Investigation on the thermohydraulic performance of high temperature superconducting (HTS) cables with heat loads using Entropy Generation Minimization (EGM) approach

摘要

High temperature superconducting (HTS) cables have revolutionized the way of power transmission in terms of carrying larger currents with lower volume of the conductor. However, such cables experience AC losses which are dissipated as heat. Hence, it is inevitable that these cables be cooled below the critical temperature of the superconductor constituting the HTS cable. In general, LN2 is circulated through the former (corrugated pipe) to retain the superconductivity of HTS tapes. The work consumed for pumping LN2 through long length HTS cables is significantly larger. Hence, it is necessary that the optimization of the pumping power with enhanced heat transfer is achieved. During the process of forced cooling of HTS cables, velocity and thermal gradients which are responsible for entropy generation is unavoidable. Hence, in the present work, an investigation on thermo-hydraulic performance in HTS cables with various heat loads is performed. Further, the volumetric rate of entropy generation is calculated and an optimum mass flow rate is identified at which minimum entropy generation rate is observed. The thermo-hydraulic performance and total entropy generation rate for internally cooled HTS cables are computationally investigated using the time averaged Reynolds Averaged Navier-Stokes (BANS) equations implemented in commercial software ANSYS (R). This implementation involves Finite Volume Method (FVM) of discretization with kappa - epsilon turbulence equations as closure to the BANS governing equations. Temperature dependent thermo-physical properties are considered for predicting the thermo-hydraulic behavior of LN2 in HTS cable. The analysis is carried out at heat loads ranging from 1-3 W/m with flow rates of 11-20 L/min at an operating temperature of 77 K and pressure of 2.7 bar. The dimensionless numbers such as Bejan number, entropy generation number and performance evaluation are studied to evaluate the optimum flow rate corresponding to lower pumping power and higher cooling capacity at various heat loads. Further, the results obtained from computational simulations are validated with the experimental results available in the literature. Furthermore, the results of volumetric entropy generation rate (EGR) are calculated to optimize the thermo-hydraulic performance in HTS cables using entropy generation minimization (EGM) approach.