Variation in (a) maximum fluid temperature and (b) mass flow rate of CO2-based loop with power and system pressure for. Variation in (a) maximum liquid temperature and (b) mass flow rate of CO2-based loop with power and sink temperature for.
Introduction
- Basic Physics of Natural Circulation Loop
- Application of NCL
- Advantages of NCL
- Simplicity
- Safety aspects
- Better flow distribution
- Flow characteristics
- Challenges of NCL
- Low driving force
- Low mass flux
- Instability effects
- Specification of a start-up and operating procedure
- Low critical heat flux
- Classification of NCL
- Supercritical Natural Circulation Loop (SCNCL)
- Motivation behind the Present Work
- Outline of the Thesis
The low mass flux also affects the critical heat flux of the system. The driving force of the system is the available driving force in the system and therefore no moving elements are required.
Review of Literature
Preamble
Supercritical water reactor (SCWR) is one of the key concepts of the generation IV initiative of nuclear reactors (Buongiorno and MacDonald, 2003). In addition, the higher steam enthalpy makes it possible to reduce the size of the turbine system, significantly reducing the capital cost of the conventional island.
Steady-state Flow Characteristics
Predicted steady-state profile of mass flow rate with heating effect was found to be quite similar to the observations of Chatoorgoon et al., 2005b. Influence of (a) loop diameter and (b) height on steady-state mass flow rate (Sharma et al., 2010a).
Heat Transfer Aspects of SCNCL
- Temperature differential between heat source and sink
- Heater and Cooler orientation
- Effect of inclination angle
- Effect of tube diameter
Under high heat flux conditions, the effect of inclination angle on heat transfer is less pronounced (Chen et al., 2013a). Yadav et al., 2012b also numerically concluded that both mass flow rate and heat transfer rate increase with increasing loop diameter.
Investigation on Flow Instability
- Numerical Investigations
Nyquist plot developed by Sharma et al., 2010c for an operating pressure of 25 MPa and a heater inlet temperature of 623 K. Comparison of stability maps developed by SUCLIN and NOLSTA for a 14 mm diameter loop (Sharma et al., 2010c).
Experimental Investigations
The design pressure of the loop was 40 MPa and both heaters were capable of delivering 20 kW of power. The temperature difference between heater and cooler was found to be dependent on the initial charge pressure in the loop and showed proportional effect on the final system pressure. None of the supercritical forced convection heat transfer correlations were found to provide reasonable prediction over the entire operating range, mainly due to failure to account for buoyancy force, nonuniform fluid temperature, and flow instability.
Due to the low critical temperature of CO2, the aggregate state of the working fluid was determined.
Observation from Literature Survey
One of the most critical issues in experimentation involving any NCL is the requirement of non-invasive instrumentation. Normally, pressure gauges and pressure transducers are used to measure the system pressure, while differential pressure transmitter is the most popular choice to get an indirect measurement of the mass flow rate of the loop. Widespread use of the coriolis mass flow meter can also be observed (Chen et al b), which is capable of producing highly accurate results, but at the cost of additional pressure loss.
Some of the early research suggested that the stability threshold corresponds to the peak mass flow rate.
Objectives of the Research
- Computational investigation
- Experimental investigation
Development of Computational Model
- Preamble
- Physical Geometry
- Scale-down Analysis of SCNCL
- Mathematical equations
- Non-dimensional Parameters
- Non-dimensional form of governing equations
- Conservation Equations and Mathematical Definitions
- Conservation of mass
- Conservation of momentum
- Conservation of energy
- Grid Generation and Sensitivity Analysis
- Thermo-Physical Property Estimation for Supercritical Fluids
- Numerical Scheme of Solution
- Selection of Turbulence Model
- Validation of Numerical Model
- Summary
The geometric dimensions of the basic model have been identified after a systematic downscaling analysis, which is detailed in §3.3. The following sections logically unfold all these features of the numerical procedure followed in this thesis. Most current numerical studies are based on this reduced model ('basic model').
Cross-sectional mesh distribution in the center plane of the source for four different models.
