An Experimental Study on the Evaporation Heat Transfer of R-22 in Small Tubes

The evaporating heat transfer of R-22 in small tubes has been experimentally studied. The tubes in present work are single square tube and single round tube. The hydraulic diameter of the tubes is 1.67㎜. The experimental apparatus consists of a refrigerant pump, a condenser and a receiver, the small-tube test section, a subcooler of liquid refrigerant, a preheater for control of refrigerant quality at the inlet of test section. The heat flux is generated by heating wire wound on the outer wall of the test section. A set of five thermocouples are embedded at the wall of the test section to measure the wall temperature at five locations. The refrigerant flow rate is measured using a high-pressure rotameter. The pressure drop across the test section is measured using a differential pressure transducer. For refrigerant mass flux of 384 ㎏/㎡s and 570 ㎏/㎡s, the inlet qualities are varied from 0.0 to 0.8 and the wall heat fluxes are varied from 4 ㎾/㎡ to 10 ㎾/㎡. The measured evaporating heat transfer coefficients in the small tubes are 620 ∼ 4760 W/㎡K, which are lower than those observed in the typical large-diameter circular tubes. The evaporating heat transfer coefficient in the square tube shows lower heat transfer coefficient than the round tube. The pressure drop increases with increasing quality.Abstract 사용기호 표목차 그림목차 제 1 장 서론 = 1 제 2 장 선행연구 고찰 = 3 제 1 절 서론 = 3 제 2 절 증발열전달에 관한 연구 = 4 2.2.1 열전달계수와 상관관계식 = 4 2.2.2 실험연구 = 7 제 3 절 요약 = 14 제 3 장 실험장치 및 실험방법 = 26 제 1 절 실험장치 = 26 3.1.1 냉매회로 = 27 3.1.2 예열기 = 28 3.1.3 시험부 = 28 3.1.4 데이터 취득장치 = 29 제 2 절 실험방법 = 30 제 3 절 데이터 처리 = 31 제 4 장 실험결과 및 고찰 = 39 제 1 절 열유속에 따른 증발열전달계수 = 40 제 2 절 질량속도에 따른 증발열전달계수 = 41 제 3 절 관의 형상에 따른 증발열전달계수 = 41 제 4 절 압력강하 = 42 제 5 장 결론 = 49 참고문헌 = 5

Paley-weiner theorem of ultradistributions without compact support

Thesis (master`s)--서울대학교 대학원 :수학과,1998.Maste

The Role of cognitive strategic questioning in the changes of student`s conceptions about heat and temperature

학위논문(박사)--서울대학교 대학원 :과학교육과,1995.Docto

야구 포수의 숙련도와 포구 자세에 따른 송구 동작 비교 연구

학위논문(석사)--서울대학교 대학원 :체육교육과,2006.Maste

깊은 굴착시 토류구조물 및 주변 지반 거동에 대한 현장연구

학위논문(석사)--서울大學校 大學院 :土木工學科,1996.Maste

수종 임플랜트 지대주나사의 반복하중 후 나사풀림에 관한 연구

학위논문(석사)--서울대학교 대학원 :치의학과 치과보철학전공,2003.Maste

Enhancement of Boiling Heat Transfer and Critical Heat Flux using Surfaces with Thermally-Induced Wetting Transition

