Filip Verplaetsen

Development of an axial microturbine for a portable gas turbine generator

A miniature gas turbine is under development with the aim of generating electrical energy from fuel. This system consists of a compressor, combustion chamber, turbine and generator. The turbine is a single-stage axial impulse turbine (Laval turbine) with a rotor diameter of 10 mm, made of stainless steel using die-sinking electro-discharge machining. It has been tested with compressed air to speeds up to 160 000 rpm and generates a maximum mechanical power of 28 W with an efficiency of 18.4%. When coupled to a small generator, it generates 16 W of electrical power, which corresponds to an efficiency for the total system of 10.5%. The power density is mainly limited by the maximal speed of the ball bearings. The main losses are the blade profile losses and the exit losses. Higher speeds can considerably reduce the exit losses and therefore increase efficiency and power density. An improved turbine has been tested at temperatures up to 360 °C and generates up to 44 W of electrical energy with a total efficiency of 16%. A 20 mm diameter centrifugal compressor matching the pressure and flow characteristics of the turbine has been designed and is currently under construction.

The influence of an electric field on the heat transfer rate during film boiling of stagnant fluids

This paper describes the influence of an electric field on the heat transfer rate during film boiling of stagnant fluids (pool boiling) on a horizontal surface. Modelling the influence of an electric field on the heat transfer during film boiling requires the knowledge of the equilibrium shape of the liquid-vapour interface. This equilibrium shape is calculated using an iterative solution technique. In each iteration step a fourth order Runge-Kutta technique is used to calculate the shape of the interface and a boundary element method is used to calculate the electric field. It is shown that the vapour bubbles formed on this interface become elongated in the presence of an electric field. This effect is accounted for in the existing heat transfer models in order to describe the influence of the electric field on the heat transfer coefficient during film boiling.

Experimental study on the minimum ignition temperature of coal dust clouds in oxy-fuel combustion atmospheres

BAM furnace apparatus tests were conducted to investigate the minimum ignition temperature of coal dusts (MITC) in O2/CO2 atmospheres with an O2 mole fraction from 20 to 50%. Three coal dusts: Indonesian Sebuku coal, Pittsburgh No.8 coal and South African coal were tested. Experimental results showed that the dust explosion risk increases significantly with increasing O2 mole fraction by reducing the minimum ignition temperature for the three tested coal dust clouds dramatically (even by 100°C). Compared with conventional combustion, the inhibiting effect of CO2 was found to be comparatively large in dust clouds, particularly for the coal dusts with high volatile content. The retardation effect of the moisture content on the ignition of dust clouds was also found to be pronounced. In addition, a modified steady-state mathematical model based on heterogeneous reaction was proposed to interpret the observed experimental phenomena and to estimate the ignition mechanism of coal dust clouds under minimum ignition temperature conditions. The analysis revealed that heterogeneous ignition dominates the ignition mechanism for sub-/bituminous coal dusts under minimum ignition temperature conditions, but the decrease of coal maturity facilitates homogeneous ignition. These results improve our understanding of the ignition behaviour and the explosion risk of coal dust clouds in oxy-fuel combustion atmospheres.

A numerical study of the influence of ammonia addition on the auto-ignition limits of methane/air mixtures

In this study the auto-ignition limit of ammonia/methane/air mixtures is calculated based upon a perfectly stirred reactor model with convective heat transfer. The results of four different reaction mechanisms are compared with existing experimental data at an initial temperature of 723 K with ammonia concentrations of 0-20 mol.% and methane concentrations of 2.5-10 mol.%. It is found that the calculation of the auto-ignition limit pressure at constant temperature leads to larger relative deviations between calculated and experimental results than the calculation of the auto-ignition temperature at constant pressure. In addition to the calculations, a reaction path analysis is performed to explain the observed lowering of the auto-ignition limit of methane/air mixtures by ammonia addition. It is found that this decrease is caused by the formation of NO and NO(2), which enhance the oxidation of methane at low temperatures.

Study of the influence of an electric field on the liquid-vapor interface during film boiling of stagnant fluids

This paper describes the influence of an electric field on the liquid-vapor interface during film boiling from a horizontal surface of stagnant fluids (pool boiling). An iterative solution technique is used to calculate the equilibrium shape of the liquid-vapor interface during film boiling. In each iteration step, a fourth order Runge-Kutta integration technique is used to calculate the shape of the interface and a boundary element method to calculate the electric field. It is shown that an electric field elongates the vapor bubbles formed on this interface and that this effect has to be accounted for in the existing heat transfer models in order to describe the influence of an electric field on the heat transfer coefficient during film boiling.

Integrated solution for micro power generation

Battery systems have a much lower energy density than fuels. The use of fuel based micro power generation units instead of batteries can therefore become interesting in certain applications. This paper describes the problems that occur when designing such a power generation unit. It is shown that operational conditions and geometrical restrictions impose tough requirements on the design of different components. The solutions as found in traditional fuel based generation systems cannot simply be downscaled, but several concepts have to be fundamentally rethought

T5-2 – Auto-ignition hazard of mixtures of ammonia, hydrogen, methane and air in a urea plant

Publisher Summary In many chemical processes, combustible gases and vapors at high pressures and high temperatures are present. To evaluate the auto-ignition hazard involved and to ensure the safe and optimal operation of these processes, it is important to know the auto-ignition temperature (AIT) of the gas mixtures. The AIT values found in literature are usually determined according to standard test methods in small vessels and at atmospheric pressure. AIT is not a constant but decreases with increasing pressures and increasing volumes. These AIT values are often not applicable to industrial environments. In addition, most available AIT data refer to single-component fuels, while information on multicomponent fuels is scarce. In the study described in the chapter, attention is focused on the auto-ignition hazard inside a urea plant. In the ammonia scrubber and the pool reactor of the plant, mixtures of ammonia, methane, hydrogen, and air are exposed to a temperature of 150°C and a pressure of 15 MPa. To evaluate these mixtures for their auto-ignition characteristics, the AIT of ammonia/air mixtures is determined experimentally for pressures up to 7500 kPa and for concentrations ranging from 20 to 80 mol%. The effect of limited methane and hydrogen additions was also investigated.