As observed in the earlier studies, helium is more efficient than neon as a refrigerant in a reverse Brayton cryocooler (RBC) from the thermodynamic point of view. However, the lower molecular weight of helium leads to higher refrigerant inventory as compared to neon. Thus, helium is suitable to realize the high thermodynamic efficiency of RBC whereas neon is appropriate for the compactness of the RBC. A binary mixture of helium and neon can be used to achieve high thermodynamic efficiency in the compact reverse Brayton cycle (RBC) based cryocooler. In this paper, an attempt has been made to analyze the thermodynamic performance of the RBC with a binary mixture of helium and neon as the working fluid to provide 1 kW cooling load for high temperature superconductor (HTS) power cables working with a temperature range of 50 K to 70 K. The basic RBC is simulated using Aspen HYSYS V8.6^®, a commercial process simulator. Sizing of each component based on the optimized process parameters for each refrigerant is performed based on a computer code developed using Engineering Equation Solver (EES-V9.1). The recommendation is provided for the optimum mixture composition of the refrigerant based on the trade-off factors like thermodynamic efficiency such as the exergy efficiency and equipment considerations. The outcome of this study may be useful for recommending a suitable refrigerant for the RBC operating at a temperature level of 50 K to 70 K.

To maintain the superconducting state of high temperature superconductor (HTS) used for power application, higher refrigeration capacity cryocooler (>2 kW @ 70 K) is required. A reverse Brayton cycle based cryocooler with long-life and high reliability is one of the suitable candidates for cooling of HTS. The basic reverse Brayton cryocooler can be modified of basic configuration in order to achieve higher FOM. An analytical study focused on modified configurations of reverse Brayton cycle for HTS has been carried out. Neon is selected as working fluid since it has higher heat capacity than helium around 70 K. Six reverse Brayton based modified cycles are standard cycle, 2-stage compression, 2-stage series expansion, 2-stage parallel expansion (same pressure ratio), 2-stage parallel expansion (different pressure ratio), and modified cycle with intermediate cooling system for recuperator. The performances of the modified reverse Brayton cryocoolers are compared and reasons for performance variations are identified.

Vortex tube (VT) is a simple low refrigeration producing device having no moving part. However, the flow inside it is very complex. Recent studies show that the performance of VT improves with the increase in the divergence angle of a flared VT. To explore the temperature separation phenomenon in the VT, a three dimensional computational fluid dynamics (CFD) analysis of VT has been carried out. For the present work, a VT having diameter of 12 mm, length of 120 mm, cold outlet diameter of 7 mm and hot outlet annulus of 0.4 mm with 6 straight rectangular nozzles having area of 0.5 sq. mm each is considered. The turbulence in the flow field of the VT is modeled by standard k-e turbulence model considering Redlich-Kwong real gas model. The effect of variation of divergence angle of hot tube in the VT is studied and compared with the experimental results available in the literature. The temperature separation between the hot outlet and cold outlet, in both straight and 2 degree flared tube is studied. Analysis results indicate that for a hot mass fraction above 0.5, the flared tube shows better cold production capacity compared to the straight tube. Effect of important parameters like temperature gradient, velocities (axial, radial and tangential), velocity gradients, effective thermal conductivity and viscosity of fluid etc., on heat transfer and shear work transfer in the VT have been investigated. To understand the temperature separation mechanism, heat transfer and work transfer along the axial direction have been evaluated in both straight and flared tubes. The isentropic efficiency and COP as a refrigerator as well as a heat pump of straight tube and flared tube have been computed.

In the present study, the performance of an RBC cryocooler with 10 kW cooling load at 65 K has been analyzed using Aspen HYSYS V8.6^®. Helium, neon and helium-neon mixture are used as working fluids for the process cycle. A thermodynamic and cost model for the RBC cryocooler has been developed using exergoeconomic tool to analyze the cycle. It has been previously reported that to have same impact on the cycle exergy efficiency improvement, a small improvement in the heat exchanger effectiveness is required in contrast to a substantial improvement in the compressor efficiency. But it will also increase the capital cost of the cryocooler. The exergy efficiency, relative cost ratio and exergoeconomic factors for each component have been found to identify the major sources of cost. The effect of using different working fluids on the thermoeconomics of the cycle have been presented in this paper. Conclusions from this study might be utilized by the scientist and design engineer in improving the cycle efficiency from the thermoeconomic point of view.