Imran Rafiq Chughtai

Numerical Simulation of Direct-contact Condensation from a Supersonic Steam Jet in Subcooled Water

Abstract The phenomenon of direct-contact condensation, used in steam driven jet injectors, nuclear reactor emergency core cooling systems and direct-contact heat exchangers, was investigated computationally by introducing a thermal equilibrium model for direct-contact condensation of steam in subcooled water. The condensation model presented was a two resistance model which takes care of the heat transfer process on both sides of the interface and uses a variable steam bubble diameter. The injection of supersonic steam jet in subcooled water tank was simulated using the Euler-Euler multiphase flow model of Fluent 6.3 code with the condensation model incorporated. The findings of the computational fluid dynamics (CFD) simulations were compared with the published experimental data and fairly good agreement was observed between the two, thus validating the condensation model. The results of CFD simulations for dimensionless penetration length of steam plume varies from 2.73-7.33, while the condensation heat transfer coefficient varies from 0.75-0.917 MW·(m 2 ·K) −1 for water temperature in the range of 293-343 K.

Study of axial mixing, holdup and slip velocity of dispersed phase in a pulsed sieve plate extraction column using radiotracer technique.

Axial mixing, holdup and slip velocity of dispersed phase which are parameters of fundamental importance in the design and operation of liquid-liquid extraction pulsed sieve plate columns have been investigated. Experiments for residence time distribution (RTD) analysis have been carried out for a range of pulsation frequency and amplitude in a liquid-liquid extraction pulsed sieve plate column with water as dispersed and kerosene as continuous phase using radiotracer technique. The column was operated in emulsion region and (99m)Tc in the form of sodium pertechnetate eluted from a (99)Mo/(99m)Tc generator was used to trace the dispersed phase. Axial dispersed plug flow model with open-open boundary condition and two points measurement method was used to simulate the hydrodynamics of dispersed phase. It has been observed that the axial mixing and holdup of dispersed phase increases with increase in pulsation frequency and amplitude until a maximum value is achieved while slip velocity decreases with increase in pulsation frequency and amplitude until it approaches a minimum value. Short lived and low energy radiotracer (99m)Tc in the form of sodium pertechnetate was found to be a good water tracer to study the hydrodynamics of a liquid-liquid extraction pulsed sieve plate column operating with two immiscible liquids, water and kerosene. Axial dispersed plug flow model with open-open boundary condition was found to be a suitable model to describe the hydrodynamics of dispersed phase in the pulsed sieve plate extraction column.

Review of Storage Techniques for Sparse Matrices

This paper reviews the current state of knowledge of the storage formats for sparse linear systems. Here we consider the ways developed so far for storing a sparse matrix and their quoted effects on computational speed. The main idea behind these formats involves keeping both the indices and the non-zero elements in the sparse matrix in a single data structure. These specialized schemes not only save storage but also yield computational savings. Since the locations of the non-zero elements in the matrix are known explicitly, unnecessary computations involving zeros can be avoided. Thus the use of these formats reduces additional memory required in the usual indexing based storage schemes and gives promising performance improvements

Axial dispersion, holdup and slip velocity of dispersed phase in a pulsed sieve plate extraction column by radiotracer residence time distribution analysis

Axial dispersion, holdup and slip velocity of dispersed phase have been investigated for a range of dispersed and continuous phase superficial velocities in a pulsed sieve plate extraction column using radiotracer residence time distribution (RTD) analysis. Axial dispersion model (ADM) was used to simulate the hydrodynamics of the system. It has been observed that increase in dispersed phase superficial velocity results in a decrease in its axial dispersion and increase in its slip velocity while its holdup increases till a maximum asymptotic value is achieved. An increase in superficial velocity of continuous phase increases the axial dispersion and holdup of dispersed phase until a maximum value is obtained, while slip velocity of dispersed phase is found to decrease in the beginning and then it increases with increase in superficial velocity of continuous phase.

