Ab0112130

Invention Journal of Research Technology in Engineering & Management (IJRTEM) ISSN: 2455-3689 www.ijrtem.com Volume 1 Issue 1 ǁ Feb. 2016 ǁ PP 01-05

CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air Conditioning Pankaj Badgaiyan1, prof. Surendra Agrawal2 Surbhi Group of Institution, Bhopal

ABSTRACT: The earths inside temperature is commonly higher than the outside air temperature in winter and lower in summer so it can makes the use of the earth convenient as warm source or cold sink respectively. Earth air heat exchanger can contributes to reduce in energy consumption. Numbers of research have been carried out on Computational Fluid Dynamics analysis of EATHE systems and still it requires optimum redevelopment. In the present work a 3D CAD model was developed for CFD analysis of earth air tube heat exchanger (EATHE) system. The obtained simulated results were compared with experimental results obtained from experimental setup installed at Bhopal situated in Central India. The temperature of earth at a certain depth about 2 m to 3 m of ground remains nearly constant throughout the year this constant temperature is called the undisturbed temperature of earth. The performance evolution was carried out to check the effect of pipe length, air flow velocity, depth of buried pipe and pipe diameter of EATHE system for summer cooling. The result shows that this system is more convenient only for summer cooling and not suitable for winter heating.

KEYWORDS: CAD Modeling, Earth‐Air Tube Heat Exchanger, CFD Analysis, etc

1. INTRODUCTION Earth‐air heat exchanger is a device which can effectively utilize the thermal energy of earth for heating/cooling applications of buildings, offices, residential, industries etc. The physical phenomenon of earth air heat exchanger is simple: the earth interior temperature commonly higher than the outside air temperature in winter and lower in summer, so it makes the use of the earth convenient as warm or cold sink respectively. Both of the above uses of earth air heat exchanger can contribute to reduction in energy consumption. The air passes through mild steel pipes buried underground at a 3 m depth gets cooled in summer season is supplied to the space to be conditioned and vice versa in winter. The thermal performance of EATHE system depends mainly on two factors: one is atmospheric conditions and soil properties; second is operating parameters like air flow velocity, pipe diameter, pipe length and depth of buried pipe. The climatic conditions and soil properties cannot be changed so, to improve the thermal performance of EATHE system operating parameters are to be optimized. A considerable amount of electrical energy can be saved if EAHE system is designed properly, so three dimensional CAD model was developed for CFD analysis of earth air tube heat exchanger (EATHE) system. Because of the high thermal inertia of the exterior climate are damped deeper in the ground. Further a delay arises between the temperature fluctuations within the ground and at the surface. Thus at a sufficient depth the soil temperature is lower than the outside air temperature in summer and higher in winter. When the fresh air is drawn through the earth tube heat exchanger the air is thus cooled in summer and heated in winter. In combination with other passive system and good thermal design of the building, the earth air heat exchanger can be used to preheat air in winter and avoid air conditioning units in building in summer, which result in a major reduction in electricity consumption of a building.

2. LITERATURE REVEIW Lot of researchers has carried out on CFD analysis of EAHE systems. By review previous research papers published by many authors we can have an idea on how it works. Mihalakakou et al. [1], Lee and Strand [2] investigated the effect of pipe radius, pipe length, air flow rate and pipe depth on the overall performance of the earth tube under various conditions during cooling season. Pipe length and pipe depth turned out to affect the overall cooling rate of the earth tube, while pipe radius and air flow rate mainly affect earth tube inlet temperature.

[3] Misra et al. analyzed the effects of time duration of continuous operation, thermal conductivity of soil pipe diameter and flow velocity on thermal performance of Earth Air Tunnel Heat Exchanger (EATHE) under transient conditions. They found that the effect of pipe diameter due to prolonged use of EATHE system on its thermal performance is least in case of soil with higher value of thermal conductivity and the increase in flow velocity leads to drop in thermal performance of EATHE system [4] Wu et al. presented a transient and implicit numerical model and implemented it on the CFD (Computational Fluid Dynamics platform), PHOENICS, to evaluate the effects of the operating parameters (i.e. the pipe length, radius, depth and air flow rate) on the thermal performance and cooling capacity of earth–air–pipe systems. The simulation results were obtained for pipe length of 20, 40 and 60 m; pipe radius of 0.1, 0.2 and 0.3 m; depth of burial of 1.6, 3.2 and 5 m; air flow rate of 1, 2 and 4 m/s. It was concluded that longer pipes and higher depth of burial results in higher cooling capacity while increase in pipe radius and air flow rate results in increase of both outlet air temperature and heating capacity.

