• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.36 no.11
• /
• pp.1073-1079
• /
• 2012

This study aims to experimentally investigate the heat transfer characteristics of a thermosyphon using nanofluids. A thermosyphon with three individual pipes, which share the internal volume of the evaporator section, was designed, and its performance was tested for various charge amounts, input powers of the evaporator section's heater, and concentrations of working fluids. The optimized charge amount of the thermosyphon using distilled water was 30%, and the thermal resistance of the thermosyphon with $TiO_2$ nanofluid was 18.1% lower than that with Ag nanofluid. In addition, the heat transfer performance of the thermosyphon with $TiO_2-1%$ was optimized at an input power of 300 W at the evaporator section's heater and a charge amount of 30%.

• Journal of Energy Engineering
• /
• v.2 no.2
• /
• pp.179-186
• /
• 1993

Heat pipes. applied to a flat plate solar collector, have a long and slender configuration with relatively low heat flux in the evaporator section. Such a heat pipe has a tendency to build-up a liquid pool at the lower part of the evaporator section. and at this pool occurs such complicated phenomena of evaporation and fluid dynamics as superheat, sudden generation of bubble, its likely explosive growth process and flooding, etc. In the present study. we tried to solve these problems by means of adjusting two principal design parameters, the liquid inventory and the installation region of the wick, using 4 heat pipes and 3 thermospheres. The corresponding results can be summarized as follows$\^$1)/. The effective thermal conductances of the heat pipe was greatly improved by eliminating the wick in the adiabatic and condenser sections$\^$2)/. The liquid inventory should be increased by about 40% larger than what is saturated the wick$\^$3)/. In the evaporator section the wick has a favorable effect to reduce both unstable operation by intermittent occurrence of nucleate boiling and response time at the initial start-up process.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.15 no.4
• /
• pp.254-260
• /
• 2003

The boiling characteristics for R134a are studied to clarify the hysteresis at low temperature on enhanced tubes of a flooded evaporator. Initial boiling conditions, refrigerant temperature, and inlet temperature of the chilled water are considered as the key parameters of the experiments. Unlike previous studies of the boiling heat transfer with uniform heat flux and uniform wall temperature, the wall temperature was varied along the tube. In, this study, it was found that the hysteresis of the temperature overshoot (705) at the onset of nucleate boiling initially at the inlet section of the tube. It is also concluded that the abnormal operation can be avoided during the low temperature boiling if the refrigeration system is started with LMTD larger than $3.4^{\circ}C$ at initial stage and larger than $1.0^{\circ}C$ at normal stage.

• Proceedings of the KSME Conference
• /
• 2000.04b
• /
• pp.19-24
• /
• 2000

The objective of this study is to develope a computer simulation model and estimate theoretically the transient performance characteristics of heat exchangers in an automotive air-conditioning system. To do that, the mathematical modelling of heat exchangers, such as evaporator and condenser, is presented first of all. For detail calculation, evaporator and condenser are divided into many sub-sections. Each sub-section is an elemental volume for transient modelling. The elemental volume is assumed to consist of three components, refrigerant, tube with fin, and air, and various properties including temperatures of three components are determined step along sub-sections. The properties of refrigerant R134a and air are calculated directly in the program. The heat transfer coefficients and pressure drop in single or two phase are also calculated by suitable empirical correlations. The overall tendencies of the simulation results were agreed well with those of actual situation.

• Journal of the Korean Society of Visualization
• /
• v.17 no.1
• /
• pp.10-18
• /
• 2019

In this study, the temperature behavior of Pulsating Heat Pipe (PHP) according to the diameter change were studied by limiting the diameter change to only the evaporator. To investigate operation of PHP in various heat input, heat input power was increased from 10 to 120 W. The results show operation can be divided into 3 regimes by temperature behavior. Thermal resistance was increased before start-up and decreased with increasing heat input. At 110 W heat input, thermal conductivity of 2 mm PHP was 8 .times higher compare to thermal conductivity of copper. Further, to investigate details of temperature behavior in higher heat input, FFT analysis was conducted. Based on the results, when the deviation of peak frequency in each section is lowest, the thermal resistance has lowest value.

