3. MATERIAL AND METHODS
3.1.2. Determination Of Outside Temperature
Outside ambient temperature is assumed as 38°C. (SAE, 2004), (Temsa, 2007), (Temsa, 2008). This value can be used for outside of the vehicle however, temperature of the vehicle floor cannot be taken as 38°C. Because there are some heat sources underneath the floor like engine, transmission, exhaust muffler, axles etc. The influence of environmental changes on underhood and underbody components of a vehicle during various driving cycles is an important issue in the
automotive industry (Apolloni, 2006). Thus real temperature values under the passenger compartment must be determined. For this purpose, vehicle floor is divided into 11 regions. And the temperatures of these regions are measured in 3 different test conditions as highway, ramp, and idle. (Fournier and Digges, 2004)
Some characteristic properties of the vehicle’s powertrain are as follow; 410 HP, 12,6 lt, 6 cylinder Euro 5 diesel Engine with 6 speed manual transmission.
Highway Test Conditions: In this test, vehicle was driven with 100km/h speed, in the last transmission speed of the vehicle, in a smooth highway.
Ramp Test Conditions: Temperatures of the regions under the floor of the vehicle were measured while the vehicle was climbing a sharp ramp. Maximum component temperatures can be seen in 40 km/h uphill condition (Weidmann et. al., 2007).
Idle Test Conditions: While this test, vehicle is parked under the direct sun light and run in idle for a certain time. And the temperatures under the floor were measured.
In all test conditions, a period of time was waited to reach a stabile temperature, and than data was recorded 30 minutes in highway and idle conditions, 10 minutes in uphill test condition. And the average ambient temperature is 35 °C.
After performing the tests mentioned above, floor region temperatures are determined by assuming the normal working conditions of the vehicle as 70%
highway, 20% ramp, and 10% idle.
In both test conditions, thermo-couples were installed to the suitable, predefined places in each of the floor regions. In Figure 3.1 thermo-couple installation plan can be seen. Also figures of some sample thermo-couples installed to the vehicle can be seen in Figure 3.2 to 3.7.
1 and 2 numbered thermo-couples were used to measure the outside temperature.
Figure 3.1. Thermo-couple installation plan
Figure 3.2. Position of sensors (8,9, and 10) to measure the temperature above the front axle
Figure 3.3. Position of sensor number 20 and 23 to measure the temperature in rear left luggage compartment
Figure 3.4. Position of sensor number 22 and 25 to measure the temperature in battery case
Figure 3.5. Position of sensor number 26 and 29 to measure the temperature above the radiator
Figure 3.6. Position of sensor number 28 and 31 to measure the temperature above the exhaust
Figure 3.7. Position of sensors number 29, 30, and 31 3.1.3. Determination of Total Heat Transfer Coefficients
For the calculation of heat transfer coefficient, the following procedure was used by considering the heat transfer section properties, inside temperature of 24 °C and outside temperatures.
Total heat transfer coefficient was calculated by employing Equation 15.
out
Here inner surface heat transfer coefficient is taken from ASHREA, (2001), as 8 W/mK for radiation and free convection conditions.
For the calculation of hout values equation 16 was employed.
k
out L
k
h Nu (16)
Nusseslt Number can be calculated by using Equation 17.
2
Local Nusselt Numbers can be calculated by using Equations 18 and 19.
5
By using the dry air properties in average temperature, Reynolds and Prandtl numbers can be calculated by employing Equations 20 and 21.
Lk
For the dry air properties in average temperature, the following equations taken from Yılmaz, (1999) were used.
85
Density ity
DynamicViscos (26)
At the end of these preparations, the amount of heat transferred from each of the regions to the passenger compartment was calculated by using Equation 27.
Q=U.F.ΔT (27)
The procedure explained above was followed for each of the 27 regions, which the components of passenger compartment and the calculation details were tabulated in Appendix A.
3.1.4. Heat Transfer by Radiation
Amount of heat transferred by solar radiation was determined by using a software program developed by a Çukurova University Mechanical Engineering department and TEMSA Global A.Ş. supported project. (Büyükalaca and Yılmaz, 2004). In the software cooling load was calculated by Radiation Time Series (RTS) method. A screen shot of the calculation algorithm of the program developed by Büyükalaca and Yılmaz, (2004) can be seen in Figure 3.8.
Figure 3.8. A screen shot of the calculation algorithm of the program (Büyükalaca and Yılmaz, 2004)
3.1.4.1. Radiation Time Series Method (RTS)
In this method, it is assumed that the instant solar radiation acting on the opaque and non-opaque surfaces of the media creates a cooling load with a phase difference. This technique employs 2 series namely, Conduction Time Series (CTS) and Radiation Time Series (RTS).