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air [5] Kabashnikov et al. developed a mathematical model for calculating the temperature of the ground and air in a ground heat exchanger for ventilation systems and calculated the degree of decrease in the efficiency of the heat exchanger on decrease of the spacing between the tubes, as well as the dependences of the heat power of the system on the length and diameter of tubes, depth of their burial, and air flow rate.

[6] Woodson et al. presented a case study to analyze the ground temperature gradient and performance of EAHE at burial depth of 0.5m, 1.0m and 1.5m in Burkina Faso.

[7] Santamouris et al. investigated the impact of different ground surface boundary conditions on the efficiency of a single and a multiple parallel earth-to-air heat exchanger system.

[8] Vikas Bansal et al. [2010] work on transient and implicit model based on CFD (FLUENT) was developed to predict the thermal performance and cooling capacity of earth–air–pipe heat exchanger systems. Results show that modeling of EPAHE system with maximum deviation of 11.4%. The 23.42 m long and 0.15 m diameter EPAHE system gives cooling in the range of 8.0–12.7 8C for the flow velocities 2–5 m/s. Cooling is found to be in the range of 1.2–3.1MWh. Velocity of air affects the thermal performance of EPAHE system. The COP of the EPAHE system is from 1.9 to 2.9 for increase in velocity from 2.0 to 5.0 m/s.

[9] Vikas Bansal et al [2012] analysis of the integrated system based on (CFD) modeling with FLUENT software. Results show that a simple EATHE system provides 4500 MJ of cooling effect during summer months, whereas 3109 MJ of additional cooling effect can be achieved by integrating evaporative cooler with the EATHE. Performance analysis shows that while ambient air itself is comfortable for 25.6% of the hours, use of integrated EATHE evaporative cooling system provides comfortable air for 34.16% hours in one year, whereas simple EATHE system is able to provide comfortable air for only 23.33% additional hours.

[10] Girja Sharan has developed some applications of earth tube heat exchangers in Gujarat, India. It is seen that the ETHE could warm-up the cold air by as much as 12 -130C. It could cool the air in May also by a similar amount, from 40.80C to 27.20C.Simulations showed that increasing the length of pipe improves the performance.

[11] Rohit Misra et al. [2013] has been evaluated for hot and dry climate of Ajmer (India) using experimental and CFD modeling.

Results show that highest derating factor of continuous operation is observed as 0.64, 0.40, 0.27 and 0.08– 0.33, 0.04–0.13 and 0.02– 0.07 for 5m and 30m location in tube of EATHE buried in soil SL1, SL2 and SL3 respectively. The analyzed cases have shown the range of derating to be as minimal as 0% to as high as 64%. The total air temperature drop under transient conditions reduced from 18.70C to 16.60C after 24 h of continuous operation for soil thermal conductivity of 0.52Wm -1 K-1.

[12] Trilok Singh Bisoniya et al. [2014] a quasi-steady state, 3-D model based on computational fluid dynamics was developed to perform the parametric analysis of EAHE system. The effect of four parameters pipe length, pipe radius, air flow velocity and depth of burial on the performance of EAHE was studied.

[13] M.K. Ghosal et al. [2005] were conducted extensively during winter period, but the model was validated against the clear and sunny days. Temperatures of greenhouse air with ground air collector were observed to be 2–30C higher of temperatures for greenhouse air during winter period. The temperature fluctuations of greenhouse air were also less when operated with ground air collector as compared to earth air heat exchanger.

[14] Kwang Ho Lee and Richard K. Strand [2012] Parametric analysis was carried out to investigate with the effect of pipe radius, pipe length, air flow rate and pipe depth on the overall performance of the earth tube under various conditions. As the pipe length and depth increases, the inlet air temperature decreases. Air flow rate and pipe radius increases, the earth tube inlet air temperature also increases. In addition, pipe length and pipe depth turned out to affect the overall cooling rate of the earth tube, while pipe radius and air flow rate mainly affect earth tube inlet temperature.

