An investigation of the two phase flow and force characteristics of a safety valve

The two phase flow of air and water is studied in a safety relief valve commonly used in the industrial refrigeration industry. In some blowdown conditions a valve which has been specified for gas only operation may encounter flows which include liquid droplets, these having been entrained from upstream processes. Of particular interest is the operation under two phase flow conditions which will be dominated by the altered flow capacity and forces acting on the valve. To examine these effects the characteristics of the flow through the valve and the forces acting on the valve disc have been examined for an air water mixtures for a range of pressures, 5-14 bar, and a wide range of gas mass fractions for various different opening positions. These characteristics determine the capacity of the valve to control pressure, the sizing of the spring and the dynamics of the valve during operation. The data on such effects for safety valves is very limited and here we show the trends that result from the introduction of two phase flow mixtures and reflect on the limited previous work in the literature that is available. The tested range of conditions the results indicate that the flow and force characteristics are influenced by liquid injection with a significant influence on the flowrate. The disc forces resulting from the two phase impacts show a small but notable effect from liquid mass fractions. This is in contrast to previous limited data found in the literature

A CFD study on two-phase frozen flow of air/water through a safety relief valve

The air-water two phase critical flows through a safety relief valve commonly used in the refrigeration industry is examined with particular emphasis on the prediction of the critical mass flowrates using CFD based approaches. The expansion of the gas through the valve and the associated acceleration is coupled to the liquid phase and results in changes to the velocity slip with the possibility of influencing the choking conditions and the magnitude of the critical mass flows. These conditions are poorly reported in the literature for safety valves. This paper presents a study where the ability of established two phase multi-dimensional modelling approaches to predict such conditions are investigated. Comparison with the simplified mixture model will show that this model tends to underestimate mass flowrates for medium to high liquid mass fraction. However, the two fluid model can adequately account for the thermal and mechanical non equilibrium for these complex flow conditions with the use of simplified droplet sizing rules

A CFD study of two-phase frozen flow of air/water through a safety relief valve

The air-water two phase critical flows through a safety relief valve commonly used in the refrigeration industry is examined with particular emphasis on the prediction of the critical mass flowrates using CFD based approaches. The expansion of the gas through the valve and the associated acceleration is coupled to the liquid phase and results in changes to the velocity slip with the possibility of influencing the choking conditions and the magnitude of the critical mass flows. These conditions are poorly reported in the literature for safety valves. This paper presents a study where the ability of established two phase multi-dimensional modelling approaches to predict such conditions are investigated. Comparison with the simplified mixture model will show that this model tends to underestimate mass flowrates for medium to high liquid mass fraction. However, the two fluid model can adequately account for the thermal and mechanical non equilibrium for these complex flow conditions with the use of simplified droplet sizing rules

Theoretical and experimental investigations on critical multiphase flows in safety relief valves and nozzles

