• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.27 no.8
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• pp.409-417
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• 2015

A simplified rotary heat-exchanger model was developed with an assumption of a linear temperature distribution along the flow direction. Based on the model, the exact fluid solution and solid temperature variations were obtained and verified from a comparison with previous numerical studies. The heat transfer in the rotary heat exchanger was investigated using the theoretical solutions. The heat exchanger's effectiveness was shown to be saturated, with a rotational-speed increase that is higher than a critical value that is solely dependent on the thermal capacity of the solid matrix but independent of the fluid flow rate; the saturated value of the effectiveness was determined only by the NTU of the heat exchanger. Where the thermal diffusivity of the solid matrix is so slight that the thermal penetration depth becomes smaller than the matrix thickness, the effective thermal capacity of the solid matrix decreased according to the penetration depth.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.26 no.12
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• pp.600-604
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• 2014

Longitudinal heat conduction is known to be an important factor in the design of a printed circuit heat exchanger(PCHE) for cryogenic applications. Parasitic heat conduction through the heat exchanger frame needs to be considered because it is known to decrease the effectiveness of the heat exchanger. In this paper, a conjugate heat transfer problem in a simple counter-flow PCHE is analyzed by a computational fluid dynamics simulation. The effect of longitudinal conduction in a straight channel is compared with the theoretical effectiveness-NTU relationship that assumes a "thin" heat exchanger frame. The calculation results suggest that the theoretical model is valid in the present calculation conditions where NTU is < 13.

• Progress in Superconductivity and Cryogenics
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• v.11 no.1
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• pp.55-59
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• 2009

Miniature Joule-Thomson refrigerators have been widely used for rapid cooling of infrared detectors, probes of cryosurgery, thermal cameras, missile homing head and guidance system, due to their special features of simple configuration, compact structure and rapid cool-down characteristics. The thermodynamic performance of J-T refrigerator highly depends on the hydraulic and heat transfer characteristics of the recuperative heat exchanger. The typical recuperative heat exchanger of the J-T refrigerator has the double helical tube and fin configuration. In this study, effectiveness-NTU approach was adopted to predict the thermodynamic behaviors of the heat exchanger for the J-T refrigerator. The thermodynamic properties from the REFPROP were used to account the real gas effects of the gas. The results show the effect of the operating conditions on the performance of the heat exchanger and refrigerator for the given heat exchanger. The influences of mass flow rate and the supply pressure on the effectiveness of heat exchanger and the ideal cooling capacity are discussed in details.

• Journal of Energy Engineering
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• v.22 no.1
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• pp.44-50
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• 2013

This study was experimentally investigated NOx and CO emissions characteristics with various equivalence ratios using premixed type of burner installed small heat exchanger. The effectiveness of heat exchanger and the entropy generation number were also calculated. As results, the heat transfer rate increases with increasing equivalence ratio due to increase the flame temperature. According to the emission characteristics and the effectiveness, the optimal operating equivalence ratio is 0.75 in the range of this experiment. Consequently, the area of the heat exchanger should be increased to reduce the entropy generation number and to increase the effectiveness.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.11
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• pp.1044-1050
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• 2004

The present study has been conducted to develop a heat pipe heat exchanger for middle-high temperature ranged from 300 to $600^{\circ}C$. Heat transfer rate, overall heat transfer coefficient and temperature effectiveness were investigated using a heat pipe heat exchanger with Dowtherm A as working fluid. Theoretical analysis was also conducted, and the followings were obtained: (1) Heat exchange rate increased as waste gas temperature supplied to evaporator and frontal velocity in condenser increased, (2) Overall heat transfer coefficient increased by $3{\sim}7\%$ as frontal velocity in evaporator and condenser increased, (3) Temperature effectiveness was about $30\%$ in evaporator and was about $40\%$ in condenser, (4) Heat recovery rate was about $38\%$, (5) Pressure drop did not exceed $8\;mmH_{2}O$ under the running condition of $1{\sim}3Nm/s$, (6) Simulation results were corresponded with experimental results.

• Journal of Energy Engineering
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• v.22 no.2
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• pp.120-127
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• 2013

This study investigated the characteristics of heat transfer and pressure drop for cross-flow heat exchanger of premixed combustion system. The premixed burner was in front of a heat exchanger, and the number of heat exchanger modules was changed to investigate the characteristics of NOx and CO emissions with various equivalence ratios. In addition, the effectiveness, entropy generation and pressure drop were calculated by various number of heat exchanger modules and the performance of heat exchanger was analyzed by the exergy loss.

• Solar Energy
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• v.9 no.2
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• pp.22-30
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• 1989

Since the energy storage method by means of the thermochemical reaction has no heat loss by separating the reactants under the storage period, it is remarked as one of promising means particularly for long-term heat storage. In this study, the heat-storage/-release characteristics of the reversible chemical reaction cycle, $Ca(OH)_2/CaO$, is numerically analysed by a mathematical modelling. As a result, the effectiveness of the heat exchanger by the chemical heat storage method is considerably higher than that by the sensible heat storage method. It is found that the major parameters, which determines the effectiveness of the heat exchanger, are the mass flow rate and inlet temperature of fluid, the residence time, etc.. The heat-storage/-release period can be controlled by changing the operation conditions. It is expected that the results obtained here will supply useful informations in designing a regenerative heat exchanger utilizing the thermochemical reaction.

• New & Renewable Energy
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• v.2 no.1 s.5
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• pp.46-55
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• 2006

The performance enhancement of heat exchanger in the drying system using solar collector with evacuated tubes is analyzed. First, for this analysis, the heat loss from a reversed trapezoidal fin attached at the pipe is calculated as a function of convection characteristic number ratio, fin base length and fin tip length. Also, the optimum heat loss and fin tip length of the fin under certain conditions are presented. The overall surface effectiveness of the cylinder with reversed trapezoidal fins in the heat exchanger are shown as a function of half fin base height, fin lateral slope and fin tip length.

• Journal of Power System Engineering
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• v.18 no.4
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• pp.36-42
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• 2014

This paper considers the influence of internal heat exchangers on the efficiency of a refrigerating system using R-1270. These internal heat exchangers(liquid-gas or suction-line heat exchangers) can, in some cases, yield improved system performance while in other cases they degrade system performance. A steady state mathematical model is used to analysis the performance characteristics of refrigeration system with internal heat exchanger. The influence of operating conditions, such as the mass flowrate of R-1270, inner diameter tube and length of internal heat exchanger, to optimal dimensions of the heat exchanger is also analyzed. The main results were summarized as follows : the mass flowrate of R-1270, inner diameter tube and length of internal heat exchanger, and effectiveness have the influences on the cooling capacity, compressor work and RCI(Relative Capacity Index) of this system. With a thorough grasp of it is possible to design the R-1270 compression refrigeration cycle using internal heat exchanger.

• Transactions of the Korean hydrogen and new energy society
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• v.15 no.3
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• pp.194-200
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• 2004

Thermal energy performance of a brazed plate heat exchanger has been evaluated experimentally. The effects of plate pitch as well as chevron angle of a plate heat exchanger on the heat transfer rate and pressure drop have been investigated in the wide range as mass flow rates in detail. This problem is of particular interest in the design of a plate heat exchanger. The results obtained indicate that both heat transfer rate and pressure drop are increased as mass flow rate is increased, as expected. It is also found that the heat transfer rate is increased with a decrease in the plate pitch while the heat transfer is decreased with a decrease in the chevron angle. Friction factor correlations are suggested based on the measured pressure drop and effectiveness of plate heat exchangers are also compared.