Jun Seok Choi

Experimental Study on the Thermal Performance of a Printed Circuit Heat Exchanger in a Cryogenic Environment

The advantages of a printed circuit heat exchanger (PCHE) are the compactness and efficiency derived from its heat-transfer characteristics; furthermore, a PCHE for which a diffusion bonding method was used during production can be applied to extreme environments such as a cryogenic condition. In this study, a micro-channel PCHE fabricated by diffusion bonding was investigated in a cryogenic environment regarding its thermal performance and the pressure drop. The test rig consists of an LN2 storage tank, vaporizers, heaters, and a cold box, whereby the vaporized cryogenic nitrogen flows in hot and cold streams. The overall heat-transfer coefficients were evaluated and compared with traditional correlations. Lastly, we suggested the modified heat-transfer correlations for a PCHE in a cryogenic condition.

Relationship between Serum Ferritin Levels and Dyslipidemia in Korean Adolescents.

Background Ferritin is associated with various cardiometabolic risk factors such as dyslipidemia, hypertension, obesity, and insulin resistance in adults. We aimed to study the association between serum ferritin levels and dyslipidemia in adolescents, because dyslipidemia is considered an important modifiable cardiovascular risk factor in the young. Methods We analyzed 1,879 subjects (1,026 boys and 853 girls) from the 2009–2010 Korean National Health and Nutrition Examination Survey IV. Subjects were categorized into quartiles according to their lipid parameters, which were classified according to age and gender. Those in the highest quartile groups for total cholesterol, low-density lipoprotein cholesterol (LDL-C), and triglyceride concentrations were diagnosed as having dyslipidemia. Those in the lowest quartile for high-density lipoprotein cholesterol (HDL-C) values were diagnosed with abnormal levels. Results In boys, total cholesterol, LDL-C, and triglyceride concentrations were significantly correlated with serum ferritin levels. In both boys and girls, serum ferritin levels were negatively associated with HDL-C values, even after adjusting for all covariates. Furthermore, there was no significant correlation between serum ferritin levels and total cholesterol, LDL, and triglyceride concentrations in girls. Conclusion Serum ferritin levels were significantly associated with major dyslipidemia parameters, more prominently in boys than in girls, and this association represents a cardiometabolic risk factor.

Serum Gamma-Glutamyltransferase Levels are Associated With Concomitant Cardiovascular Risk Factors in Korean Hypertensive Patients: A Nationwide Population-Based Study

AbstractPrevious studies suggested that serum gamma-glutamyltransferase (GGT) levels were associated with the prevalence of cardiovascular disease (CVD) risk factors including hypertension, diabetes mellitus (DM), and metabolic syndrome (MetS) in the general population. We aimed to investigate the relationship between serum GGT levels and CVD risk factors in Korean hypertensive patients.This cross-sectional study was based on data from the Korea National Health and Nutrition Examination Survey (KNHANES) 2011 to 2012. The analysis included 1541 hypertensive participants. Study participants were divided into groups according to tertiles of serum GGT with cutoff points of 20 and 35 U/L.Serum GGT levels were positively associated with the components of MetS (P value < 0.05, except for systolic blood pressure and high-density lipoprotein cholesterol). After adjusting for possible confounders, serum GGT levels were associated with an increased risk of MetS, high waist circumference, high triglyceride level, fasting plasma glucose, DM, and the urinary albumin-to-creatinine ratio (P = 0.001).In hypertensive patients, serum GGT levels are positively associated with major cardiovascular risk factors such as MetS, DM, and urinary albumin excretion.

Numerical Analysis on Longitudinal Heat Conduction in Printed Circuit Heat Exchanger

Abstract Longitudinal heat conduction is known to be an important factor in the design of a printed circuit heat exchanger(PCHE) for cryogenic applications. Parasitic heat conduction through the heat exchanger frame needs to be considered because it isknown to decrease the effectiveness of the heat exchanger. In this paper, a conjugate heat transfer problem in a simple counter-flow PCHE is analyzed by a computational fluid dynamics simulation. The effect of longitudinal conduction in a straight channel is compared with the theoretical effectiveness-NTU relationship that assumes a “thin” heat exchanger frame. The calculation results suggest that the theoretical model is valid in the present calculation conditions where NTU is < 13. Key words Longitudinal heat conduction(유동방향 전도열전달), Printed circuit heat exchanger(인쇄기판형 열교환기), Cryogenic heat exchanger(극저온 열교환기)†Corresponding author, E-mail: shyoon@kimm.re.kr 기호설명 A s : 열교환기 프레임 단면적 c p : 비열값 D h : 유로 수력직경 f : 마찰계수(Darcy friction factor)

Relationship Between Dyslipidemia and Albuminuria in Hypertensive Adults: A Nationwide Population-Based Study.

