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One of the highly efficient sources of renewable energy is the Ground Source Heat Exchangers (GSHE). Thermal interaction effects between boreholes cause a decrease in the efficiency of GSHEs. In the present study, the effects of the distance between boreholes, soil properties, grout heat conduction coefficient, and boreholes’ arrangement are investigated on the outlet fluid temperature in the GSHEs. The Finite Line Source (FLS) model is coupled with the Simplified Thermal Resistance-Capacity Model (STRCM) and a transient modified model called FLS-STRCM is developed. The FLS model considers the thermal interaction between boreholes while the STRCM model solves the flow field inside the boreholes. The obtained results of the FLS-STRCM are compared with the transient results of Computational Fluid Dynamics (CFD) simulations and available experimental data. To simulate the heating and cooling cycles of a building, the present model of GSHE is coupled with the water to air heat pump in the TRNSYS software. The available models of GSHEs in the TRNSYS software are not capable of considering different arrangements of boreholes. However, the present model, in addition to the transient simulation of GSHEs, can examine any desired layouts of boreholes. The results of the present model show that for a set of 6 boreholes in a rectangular arrangement, a reduction of the distance between boreholes from 7 m to 3 m results in 2.8% changes in the average temperature of outlet fluid. Also, the investigation of soil and grout properties shows that the soil properties has a dominant effect on the outlet fluid temperature. Considering the computational time and the acceptable accuracy of the presented model compared to the CFD methods, the FLS-STRCM model can be employed for the engineering applications in the construction of the GSHE system and the long-term simulations.

Rotating Vortex Rope (RVR) resulting from flow instabilities inside draft tube affects the hydraulic turbine's efficiency and wear. Among the passive and active RVR mitigation methods, the water injection method has been widely investigated lately. The present research focuses on the dynamics of RVR mitigation using the water injection method and the optimization of exergy loss during the water injection. The numerical simulations are performed using a reduced turbine geometry consisting of one stay vane, two guide vanes, one runner passage by applying periodic boundary conditions at side boundaries, and a complete draft tube. The pressure fluctuations, velocity field, and the RVR structure are well captured using the Shear Stress Transport - Scale Adaptive Simulation (SST-SAS) turbulence model. The results of the local swirl number during the load variation show that the onset of flow instabilities and RVR formation are created due to a decrease of the axial momentum below the runner cone. The required axial momentum flux of water injection is calculated for a stable swirl number and the local minimum ratio of dimensionless loss to the pressure recovery factor. The numerical results show that the rotating and plunging components of the pressure fluctuations inside the draft tube reduce during the water injection. The pressure recovery of the draft tube improves. The dynamics of RVR mitigation are visualized using λ2 criterion and the spectrogram of pressure probes on the draft tube wall. An optimum water flow rate and jet velocity for a fixed axial momentum are proposed. The results show that a water jet with a large radius and low velocity is more effective to mitigate the RVR and minimizing the draft tube losses.

Rotating Vortex Rope (RVR) has been a matter of focus for years due to the major effects on hydraulic turbine’s efficiency. The exact procedure of RVR formation is still vague. The present research focuses on the dynamics of the RVR formation during the load variation employing transient numerical simulations. Two different geometries including the full geometry and the reduced one, which consists of one stay vane, two guide vanes, one runner blade, one splitter blade and full draft tube, are considered. In order to capture the transient swirling flow features inside the draft tube, the Shear Stress Transport-Scale Adaptive Simulation (SST-SAS) model is utilized to approximate the turbulent stresses. The pressure results inside the draft tube agree well with the experimental measurements. Moreover, the velocity results show the central low-axial-velocity and high-tangential-velocity region in the draft tube properly. The flow structure is visualized using λ2 criterion. The dynamic of RVR and the physics behind the RVR formation are investigated during the load variation. The results indicate four flow regimes with different characteristics during RVR formation. The first flow regime is a stable swirling structure occurring at Best Efficiency Point (BEP). The second flow regime occurs at the beginning of the load variation where signs of flow instabilities appear. These instabilities are temporary and washed down by the upstream flow. Expanding the instabilities and creating the vortical structures in the draft tube are the important flow features in the third flow regime. The fourth flow regime is the presence of a developed rotating rope occurring at the Part Load (PL) condition. The flow regimes differ according to the size and shape of the stalled region during load rejection inside the draft tube cone. They also reveal that despite some shortcomings, the reduced model is reliable to simulate the RVR transient formation. The full geometry simulations could be also applicable for practical problems provided that the modified time step is slightly greater than the main blade rotational angle is used.