Young-Soo Chang

Experimental investigation on the drop-in performance of R407C as a substitute for R22 in a screw chiller with shell-and-tube heat exchangers

A drop-in test of a mixed refrigerant R407C is performed in a commercial screw chiller with shell-and-tube heat exchangers originally designed for R22. The test results show a severe performance reduction when substituting the refrigerant from R22 to R407C. The reason for the performance reduction is analyzed comprehensively, and the influence of thermodynamic properties, compressor efficiency, and heat transfer is evaluated quantitatively. The major factor causing the performance reduction is assessed as the degradation of the heat transfer in using the mixed refrigerant, R407C. The heat transfer degradation in the evaporator is found to be larger and influences more on the chiller performance reduction. The performance reduction caused by the evaporator is approximately two times compared with that of the condenser.

High performance piezoelectric MEMS energy harvester based on D33 mode of PZT thin film on buffer-layer with PBTIO3 inter-layer

In this paper, a MEMS energy harvester was investigated to scavenge power from ambient vibration source. It was designed to convert low level vibration to electrical power via the piezoelectric effect. The proposed energy harvester was fabricated by patterning Pt electrodes into inter-digital geometry on top of the sol-gel-spin coated Pb(Zr,Ti)O<inf>3</inf> thin film for d33 mode on silicon cantilever with proof mass. In order to obtain well grown PZT thin film, PbTiO3 seed-layer was newly applied as an inter-layer between PZT and ZrO<inf>2</inf> layer. The fabricated energy harvester generated 1.1uW of electrical power to 2.2MΩ of load with 4.4V<inf>peak-to-peak</inf> from a vibration of 0.39g acceleration at it resonant frequency of 528Hz. The corresponding power density was 2.8mW·cm^−3·g⁻¹.

Performance Simulation of a Ventilation System Adopting a Regenerative Evaporative Cooler

Cooling load reduction was analysed of a ventilation system adopting a regenerative evaporative cooler. The regenerative evaporative cooler is a kind of indirect evaporative cooler which cools the air down to its inlet dewpoint temperature in principle without change in the humidity ratio. The regenerative evaporative cooler was found able to cool the ventilation air to when the outdoor condition ranges and 0.01~0.02 kg/kg. When the outdoor humidity ratio is lower than 0.018 kg/kg, the regenerative evaporative cooler was found to provide cooling performance enough to compensate the ventilation load completely and to supply additional cooling as well. Energy simulation during the summer was carried out for a typical office building with the ventilation system using the regenerative evaporative cooler. The results showed that the seasonal cooling load can be reduced by about 40% by applying the regenerative evaporative cooler as a ventilation conditioner. The reduction was found to increase as the outdoor temperature increases and the outdoor humidity ratio decreases.

Fault Symptom Analysis and Diagnosis for a Single-Effect Absorption Chiller

In this study, fault symptoms were simulated and analyzed for a single-effect absorption chiller. The fault patterns of fault detection parameters were tabulated using the fault symptom simulation results. Fault detection and diagnosis by a process history-based method were performed for the in-situ experiment of a single-effect absorption chiller. Simulated fault modes for the in-situ experimental study are the decreases in cooling water and chilled water mass flow rates. Five no-fault reference models for fault detection of a single-effect absorption chiller were developed using fault-free steady-state data. A sensitivity analysis of fault detection using the normalized distance method was carried out with respect to fault progress. When mass flow rates of the cooling and chilled water decrease by more than 19.3% and 17.8%, respectively, the fault can be detected using the normalized distance method, and COP reductions are 6.8% and 4.7%, respectively, compared with normal operation performance. The pattern recognition method for fault diagnosis of a single-effect absorption chiller was found to indicate each failure mode accurately.

