Suhyeon Park

Micromix combustion is a promising approach to addressing issues such as flashback, combustion instability, and nitrogen oxide (NO_x) emissions in hydrogen-fueled gas turbines. In this study, a numerical analysis was conducted to investigate key factors influencing NO_x emissions in a lab-scale micromix combustor under atmospheric pressure and room temperature. The effects of air flow rate, equivalence ratio, and combustor component surface temperature on NO_x formation were systematically evaluated. The results show that increasing the air and hydrogen flow rates at a constant equivalence ratio reduced NO_x emissions due to shorter residence times. In addition, higher surface temperatures of combustor components increased flame temperatures and NO_x production. Lastly, high equivalence ratios led to partial flame merging, increasing NO_x emissions. These findings highlight that the efficient cooling of combustor components and proper selection of air flow rate and equivalence ratio are critical to minimizing NO_x emissions in micromix combustors. The study provides design and operational guidelines for the future development of hydrogen-fueled gas turbines.

Spiders utilize their fine silk fibers for their aerial dispersal, known as ballooning. With this method, spiders can disperse hundreds of kilometers, reaching as high as 4.5 km. However, the passive dynamics of a ballooning model (a highly flexible filament and a spider body at the end of it) are not well understood. Here, we introduce a bead–spring model that takes into account the anisotropic drag of a fiber to investigate the passive dynamics by the various non-uniform flows: (i) a shear flow, (ii) a periodic vortex flow field, and (iii) a homogeneous turbulent flow. For the analysis of the wide range of parameters, we defined a dimensionless parameter, which is called “a ballooning number.” The ballooning number is defined as the ratio of Stokes’ fluid-dynamic force on a fiber by the non-uniform flow field to the gravitational force of a body. Our simulations show that the present model in a homogeneous turbulent flow exhibits the biased characteristic of slow settling with increasing turbulence. Upon investigating this phenomenon for a shear flows, it was found that the drag anisotropy of the filament structure is the main cause of the slow settling. Particularly, the cause of slow settling speed lies not only in the deformed geometrical shape but also in its generation of fluid-dynamic force in a non-uniform flow. Additionally, we found that the ballooning structure could become trapped in a vortex flow. These results help deepen our understanding of the passive dynamics of spiders ballooning in the atmospheric boundary layer.

Background: Non-chemical methods are as important as pesticides to control bed bugs. The most effective method is heat treatment. Underfloor heating is widely used in residential buildings in Korea, and the benefits of floor heating can be utilized in heat treatment for bed bug control.Methods: Simulations of indoor air flow and heat transfer were performed to compare and analyze the temperature distributions with different heating methods for bed bug control using floor heating and fan heater methods.Results: Even though the average indoor temperature of floor heating was lower, it was more uniform with floor heating than that of fan heating under the assumed conditions. Local cold spots disappeared with the floor heating method.Conclusions: It was confirmed that floor heating has advantages over fan heating to achive uniform high temperatures during heat treatment for exterminating bed bugs.

In this study, a numerical investigation is conducted on combustion-driven pulsed-jet actuation to control the flow around a lifting surface. Based on relevant experimental measurements and computations, high-speed jets are generated from the impulsive variation in pressure at the actuator boundary. A supersonic jet flow is momentarily generated by combustion in a reaction chamber of the actuator, and the flow interacts with the external flow around the lifting surface and alters the aerodynamic characteristics. The computational results indicate that the flow control performance of the jet actuation is significant at a high-incidence angle of attack, such as beyond the stall angle, whereas the impact is minimal at low angles of attack, such as in the linear lift region. Repetitive jet actuation can produce additional momentum to the external flow and alters the pressure distribution on the suction surface, particularly downstream of the actuator location. This pressure variation from the actuation yields an additional lift force on the lifting surface and reduces the amplitude of the aerodynamic moment at a given angle of attack, thus enhancing the aerodynamic performance of the airfoil.