• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.18 no.5
• /
• pp.402-409
• /
• 2006

For the design of a liquid direct contact cooling system, thermal and hydraulic analysis has been carried out. Well-known Zukauskas correlations are used to estimate the Nusselt number between the liquid refrigerant columns and the inlet airflow. The inlet air velocity is set at a typical value used in an actual showcase. For a constant column number, the best nozzle arrangement is determined for the maximum heat transfer. Heat transfer increases as the transverse pitch of the refrigerant column decreases. Among all the cases dealt with in the present study, the staggered arrangement with 140-columns of $14{\times}10$ shows the best thermal peformance and the expected temperature drop is $27.8^{\circ}C$. The effect of downstream refrigerant columns on the overall thermal performance is investigated as well.

• Nuclear Engineering and Technology
• /
• v.52 no.4
• /
• pp.734-741
• /
• 2020

A new air-cooled waste heat removal system with a direct contact heat exchanger was designed for SMRs requiring 200 MW of waste heat removal. Conventional air-cooled systems use fin structure causing high thermal resistance; therefore, a large cooling tower is required. The new design replaces the fin structure with a vertical string type direct contact heat exchanger which has the most effective performance among tested heat exchangers in a previous study. The design results showed that the new system requires a cooling tower 50% smaller than that of the conventional system. However, droplet formation on a falling film along a string caused by Rayleigh-Plateau instability decreases heat removal performance of the new system. Analysis of Rayleigh-Plateau instability considering drag force on the falling film surface was developed. The analysis results showed that the instability can be prevented by providing thick string. The instability is prevented when the string radius exceeds the capillary length of liquid by a factor of 0.257 under stagnant air and 0.260 under 5 m/s air velocity.

• Journal of the Korean Society of Manufacturing Process Engineers
• /
• v.14 no.5
• /
• pp.68-74
• /
• 2015

In the present work, a series of numerical calculations have been conducted on the cooling of a hot surface using an air/water mist jet. In some cooling processes, such as in the glass-tempering process, direct contact between the cold water drops and the hot surface should be avoided, because this may cause surface cracks due to the sharp temperature gradients. Thus, the main focus of this study is finding the appropriate operating conditions for maximum cooling without direct contact between the drops and the surface. A series of numerical experiments have been performed, and, at the same time, those results were compared with those of the previous experiments for verification purposes. The effects of droplet impinging velocity, hot plate temperature, and liquid loading ratio for mono-dispersed drops of various sizes were studied in detail.

• Analytical Science and Technology
• /
• v.19 no.4
• /
• pp.323-332
• /
• 2006

This paper discusses several low-temperature heat-tranfer fluids, including water-based inorganic salt, organic salt, alcohol/glycol mixtures, silicones, and halogenated hydrocarbons in order to choose the best heat-transfer fluid for the newly designed direct contact refrigeration system. So, it contains a survey on commercial products such as propylene glycol and potassium formate as newly used in super market and food processing refrigeration. The stability of commercial fluids at the working temperature of $-20^{\circ}C$ was monitored as a function of time up to two months. And organic and inorganic compositions of candidate fluids were obtained by analytical instruments such as ES, XRF, AAS, ICP-AES, GC, and GC-MS. Analysis results indicate that commercial propylene glycol is very efficient and safe heat transfer fluids for the direct cooling system with liquid phase.

• Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
• /
• 1999.02a
• /
• pp.1-6
• /
• 1999

The recent progress in the new cooling technology for superconducting systems without liquid cryogens is briefly presented. In these conduction-cooled systems, the super-conducting magnets are cooled by a direct contact with closed-cycle cryocoolers and only electricity is supplied to maintain the cryogenic temperatures. It is reported that at least 20 conduction-cooled (low Tc or high Tc) super- conducting systems have been constructed, tested, or commercially used worldwide. Some of the significant design issues are discussed in comparison with the conventional liquid-helium cooled systems.

• Proceedings of the KSME Conference
• /
• 2007.05b
• /
• pp.2291-2296
• /
• 2007

Thermal transport from vertical heated surface to falling liquid film in a channel has been investigated experimentally. Air-flow is introduced into channel to make a counter flow against falling liquid film. This problem is of particular interest in the design of direct contact heat exchange system, such as cooling tower, evaporative cooling system, absorption cooling system, and distillation system. The effects of channel width and air flow rate on the heat transfer to falling liquid film are studied in detail. The results obtained indicate that heat transfer rate is gradually decreased with an increase in the channel width without air flow as well as with air flow in a channel. It is also found that heat transfer rate of air-flow is increased while heat transfer rate of falling liquid film is decreased with an increase in the air flow rate at a given channel width. However, total heat transfer rate form the heated surface is increased as the air flow rate is increased.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.32 no.5
• /
• pp.335-341
• /
• 2008

