• Journal of Advanced Marine Engineering and Technology
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• 제31권1호
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• pp.74-83
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• 2007

This paper presents the experimental results of evaporation heat transfer coefficients of HC refrigerants (e.g. R290 and R600a). R-22 as a HCFCs refrigerant and R-l34a as a HFCs refrigerant in horizontal double pipe heat exchangers, having four different inner diameters of 10.07, 7.73, 6.54 and 5.80 mm respectively. The experiments of the evaporation process were conducted at mass flux of $35.5{\sim}210.4 kg/m^2s$ and cooling capacity of $0.95{\sim}10.1 kW$. The main results were summarized as follows : The average evaporation heat transfer coefficient of hydrocarbon refrigerants(R-290 and R-600a) was higher than the refrigerants, R-22 and R-l34a. In comparison with R-22 the evaporation heat transfer coefficient of R-l34a is approximately $-11{\sim}8.1 %$ higher. R-290 is $56.7{\sim}70.1 %$ higher and R-600a is $46.9{\sim}59.7 %$ higher. respectively. In comparison with experimental data and some correlations, the evaporation heat transfer coefficients are well predicted with the Kandlikar's correlation regardless of a type of refrigerants and tube diameters.

• International Journal of Air-Conditioning and Refrigeration
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• 제14권4호
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• pp.156-165
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• 2006

Convective boiling heat transfer coefficients were measured in a horizontal minichannel with R-l34a. The test section was made of stainless steel tube with an inner diameter of 3.0 mm and a length of 2m. It was uniformly heated by applying electric current directly to the tube. Local heat transfer coefficients were obtained for heat fluxes from 10 to $40kW/m^2$, mass fluxes from 200 to $600kgT/m^2s$, qualities up to 1.0, and the inlet saturation temperature of $10^{\circ}C$. The experimental results were mapped on Wojtan et $al.'s^(7)$ and Wang et $al.'s^(8)$ flow pattern maps. The nucleate boiling was predominant at low vapor quality whereas the convective boiling was predominant at high vapor quality. Laminar flow appeared in the flow with minichannel. The experimental results were compared with six existing two-phase heat transfer coefficient correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants was developed with mean and average deviations of 10.39% and -3.66%, respectively.

• 한국항공우주학회지
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• 제44권12호
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• pp.1062-1070
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• 2016

본 연구에서는 전산해석을 통해 냉각 터빈 블레이드 팁 간극의 유동 및 열전달 특성을 연구하였다. 1단 고압터빈 노즐 출구에서 획득한 속도, 온도 프로파일을 로터의 입구에 적용하여 로터 도메인을 대상으로 전산해석을 하였다. 스퀼러 팁이 적용된 블레이드의 팁 간극을 스팬의 1%부터 2.5%로 조절하여 팁 간극의 공력 손실, 열전달 계수와 막냉각 효율의 영향을 고찰했다. 팁 간극이 커질수록 출구에서 공력 손실과 블레이드 끝단 표면에서 열전달 계수는 증가하였다. 특히 팁 간극이 스팬의 2%일 때 평균 열전달 계수가 급격히 증가하였다. 팁 영역의 막냉각 효율은 팁 간극이 작을수록 높았고, 캐비티 내부 냉각 홀 근처의 막냉각 효율이 높았다.

• 수산해양기술연구
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• 제30권1호
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• pp.33-44
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• 1994

This work studies for boiling and condensation heat transfer performance of trapezoidally shaped integral-fin tubes having fin densities from 748fpm to 1654fpm. For comparison, tests are made using a plain tube having the same inside and outside diameter as that of the root of fins of finned tubes. Hahne's theoretical model and Webb's theoretical model are used to predict the R-11 boiling heat transfer coefficient and condensing heat transfer coefficient respectively for plain tube and all integral-fin tubes. Experiments are carried out using R-11 as working fluid. This work is limited to film-wise condensation and pool boiling on the outside surface of plain tube and 4 low integral-fin tubes. In case of condensation, the refrigerant condenses at saturation state of 32$^{\circ}C$ on the outside tube surface cooled by coolant and in case of boiling. the refrigerant evaporates at saturation state of 1bar on the outside tube surface. The amount of non-con-densable gases in the test loop is reduced to a negligible value by repeated purging. The actual boiling and condensing processes occur on the outside tube surfaces. Hence the nature of this surface geometry affects the heat transfer performances of condenser and evaporator in refrigerating system. The condensation heat transfer coefficient of integral-fin tube is enhanced by both extended tube surface area and surface tension. The ratio of the condensation heat transfer coefficients of finned to plain tubes is greater than that of surface area of finned to plain tubes, while ratio of the boiling heat transfer coefficient of finned to plain tubes shows reverse result. As a result, low integral-fin tube can be used in condenser more effectively than used in evaporator.

