Theodorus H. van der Meer

Experiments on a rotating-pipe swirl burner

Laser-Doppler measurements of mean velocity components and Reynolds stresses are reported in the near-field of a stable swirling flame of a newly designed natural-gas-fired experimental burner. The swirling motion is generated by the rotating outer pipe of the annular air passage, thus providing well defined inflow conditions which can be easily reproduced in computational and modelling studies. Prior to LDA measurements, burner stability characteristics were determined in terms of gas flow rate and pipe rotation speed. The measurements, corresponding to a stable, short, blue flame regime, show that the flow and combustion are dominated by a distinct, standing toroidal vortex, providing a hot core as source of stabilisation.

A numerical investigation on the vortex formation and flow separation of the oscillatory flow in jet pumps.

A two-dimensional computational fluid dynamics model is used to predict the oscillatory flow through a tapered cylindrical tube section (jet pump) placed in a larger outer tube. Due to the shape of the jet pump, an asymmetry in the hydrodynamic end effects will exist which will cause a time-averaged pressure drop to occur that can be used to cancel Gedeon streaming in a closed-loop thermoacoustic device. The performance of two jet pump geometries with different taper angles is investigated. A specific time-domain impedance boundary condition is implemented in order to simulate traveling acoustic wave conditions. It is shown that by scaling the acoustic displacement amplitude to the jet pump dimensions, similar minor losses are observed independent of the jet pump geometry. Four different flow regimes are distinguished and the observed flow phenomena are related to the jet pump performance. The simulated jet pump performance is compared to an existing quasi-steady approximation which is shown to only be valid for small displacement amplitudes compared to the jet pump length.

Prediction and Measurement of the Product Gas Composition of the Ultra Rich Premixed Combustion of Natural Gas: Effects of Equivalence Ratio, Residence Time, Pressure, and Oxygen Concentration

The ultra rich combustion (partial oxidation) of natural gas to hydrogen and carbon monoxide is theoretically and experimentally investigated. The effect of the process parameters equivalence ratio, residence time, pressure, and composition of the oxidizer is explored. Computations are performed with the use of the chemical kinetics simulation package CHEMKIN. First, the ultra rich combustion process is modeled as a freely propagating flame in order to establish the rich flame propagation properties. An Arrhenius correlation of the laminar flame speed with the adiabatic flame temperature is found with activation temperature 20,000 K. Subsequently, perfectly stirred reactor (PSR) computations were performed. From these, it is concluded that optimal natural gas conversion to hydrogen and carbon monoxide requires a residence time of at least 50 ms and, depending on residence time, an equivalence ratio between 2 and 4. To reach chemical equilibrium in ultra rich mixtures, the residence time is very long (> 1000 ms). The model predictions are validated by experiments on ultra rich combustion of natural gas by means of air enriched to 40% oxygen concentration at up to 3 bar and 300 kW. The effect of equivalence ratio at residence time 50 ms was investigated. Good comparison was found between measurements and model predictions on carbon monoxide, hydrogen, and the soot precursor acetylene. It can be concluded that the model provides reliable information on product gas concentrations as a result of ultra rich combustion of natural gas.

Co-production of synthesis gas and power by integration of Partial Oxidation reactor, gas turbine and air separation unit

A new synthesis gas production process based on partial oxidation (POX) of natural gas is investigated. Synthesis gas and mechanical power are generated in a plant that integrates a POX reactor, a synthesis gas turbine and an air separation unit. The synthesis gas is expanded in the turbine which runs the air separation unit. By means of an exergy analysis, it is shown that the synthesis gas turbine is a very efficient way of synthesis gas heat recovery. When compared with a conventional system (which uses a heat recovery steam generator), the new plant delivers 10 times more power, reduces with 26.4% the exergy losses and has an increased efficiency with 3.7%.

Characterization and reduction of flow separation in jet pumps for laminar oscillatory flows

A computational fluid dynamics model is used to predict the oscillatory flow through tapered cylindrical tube sections (jet pumps). The asymmetric shape of jet pumps results in a time-averaged pressure drop that can be used to suppress Gedeon streaming in closed-loop thermoacoustic devices. However, previous work has shown that flow separation in the diverging flow direction counteracts the time-averaged pressure drop. In this work, the characteristics of flow separation in jet pumps are identified and coupled with the observed jet pump performance. Furthermore, it is shown that the onset of flow separation can be shifted to larger displacement amplitudes by designs that have a smoother transition between the small opening and the tapered surface of the jet pump. These design alterations also reduce the duration of separated flow, resulting in more effective and robust jet pumps. To make the proposed jet pump designs more compact without reducing their performance, the minimum big opening radius that can be implemented before the local minor losses have an influence on the jet pump performance is investigated. To validate the numerical results, they are compared with experimental results for one of the proposed jet pump designs.

