Air quality in vehicle cabins
15-12-2021 | Posted by Joaquín Martí
With increasing concerns for emissions and pandemics, vehicle Heating, Ventilation and Air Conditioning (HVAC) units are not only expected to provide comfort, but also safety. Air quality inside the vehicle cabin depends on the surrounding air and the performance of the HVAC unit. Some pollutants inside the car may come from the vehicle ahead, but contaminants released by occupants are also a concern. The unit must ensure that occupants are safe whilst meeting performance and efficiency imperatives, with regulations requiring a reduction in polluting refrigerants, and with controlled manufacturing costs.
The air filter is a most important and underrated component of the HVAC system, which traps the entering airborne debris. There are filters with different levels of efficiency and, depending on the level required, the air travels through one or more filters. The Minimum Efficiency Reporting Value (MERV) describes the ability to block particles between 0.3 and 10 microns. MERV ratings range from 1 to 16, with the higher values indicating higher trapping efficiency. In vehicles, High Efficiency Particulate Air (HEPA) filters are widely used; HEPA filters must remove 99.97% and 99.95% of contaminants in the US and Europe, respectively.
OEMs must design vehicle cabins to minimize air cleaning time at an early stage, as there is little flexibility later. This can be digitally addressed by detailed modelling of the filter to evaluate contaminant transport, filter efficiency, filter life, and contaminant removal within a reasonable turnaround time.
SIMULIA solutions help to replicate real world scenarios by virtually testing different filter designs and cabin configurations. Simulation also provides insights on the impact of fan position, filter rating and clogging on aerodynamics, thermal and acoustic comfort, and energy efficiency.
A highly accurate Lattice-Boltzmann-based PowerFLOW solver with Particle Modeling is used to model the flow behaviour, contaminant transport, accumulation on the filter and pressure drop. The filter design is integrated into the cabin comfort simulation framework. In the simulation, contaminants are injected and the filter is modelled as a screen that controls the pass-through fraction, as a function of its rating. It provides the user with insights about filter/system efficiency and models the effects of clogging. More details are given here.
Designing a filter involves a careful balance between pressure drop and filter efficiency. The flow is affected by the accumulation of particles on the air filter. Coarser filters like MERV8 let more particles through, but last longer between replacements and require less powerful blowers. Finer filters like MERV12 and MERV13 block more particles, but require a more powerful blower and will clog up more easily. Contaminant accumulation on the filter governs its lifecycle and the efficiency of the system. Efficiency and air quality will be higher for the finer filters, but flow resistance will also be higher, resulting in higher energy consumption and more frequent filter replacement.
Simulation can also predict the particle age, indicating the contaminant removal rate. As the HVAC operates in time, the simulation can determine the fraction of particles captured by the filter. The time taken for contaminant removal is predicted by detailed modelling of the cabin to assess real-life removal rates. Driver and passenger breathing rates are specified as contaminant emission. During the simulation, contaminants leave the cabin through the recirculation outlet and clean air is recirculated back into the cabin through the vent inlets.
Initially the cabin will have contaminants from occupants or from outside. During the simulation, the heavier particles settle on surfaces. The lighter aerosols circulate inside the cabin; some of them reach the recirculation outlet, go through the HVAC filter where contaminants are filtered and accumulated, and clean air enters the cabin through the HVAC vents. This process is carried out until most of the aerosols are filtered. The position of HVAC filters and outlets plays an important role in the removal rate; in low-flow, damp areas, aerosols are trapped and remain for a long time. Simulation provides guidance on filter selection and placement, as well as time taken to remove contaminants.
In summary, SIMULIA PowerFLOW has a capability to model the transport and filtering of contaminants. The flow and air quality performance of the HVAC can be assessed and improved.
The placement of filters can be optimized to reduce maintenance, minimize cabin cleaning time and improve interior air quality. The filter properties can be tuned to maintain optimal HVAC performance for thermal and acoustic comfort, as well as energy efficiency. The time taken to remove the contaminants and their time evolution inside the cabin can be predicted to ensure the safety of occupants.
SIMULIA solutions enable you to overcome new challenges in cabin air quality, whilst maximizing passenger comfort and energy efficiency. If you have any queries, we at Principia have the professionals with the desire to help and the expertise to do it successfully.
Pictures taken from Dassault Systèmes’ SIMULIA blog (https://blogs.3ds.com/simulia)