Effect Of Employing Exit Channels Having Different Diameters

After giving up the zoning application, since it was observed from the previous analyses that altering the main channel diameter while keeping the exit channel diameter constant, has very small effect on fluctuation. It was decided to employ another design strategy based on keeping the main channel diameter constant and employing exit channels having different diameters on a single main channel.

Another reason to use this technique is the manufacturing simplicity, standard part availability on the market, and no need to use expensive molds to manufacture the main channel over 10m long.

By considering the positive effect of decrease in exit channel on fluctuations on exit mass flow rates, and exit velocity limits. It was decided to build up a channel having 40 and 45 mm of exit channels.

By remembering the results of the analysis of 150/50 channel, it was observed that mass flow rates of exit 1, Which is located at the end and positioned parallel to the main channel, and Exits numbered 6 and 7, which are located near to the inlet region, are higher than the other exits. Because of that reason, it was decided to use 40 mm exit diameters of the exits numbered 1,6 and 7.

Consequently, the air channel, whose exit diameters and general view are showed in Table 4.25 and Figure 4.52 and having 150 mm main channel diameter, was designed.

Table 4.25. Table of exit diameters of the channel having variable exit diameters

Exit Number

Exit Diameter

(mm) Exit 1 40 Exit 2 45 Exit 3 45 Exit 4 45 Exit 5 45 Exit 6 40 Exit 7 40 Exit 8 45 Exit 9 45 Exit 10 45 Exit 11 45 Exit 12 45 Exit 13 45 Exit 14 45

Figure 4.52. General view of the channel having various exit diameters

Analyses were run with this geometry under the same conditions and area weighted average exit velocity and mass flow rate values obtained from the analyses of channel having variable exit diameters are tabulated in Table 4.26 and graphed in Figure 4.53 and 4.54 respectively.

Table 4.26. Analysis results of the channel having variable exit diameters

Area Weighted Average Exit Velocities of Channel 150/40/45

10

Area Weighted Average Exit Velocity (m/s)

150/40/45

Figure 4.53. Graphic of area weighted average exit velocities of the channel having variable exit diameters

Mass Flow Rate of Channel 150/40/45

0,01 0,02 0,03 0,04

Exit 1 Exit 2 Exit 3 Exit 4 Exit 5 Exit 6 Exit 7 Exit 8 Exit 9 Exit 10

Exit 11

Exit 12

Exit 13

Exit 14 Exit Num ber

Mass Flow Rate (kg/s)

150/40/45 Mean

Figure 4.54. Graphic of mass flow rates of the channel having variable exit diameters It is observed from results of analysis of the channel having variable exit diameters that, decrease in diameters of Exits 1, 6 and 7 causes an increase in velocity but decrease in mass flow rates. Deviation of this channel was calculated as 9,6%, which is still high from our criteria.

As a result of investigations of the mass flow rate values, it was seen that mass flow rates of exits 2,3,4,12,13 and 14 were higher than the average value. So employing smaller diameter on these exits might provide a homogeneous air distribution. So, the air channel, whose exit diameters are listed in Table 4.27, was designed.

Table 4.27. Table of exit diameters of the channel having variable exit diameters weighted average exit velocity and mass flow rate values obtained from the analyses of the channel having variable exit diameters are tabulated in Table 4.28 and graphed in Figure 4.54 and 4.55.

Table 4.28. Analysis results of the channel having variable exit diameters

Exit Number Exit Diameter (mm) Exit Velocity (m/s) Mass Flow Rate (kg/s)

Exit 1 40 16,01 0,02499

Area Weighted Average Exit Velocities of Channel 150/40/45

Area Weighted Average Exit Velocity (m/s)

150/40/45

Figure 4.54. Graphic of area weighted average exit velocities of the channel having variable exit diameters

Mass Flow Rate of Channel 150/40/45

0,01

Figure 4.55. Graphic of mass flow rates of the channel having variable exit diameters

As a result of investigations of the mass flow rate values, it was seen that mass flow rates of decreased exits were closed to the mean value. However, mass flow rate of 5 and 11 numbered exits, having 45 mm diameter, has higher mass flow rate values. Also it was observed an increase in mass flow rate of exit 11, because of the decrease in diameter of exits numbered 12,13 and 14. Deviation of this channel was calculated as 6,8%, which is very close to our criteria.

