Abstract
urrent research in the field of automotive thermal heat rejection systems is limited. The advancements in high-performance vehicles and electrification fields haven’t been considered yet, and studies carried out for aggressive testing conditions are limited. Less than half of the studies reviewed throughout the course of this research tested under high-performance and aggressive testing. Many of these studies were for niche applications such as in-cycle radiators or finned-tube evaporators. Therefore, there is a need to test heat the ejection of automotive radiators under high-performance systems. This would not only improve the thermal heat rejection systems' efficiency but also indirectly improve emissions.A modular and mobile rig with a length of 7-8m was constructed (depending on the test case). The heat rejection and airside pressure loss were then measured by changing the flow rate from 30-180lpm and airflow velocity from 2-10m/s. These graphs were then plotted, and delta HR plots (comparing two different cases) were then analysed for 14 other cases with varying angles, fan configurations and blockages.
The LTR was modelled using the ɛ-NTU method to provide a base for further testing. Due to limitations in data, an NTU of 3 was assumed, which correctly predicted the heat rejection within a 4% margin. For the HTR case, the air velocity significantly affected the heat rejection, whilst the coolant flow rate effect was much smaller. Higher coolant flow rates showed that the heat rejection was 19% higher for an air velocity of 2m/s when comparing the 180lpm case to the 30lpm case. However, this was 76% higher for the 10m/s case. The average heat rejection at 180lpm only showed an overall improvement of 51% over that of the 30lpm case. This is possibly due to the radiator's sizeable effective surface area, which enhances the heat rejection rate.
Fan operation generally improved the heat rejection; however, a fan did not show much change at 8m/s and 10m/s when the fan at 13.6V and fan at 0V cases were compared for the 90° case. This could imply that the fan acted as a blockage for air velocities of over 6m/s.
Although angled configurations of 15° and 25° showed slight improvements in heat performance, the 35° case showed an improvement of about 14kW for air velocities of the 8m/s and 10m/s cases. A possible reason for the enhancement of heat rejection in only the 35° case would be the additional direct convection caused by the inner geometry of the radiator, which would be the louvres in this case.
When the 1m blockage, 13.6V case, was compared with the baseline case, it was found that the average heat rejection for 180lpm was 15.4kW more than that of the baseline case. The same averaged heat rejection was more than 15kW(for 180lpm) for the 1m blockages, with no fan case compared to the baseline case. The effects of the fan were pronounced for the 10m/s air velocity case, as a fan showed an average increase of 4.5kW in heat rejection. The same heat rejection for a 0.5m blockage was 14.1kW without a fan at 180lpm and 16.4kW with a 13.6V fan. Hence, a 1m blockage without a fan provided better heat rejection (by 0.4kW at the highest point). However, with a 13.6V fan, this changed as 0.5m blockage showed better heat rejection (by 1.5kW at the highest point) with the fan on compared to the 1m blockage case.
For the angular 35° case, the airflow was accelerated over a smaller area, and the adverse effects of the blockage were more pronounced, while the 35°, 1m blockage case saw a heat rejection decrease of 2.7kW, this was brought down to 1.9kW when a fan was used. Like the 0.5m blockage case, heat rejection decreased from 4.7kW (no fan) to 1.9kW (with 13.6V fan).
Thus, a comprehensive analysis of automotive thermal systems was completed with certain limitations, such as non-uniformity of airflow and lack of temperature control. This leaves avenues for future work on the rig, which could include using an auxiliary heater to maintain the desired temperature, CFD analysis to get a better idea of turbulence intensity and modifying the wind tunnel to enable closed-loop testing, as it would allow for more uniform airflow.
| Date of Award | 2 Nov 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Sam Akehurst (Supervisor), Ian Kennedy (Supervisor), Colin Copeland (Supervisor) & Andrew Lewis (Supervisor) |