Thermalhydraulic Characterization of
- Preamble
- Validation of Numerical Model
- Comparison of Thermalhydraulic Parameters
- Comparison of Mass Flow Rate
- Mass Inventory Requirement
- Heat Transfer Aspects
- Epilogue
Variation of heat transfer coefficient at the source with (a) source temperature for 𝑇𝑐 = 315 K and (b) sink temperature for 𝑇ℎ = 331 K with. Variation of heat transfer coefficient of CO2 at source with (a) source temperature for 𝑇𝑐 = 315 K and (b) sink temperature for 𝑇ℎ = 331 K at. Variation of the heat transfer rate at the source with (a) source temperature for 𝑇𝑐 = 315 K and (b) sink temperature for 𝑇ℎ = 331 K with.
CO2 provides the greatest heat transfer rate at higher pressures and also when the wash temperature is reasonably below the pseudocritical value.
Characterization of Heat Flux Supported
Preamble
A rectangular 3D model of SNCCL was developed by Yadav et al., 2012c, 2012b to compare the flow and heat transfer behavior of subcritical single-phase water and sub- to supercritical CO2. Review of the literature suggests that thorough steady-state thermal-hydraulic analysis of SNCCL for various working fluids with heat flux-coupled conditions is still not well explored. Thus, it is imperative to conduct a comparative thermal-hydraulic analysis of loops under identical operating conditions, to check the feasibility of the choice of a supercritical system and also to propose a guideline for selecting the nature of the loop, subcritical or supercritical, based on the requirement. of energy transport.
Influences of operating parameters such as system pressure, input heat flux and coolant temperature on loop thermal hydraulics are established for all three fluids with a focus on performance comparison.
Validation of Numerical Model
Experimental validation is mandatory for any numerical model to assess its correctness and therefore current predictions are compared with the experimental correlation proposed by Swapnalee et al., 2012. As can be observed from Figure 5-1a, excellent agreement over the full range of parameters considered on a non-dimensional. Direct comparison is also made with the experimental data of Lomperski et al., 2004 and the present model is able to achieve better accuracy than the earlier correlation, especially at higher power levels, while the fit is exact at lower powers.
Lack of complete geometric information on the cooler side of the experimental facility of Lomperski et al., 2004 thwarts any further attempt to refine the computational model for improved validation at higher power levels.
Effect of Operating Pressure and Sink Temperature on Steady-
Variation of (a) peak fluid temperature and (b) CO2-based loop mass flow rate with constant sink system power and pressure. Variation of (a) peak liquid temperature and (b) CO2-based loop mass flow rate with power and sink temperature for a constant system. Variation of (a) peak liquid temperature and (b) mass flow rate of the R134a-based loop with constant sink system power and pressure.
Variation of R134a-based loop mass flow with power and sink temperature for a constant system pressure of 5 MPa.
Appearance of Flow Induced Heat Transfer Deterioration
Variation of the average heat transfer coefficient of the CO2-based loop with power for (a) a constant sink temperature of 305 K and (b) a constant. Axial variation of the surface-averaged heat transfer coefficient along the sink of a CO2-based loop for a system pressure of 8 MPa and a sink of 285 K. A low mass flow rate and thus a low 𝑅𝑒 results in a lower heat transfer coefficient with a decrease in the sink temperature.
Variation in average heat transfer coefficient of (a) R134a and (b) water-based loop with power, system pressure, and sink temperature.
FiHTD for Different Working Fluids
It has been recognized that the non-dimensional variation of enthalpy and density plots for selected fluids is almost identical to that of CO2 at a pressure of 8.6 MPa. Here the process of non-dimensionalizing the temperature is similar to non-dimensional enthalpy (section 3.3.2.). For healthy operation of SNCCL, the values of non-dimensional density and enthalpy should be greater than "1" and less than "0", respectively.
The correlations of Su with nondimensional density and enthalpy are also configured and shown in Figure 5-18.
Epilogue
Under identical operating conditions, the fluid temperature level for the CO2-based loop is the lowest among the three fluids until FiHTD occurs and thus the supercritical condition can be adopted. At higher powers, the inventory requirement for R134a is less than water, with similar peak temperature ordering, and therefore R134a may be a viable liquid in such situations, provided the chemical stability limit is respected . Overall, it can be concluded that sCO2-based SCNCL may be a better choice, as long as the power level can be limited to FiHTD, due to the higher flow rate and lower temperature levels of liquid.
Also, the stability analysis and dynamic performance assessment need to be done for SCNCL before a final conclusion is drawn and it can be seen as the next step in the research.
