Topics

Liquid helium has a negative JT coefficient. If the heat transfer process of the liquid helium is carried out with a continuous pressure drop in the hot stream (heat source), an extra enthalpy reduction is expected. The isothermal line o liquid line of helium has a positive slope as a unique physical property, and it means JT coefficient is a negative value. Despite the same temperature, high-pressure liquid helium has a lower enthalpy than lower-pressure liquid helium by the sign of the JT coefficient (B and C in the P-h diagram). Thus, for efficient heat exchange from an enthalpy point of view, the heat transfer process of a hot stream with a continuous pressure drop (1 to 2) is superior to the heat transfer process without a pressure drop (1 to 2'). Since liquid helium has small latent heat (20.7 J/g at ambient pressure) when compared to other cryogenic fluids, it is easy to be vaporized by heat ingress. Furthermore, a large amount of power input is required to reach and maintain a low temperature. The power input becomes larger as the target temperature becomes lower. Hence, distributed JT effect heat exchanger for liquid helium is needed to be studied for a more efficient cooling system. 

Magnetocaloric effect (MCE) is the phenomenon that can change the temperature or the entropy of materials by alternating magnetic field. The magnetic refrigeration which utilizes MCE replaces conventional gas compression/expansion process by magnetization/demagnetization of magnetic materials. Adiabatic demagnetization refrigerator (ADR) is one way of magnetic refrigeration using MCE. 

The ADR is composed of the magnetic material, magnet which produces alternating field on magnetic refrigerant, and heat switch. The heat switch allows the heat flow between the magnetic material and heat sink or cooling target by changing state (on/off) of the heat switch. In common, active control of heat switch is needed. If the heat switch operates passively, the complexity of the control system can be reduced.

ADR with passive heat switch is to be operated with one passive superconducting heat switch at the warm end and one passive thermosiphon heat switch at the cold end. Because the state of the conductor and thermosiphon is determined by temperature and magnetic field of ADR cycle, we call this ‘passive’ heat switch ADR. 

A mixture of water and cryoprotectant agent (CPA) is investigated for the substance of a cryogenic thermal storage unit (TSU). Since water is liquid at room temperature with high specific heat, it is an ideal material for thermal storage. The undesirable characteristic of volume increase in water upon freezing can be negated by CPA. The experiments are conducted with various concentrations of CPA aqueous solution to find the most suitable TSU material especially for liquid air energy storage (LAES) system. The volume change characteristic of CPA mixture is directly measured at varying temperature by an in-situ strain gauge.

The sorption J-T refrigerator can provide cryogenic cooling without vibrations, since the sorption compressor operates thermally, not mechanically. It also does not cause electromagnetic interference (EMI) to sensitive measuring device. Furthermore, a compact refrigeration system can be manufactured by using sorption J-T refrigerator since the size of the sorption compressor can be minimize-able. For these reasons, sorption J-T refrigerator is especially attractive for space applications (i.e. optical detectors).The sorption J-T refrigerator is a J-T refrigerator using a sorption compressor and a recuperative heat exchanger. The sorption compressor is operated thermally by repeating adsorption and desorption of working fluid in a “sorption cell”. Activated charcoal is generally used as an adsorbent of helium because of its large surface area per unit mass at cryogenic temperature.

It is important to minimize the cycle period to increase cooling performance. Therefore, the sorption cell should be designed to rapidly diffuse heat in the cell. In addition, its overall thermal inertia should be minimized for fast response characteristics.

Cryogenic line chill down is the transient process of the pipe cooling by internal flow of cryogenic fluid. Cryogenic line chill-down process is accompanied with complicated heat transfer phenomena. Firstly, the properties of boil-off gas change significantly respect the temperature changes. Secondly, the whole process is transient. The wall temperature, flow pattern, and heat flux constantly change over time. Thirdly, pressure and mass flux oscillation occur due to fast evaporation. As a result, the complexity of heat transfer phenomena brings about the main difficulty to predict the cryogenic line chill-down process. We aim to extend the quenching database with various cryogenic fluids and develop empirical heat transfer correlations to predict the cryogenic line chill-down process.

The lowest attainable refrigeration temperature of a nitrogen based Joule–Thomson refrigerator is generally limited to 77 K since the compressor suction pressure is usually higher than atmospheric pressure.The Joule–Thomson process with an ejector is proposed to achieve a refrigeration temperature as low as 68 K by adjusting the evaporation pressure down to 28 kPa and boosting the return stream pressure up to 147 kPa. A one-dimensional numerical model is developed to predict the performance of the ejector at cryogenic temperature, and its accuracy is compared with experimental data. The analysis results show that the addition of the ejector in the Joule–Thomson refrigeration cycle increases up to 5 times the overall efficiency, where the maximum achievable COP and exergy efficiency are 0.0195 and 6.65%, respectively. Other featured advantages of the proposed Joule–Thomson refrigeration cycle with ejector are the simplicity of cycle, minimization of mechanical moving components, cost effectiveness, and high reliability compared to other cryogenic refrigeration methods using pumps or cold compressors in Joule–Thomson cycles.

