• 한국신재생에너지학회:학술대회논문집
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• 2010.06a
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• pp.172.1-172.1
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• 2010

In Jeju, underground air is used for heating greenhouse and fertilizing natural $CO_2$ gas by suppling directly into greenhouse. But greenhouse heating method by direct supply of underground air has several problems as like low temperature below $20^{\circ}C$ or high relative humidity over 90%. The underground air is inadequate in heating of crops such as mangos, oranges with the growing temperature over $20^{\circ}C$. Also if the relative humidity of greenhouse is kept with over 90%, diseases can strike almost of the crops. And also the ventilation loss becomes larger because the air pressure of inside greenhouse by direct supply of underground air is higher. In this study the heat pump system using underground air as heat source was developed and heating performance of the system was analyzed. Heating COP of the system was 2.5~5.0 and rejecting heat into greenhouse and extracting heat from underground air in this heat pump system were 46.5~31.4 kW, 34.9~20.9 kW respectively.

• Journal of Biosystems Engineering
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• v.26 no.2
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• pp.155-162
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• 2001

The greenhouse heating system with heat pump was built for development of simulation model and validation. The computer simulation model for the system to predict temperature of air and soil and moisture content of soil in the greenhouse were developed, and its validity was justified by actual data. From the analysis of experimentally measured data and the simulation output, following results were obtained. 1. The expected values of inside air temperature for the heating system with heat pump were very much close to the experimental values. 2. In the heating system with heat pump, the expected values of day time surface temperature of soil by computer simulation were very much similar to the measured values, but those of night time were higher than the measured value by at most 2.0$\^{C}$. 3. The simulation model predicted temperature of greenhouse film as of 1$\^{C}$ below than the mean value of ambient air and greenhouse air temperature. 4. Heat loss value of daytime was found to be larger than that of nigh as much as 1.3 to 2.3 times for the heating system with heat pump. 5. In the heating system with heat pump, when the lowest ambient temperature was -8$\^{C}$∼-7$\^{C}$ the air temperature of greenhouse was 5$\^{C}$∼6$\^{C}$, thus the heat pump heating system contributed in greenhouse heating by 13$\^{C}$.

• 한국신재생에너지학회:학술대회논문집
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• 2009.06a
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• pp.422-425
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• 2009

A sea water source cascade heat pump was designed and tested in this study. The system was designed to perform a single stage operation in summer, as well as a cascade operation in winter to ensure the high temperature lift. A steady-state simulation model was developed to analyze and optimize its performance. The simulation results show that the R717 exhibits best performance among combinations considered in this study. A R410A also exhibits the highest performance among HFCs with the smallest compressor displacement. A 15-RT R410A-R134a pilot system was installed in the 5-story commercial building at Samcheok City by the East Sea. A scroll type R410A compressor, a reciprocating type R134a compressor, plate type condenser/evaporator/ cascade heat exchanger and two electronic expansion valves were used to build a pilot. A titanium plate type heat exchanger is also used for the heat exchanging with a sea water. The heat source/sink water is supplied from the well below the seashore in the depth of 5 m. In the initial test of the system, supply water temperature was rising up to $67^{\circ}C$ using a sea water heat source of $9^{\circ}C$, while an ambient temperature was $4.5^{\circ}C$.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.1491-1498
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• 2003

Droplet evaporation can be used to transfer large amounts of energy since heat is transferred across a thin liquid film. Spreading the drop over a larger area can enhance this heat transfer. One method of accomplishing this is to dissolve gas into the liquid. When the drop strikes the surface, a gas bubble nucleates and can grow and merge within the liquid, resulting in an increase in the droplet diameter. In this study, time and space resolved heat transfer characteristics for a single droplet striking a heated surface were experimentally investigated. The local wall heat flux and temperature measurements were provided by a novel experimental technique in which 96 individually controlled heaters were used to map the heat transfer coefficient contour on the surface. A high-speed digital video camera was used to simultaneously record images of the drop from below. The measurements to date indicate that significantly smaller droplet evaporation times can be achieved. The splat diameter was observed to increase with time just after the initial transient dies out due to the growth of the bubble, in contrast to a monotonically decreasing splat diameter for the case of no bubbles. Bursting of the bubble corresponded to a sudden decrease in droplet heat transfer.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.7
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• pp.642-648
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• 2000

Falling film heat transfer analyses with aqueous lithium bromide solution were peformed to investigate the transfer characteristics of the copper tubes. Finned(knurled) tube and a smooth tube were selected as test specimens. Averaged generation fluxes of water and the heat transfer performances(heat flux, heat transfer coefficient) were obtained. The results of this work were compared with the data reported previously. As the film flow rate of the solution increased, the generation fluxes of water decreased for both tubes. The reason is estimated by the fact that the heat transfer resistance with the film thickness increased as the film flow rate increased. The effect of the enlarged surface area at the knurled tube was supposed to be dominant at a small flow rate. The generation fluxes of water increased with the increasing degree of tube wall superheat. Nucleate boiling is supposed to occur at a wall superheat of 20 K for a smooth tube, and at 10 K for a knurled tube. The heat transfer performance of the falling film was superior to pool boiling at a low wall superheat below 10 K for both tubes tested. The knurled tube geometry showed good performance than the smooth tube, and the increased performance was mainly came from the effect of the increased heating surface area.

