• Nuclear Engineering and Technology
• /
• v.54 no.3
• /
• pp.867-876
• /
• 2022

An experimental study has been conducted to investigate the heat transfer characteristics of supercritical carbon dioxide (sCO₂) uniformly heated in the horizontal circular smooth tube. The results illustrated that there was a significant difference in heat transfer between the top wall and bottom wall due to the buoyancy. Bulk flow acceleration cannot be negligible in the high heat flux region, which leads to heat transfer deterioration. A new heat transfer correlation is proposed, in which the buoyancy parameter and bulk flow acceleration have been taken into account. The new correlation and six classic correlations for sCO₂ are examined in horizontal tubes. The comparison indicates that the new correlation has a better performance for sCO₂ flowing through a horizontal heating tube under natural circulation conditions. For example, 94.9% of the calculated results using the new heat transfer correlation were within ±30% of the experimental results while only 87.9% of that using the Jackson correlation (the best of the six) were within the same error bands.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.28 no.8 s.227
• /
• pp.989-995
• /
• 2004

This paper presents a semi-empirical model to predict the frost growth formed on the cold cylinder surface. The model is composed of the correlations for frost properties including the various frosting parameters and local heat transfer coefficient. The effects of varying the correlations for local heat transfer coefficient on the frost growth are examined to establish the model. The numerical results are compared with experimental data obtained by the previous researchers. The results agree well with the experimental data within a maximum error of $13\%$. As the results, the frost thickness decreases with changing angular position from front stagnation to separation point. Also, the effects of air velocity on the frost growth are negligible, as compared to the other frosting parameters.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.8 no.2
• /
• pp.254-266
• /
• 1996

Boiling heat transfer coefficients of pure refrigerants(R22, R32, R125, R134a, R290, and R600a) and refrigerant mixtures(R32/R134a and R290/R600a) are measured experimentally and compared with several correlations. Convective boiling term of Chen's correlation predicts experimental data for pure refrigerants fairly well(root-mean-square error of 12.1% for the quality range over 0.2). An analysis of convective boiling heat transfer of refrigerant mixtures is performed for an annular flow to study degradation of heat transfer. Annular flow is the subject of this analysis because a great portion of the evaporator in refrigeration or air conditioning system is known to be in the annular flow regime. Mass transfer effect due to composition difference between liquid and vapor phases, which is considered as a driving force for mass transfer at interface, is included in this analysis. Correction factor $C_F$ is introduced to the correlation for the pure substances through annular flow analysis to apply the correlation to the mixtures. The flow boiling heat transfer coefficients are calculated using the correlation considering nucleate boilling effect in the low quality region and mass transfer effect for nonzazeotropic refrigerant mixtures.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.25 no.2
• /
• pp.171-179
• /
• 2001

Experimental study has been carried out on the characteristics of pressure drop and heat transfer of brazed plate heat exchangers using R-22. Data are presented for the following range of variables: the mass flux (40∼90kg/$m^2$s), chevron angle ($20^{\circ}$, $35^{\circ}$, $45^{\circ}$) and inlet pressure of the refrigerant (1.4 and 1.6MPa). For both subcooled and two-phase flow, as chevron angle increases, pressure drop and heat transfer coefficient decrease. Condensation heat transfer coefficient and pressure drop were compared with the previously proposed correlations. Among therm, Traviss correlation agreed with experimental results within -40%∼-84% for heat transfer coefficient and -59%∼62% for pressure drop.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.12 no.10
• /
• pp.941-949
• /
• 2000

Pool boiling heat transfer coefficients (HTCs) of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HFC22, HFC125 and HFC32 on a horizontal smooth tube have been measured. The experimental apparatus is specially designed to simulate the real heat transfer tube with the use of the secondary fluid of water as a heat source rather than a conventional electric heat source. Data were taken in the order of decreasing heat flux starting at $80 ㎾/m^2\; and \;ending\; at\; 5㎾/m^2\;in\; the\; poo\;l temperature\; at\; 7^{\circ}C$, Test results showed that HTCs of HFC125, and HFC32 are 50~67% higher than those of HCFC22. It is also found that some of the popular pool boiling heat transfer correlations in the literature are not good to predict the HTCs of newly developed alternative refrigerants. A new correlation was developed by a regression analysis which is based upon the consistent data obtained in this study and it showed an excellent agreement with all experimental data having an absolute mean deviation of less than 10%.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.4 no.3
• /
• pp.187-198
• /
• 1996

In this study, the performance of an automobile heat storage system using PCM is numerically simulated. For the analysis of system performance. The phase-change of the PCM and the transient forced convective heat transfer for the HTF are considered simultaneously as a conjugate problem. The phase-change behavior is effectively analyzed using a concept of thermal resistance. From the correlations of phase change rate and heat transfer due to the variations of flow rate of HTF around PCM, the automobile heat storage system performance is predicted. The present results amy be used as the fundamental information for the design of automobile heat storage system.

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

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.28 no.12
• /
• pp.495-501
• /
• 2016

Condensation heat transfer and pressure drop of R245fa were investigated experimentally in a plate-shell heat exchanger which consisted of thirty seven counter flow channels formed by thirty-eight plates with a chevron angle of $50^{\circ}$. The upflow of the water in one channel receives heat from the downflow of R245fa in the other. The effects of refrigerant mass flux, imposed heat flux, refrigerant saturation pressure, and mean vapor quality on the heat transfer characteristics were explored in detail. Experimental correlations were proposed to predict the condensation heat transfer coefficient and friction factor in terms of the Boiling number, Reynolds number, and Prandtl number. In the experiments, the mean vapor quality in the refrigerant channel was varied from .22 to .82, mass flux from 3 to $5kg/m^2$, imposed heat flux from 1 to $3kW/m^2$, and system pressure from .61 to .81 MPa.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.29 no.2 s.233
• /
• pp.279-286
• /
• 2005

This study presents experimental results on the heat transfer coefficients in the film boiling region of spray cooling for actual metallurgical process. In this study, the heat flux distributions of a two dimensional dilute spray impinging on a hot plate were experimentally investigated. A stainless steel block was cooled down from intial temperature of about $800^{\circ}C$ by twin fluid (air-water) flat spray. It was found from the experimental results that the heat transfer area was classified into the stagnation region and wall-flow region. In the stagnation region, the experimental data of local heat transfer coefficient was closely correlated with the local droplet-flow-rate supplied from the spray nozzle directly. Thus, the local heat transfer coefficients are in good agreement with the predicted values from the correlations proposed by our previous study. In wall-flow region, however, remarkable differences are observed between experimental data and predicted values because the number of rebound droplets increase with increasing the distance from the stagnation point.

• Proceedings of the SAREK Conference
• /
• 2009.06a
• /
• pp.1528-1535
• /
• 2009

Characteristics of R-22 and R-134a two-phase vaporization in horizontal small tubes were investigated experimentally. In order to obtain the local heat transfer coefficients, the test was ran under heat flux range of 10 to $40\;kW/m^2$, mass flux range of 200 to $600\;kg/m^2s$, saturation temperature range of 5 to $10^{\circ}C$, and quality up to 1.0. The test section, which was made of stainless steel tube and heated uniformly by applying an electric current to the tube directly, have inner tube diameters of 0.5, 1.5 and 3.0 mm, and lengths of 0.33 and 2.0 m. The effects on heat transfer coefficient of mass flux, heat flux and inner tube diameter were presented. The experimental heat transfer coefficients were compared with the predictions using existing heat transfer coefficient correlations. A new boiling heat transfer coefficient correlation based on the superposition model, with considering the laminar flow, was developed.