Chapter 2: Review of Literature
2.6. Observation from Literature Survey
oscillate between two-phase and single-phase, even leading to partial condensation at the cooler outlet during shutdown. Asymmetry in configuration was found to impose a flow direction. However, no flow reversal or unstable fluctuations were observed over the entire range of test conditions.
One of the most critical issues in experimentation involving any NCL is the requirement of non-intrusive instrumentation. NCLs are characterized by low driving head due to the absence of any designated prime mover. Presence of any kind of intrusion, in the form of thermocouple bids or anemometer port, leads to additional pressure drop, which is highly undesirable. Unfortunately, none of the experimental studies available in open literature none reports any novel attempt of developing sensing tools suited exclusively for natural circulation based systems, rather than opting for only the conventional ones.
Most of the researchers opted for an electric heater as the source (Chen et al., 2013b; Kiss et al., 2017; Sharma et al., 2014, 2013, 2012, 2010a; Swapnalee et al., 2012), while employing a variac for controlling the power input. Only a few works used a hot-side heat exchanger in the source section, and a thermostatic bath to control the concerned temperature (Yadav et al., 2017).
The cooling section is invariably a tube-in-tube type heat exchanger, with water being the most common choice as the secondary coolant. Recently Sadhu et al., 2018 developed an air-cooled heat exchanger as the cooler.
According to the level of temperatures involved, various types of thermocouples and resistance temperature detectors are use for measuring the temperature. Normally pressure gauges and pressure transducers are used for measuring the system pressure, whereas differential pressure transmitter is the most popular choice to get an indirect measurement of the mass flow rate of the loop. Scattered use of the coriolis mass flow meter can also be observed (Chen et al., 2016, 2013b), which is capable of providing highly accurate results, but at the expense of additional pressure loss.
various codes like SPORTS, FIASCO, NOLSTA and SUCLIN as examples, mostly for stability characterization of SCNCLs. Some sparse application of commercial software like ANSYS-Fluent and RELAP5 can also be found in recent times.
Both theoretical and experimental observations indicate that the steady-state mass flow rate increases with power supply, till it attains a maxima, and decreases afterwards, quite similar to two-phase NCLs. Some of the early research suggested the stability threshold to correspond to the peak mass flow rate. It was identified later, however, not to be a perfect guideline.
CO2 as a working fluid was predicted to result in higher velocity magnitude, along with an asymmetric cross-sectional variation, and larger heat transfer rates compared to water, thereby indicating CO2 as a better working medium.
Quite a few of such observations were also supported by experimental data. A systematic effort has been put forward to summarize the influence of geometric and operating parameters on steady-state performance of SCNCL.
Increase in loop height and diameter provides higher mass flow rate. Also the HHHC orientation results in highest mass flow rate among all possible configurations. Both mass flow rate and heat transfer rate increases with system pressure. With an increase in heater inlet temperature in an open loop, mass flow rate decreases. Inclination angle also has a similar effect. For high heat flux conditions, Nusselt number decreases with increasing inclination.
A thorough literature appraisal suggest that, both time-domain and frequency-domain approaches were employed for numerical stability predictions. Qualitative matching can be observed between the results obtained from the two approaches, but with a vast quantitative difference.
Unstable zone predicted by the time-domain approach is generally larger than the same predicted by frequency-domain approach. Detailed parametric analysis found that the increase in diameter and height increases the instability in the loop. Local losses provided in the cold leg has a stabilizing influence rather than that in the hot leg. The need of optimizing the coolant flow rate was experimentally stressed upon, as lower coolant flow was found to enhance instability. A power step-up was observed to have a stabilizing effect on the loop compared to power step-down. RELAP5 simulations predicted that slow increase of input heat flux can induce less flow instability
than a quick increase. The nature of such dynamic response, however, is still an open topic of further analysis.
A detailed scrutiny of the available literature clearly shows that the researchers have tried to explore different aspect of SCNCL, starting from steady-state thermalhydraulics to intricate transient and stability response, following theoretical, computational and experimental loops. While several facets of this relatively-new, but with tremendous potential, technology of SCNCL have been documented over the last decade and half, there are still quite a few glaring gray areas, which must be addressed before its widespread commercialization. At the vary onset, the technologists must justify the need of adopting supercritical condition over the conventional subcritical systems and must establish the relative gain in heat transfer performance over the complicacies brought in by the drastic transformation experienced by the supercritical fluid. Nearly all the available studies dealing with supercritical system, both numerical and experimental, focus on demonstrating the enhancement or deterioration in heat transfer for a single fluid, without attempting to compare with another one working under the same conditions.
It is more logical to assess the performance of different fluids, subcritical or supercritical, within the same regime of operating parameters, before deciding on a particular one. While water and CO2 can be identified as the two most popular fluid among researcher, the disparity in there critical point values is really striking. Among the common fluids, CO2 is characterized by one of the lowest critical temperature and moderate critical pressure. On the contrary, both values are quite extreme for water from lab-scale point of view. Therefore it is very much possible to envisage different thermodynamic states of CO2, with water continuing to be compressed liquid, which are expected to present very contrasting heat transport characteristics. Hence a systematic performance comparison between natural circulation loops operating with different working fluids characterized by contrasting thermodynamic states, is the need of the hour, in order to establish the superiority of SCNCL as a heat transport system. The lack of reliable experimental data and consequent heat transfer correlations for SCNCL is another major area of concern. The requirement of non-intrusive instrumentation and the involvement of higher levels of pressure and temperature, often makes it difficult to have precise experimental measurement, despite the loop having a simple geometrical look and very specific areas of energy interaction. That can be recognized as the
principal reason for contrasting conclusions reported in literature. Therefore sophisticated experimentation with SCNCL should be attributed as another one with topmost priority. The knowledge base regarding the stability response of SCNCL is also quite thin and is expected to receive enhanced attention in the coming days.