• 한국산학기술학회논문지
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• 제20권3호
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• pp.476-482
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• 2019

본 기후변화협약, 온실가스배출규제와 같은 규제적 규약들은 고효율 에너지 기자재 개발과 에너지의 효율적 이용을 촉진하는 촉매로 작용하고 있다. 에너지소비가 큰 분야 중 전열기기가 효율이 좋지 못한 편이다. 전기보일러는 히터에 물을 접촉시켜 순환시키는 방식을 주로 사용하고 있다. 기존 전기보일러를 활용할 경우 물의 미네랄이 고온의 히터와 접촉하여 탄화되어 흡착되는 과정에서 히터의 부식을 촉진시키고 히터의 효율을 저하시킨다. 이러한 이유로 에너지 효율이 높고, 전도나 대류가열보다 피가열물을 균일하게 가열시킬 수 있는 유도가열방식을 보일러에 적용시킨 전기유도보일러가 각광을 받고 있는데, 이 방식은 물이 담겨 있는 보일러 관로를 유도가열방식으로 가열하여 직접가열시 발생하는 문제를 해결시킬 수 있는 대안으로 인정받고 있다. 비교적 오랜 기간 유도가열에 대한 연구가 이루어졌음에도 불구하고 유도가열을 이용한 전열기술에 대한 연구결과는 그리 많지 않은 실정이다. 따라서 본 논문에서는 전기유도보일러의 발열효율을 개선하기 위하여 매개변수연구방법을 통하여 권선의 단면적, 권선의 수 및 권선의 겹수에 의한 영향을 유한요소법을 이용하여 해석하고 발열의 요소가 되고 있는 동손과 철손을 분석하여 발열을 극대화시킬 수 있는 권선설계방법으로 총손실이 최대가 되는 설계점을 찾는 방법을 제시하였다.

• 대한기계학회논문집B
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• 제24권1호
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• pp.124-131
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• 2000

In this study, pool boiling performance of Turbo/B-type metal-formed tubes was investigated. Tubes with three different cavity gap width(0.04 mm, 0.07 mm, 0.1 mm) were manufactured and tested using R-11, R-123 and R-134a. Tests were conducted at two different saturation temperatures $4.4^{\circ}C$ and $26.7^{\circ}C.$ Heat flux was varied from 10 kW/m2 to 50 kW/m2. It was found that optimum gap width varied for different refrigerants. For low-pressure refrigerants such as R-11 or R-123, optimum gap width was 0.07 mm. For high-pressure refrigerant R-134a, however, the optimum value was 0.1 mm. Compared with the heat transfer performance of the smooth tube, the metal-formed tubes enhanced the heat transfer coefficients significantly - 6.5 times for R-11, 6.0 times for R-123 and 5.0 times for R-134a (at $4.4^{\circ}C$ saturation temperature and 40 kW/m2 heat flux), which are comparable with the performance of foreign products. The heat transfer coefficients of R-134a were larger than those of R-11 or R-123, and they increased as the saturation temperature increased.

• 대한기계학회논문집
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• 제18권5호
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• pp.1264-1274
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• 1994

Heat transfer performance of integral-fin tube which is used in recipro turbo refrigerator or high compact heat exchangers is studied. Eight tubes with trapezoidal shaped integral-fins having fin densities from 748 to 1654 fpm and 10, 30 internal grooves are tested. A plain tube having the same(inner and outer) diameter as the fin tubes is also tested for comparison. Pool boiling heat transfer of R-11 is investigated experimentally and theoretically on single tube arrangement. The refrigerant evaporates at saturation state of 1 bar on the outside tube surface and heat is supplied by not water which circulates inside of the tube. From the result of eight fin tubes and one plain tube tested, a tube having 1299 fpm-30 grooves shows the best performance. A maximum overall heat transfer coefficient of this tube is about 4000 $W/m^{2}K$ at 2.8m/s of water velocity. The maximum heat transfer enhancement (i.e., the ratio of overall heat transfer coefficients of finned to plain tubes)is about 2.1.

• 대한기계학회논문집B
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• 제23권2호
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• pp.210-217
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• 1999

This paper reports the experimental results on heat transfer characteristics of R-22 and R-407C(HFC-32/125/134a 23/25/52 wt%) condensing inside horizontal smooth and finned tubes. The test condensers used In the study are double pipe heat exchangers of 7.5 mm ID, 9.5 mm OD smooth tube, and 60 finned micro-fin tube with 8.53 mm ID, 9.53 mm OD. Each of these tubes was 4 000 mm long tubes connected with an U-bend. These U type two-path test tubes are divided In 8 local test sections for the identification of the local condensing heat transfer characterisitcs and pressure drop, U-bend effects on condensing flows. Inlet quality is maintained 1.0, and refrigerant mass velocity is varied from 102.0 to $301.0kg/m^2{\cdot}s$. From the results, it was found that the pressure drop of the R-407C Increased, and heat transfer coefficient decreased compared to those of R-22. In comparison condensing heat transfer characteristics of micro-fm tube with those of smooth tube, increasing of condensing heat transfer coefficient was found outstanding compared to the increasing ratio of pressure drop. Furthermore, pressure drop In U-bend showed at most a 30 % compared to the total pressure drop in the test section.

