Analytical analysis of the thermal comfort of the occupants inside a truck cabin

Thermal comfort inside a truck cabin is a parameter that influences the well-being of the driver and passengers. From the very first steps of designing the driver’s seat of a vehicle, the driver’s well-being is taken into account through ergonomic parameters. To analyze the ultimate comfort inside a truck cabin, a simplified sketch of the cabin will be proposed. The three-dimensional analytical calculation model is solved using the Ansys program in the Fluid Flow (CFX) module. Through modeling and numerical simulations, the temperature inside a truck cabin, the mode of movement of the air flow and the speed of the air flow are determined. Following these tests, it is checked whether the thermal comfort conditions for the driver and passengers in a truck cabin are respected and fulfilled. These term comfort conditions are evaluated based on some comfort indices available in specialized literature.


Introduction
The ergonomic quality of a vehicle is related to the quality of the relationship between the user and the vehicle during it is use.Thermal comfort is among the main ergonomic parameters.
Thermal comfort represents the quality of the microclimate inside the passenger compartment and the thermal sensation felt by the driver when in contact with the interior surfaces of the vehicle.This is a parameter inside the body that can psychologically influence the pleasure of using a vehicle.
Occupants must feel comfortable even in adverse environmental conditions (high or low temperatures, solar radiation, heavy rain, and snow) [2].
Also, the driver's vigilance and his ability to concentrate in traffic depend to a large extent on his comfort level.
The internal temperature of the human body is naturally controlled and is maintained within a narrow range around 37°C.Thus, thermal discomfort and dangerous situations of thermal stress are avoided [1].
If the temperature of the human body increases by a few tenths of a degree, this increase is sufficient to stimulate the sweating mechanism which is capable of multiplying the heat dissipated by the human body four times [1].
During the hot season, if a vehicle's cooling system is not working properly, the bodies of the occupants of the vehicle heat up.If the internal temperature of the human body increases by more than 2°C compared to the optimal figure of 37°C, the subject's behavior changes for the worse, a feeling of weakness appears.The consequences of the increase in human body temperature are vasodilatation (increased blood flow through the skin which leads to an increase in heat conduction to the outside, and the temperature of the skin becomes similar to that of the internal tissues) and sweating (the cooling 1303 (2024) 012023 IOP Publishing doi:10.1088/1757-899X/1303/1/012023 2 mechanism for the body, the energy required to evaporate sweat is directly subtracted from the skin) [1].
During the cold season, the heating of the passenger compartment must be taken into account.If the measures put in place to prevent cooling are inadequate, the human body gets cold.Also, it must be taken into account when evaluating thermal comfort that the hands and feet, being far from the center of the body, are the first prone to cooling.And in this case, if the internal temperature drops by more than 2°C below the optimal value, the normal behavior of the occupants changes, a feeling of numbness or stiffness appears.
In studies of the physiology of the human body, it has been shown that the body's internal temperature does not change (no cooling, heating or evaporation occurs) if the body is exposed to an ambient temperature in the range of 23°C ÷27°C (subject is clothed and in sedentary activity).Also, skin temperature (Tsk) and internal temperature (Tcr) are in neutral state at the following values: Tsk =33.7°C and Tcr= 36.8°C[2].
The main parameters that directly affect the comfort state and microclimate of the occupants are the following: temperature, air speed, humidity, solar radiation, radiation from components, particles in the air [2].
The interior temperature of the passenger compartment depends a lot on the ambient temperature.Because of this, a constant temperature of around 22°C cannot be established.During the cold season the temperature inside the cabin must be higher to compensate for this low temperature level.During the summer, the human body adapts more easily to high temperatures.Thus, the temperature inside the cabin can exceed the value of 22°C and be evaluated as a comfortable temperature.Some vehicles are equipped with powerful air conditioning systems that cool the passenger compartment by up to 20°C at an ambient temperature of 45°C, solar radiation of 800 W/m2 and humidity of 30%.But after a period on board the vehicle this temperature is too low.Also, the human body must cope with the impact of the sudden change in temperatures when getting out of the vehicle.For this reason, studies have been done that recommend certain temperatures in the passenger compartment [2].
Also, to increase the comfort of the occupants, in addition to the optimal temperature, they can be exposed to air flow.According to [3], one can see what air speed is suitable depending on the temperatures.In the specialized literature, there are many data from different authors regarding the air flow.In conclusion, the higher the ambient temperature, the higher the air flow speed should be to reach the same level of comfort.However, the effect of an air flow is reversed if the air temperature is too high.The effect of an air flow becomes negative at temperatures above 35°C (temperature close to that of the human body) the heat is transferred to the human body.
The paper presents a model made for the study of the air flow introduced by the HVAC system in the cabin of a truck, to control the temperature level inside it.