Solar heat acting on the opaque surfaces of the media firstly conveyed to the inner surfaces of the walls, and than transferred to the media. In order to take account this phase difference, Conduction time series (CTS) values was employed in RTS method. In this method, cooling load at the calculation time is effected by the previous times heat gains, depending on the heat storage properties of the wall.
Heat coming to the inner surfaces is transferred to the media by both convection and radiation. Convection heat load directly converted to cooling load.
Radiation heat load firstly is exerted on the surfaces of the goods in side the media and heated up the surface of the goods. When the surface temperature of the goods reaches the medium air temperature, it starts to heat up the inside air temperature.
Since it is needed a period of time to do these operations, radiation heat gain converted to cooling load with a phase difference. In order to taking in to consideration this phase difference “Non-Solar Radiation Time Series” values are employed in the calculations. RTS values depend on the properties of the surfaces (carpet etc.) and density of the goods in the media. RTS values of buildings are provided in ASHREA Fundamentals Handbook as light, medium and heavy depending on the structure of the building.
Heat gain coming from opaque surfaces is calculated as conduction, direct radiation, dissipated radiation, and reflective radiation. Direct radiation entering to the media is completely converted to cooling load with a phase difference. In order to taking in to consideration this phase difference “Solar Radiation Time Series” values are employed in the calculations. The other three components (conduction, dissipated, and reflective) are summed and they are evaluated as a single heat gain type. As opaque surfaces, heat is transferred to the media by both convection and radiation. While convective heat gain converted directly to cooling load, radiation heat gain converted to cooling load by considering non-Solar RTS values.
While convection part of the latent and sensible heats produced by internal heat sources (People, Lightening, and equipments) is converted cooling load directly, radiation part of the sensible heat is converted to cooling load with a phase difference. In order to taking in to consideration this phase difference “Non-Solar Radiation Time Series” values are employed in the calculations.
Infiltration and ventilation heat gains are converted to cooling load instantly.
In order to apply RTS method to a bus, some adaptations must be done.
Because, CTS values in ASHRAE handbook are defined only some special types of walls (35 different types of walls and 19 roofs). And these types of walls are not applicable to a bus structure walls. In this scope an analytical work was conducted and a new method was developed that can give CTS values of opaque surfaces of a bus structure, by considering the wall types mentioned in ASHRAE handbook by
Büyükalaca and Yılmaz, (2004). It is found that CTS values are a function of heat capacity of the wall (density x Specific heat x thickness= ρcS) and heat resistance (R). And unknown CTS values were extrapolated or interpolated from ASHRAE values.
Another problem in calculating the radiation-cooling load of a bus is the position to Sun. In this scope, 2 calculation situations can be considered. In the first situation it is assumed that while the sun doing its neutral movements, vehicle is driven in the same direction continuously. In this case 4 different calculation options were considered. (A: West, B: North, C: East, D: South) (Büyükalaca et al, 2011).
These 4 different calculation options are shown in Figure 3.9.
Figure 3.9. Calculation options when vehicle is driven in the same direction continuously
In the second situation, it is assumed that the vehicles one side follows the sun continuously. In this case again 4 different calculation options were considered.
(E: Front, F: Right, G: Left, H: Rear). These 4 calculation options are shown in Figures 3.10 to 3.13.
Figure 3.10. Front of the vehicle follows the Sun
Figure 3.11. Right side of the vehicle follows the Sun
Figure 3.12. Left side of the vehicle follows the Sun
Figure 3.13. Rear of the vehicle follows the Sun
3.1.5. Cooling Load by Internal Sources
By considering 50 passengers and 2 crews inside the vehicle, and in order to take the worst-case consideration, it is assumed that vehicle is in its full position and all the passengers are adult. Also cooling loads by lightening and equipments was taken into consideration.
Table 3.30. Heat generation by passengers (Büyükalaca and Yılmaz, 2004)
HEAT GAIN BY PASSENGERS
Heat
Total Heat Generation 130 Latent Heat 55
Child
Sensible Heat 50
Total Heat Generation 87 Latent Heat 37
3.1.6. Cooling Load by Ventilation
In order to calculate the Ventilation cooling load, firstly number of adult and child in the media must be determined. And than according to the ASHRAE tables and number of passenger, amount of fresh air requirement can be determined. Here in order to take the worst-case consideration, it is assumed that vehicle is in its full position and all the passengers are adult.
