will be lost to diffusion through the walls of the concrete pipe. Thus, these pipes might not prove to be efficient.So, due to loss we avoid for CFD analysis

Poly-Vinyl Chloride (PVC)

Poly-vinyl chloride or PVC pipes have started being widely used in home applications and various industrial plants. Their non- reactivity with a wide range of environmental and chemicals agents has made them increasingly popular with a wide range of applications. Their other advantages include the ease of handling because of their light weight. Moreover they have good ageing properties. It has capacity to hold higher temperature as required by environment.  They have high coefficient of thermal expansion. But since we will require our system to heat or cool environment air only, we will not encounter such high temperatures. The main factor for considering PVC pipe was its cost and durability. So, we take PVC material for CFD analysis

3.1.2 Tube Depth

The ground temperature is explained by the external climate and the soil composition, according to its thermal properties and water content. The ground temperature fluctuates with respect to time, but the amplitude of fluctuation diminishes with increasing depth of the tubes, and the temperature practically constant value in deeper of the ground throughout the year. On the basis of temperature distribution, ground has been distinguished into three type zones.

Surface zone:  Extended up to 1 m depth in ground is very sensitive to external temperature.

Shallow zone: Extended up to 1-8 m depth and temperature is almost constant and remains close throughout the yearto the average annual air temperature.

Deep zone: Extended up to 20 m and ground temperature is practically constant. Soil temperature at a depth of about 10 feet or more stays mostly constant throughout the year and stays equal as average annual temperature. After a depth of 3-4 m in the ground, temperature remains nearly constant.

3.1.3 Tube Diameter, Tube Length and Air Flow rate:

The total surface area of the earth tube heat exchangers is a very important factor in overall cooling capacity, by two ways it can be increased, either increasing the tube diameter or tube length. Optimum tube diameter varies widely with tube costs, tube length,mass flow rateand flow velocity. For the best performance at the lowest cost a diameter should be selected that it can balance the thermal and economic factors. The optimum is determined bythe excavation and the actual cost of the tube. Excavation costs in particular vary greatly from one location to another. The optimum tube length was determined by passing the air from the blower at different velocity. The air was passed through the inlet at the minimum speed of the blower i.e. 3-7 m/s and at the length of 13 m, the outlet velocity varies, any further increase in length used to reduce the velocity at outlet which was not required. The 0.05 m diameter pipe was considered for the experiment.

________________

3.2 ETHE Simulation Studies

With the reference Girja Saran et.al [1] it make an experimental work on  Performance of Earth Tube Heat Exchanger Cooling of Ai, by using MS pipe of 0.05 m diameter and was buried at a depth of 3 m. They used air velocity range 0.3 to 45 m/s to drive the air through the pipe which was circulated throughout the pipe.

They start the experimentation, the blower was switched on and the air passes through the pipe. After some time it achieved the steady state. The velocity at the inlet and outlet was calculated. The thermocouple wire is attached at inlet, middle and outlet portion which continuously displays the readings of thermocouple. The above procedure was repeated with different ambient conditions, it conducted 3day of summer season (28, 29 & 30 May-2016) and 3 day of winter season (28, 29 & 30 Jan-2017). All the data thus obtained was compiled into a single table.

The total cooling and heating has been calculated for flow velocities 11m/s by the following equation:

For Summer Season

Qc = mCp(Tinlet – Toutlet)

For winter Season

Qc = mCp(Tinlet – Toutlet)

where m= mass flow rate of air through the pipe

Cp= specific heat capacity of air

Tinlet = inlet temperature of air

Toutlet = outlet temperature of air

Coefficient of performance (COP) of the system has been calculated from the following expression:

For Summer Season

COP mCp(Tinlet – Toutlet)/ Power Input

For winter Season

COP= mCp(Tinlet – Toutlet)/ Power Input

The result come out by performing experiment by them are for the pipe of 9 m length and 0.05 m diameter, the temperature increased/decreased of 3.23o C – 6.1o C has been observed for the outlet flow velocity 11 m/s. In the summer season the maximum COP obtained is 2.817 at time 14:00 and maximum COP obtained in winter season is 2.25 at time 17:00

3.3 Investigation of ETHE in Bhopal, M.P

The preceding discussion on heat exchanger provides a reference for the complexity of the ETHE problem. Therefore, a field investigation was firstly conducted at the previously mentioned and the results are shown. Then a detail numerical technique, using a Computational Fluid Dynamic method is carried out to study the air flow and the temperature distribution in large ETHE

There were two major purposes for conducting the site investigation: the first one was to observe air flow and heat transfer phenomena in an ETHE under supposed condition or working condition, and the second one was to collect detailed boundary conditions and flow field data for CFD modeling validation. Since the climate condition is considered of Bhopal located in Madhya Pradesh, India. In which I am calculating the annual mean air temperature in Bhopal with the lowest daily means value of seven days i.e. from 5 to 11 in January (2017) and the highest daily mean value of seven days i.e. from 5 to 11 in May (2016).  The test was planned to observe the cooling and heating performance of the ETHE system in order to change in parameter.

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3.4 Experimental Design

If the experimental data were collected in such mode, they would not be able to serve for steady- state CFD model validation. Therefore, some temporary modifications to the system were implemented as follows and modified ETHE is shown in Figure 1

• Measurement were performed under quasi- steady conditions, and manual control were used to over ride the automatic control.

