**3.1 Grid independence study**

The result of CFD analysis is based upon the number of grid elements or size of meshing. If the number of elements is increased, the computational result quality will be good at a certain limit and if the number of elements is decreased, the computational result quality will be poor. In the present study number of an element is depending on the size of the element, if the size of an element increases the number of an element is decreases and vice versa. For good quality of computational fluid dynamic result, we need to reduce the size of an element. Due to reduce in size the resulting quality will improve at a certain size of an element. After a certain size, the CFD result will be constant. Performance of computational fluid dynamic evaluation at the specific variety of detail is called grid independence. For the heat exchanger for the earth-to-air model, the grid independence study has been executed with an exceptional quantity of mesh elements is shown in **Table 2** and **Figure 9**.

**3.2 Validation of model to experimental data**

*DOI: http://dx.doi.org/10.5772/intechopen.99348*

*Earth Air Tunnel Heat Exchanger for Building Cooling and Heating*

**3.3 Optimizing the design of heat exchanger for earth to air**

**Material Thermal conductivity (w/m K) Density (kg/m3**

*Physical and thermal parameters used in validation [3].*

Air 0.0242 1.225 1006 Soil 0.52 2050 1840 PVC 0.16 1380 900

for the pipe material.

**Figure 10.**

**273**

**Table 3.**

*Graph of temperature v/s length of EATHE [8].*

A CFD-based EATHE model was developed to forecast the thermal performance of the EATHE system. The developed CFD model was validated using the previously published results by Bansal et al. [3]. They conducted the experiments to determine the cooling performance of the EATHE system for the hot and dry climate of Ajmer (Western India) with pipes of two different materials-PVC and mild steel. Both the pipes have 0.15 m inner diameter, 23.42 m length, and were enshrouded 2.7 m below the ground surface in the horizontal position. The properties of air, soil and PVC is shown in **Table 3**. Experimental and simulation temperature results along the pipe length for PVC pipe at a velocity of 5 m/s are shown in **Figure 10**. The temperature predicted by the CFD model at all the points, except at the inlet point, along the pipe length is lower than the experimental results. It is also observed that temperature deviation varies between 0% to a maximum of 7.62%, and this deviation in temperatures may be contributed by the variation in the coefficient of friction (from actual friction coefficient of pipe) used in a simulation

The fluid flow analysis for a different design of earth to air heat exchanger has been evaluated by using the computational fluid dynamic fluent model for the cooling mode of the hot and dry climate of the Bikaner region, the ambient temperature of the Bikaner region is considered 47.6°C for the inlet temperature of optimizing the design of heat exchanger for the earth to air. In this analysis, the

**) Specific heat capacity (j/kg K)**

The maximum and minimum detail size 0.15 and 0.011 is a stander heat exchanger for earth to air model. This size of an element is further utilized for the earth air tunnel model optimization of the design.


**Table 2.**

*Grid independence study with different number of mesh element.*

**Figure 9.** *Graph of temperature v/s number of grids [11].*
