• Energy Research

Abstract The hydro energy of the gravity water flow from the coal-fired thermal power plant units to the river in an open cooling system of turbine condensers is determined. On the basis of statistical data for a long time period, the water net head duration curve due to the river annual level change, as well as the reduction of the hydro energy potential due to the thermal power plant overhauls periods, are evaluated in the case study of the Thermal Power Plant “Nikola Tesla B” in Serbia. A small hydro power plant is designed for the utilization of this hydro energy, and the economic benefits of the project are calculated. The internal rate of returns and pay back periods are calculated in dependence of the electricity price and total investment costs. The increase of profitability is assessed, bearing in mind that the plant might be realized as the Clean Development Mechanism project according to the Kyoto protocol. The obtained results show that the project is economically attractive, and it can be carried out with standard matured solutions of hydro turbines available at the market. Even for the relatively low electricity price from small hydro power plants in Serbia of 0.08 €/kW h the internal rate of return and the pay back period are 17.5% and 5.5 years.

A model of two-component two-phase critical flow is presented. The modelling approach is based on one-dimensional homogeneous gas-liquid two-phase isentropic flow of mixture. The homogeneous model is modified by taking into account the void fraction and two-phase mixture density dependence on velocity slip. The velocity slip is calculated using Chisholm correlation that depends on the gas phase quality and Zivi correlation for the prediction of the maximum velocity slip values. At the location of the critical flow the two-phase mixture velocity equals sonic velocity and it is calculated with the so-called ''frozen sonic velocity' model. The model is validated against data measured in air water flow at the PUMA experimental facility. Obtained results are presented together with the predictions by the well-known Fauske model. It is shown that Fauske model overpredicts measured critical mass fluxes, while the present model shows acceptable agreement with the measured data.

Editorial of ECOS 2012 Special Issue

Steam accumulators are applied as buffers between steam generators and consumers in cases of different steam production and consumption rates. The application of the steam accumulator saves energy, reduces pressure fluctuations, and prevents aging of tubes and pressurized vessels in steam generators. In this paper, modes of the steam accumulator operation are analyzed and the general design of the steam accumulator control system is defined. Equilibrium and nonequilibrium thermodynamic models of the steam accumulator are presented with the aim of predicting the steam accumulator capacity and as support to the design of the control system. The equilibrium model is based on the mass and energy balance equations of the total water and steam content in the accumulator, while the nonequilibrium model is based on the mass and energy balance equations for each phase and closure laws of nonequilibrium evaporation and condensation rates. The steam accumulator pressure transients are simulated for constant steam ch...

Abstract Pressure changes in annular downward flows of condensing steam in vertical tubes are predicted with a three-fluid model. Several available correlations for the steam–liquid film interfacial friction are applied in the three-fluid model. Since discrepancies are obtained between calculated and measured pressure changes, a new correlation for the interfacial friction coefficient is proposed by taking into account the influence of the steam and liquid density ratio. The new correlation provides good agreement. The influence of the gravitational, frictional and acceleration pressure changes on the total pressure change is analysed, as well as the droplets entrainment from the liquid film surface to the steam core.

A steam turbine trip is followed by a prompt closure of stop valves in front of the turbine and consequently to a pressure rise in the main steam pipeline. This steam hammer transient leads to the generation of intensive fluid dynamic forces that act along the pipeline axis and induce additional dynamic loads on the main steam pipeline. It is a common practice to assume a simultaneous closure of all stop valves in the safety analysis of the main steam pipeline. In the present paper computer simulations and analyses of the fluid dynamic forces are performed for several scenarios that take into account the possibility of delayed closure of the stop valve in front of the turbine. The influence of the failure of the steam by-pass line opening is considered too. The results show that the delay of the stop valve closure increases the maximum intensity of fluid dynamic force in the pipeline segment in front of the stop valve and decreases the intensity of fluid dynamic forces in segments along the pipeline. The failure of the by-pass line to open leads to prolonged steam pressure and fluid dynamic forces oscillation in pipeline segments. The simulations were performed with the in-house computer code based on the method of characteristics for the solving of the hyperbolic system of PDE that represent the mass, momentum and energy balance equations of the 1-D, compressible and transient fluid-flow. The obtained results are a support to safety analyses of thermal power plants under transient conditions.

Abstract Steam accumulators are applied as buffers between steam generators and consumers in cases of different steam production and consumption rates. The steam accumulator is filled with water and steam and it operates with a sliding pressure. Dynamics of the steam accumulator charging and discharging depends on the accumulator volume, initial and boundary process parameters and condensation and evaporation rates. The non-equilibrium numerical model of steam accumulator is developed with the aim of predicting a steam accumulator capacity and as a support to the design of a control system. The model is based on the mass and energy balance equations for each phase and closure laws of phase transitions. The applied methodology is new regarding the commonly used equilibrium approach. The steam accumulator pressure transients are predicted for the case of periodic variable steam supply to a coal drying plant. The influence of the inlet steam enthalpy on the water mass and level change is investigated. The importance of the non-equilibrium evaporation and condensation modelling for the reliable prediction of steam accumulator transients is demonstrated by examples of charging and discharging tests.