So it was predicted that employing smaller diameter on exits 5 and higher diameter on exit 12, by keeping the others constant might provide a homogeneous air distribution.

Consequently, air channel, whose exit diameters are listed in Table 4.29, was designed.

Area weighted average exit velocity and mass flow rate values obtained from the analyses of the modified channel having variable exit diameters are tabulated in Table 4.30 and graphed in Figure 4.56 and 4.57.

Table 4.30. Analysis results of the modified channel having variable exit diameters

Area Weighted Average Exit Velocities of Channel 150/40/45

11

Area Weighted Average Exit Velocity (m/s)

150/40/45

Figure 4.56. Graphic of area weighted average exit velocities of the modified channel having variable exit diameters

Mass Flow Rate of Channel 150/40/45

0,01 0,02 0,03 0,04

Exit 1 Exit 2 Exit 3 Exit 4 Exit 5 Exit 6 Exit 7 Exit 8 Exit 9 Exit 10

Exit 11

Exit 12

Exit 13

Exit 14 Exit Num ber

Mass Flow Rate (kg/s)

150/40/45 Mean

Figure 4.57. Graphic of mass flow rates of the modified channel having variable exit diameters

As a result of modifications, deviation value raised to 7,3% from 6,8%. So this modification has a negative effect on fluctuations of mass flow rates. Here it was seen that increasing the diameters of exits, which are near to the end of the channel, increases the mass flow rate and also fluctuations. After seeing the increase in fluctuation when diameters of the exits near to the end of the channel increased. It was decided to do opposite of this. So diameters of the last 4 exits were decreased to 40 mm.

Resultant air channel dimensions are listed in Table 4.31 was designed.

Table 4.31. Table of exit diameters of the modified channel having variable exit diameters

Exit Number Exit Diameter (mm)

Exit 1 40

Area weighted average exit velocity and mass flow rate values obtained from the analyses of the modified channel having variable exit diameters are tabulated in Table 4.32 and graphed in Figure 4.58 and 4.59.

Table 4.32. Analysis results of the modified channel having variable exit diameters

Exit

Area Weighted Average Exit Velocities of Channel 150/40/45

Area Weighted Average Exit Velocity (m/s)

150/40/45

Figure 4.58. Graphic of area weighted average exit velocities of the modified channel having variable exit diameters

Mass Flow Rate of Channel 150/40/45

0,01

Figure 4.59. Graphic of mass flow rates of the modified channel having variable exit diameters

As a result of the last modification, deviation from the mean mass flow rate decreases to 5,5% which is almost out target value.

General evaluation for employing a channel having variable exit diameters;

Using variable exit diameters may cause to use non-standard parts to use in the channel and rest of the system components. When the criteria using standard parts in the system was considered, this situation may increase the cost of the system, and cause some labor mistakes in the installation stages. Also when the exit diameters of the resultant channel is examined, it is seen that only 3 of the channels have 45 mm and the rest of 11 channels have 40 mm exit diameter.

So, designing a channel having standard constant exit diameter of 40 mm may provide the optimum solution from the engineering point of view by considering standard part usage, cost, homogeneous mass flow rate, velocity limits, and internal noise.

Analyses were run with this geometry under the same conditions and area weighted average exit velocity and mass flow rate values obtained from the analyses of the channel having 40 mm of exit diameter are tabulated in Table 4.33 and graphed in Figure 4.60 and 4.61.

Table 4.33. Analysis results of the channel having variable exit diameters

Exit

Area Weighted Average Exit Velocities

12 14 16 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Exit Number

Area Weighted Average Exit Velocity (m/s)

150/40

Figure 4.60. Graphic of area weighted average exit velocities of the channel 150/40

Mass Flow Rate of Channel 150/40

0,01 0,02 0,03 0,04

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Exit Num ber

Mass Flow Rate (kg/s)

150/40

Mean

Figure 4.61. Graphic of mass flow rates of the channel 150/40

Standard deviation of mass flow rate obtained by means of CFD is 0,0018 kg/s. This value is approximately 7% of average mass flow rate of exits. This amount of deviation can be assumed as proper for homogeneous air distribution along the passenger cabin from practical, cost effect and engineering points of view.