3.1.7. Cooling Load by Infiltration
Amount of infiltration to the vehicle depends on several conditions, like vehicle speed, position (Open/Closed) and leakage rate of doors and windows,
ventilation system operation (On/Off). Here ACH (Air Change per Hour Method) were used to calculate the infiltration-cooling load.
Here, in Table 3.31, the values for the vehicle situation is assumed as;
parked, doors and windows fully opened, ventilation on. And speed 60 Mph, doors and windows fully closed, A/C on.
Table 3.31. ACH values of a vehicle (Ott et. al., 2007)
Speed
(mph) Windows and Doors Ventilation System
Air Change Rate (1/h) Front
As a result of whole calculations, a proper A/C unit is selected to the vehicle.
After determining the A/C unit to be used in the vehicle, the unit was mounted on the test vehicle Photos of selected A/C unit, installation area on the roof of the vehicle and a view from inside the vehicle showing the inlet suction areas and evaporator outlets can be seen in Figure 3.14, 3.15 and 3.16 respectively.
Figure 3.14. Photo of the selected A/C unit
Figure 3.15. Installation area on the roof of the vehicle
Figure 3.16. A view from the test vehicle
3.2. Current Situation
In the current situation, as indicated previously, an air channel created by the boundaries of roof structure and luggage racks is employed in the vehicle. Also this air channel accommodates luggage rack connection brackets, heating water and refrigerant fluid pipes of A/C unit, electrical harnesses and some small brackets to mount electrical components. Since this structure of air channel creates a non-uniform air distribution, a non-non-uniform temperature distribution along the vehicle is observed.
Assemble of the roof top unit to the roof of the bus with the structure of the current air channel with a section view can be seen in Figure 3.17.
Figure 3.17. Current cross-section of the air channel and roof top A/C unit
In order to see the current situation of the vehicle and air conditioning channel in detail a set of tests was performed. During the tests, air velocity at the outlet of the nozzles, temperature distribution along the vehicle, temperature distribution in different levels of the passenger compartment (Head, Lumbar and ankle levels of the passenger) and internal noise level in different locations of the passenger cabin were measured.
3.2.1. Test Preparations
In order to determine the temperature distribution along the vehicle and in different levels of the passenger compartment, some of the passenger seats indicated
in Figure 3.18 and driver seat were equipped with thermo-couples as shown in Figure 3.19.
Figure 3.18. Seats equipped with thermo-couples
Figure 3.19. Thermo-Couple installation points of each seat (SAE, 2004)
Thermo-Couples were so positioned that to measure the temperature of the regions in head, lumbar and ankle region of the passenger. A photo of a sample seat thermo-couple installation and all the seats to be equipped with thermo-couples can be seen in Figure 3.20 and 3.21 respectively.
Figure 3.20. A photo of a sample seat thermo-couple installation
Figure 3.21. Seats to be equipped with thermo-couples
In order to see the effect of radiation heat transfer from sun light, a device measuring the effect of Sun light radiation was attached to the roof of the vehicle in a region near to the front. The installation photos of the radiation-measuring device can be seen in Figure 3.22.
Figure 3.22. Installation photos of the Sun light radiation-measuring device
After completing the installation of all the measurement sensors, data logger, which collects data from sensors and transfers to the computer, was installed to the vehicle. Data logger and the computer can be seen in Figure 3.23.
Figure 3.23. Data logger and the computer
The passenger compartment was allowed to soak in the sun for approximately 4 hours prior to initialization of airflow and data acquisition (Huang and Han, 2002).
The suitable time for soaking and testing was determined as follows;
Mansour et. al., (2007) developed a computer code according to ASHRAE, (1997) to estimate the cooling load variation (Figure 3.24).
Figure 3.24. Hourly cooling load distribution at different typical day times around year (Mansour et al, 2007)
As shown in Figure 3.24, the peak load occurred between 10.00 to 15.00 hours and this is commanding the AC system to operate at maximum capacity (Mansour et. al., 2007). The cooling load calculations conducted in the present study also showed that the maximum cooling load was achieved at 15:00 hour. So by referencing this information, soaking time is determined as between 10:00-14:00 and time to start testing was determined as 14:00.
After all the preparations were completed, the vehicle was parked under direct Sun light radiation with a magnitude of 890 W/m2, and 38 °C of average ambient temperature, between 10:00 and 14:00 times, 4 hours. The vehicle so
positioned that the right side of vehicle was subjected to direct Sun light, which is the worst case in the cooling load calculations as determined in the current study. During the heat up stage the doors and the windows of the vehicle was kept closed. Photos of the test vehicle during heat up stage can be seen in Figure 3.25.