Jet pumps for thermoacoustic applications: design guidelines based on a numerical parameter study

The oscillatory flow through tapered cylindrical tube sections (jet pumps) is characterized by a numerical parameter study. The shape of a jet pump results in asymmetric hydrodynamic end effects which cause a time-averaged pressure drop to occur under oscillatory flow conditions. Hence, jet pumps are used as streaming suppressors in closed-loop thermoacoustic devices. A two-dimensional axisymmetric computational fluid dynamics model is used to calculate the performance of a large number of conical jet pump geometries in terms of time-averaged pressure drop and acoustic power dissipation. The investigated geometrical parameters include the jet pump length, taper angle, waist diameter, and waist curvature. In correspondence with previous work, four flow regimes are observed which characterize the jet pump performance and dimensionless parameters are introduced to scale the performance of the various jet pump geometries. The simulation results are compared to an existing quasi-steady theory and it is shown that this theory is only applicable in a small operation region. Based on the scaling parameters, an optimum operation region is defined and design guidelines are proposed which can be directly used for future jet pump design.

Flow Separation and Turbulence in Jet Pumps for Thermoacoustic Applications

The effect of flow separation and turbulence on the performance of a jet pump in oscillatory flows is investigated. A jet pump is a static device whose shape induces asymmetric hydrodynamic end effects when placed in an oscillatory flow. This will result in a time-averaged pressure drop which can be used to suppress acoustic streaming in closed-loop thermoacoustic devices. An experimental setup is used to measure the time-averaged pressure drop as well as the acoustic power dissipation across two different jet pump geometries in a pure oscillatory flow. The results are compared against published numerical results where flow separation was found to have a negative effect on the jet pump performance in a laminar flow. Using hot-wire anemometry the onset of flow separation is determined experimentally and the applicability of a critical Reynolds number for oscillatory pipe flows is confirmed for jet pump applications. It is found that turbulence can lead to a reduction of flow separation and hence, to an improvement in jet pump performance compared to laminar oscillatory flows.

Numerical investigation of heat transfer enhancement by carbon nano fibers deposited on a flat plate

Numerical simulations of flow and heat transfer have been performed for flow over a plate surface covered with carbon nano fibers (CNFs). The CNFs influence on fluid flow and heat transfer has been investigated. Firstly, a stochastic model for CNFs deposition has been explained. Secondly, the lattice Boltzmann model for simulating fluid flow and heat transfer is described. Finally, results from calculations for the heat transfer are presented. The results show substantial heat transfer enhancement for a densely covered surface with CNFs of varying length. From a detailed analysis of the results it is concluded that the enhancement is caused by boundary layer regeneration.

Plug-and-play liquid PV thermal panels - integrated design for easy manufacturing and installation

A photovoltaic-thermal panel or PVT panel simultaneously generates heat and electricity. This paper reports about the design of a new PVT panel by paying specific attention to user requirements, costs, manufacturing, building integration and installation. The panels technical aspects and energy performance where also optimized. The result is a plug-and-play liquid PVT panel which consists of a PV laminate glued on top of a plastic channel absorber. The panel is covered by a plastic layer to reduce heat losses. It is equipped with four connection points which enhance the flexibility of the positioning of PVT panels on a tilted roof. Because of this feature the installation process will be simplified and installation costs will be decreased. Since part of the production process can be automated, production costs can be decreased as well. In Western Europe at an irradiation of 1000 kWh/m2.year, the PVT panel is expected to yield about 100 kWh/m2 of electricity and 1.0 GJ/m2 of heat.

On the performance and flow characteristics of jet pumps with multiple orifices

The design of compact thermoacoustic devices requires compact jet pump geometries, which can be realized by employing jet pumps with multiple orifices. The oscillatory flow through the orifice(s) of a jet pump generates asymmetric hydrodynamic end effects, which result in a time-averaged pressure drop that can counteract Gedeon streaming in traveling wave thermoacoustic devices. In this study, the performance of jet pumps having 1-16 orifices is characterized experimentally in terms of the time-averaged pressure drop and acoustic power dissipation. Upon increasing the number of orifices, a significant decay in the jet pump performance is observed. Further analysis shows a relation between this performance decay and the diameter of the individual holes. Possible causes of this phenomenon are discussed. Flow visualization is used to study the differences in vortex ring interaction from adjacent jet pump orifices. The mutual orifice spacing is varied and the corresponding jet pump performance is measured. The orifice spacing is shown to have less effect on the jet pump performance compared to increasing the number of orifices.