We investigate the application of ejector in the cryogenic refrigeration system. A new refrigeration cycle is proposed using ejector technology to enhance the thermodynamic efficiency of Joule-Thomson (JT) refrigeration system. The ejector combined with the JT refrigeration cycle works with nitrogen and neon mixed refrigerant. The target refrigeration temperature is aimed to near the freezing point of nitrogen (63.15 K). A performance map of ejector-JT cycle is accomplished according to the ejector entrainment ratio and the ejector exit pressure. The effect of the ejector on the refrigeration cycle COP is analyzed by calculating the exergy destruction rate at each component of the refrigeration cycle. The exergy analysis shows that the losses at every component of the refrigeration cycle can be reduced using ejector when the entrainment ratio is properly designed. A lab-scale prototype ejector which has a rectangular cross sectional area is fabricated by stacking thin layers of etching plates. The fabricated lab-scale ejector is tested with JT refrigeration circuit, and the nozzles and the diffuser showed 80% isentropic efficiency.

A Joule-Thomson refrigerator of millimeter size has been recently developed for cryosurgical ablation. This small size Joule-Thomson refrigerator uses a compact heat exchanger of micro channels and mixed refrigerant for high exergy efficiency. In the compact heat exchanger of micro channels, phase change of mixed refrigerant (like boiling and condensation) occurs. The study on boiling or condensation heat transfer of such cases is, however, very rare. The research has been performed with R134a/R123 mixture in the single micro channel of approximately 0.2 mm in diameter. The effects of reduced pressure, heat flux, mass flow rate and mixture composition are investigated.

The J-T (Joule-Thomson) refrigerator produces a refrigeration effect by isenthalpic expansion of high pressure gas. It has been developed in many forms for various cryogenic applications since it has a very simple expansion device. For example, it can be fabricated in a few millimeter diameters to be inserted into a human body and operated as a cryosurgical probe. The J-T refrigerators using mixed refrigerant have significant refrigeration power compared to that of conventional J-T refrigerators using pure gas. Their performances are, however, considerably degraded even in a slight change of operating condition. The miniature J-T refrigerator using mixed gas is researched in the topics of optimizing the composition of mixture and the design of micro heat exchanger and refrigerator.

Cryogenic refrigerators require efficient regeneration process. It is achieved either by a regenerator or by a recuperator; a regenerator for periodically operating refrigerators (Stirling refrigerator, GM refrigerator, pulse tube refrigerator, etc.) and a recuperator for steadily operating refrigerators (J-T refrigerator, reverse Brayton refrigerator, etc.). In general, the periodically operating refrigerators are applied to small scale, and the steadily operating refrigerators are applied to large scale. Therefore, the recuperators are commonly large scale heat exchangers such as Giauque-Hampson heat exchangers or plate-fin heat exchangers. However, some applications demand small scale recuperators. We are investigating PCHE (Printed Circuit Heat Exchanger) and micro perforated heat exchangers for such applications.

 Magnetic refrigeration is based on the magnetocaloric effect, which is entropy change in a material due to the variation of external magnetic field, and has a potential for higher efficiency and reliability than conventional gas-cycle refrigeration. Magnetic refrigeration is free from environmental problems due to ODS (Ozone Depletion Substance) seen in conventional refrigerants since it employs solids as refrigerant which is called magentocaloric materials An active magnetic regenerator (AMR) is a part of magnetic refrigeration system, which utilizes magnetocaloric material as a regenerator. It is inherently difficult for AMRs to be operated over a wide temperature range as conventional cryocoolers. Multi-material AMR has a large potential to improve refrigeration performance and operating temperature span. Therefore, it is necessary to find the optimum operating conditions and materials and its distribution. Currently, numerical investigation about the multi-material AMR is conducted as a preliminary state of AMR development.

  A thermosiphon, wickless heat pipe, is an efficient heat transfer device utilizing phase change phenomenon of fluid in closed volume. A conventional thermosiphon is best described by dividing it into three sections; evaporator, condenser, and adiabatic part. Thermal load is connected on the evaporator where the liquid evaporates by boiling heat transfer. The vapor rises and passes through the adiabatic section to the condenser part. In the condenser, the vapor condenses and gives up its latent heat. Gravity then returns the condensate back to the evaporator section.

The current research work improves a conventional thermosiphon that peculiarly utilizes two evaporators to overcome spatial restriction. The aim of the double-evaporator thermosiphon is to be used for cooling two thermal loads that are separated vertically. The current research is focused on the heat transfer characteristics of the double-evaporator thermosiphon that is experimentally investigated.

An efficient cryocooler promises the development of HTS (High Temperature Superconductor) applications. Especially a Stirling-type PTR is generally suitable for small-size application due to its overall low-volume characteristic. In order to design an efficient Stirling-type PTR, it is needed the accurate prediction of its cooling performance along with the accompanied behavior of the linear compressor which drives the cooler. The objective of this research is to establish the design method for a Stirling-type PTR. In this research, the performance analysis code is developed with considering both the dynamics of a linear compressor and the thermo-hydraulic behavior of a PTR. In the experiment, the cooling performance of a cryocooler and the piston displacement are measured. This research would be helpful to design more efficient cryocooler.