• Proceedings of the SAREK Conference
• /
• 2008.06a
• /
• pp.304-308
• /
• 2008

The optimal design of two stage evaporation & absorption system which is related to the large temperature difference system was investigated numerically in the absorption refrigeration system. The concentrations at inlet & oulet of absorber are 62.9% and 56.9%, but in two stage absorption system the values are 62.2% and 56.2%. Therefore strong solution & weak solution became diluted than the standard value. The amount of weak solution circulation can be reduced in absorption refrigeration system, and the sensible heat load is more reduced to enhance the COP of system. As UAR is increased, COP becomes larger, and this means the role of top section is more important than bottom section in two stage evaporation & absorption system. But the increase of COP becomes slower at 0.7 of UAR ratio. The performance of Type2 is higher than Type1 in COP with the flow direction of cooling waters. This phenomena is due to the active absorption of vapor -absorption & lower temp. cooling water is more effective. The pressure at bottom section becomes higher & that at top section becomes lower and therefore the circulation rate can be diminished more.

• Proceedings of the SAREK Conference
• /
• 2008.11a
• /
• pp.203-211
• /
• 2008

The optimal design of two stage evaporation & absorption system which is related to the large temperature difference system was investigated numerically in the absorption refrigeration system. The concentrations at inlet & oulet of absorber are 62.9% and 56.9%, but in two stage absorption system the values are 62.2% and 56.2%. Therefore strong solution & weak solution became diluted than the standard value. The amount of weak solution circulation can be reduced in absorption refrigeration system, and the sensible heat load is more reduced to enhance the COP of system. As UAR is increased, COP becomes larger, and this means the role of top section is more important than bottom section in two stage evaporation & absorption system. But the increase of COP becomes slower at 0.7 of UAR ratio. The performance of Type2 is higher than Type1 in COP with the flow direction of cooling waters. This phenomena is due to the active absorption of vapor -absorption & lower temp. cooling water is more effective. The pressure at bottom section becomes higher & that at top section becomes lower and therefore the circulation rate can be diminished more.

• Proceedings of the KSME Conference
• /
• 2007.05b
• /
• pp.2119-2123
• /
• 2007

This study deals with performance characteristics of heat pipe combined with a solid-liquid phase change material(PCM). The outer diameter of the heat pipe was 9.5 mm and the total length was 600 mm, where the evaporator, the adiabatic section and the condenser lengths were equally 200 mm. A paraffin wax having a melting point of 58.5$^{\circ}C$ was used as PCM. The paraffin container was attached to the adiabatic section of the heat pipe. The paraffin container had outer diameter of 18 mm, wall thickness of 1.2 mm and the total length of 100 mm. The heat pipe was tested with tilt angle of horizontal degree and favorite angle 10 degree, with evaporator lower position to provide stable operation of the heat pipe. Input thermal load was varied from 40 W, with increment of 40 W, to above 100 W until the maximum temperature of the heat pipe wall reached 200$^{\circ}C$. Test results of the PCM heat pipe were presented in comparison with conventional heat pipe of the same basic dimensions. The performance was analyzed in terms of temperature distribution, thermal resistance and heat transport capability.

• Journal of Power System Engineering
• /
• v.12 no.4
• /
• pp.26-31
• /
• 2008

The evaporation heat transfer coefficient and pressure drop of R-22 and R-407C in a small diameter copper tube were investigated experimentally. The main components of the refrigerant loop are a receiver, a compressor, a mass flow mete, a condense and a double pipe type evaporate (test section). The test section consists of a smooth copper tube of 4.3 mm inner diameter. The refrigerant mass fluxes were varied from 100 to $300[kg/m^{2}s]$ and the saturation temperature of evaporator were $5[^{\circ}C]$. The evaporation heat transfer coefficients of R-22 and R-407C increase with the Increase in mass flux and vapor quality. The evaporation heat transfer coefficient of R-22 is about $7.3\sim47.1%$ higher than that of R-407C. The evaporation pressure drop of R-22 and R-407C increase with the increase of mass flux. The pressure drop of R-22 is about $8\sim20%$ higher than that of R-407C.

• Journal of the Korean Institute of Gas
• /
• v.12 no.3
• /
• pp.43-49
• /
• 2008

The evaporation heat transfer coefficient and pressure drop of R-22 and R-407C in horizontal copper tubes were investigated experimentally. The main components of therefrigerant loop are a receiver, a compressor, a mass flow meter, a condenser and a double pipe type evaporator (test section). The test section consists of a smooth copper tube of 4.3 mm and 6.4 mm inner diameter. The refrigerant mass fluxes were varied from 100 to $300[kg/m^2s]$ and the saturation temperature of evaporator were 5 [$^{\circ}C$]. The evaporation heat transfer coefficients of R-22 and R-407C rise with the increase in mass flux and vapor quality. The evaporation heat transfer coefficient of R-22 for inner diameter tube of 4.3 mm and 6.4 mm is about $7.3{\sim}47.1%$ and $5.68{\sim}46.6%$ higher than that of R-407C, respectively.