CFD simulation and test of unconventional pollutant emissions in tractor cab

Abstract: The widespread distribution of unconventional pollutants potentially exposes to the machinery cab, particularly with the continuous improvement of agricultural mechanization in China in recent years. Taking a tractor of Dongfanghong 1 804 as the research object, this study aims to clarify the emission characteristics of unconventional pollutants in the cab using the combination of simulation and test. 1) The diffusion process of unconventional pollutants in the cab was simulated by a Computational Fluid Dynamics (CFD) software (Fluent). The initial conditions were set at the ambient temperature of 23 ℃, where benzene was selected as a representative pollutant, considering the high carcinogenicity of benzene. Two reference sections were selected as the cross-sectional area of the breathing belt and the middle sectional area of the air supply outlet in a cab. The reason was that the breathing belt area of drivers with a great impact on the human body, while, the outlet area of air supply with a large air flow in the cab. In addition, the reference point was selected from the human breathing area, each of which was located on the connecting line between the upper part of the steering wheel and the top of the seat. The height was aligned with the height of the breathing belt of the driver and passengers. After that, a CFD simulation was conducted to obtain the temporal and spatial distribution of benzene concentration in different sections and reference points. The simulation results showed that the benzene concentration in each section basically reached equilibrium under the condition of a closed cab after 12 hours. The average benzene concentrations at the reference point and two reference sections were 1.81, 2.70, and 3.03 μg/m3, respectively, which were below the limit value required for the concentration of organic matter in the air in the vehicle. Furthermore, the pollutant concentration in the cab decreased significantly under the condition of external circulation ventilation, where the average mass percentage of benzene in two sections decreased to 1.71 and 1.98 μg/m3. Correspondingly, the entry of fresh air was conducive to the improvement of air quality in the cab. 2) A systematic test was carried out to verify the concentration of pollutants in the cab. According to HJ/t400-2007 sampling and determination s of volatile organic compounds and aldehydes and ketones in vehicles and the standard TO-17 of the National Environmental Protection Agency (EPA), the volatile organic compounds in the cab were collected by a Tenax TA tube, afterward they were analyzed by thermal desorption gas chromatography/mass spectrometry. It was found that there were a wide variety of volatile organic compounds detected in the cab of agricultural machinery, including 241 kinds of alkanes, olefins, halogenated hydrocarbons, alcohols, aldehydes, ketones, esters, and other compounds, of which alkanes accounted for the largest proportion, followed by aromatic compounds and esters. The detected concentrations of benzene, toluene, xylene, ethylbenzene, and styrene were 3.08, 30.49, 46.84, 8.39, and 2.43 μg/m3, respectively, which were lower than the maximum concentration specified in the national standard of GB/T27630-2011 guidelines for the evaluation of air quality in passenger cars. A comparison was made on the measured benzene concentration at the reference point after being closed for 16 hours with the simulation. It was found that the error range was 7.7%-10.0%, indicating the more reliable simulation. In addition, a portable handheld detector was used to detect the changing trend of total volatile organic compounds (TVOC) concentration in the cab within 18 hours at the reference point under the condition of the closed cab, in order to further verify the reliability of the numerical simulation. It was found that the TVOC concentration in the tractor cab basically reached equilibrium after 12 hours, and the changing trend was roughly the same as that of benzene concentration at the reference point during simulation, indicating the high reliability and accuracy of the model. This finding can provide a theoretical basis for further study of air quality in an agricultural machinery cab.