Two-phase gas/liquid critical flows are in general complex due to the existence of mass, momentum and heat transfer between phases. In this study these processes have been investigated in a safety relief valve where the complex geometry introduces multiple choking points, shock waves, rapid accelerations involving rapid state changes of velocity, pressure and temperature and flow regime changes. These effects influence to a different degree the valve mass flowrate and the forces acting on the valve and thus the rating and dynamic response when in operation. At present no predictive tools are available for design evaluation of safety valves operating under two phase conditions. In this thesis the ability of the standard two fluid model to predict such processes is investigated.Two fluid (Euler-Euler) multi-dimensional modelling approaches with a mixture k-Ɛ turbulence model to predict such conditions are investigated. The study has been divided into two stages: firstly, for fixed mass fraction conditions air-water two phase critical flows through a conventional spring loaded safety relief valve commonly used in the refrigeration industry have been carried out experimentally and computationally. Secondly, to extend the study to thermal non equilibrium conditions investigations of steam-water two phase flows through a converging-diverging nozzle have been performed computationally. In the first stage, experiments have been carried out for a range of pressures (4.8 - 13.8 bar), a wide range of water fractions (0 - 0.89) and for various different opening positions of the valve disc. Quasi steady flow has been assumed appropriate and valve flow-lift and force-lift characteristics have been obtained, which determine the capacity of the valve to control pressure, the sizing of the spring and the dynamics of the valve during operation. Comparison with the simplified homogenous mixture model will show that this model tends to underestimate mass flowrates for medium to high liquid mass fraction. Force and flow scaling parameters have been explored for use in valve design under various multiphase flow conditions. In the second stage, for thermal non equilibrium conditions the numerical calculations have been made for a range of pressures (1.34 - 1.89 bar), a range of water mass fractions (0 – 0.36). The computational predictions of mass flowrate compare well with experimental data from the literature. In general, the two fluid model can adequately account for mechanical non equilibrium for these complex flow conditions with the use of simplified droplet sizing rules. Thus the CFD Euler-Euler model predictions have been found in good agreement with the experimental data. For thermal non equilibrium where phase changes dominate less progress was achieved and further investigated required.Two-phase gas/liquid critical flows are in general complex due to the existence of mass, momentum and heat transfer between phases. In this study these processes have been investigated in a safety relief valve where the complex geometry introduces multiple choking points, shock waves, rapid accelerations involving rapid state changes of velocity, pressure and temperature and flow regime changes. These effects influence to a different degree the valve mass flowrate and the forces acting on the valve and thus the rating and dynamic response when in operation. At present no predictive tools are available for design evaluation of safety valves operating under two phase conditions. In this thesis the ability of the standard two fluid model to predict such processes is investigated.Two fluid (Euler-Euler) multi-dimensional modelling approaches with a mixture k-Ɛ turbulence model to predict such conditions are investigated. The study has been divided into two stages: firstly, for fixed mass fraction conditions air-water two phase critical flows through a conventional spring loaded safety relief valve commonly used in the refrigeration industry have been carried out experimentally and computationally. Secondly, to extend the study to thermal non equilibrium conditions investigations of steam-water two phase flows through a converging-diverging nozzle have been performed computationally. In the first stage, experiments have been carried out for a range of pressures (4.8 - 13.8 bar), a wide range of water fractions (0 - 0.89) and for various different opening positions of the valve disc. Quasi steady flow has been assumed appropriate and valve flow-lift and force-lift characteristics have been obtained, which determine the capacity of the valve to control pressure, the sizing of the spring and the dynamics of the valve during operation. Comparison with the simplified homogenous mixture model will show that this model tends to underestimate mass flowrates for medium to high liquid mass fraction. Force and flow scaling parameters have been explored for use in valve design under various multiphase flow conditions. In the second stage, for thermal non equilibrium conditions the numerical calculations have been made for a range of pressures (1.34 - 1.89 bar), a range of water mass fractions (0 – 0.36). The computational predictions of mass flowrate compare well with experimental data from the literature. In general, the two fluid model can adequately account for mechanical non equilibrium for these complex flow conditions with the use of simplified droplet sizing rules. Thus the CFD Euler-Euler model predictions have been found in good agreement with the experimental data. For thermal non equilibrium where phase changes dominate less progress was achieved and further investigated required

Quality assurance of Cyberknife robotic stereotactic radiosurgery using an angularly independent silicon detector

Purpose: The aim of this work was to evaluate the use of an angularly independent silicon detector (edgeless diodes) developed for dosimetry in megavoltage radiotherapy for Cyberknife in a phantom and for patient quality assurance (QA). Method: The characterization of the edgeless diodes has been performed on Cyberknife with fixed and IRIS collimators. The edgeless diode probes were tested in terms of basic QA parameters such as measurements of tissue-phantom ratio (TPR), output factor and off-axis ratio. The measurements were performed in both water and water-equivalent phantoms. In addition, three patient-specific plans have been delivered to a lung phantom with and without motion and dose measurements have been performed to verify the ability of the diodes to work as patient-specific QA devices. The data obtained by the edgeless diodes have been compared to PTW 60016, SN edge, PinPoint ionization chamber, Gafchromic EBT3 film, and treatment planning system (TPS). Results: The TPR measurement performed by the edgeless diodes show agreement within 2.2% with data obtained with PTW 60016 diode for all the field sizes. Output factor agrees within 2.6% with that measured by SN EDGE diodes corrected for their field size dependence. The beam profiles\u27 measurements of edgeless diodes match SN EDGE diodes with a measured full width half maximum (FWHM) within 2.3% and penumbra widths within 0.148 mm. Patient-specific QA measurements demonstrate an agreement within 4.72% in comparison with TPS. Conclusion: The edgeless diodes have been proved to be an excellent candidate for machine and patient QA for Cyberknife reproducing commercial dosimetry device measurements without need of angular dependence corrections. However, further investigation is required to evaluate the effect of their dose rate dependence on complex brain cancer dose verification