The application of CFD to simulate cabin climatic conditions has become an integral part of vehicle development process and complements well with wind tunnel tests. However, determining thermal comfort inside the car cabin using CFD requires considering large number of factors to ensure that the simulation results are well in sync with the experimental test values.
The thermal analysis of car compartment involves not only the complex geometry, but also a strong interaction between airflow and all the three modes of heat transfer: conduction, convection and radiation. Failing to comprehensively include the details in the CFD model will lead to inaccuracy in results and drastically increase the consumption of computational power.
To ensure that the end results from CFD yields expected values, here are six most important factors that you should keep in mind to ensure accurate replication of real conditions on computational domain.
The air temperature in context to car cabin is the temperature of air surrounding the occupant body with the respect of location and time, which is obtained by measuring dry-bulb temperature. However, the air temperature zone inside the vehicles is inhomogeneous mainly due to the installed air conditioning system and narrow space allowing the air temperature to easily get influenced by heat exchange.
As such the air temperature difference between the head and ankle level must not exceed by 30C. The air temperature also largely depends on the available space inside the vehicle cabin. A small sized car may have different air temperature compared to a larger one.
The operative temperature is the integrated effect of air and mean radiant temperature, which can be measured using unheated ellipsoid shaped sensor. This temperature is introduced to avoid the complexity in measuring the radiant temperature, which is a temperature measured during the heat loss from the body due to radiation.
It is the average speed of the air to which the body is exposed with respect to location and time. The velocity has a significant influence on many other factors; an increase in velocity will also increase convective heat transfer rate. Moreover, human body movement can also promote in increase in air velocity. However, an increase in velocity will also lead to human discomfort. Usually, in vehicles the air velocity value must lie between 0.1 m/s – 0.4 m/s. Besides, the heated air flow will have the tendency to rise upwards towards the human head, which must be compensated by the cold air flow.
Relative Humidity (RH) is the ratio of amount of water vapor in the air to the amount of water vapor in the air at specific temperature and pressure. In case when the water content in air is heated, it will raise the humidity and promote evaporation of sweat from the skin. Ideally, the RH should fall between 30%-70%. A RH more than 70% will lead to sweating, while a RH below 30% will cause dry sensation and affect the mucous membranes, leading to extreme discomfort.
Human Activity and Clothing Insulation
Human activity adds heat inside the cabin due to perspiration, and to consider this effect, some common values provided by ASHRAE Standards must be considered. Additionally, clothing insulation also plays a vital role in maintaining thermal comfort inside the cabin. Too much insulation will disrupt the heat balance between the body and the surrounding environment, while too less insulation will put the occupants at the risk of cold injury. The insulation of each part of the human body must be considered to ensure optimum thermal comfort.
Apart from the above, factors such as time of travel, shades and glazing, noise, vehicle speed, human physiology and psychology as well as inside and outside colors also significantly impact the thermal comfort inside the cabin. As such it has been difficult to define a criterion for thermal comfort due to varied thermal sensitivities of people. However, to overcome this complexity, a rule of thumb is to develop a thermal environment which satisfies the comfort conditions for 80% of occupants.
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