Airborne respiratory diseases such as COVID-19 pose significant challenges to public
transportation. Several recent outbreaks of SARS-CoV-2 indicate the high risk of
transmission among passengers on public buses if special precautions are not taken. This
study presents a combined experimental and numerical analysis to identify transmission
mechanisms on an urban bus and assess strategies to reduce risk. The effects of the
ventilation and air-conditioning systems, opening windows and doors, and wearing masks are
analyzed. Specific attention is paid to the transport of submicron- and micron-sized
particles relevant to typical respiratory droplets. High-resolution instrumentation was
used to measure size distribution and aerosol response time on a campus bus of the
University of Michigan under these different conditions. Computational fluid dynamics was
employed to measure the airflow within the bus and evaluate risk. A risk metric was
adopted based on the number of particles exposed to susceptible passengers. The flow that
carries these aerosols is predominantly controlled by the ventilation system, which acts
to uniformly distribute the aerosol concentration throughout the bus while simultaneously
diluting it with fresh air. The opening of doors and windows was found to reduce the
concentration by approximately one half, albeit its benefit does not uniformly impact all
passengers on the bus due to the recirculation of airflow caused by entrainment through
windows. Finally, it was found that well fitted surgical masks, when worn by both infected
and susceptible passengers, can nearly eliminate the transmission of the disease.
Polyoxymethylene dimethyl ethers (PODE)
with high oxygen content and ethanol with high octane number are ideal
additives for diesel and gasoline, respectively. Previous studies
have shown that diesel/gasoline fuels and diesel/gasoline/PODE fuels
can significantly reduce soot emissions. To further reduce soot emissions,
ethanol was blended with these two fuels in this study and the effect
of ethanol on fuel ignitability was investigated by experimental study
and numerical simulation. Experiments were conducted in a cooperative
fuel research engine and a cetane ignition delay instrument under
various temperature and exhaust gas recirculation ratio. Results show
that the effect of ethanol on chemical ignition delay is higher than
that on physical ignition delay. With the addition of ethanol, the
combustion phasing is retarded and the maximum apparent heat release
rate of both low-temperature heat release and high-temperature heat
release become lower. At the same time, an increase in the critical
compression ratio (CCR) is observed with the addition of ethanol,
while improving ignitability can be observed with the addition of
PODE. It is worth noting that ethanol addition affects the CCR more
for CCR of diesel/gasoline/PODE/fuels than that for diesel/gasoline
fuels. Furthermore, kinetic simulation shows that there is an antagonism
between ethanol and PODE during the decomposition process because
OH is the key reactant in both ethanol and PODE decomposition.
A modified cooperative fuel research (CFR) octane rating engine and a constant-volume combustion chamber (CVCC) (CID510) were adopted to investigate the effect of the addition of polyoxymethylene dimethyl ether (PODE) on the ignition characteristics of diesel fuels. PODE was blended in diesel fuel with a volume fraction from 0 to 30%. The ignition delay times of PODE/diesel blends were obtained on a CVCC at a temperature of 833−893 K, and a reduction of environmental oxygen (O 2 ) content ranges from 21 to 12.8%. With PODE addition, the ignition delay time decreases, whereas the onset of low-temperature heat release (LTHR) moves forward and the peak of LTHR increases. The blends with a higher PODE fraction can weaken the negative effect on ignition delay caused by the reduction of oxygen content in air. The critical compression ratio (CCR) and the apparent heat release rate of binary blends were measured on the CFR engine at an equivalence ratio of 0.5 under homogeneous charge compression ignition combustion mode. The CCR becomes lower with higher PODE content in the blend. All blends exhibited two-stage ignition. The blends with a higher PODE fraction give a higher percentage of LTHR. Numerical simulation reveals that decomposition of PODE produces more CH 2 O, which accelerates the low-temperature reaction. The reactions related to O • and O 2 are enhanced by oxygen available in PODE, which, along with the molecular structure of PODE, results in higher reactivity for the PODE/diesel binary blends.
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