• It is considered as a closed system

• Parameter of ETHE is 30 m tube length, 0.15 m tube diameter, 3 mm pipe thickness

• The constant basic soil temperature in summer season and winter season is 25.4o C through out the year.

• The temperature value considered for inlet temperature is 7 days in January 2017 from 5 to 11 and 7 days in May 2016 from 5 to 11 at hourly ( 10 am to 5 pm)

• Inlet air velocity is considered for CFD analysis is 3 m/s, 5 m/s and 7 m/s

Analysis Deign Set Up

Fig.1

3.5 METHODOLOGY

3.5.1 Description of CFD model

Computational fluid dynamic (CFD) is a method which is based on computer simulation for the analyzing heat transfer, fluid flow, and related phenomena such as chemical reaction. CFD analysis carried out in the various project, industries, engine etc. In this project CFD used for analysis of flow and heat transfer. A large amount of expense take place during experimental study but in CFD analysis the number of configurations desired for testing as compare to practically. A lot of CFD software available such as Fluent, Ansys, CFX, ACE.

Three main elements are needed to run a simulation.

1. Pre-Processor: Calculation the geometry for the computational domain of interest and generate the mesh of control volumes

2. Solver: The calculation using a numerical solution technique, which can use finite element, finite difference or spectral methods.

3. Post-Processer: Provide for visualization of the result and includes the capability to display the geometry, mesh, counter, create and 2D & 3D surface plots.

3.5.2 Problem Solving with CFD

There are many decisions to be made before setting up the problem in the CFD code. Decision for problem should be 2D or 3D, which turbulence model to use, which type of boundary conditions to use, whether or not to calculate temperature or variations based on the air flow density etc. Level of problem should be made simple by considering assumption. It reaches to an accurate solution.

CFD based analysis has been employed to resolve the transient temperature field around the horizontal buried pipe of ETHE, using an unstructured grid. A transient and implicit numerical model based on coupled simultaneous heat transfer and turbulent flow was developed to predict the thermal performance and evaluate the cooling capacity of Earth Air Tunnel Heat Exchanger system. The model incorporates the effect of turbulent air flow on the thermal performance of ETHE. According to the sensitivity of temperature quantity, the element type and grid density were selected to be variable, so that the calculation can adapt to the actual situation and reach a high level of accuracy. Since the temperature changes more sharply around the pipe wall, the grid is designed to be denser in that area, while it is sparser farther away from the pipe wall. In the present study it has been assumed that air is incompressible and the soil is homogeneous and its physical properties are constant. It was also assumed that the property of the pipes and ground materials do not change with temperature and engineering materials used in the CFD model are isotropic and homogeneous. The fundamental equations of fluid flow and heat transfer have been used in the analysis. The geometric modeling and meshing have been prepared Fluent/CAD. The main objective of the CFD study was to investigate the transient behavior of simple ETHE system used in continuous heating mode and compare it’s thermal performance with ETHE operating under steady state condition (assuming that the temperature of soil surrounding the pipe remains constant) in terms of derating factor. Physical and thermal parameters of different engineering materials used in the simulation are listed below.

Physical and thermal parameters used in simulation.

1. Material Density

2. Specific heat capacity

3. Thermal conductivity

4. Temperature( inlet winter)

5. Temperature (inlet summer)

6. Soil (winter)

7. Soil(summer)

8. Air velocity

9. PVC

10. Pipe Length

11. Pipe diameter

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3.6 CFD Analysis for ETHE with velocity change using Ansys Fluent

3.6.1 Preprocessing:

CAD Model: Generation of 3D CAD model of ACC using Unigraphics. Then import the CAD model into Ansys Design modeller in Para solid format (.xtl or .x_t)

Fig.2 Mesh: Generate the mesh of ETHE in the Ansys software

Fig.3 Mesh type: Tetrahedral

 Element length of pipe = 30 m

Number of Nodes = 59177

Number of Element = 178479

3.6.2 Fluent Setup: After mesh generation define the following setup in the Ansys fluent.

• Problem Type: 3D

• Type of Solver: Pressure‐based solver.

• Physical Model: Viscous: K, e two equation turbulence model.

• Material Property:    Flowing fluid is air

 Density of air = 1.225 kg/m3

 Viscosity = 1.7894e‐05

Cp  = 1006.43 J/kg k

PVC Pipe

 Density = 1400 kg/m3

 Cp  = 1046.7 j/kg k   

• Boundary Condition (Operation Condition)

Inlet Velocity = 3 m/s, 5 m/s & 7 m/s

Turbulent intensity = 5%

Hydraulic Diameter =   0.15 m

Length of pipe= 30 m

Thickness of pipe= 0.003 m

Constant basic soil temperature = 25.4o C

• Solution Method:

Pressure‐ velocity coupling – Scheme ‐SIMPLE

Pressure – Standard

Momentum – Second order

Turbulent Kinetic Energy (k) – First order

Turbulent Dissipation Rate (e) ‐ First order

• Solution Initialization: Initialized the solution to get the initial solution for the problem.

• Run Solution: Run the solution by giving 500 number of iteration for solution to converge.

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