Objective
The objective of this report is to determine the best method for directing airflow through a parked car using ventilation fans.
Introduction
In the development of the group’s senior design proposal for project A.S.H.E., the main challenge is removing heat from a parked car. The group settled on using fans to ventilate the vehicle and must now decide on a method which will best accomplish this. Many configurations are possible for directing airflow through the vehicle. The group originally proposed a concept using an MCU driven Control Module to monitor temperatures and shift the direction of airflow based on which direction will bring in the coolest air and exhaust the hottest air. Without test data or research on this method, it was decided that further research was necessary to justify the method and thus the Control Module. In this report, prior studies on temperatures and ventilation in parked cars will be reviewed and analyzed, along with the group’s freshly conducted study on temperature differences between the four windows and heat removal provided by purely exhaust, crossflow, and controlled crossflow ventilation strategies.
Background Analysis
Many studies have been conducted on ventilation fans for parked cars. Some have utilized computational fluid dynamics (CFD) to model and visualize the temperature gradient inside the car. Kamar et. al used CFD to make a model and then created flow simulations to make predictions about temperatures in different sections of the car under ventilated and non-ventilated conditions [1].
Figure 1 - Predicted and measured temperatures in the front and rear of the vehicle throughout the day. [1]
The results of these predictions can be observed in figure 1. The figure shows predicted and measured temperatures for hour intervals throughout the day in the front and rear of the car. It is shown that the greatest variation between prediction and measurement was 4% and occurred at 15:00 for the rear of the car, and the average variation between prediction and measurement was calculated to be 3%.
An important observation from figure 1 is that the temperature in the front section of the cabin is measurably and predictably higher than that of the back section. It also appears the temperature difference between the front and back sections does not change much throughout the day. The temperatures in the car are further illustrated in figure 2 by the CFD model produced by Kamar et. al, which shows how varying air temperatures are distributed throughout the cabin without ventilation at 1 pm. The highest temperatures occur on the dashboard, while the lowest temperatures are observed underneath the dashboard.
Similar results were obtained in another study conducted by Chen et. al [2]. Their study also used CFD models to showcase temperature distribution, but took car orientation into account. It also gives CFD models of a driver plane and an infant plane. Figure 3 shows how the driver and infant are placed in the model. The simulation was conducted at 14:00, and the results can be seen in figure 2, which shows that front passenger seats are always higher in temperature than rear passenger seats, regardless of car orientation [2]. The figure also shows that temperature differences between front and back are consistently greater than differences between driver plane and infant plane.
Figure 2 - Contour of air temperature (K) inside the car cabin at 1 pm (a) isometric view, (b) symmetrical plane view [1].
Figure 3 - 3-D model of a car with driver and baby [2].
Another study from Sevilgen et. al conducted a similar simulation which provided the transient response of the temperature distribution when the AC was first turned on after the hot soak [3]. Again, a CFD model was used. From the horizontal plane shown in figure 4, it is observed that the temperatures in the front passenger section once again exceed those of the rear passenger section even as the cabin begins to cool down. From 1 minute of AC to 20 minutes of AC, the area just in front of the driver decreases from 41.9 °C to 30.0 °C while the area in the passenger space behind the driver seat decreases from 36.9 °C to 24.3 °C. Once again, the rear passenger space remains cooler than the front passenger space.
Figure 4 - Hot soak temperature distribution of different cars in different orientations [2].
Figure 5 - Horizontal plane temperature predictions after 1 minute and 20 minutes [3].
Table 1 - Computed mean surface and air temperatures [3].
Table 1 is a subsection of a table in the study by Sevilgen et. al. Because the proposed design for A.S.H.E. will have ventilation fans mounted in vent shades with fans flush to the windows, it is of interest to consider window temperatures. The table shows the temperature of each window 30 seconds after the AC has been turned on from a hot soak. Both frontal windows had mean temperatures higher than the rear windows. The right-side windows were also hotter than left side.
Another study from Ciocanea et. al took a different approach, finding the transient response of the air temperature in a parked car after the AC was turned off [4]. This, along with the external temperature and car body temperature are shown in figure 6. The main takeaway from this graph is that it takes about 20 to 25 minutes for the car’s internal air temperature to become equal to the external air temperature.
Figure 6 - tin tout and tsteel against time from the moment the AC is shut off [4].
The focus of discussion is now shifted to the effect of different ventilation methods on car temperatures. Returning to [1], the table below was obtained after simulating the CFD model for various vent fan configurations. It shows that a maximum of 23.6% temperature reduction was achieved by placing 4 ventilation fans spaced on the roof. The table also shows that a range of 6.2% temperature reduction existed for the different cooling methods, leading to a 3 °C difference between the best and worst ventilation methods. Since a 1 °C decrease in parked car air temperatures can lead to 4.1% AC power savings [5], a 3 °C decrease due to change in ventilation method can be considered significant. In the procedure section which follows, three different window ventilation methods will be tested against each other.
Table 2 - Comparison of effectiveness of 5 ventilation methods [1].
Conclusion
From the above studies, it can be concluded that temperature differences between different parts of the car are minimal. However, the front passenger section was repeatedly observed to have higher air temperature than the rear passenger section. This corresponds with a maximum temperature observation at the vehicle’s dashboard. Since the A.S.H.E. design relies on ventilation from the windows, it may be worthwhile to design the system such that the rear windows bring air in and the front windows exhaust air so that the cooler rear section air is brought to the front by the air flow. To improve battery life, the device should be automatically turned on 25 minutes after the car has turned off [4], or when the internal temperature exceeds the external.
References
[1] H. M. Kamar, N. Kamsah, I. S. Sabri, and M. N. Musa, “Reducing soak air temperature inside a car compartment using ventilation fans,” J Teknol, vol. 78, no. 8–4, pp. 155–166, 2016, doi: 10.11113/jt.v78.9597.
[2] S. Chen, B. Du, Q. Li, and D. Xue, “The influence of different orientations and ventilation cases on temperature distribution of the car cabin in the hot soak,” Case Studies in Thermal Engineering, vol. 39, p. 102401, Nov. 2022, doi: 10.1016/j.csite.2022.102401.
[3] G. Sevilgen and M. Kilic, “Investigation of transient cooling of an automobile cabin with a virtual manikin under solar radiation,” Thermal Science, vol. 17, no. 2, pp. 397–406, 2013, doi: 10.2298/TSCI120623150S.
[4] A. Adrian CIOCANEA and L. Dorin Laurenţiu BUREŢEA, “CABIN HEAT REMOVAL FROM PARKED CARS.”
[5] J. P. Rugh, T. J. Hendricks, and K. Koram, “Effect of Solar Reflective Glazing on Ford Explorer Climate Control, Fuel Economy, and Emissions,” 2001.
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