No abstract

In areas with strong wind, windbreaks are built along railways to reduce the impact of wind on trains. However, because of the restrictions imposed by actual terrain, windbreak structures are often not uniform, such as from a cutting to an embankment, resulting in a discontinuous transition region. When a train runs through this region, a distinct yawing phenomenon occurs. This study numerically explored the aerodynamic features of a train running through a rectangular windbreak transition region. The variations in the pressure, side force, and moment of a train were analyzed, and the flow field features were clarified. Furthermore, the yawing motion of the car body with time was described. Finally, based on EN14067-6, the critical wind speed was obtained using the safety assessment of a train running through a windbreak transition region.

Summary For a heat pump, three injection styles including liquid, two‐phase, and vapor injection are used to improve its performance. But there is no quantitative index to define the injection styles. They are distinguished from the injection configurations. The subcooler refrigerant injection (SCRI) is generally considered as the vapor injection. In this study, the injection super‐heating degree (ΔTinj) is introduced as a technical parameter to define the injection style. The ΔTinj is affected by the subcooling degree (ΔTsc). These two parameters are used to study their effects on the heat pump performance. Experimental and theoretical analysis reveals that three injection styles can be selectively achieved in the SCRI configuration by the suitable control of the main and subcooling electronic expansion valves. Additionally, for liquid and two‐phase injection, it can improve heating capacity (Qh) by heightening the ΔTsc to increase the heat exchange amount in the evaporator. For vapor injection, in addition to increase of the ΔTsc, the Qh can be enhanced by raising the ΔTinj to lift the compression power of the compressor. The two‐phase injection is more beneficial to enhance the heat pump performance than the liquid injection and two‐phase injection. The largest Qh and corresponding increase amplitude (δQh) were, respectively, obtained with two‐phase injection to be 13 577 W and 13.5%, and the corresponding COP and increase amplitude (δCOP) were, respectively, 2.36 and 16.8%.

Tracer analysis is commonly used to evaluate the hydraulics of environmental and chemical engineering systems. The traditional tracer analysis is conducted through physical experiments that are usually complex, costly, time-intensive, and may be impractical. Because of the continued advancement of computing technology, computational fluid dynamics (CFD) has demonstrated its applicability in simulating tracer transport. CFD can provide advantages, including no interruptions of existing water treatment process, no impacts of background concentration, and a relatively low cost. However, no reports have quantitatively studied the cost that CFD can save on tracer analysis. This study first proved the accuracy of CFD tracer analysis for an existing ozone disinfection tank and then compared the economic expenses and environmental impacts of CFD tracer analysis with those of a field tracer analysis. It was found that CFD-based tracer analysis has accuracy on par with the physical-based study but at relatively low economic cost and environmental impacts. K E Y W O R D S computational fluid dynamics, cost, environmental impacts, tracer study