Thermalhydraulics & Gasification Laboratory

Thesis title

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Abstract

Natural circulation loops (NCLs) offer a very efficient option of heat transport from source to sink without any prime mover. Its structural simplicity and inherent reliability have resulted in diverse applications and particularly in nuclear reactor core cooling. The present dissertation focuses on theoretical modelling and experimental investigations of different aspects of NCLs.
A geometry independent unified model for single-phase NCLs is proposed through a set of generalized governing equations accompanied by the associated correlations and reference parameters. Comparison with five sets of experimental data from the literature exhibits good agreement. Non-linear stability analysis has been performed to identify the neutral condition. Rectangular loop has been found to be inherently more unstable compared to its equivalent toroidal configuration. Several options of enhancing stability margin of the rectangular loop, without greatly affecting the steady-state, has been analyzed and increase in loop height has been identified as the most beneficial option with respect to geometric parameters. Tilting the adiabatic arms by a small angle, while maintaining the rectangular shape, can be another excellent solution. The unified model was upgraded to a conjugate form as well incorporating wall conduction and ambient heat leakage. Heat loss is found to have significant adverse effect on the steady-state temperature profiles, thereby lowering the effectiveness. However, effectiveness increases with ambient temperature and approaches the ideally insulated condition at some particular ambient condition, which is a function of loop geometry and power level. Effectiveness variation with ambient temperature for different power input exhibits a crossover point.
A two-phase NCL analysis incorporating a mechanistic model for subcooled boiling has been performed to identify the influence of thermal non-equilibrium. Major effects were observed around the transition regions. A generalized working-regime map was developed to identify possible combination of fluid stream condition at boiler exit and condenser entry. Finally an extensive experimental investigation of phasic entrainment has been performed employing an appropriately scaled-down AHWR air-water loop. Droplet carryover has been observed to increase rapidly with air supply. Incorporation of a suitably porous suppressor plate has been demonstrated as an effective option leading to substantial reduction in carryover.

Thesis title

Defended on

Thesis supervisors

Abstract

A numerical model has been developed to predict the flame characteristics of laminar diffusion flame in a confined environment. A cylindrical stream of fuel (methane), surrounded by a co-flowing stream of oxidizer (air), is considered to produce an axisymmetric laminar diffusion flame. A two-step global reaction kinetics has been assumed. The numerical model adopts the SOLA (Solution Algorithm) technique for iterative solution. All the field variables, including the flow velocity, the temperature and concentrations of all the species involved in the reactions, are determined throughout the entire reaction domain by solving the conservation equations for mass, momentum, energy and species concentrations in their fully elliptic form, using the appropriate boundary conditions.
Velocity, temperature and species concentration profiles are observed to determine the flame characteristics. The contour of unity equivalence ratio serves as a representation of the flame surface. The flame contour is also represented by the heat release zone within the solution domain. The predictions from the present model agree reasonably well with an identical experimental study conducted by Bennett et al.
Parametric variation is done on the present code to have some very interesting and practically important results. By increasing the rim thickness of the inner cylinder, lifted flame is obtained, which shows a clear sign of partial premixing and a double-flame structure.
Increasing fuel flow rate increases the flame height, but not much variation happens with increasing oxidizer flow rate. The flame height increases when the oxygen concentration in air is lowered by diluting it with Nitrogen, but huge reduction is predicted in the peak flame temperature. It also reduces the formation of pollutant like soot and NO_x. Dilution of the fuel stream with CO₂ increases the flame height along with an increase in the peak flame temperature. All these results provide fundamental information about laminar non-premixed flame, which can be useful to the industries.