• Journal of Advanced Marine Engineering and Technology
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• v.21 no.5
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• pp.564-570
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• 1997

The most important factor for the desalination system is the fresh water production cost dependent upon the possible energy source which should be obtained easily and with low price. Recently in Korea the demand of LNG, as a cheap and clean energy which does not cause an environmental problem, has sharply been increased. In general, LNG is storaged in a tank as a liquid state below -162 'C. When it is serviced, however, the LNG absorbs energy from a heating source and transforms to the gaseous state with high pressure. During this process a huge amount of cold energy accumulated in LNG is wasted. This waste cold energy can be utilized for producing fresh water from sea water using a sea water freezing desalination system. In order to develop a sea water freezing desalination system and to establish its design technique, a qualitative and quantitative data regarding the freezing behavior of sea water is needed in advance. The goal of this study, therefore, are to reveal the freezing mechanism of sea water, to measure the freezing rate, and to investigate the freezing heat-transfer characteristics. The experimental results help to provide a general understanding of the sea water freezing behavior in a Rectangular vessel cooled from below.

• Journal of the Korean Society for Heat Treatment
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• v.19 no.1
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• pp.3-9
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• 2006

The phase changes, nitride precipitation and hardness variations of 14%Cr-6.7Ni-0.65Mo-0.26Nb-0.05V-0.03C super martensitic stainless steel were investigated after nitrogen permeation heat treatment at a temperature range between $1050^{\circ}C$ and $1150^{\circ}C$. The nitrogen-permeated surface layer was transformed into austenite. The rectangular type NbN, NbCrN precipitates and fine round type precipitate were coexisted in the surface austenite layer, while the interior region that was free from nitrogen permeation kept the martensitic phase. The hardness of surface austenite showed 280 Hv, while the interior region of martensite phase represented 340 Hv. When tempering the nitrogen-permeated steel at $450^{\circ}C$, a maximum hardness of 433 Hv was appeared, probably this is attributed to the secondary hardening effect of the precipitates. The nitrogen concentration decreased gradually with increasing depth below the surface after showing a maximum of 0.3% at the outmost surface. The strong affinity between nitrogen and Cr enabled the substitutional element Cr to move from interiors to the surface when nitrogen diffuse form surface to the interior. Corrosion resistance of nitrogen permeated steel was superior to that of solution-anneaed steel in the solution of 1N $H_2SO_4$.

• Journal of the Korean Society for Heat Treatment
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• v.33 no.5
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• pp.232-238
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• 2020

Heating experiments using the 7075 aluminum alloy in the state of billet and extrudate have been performed to investigate the pertinent ranges of working temperatures and holding times for the application to the various automobile parts. The 7075 specimens from raw billet of 152 mm in diameter and 400 mm in length prior to extrusion were used for heating with a holding time of 10 minutes at temperatures between 380℃ and 550℃. Then, an extrusion process using the billet has been fulfilled at 380℃ with extrusion speed of 0.8 mm/min to get an plate-type extrudate of 75 mm in width and 4.2 mm in thickness. The samples from the extrudate were subjected to heating experiments at temperatures between 380℃ and 440℃ with holding times such as 10 min, 30 min, 60 min and 120 min at each heating temperature. The microstructures were investigated on the optical and EBSD micrographs. The hardness measurement and the tensile test have been performed to investigate the effect of the heat treatment on the mechanical property. The results showed for the 7075 extrusion process that the safe heating of billet can be performed below 450℃ and the extrusion can be done safely up to 400℃.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.2
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• pp.109-120
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• 1997

Microstructure and mechanical properties of Fe-Si-Mn-P high strength steel sheet have been investigated by controlling the cooling rate. Bainite and ferrite were obtatined by annealing in the ferrite pluse austenite region, and ferrite and austenite were obtatined after annealing in the fully austenite region. Ferrite and pearlite were obtained when the cooling rate was controlled from the annealing temperature above $760^{\circ}C$ and bainite showed with increasing cooling rate, however below $760^{\circ}C$ ferrite and bainite were obtained. Tensile strengths and hardness nearly unchanged with increasing cooling rate after control the cooling rate from the temperature above $760^{\circ}C$, while tensile strengths increased and elongation decreased with increasing cooling rate when the cooling rate was controlled from the tempeature below $760^{\circ}C$. Without regard to annealing temperature, tensile strength increased and elongation decreased with increasing cooling rate. Tensile strengths and elongation values heat treated in the ferrite plus austenite region were higher than those in the fully austenite region. Retained austenite and strength-elongation balance showed the maximum value at $780^{\circ}C$ and decreased with increasing annealing temperature. Strength-elongation balance value was controlled by the retained austenite.

• Journal of computational fluids engineering
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• v.6 no.1
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• pp.1-13
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• 2001

The natural convection heat transfer and solidification in a stratified pool are studied. The flow and heat transfer characteristics in a heat generating pool are compared between single-layered and double-layered pools. And local Nusselt number distributions on outer walls are obtained to consider thermal loads on a vessel wall. The cooling and solidification of Al₂O₃/Fe melt in a hemispherical vessel are simulated to study the mechanism of heat transfer and temperature distribution. A unstructured mesh is chosen for this study because of the non-orthogonality originated from the boundaries of double-layered pool. Interface between the layers is modeled to be fixed. With this assumption mass flux across the interface is neglected, but shear force and heat flux are considered by boundary conditions. The colocated cell-centered finite volume method is used with the Rhie-Chow interpolation to compute cell face velocity. To prevent non-physical solutions near walls in case body force is large the wall pressure is extrapolated by the way to include body force. The numerical solutions calculated by current method show that averaged downward heat flux of the double-layered pool increases compared to single-layered pool and maximum temperature occurs right below the interface of the layers.