Figure 3.25. Photos of the test vehicle during heat up stage 3.2.2. A/C Performance Test and the Measurements
After the 4-hour heat up stage, average inside temperature of the vehicle was approximately 45 °C, even in some regions up to 50 °C. Because of the high rate of Sun light radiation, especially surface temperature of some plastic parts, which is dark colored and subjected to direct sun light, measured as 47 °C with a laser type thermometer. However, since the inside roof panels have isolation, light color and are not subjected to direct sun light, the temperature of the inside roof panels were observed as lower than the black parts like dashboard and steering wheel. Photos of surface temperature measurements of dashboard, steering wheel and inside roof panels can be seen in Figure 3.26, 3.27 and 3.28 respectively.
Figure 3.26. Measurement of surface temperature of the dashboard of the vehicle with laser thermometer
Figure 3.27. Measurement of surface temperature of the steering wheel of the vehicle with laser thermometer
Figure 3.28. Measurement of surface temperature of the inside roof panels of the vehicle with laser thermometer
The vehicle, whose inside material surface and inside air temperatures raised, was driven in Adana-Gaziantep highway with a constant speed of 90 km/h so that, the right side of the vehicle subjected to sun light during all the test. During the test, temperature distribution of the test vehicle and sun light radiation were measured and recorded by means of 41 thermo-couples (2 for outside ambient temperature) and radiation measurement device installed to the vehicle and data logger. At the same time, the exit velocities of the nozzles were measured with an anemometer and the results were tabulated. The figure showing the measurement of exit velocity of the nozzles can be seen in Figure 3.29.
Figure 3.29. Measurement of exit velocity of the nozzles 3.2.3. Test Results
Air velocities at the outlet of the nozzles of each passenger measured from the current vehicle is shown in Figure 3.30.
AIR VELOCITIES AT NOZZLES
LEFT 5,27 5,44 5,47 5,49 6,50 7,03 5,65 5,49 5,46 5,36 4,74 4,62 3,43 3,21 RIGHT 4,15 4,28 4,43 5,33 6,04 6,54 6,06 5,56 4,62 4,42 4,27 4,12 4,04 3,66
Drive
Figure 3.30. Air velocities at the nozzles measured from the current vehicle
As can be seen obviously from Figure 3.30, there are great amount of differences between air velocities at the outlet of each passengers. In the regions under the A/C unit the nozzle exit velocities are very high, while in the front and rear portions of the vehicle, where are subjected to high level of solar radiation and excessive heat transferred from the engine room, is very low. As a result of this uniform air distribution, temperature distribution along the vehicle is also non-uniform. This can also be observed from the results of the temperature distribution test.
At the end of 100-minute test, the temperature distribution in the level of head, lumbar and feet ankle of the passenger can be seen in Table 3.32.
Table 3.32. Temperature distribution along and different levels of the passenger
When Table 3.32 was investigated, because of the high rate of solar radiation from the front windshield of the vehicle, it was observed that, the starting temperature of the driver head region is about 51 °C. Since the rear glass of the vehicle is smaller than the front windshield, the temperature of the rear region is lower than the front but still it is higher than the middle portion of the vehicle. Also it was seen that, since the vehicle is stationary during the heat up stage, the air heated
up was lifted up and the effect of radiation of side glasses near to the head region, the temperatures of head regions in all locations are higher than the lumbar and ankle regions.
The graphic representing the temperature distribution along the vehicle in different levels at the 60th minute of the test can be seen in Figure 3.31.
L6R6
Temperature Distribution at 60th min.
Head Lumbar Ankle
Figure 3.31. Temperature distribution at the 60th minute of the test
Under normal conditions, it is targeted to reach an average inside temperature of 25 °C after a 60-minute test. (Temsa, 2007). When we investigate the test results without considering the differences between the regions, it can be said that the target was achieved, because the average temperature inside the vehicle after 60 minutes was determined as approximately 25 °C. On the other hand, from the temperature distribution point of view, there are serious differences between regions. For example; while the maximum temperature was observed in the driver head region as 33,1 °C, minimum temperature was recorded as 20 °C in L3 and R3 ankle regions, where the locations just under the A/C unit. At the end of 60th minute, although the temperatures in the regions near to the A/C unit (L2, L3, R2, R3 and R4) were under 23 °C. In front and rear regions of the vehicle, temperatures changed between 27-30
°C because of the non-uniform air distribution. In that situation, while the passengers in the middle regions, where is near to the A/C unit, complaint about the low
°C because of the non-uniform air distribution. In that situation, while the passengers in the middle regions, where is near to the A/C unit, complaint about the low