AbstractThis study aimed to estimate the relationship between various lipid abnormalities and albuminuria in hypertensive Korean adults. Data obtained from the Korea National Health and Nutrition Examination Survey in 2011 to 2012 were analyzed. The study included 2330 hypertensive participants. Total cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) levels were measured. Dyslipidemia parameters were defined as high TG ≥200 mg/dL, low HDL-C as HDL-C <40 mg/dL, high TC/HDL-C as TC/HDL-C ratio ≥4, high TG/HDL-C as TG/HDL-C ratio ≥3.8, and high LDL-C/HDL-C as LDL-C/HDL-C ratio ≥2.5. Albuminuria was defined as a urine albumin to creatinine ratio (ACR) ≥30 mg/g. Women with albuminuria showed significantly higher levels of TG, TC/HDL-C, and TG/HDL-C and a lower level of HDL-C than women without albuminuria (all P < 0.05). LogTG, TC/HDL-C, and logTG/HDL-C were positively correlated with ACR in both men and women; however, HDL-C was negatively correlated with ACR in women and non-HDL-C was positively correlated with ACR in men. In men, there was no association between ACR and lipid parameters. However, in women, higher values for logTG, TC/HDL-C, and logTG/HDL-C were associated with an increased odds ratio (OR) for albuminuria (OR [95% confidence interval]: 1.53 [1.06–2.21], 1.21 [1.02–1.45], and 1.78 [1.21–2.63], respectively) and HDL-C with a decreased OR for albuminuria (0.78 [0.67–0.92]) after adjusting for all covariates. LogTG, TC/HDL-C, and logTG/HDL-C were associated with an increased prevalence of albuminuria in hypertensive women. Screening and treatment for dyslipidemia may be necessary for hypertensive women to address potential albuminuria.

Thermal Characterization of Polymer Composites by Using the 3-Omega Method

Polymer composites having comparable thermal conductivity to stainless steel at room temperature are commercially available nowadays. Metal or carbon fiber and particles are added to base polymers to enhance mechanical and thermal performance. However for polymer composites having high additive concentration, characterizing mechanical and thermal properties of the composite may be a challenging problem due to an-isotropic natural and non-homogeneity. In this paper, a novel thermal property measurement method based on the 3-omega (3ω) is proposed for thermal analysis of polymer composites. Sensitivity and feasible limit of the 3ω method with “boundary mismatch assumption” is analyzed for measurement of polymer composites having broad range of thermal conductivity.Copyright

Experimental Study on the Heat Transfer Performance of the Diffusion-Bonded Micro Channel Heat Exchanger in the Pilot-Scale Test Facility

The heat exchangers using micro channel structure have been studied due to its high efficiency and compactness. If the micro channel heat exchanger is applied to the natural gas liquefaction process, the efficiency of LNG plant can be improved. In this study, the micro channel was fabricated by chemical etching and the heat exchanger core was made by the diffusion bonding method for cryogenic reliability. For applying to the large scale plant such as LNG plant, the pilot-scale thermal performance test setup of the heat exchanger was built. Tests can be performed in the cryogenic environment. The working fluid is cryogenic nitrogen gas. Two different temperature level of nitrogen gas was made by vaporizing from liquid nitrogen. And these fluids exchange the heat through the micro channel heat exchanger. Test rig is an open loop. Therefore nitrogen gas is discharged to the ambient. Temperatures are measured by RTD sensors. Inlet pressures of heat exchanger are measured by the cryogenic pressure transducer and pressure differences of heat exchanger are measured by the differential pressure transmitters. And all the measured data is acquired by DAQ module and saved into PC. The heat transfer coefficients of the micro channel heat exchanger are calculated and the heat transfer characteristics are investigated. And the test result was compared with the existing heat transfer correlations. And the modified heat transfer correlation of the micro channel heat exchanger in the cryogenic environment is suggested.Copyright

Experimental Study on the Heat Transfer Characteristics of Cryogenic Micro Channel Heat Exchanger

In recent years, many researchers investigated micro channel heat exchangers because of its high efficiency and compactness. However, few experimental studies about micro channel heat exchanger in cryogenic environment have been conducted. In this study, micro channel was fabricated by chemical etching and heat exchanger core was made by diffusion bonding method for cryogenic reliability. Performance test was conducted in cryogenic test rig. Working fluids are liquid nitrogen and methane gas. Methane gas was condensed in the micro channel heat exchanger. Heat transfer coefficients and pressure drop were measured and the heat transfer characteristics were investigated. These results can be used to design the heat exchanger of gas liquefaction plant.Copyright