Academic literature on the topic 'Thermohydraulic instabilities'

This is the first of a series of two papers reporting on a study of reflooding of a hot vertical tube. A mechanistic model of reflood is developed using a multifield modeling approach to analyze experimental data reported previously [1]. In Part 1 of this paper, the mathematical model for the thermohydraulic processes during reflood is derived from the general two-field conservation equations, and its structure is examined. Linear stability of the equation system is analyzed incorporating consideration of phase pressure differences. For inverted annular flow, the system is stable to short-wavelength perturbations and captures long-wavelength interfacial instabilities. The length of the most unstable waves is also derived in the analysis and agrees well with the available data. For dispersed flow, the system is predicted to become unstable if the Weber number exceeds a critical value of 8. In Part 2 [2], constitutive relations for reflood are fomulated and model is numerically solved for comparison with experimental data.

The possibility of instabilities in future nuclear reactors cooled by supercritical water is a matter of concern due to sharp changes in thermodynamic properties of coolant within the core. In the present work, a lumped parameter dynamic model of supercritical water-cooled reactor has been developed for analysis of coupled neutronic-thermohydraulic instabilities. The coolant channel is divided into two nodes with a moving boundary between them. The heater wall dynamics is described by a lumped parameter energy balance. Point neutron kinetics with one group of delayed neutrons has been used to model the power dynamics. Simple non-dimensional equations of state have been obtained for evaluating thermodynamic properties. Stability analysis has been done for various values of parameters such as the reactor power, coolant mass flow rate and inlet temperature. Stability maps have been plotted in the parameter planes. Dynamic simulations have been performed in the time domain to study the nature of operating transients. The stability analysis with neutronics is found to be more conservative. Transient simulations without neutronics indicate a supercritical Hopf bifurcation and the existence of a stable limit cycles in the unstable region. However, simulations with coupled neutronics indicate a subcritical Hopf bifurcation and the existence of unstable limit cycles in the stable region. Therefore, the analysis with neutronics is more conservative and shows that the system can be unstable for large perturbations, even if it is stable for small perturbations.

Passive systems like natural circulation (NC) loops can offer reliable and cost efficient alternatives to common active systems for decay heat removal in nuclear power plants. During the transition between stable single and stable two phase flows, instabilities e. g. flashing and geysering may occur in the riser due to low system pressure and saturation temperature conditions. These instabilities may cause severe stress to the system components. This paper presented some results of the study on the decay heat removal system based on natural circulation, performed on the open loop NC test facility GENEVA, built at TU Dresden in 2013. 16 probes were used to determine void fraction along the riser on nine different levels in high time and spatial resolution, and stability maps was created for riser with inner diameters of 20 mm and 38 mm and up to 85 kW evaporator power.

In thermohydraulic analysis, unsteady subcooled flow boiling is of vital importance to both natural circulation systems where flow instabilities are frequently encountered and offshore nuclear power systems which operate under the influence of ocean waves. An experimental study was conducted here to investigate how an imposed periodic flow oscillation affects the subcooled flow boiling heat transfer of water in a vertical tube. The average heat transfer characteristics and variations of the transient parameters are investigated. The result shows that there is a wall temperature overshoot as a consequence of boiling onset and the wall temperature downstream of boiling front could even drop below the saturation temperature under the high inlet subcooling of 75 °C. Under flow pulsation, intermittent flow boiling appears, when the imposed heat flux, q, is close to the boiling onset heat flux of steady flow, qs,onset. As a result of intermittent flow boiling, the average wall temperature of pulsating flow is lower than the wall temperature in steady flow when the q<qs,onset and the average wall temperature of pulsating flow is higher than the wall temperature in steady flow when the q>qs,onset. Moreover, during intermittent flow boiling, the boiling induced a decrease of the total pressure drop and can cause a large pressure fluctuation. In addition, two types variation of outlet fluid temperature fluctuations were observed and the wall temperatures present an extra local maximum and minimum values beside the extrema corresponding to the extremal mass flux.

In many industrial applications, mechanical structures like heat exchanger tube bundles are subjected to complex flows causing possible vibrations and damage. Part of fluid forces are coupled with tube motion and these so-called fluid-elastic forces can affect the structure dynamic behaviour generating possible instabilities and leading to short term failures through high amplitude vibrations. Most classical fluid force identification methods rely on structure response experimental measurements associated with convenient data processes. Owing to recent improvements in Computational Fluid Dynamics, numerical simulation of flow-induced vibrations is now practicable for industrial purposes. The present paper is devoted to the computation of fluid-elastic forces acting on tube bundles subjected to one-phase cross flows. What is the numerical process ? In the case where fluid-elastic effects are not significant and are restricted to added mass effects, there is no real coupling between structure and fluid motion. The structure displacement is not supposed to affect flow patterns. Thus it is possible to solve the fluid and the structure problems separately by using a fixed non-moving mesh for the fluid dynamic computation. Lift and drag forces acting on tube bundles can be computed numerically by using Large Eddy Simulation. Their spectrum and time history can be introduced as inlet conditions in the mechanical calculation providing the tube vibratory response. On the contrary when fluid-elastic effects can not be neglected, in presence tube bundles subjected to cross flows for example, a coupling between flow and structure computations is required. Such a calculation is performed in the present work. An improved numerical approach has been developed and applied to the fully numerical prediction of the dynamic behaviour of a flexible tube belonging to a fixed tube bundle subjected to cross flows. The purpose is to be able to provide a numerical estimate of the critical flow velocity for the threshold of fluidelastic instability of tube bundle without experimental investigation. The methodology consists in simulating in the same time thermohydraulics and mechanics problems by using an Arbitrary Euler Lagrange (ALE) formulation for the fluid computation. Numerical results turn out to be consistent with available experimental data obtained in the same configuration. This work is a first step in the numerical prediction of tube bundle vibrations in presence of cross flows.