[15] Alexander de jesus Freire et al. [2013] Comparing the result of the day simulation with 1D and 2D models, there are variations of 6.5% in the average exit temperature of about 10C. Under more extreme temperature, the variation of result increase without significant error, recording a maximum difference of 2.50C. It was found that a deviation of 30C below the reference temperature 250C occurs with a frequency of 65%. The acceptable room temperature is kept for comfort from 20 0C to 250C which is corresponding to 88% of the considering time.

[16] F. Al-Ajmi et al. [2006] has been implemented and all the models have been encoded within the TRNSYS-IISIBAT environment. Simulation results showed that reduction of 1700Win the peak cooling load, with an indoor temperature reduction of 2.80C during summer. The indoor temperature varies in the summer season between 32 and 28.1 0C. The EAHE is shown to have the potential for reducing cooling energy demand in a typical house by 30% over the peak summer season. EAHE system alone cannot maintain indoor thermal comfort within the acceptable range 22–270C. [17] Girja Sharan and Ratan Jadhav [2002] have been worked on a single pass earth-tube heat exchanger (ETHE) was installed to study its performance in cooling and heating mode. ETHE reduce the temperature of hot ambient air by as much as 14 oC in May. The basic soil temperature in January was 24.2oC.The coefficient of performance (COP) in cooling mode averaged to 3.3 and heating mode it averaged to 3.8.

[18] Vikas Bansal et al. [2013] were performed experimentally and computational fluid dynamics modeling with FLUENT software.

The maximum air temperature drop for 100 m is 18.40C, 18.70C and 18.40C for soil thermal conductivity of 0.52, 2.0 and 4.0W/mK respectively. However, the maximum air temperature drop obtained 18.3 0C and 14.0C, 18.30C and 17.20C and 18.60C and 18.0C for soil thermal conductivity of 0.52, 2.0 and 4.0W/mK respectively. The value of derating factor at a section situated at 20 m, 50 m and 90 m from inlet of EATHE pipe varies between 0.20- 0.59, 0.08- 0.44 and 0.01–0.24 respectively for soil thermal conductivity of 0.52Wm-1 K-1 during 24 h of operation. Derating minimum value of 0.2% and high as 68%. | Volume 1| Issue 2 |

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air [19] Jian Zhang and Fariborz Haghighat [2009] was performed on numerical experiments using computational fluid dynamics (CFD) were conducted to investigate the airflow and thermal behavior in the large ducts. The three-layered ANN structure was constructed with the six parameters as the inputs and the three Nusselt numbers as the outputs. After the ANN model was trained by thirty CFD simulation cases to predicted very good results. ANN for predicting average Nusselt numbers of the duct ceiling, wall, and floor.

[20] Vikas Bansal et al. [2009] work on transient and implicit model based on CFD (FLUENT) was developed to predict the thermal performance and cooling capacity of EPAHE systems. Experimental and simulation results for modeling of EPAHE system with maximum deviation of 2.07%. The 23.42 m long and 0.15 m diameter EPAHE system gives heating range of 4.1–4.80C for the flow velocities 2–5 m/s. The hourly heat gain is found to be in the range of 423.36–846.72 kWh. The maximum rise in temperature for PVC and steel pipe is 4.5 and 4.80C, respectively.

3. CFD MODELIND AND SIMULATION The effect of operating parameter, pipe length, and diameter, depth of burial and air flow rate on thermal performance and cooling /heating capacity of EATHE can be analyzed. Computational Fluid Dynamic (CFD) is very popular tool for modeling and to predict the effect of operating parameter of EATHE system. Some of the commercial CFD codes in use are FLLUENT, STAR CD, CFX, FIDAR, ADINA, CFD2000 and PHOENICS. CFD (FLUENT) is used for the simulation of EATHE system. CFD (FLUENT) is used for the simulation of EATHE system. The fallowing assumptions were mainly considered in CFD modeling.  Analysis is based on steady state conditions  The soil properties around the pipe are isotropic, homogeneous and its conductivity along vertical and horizontal directions has a constant value;  The cross sectional area of pipe is uniform in axial direction.  The pipe material having thermal resistance is negligible (thickness of the pipe is very small).  The convective flow inside the pipe is thermally and hydro dynamically developed.  The temperature on the surface of pipe is uniform in axial direction.

Thermal model of EATHE system with specified dimension (length of pipe, 24m; diameter, 0.1524m and depth of pipe 3m) is developed in CATIA as shown in figure-02. The model is developed in CFD (FLUENT) where mesh of geometry is generated. The purpose of meshed model is discredited equation and boundary condition on single model. The CFD simulations were performed considering three dimensional steady state, turbulent flow (k-epsilon model) enabling heat transfer and thermal energy. The total number of nodes and element generated in meshing of geometry of EATHE were 357700 and 916068 respectively. So that approximately 916068 elements were used in CFD analyses.

