Beomjoon Lee

Development of the Supercritical Carbon Dioxide Power Cycle Experimental Loop in KIER

Three supercritical carbon dioxide (CO2) power cycle experimental loops have been developed in Korea Institute of Energy Research (KIER) from 2013. As the first step, a 10 kWe-class simple un-recuperated Brayton power cycle experimental loop was designed and manufactured to test its feasibility. A 12.6 kWe hermetic turbine-alternator-compressor (TAC) unit which is composed of a centrifugal compressor, a radial turbine and the gas foil bearings was manufactured. The turbine inlet design temperature and pressure were 180 °C and 130 bar, respectively. Preliminary operation was successful at 30,000 RPM which all states of the cycle existed in the supercritical region. Second, a multi-purpose 1 kW-class test loop which operates as a transcritical cycle at a temperature of 200 °C was developed to concentrate on the characteristics of the cycle, control and stability issues of the cycle. A high-speed turbo-generator was developed which is composed of a radial turbine with a partial admission nozzle and the commercial oil-lubricated angular contact ball bearings. Finally, a 60 kWe-class Brayton cycle is being developed which is composed of two turbines and one compressor to utilize flue-gas waste heat. As the first phase of development, a turbo-generator which is composed of an axial turbine, a mechanical seal and the oil-lubricated tilting-pad bearings was designed and manufactured.Copyright

Droplet Traffic Control in Microchannel by Droplet Bistability

This paper presents novel methods for precisely controlling water droplets by use of a microfluidic bifurcation channel with outlet restrictions, based on droplet bistability which utilizes the Laplace pressure due to interfacial tension arising when a droplet encounters a narrow restriction. We implement droplet bistable geometry, which has two symmetric branches and restrictions, to operate as capillary valves, so that a droplet can be trapped in front of a restriction and released through it when the next droplet arrives at the other restriction. It is observed that this trap-and-release occurs repeatedly and regularly by the succeeding droplets. It is also found that there is a critical flow rate to achieve droplet bistability which occurs only when the apparent Laplace pressure surpasses the pressure drop across the droplet. By adopting a simplified hydrodynamic resistance model, droplet bistable mechanism is clearly explained. Droplet bistability enables simple and precise control of droplets at a bifurcation channel. Thus, by an appropriate channel design to induce droplet bistability, precise control of droplet traffic is achieved at a bifurcation channel connected with a single inlet channel and two outlet channels. In particular, we are able to distribute droplets at a junction in a manner of perfect alternation between the two outlet channels. Bistable components can be used as an elaborately embedded droplet traffic signal for red light (trap) and green light (release) in complex microfluidic devices, where droplets provide both the chemical or biological materials and the processing signal.Copyright

Droplet Bistability in Microchannel and its Application to Flow Control

Abstract. We demonstrate the droplet bistability in a microchannel which has two symmetric necks thatoperate as capillary valves. It is shown that there are certain flow conditions, determined by droplet veloc-ity and droplet size, to achieve bistability. Droplet bistabililty allows simple but precise control of dropletat a bifurcation channel. Therefore, by an appropriate channel design to induce droplet bistability, we candistribute droplets at a junction passively in the manner of perfect alternation and perfect switching inthe choice of the outlets. Key Words: Droplet(액적), Bistability(쌍안정성), Microchannel(미세채널) 1. 서론 미세 채널 구조는 스펀지, 멤브레인, 생체 내 순환계등 일상 생활에서 쉽게 발견할 수 있다. 이러한 미세 채널 구조는 흔히 내부에 복잡한 네트워크 구조를 이루고있으며, 채널 내부에 다상의 유동을 형성한다. 복잡한미세 채널 내부의 다상 유동은 층류 유동을 특징으로하는 단상 유동과는 달리 버블이나 액적으로 발생하는비선형성으로 인해 복잡한 유동 특성을 나타낸다. 이러한 복잡한 유동을 제어하기 위해서 많은 연구진들이 능동적 또는 수동적 제어 기법을 개발하여 왔다.능동적 제어 (1)기법으로는 공압 제어 , 동전기 제어 (2) , 초음파 제어 (3) 등이 있으나, 이를 위해서는 외부 장치 및외부 에너지가 필요하다는 단점이 있다. 반면 채널의기하학적 구조를 활용하는 수동적 제어는 주기적 유동특성을 보이는 다상 유동 제어에 효과적이며, 간단하고 정밀한 유동 제어에 적합하다. Prakash

Simultaneous Measurement of Velocity and Temperature Fields in Micro-Scale Flow and Its Application to Electrokinetic Flow

In this paper, a technique of simultaneously measuring the velocity and the temperature in micro-scale flow is proposed. This method uses particle tracking velocimetry (PTV) for measuring the velocity and laser induced fluorescence (LIE) for measuring the temperature. To measure the accurate velocity and temperature, images for PTV and for LIE are separated by using two light sources and a shutter which is synchronized with a camera. By using only one camera, measurement system can be simplified and the error from complicate optical system can be minimized. Error analyses regarding the concentrations of fluorescent dye and particle and the light source fluctuation are also conducted. It is found that the error of the temperature and the velocity highly depends on the concentration of fluorescent particles which are used for PTV. This technique is applied to the simultaneous measurement of the velocity and the temperature in the electrokinetic flow. It is found that the velocity and temperature vary with the electric field strength and the concentration of electrolyte.