• Title, Summary, Keyword: Heat Transfer Coefficient

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A Study on the Greenhouse Water Curtain System: Heat Transfer Characteristics

  • 손원명;한길영
    • Magazine of the Korean Society of Agricultural Engineers
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    • v.32 no.E
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    • pp.80-87
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    • 1990
  • Energy balance equations Were developed to describe the heat transfer mechanisms in a double layer plastic greenhouse with a water curtain system. Heat transfer variables were determined by using various temperature data measured in a conventional prototype semicircular cross-section greenhouse over a range of water temperatures and water flow rates. The heat transfer coefficient between flowing water and greenhouse air was independent of water flow rates. But the heat transfer coefficient between water surface and the stagnant air space within the double plastic layer was dependent on water flow rates. Substituting the heat transfer coefficients, determined from the energy balance equations in the heat transfer equations, demonstrated various relationships among ambient air temperature, greenhouse air temperature, water temperature, and water flow rates. The heating benefits were linearly related to not only the inside and outside air temperatures but also to the water temperature. The energy conservation effects of the water curtain system were found even initial water temperatures were considerably lower than the greenhouse setting temperatures. Sensitivity analysis for heat transfer coefficients demonstrated that the heat transfer coefficient between greenhouse air and the stagnant air within the plastic layers was the most significant coefficient in the estimation of heating effects.

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Numerical Analysis on the Thermal and Fluid in Air Conditioning Duct for Marine Offshore (해양 구조물용 공조덕트 열유동에 관한 수치해석)

  • Yi, Chung-Seob;Lee, Byung-Ho;Chin, Do-Hun
    • Journal of the Korean Society of Manufacturing Process Engineers
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    • v.17 no.5
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    • pp.23-29
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    • 2018
  • This study is about the distribution of heat transfer in air conditioning ducts used for marine vessels and oil drilling platforms. As the convective heat transfer coefficient increased, heat transfer was conducted dynamically to inside as it exited to the outlet of duct. The experiment was to determine if the amount of heat transfer generated at the duct exit increased as the convective heat transfer coefficient increased. When the convective heat transfer coefficient was low, the temperature of the duct showed a relatively high temperature difference between the outside and inside of the duct due to the temperature influence of the internal fluid. In case of temperature distribution generated the volume of the duct along the change of the convective heat transfer coefficient, the temperature descended as heat transfer was promoted and the convective heat transfer coefficient increased.

Boiling Heat Transfer of Ammonia inside Horizontal Smooth Small Tube (수평미세관내 NH3 비등열전달 특성)

  • Choi, Kwang-Il;Oh, Jong-Taek
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.25 no.2
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    • pp.101-108
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    • 2013
  • This paper is presented an experimental study of flow boiling heat transfer characteristics of ammonia, and is focused on pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal small tube with inner diameter of 3.0 mm and length of 2000 mm. The direct heating method is applied for supplying heat to the refrigerant, where the test tube is uniformly heated by electric current. The local heat transfer coefficients were obtained over a heat flux range of 20 to $80kW/m^2$, a mass flux range of 50 to $500kg/m^2s$, a saturation temperature range of 0 to $10^{\circ}C$, and quality up to 1.0. The pressure drops increase with increasing mass flux and heat flux, and with decreasing saturation temperature. The heat transfer coefficients increase with increasing mass flux and saturation temperature in middle and high quality region. And the local heat transfer coefficient increase with increasing heat flux in low quality region. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coefficient correlation. A new boiling heat transfer coefficient correlation based on the superposition model for ammonia in small tubes is developed average deviation of -0.17% and mean deviation of 10.85%.

Optimal design of a micro evaporator to maximize heat transfer coefficient (열전달 계수 최대화를 위한 마이크로 증발기의 최적 설계)

  • Sung, Tai-Jong;Oh, Dae-Sik;Seo, Tae-Won;Kim, Jong-Won
    • Proceedings of the KSME Conference
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    • pp.2097-2101
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    • 2007
  • This paper presents an optimal design of a micro evaporator which maximizes the heat transfer coefficient. Number of gaps, spanwise distance and streamwise distance are selected as the geometric design parameters. Mass flow rate of the refrigerant is selected as the non-geometric design parameter. Temperature at the surface of the heater is measured to valuate the heat transfer coefficient. Nine experiments are conducted using $L_9(3^4)$ orthogonal array. Maximum heat transfer coefficient is 640 W/$m^2K$ at the parameters of 2 gaps, 0.2 mm spanwise distance, 1.0 mm streamwise distance and 0.72 g/s mass flow rate. Among the 3 geometric parameters, the spanwise distance is the most sensitive parameter influencing the heat transfer coefficient. We conduct a second stage of experiment to increase the heat transfer coefficient by reselecting the mass flow rate. We concluded that 0.87 g/s is the optimized flow rate for an active micro cooler resulting in a heat transfer coefficient of 651 W/$m^2K$.