DoctorBoiling heat transfer (BHT) and critical heat flux (CHF) are important factors for boiling heat transfer systems. The factors govern the efficiency and safety of the system. In this reason, there have been many studies for the enhancement of BHT and CHF. A surface modification is a promising technique to enhance BHT and CHF without changing the working fluid. Especially, the surface modifications affect wettability of the surface that changes boiling characteristics significantly. However, there is a trade-off between BHT and CHF when the wettability is modulated. A hydrophobic surface induces a nucleation at low wall superheats with increased nucleation site density; it results in an improvement of BHT. At the same time, a premature CHF occurs on the hydrophobic surface due to the vigorous bubble generation. In contrast, a hydrophilic surface enhances CHF with supplying liquid on the heated surface while BHT deteriorates in comparison to the hydrophobic surface. In this respect, if a surface changes its wettability by itself during boiling, then, the enhancements of BHT and CHF are attainable. In the present study, TiO2 and ZnO thin films were used as coating materials on a silicon heating substrate in pool and flow boiling. TiO2 and ZnO thin films were initially hydrophobic, but they became hydrophilic as the temperature increases. For an achievement of the wetting transition at a low temperature which corresponds to the wall temperature during boiling, the radio frequency (RF) sputtering condition was adjusted. For the fabricated surfaces, the wetting transition was confirmed by heat treatment for 10 h in air atmosphere; the contact angles was 83.1 on TiO2 and 101.3 on ZnO after the heat treatment at 100 C. The complete wetting transition was achieved under the heat treatment near 200 C. After the heat treatment at 200 C, they became hydrophilic showing the contact angle of 32.7 and 26.7 on TiO2 and ZnO, respectively. Using the surfaces and an SiO2-deposted surface as a reference, the pool boiling and flow boiling experiments were conducted with deionized (DI) water. The pool boiling experiments were conducted at 1.0, 2.0, 3.3, and 4.1 bars for the regulation of the saturated temperature of the working fluid because the complete wetting transition was postulated to occur near 200 C. BHT was improved on both TiO2 and ZnO regardless of the pressure conditions; CHF on TiO2 was also enhanced at 4.1 bar. Furthermore, additional enhancements of CHFs on TiO2 and ZnO were achieved by the time effect test that maintained heat flux of 8090 % of CHF (at high wall temperature) for 20 min to induce complete wetting transition. As a result, CHFs were enhanced on both surfaces. The time effect of the wetting transition was also examined by heat treatment tests. An empirical correlation for the dependence of the receding contact angle on the heat treatment temperature and time was made and combined with a CHF prediction model. Using the empirical correlation, the CHF enhancement ratio was predicted. In flow boiling experiment, the wetting transition effect was investigated in a macrometer channel (hydraulic diameter was 7.5 mm) using DI water. The mass flux was ranged from 600 to 1200 kg/m2s and the inlet temperature was set 97 C for the prevention of damage in the acrylic channel. BHTs of TiO2 and ZnO were improved in comparison with SiO2 for the examined mass flux conditions. In addition, CHF degradation was compensated on TiO2, and CHF was enhanced on ZnO at the mass flux of 1200 kg/m2s. Also, CHFs on TiO2 and ZnO were more enhanced by the time effect tests. However, the enhancement ratio of CHF was lower than other results in literature. The flow and structure effects are possible reasons. Some of previous studies showed that CHF enhancement ratio decreased as mass flux increased on the modified surface. In the present study, although TiO2 and ZnO became hydrophilic as the wall temperature increased at high mass flux, the increased mass flux may attribute to the reduction in an increase of CHF. Another reason could be that structures in previous studies contributed to the additional enhancement of CHFs on the modified surface. In summary, the thermally-induced wetting transition of TiO2 and ZnO was adopted to pool and flow boiling for the enhancement of both BHT and CHF. There was no defect on the surfaces and contamination of the working fluid. Furthermore, the roughness was nanometer scale which should not provide nucleation sites. The wetting transition was the only plausible factor for the explanation of the boiling characteristics. For both pool and flow boiling, the nucleation on TiO2 and ZnO was more easily activated at the low wall temperature in comparison to SiO2 due to their hydrophobicity. Also, at the high temperature, BHT was enhanced without degradation of CHF. It is explained by that TiO2 and ZnO became hydrophilic at a temperature near CHF, based on the observed variation of the contact angle after the heat treatment. Therefore, TiO2 and ZnO are suggested as potential surfaces which enhance both BHT and CHF without any external control during an operation

RANKL에 의한 파골세포 분화과정에서의 포도당대사 경로 변화

Thesis(doctors)--서울대학교 대학원 :치의학과,2008.2.Docto