• 대한기계학회논문집B
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• 제31권4호
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• pp.335-340
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• 2007

A new correlation between the Nusselt number based on modified heat transfer coefficient and Reynold number based on droplet-flow-rate was developed for the experimental data. The modified heat transfer coefficient was defined as ratio of wall heat flux to droplet subcooling. In the previous reports, the local heat flux of spray cooling in the film boiling region was experimentally investigated for the water spray region of $D_{max} = 0.0007{\sim}0.03m^3/(m^2s)$ . In the region near the stagnation point of spray flow, a new heat transfer correlation is recommended which shows good predictions for the water spray region of $D_x{\le}0.01m^3/(m^2s)$.

• 대한기계학회논문집
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• 제13권3호
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• pp.528-537
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• 1989

The objective of this paper is to investigate effects of the specific heat and the diameter of suspending particles on the heat transfer coefficient of two phase turbulent flow with suspension of solid particles in a circular tube with constant heat flux. Heat transfer coefficients of two phase turbulent flow in pipe with suspension of graphite powder were measured with variations of particle sizes and solid-gas loading ratio. Measured data were compared with predictions by numerical analysis in which the turbulece models are closed on the first order level. Results show that heat transfer coefficient increases with increasing the solid-gas loading ratio and the specific heat of suspending material, however, it decreases as the average diameter of particles decreases below $24{\mu}m$.

• 한국기계가공학회지
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• 제20권6호
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• pp.82-91
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• 2021

We compared the heat transfer characteristics of the parallel and the counterflow flow in the concentric double tube of the Al₂O₃/water nanofluids using numerical methods. The high- and low-temperature fluids flow through the inner circular tube and the annular tube, respectively. The heat transfer characteristics according to the flow direction were compared by changing the volume flow rate and the volume concentration of the nanoparticles. The results showed that the heat transfer rate and overall heat transfer coefficient improved compared to those of basic fluid with increasing the volume and flow rate of nanoparticles. When the inflow rate was small, the heat transfer performance of the counterflow was about 22% better than the parallel flow. As the inflow rate was increased, the parallel flow and the counterflow had similar heat transfer rates. In addition, the effectiveness of the counterflow increased from 10% to 22% rather than the parallel flow. However, we verified that the increment in the friction factor of the counterflow is not large compared to the increment in the heat transfer rate.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 춘계학술대회논문집
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• pp.299-302
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• 2003

The interface heat transfer coefficient was measured for non-isothermal bulk forming of Ti-6Al-4V. FE analysis and experiments were conducted. Equipment consisting of AISI H13 die was instrumented with thermocouples located at sub-surface of the bottom die. Die temperature changes were investigated in related to the process variables such as reduction, lubricant and initial die temperature. The calibration approach based on heat conduction and FE analysis using an inverse algorithm were used to evaluate the interface heat transfer between graphite-lubricated die and glass-coated workpiece. The coefficients determined determined were affected mainly by the contact pressure. The validation of the coefficients was made by the comparison between experimental data and FE analysis results.

• 전기학회논문지
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• 제58권11호
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• pp.2155-2161
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• 2009

In this paper, we dealt with the electro-thermal coupling analysis for temperature prediction of power transformer. Heat transfer coefficient are calculated using Nusselt number in accordance with heat source generated from transformer windings and core materials. The calculated temperatures in power transformer were compared to those of measured ones and showed good agreement. This coupling method using heat transfer coefficient can be used at the design stage of power transformer efficiently.

• International Journal of Air-Conditioning and Refrigeration
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• 제7권
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• pp.45-54
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• 1999

In this study, condensation heat transfer experiments were conducted in two small diameter (ø17.5, ø4.0) tubes. Comparison with the existing in-tube condensation heat transfer correlations indicated that these correlations over predict the present data. For example, Akers correlation over predicted the data up to 104 %. The condensation heat transfer coefficient of the ø4.0 I.D. tube was smaller than that of the ø7.5 I.D tube; at the mass velocity of 300 kg/$m^2$s, the difference was 12 %. The pressure drop data of the small diameter tubes were highly (two to six times) over predicted by the Lockhart-Martinelli correlation. Sub-cooled forced convection heat transfer test confirmed that Gnielinski's single phase heat transfer correlation predicted the data reasonably well.