수평관내 이산화탄소의 증발열전달 특성에 관한 실험적 연구

Abstract

친환경 냉동·공조기 개발의 일환으로 자연냉매인 CO₂의 냉매적 특성을 파악하고자, 수평관 내 증발열전달 특성을 연구하였다. 질량유속은 200 ∼ 1200 kg/(㎡s), 열유속은 10 ∼ 80 kW/㎡ 그리고 입구 포화온도는 -5 ∼ 5℃의 실험조건에서 평활관, 마이크로핀관 그리고 마이크로채널관에 대해 실험하였다. 또한, 오일영향을 조사하기 위해 오일농도를 0 ∼ 5 w% 변화시켰다. 평활관의 경우, 기존 연구결과와 동일하게 증발이 진행되면 열전달계수가 감소하였으며, 질량유속의 영향은 적었다. 그러나 질량유속이 600 kg/(㎡s)이상일 때, 저건도 영역에서 드라이아웃을 확인하였다. 이를 예측할 수 있는 실험식을 도출하였고, 오차범위는 ±28% 내에서 만족하였다. 마이크로핀관의 경우, 건도가 증가하여도 열전달계수는 일정한 범위 내에서 유지되었다. 이는 관내 젖음성(wetting)과 핀에 의한 난류효과에 기인한 것으로, 평활관에 비해 향상계수(enhancement factor)는 2.4 ∼ 3.4이고 손실계수(penalty factor)는 1.8 ∼ 2.3으로 평활관보다 전열특성이 우수하였다. 또한, 드라이아웃이 발생하는 건도가 평활관에 비해 증가하였다. 두 종류의 마이크로채널관은 드라이아웃 발생이전에 증발 열전달계수가 일정하게 유지되었다. 서로 다른 수력직경에 대한 전열특성을 비교하면 향상계수는 1.8 ∼ 2.3이고 손실계수는 2.6 ∼ 3.3로 조사 되었으며, 수력직경이 작을수록 열전달성능 향상보다는 압력강하에 대한 손실이 더 크게 나타났다. 오일혼합은 모든 전열관에서 열전달계수를 감소시켰으나, 드라이아웃 발생건도 이후에서는 오일이 혼합되지 않은 경우보다 열전달계수가 크게 나타났다. 이는 전열관 내부 표면에 형성된 오일막에 의해 드라이아웃 발생이 지연되었기 때문이다. 또한, 오일농도에 의한 열전달계수의 영향은 오일농도 5 w%일 때 가장 크게 나타났으며, 감소폭은 최대 35%이었다. 압력강하 특성은 모든 전열관에서 동일한 경향을 보였고, 질량유속과 열유속이 클수록 그리고 입구 포화온도가 낮을수록 높게 나타났다. 오일이 혼합되면 전열관 내 오일점성에 의한 마찰력 증가로 압력강하가 증가하며, 증가 정도는 약 15% 내외로 관찰되었다. 실험에서 획득한 데이터를 이용하여 건도 증가에 따라 증발 열전달계수가 감소하는 경향과 저건도 영역에서 핵비등 효과를 보다 잘 예측할 수 있는 새로운 실험식을 도출하였다. 이 실험식에 의한 오차범위는 ±30% 내에서 실험 데이터를 만족하였다.; Because of the ozone layer depletion and global warming, new alternative refrigerants are being developed. In recent years, CO₂ among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In the present study, the heat transfer characteristics of natural refrigerant CO₂ are investigated, as a part of the development of clean refrigerants for environmental protection. Experiments were conducted at mass fluxes of 200 to 1200kg/(㎡s), heat fluxes of 10 to 80 kW/㎡ and saturation temperatures of -5 to 5℃. This paper presents the measured heat transfer coefficients and pressure drops in a horizontal smooth tube, micro fin-tube and micro-channel during evaporation process of CO₂. Also, oil concentration was changed at the range of 1 to 5 w%. to examine the oil effect. The results of this study can improve theoretical understanding of thermal-fluid phenomena, such as two-phase and dryout, in a smooth tube, a micro fin-tube and a micro-channel tube. Also, the present results can be used in air conditioners for CO₂. These results can provide the basic design data of a HVAC&R system. The main results were summarized as follows : In the case of the smooth tube, the heat transfer coefficient decreased during evaporation processor, and the effect of the mass flux was insignificant, as shown the existing results. When mass flux was above 600 kg/(㎡s), however, dryout was occurred at low quality region. When mass flux was above 600 kg/(㎡s) and heat flux was above 40 kW/㎡, dryout on the upper side of the tube was occurred. From theses, new correlations to predict dryout position proposed. The deviation between the measured values and the calculated values by the present correlation was above ±28%. The evaporation heat transfer coefficients of CO₂ in the micro-fin tube were uniform though vapor quality increased. This was because the wetting of inner-side of a tube and the turbulence effect due to fin. Enhancement factor and penalty factor of the micro-fin tube were 2.4 ∼ 3.4 and 1.8 ∼ 2.3 respectively. Thus the heat transfer characteristics was better than the smooth tube. Also, it was found the quality occuring dryout was shifted toward high quality. By the difference of the inner diameter of the micro-channel, enhancement factor was 1.8 ∼ 2.3 and penalty factor was 2.6 ∼ 3.3, on the basis of the micro-channel with tube I.D. 0.68 mm. And the loss by the pressure drop was larger than the enhancement of the heat transfer performance. The oil addition decreased the heat transfer coefficient for all heat transfer tube. But the heat transfer coefficient increased after the quality occuring dryout. This was because the oil film with high boiling temperature prevented the occurrence of dryout. Also, the evaporation heat transfer coefficient by oil addition was largest at oil concentration 5 w%, the decrease of the heat transfer coefficient was the maximum 35%. The pressure drop characteristics were similar to all heat transfer tube, and it increased with increasing mass flux and heat flux, and decreasing saturation temperature. The pressure drop increased with increasing the oil addition. The increase of the pressure drop was measured about 15%. This is because the friction force was large due to oil viscosity. The experimental data in this study was compared to the correlations proposed by Shah's correlation, Jung's correlation, Kandalikar's correlation and Winterton's correlation. Among these correlations, the Kandalikar's correlation agrees quite well with the experimental data within 23.9%. When the quality increased, however, the trend on the decrease of evaporation heat transfer coefficient was not predicted and the heat transfer effect due to nucleate boiling was under estimate. From these, the deviation between the measured values and the calculated values was large at high mass flux. Therefore, the propose of the new correlation was needed. From theses, new correlations to predict heat transfer coefficient proposed. The deviation between the measured values and the calculated values by the present correlation was above ±30%.