Guidelines for Selection of Geometric
- Computational Model Development
- Loop Diameter
- Loop Height
- Width
- Heater and Cooler Length
- Corner Bends
- Role of Relative Positioning of Source and Sink
- Effect of Inclination Angle
- Epilogue
Almost identical values of the average heat transfer coefficient can be observed for all loops up to the maximum. Variations in mass flow rate with heater power for three different lengths of the horizontal arms. Variations in mass flow rate with heater power for three different positions of the cooler in the upper horizontal arm 0.00.
Effect of relative positioning of source and sink on velocity vectors with 2 kW power supply.
Experimental Characterization of a
Preamble
Orientation of heater and cooler has a major influence on the flow pattern and thermal hydraulics of the system (Swapnalee et al., 2012). Scale-down study for a supercritical water loop was done by Rohde et al., 2011, considering R-23 as working fluid. Due to the non-toxic and environmentally friendly nature of R134a, it has been identified as possible substitutes for water, especially for laboratory-scale testing facilities or smaller energy transport systems.
Prior to development of the test rig, a systematic reduction analysis was performed and layout dimensions were finalized (see § 3.3).
Design and Development of Components
- Main structure
- Heater
- Power input circuit
- Cooler
- Cooling water connection
- Calibration of thermocouple
- Experimental procedure
To control the maximum temperature of the heater, i.e., as a safety device, the temperature controller is used in the circuit. The thermocouples are fixed to the surface of the inner tube with the help of a thermocouple bonding machine. The cooling circuit is the combination of the cooler, the thermostatic bath and the insulated pipes; which are used to connect the inlet and outlet of the cooler.
Initially the recirculation chiller is set at 15°C, when cooler temperature reaches 15°C both temperatures are recorded ie. the temperature of the cooler and thermocouple.
Numerical Comparison of Model and Prototype: Operating at
The fluid near the wall of the heater has a higher temperature and lower density, compared to the cooler bulk fluid near the centerline. Due to such local phenomenon, fluid near the top wall of the sink and the bottom wall of the source directly provides the heat and mass transfer to the main stream, leading to enhanced heat transfer at those locations. The range of non-dimensional numerical results is almost identical for both cases (Figure 7.8).
The presence of local uplift or broadly asymmetric profiles in both the model and the prototype ensures the similarity of the systems.
Results and Discussion
- Variation of temperature at different location
- Effect of sink temperature
- Effect of tilt
- Tilt on x-y plane
- Tilt on y-z plane
- Effect of pressure
- Loss-of-coolant experiment
- Inspection of insulation leakage
- Sudden increase/decrease of load over a steady state
- Step input load
Temperatures are measured using thermocouples placed on the outer surface of the tube. Most of the experimental results are compared with simulated data, here simulations were performed using ANSYS-Fluent 15. Yadav et al., 2017 also found a stable system, while Chen et al., 2013c observed small periodic waves has in temperature profile of the heater wall.
So, according to real situation, an experimental investigation is essential to study the system behavior, mostly stability performance of the system for such circumstances.
Epilogue
The center or average vertical distance between the heater and the cooler decreases as the tilt angle increases. Whereas, after tilting in the y-z plane, the system remains symmetrical in nature and therefore the influence of the tilt in the y-z plane is less pronounced than the x-y plane. For the selected range of study, no unstable results were found in the experiments and for both cases the system remains in steady state.
Detailed experimental analyzes have confirmed that the system is stable under all selected operating conditions.
Transient Simulations of a 2D SCNCL for
- Preamble
- Physical Geometry
- Grid Generation and Mesh Sensitivity Analysis
- Time Step Refinement Analysis
- Results and Discussion
- Stability Analysis of SCNCL
- Analysis of Flow Transients
- Dynamic Performance Assessment of SCNCL
- Epilogue
To apply the perturbation in the input heat flow of the system, different UDFs are used. The operating condition of the system was selected at a source heat flux of 700 W/m2, a well temperature of 355 K and a pressure of 10 MPa. Here the input power for the upper threshold is directly related to the sink temperature of the system.
The sign of the mass flow changes when the flow direction is changed.
Conclusions
Conclusions of the Present Work
- Numerical characterization of temperature coupled loop 179
- Influences of geometric parameters on steady-state
- Experimental assessment of steady-state and stability
- Transient analysis for stability appraisal of SCNCL
Scopes for Future Work
