• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.6
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• pp.403-410
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• 2009

This paper proposes a new approach to find the optimum ratios between sizes of the heat exchangers of the heat recovery steam generator (HRSG) system with limited size to maximize the efficiency of the steam turbine (bottom) cycle of combined cycle power plants (CCPP), but without performing the bottom cycle analysis. This could be achieved by minimizing the unavailable exergy (the sum of the destroyed and the lost exergies) resulted from the heat transfer process of the HRSG system. The present approach is relatively simple and straightforward because the process of the trial-and-error method, typical in performing the bottom cycle analysis for the system optimization, could be avoided. To demonstrate the usefulness of the present method, a single-stage HRSG system was chosen and the optimum evaporation temperature was obtained corresponding to the condition of the maximum useful work. The results show that the optimum evaporation temperature based on the present exergy analysis appears similar to that based on the bottom cycle analysis. Also shown is the dependency of size (NTU) ratios between the heat exchangers on the inlet gas temperature, which is another important factor in determining the optimum condition once overall size of the heat recovery steam generator is given. The present approach turned out to be a useful tool for optimization of the singlestage HRSG systems and can easily be extended to multi-stage systems.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.4
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• pp.395-404
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• 2013

In this study, R-410A evaporation heat transfer tests were conducted in flattened tubes made from 5-mm round tubes. The test range covered a saturation temperature of $15^{\circ}C$, heat flux of $5{\sim}15kW/m^2K$, and mass flux of $200-400kg/m^2s$. The results showed that both the condensation heat transfer coefficient and the pressure drop increased as the aspect ratio increased, with a pronounced increase for an aspect ratio of 4. A comparison of the flattened tube data with existing correlations revealed that the heat transfer coefficients were reasonably predicted by the Shah correlation, and the pressure drops were reasonably predicted by the Jung and Radermacher correlation.

• YAKHAK HOEJI
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• v.3 no.1
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• pp.26-34
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• 1957

The solar evaporation method is one of the most important and popular salt manufecturing method in Korea. The rate of evaporation of sea water depends on a complex climate factors. Of these factors, the most important is solar radiation and in particular the extent to which it is absorbed in the brine. By the addition of suitable dyestuff, a further increase in absorption is obtained and can result in all the radiation entering the brine being made available as heat. "Solivap Green", one of several dyestuffs which have been suggested for accelerating solar evaporation, was tested in this experiment. The results of the experiment. 1. Increase the evaporation rate of brine up to 20-25%. 2. Elevate the temperature of brine 2-$4^{\circ}C$. higher than that of brine adding no dyestuff. 3. Optimum dyestuff concentration is 25-30 mg/L and allowable maximum concentration can not exceeded 1000 mg/$m^2$ (50 mg/L). 4. Addition of dyestuff does not cause the degradation of salt produced. 5. A conversion table which indicates the concentrations for various depths of brine was prepared for engineering purpose. 6. Absorption spectrum of the dyestuff was studies, but toxicological and structural studies for the dyestuff have not been done in this experiment.

• Journal of ILASS-Korea
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• v.25 no.2
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• pp.68-73
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• 2020

In the present study, experiments on the film boiling of liquid droplets on oxidized copper surface was conducted. The shape of pure water droplets was observed, and the evaporation rate of them was measured during the film boiling evaporation process. The droplet of initial volume 16 ~ 30 µl was applied onto the oxidized copper surface heated up to 300 ~ 500℃, then the shape of the droplet was analyzed during the film boiling evaporation. Experimental results showed that there was good correlation between dimensionless volume and dimensionless time. However, a significant difference in evaporation rate for small and large droplets discussed in previous study was not found.

• Water for future
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• v.6 no.1
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• pp.5-12
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• 1973

A simple regression formula for estimating the lake evaporation rate from the copper-plated pan (diameter 20cm, height 10cm) is derived. A comparison with other formulae indicates that the formula is more accurate than others. An annual map of man evaporation in Korean peninsula has been prepared using the relation. It demonstrates the areal average distribution of mean annual evaporation from a free water surface with no heat storage effect and avected energy owing to differences in the temperature of in-and outflow. The mean annual ratio of the lake to the copper-plated pan evaporation is found to be 0.64. The ratio varies with local conditions from 0.62 to 0.66, and hence it can be considered fairly uniform. However the seasonal variation of the ratio appears to be rather significant. It changes from the lowest of 0.61 to the highest of 0.75.

