P.K. Vijayan

Experimental observations on the general trends of the steady state and stability behaviour of single-phase natural circulation loops

The steady state flow in uniform or non-uniform diameter single-phase natural circulation loops (NCLs) can be expressed as Re ss = C[Gr m /N G ] r where the constants C and r depend on the nature of the flow (i.e. laminar or turbulent). Testing these correlations with experimental data showed reasonable agreement. The above equation suggests that simulation of the steady state flow in single-phase NCLs can be achieved by simulating the non-dimensional parameter (Gr m /N G ). The stability behaviour of a non-uniform diameter loop is found to depend on a large number of geometric parameters in addition to the modified Stanton number (St m ) and the ratio (Gr m ) b/(3 - b) / (N G ) 3/(3 - b) . The transient and stability behaviour of a uniform diameter loop can be simulated by simulating the product (Gr m ) b/(3 - b) × (D/L t ) 3/(3 - b) and St m if the length scales are preserved.

CFD analysis of single-phase flows inside helically coiled tubes

It has been well established that heat transfer in a helical coil is higher than that in a corresponding straight pipe. However, the detailed characteristics of fluid flow and heat transfer inside helical coil is not available from the present literature. This paper brings out clearly the variation of local Nusselt number along the length and circumference at the wall of a helical pipe. Movement of fluid particles in a helical pipe has been traced. CFD simulations are carried out for vertically oriented helical coils by varying coil parameters such as (i) pitch circle diameter, (ii) tube pitch and (iii) pipe diameter and their influence on heat transfer has been studied. After establishing influence of these parameters, correlations for prediction of Nusselt number has been developed. A correlation to predict the local values of Nusselt number as a function of angular location of the point is also presented.

Scaling laws for single-phase natural circulation loops

Abstract The power-to-volume scaling laws used for the construction of scaled test facilities simulating the primary system of nuclear power plants result in loops of the same elevation (and length) with reduced diameters. The adequacy of these scaling laws for simulating single-phase natural circulation was tested in three rectangular loops, each having the same elevation but different loop diameters of 6 mm, 11 mm and 23.2 mm respectively. The experiments showed that the power-to-volume scaling principles adequately describe the steady state behaviour. The stability behaviour observed in the loop 23.2 mm in diameter, however, could not be reproduced in the smaller diameter loops. Subsequent theoretical investigation of the single-phase natural circulation phenomenon showed that the transient and stability behaviour can be simulated only if the diameter ratio D p / D m is also simulated. The theoretical investigation suggested the following scaling laws for single-phase natural circulation: ( Gr m ) p =( Gr m ) m L p L m = D p D m = H p H m = (L h ) p (L h ) m ( St m ) p =( St m ) m For simulating the steady state behaviour alone, it is sufficient to simulate the product Gr m ( D / L ).

Hepatoprotective effect of the total alkaloid fraction of Solanum pseudocapsicum leaves

The total alkaloid fraction of the methanol extract of leaves of Solanum pseudocapsicum was tested for its hepatoprotective activity against CCl 4 induced toxicity in freshly isolated rat hepatocytes, HepG2 cells and animal models. The total alkaloid fraction was able to normalise the levels of aspartate amino transferase (ASAT), alanine aminotransferase (ALAT), alkaline phosphatase (ALP), triglycerides (TGL), total proteins, albumin, total bilirubin and direct bilirubin, which were altered due to CCl 4 intoxication in freshly isolated rat hepatocytes and also in animal models. The antihepatotoxic effect of the total alkaloid fraction was observed at very low concentrations (6–10µg/ml) and was found to be superior to that of the standard used. A dose dependent increase in the percentage viability was observed when CCl 4 exposed HepG2 cells were treated with different concentrations of the total alkaloid fraction. The highest percentage viability of HepG2 was observed at a concentration of 10µg/ml. Its in vivo hepatoprotective effect at 20 mg/kg body weight was comparable with that of the standard at 250mg/kg body weight. The total alkaloid fraction merits further investigation to identify the active principles responsible for the hepatoprotective properties. The results from the present investigation also indicate well correlation between the in vivo and in vitro studies.

Analytical study of nuclear-coupled density-wave instability in a natural circulation pressure tube type boiling water reactor

Abstract An analytical model has been developed to study the nuclear-coupled density-wave instability in the Indian advanced heavy water reactor (AHWR) which is a natural circulation pressure tube type boiling water reactor. The model considers a point kinetics model for the neutron dynamics and a lumped parameter model for the fuel thermal dynamics along with the conservation equations of mass, momentum and energy and equation of state for the coolant. In addition, to study the effect of neutron interactions between different parts of the core, the model considers a coupled multipoint kinetics equation in place of simple point kinetics equation. Linear stability theory was applied to reveal the instability of in-phase and out-of-phase modes in the boiling channels of the AHWR. The results indicate that the stability behavior of the reactor is greatly influenced by the void reactivity coefficient, fuel time constant, radial power distribution and channel inlet orificing. The delayed neutrons were found to have a strong influence on the Type I and Type II instabilities observed at low and high channel powers, respectively. Also, it was found that the coupled multipoint kinetics model and the modal point kinetics model predict the same threshold power for out-of-phase instability if the coupling coefficient in the former model is half the eigen value separation between the fundamental and the first harmonic mode in the latter model. Decay ratio maps were predicted considering various operating parameters of the reactor, which are useful for its design.