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Heat and mass transfer analysis in air gap membrane distillation process for desalination

  • Pangarkar, Bhausaheb L.;Sane, Mukund G.
    • Membrane Water Treatment
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    • v.2 no.3
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    • pp.159-173
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    • 2011
  • The air gap membrane distillation (AGMD) process was applied for water desalination. The main objective of the present work was to study the heat and mass transfer mechanism of the process. The experiments were performed on a flat sheet module using aqueous NaCl solutions as a feed. The membrane employed was hydrophobic PTFE of pore size 0.22 ${\mu}m$. A mathematical model is proposed to evaluate the membrane mass transfer coefficient, thermal boundary layers' heat transfer coefficients, membrane / liquid interface temperatures and the temperature polarization coefficients. The mass transfer model was validated by the experimentally and fitted well with the combined Knudsen and molecular diffusion mechanism. The mass transfer coefficient increased with an increase in feed bulk temperature. The experimental parameters such as, feed temperature, 313 to 333 K, feed velocity, 0.8 to 1.8 m/s (turbulent flow region) were analyzed. The permeation fluxes increased with feed temperature and velocity. The effect of feed bulk temperature on the boundary layers' heat transfer coefficients was shown and fairly discussed. The temperature polarization coefficient increased with feed velocity and decreased with temperature. The values obtained were 0.56 to 0.82, indicating the effective heat transfer of the system. The fouling was observed during the 90 h experimental run in the application of natural ground water and seawater. The time dependent fouling resistance can be added in the total transport resistance.

A Study on Prediction Model of Heat Transfer Coefficient in the Circulating Fluidized Bed Heat Exchanger with Multiple Vertical Tubes (다관형 고밀도 순환유동층 열교환기의 열전달계수에 대한 예측모델 연구)

  • Park, Sang-Il
    • Proceedings of the SAREK Conference
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    • pp.288-293
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    • 2005
  • The pressure distribution and heat transfer coefficient were measured at room temperature in the high suspension density CFB heat exchanger with multiple vertical tubes and the effective density of CFB was determined. The theoretical model for predicting heat transfer coefficient was developed in this study. The model predictions were compared with the measured heat transfer coefficient to show relatively good agreement between them.

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A Study on Heat Transfer Characteristics of Helical Coiled Tube (나선코일의 열전달 특성에 관한 연구)

  • PARK, Jong-Un;CHO, Dong-Hyun
    • Journal of Fisheries and Marine Sciences Education
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    • v.16 no.2
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    • pp.257-270
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    • 2004
  • The two-phase closed thermosyphon is a heat transfer device capable of transfer large quantities of heat from a source to a sink by taking advantage of the high heat transfer rates associated with the evaporation and condensation of a working fluid within the device. A study was carried out with the performance of the heat transfer of the thermosyphon having 50, 60, 70, 80, 90 internal micro grooves in which boiling and condensation occur. A plain thermosyphon having the same inner and outer diameter as the grooved thermosyphon is also tested for comparison. Water, methanol and ethanol have been used as the working fluids. The liquid filling as the ratio of working fluid volume to total volume of thermosyphon, the inclination angle, micro grooves and operating temperature have been used as the experimental parameters. The heat flux and the boiling and the condensation heat transfer coefficient and overall heat transfer coefficient at the condenser and evaporator zone are estimated from the experimental results. The experimental results have been assessed and compared with existing correlations. Imura's and Kusuda's correlation for boiling showed in good agreement with experimental results within ${\pm}20$% in plain thermosyphon. The maximum heat transfer rate was obtained when the liquid fill ratio was about 25%. The high heat transfer coefficient was found between 25o and 30o of inclination angle for water and between 20o and 25o for methanol and ethanol. The relatively high rates of heat transfer have been achieved in the thermosyphon with internal micro grooves. The micro grooved thermosyphon having 60 grooves shows the best heat transfer coefficient in both condensation and boiling. The maximum enhancement (i.e. the ratio of the heat transfer coefficients of the micro grooved thermosyphon to plain thermosyphon) is 2.5 for condensation and 2.3 for boiling.