• Proceedings of the SAREK Conference
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• 2005.11a
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• pp.335-340
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• 2005

Nonlinear and forced oscillations of supported viscous droplet were focused in the present study. The droplet has a free contact line with solid plate and inviscid fluid. Natural frequencies of a pendant droplet have been investigated experimentally by imposing the acoustic wave while the frequency is being increased at a fixed amplitude. The evaporation was observed at atmosphere pressure. The droplet was recorded throughout the entire evaporation process and transient variations of the volume was measured. The evaporation process of oscillating droplet with thermofoil has been also observed to investigate analyzing the resonance effect on the thermal characteristics of droplet. It is found that a pendant droplet shows the resonant behaviors at each mode similar to the theoretical analysis. During imposing the acoustic wave, the pendant droplet makes a rotating motion in its longitudinal axis which is a new shape oscillation mode. The evaporation rate of a pendant droplet at resonant frequency is significantly enhanced.

• Journal of Power System Engineering
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• v.16 no.2
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• pp.24-29
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• 2012

In this paper, an analysis on performance and exergy of R404A refrigeration system using R744 secondary refrigerant was performed numerically to optimize the design for the operating parameters. The operating parameters considered in this study include subcooling and superheating degree, internal heat exchanger and compression efficiency, evaporation and condensation temperature in the R404A refrigeration cycle and temperature difference of cascade heat exchanger. The main results are summarized as follows : The COP(coefficient of performance) of R404A refrigeration system increases with increasing evaporation temperature. The evaporation capacity of R744 as secondary refrigerant increases with the increase in evaporation pressure of R744 secondary refrigeration. And the enthalpy in the evaporator outlet of R744 increases with the increasing evaporation pressure of R744 secondary refrigeration. Therefore, it is important to analysis for the relationship between COP of R404A refrigeration system and refrigeration capacity of R744. As cascade evaporation temperature increase, the exergy loss of condenser and compressor using R404A is the largest among all components. Therefore, the exergy loss in the condenser and compressor using R404A must be decreased to enhance the COP of R404A refrigeration system with R744 secondary refrigerant.

• Journal of Mechanical Science and Technology
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• v.18 no.3
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• pp.513-525
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• 2004

This paper reports an experimental study on flow boiling of pure refrigerants R l34a and R l23 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10㎜ located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5-50㎾/㎡, vapor quality 0-100 percent and mass velocity of 150-600㎏/㎡s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen's superposition model, a new correlation is presented for heat transfer coefficients of mixture.

• Proceedings of the SAREK Conference
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• 2005.11a
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• pp.268-273
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• 2005

Because of environmental issues caused by CFC, HCFC or HFC refrigerants, new alternative refrigerants has gained a significant attention. This paper presents experimental information on heat transfer coefficient and pressure drop behavior during evaporation process of R32/290 mixtures in a horizontal smooth tube. A smooth tube with outer diameter of 5 mm and length of 5 m was selected as a test tube. Heat transfer coefficients and pressure drop characteristics were measured for a range of mass fluxes from 497 to 994 $kg/m^2s$, heat fluxes from 12 to 20 $kW/m^2$ and for several mixture compositions(100/0, 75/25, 58.4/41.6, 2s/75, 100/0 by wt% of R32/290). The differences of measured heat transfer characteristics among various R32/290 refrigerant mixtures were analyzed for various compositions.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.21 no.4
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• pp.243-251
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• 2009

Refrigerant mixtures provide an opportunity to adjust their properties to fit design criteria and a possibility to create new blends that can improve heat transfer characteristics. Therefore, mixture of $CO_2$ and propane is chosen which may be a promising refrigerant and has good environmental compatibility. This paper presents measured heat transfer coefficient data during evaporation process of $CO_2$/propane mixtures flowing upward in vertical smooth and. micro-fin tubes. Smooth and micro-fin tubes with outer diameters of 5 mm and length of 1.44in were selected as test tubes. The tests were conducted at mass fluxes of 212 to $656kg/m^{2}s$, inlet temperatures of -10 to $30^{\circ}C$, heat fluxes of 15 to $60\;kW/m^2$ and for several compositions (75/25, 50/50, 25/75 wt%). Among $CO_2$/propane refrigerant mixtures, the heat transfer characteristics are much better than that of any compositions when the composition is 75/25 (wt%).