• International Journal of Air-Conditioning and Refrigeration
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• 제15권4호
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• pp.163-168
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• 2007

The feature of six existing condensation heat transfer correlations for microfin tubes were evaluated with the consideration of vapor quality, mass flux, geometries, and various refrigerants. The Kosky and Staub [15] and the Jaster and Kosky [16] correlations for smooth tube were used for the evaluation of the heat transfer enhancement factor (EF). For the prediction of zeotropic mixtures, most correlations show discrepancy with previous measurements. The Yu and Koyama [4] and the Shikazono et al. [8] correlations do not consider spiral angle effect. The Han and Lee [10] correlation shows fin height growth deteriorates heat transfer. Experimental verification to develop reliable condensation heat transfer correlation for microfin tubes is still needed with the consideration of geometrical effects and working conditions.

• 한국산학기술학회논문지
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• 제13권4호
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• pp.1440-1446
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• 2012

본 논문은 수평 마이크로핀관내 초임계 $CO_2$와 PAG 혼합물의 열전달과 압력강하 특성에 대해서 실험적으로 연구한 것이다. $CO_2$와 PAG 혼합물의 열전달계수는 압력이 10 MPa이고, 질량유속은 520 kg/$m^2s$이며, PAG 오일 농도는 0.06~2.26%에서 측정하였다. PAG 오일농도가 0.3%인 경우, $CO_2$와 PAG 혼합물의 열전달계수와 압력강하는 순수 $CO_2$ 냉매의 열전달계수와 동일한 경향을 나타내었다. 그리고 PAG 질량농도가 2.26%인 경우, 초임계 온도근처에서 측정한 열전달계수는 순수 $CO_2$의 열전달계수 보다 약 50%정도 낮게 나타났다. 마찰압력강하는 $60^{\circ}C$의 $CO_2$ 평균온도에서 순수 $CO_2$의 압력강하보다 약 1.6배 더 높게 나타났다.

• 에너지공학
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• 제19권4호
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• pp.234-239
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• 2010

원형 파이프를 유동하는 6.5% EG 수용액으로부터 만들어진 아이스슬러리의 열전달 특성을 분석하기 위한 실험을 수행하였다. 실험 장치의 시험부는 13.84 mm 내경과 1,500 mm의 길이를 가진 동관으로 제작되었다. 아이스슬러리는 시험부 주변에 이중관 형태로 만들어진 원형관 내부를 유동하는 온수에 의해 가열되었다. 본 연구의 실험에 적용된 IPF와 질량 유속은 각각 0 ~ 25% 그리고 1000 ~ 3,000 kg/$m^2s$의 범위이었고, 온수의 온도와 유량은 일정하게 유지하였다. 측정된 열전달량은 질량 유량과 IPF가 증가함에 따라 증가하였으나, IPF의 영향은 높은 질량 유량에서는 작은 것으로 나타났다. 낮은 질량유량에서는 열전달 계수의 급격한 상승이 15 ~ 20%의 IPF에서 나타났다. 마지막으로 측정된 열전달계수는 기존의 열전달상관식들에 의해 계산된 열전달계수와 비교하여 제시하였다.

• Journal of Advanced Marine Engineering and Technology
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• 제27권1호
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• pp.108-116
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• 2003

In this paper, the experimental results of condensation heat transfer were reported for the plate and shell heat exchangers(P&SHE) using R-l34a. An experimental refrigerant loop has been established to measure the condensation heat transfer coefficient of R-l34a in a vertical P&SHE. Two vertical counter flow channels were formed in the P&SHE by three plates of geometry with a corrugated trapezoid shape of a chevron angle of 45$^{\circ}$. Downflow of the condensing R-l34a in one channel releases heat to the cold up flow of water in the other channel. The effect of the refrigerant mass flux, average heat flux, system pressure and vapor quality of R-l34a on the measured data were explored in detail. The results indicate that at a higher vapor quality the condensation heat transfer coefficients are significantly higher. Condensation heat transfer coefficients were increased when the refrigerant mass flux was increased. A rise in the average heat flux causes an slight increase in the hr. Finally, at a higher system pressure the hr is found to be lower. Correlation is also provided for the measured heat transfer coefficients in terms of the Nusselt number.

• 설비공학논문집
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• 제23권11호
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• pp.718-725
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• 2011

Characteristics of flow boiling heat transfer in microchannels were investigated experimentally. The microchannels consisted of 9 parallel trapezoidal channels with each channel having 205 ${\mu}m$ of bottom width, 800 ${\mu}m$ of depth, $3.6^{\circ}$ of sidewall angle, and 7 cm of length. Tests were performed with R113 over a mass velocity range of 150~920 $kg/m^2s$, heat flux of 10~100 $kW/m^2$ and inlet pressures of 105~195 kPa. Flow boiling heat transfer coefficient in microchannels was found to be dominated by heat-flux. However the effect of mass velocity was not significant. Contrary to macrochannel trends, the heat transfer coefficient was shown to decrease with increasing thermodynamic equilibrium quality. A new correlation suitable for predicting flow boiling heat transfer coefficient was developed based on the laminar single-phase heat transfer coefficient and the nucleate boiling dominant equation. Comparison with the experimental data showed good agreement.

