Preliminary research on virtual thermal comfort of automobile occupants

Numerical simulation of climate conditions in automotive industry for the study of thermal comfort had become more and more prominent in the last years compared with the classical approach which consists in wind tunnel measurements and field testing, the main advantages being the reduction of vehicle development time and costs. The study presented in this paper is a part of a project intended to evaluate different strategies of cabin ventilation for improving the thermal comfort inside vehicles. A virtual thermal manikin consisting of 24 parts was introduced on the driver seat in a vehicle. A heat load calculated for summer condition in the city of Cluj-Napoca, Romania was imposed as boundary condition. The purpose of this study was to elaborate a virtual thermal manikin suitable for our research, introduction of the manikin inside the vehicle and to examine his influence inside the automobile. The thermal comfort of the virtual manikin was evaluated in terms of temperature and air velocity.


Introduction
Computational Fluid Dynamics (CFD) methods are applied nowadays more and more in all stages related with conception of a motor vehicle.Among them, a particular interest for us represents the use of numerical simulations in predicting the thermal comfort of automobile occupants.In the last years regardless of the vehicle brand, an increased attention is oriented to the thermal comfort of the automobile occupants.The number of people spending a significant portion of their time in transportations is rising, and hence, there is an increasing demand for thermal comfort while traveling.A more comfortable climate in automobile in almost all cases will work towards reducing driver's stress and thus contributes to a safer driving.In addition, today's demands for energy efficiency and performance, have led to an increased interest in investigating and analyzing the system and design requirements for good quality of the vehicular environment.However, there are a few constraints which are making difficult this task: demands in increasing the fuel economy and tendency to use more glass in automobile design being among them.In order to achieve the thermal comfort inside the automobile a first step will be to evaluate the heat load that enter in the occupant's area.Is important that this evaluation to be obtained in the early stage of the vehicle design and this can be accomplished using CFD method, studies in the literature successfully applied this method [1-7].
The common interpretation of "thermal comfort" regards the state of a person that would express a feeling of wellbeing regarding the thermal conditions in an occupied space.However, thermal comfort is a term difficult to define and a universal definition of its meaning is almost impossible to obtain [8].Nevertheless, it is the tangible interest of engineering applications that motivated a never-ending quest of quantitative models for estimating, predicting or classifying the state of thermal comfort in occupied spaces.Almost all these attempts are focusing on the physiological component.Most of them are relying on the assumption that the state of thermal comfort might be ensured when the heat produced in excess by the human body through its metabolic sources is dissipated in the environment and the thermoregulatory system intervention is infinitesimal.This way it is universally accepted nowadays that an environment is considered comfortable from the thermal point of view, when 80% to 90% of people from that environment do not express thermally dissatisfaction [9-12].Achieving thermal comfort assume that both psychological and physiological factors are fulfilled with some degree.The air velocity, thermal radiation and temperature fields are among the most important factors that influence thermal comfort, thus in order to improve the design of a vehicle in terms of thermal comfort it is necessary to investigate the air-flow field and the temperature distributions inside the automobile.

Geometry
In a previous study [4], we evaluated a simple approach of calibrating and validating a CFD model that reproduces the thermal environment and the flow dynamics inside a simple vehicular cabin.In this new study, a virtual manikin was installed in the automobile on the driver's seat.For this purpose, we studied tens of virtual manikins available on the web, we tried to pose them in driving position.We failed with almost all the geometrical models for the virtual manikins from different reasons.We succeed with one virtual manikin by reconstructing parts from his body, an image with the reconstructed virtual manikin in driving position is presented in Fig. 1a.The construction of the virtual manikin was made taking into account other real thermal manikins developed in our laboratory [13].The virtual manikin was split in 24 parts (Fig. 1b) related with the anatomic parts of a real human.

Fig. 1 Virtual thermal manikin used in this study
The studied vehicle geometry was built in SolidWorks.The virtual manikin was introduced into the vehicle studied in [4] with the to simulate the presence of the driver (Fig. 2 and Fig. 3).

Boundary conditions and setup
The introduction of air in the interior of the automobile was considered through the four air diffusers positioned on the dashboard of the vehicle using the appropriate airflow for the middle level of the air cooling system (35m