Inmyong Park

Ramping Operation of the Conduction-Cooled High-Temperature Superconducting Magnet for an Active Magnetic Regenerator System

An active magnetic regenerator (AMR) system requires fast ramp rate of magnetic field variation. The magnetic field change enables a magnetic refrigerant to create magnetocaloric effect for magnetic refrigeration. In this paper, a conduction-cooled high-temperature superconducting (HTS) magnet has been thermally analyzed and tested for an AMR system. A GdBCO conductor insulated with polyimide tape was wound by dry winding and standard double-pancake coil method. Stycast 2850FT was applied to the edge of the tape conductor so that the cooling plates, which were located at the top and bottom of the coil, should effectively cool the magnet. The whole thermal bridging between the magnet and a two-stage GM cryocooler is analyzed by the numerical simulation and a Joule heating experiment of the HTS magnet at normal state. The inductance of the HTS magnet was measured as 350 mH. The HTS magnet is intended to be operated with maximum 1 T/s at 20 K, cooled by the cryocooler. A solenoid switch and an external dump resistor were employed in order to discharge the HTS magnet. A continuous ramping operation test was conducted with a 60-A peak current and a maximum 20-A/s ramp rate by 3-kW dc power supply. This paper describes the experimental results of the ramping operation of the HTS magnet for an AMR system, and the technical issues on the results are discussed.

Experimental Investigation on Conduction-Cooled Fast-Ramping Layer-Wound (RE)BCO Superconducting Magnet for Magnetic Refrigeration

Magnetic refrigeration requires a strong magnetic field and its alternation for the magnetization and demagnetization processes. A conduction-cooled (RE)BCO coil was designed and fabricated to exert the maximum central magnetic field of 3.5 T with a field homogeneity of 90% on the magnetic refrigerant bed for an adiabatic demagnetization refrigerator (ADR) operating between 22 and 20 K. The polyimide tape insulated (RE)BCO tape conductor was wound on the stainless steel bobbin with standard layer-wound method whose outer diameter and thickness were 25.4 and 0.3 mm, respectively. The fast-ramping (RE)BCO coil was conductively cooled by a GM cryocooler to approximately 20 K. The (RE)BCO coil winding has a novel thermal drain structure to withstand the high thermal loads during alternating current (AC) operation which consists of high purity copper strip arrays installed between the inter-layers of the coil winding. Alternating magnetic field was stably generated with the peak of 3.0 T at the current of 130 A (10.5 A s-1). In this paper, the technical issues regarding to the fast-ramping conduction-cooled (RE) BCO coil itself and the integrated operation with the magnetic refrigeration system are discussed.

Measurement and analysis of energy flow in Stirling-type pulse tube refrigerator

The analysis of energy flow is a useful method for understanding a system. This paper focuses on the instantaneous measurement of physical conditions and analysis of energy flow in the Stirling-type pulse tube refrigerator. For measuring physical conditions of the working fluid such as the mass flow rate, the temperature, and the pressure, several cryogenic sensors are installed in the Stirling-type pulse tube refrigerator optimally designed in the operating condition of 60 Hz and 2.5 MPa. The physical conditions of the working fluid in the Stirling-type pulse tube refrigerator are carefully measured as varying the operating frequency and the charging pressure. From the measured results, the enthalpy, the PV work, and the loss are quantified in each location of the PTR and the conversion of energy flows is experimentally confirmed. The results of this paper can be used for understanding the optimal operating condition and modifying correlations of various losses in the Stirling-type pulse tube refrigerato...

