Kannan Iyer

Influence of Initial Water Content and Specimen Thickness on the SWCC of Fine-Grained Soils

AbstractEarlier researchers have demonstrated that the compaction effort and water content, type of soil and mineralogy, void ratio, fabric, and stress history influence the soil-water characteristics curve (SWCC) of fine-grained soils to a great extent. However, it should be noted that compacting the soil and retrieving specimens from the compacted mass for suction measurements is a tedious task. Also, many times, identical soil specimens with similar compaction states may result in varied pore-size distribution characteristics. Hence, for establishing the SWCC, soil specimens in the form of slurry of adequate consistency would be quite handy, particularly for maintaining uniformity in specimen preparation. With this in view, the current study was carried out to understand the influence of initial water content on the SWCC and to recommend its best suited value for conducting experiments. It has also been noted that suction measurement devices, such as pressure membrane extractor and the dew point potent...

Influence of Drying and Wetting Cycles on SWCCs of Fine-Grained Soils

The soil-water characteristics curve (SWCC) is greatly influenced by the path that is followed for suction measurement (i.e., drying or wetting path) and the number of cycles of these paths to which the soil is exposed. To evaluate the influence of these parameters, drying- and wetting-path SWCCs of three fine-grained soils were developed for single and multiple cycles by employing the Aquasorp V R Isotherm generator. This device has been primarily employed for food products, powders, and amorphous materials for determination of their moisture sorption isotherm, which relates moisture content to water activity. As water activity can be mathematically related to soil suction, the Aquasorp V R can be used for developing the SWCC. Hence, demonstrating the potential of the Aquasorp V R for characterizing fine-grained soils becomes essential. The present study has been focused at highlighting the possible uncertainties associated with establishment of the SWCCs (both during drying and wetting paths) and the benefits and limitations of the Aquasorp V R in achieving these objectives. Furthermore, by employing mercury intrusion porosimetry (MIP), the significance of capillarity on soil suction, which in turn governs the SWCC, has been demonstrated.

Continuous determination of drying-path SWRC of fine-grained soils

Establishing the soil water retention curve, SWRC or the soil water characteristic curve, SWCC, is very useful for determination of unsaturated properties of soils. However, it has been observed that SWRC of a soil is not unique and depends on various factors such as the initial moisture content, density of soil, method of compaction, soil fabric and the path (drying or wetting) adopted for establishing it. In this context, many techniques and instruments have been employed by earlier researchers for determination of the SWRC of soils. However, these techniques entail weighing of the samples during prolonged testing, manually, and hence yield discrete data points. In this situation, AquaSorp® Isotherm Generator (manufactured by Decagon Devices Inc., USA) has been found to be quite useful for continuous determination of the drying-path SWRC of fine-grained soils. This device has been primarily employed for food products, powders and amorphous materials. Hence, demonstration of the utility and limitations of this device for SWRC determination of fine-grained soils becomes essential. With this in mind, extensive studies were conducted on commercially available soils (Kaolinite and Bentonite) by employing this device. In order to understand the influence of specimen specific parameters on the obtained SWRCs, the molding water content and thickness of the specimens were varied and the results have been evaluated critically. Details of the methodology adopted for these investigations, and the findings of the study are presented in this technical note. Based on a critical comparison of the results obtained from this device with those obtained from the dewpoint potentiameter, WP4®, the utility of this device for continuous determination of drying-path SWRC of the soils has also been demonstrated.

Effect of Loop Configuration on Steam Drum Level Control for a Multiple Drum Interconnected Loops Pressure Tube Type Boiling Water Reactor

For AHWR (Advanced Heavy Water Reactor), a pressure tube type Boiling Water Reactor (BWR) with parallel inter-connected loops, the Steam Drum (SD) level control is closely related to Main Heat Transport (MHT) coolant inventory and sustained heat removal through natural circulation, hence overall safety of the power plant. The MHT configuration with multiple (four) interconnected loops influences the SD level control in a manner which has not been previously addressed. The MHT configuration has been chosen based on comprehensive overall design requirements and certain Postulated Initiated Event (PIEs) for Loss of Coolant Accident (LOCA), which postulates a double ended break in the four partitioned Emergency Core Cooling System (ECCS) header. A conventional individual three-element SD level controller can not account for the highly coupled and interacting behaviors, of the four SD levels. An innovative three-element SD level control scheme is proposed to overcome this situation. The response obtained for a variety of unsymmetrical disturbances shows that the SD levels do not diverge and quickly settle to the various new set points assigned. The proposed scheme also leads to enhanced safety margins for most of the PIEs considered with a little influence on the 100% full power steady-state design conditions.

Effect of Coolant Inventories and Parallel Loop Interconnections on the Natural Circulation in Various Heat Transport Systems of a Nuclear Power Plant during Station Blackout

Provision of passive means to reactor core decay heat removal enhances the nuclear power plant (NPP) safety and availability. In the earlier Indian pressurised heavy water reactors (IPHWRs), like the 220 MWe and the 540 MWe, crash cooldown from the steam generators (SGs) is resorted to mitigate consequences of station blackout (SBO). In the 700 MWe PHWR currently being designed an additional passive decay heat removal (PDHR) system is also incorporated to condense the steam generated in the boilers during a SBO. The sustainability of natural circulation in the various heat transport systems (i.e., primary heat transport (PHT), SGs, and PDHRs) under station blackout depends on the corresponding systems coolant inventories and the coolant circuit configurations (i.e., parallel paths and interconnections). On the primary side, the interconnection between the two primary loops plays an important role to sustain the natural circulation heat removal. On the secondary side, the steam lines interconnections and the initial inventory in the SGs prior to cooldown, that is, hooking up of the PDHRs are very important. This paper attempts to open up discussions on the concept and the core issues associated with passive systems which can provide continued heat sink during such accident scenarios. The discussions would include the criteria for design, and performance of such concepts already implemented and proposes schemes to be implemented in the proposed 700 MWe IPHWR. The designer feedbacks generated, and critical examination of performance analysis results for the added passive system to the existing generation II & III reactors will help ascertaining that these safety systems/inventories in fact perform in sustaining decay heat removal and augmenting safety.