Implementation and Evaluation of Parallel Sparse Matrix-Vector Products on Distributed Memory Parallel Computers

The sparse matrix vector product (SMVP) is the kernel operation in many scientific applications. This kernel is an irregular problem, which has led to the development of several compressed storage formats. This paper discusses scalable implementations of sparse matrix-vector products, which are crucial for high performance solutions of large-scale linear equations, on distributed memory parallel computers using message passing. Five storage formats for sparse matrices are evaluated. We conduct numerical experiments on several different sparse matrices and show the parallel performance of our sparse matrix-vector product routines

Low cost indigenous particle imaging velocimetry

This study investigates the flow visualization using PIV technique. It is tried to indigenously develop PIV technique with the help of low power laser, ordinary camera, and available tracer particles. PIV results are obtained with the help of some open software. Flow patterns of water containing tracer particles are studied. Flows are induced by magnetic stirrer and rotor. Flow visualization study is very useful in finding velocity profiles of different flows. Instantaneous vector maps of flow field and calibration of velocities are presented. Further studies are under consideration in PIEAS to develop the mature PIV technique.

CFD study of natural convection heat transfer from an enclosed assembly of vertical cylinders

In the present work, CFD study was performed for a computational domain of a 3 × 3 square array of vertical heat generating cylinders with a pitch to diameter ratio (P/D) equal to 2. The assembly was enclosed in a 127 mm diameter pipe. Each rod had a diameter of 12.7 mm and a heating length of 570 mm. The fluid entered and left the computational domain radially. Constant heat flux boundary conditions were employed for different heat flux values. Due to the radial entrance of fluid, swirls are formed at both inlet and outlet which resulted in turbulence. K-Omega model was used to include the effects of turbulence. Surface temperature for different positions of several cylinders was calculated and compared with experimental results. A good agreement was found between experimental data and computational results. The heat transfer coefficient, modified Rayleigh number and Nusselt number can also be calculated. After developing the method, temperature and velocity profile for different values of heat flux, heating rods arrangements and dimensions of the assembly can be predicted for design purposes. Correlations based on simulations results can be proposed for natural convection calculations in assembly of vertical heating rods.

Analysis of natural convection in IPWR type SMR test facility

In advanced and conventional reactors natural circulation plays an important role in cooling of core. This dissertation is based on modeling of a test facility using RELAP5/MOD 4.0. Test facility (TF) comprises of heating core, riser, helical coil heat exchanger, and shell and tube heat exchanger and down comer these components are joined with associate piping and fittings. In the TF, heating core is cooled by natural circulation. In this thesis, analysis of natural circulation depends on geometry and operational parameters. The primary objective of this thesis is to investigate the effects of riser height, riser diameter, Source and sink distance, power and secondary side mass flow rate on primary loop mass flow rate, which is established due to natural convection. Natural circulation loop of TF modeled with the aid of RELAP5/MOD 4.0 thermal hydraulic system code. Riser length=78 inch, Pressure= 1Bar, Pitch to diameter = 1.4, SS Flow rate = 0.45kg/s, ΔH = −5 inch, Riser Diameter = 6 inch and power 37.0kw. Primary loop mass flow rate is 0.17kg/s (612kg/hr), analytical solution mass flow rate is 0.18kg/s, average error is 12.8% in computational solution. This error reduces as power increases with constant intervals. The RELAP5/MOD 4.0 results give good agreement with analytical solution.

Experimental study and CMFD modeling of transport processes in flow of two Immiscible liquids

Rectangular shaped sieve plate column was installed for study of transport parameters through high speed photography and image analysis. Two Immiscible liquids consisting of the dispersed phase (water) and continuous phase (kerosene oil) were introduced in countercurrent operation. Experiments were performed for dispersed and continuous phase superficial velocities 0.16-0.44 cm/sec and 0.37-0.47 cm/sec respectively, pulsation frequency 0.95 -1.72 rev/sec and pulsation amplitude 1cm. Upon increasing the superficial velocity of dispersed phase, its void fraction increases up to its asymptotic value and droplet velocity increase while the droplet size decreases. Upon increasing the pulsation frequency, its void fraction increases while the droplet size and droplet velocity decreases. A Computational Multi Fluid Dynamics simulation has been carried out in order to verify the experimental results using the FLUENT. The CMFD calculations for void fraction have been compared with the experimental results which have shown a maximum deviation of 2-3%.