• International Journal of Air-Conditioning and Refrigeration
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• 제11권4호
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• pp.188-198
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• 2003

R410A and R407C are considered to be alternative refrigerants of R22 for the air-conditioners. An experimental study is carried out to investigate the effect of the change of mass flow rate on the characteristics of heat transfer and pressure drop in three row slit finned-tube heat exchanger for R407C, R410A and R22. R407C, a non-azeotropic refrigerant mixture, exhibited a quite different condensation phenomenon from those of R22 and R410A and its condensation heat transfer coefficient was much lower than that of R22 and R410A. On the other hand, the condensation heat transfer coefficient of R410A, near-azeotropic refrigerant mixture, was a little higher than that of R22. R410A also showed the lowest condensation pressure drop across the test section. For all refrigerants, the condensation heat transfer coefficient and pressure drop increase as the mass flux increases. The condensation heat transfer coefficient correlation proposed by Kedzierski shows the best agreement with the experimental data within $\pm$20%.

• 대한기계학회논문집B
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• 제37권9호
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• pp.815-821
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• 2013

고분자 전해질 연료전지(이하 PEMFC) 시스템의 효율과 수명은 유입되는 공기의 습도에 직접적인 영향을 받는다. 그러므로 공기는 정상 운전조건에서 적절한 습도를 유지시켜 주어야 한다. 하지만 가습 장치의 특성들에 대해서는 연구가 부족한 상태이다. 본 연구에서는 정상상태에서 다양한 입구조건에 따른 막 가습기의 수분전달 특성을 알아보기 위해 실험을 수행하였다. 실험에 이용할 평판형 막 가습기를 제작하였으며, 실험에 적합한 환경을 조성하였다. 우선 일정한 온도 조건하에서 막을 통과하는 수분 전달 능력을 실험하였고 이후 다양한 입구 조건에 따른 수분 전달 특성을 알아보았다. 본 실험에서 사용된 입구조건의 변수는 건공기와 습공기의 유량, 작동온도, 작동압력 및 유동배열이 있으며 각각의 입구 조건이 가습기의 성능에 미치는 영향을 논의하였다.

• 한국소음진동공학회논문집
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• 제25권12호
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• pp.822-829
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• 2015

A transfer matrix method has been developed to determine the more accurate natural frequencies for the bending vibration of Bernoulli-Euler beam with linearly reduced width and a concentrated tip mass. The proposed method can be computed an infinite number of the natural frequencies using a single element. Using the differential equation, shear force, and bending moment in which can be deduced by the diverse variational principles, a transfer matrix is formulated. The roots of the differential equation are computed by the Frobenius method. The effect of the concentrated mass for the natural frequencies of width-tapered beams is examined through a parametric study, and to show the accuracy of the proposed method, the computed results compared with those obtained from commercial finite element analysis program(ANSYS).

• 대한기계학회논문집
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• 제18권10호
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• pp.2675-2685
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• 1994

A study has been conducted to provide the experimental information for the velocity and temperature profiles of steam-air mixutre and to investigate their roles on the film condensation with wavy interface. Saturated gas mixture of steam-air was made to flow through the nearly horizontal$(4.1^{\circ})$ square duct of 0.1m width and 1.56m length at atmospheric pressure, and was condensated on the bottom cold plate. The air mass fraction in the gas mixture was changed from zero(W =0, pure steam) to one(W =1, pure air), and the bulk velocity was varied from 2 to 4 m/s. Water film was injected concurrently to investigate the effect of wavy interface on the condensation. The velocity and temperature profiles were measured by LDA system and thermocouples along the three parameters ; air mass fraction, mixture velocity and film flow rate. The profiles moved toward the interface with increasing steam mass fraction, mixture velocity and film flow rate. The Prandtl and Schmidt numbers were near one in the present experimental range, however there was no complete similarity between the velocity and temperature profiles of gas mixture. And the heat transfer characteristics and interfacial structure were coupled with each other.

• 한국유체기계학회 논문집
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• 제16권6호
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• pp.19-25
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• 2013

The effect of tip gap height on heat/mass transfer characteristics on the floor of cavity squealer tip has been investigated in a turbine cascade for power generation by employing the naphthalene sublimation technique. The squealer rim height is chosen to be an optimal one of $h_{st}/c$ = 5.51% for the tip gap height-to-chord ratios of h/c = 1.0, 2.0, 3.0 and 4.0%. The results show that heat transfer on the cavity floor is strongly dependent upon the behavior of the cavity flow falling down onto the floor. For lower h/c, the floor heat transfer is influenced by the tip leakage flow falling down along the inner face of the suction-side squealer, whereas the floor heat transfer for higher h/c is augmented mainly due to the impingement of leakage flow on the floor near the leading edge. Compared to the plane tip surface heat transfer, the cavity floor heat transfer is less influenced by h/c. For h/c = 1.0%, the average thermal load is as low as a half of the plane tip surface one, and the difference in the thermal load between the two cases tends to decrease with increasing h/c.