• 대한기계학회논문집B
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• 제20권5호
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• pp.1725-1734
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• 1996

Experimental condensation and evaporation heat transfer coefficients were measured in a horizontal smooth tube and a horizontal micro-finned tube with HFC-134a. The test sections are straight, horizontal tubes with have a 9.52mm outside diameter and about 5000mm long. The micro-finned tube had 60 fins with a height of 0.12mm and a spiral angle of 25.deg.. The condensation test section was a double-pipe type with counter flow configuration. The evaporation test section employed an electic heating method. Enhancement factors which is defined as a ratio of the heat transfer coefficient for micro-finned tube to that for smooth tube, varied from 1.3 to 1.6(mass flux:110~190kg/m$^{2}$s) for condensation and 1.2 to 1.5 (mass flux:70~160kg/m$^{2}$s) for evaporation. The experimental data of condensation and evaporation heat transfer coefficients were compared to several empirical correlations. Based on these comparisons, modified correlations of the condensation and evaporation heat transfer coefficient for both smooth and micro-finned tubes were proposed.

• 대한기계학회논문집B
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• 제29권7호
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• pp.854-860
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• 2005

Pool boiling heat transfer characteristics of R-134a were investigated in titanium plain and low finned tubes. The diameter of test tube was 15.88 mm and the fin density was 33 fpi. Tests were conducted at saturation temperatures of $20^{\circ}C$ and $30^{\circ}C$. Heat fluxes varied from 5000 W/$m^2$ to 50,000 W/$m^2$ based on surface area of the plain tube. The pool boiling heat transfer coefficients of the titanium horizontal plain tube are lower than those of the copper plain tube by $8.2\%$. The boiling heat transfer coefficients of the low finned tube are averagely higher than those of the plain tubes by $34\%$. The average deviation of the Slipcevic correlation from the present data for the low finned tube is $20\%$.

• 대한기계학회논문집
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• 제18권5호
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• pp.1275-1287
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• 1994

Heat transfer performance improvement by fin and grooves is studied for condensation of R-11 on integral-fin tubes. Eight tubes with trapezoidal shaped integral-fins having fin densities from 748 to 1654 fpm and 10, 30 grooves are tested. A plain tube having the same diameter as the finned tubes is also tested for comparison. R-11 condenses at saturation state of $32^{\circ}C$ on the outside tube surface cooled by inside water flow. All of test data ate taken at steady state. Beatty and Katz's, Rudy's and Webb's theoretical models are used to predict the R-11 condensation coefficient of tubes having 748, 1024 and 1299 fpm. The predicted value by Betty and Katz's model is within 10% of experimental values in this study at fpm<1024 and Rudy's model predicted the experimental data at fpm>1024 within 15%. The tube having fin density of 1299 fpm and 30 grooves has the best overall heat transfer performance. This tube shows the overall heat transfer coefficient of 11500 $W/m^{2}K$,/TEX> at coolant velocity of 3.0m/s.

• 설비공학논문집
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• 제13권9호
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• pp.827-835
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• 2001

In this study, condensation heat transfer characteristics were conducted with special heat transfer tubes of SH-C type. Experiments were carried out the saturated vapor temperature of 334K and the wall subcooling of 1.5-4.5K. The refrigerant was R-113 and the enhanced tubes used in the present study were SH-CDR, SH-CYR and SH-CHR. The experimental results showed that the condensation heat transfer coefficients of SH-C type tubes were about 23-66% higher than those of a low integral-fin tube. It was visualized that the condensed liquid on the outer surface of SH-C type tubes flowed continuously down unlike a low integral-fin tube and a plain tube, due to a 3-D extending fin on the outer surface of SH-C type tubes. As a result, the thermal resistance of the condensed liquid decreased and the heat transfer coefficient increased. Also, the enhancement ratio of SH-CDR tube was the highest, and it was about 9-11 times as compared to that of a plain tube.

• 설비공학논문집
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• 제11권6호
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• pp.808-814
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• 1999

Pool boiling performance of a metal-formed enhanced tube for a flooded refrigerant evaporator was experimentally investigated. Tests were performed for three different refrigerants(R-11, R-123, R-l34a), at two different saturation temperatures $4.4^{\circ}C \;and \;26.7^{\circ}C$ .Heat flux was varied from 10㎾/$m^2\;to\ 50㎾/$m^2$. Compared with the heat transfer coefficients of the smooth tube, the heat transfer coefficients of the enhanced tube were 6.6 times higher for R-11, 6.0 tines higher for R-123 and 3.5 times higher for R-l34a. The enhancements are comparable with those of foreign products. The heat transfer coefficients of R-l34a were higher than those of R-11 and R-123, either for the enhanced tube or for the smooth tube. At $4.4^{\circ}Csaturation temperature, however, the heat transfer coefficients of R-l34a were approximately the same as those of R-11, The effect of the saturation pressure on the boiling performance was similar to that of the smooth tube - the heat transfer coefficient increases as the saturation pressure increases.

• 설비공학논문집
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• 제18권4호
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• pp.297-303
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• 2006

Friction and heat transfer coefficients were measured inside the corrugated tube using water as the working fluid. The test is performed for 16 tubes which outer diameter of tubes are 12.7 mm. These specifications are 4 indentation depths and 4 indentation pitches, respectively. The range of the water velocity inside the tube is from 0.5 to 3.0 m/s (8,500