Roger Hermansson

Heat and mass transfer simulations of the absorption process in a packed bed absorber

Numerical modelling of the absorption process in a cross-current absorber has been performed with FLOW3D, a commercially available software. The simulations are verified by comparisons with experimental results. The modelling of mass and heat transfer is discussed. Comparisons regarding the overall capacity as well as transfer rates show good agreement between experiments and simulations. It is possible to model the mass and heat transfer for a cross-current absorber if the equilibrium line for the absorption solution is known.

Particle-tracking option in Fluent validated by simulation of a low-pressure impactor

Abstract To decrease the emission of particles from a domestic wood-log boiler, particle traps based on inertial separation could be used. These could be designed and optimized with simulation tools, i.e. computational fluid dynamics (CFD) modeling. To find out whether the particle-tracking option in a commercial CFD code is reliable, a low-pressure impactor has been studied and each stage of the impactor simulated. By comparing experiments, it has been possible to determine the accuracy of the particle-tracking option for the CFD code. It has been shown that the particle-tracking procedure in the CFD software Fluent V6.0 offers good accuracy at velocities below 12 m/s with Reynolds numbers between 790 and 2150. Acceptable accuracy was shown for velocities of 20–170 m/s at Reynolds numbers between 960 and 2980. Considering actual flow velocities in a wood-log boiler and many other similar applications, the particle-tracking procedure should be accurate enough, at least for flows with a Reynolds number lower than 3000. Thus, it is recommended to use the software Fluent for simulations to find ways to reduce the particle emissions by use of mechanical traps. Three-dimensional simulations with either the Reynolds Stress Model (RSM) turbulence model, for accuracy, or the Renormalization Group (RNG) k–e model, to save calculation time, should be used together with the stochastic particle tracking.

Use of video-based Particle Image Velocimetry technique for studies of velocity fields in a water heat storage vessel

A video-based Particle Image Velocimetry technique has been developed. The technique is particularly suitable for measurement of small velocities, below 3 cm/s. It has proved to be useful for the documentation of non-stationary velocity fields in a scaled-down model of a water heat storage vessel. An ordinary video camera is used to record the in-plane movements of particles in a light sheet in seeded water. The hardware used, the experimental method and the accuracy of the method are discussed. The use of two commercially available software packages (NIH-Image and IGOR) for the analyses is described. Examples of velocity fields are presented, showing that the measuring technique can be used for studies of mixing near the inlet of the storage vessel and exchange of water between the boundary layer and the core.

Experimental studies during heat load fluctuations in a 500 kW wood-chips fired boiler

Abstract Several long-term experiments with fluctuating thermal outputs have been carried out in a newly developed biomass fuelled boiler suitable for small district heating networks. The experiments have been performed by either using the furnace only or the furnace together with a water heat store. Comparisons between these two operation strategies have been made concerning emissions and overall performance. Furthermore, the plant has been run to match a simulated heat demand during different seasons, in order to study the performance of the system during more realistic operation conditions. The results are very satisfactory concerning both performance and emissions, using any of the control strategies. Typical emissions of CO and NOx during the experiments are in the range of 10– 50 mg Nm −3 (5– 25 mg MJ −1 ) and 130– 175 mg Nm −3 (60– 90 mg MJ −1 ), respectively. However, during summer when the heat demand is low or zero, operational problems will occur if the heat store is excluded. Therefore, the main conclusion is that the most appropriate solution for a small district-heating system is to use a water heat store to match the heat load variations, while the furnace operates at as constant thermal output as possible.