Production Of Turbulence Model Determination

Solving CFD problem usually consists of four main components: geometry and grid generation, setting-up a physical model, solving it and post-processing the computed data. The way geometry and grid are generated, the set problem is computed and the way acquired data is presented is very well known. Precise theory is available. Unfortunately, that is not true for setting-up a physical model for turbulence flows. The problem is that one tries to model very complex phenomena with a model as simple as possible. (Sodja, 2007).

The most important stage of a CFD analysis study is to determine the most suitable turbulence model for your flow domain geometry.

In the present study, in order to determine the most suitable turbulence model, an experimental study was carried out. In the previous section general geometry of the air channel was determined. So an experimental setup was designed by employing parts and manufacturing processes available in the market. By considering standard part usage, it was decided to use a simply available standard pipe, which has 153 mm inner and 160 mm outer diameters, made of plastic, as the main channel. And again by considering standard part usage, it was decided to employ a plastic pipe, available in the market, having 50 mm outer and 45 mm inner diameter as the exit channels of the experimental setup. Also a fan was selected for the experimental setup, to execute the function of evaporator fans. The photo showing the fan used in A/C roof top unit and 3D CAD model of the fan to be used in the experimental setup are showed in Figure 3.41 and 3.42 respectively.

Figure 3.41. Evaporator fans used in the selected A/C roof top unit.

Figure 3.42. 3D CAD model of the fan unit used in the experimental setup

An experimental setup was designed by using the parts mentioned above and referencing the dimensions previously determined. 3D CAD view of the experimental setup can be seen in Figure 3.43.

Figure 3.43. 3D CAD view of the experimental setup

After completing the design studies of the experimental setup, it was started to production process.

6-m long 2 pipes having 153 mm inner diameter were connected to each other and than one of the pipe was cut with respect to the design. Than a linear line was signed on the surface of the pipe and then the center points of the exit channels were marked and drilled with distances indicated in design. Some views from drilling operation and situation of the experimental setup production, after drilling operations can be seen in Figure 3.44.

Figure 3.44. Drilling operation and some of the exit holes on the main channel

Exit channels having 50 mm outer diameter were cut with respect to design and in order to prevent the misalignments at junction region of exit channels with main channel, a reference ring was also added to each of the exit channels. Some views of the exit channel with ring can be seen in Figure 3.45.

Figure 3.45. Sample of exit channels and reference rings

After that, produced exit channels are bonded to the main channel by following bonding procedure, that contains sanding, application of cleaner and primer than sealer mastic application. A sample view from the sealer mastic application and bonded exit channel can be seen in Figure 3.46.

Figure 3.46. A sample view from the sealer mastic application and bonded exit channel

At the end of exit channel bonding operation, main channel was placed to the previously manufactured steel construction stand. And the general view of the experimental setup in that situation can be seen in Figure 3.47.

Figure 3.47. General view of the experimental setup at the production stage

Since the inlet channel is a complex geometry having 3D surfaces, a CNC machine was employed to produce the wooden mold that will be used in production of inlet channels, which is made of composite material (Fiber Reinforced Plastic, FRP). Views from CNC machining stage and the resultant machined base mold can be seen in Figure 3.48.

Figure 3.48. Views from CNC machining stage and the resultant machined base mold

A mold was manufactured by adding some parts to the wooden base mold.

Final mold, semi-product and final inlet channel can be observed in Figure 3.49.

Figure 3.49. Mold and production stages of an inlet channel

By following the production procedure 6 pieces of inlet channels were produced. Photos of produced inlet channels from different sides can be seen in Figure 3.50.

Figure 3.50. Inlet channels

Produced inlet channels were bonded to the main channel by following bonding procedure, that contains sanding, application of activator and primer than sealer mastic application. Stages of bonding procedure can be seen in Figure 3.51.

Figure 3.51. Stages of bonding procedure (a): Sanding, (b): Activator, (c): Primer, (d): Sealer mastic

At the end of the installation of 6 inlet channels, final appearance of inlet region of experimental setup is shown in Figure 3.52.

Figure: 3.52. Inlet region of experimental setup

After completing the installation, whole setup was painted in grey, than the inlet and exit regions were painted in blue and red respectively. Painting operation of inlet region and a sample of painted exit channel can be seen in Figure 3.53.

Figure 3.53. Painting operation of inlet region and a sample of painted exit channel After painting operation, 3 pieces of 2-stage fans, which demonstrates A/C evaporator fans, were installed to the inlet channels. In order to control the fans, 3 pieces of 2-stage control switches and 6 pieces of relays (for each of stages of fans) were mounted with a suitable harness to the experimental setup. Finally 2 pieces of 24V battery were connected to the system. Fans, harness, control switches and relays can be seen in Figure 3.54 and 3.55.

Figure 3.54. Installation of fans and harness

Figure 3.55. Installation of control switches and relays

At the end of all these operations experimental setup was completed and ready to use. Final situation of the experimental setup can be seen in Figure 3.56.

Figure 3.56. General appearance of experimental setup

3.3.2.2. 3D Flow Domain Design Of Experimental Setup

3D Computer Aided Drawing (CAD) program CATIA V5R20 was used to create 3D model of the flow domain to be analyzed. Quality of a mesh has an important role on the results of analysis. The key factor, most effective on quality of a mesh, is shape of the elements forming mesh. Shape of elements can be triangular, rectangular or other. However rectangular elements provide the best quality mesh for FLUENT^® solver, which works with finite volume technique. If these kinds of geometries having multiple bodies are modeled in a single part body. Mesh generated with triangular elements providing lower quality with respect to rectangular elements. (Mezarcıöz et al, 2014).

In the current study, In order to get a high quality mesh, the geometry was divided in to 21 volumes, which are 6 inlets, 14 exits, and 1 main channel. So a mesh, which will be composed of rectangular elements, can be created with high quality. Samples of volumes forming the flow domain can be seen in Figure 3.57, 3.58 and 3.59.

Figure 3.57. A sample of inlet channel

Figure 3.58. A sample of exit channel

Figure 3.59. 3D model of flow domain