New study finds thermo solutions for aerodynamics, truck cabin comfort

Findings from a new study regarding in-cabin temperature control are expected to influence future design of truck air conditioning systems.

An international research team involving Monash University, Zhejiang University in China and the University of Pennsylvania in the US have spent the past two years investigating the air inlet mode, temperature, relative humidity and flow speed of ventilation of air conditioners in truck cabins, and how this impacts dewing and driver comfort.

Some 33 different working conditions of air conditioners – including temperatures, humidity and flow velocities – were used to improve cabin defogging and maximise driver comfort in a retrofitted truck cabin.

The study, published in Applied Thermal Engineering, found highest anti-dewing efficiency was achieved when the air conditioning airflow was set to speed above 0.6 metres per second, a relative humidity of 20 per cent and temperature above 320 Kelvin (46.85 Celsius) within 200 seconds.

However, the optimal zone – where the combined requirements of occupant comfort, energy efficiency and safety were met – was when the relative humidity was within the range of 20-60 per cent and the temperature was between 292K-298K (19°C-25°C).

Researchers have collaborated with automobile manufacturers in Australia and China on thermo-fluid problems, such as vehicle aerodynamics, drag reduction, and thermo-fluid flows in engines.
It is anticipated that the findings from this study could influence the future design of automotive air conditioning systems. 

According to Dr Jisheng Zhao, Research Fellow in the Department of Mechanical and Aerospace Engineering at Monash University the other interesting finding is that the optimised settings can effectively and efficiently control the flow velocity and temperature distributions of the window surfaces and inner cabin space.

“These findings should provide us with a basic design guideline for the air conditioning system in trucks when considering the combined requirements,” he said.

“The airflow velocity and temperature distributions should also help locate comfortable positions in the cabin,” Dr Zhao said.

Vehicle window dewing not only affects the occupants comfort, but also interferes with the driver’s sight which can cause a potential threat to the safety of driving as well as electronic equipment in the vehicle.

Vapor condensing is caused by the differences in temperature and moisture in the air. The problem can be resolved by adjusting the inside temperature to avoid the dewing point and reducing the relative humidity through the ventilation system. 

But, in order to reach maximum defogging capacity, cabin temperatures could soar to nearly 50°C – making drivers extremely uncomfortable and distracted while on the road.

Researchers, led by Professor Yuqi Huang at Zhejiang University, simulated over 30 working conditions of air conditioners as they sought to improve cabin defogging while maximising driver comfort in the aforementioned retrofitted truck cabin.

The results showed that reducing humidity could not only effectively control condensation, but also optimise the distribution of the internal airflow and increase the heating effect.

“On the other hand, low cabin pressure affects the occupants’ thermal comfort, whereas high temperatures will increase the content of water vapour in the cabin, thereby worsening the problem of condensation,” Professor Huang said.

To satisfy the comfort, efficiency and safety requirements simultaneously, the research identified that automobile air conditioners could be controlled with the relative humidity range of 20-60 per cent, and a temperature range of 292K-298K (19°C-25°C).

Dr Zhao said the findings were significant, especially for vehicle batteries, which could see losses of up to 50 per cent of the total driving distance due to the energy consumption of air conditioners.

“Trucks play an important role in our freight transport system, carrying more than 70 per cent of the freight across land. The need for energy reductions and environmental compatibility of truck design, by improving aerodynamic performance for example, is now a worldwide priority,” Dr Zhao said.

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