Performance of the Fast-Ramping High Temperature Superconducting Magnet System for an Active Magnetic Regenerator

Fast magnetic field alternation is indispensable for continuous magnetic refrigeration. An active magnetic regenerative refrigerator (AMRR) utilizes magnetocaloric effect of magnetic materials which occurs during magnetization and demagnetization processes. A conduction cooled high temperature superconducting (HTS) magnet can be one of the prospective candidates as an alternating magnetic field generator. This paper describes the development effort of the cryogen-free HTS magnet for the AMRR. The magnet consists of twelve double pancake GdBCO coils which are insulated with polyimide tape. A two-stage GM cryocooler was employed to cool down the magnet. The critical current of the magnet was measured at the operating temperature before alternating current (AC) operation. In order to remove heat produced by AC loss with small temperature difference, thermal links between the cryocooler and the magnet were carefully designed. This paper presents the test results, AC loss analysis of the HTS magnet in the AMRR system. Maximum central magnetic field of 3 T (150 A) was achieved with the maximum ramping rate of 1 T/s (50 A/s). The AC loss was measured as 11.2 W at the operating conditions and the generated heate was effectively removed by the cryocooler. The AC loss was predicted by the numerical simulation and the simulation results were compared with the experimental results.

Experimental investigation of 20 K two-stage layered active magnetic regenerative refrigerator

The performance of a two-stage layered AMRR is experimentally investigated. The test apparatus includes two-stage layered AMRs, low temperature superconducting (LTS) magnet which generates maximum magnetic field of 4 T, and the helium gas flow system. The helium compressor with the tandem rotary valve is employed to generate the oscillating flow of the helium gas minimizing the pressure swing effect. The mass flow rate of working fluid is controlled separately at the first and second stages of the AMR by solenoid valves. The mass flow rate of the AMRs is measured by the mass flow meter and the cryogenic hot-film sensor which is calibrated at cryogenic temperature range from 20 K to 77 K. In order to reduce the heat leak by shuttle heat transfer of the working fluid, void volumes have been implemented and connected to the cold ends of the AMR1 and AMR2. The temperature span of the AMR is recorded as 52 K and the performance of the AMR with the variation of the mass flow rate is analysed. The results show that the mass flow rate and the heat leak due to the shuttle heat transfer by oscillating working fluid are crucial factors in the AMR performance.

Investigation on the two-stage active magnetic regenerative refrigerator for liquefaction of hydrogen

An active magnetic regenerative refrigerator (AMRR) is expected to be useful for hydrogen liquefaction due to its inherent high thermodynamic efficiency. Because the temperature of the cold end of the refrigerator has to be approximately liquid temperature, a large temperature span of the active magnetic regenerator (AMR) is indispensable when the heat sink temperature is liquid nitrogen temperature or higher. Since magnetic refrigerants are only effective in the vicinity of their own transition temperatures, which limit the temperature span of the AMR, an innovative structure is needed to increase the temperature span. The AMR must be a layered structure and the thermophysical matching of magnetic field and flow convection effects is very important. In order to design an AMR for liquefaction of hydrogen, the implementation of multi-layered AMR with different magnetic refrigerants is explored with multi-staging. In this paper, the performance of the multi-layered AMR using four rare-earth compounds (GdNi2...

Flow Boiling Heat Transfer Characteristic of Ternary Mixture in a Micro-Channel

This paper describes the flow boiling heat transfer of R123/R134a/R22 mixture in a single round micro-channel with 0.19 mm ID. The flow boiling heat transfer coefficients were measured for ternary mixture (R123/R134a/R22 mole fraction: 0.194/0.62/0.186) at various experimental conditions: mass velocities (314, 392, 470 kg/m2 -s), heat fluxes (10, 15, 20 kW/m2 ) and vapor qualities (0.2–0.8). The heat transfer characteristics of the R123/R134a/R22 mixture are similar to those of the R123/R134a mixture (mole fraction: 0.502/0.498) observed in the previous flow boiling experiment which indicates that major heat transfer mechanism in the microchannel is dominated by evaporation of thin liquid film around the elongated bubbles. The large reduction of heat transfer coefficients compared with pure refrigerant is observed in micro-channel flow boiling by mass transfer effect of mixed refrigerant.Copyright