Study on Heat Transfer Characteristics of Evaporator with Horizontal Small Diameter Tubes using Natural Refrigerant Propane (자연냉매 프로판을 이용한 수평세관 증발기의 열전달 특성에 관한 연구)

  • Ku, H.K.
    • Journal of the Korea Society For Power System Engineering
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    • v.14 no.4
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    • pp.11-16
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    • 2010
  • The evaporation heat transfer characteristics of propane(R-290) in horizontal small diameter tubes were investigated experimentally. The test tubes have inner diameters of 1 mm and 4 mm. Local heat transfer coefficients were measured at heat fluxes of 12, $24\;kW/m^2$, mass fluxes of 150, $300\;kg/m^2s$, and evaporation temperature of $15^{\circ}C$. The experimental results showed that the evaporation heat transfer coefficient of R-290 has an effect on heat flux, mass flux, tube diameter, and vapor quality. The evaporation heat transfer of R-290 has an influenced on nucleate boiling at low quality and convective boiling at high quality. The evaporation heat transfer coefficient of R-290 increases with decreasing inner tube diameter. And the evaporation heat transfer coefficient of R-290 is about 1~3 times higher than that of R-134a.

A study on condensation heat transfer performance in microchannel tube (마이크로 채널 관에서의 응축 열전달 성능에 관한 연구)

  • Lee, Jeong-Kun
    • Journal of the Korea Society of Die & Mold Engineering
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    • v.13 no.2
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    • pp.22-29
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    • 2019
  • This study conducted a research as to condensation heat transfer by using three types of flat micro multi-channel tubes with different processing of micro-fin and number of channels inside the pipes and different sizes of appearances. In addition, identical studies were conducted by using smoothing circular tubes with 5mm external diameter to study heat transfer coefficient. The condensation heat transfer coefficient showed an increase as the vapor quality and mass flux increased. However, each tube shows little differences compared to 400kg/m2s or identical in case the mass flux are 200kg/m2s and 100kg/m2s. The major reason for these factors is increase-decrease of heat transfer area that the flux type of refrigerant is exposed to the coolant's vapor with the effect of channel aspect ratio or micro-fin. In addition, the heat transfer coefficient was unrelated to the heat flux, and shows a rise as the saturation temperature gets lower, an effect that occurs from enhanced density. The physical factor of heat transfer coefficient increased as the channel's aspect ratio decreased. Additionally, the micro pin at the multi-channel type tube is decided as a disadvantageous factor to condensation heat enhancement factor. That is, due to the effect of aspect ratio or micro-fin, the increase-decrease of heat transfer area that the flux type of a refrigerant is exposed to the vapor is an important factor.

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Evaporation Heat Transfer and Pressure Drop Characteristics of Refrigerant R-22 in a P1ate and Shell Heat Exchanger (Plate and Shell 열교환기내의 R-22 증발열전달 및 압력강하 특성에 관한 실험적 연구)

  • Seo, Mu-Gyo;Park, Jae-Hong;Kim, Yeong-Su
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.25 no.10
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    • pp.1318-1326
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    • 2001
  • The evaporation heat transfer coefficient and pressure drop for refrigerant R-22 flowing in the plate and shell heat exchanger were investigated experimentally in this study. Two vertical counterflow channels were farmed in the exchanger by three plates of commercial geometry with a corrugated trapezoid shape of a chevron angel of 45 ° Upflow boiling of refrigerant R-22 in one channel receives heat from the hot downf1ow of water in the other channel. The effects of the mean vapor quality, mass flux, heat flux and pressure of R-22 on the evaporation heat transfer and pressure drop were explored. The quality change of R-22 between the inlet and outlet of the refrigerant channel ranges from 0.03 to 0.05. The present data showed that both the evaporation heat transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux, the evaporation heat transfer coefficient and pressure drop are higher for the entire range of the vapor quality Raising the imposed wall heat flux was found to slightly improve the heat transfer, while at a higher refrigerant pressure, both the heat transfer and pressure drop are slightly lower.