Thermal transport from vertical heated surface to falling liquid film in a channel has been investigated experimentally. Air-flow is introduced into channel to make a counter flow against falling liquid film. This problem is of particular interest in the design of direct contact heat exchange system, such as cooling tower, evaporative cooling system, absorption cooling system, and distillation system. The effects of channel width and air flow rate on the heat transfer to falling liquid film are studied in detail. The results obtained indicate that heat transfer rate is gradually decreased with an increase in the channel width without air flow as well as with air flow in a channel. It is also found that heat transfer rate of air-flow is increased while heat transfer rate of falling liquid film is decreased with an increase in the air flow rate at a given channel width. However, total heat transfer rate from the heated surface is increased as the air flow rate is increased.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.11 no.3
• /
• pp.349-358
• /
• 1999

Thermodynamic cycle analysis has been performed to maximize the liquid amount for various hydrogen liquefaction systems using GM(Gifford-McMahon) refrigerator. Since the present authors' previous experiments showed that the liquefaction rate was approximately 5.1mg/s in a direct contact with a commercial GM refrigerator, the purpose of this study is to predict how much the liquefaction rate can be increased in different configurations and with improved heat exchanger performance. The optimal operating conditions have been analytically sought with real properties of normal hydrogen for the single-stage GM precooled L-H(Linde-Hampson) system, the two-stage GM direct contact system, the two-stage GM precooled L-H system and the two-stage helium GM-JT (Joule-Thomson) system. The maximum liquefaction rate has been predicted to be only about 7 times greater than the previous experiment, when the two-stage precooling is employed and the effectiveness of heat exchangers approaches to 99.0%. It is concluded that the liquefaction rate is limited mainly by the cooling capacity of the current GM refrigerators and a larger scale of hydrogen liquefaction is possible with a greater capacity of cryocooler at 60-70 K range.

• International Journal of Air-Conditioning and Refrigeration
• /
• v.8 no.2
• /
• pp.39-50
• /
• 2000

Thermodynamic cycle analysis is presented to estimate the maximum liquefaction rate of hydrogen for various systems using a Gifford-McMahon(GM) cryocooler. Since the present authors` previous experiments showed that the gaseous hydrogen was liquefied approximately at the rate of 5.1 mg/s from the direct contact with a commercial two-stage GM refrigerator, this study has been proposed to predict how much the liquefaction rate can be increased in different configurations using the GM cooler and with improved heat exchangers. The optimal operating conditions have been analytically sought with real properties of normal hydrogen for the Linde-Hampson(L-H) system precooled by single-stage GM, the direct-contact system with two-stage GM, the L-H system precooled by two-stage GM, and the direct-contact system with helium GM-JT (Joule-Thomson). The maximum liquefaction rate has been predicted to be only about 7 times greater than the previous experiment, even though the highly effective heat exchangers may be employed. It is concluded that the liquefaction rate is limited mainly because of the cooling capacity of the commercially available GM cryocoolers and a practical scale of hydrogen liquefaction is possible only if the GM cooler has a greater capacity at 70-100 K.

• Electronics and Telecommunications Trends
• /
• v.38 no.6
• /
• pp.41-51
• /
• 2023

Heat dissipation technology for semiconductors and electronic packaging has a substantial impact on performance and lifespan, but efficient heat dissipation is currently facing limited improvement. Owing to the high integration density in electronic packaging, heat dissipation components must become thinner and increase their performance. Therefore, heat dissipation materials are being devised considering conductive heat transfer, carbon-based directional thermal conductivity improvements, functional heat dissipation composite materials with added fillers, and liquid-metal thermal interface materials. Additionally, in heat dissipation structure design, 3D printing-based complex heat dissipation fins, packages that expand the heat dissipation area, chip embedded structures that minimize contact thermal resistance, differential scanning calorimetry structures, and through-silicon-via technologies and their replacement technologies are being actively developed. Regarding dry cooling using single-phase and phase-change heat transfer, technologies for improving the vapor chamber performance and structural diversification are being investigated along with the miniaturization of heat pipes and high-performance capillary wicks. Meanwhile, in wet cooling with high heat flux, technologies for designing and manufacturing miniaturized flow paths, heat dissipating materials within flow paths, increasing heat dissipation area, and reducing pressure drops are being developed. We also analyze the development of direct cooling and immersion cooling technologies, which are gradually expanding to achieve near-junction cooling.