Modeling of Two-Phase Flow Instabilities in Microchannels

This paper addresses a non-dimensional analytical stability model aimed at predicting the occurrence of flow instabilities at micro-scale. In this context, linear stability model using homogenous flow was considered. Towards that, a linear stability model was developed using perturbation method. A characteristic equation (the response of pressure drop to a hypothetical perturbation in inlet velocity) obtained in this analysis, is shown to be a function of sub-cooling number, Zuber number, Froude number, friction number and inlet and outlet restriction coefficients. Then, a neutral dynamic stability curve is obtained using D-Partition approach. Similarly, static or excursive stability curve is also obtained from the characteristic equation. The derived analytical form for static and dynamic instability threshold is represented in the form of simplified correlations. The experimental data reported by other researchers agree well with these correlations. From the results, it is amply clear that for all practical purposes, two-phase cooling will be unstable. The question to be answered in future is, therefore, whether the oscillations that accompany can be tolerated from the application viewpoint.Copyright

Numerical simulation of contact melting using the cell-splitting modified enthalpy method

ABSTRACT The present work deals with the numerical simulation of contact melting using a cell-splitting enthalpy method, which is an improvement over the conventional enthalpy-porosity method. It is demonstrated that such a method is far superior to the enthalpy-porosity method which not only is unable to capture the interface precisely but is also unable to capture the melt rates, unless the coefficients are back-fitted with the experimental data. In contact melting, the contact layer is thin and hence to resolve the flow, fine grids have to be used. A novel integral model is proposed where a single control volume is used in the contact layer. A parametric study is performed for contact melting in a square geometry and a correlation is evolved for the melt rates. The shapes of the solid during contact and noncontact melting are discussed and the physical mechanisms that decide the evolution are articulated.

Compressible flow behaviour through wavy channels

The flow of compressible and incompressible fluids through channels with wavy walls is encountered in many engineering applications. There have been attempts to predict the distribution of fluid parameters like pressure, temperature etc using numerical and experimental investigations. In some cases mass flow estimation is the objective of analysis and in other cases heat flux enhancement is the objective. The present study aims at finding the mass flux through channels of varying wall surface geometry. The flow behavior through wavy channels changes drastically as the spacing between surfaces is reduced. Hence, in present work channels with sinusoidal, triangular and rectangular wavy surfaces are considered. The effects of wavelength, amplitude and pressure ratio on flow rate through wavy channels are analyzed. Correlations for non-dimensional mass flux are proposed for sinusoidal, triangular, rectangular wavy channels, which may be used in case of crack geometry.

Water retention characteristics of swelling clays in different compaction states

ABSTRACT The soil–water retention (SWR) characteristics of the clays play an important role in controlling their engineering behaviour, particularly, in the unsaturated state. Although, researchers have attempted to understand the water retention characteristics of the clays in their reconstituted or remoulded state, such studies are rare for the clays in their intact state. In this context, it becomes important to understand the influence of initial state of compaction, which would create different pore and fabric structure (viz., microstructure), on the water retention characteristics of the clays. With this in view, SWR behaviour was experimentally determined for the swelling clays (dried from different compaction states, viz., intact, reconstituted and remoulded) by employing Dewpoint PotentiaMeter (WP4C®). The changes in the pore size distribution of the clays at different stages of drying cycle were also studied by employing the Mercury Intrusion Porosimetry. The study reveals that the SWR curves for the intact and reconstituted specimens of the clays converge beyond a certain stage of drying. Also, a critical analysis of changes in the pore structure of the swelling clay specimens, during drying, indicates that the progressively deforming pore structure plays an important role in controlling its water retention characteristics to a great extent.

Some investigations to quantify hysteresis associated with water retention behaviour of fine-grained soils

ABSTRACT The soil water retention characteristics curve (SWRC) has been reported to be quite useful for estimation of unsaturated soil properties. However, the uniqueness of SWRC is questionable due to hysteresis associated with the drying- and wetting-path SWRCs and this poses great challenge in utilising the SWRC for reliable estimation of unsaturated soil properties. Although hysteresis associated with SWRCs has been extensively studied for coarse-grained soils, due to limited studies on wetting-path SWRC for fine-grained soils, the hysteresis for fine-grained soils is not well understood. The present work attempts to address this gap, by studying the drying- and wetting-path SWRCs for eight different fine-grained soils by employing Dew point Potentiameter (WP4C®), Environmental Chamber and Controlled Water Sprinkling method. The study employs the concept of ‘Suction Hysteresis’, ψh, for quantification of hysteresis. Further, the influence of various soil-specific properties on the variation of ψh-water content relationship (viz., slope of variation of suction hysteresis, Sψh) has also been studied and demonstrated. The findings of the study are quite encouraging and it has been realised that extensive studies on soils of different characteristics would be quite useful in quantifying the variation of SWRC during drying and wetting cycles.