4. EXPERIMENTAL SETUP: EATHE is consists of a 24 m long and 0.1524 m inner diameter with wall thickness of 3 mm and the pipe is made up of mild steel. It is buried inside the ground surface at the depth of 3.048 m. The length and spacing of serpentine pipes were 6 m and 1.5 m respectively with 3 turns. The turns which are created through a 900 short elbow at the end makes the outlet horizontal to the entry of air. The inlet and outlet of the EATHE rise 0.5 m above ground surfaces. Inlet is coupled to the delivery end of blower and outlet is connected with Room. Air tight cover was used to join the buried pipes with the blower. The isometric view of the experimental greenhouse with the integrated earth air tube heat exchanger is shown in figure no. 1.

Figure No. 1: Actual View Of Ms Pipe Buried Inside Earth

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air 5. EARTH UNDISTRIBUTED TEMPERATURE In the design process, the undisturbed ground temperature is one of the main input parameters. Still, its accurate modeling is difficult because the soil parameters are often unknown. Additionally, the definition of “undisturbed ground temperature� is problematic due to the thermal influence of a building or different soil properties at an EATHE. In the following, the undisturbed ground temperature is influenced by the building but not by the EATHE and is defined for mean soil properties. Hence, the undisturbed ground temperature is a hypothetical value. For homogeneous soil of constant thermal diffusivity, the ground temperature at any depth z and time t is [ASHRAE-HVAC systems and equipment, 2000]: This equation shows that the soil temperature at a certain depth mainly on the surface temperature and on the thermal characteristic of the soil.

Tz,t = Tm–As exp–Z

đ?œ‹ 365 đ?›źđ?‘

1 2

1

2đ?œ‹

đ?‘?

cos{ 365 [t-t0 2

365 2 đ?œ‹đ?›źđ?‘

]}

Where Tz,t is the ground temperature at time t (s) and depth z (m), T m is the average soil surface temperature (oC), As is the amplitude of soil surface variation (oC), Îąs is the soil thermal diffusivity (m2/s, m2/day), t is the time elapsed from the beginning of the calendar year (day) and to is the phase constant of soil surface (s; day).

BOUNDARY CONDITION In three dimensional model of EATHE the fallowing boundary condition were used.

INLET BOUNDARY CONDITION At the inlet of EATHE pipe, the static temperature of air, T in (oC) and the value of air flow rate, Va (m/s) at inlet were to be defined.

The thermodynamic properties (density and specific heat capacity) and transport properties (dynamic viscosity and thermal conductivity) of air were to be defined at static temperature of air at inlet.

OUTLET BOUNDARY CONDITION In the subsonic flow regime, the relative pressure at the outlet of the EATHE pipe was defined as equal to zero temperature.

WALL The surface of the EATHE pipe (wall) is uniform and temperature on the surface of EATHE pipe (wall) was uniform in axial direction and equal to the earthâ€&#x;s undistributed temperature at Bhopal city (21 oC). Smooth wall is assumed at the inner surface of EATHE pipe.

6. EXPERIMENTAL AND SIMULATED RESULT VARIFICATION Given more importance to ventilation, fresh and better quality air supply to the building, open loop system is adopted in experimental setup on another hand close loop system result in the better efficiency of the earth air tube heat exchanger system and less moisture condensation inside the pipe and less moisture condensation inside the pipe.

The experimental setup consists of 24m mild steel pipe and 0.1524m diameter which is buried 3m under the ground surface.

The results obtain by the simulation of EATHE system in CFD is validated by the experimental data taken on actual experimental setup installed at Bhopal (central India) as shown in figure-02. In EATHE pipe arrangement open loop system is preferred over the close loop system for the clean, pure and quality of air supplied to the building.

Figure -2 Placed buried tube inside earth.