In Vitro Cytotoxicity and Anti-tumor Properties of the Total Alkaloid Fraction of Unripe Fruits of Solanum pseudocapsicum

The total alkaloid fraction of the methanol extract of unripe fruits of Solanum pseudocapsicum was tested for in vitro cytotoxicity on HEp-2, RD and Vero cell lines and anti-tumor activity using DLA and HEp-2 cell lines. Cell viability and morphological changes were assessed. The total alkaloid fraction exhibited strong cytotoxic activity against all the cell lines tested. The CTC 50 (50 % of cytotoxicity inhibition) was found to be 1.65 µg/ml for the RD cell line, 6.32 µg/ml for the HEp-2 cell line and 12.01 µg/ml for the Vero cell line. In the clonogenic assay, no colony formation was observed even up to a concentration of 25 µg/ml. In the short term, antitumor, studies using DLA cells, the total alkaloid fraction was associated with 50 % viability in the concentration range of 6.25-12.5 µg/ml. In long term, anti-tumor activity using the HEp-2 cell line, no colony formation was observed up to a concentration of 20 µg/ml. Hence, there is a correlation between the results obtained in the cytotoxicity and antitumor studies carried out. Morphological observation by phase contrast microscopy revealed intense damage on all the cell lines. The total alkaloid fraction has the potential for further investigation in animal models.

Effect of Loop Diameter on the Steady State and Stability Behaviour of Single-Phase and Two-Phase Natural Circulation Loops

In natural circulation loops, the driving force is usually low as it depends on the riser height which is generally of the order of a few meters. The heat transport capability of natural circulation loops (NCLs) is directly proportional to the flow rate it can generate. With low driving force, the straightforward way to enhance the flow is to reduce the frictional losses. A simple way to do this is to increase the loop diameter which can be easily adopted in pressure tube designs such as the AHWR and the natural circulation boilers employed in fossil-fuelled power plants. Further, the loop diameter also plays an important role on the stability behavior. An extensive experimental and theoretical investigation of the effect of loop diameter on the steady state and stability behavior of single- and two-phase natural circulation loops have been carried out and the results of this study are presented in this paper.

Linear and Nonlinear Stability Analysis of a Supercritical Natural Circulation Loop

Supercritical water (SCW) has excellent heat transfer characteristics as a coolant for nuclear reactors. Besides it results in high thermal efficiency of the plant. However, the flow can experience instabilities in supercritical water cooled reactors, as the density change is very large for the supercritical fluids. A computer code SUCLIN has been developed employing supercritical water properties to carry out the steady-state and linear stability analysis of a SCW natural circulation loop (SCWNCL). The conservation equations of mass, momentum, and energy have been linearized by imposing small perturbation in flow rate, enthalpy, pressure, and specific volume. The equations have been solved analytically to generate the characteristic equation. The roots of the equation determine the stability of the system. The code has been benchmarked against published results. Then the code has been extensively used for studying the effect of diameter, heater inlet temperature, and pressure on steady-state and stability behavior of a SCWNCL. A separate computer code, NOLSTA, has been developed, which investigates stability characteristics of supercritical natural circulation loop using nonlinear analysis. The conservation equations of mass, momentum, and energy in transient form were solved numerically using finite volume method. The stable, unstable, and neutrally stable points were identified by examining the amplitude of flow and temperature oscillations with time for a given set of operating conditions. The stability behavior of loop, predicted using nonlinear analysis has been compared with that obtained from linear analysis. The results show that the stability maps obtained by the two methods agree qualitatively. The present paper describes the linear and nonlinear stability analysis models and the results obtained in detail.

Study on the stability behaviour of a natural circulation pressure tube type boiling water reactor

The stability behaviour of a natural circulation pressure tube type boiling water reactor (BWR) has been investigated analytically. The analytical model considers homogeneous two-phase flow, a point kinetics model for the neutron dynamics and a lumped heat transfer model for the fuel dynamics. The results indicate that both Type I and Type II density-wave instabilities can occur in the reactor in both in-phase and out-of-phase mode of oscillations in the boiling channels of the reactor. The delayed neutrons were found to have strong influence on the stability of Type I and Type II density-wave instabilities. Also, the stability of the reactor is found to increase with increase in negative void reactivity coefficient unlike that observed previously in vessel type BWRs. Decay ratio map was predicted considering the effects of channel power, channel inlet subcooling, feed water temperature and channel exit quality, which are useful for the design of the reactor.

The limits of conditional stability for single-phase natural circulation with throughflow in a figure-of-eight loop

Abstract This paper is based on the investigations on the stability of natural circulation with throughflow in a figure-of-eight loop, a configuration that is employed in the pressure tube type heavy water reactors. The investigations have been both experimental as well as theoretical. The latter include both the linear stability analysis as well as the nonlinear finite difference analysis. The finite difference analysis has been used not only to identify the limits of the stable and unstable regimes but also used to identify the limits of the conditionally stable regime. Good agreement is found between theory and experiment.