The model
The geometric model was made based on the shape and dimensions of the passenger compartment of a truck cabin with two seats and equipped with two beds (fig.1.a).The mannequins were modeled considering the main dimensions of a 90% mannequin.Geometric modeling was performed in the geometric modeling module of ANSYS software.Discretization with elements (fig.1.b) was done only on the interior space of the passenger compartment that contains air.The discretization density is higher on the interior geometric elements and in the area of the air inlet and outlet.
The setting of the CFD analysis took into account the establishment of the main parameters that define its boundary conditions, considering that the analysis will only be performed for heating the cabin in winter conditions.
The total air flow given by the HVAC system was considered to be 0.05 kg/s [4], [5], [6], this being distributed, depending on the setting, on the ventilation panels from the windshield, dashboard and feet.The exit of air from the cabin is done at normal atmospheric pressure, through two openings, located at the bottom of the rear wall.
The boundary walls were considered no slip walls with different thermal conductivity properties [3]: windshield and side windows (glass) -1.172 W/m 2 K; body and dashboard (plastic) -2.7 W/m 2 K; floor (carpet) -0.294 W/m 2 K, seats and beds (foam) -0.05 W/m 2 K.The temperature of the outer walls was assumed to be equal to the outside temperature, except for the tunnel wall, whose temperature is influenced by the temperature of the engine compartment.

Results
To study the variation of temperature inside the cabin over time, the six cases were studied using a transient CFD analysis using ANSYS software.It was analyzed how the air inside the cabin is heated for one hour, from the initial time when warm air was introduced inside the cabin through the ventilation panels.
The analysis of the temperature variation over time was done especially at eight points near the driver and passenger.Points 1 and 2 are next to the two dummies, Point 3 being positioned at the same level, in the median vertical plane of the cabin.Points 4 and 5 are located at an almost equal distance from the face and palms of the mannequins, Point 6 being positioned at the same level, in the median vertical plane of the cabin.Points 7 and 8 were considered in the leg area of the two mannequins (fig.2).From the analysis of the temperature variation in the six cases (figure 3 -figure 8), it can be seen that in the first 5-6 minutes there is a rather rapid increase in temperature, followed by several oscillations (decreases followed by increases).After about 15-20 minutes.these oscillations subside and there follows a slow increase in temperature, which after about an hour remains approximately constant.The oscillations that occur after the first heating can be explained by the return of the cooled air stream when it passes through the rear area near the beds, towards the front of the passenger compartment.After this stage, the air behind the passenger compartment also starts to heat up and cooling no longer occurs in the analyzed points, even if the air returns to the front (figure 9).
From the analysis of the graphs, it is observed that the shape of the curves is similar for the cases C1 and C4, C2 and C5, C3 and C6, differing in the initial temperature and the temperature up to which the first heating is done, before the temperature oscillations, this temperature being slightly higher in cases with initial temperature 0 o C. In contrast, the temperature reached after one hour is almost identical.In points 7 and 8, the heating is stronger already in the first 5 minutes, probably due to the fact that the air returns more difficult from the colder areas.Towards the end, the temperature remains constant but lower by about 5 o C (figure 10).Of course, in cases C3 and C6, the temperature at the feet is almost 10 o C lower than the other cases, but it is still at the level of 18-20 o C. In cases with an outside temperature of -10 o C, the windshield heating is done at temperatures above 15 degrees only in cases C4 and C5, in case 6, even after an hour, there may be areas that are not defrosted.

Conclusions
This model represents a first step in the study of the main parameters that characterize the flow of air inside the cabin, to ensure an optimal climate for the driver and passenger.The transient CFD analysis can show the evolution of these parameters over time, being possible to determine the time when favorable temperatures are obtained in the seat area, but also in the bed area, in the case of this cabin.
It is desired to continue the study with the performance of some experimental tests, in order to be able to validate the created model.Of course, the improvement of this model will also be pursued, so that the calculation errors are reduced as much as possible.We believe that the thermal conductivity properties of the walls, as well as the air flow rates through the cabin ventilation panels, should be determined as accurately as possible, through experimental studies.

Figure 1 .Tabel 1 .
Geometric model (a) and finite element model (b) of the passenger compartment.Six cases of passenger compartment heating were studied, for three modes of regulation of the ventilation system and for two values of the outside temperature, 0 o C and -10 o C (table1).The analysed cases.

Figure 2 .
Figure 2. The points where the temperature variation was analyzed.

Figure 9 .
Figure 9.The appearance of temperature oscillations in the first 15 min in the case of C1.

Figure 10 .
Figure 10.The temperature in the median plane of the driver, case C1.

7 Figure 11 .
Figure 11.Temperature on the surface of the windshield and side windows.