The experiments were carried out at Bhopal during summer. Both simulation and experimental observation were taken at air flow velocity of 4, 5 and 6 m/s. In figure-3 and 4 shows the variation of experimental and simulated temperature at different location along the length of mild steel pipe at velocity 4 m/s. In figure-5 and 6 shows the variation of experimental and simulated temperature | Volume 1| Issue 2 |

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air at different location along the length of mild steel pipe at velocity 5 m/s. In figure-7 and 8 shows the variation of experimental and simulated temperature at different location along the length of mild steel pipe at velocity 6 m/s. In figure-9 and 10 shows the variation of experimental and simulated COP at hourly time interval along the length of mild steel pipe at different air flow velocity. In figure11, 12 and 13 represent hourly variation of experimental and simulated COP along the length EATHE pipe at velocity 4 m/s, 5 m/s and 6 m/s. In figure-14, 15 and 16 shows hourly variation of experimental and simulated temperature at the outlet of EATHE pipe at 4 m/s, 5 m/s and 6 m/s air flow velocity. In figure-17 and 18 shows the influence of air velocity and pipe depth on inlet air temperature.

In figure-3, 4 and 5 the location of nine thermocouple (T1 to T9), Tin and Tout are shown on horizontal axis. The variation of air temperature along the length of pipe from inlet to outlet is shown in vertical axis.

Figure-3 Hourly variation of experimental temperature at different location along the length of EATHE pipe at 4 m/s air flow velocity.

Figure-4 Hourly variation of simulated temperature at different location along the length of EATHE pipe at 4 m/s air flow velocity

Figure-5 Hourly variation of experimental temperature at different location along the length of EATHE pipe at 5 m/s air flow velocity.

Figure-6 Hourly variation of simulated temperature at different location along the length of EATHE pipe at 5 m/s air flow velocity

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air

Figure-7 Hourly variation of experimental temperature at different location along the length of EATHE pipe at 6 m/s air flow velocity.

Figure-8 Hourly variation of simulated temperature at different location along the length of EATHE pipe at 6 m/s air flow velocity.

Figure-9 Hourly variation of experimental COP at different air flow velocity along the length of EATHE pipe.

Figure-10 Hourly variation of simulated COP at different air flow velocity along the length of EATHE pipe.

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air

Figure-11 Hourly variation of experimental and simulated COP along the length EATHE pipe at velocity 4 m/s.

Figure-12 Hourly variation of experimental and simulated COP along the length EATHE pipe at velocity 5 m/s.

Figure-13 Hourly variation of experimental and simulated COP along the length EATHE pipe at velocity 5 m/s.

Figure-14 Hourly variation of experimental and simulated temperature at the outlet of EATHE pipe at 4 m/s air flow velocity.

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air

Figure-15 Hourly variation of experimental and simulated temperature at the outlet of EATHE pipe at 5 m/s air flow velocity.

Figure-16 Hourly variation of experimental and simulated temperature at the outlet of EATHE pipe at 6 m/s air flow velocity.

Figure-17 Influence of air velocity on inlet temperature.

Figure-18 Influence of pipe depth on inlet temperature.

7. CONCLUSION The analysis is based on to determine the effect of four important parameter namely pipe length, pipe diameter ,depth of buried pipe and air flow velocity on outlet air temperature of EATHE system. The performance of EATHE system is depending on total contact surface area of pipe which can be increased by increasing the length of pipe or pipe diameter. Therefore after taking the CFD FLUENT analysis fallowing conclusion were made.

1. The outlet air temperature of EATHE system decrease with increase in length of pipe.

2. It is observed that greater the length of pipe increase the pressure drop along the length of pipe.

3. The outlet air temperature of EATHE system decrease with increase depth of pipe. So, pipe of EATHE system can be installed as deeply as possible.

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CFD Base Performance Evaluation of Earth-Air Tube Heat Exchanger for Natural Air 4. It is observed that due to increase in air flow velocity, the time to which air remain in contact with ground is reduced. So, as a result outlet air temperature of EATHE system is increase. The performance of EATHE system can be increased only by decreasing the air flow velocity.

5. The total average COP in the experimental period is found to be range of 2.76 to 6.55 at different air velocity like as 4 m/s, 5 m/s and 6 m/s respectively. Based on the outcome it can be confirmed that EATHE hold considerable promise as a mean to cool ambient air for a variety of application such as a livestock building and greenhouse.

6. The difference between the simulated and experimental value of outlet air temperature are 0.77-1.86, 0.21-3.59 and 0.85-2.79 for different air velocity of 6 m/s, 5 m/s and 4 m/s respectively. The simulated and experimental temperature of greenhouse air in the developed system showed fair agreement. It is completed that there is possible EATHE system to make a useful involvement in energy saving.

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