Modelling of the Heat and Mass Transfer in a Liquid Desiccant Dehumidifier with Extended Surface

This study presents a new idea of liquid desiccant dehumidifier with extended surface to improve the compactness. Extended surface is inserted between vertical cooling tubes, and the liquid desiccant flows down along the tube walls and the extended surface as well. Though the extended surface contributes to the increase in the mass transfer area, the effect tends to be limited because less conductive non-metallic materials need to be applied due to the high corrosiveness of liquid desiccant. To analyze the effects of the extended surface insertion, mathematical modelling and numerical integration are performed for the heat and mass transfer in the liquid desiccant dehumidifier. The results show that, though the liquid desiccant on the extended surface is heated due to the moisture absorption, the temperature can be maintained by periodic mixing at the contact points between the tube and the extended surface with the liquid desiccant stream from the tube side at a relatively low temperature. This implies the absorption heat from the extended surface side can be removed effectively by mixing, which leads to a substantial improvement of the dehumidification in the liquid desiccant dehumidifier with extended surface. When the interval of the extended surface, , is less than 0.1, the dehumidification is shown to increase by more than two times compared with that without extended surface.

An Experimental Study on Oil Effect of CO 2 in Heat Pump Outdoor Heat Exchanger

In order to investigate the effects of PAG oil concentration on heat transfer performance and pressure drop during gas cooling process of , the experiments on fin-tube heat exchanger of heat pump were performed. The experimental apparatus consists of a gas cooler, a heater, a chiller, a mass flow meter, a pump and measurement system. Experiments were conducted in various experimental conditions, which were inlet temperature(), mass flow rates (50, 55, 60, 65, 70 g/s) and PAG oil concentration(0 to 2.6 wt%). Heat transfer rate decreased with the increase of the oil concentration and the decrease of inlet pressure. And pressure drop increased with the increase of the oil concentration and mass flow rate of refrigerant. The COP reduction by deterioration of gas cooler performance with oil concentration was analyzed. When inlet pressure of gas cooler is 100 bar, the COP reduction was estimated by 6% under 1 wt% of oil concentration.

Performance Analysis of a Desiccant Rotor for Rotational Period in a Desiccant Cooling System

The performance simulation of a desiccant rotor, which is a core component of a desiccant cooling system, was conducted on the basis of a theoretical solution of the heat and mass transfer process in the rotor. The simulation model was validated by comparing simulation results with experimental data; reasonable agreement was observed. The effect of the rotation speed on the performance of the desiccant rotor was investigated for various operation conditions: temperature (50 to ), humidity ratio (0.01 to 0.02 kg/kg DA), and flow rate of regeneration air. The optimum rotation speed was determined from the maximum moisture removal capacity (MRC) of the desiccant rotor, and it was found to vary with the operation conditions. Further, the correlation for the optimum rotation speed was determined by regression analysis.

Experiment of Micro Cantilever Deflection by Thermal Bubble Growth in Liquid

Micro cantilevers are significant structure for MEMS devices, such as bio-chips, sensors and STEM/AFM probes. The beam deflection and its characteristics have been studied for various purposes. In this study, expending bubbles from thermal surface exert force on micro-cantilever beam and causes deflection of the beam. Cantilevers were fabricated by classic MEMS fabrication method; photolithography and dry etching. The micro-beam was fabricated from n-type silicon wafer and its thickness varies from 10 micron to 30micron with various geometry (length, width and tip shapes). The distance from thermal surface and cantilever beam is also significant variables for analysis of bubble-beam interaction. We observed beam deflection with respect to various bubble generation conditions (bubble size, contact area and generating frequency). Simple analysis of bubble-beam interaction were performed and compared with experimental results.Copyright

Enhancement of Thermal Bubble Nucleation From Micro Cavity Filled With Porous Materials

In many applications, the study of nucleation of thermal bubble is requisite for design of micro geometry such as micro-actuator, chip cooling system and ink-jet device operation and so on. In heterogeneous nucleation boiling, a bubble generally initiated from a small gas trapped cavity on heated surface. Experimental observations were undertaken on the bubble generation phenomena from fabricated micro-cavity on the silicon wafer, which is immersed in FC-72. Especially, the nucleation of thermal bubble is much enhanced in the case of the cavity filled with porous material compared to vacant cavity. Incipience boiling occurs much less degree of superheat, which is the difference of surface temperature and bulk fluid temperature. It is also observed that the generated bubble size and frequency are dependent on the specifications of cavities. In this study, we performed various experiments to minimize the degree of superheat and to generate specific bubble size and purposed frequency by modification of cavity size and filling porous materials.Copyright