Sven B Andersson

Expansion of a freely bubbling fluidized bed

Abstract The expansion of a freely bubbling fluidized bed is studied over a range of particle properties and gas velocities that applies to fluidized bed boilers. A bed expansion model is derived from a modified two-phase flow model. The results from the model are compared with measurements in both a cold two-dimensional bed and a 16 MW th fluidized bed boiler, as well as with data found in the literature. The model represents experimental results for sand particles of a diameter ranging from 0.15 mm to 4.0 mm and with gas velocities up to 3 m s −1 .

Effects of Multiple Injections on Engine-Out Emission Levels Including Particulate Mass from an HSDI Diesel Engine

The effects of multiple injections on engine-out emissions from a high-speed direct injection (HSDI) diesel engine were investigated in a series of experiments using a single cylinder research engine. Injection sequences in which the main injection was split into two, three and four pulses were tested and the resulting emissions (NOx, CO HC and particulate matter), torque and cylinder pressures were compared to those obtained with single injections. Together with the number of injections, the effects of varying the dwell time were also investigated. It was found that dividing the main injection into two parts lowered the engine-out particulate and CO emissions and increased fuel efficiency. However, it also resulted in increased NOx emissions. Further, using double injections reduced the peak rate of heat release (RoHR) and increased RoHR in the later stages of the combustion without changing the combustion duration, resulting in a more even distribution of RoHR during the combustion, which is believed to be the main reason for the changes in fuel consumption and engine-out emission levels. When the number of injections was increased to three or four and the dwell time was prolonged the RoHR decreased, the combustion duration increased and the CA50 was retarded. Consequently, NOx emissions were reduced but the fuel efficiency also declined, and emissions of particulate mater, CO and HC rose.

Combustion characteristics of diesel sprays from equivalent nozzles with sharp and rounded inlet geometries

The ignition delay, flame structure, temperature, and soot distribution in a diesel spray injected at 80 MPa in a high-temperature (830 K) and high-pressure (6 MPa) quiescent air was studied for two nozzles, one with 0% hydrogrinding (HG) and another with 20% HG. HG in diesel nozzles is the process of forcing an abrasive fluid through the nozzles with sharp inlets; the abrasive fluid wears the sharp inlet edge of the spray holes until a prescribed flow rate is achieved. The percentage of HG used in this article is a measure of an increase in the volume flow rate after the HG process in a low-pressure flow test. The difference is substantial. For convenience, at some instances 0% HG is referred to as the sharp inlet and 20% HG as the rounded inlet. Spray impingement studies were made to evaluate the time-resolved spray momentum, nozzle discharge coefficient, and turbulence kinetic energy to characterize the nozzle internal flow effects on spray combustion. Equivalent nozzles were selected such that the momentum rates of the spray from both nozzles, as determined by the spray impingement, were the same. This was obtained by increasing the orifice diameter of the nozzle with 0% HG to compensate for the higher friction losses and lower discharge coefficient of the nozzle. The differences in discharge coefficient indicate that the flows inside the nozzles have different turbulence and cavitation levels. Inspite of the strong differences in internal flow, the sprays, which had the same momentum rate, behaved identically. In particular, the spray dispersion, penetration, ignition delay, combustion temperatures, flame volumes, soot concentration, and liftoff distances were almost the same for both sprays. Also, the use of noncircular injection orifices was shown not to change the combustion and emission performance of a diesel engine when the momentum of the fuel jets is the same. The work thus shows that diesel spray combustion is fully controlled by the spray momentum and that for realistic injection and combustion conditions the internal nozzle flow structure does not matter as long as it does not change the momentum.

Comparison of Working Fluids in Both Subcritical and Supercritical Rankine Cycles for Waste-Heat Recovery Systems in Heavy-Duty Vehicles

In a modern internal combustion engine, most of the fuel energy is dissipated as heat, mainly in the form of hot exhaust gas. A high temperature is required to allow conversion of the engine-out emissions in the catalytic system, but the temperature is usually still high downstream of the exhaust gas aftertreatment system. One way to recover some of this residual heat is to implement a Rankine cycle, which is connected to the exhaust system via a heat exchanger. The relatively low weight increase due to the additional components does not cause a significant fuel penalty, particularly for heavy-duty vehicles. The efficiency of a waste-heat recovery system such as a Rankine cycle depends on the efficiencies of the individual components and the choice of a suitable working fluid for the given boundary conditions. Commonly used pure working fluids have the drawback of an isothermal evaporation and condensation, which increases irreversibility, and consequently decreases the efficiency during the heat transfer. Previous work has suggested that one way to overcome this problem is to use zeotropic mixed working fluids. These have already been applied in several stationary systems and refrigerant cycles but not yet in waste-heat recovery systems for portable applications. This theoretical study compares different pure working fluids and zeotropic mixtures in both subcritical and supercritical Rankine cycles. The main objective was to analyze the respective energy and exergy efficiencies by modeling the Rankine cycles. The results suggested that the final fluid and cycle choice is limited by the exhaust-gas temperature range of a heavy-duty diesel engine and realistic condensation conditions for the fluid. Further, environmental and safety concerns over working fluids in portable applications are important challenges, which need to be taken into account in selecting an appropriate fluid. Copyright

Optical studies of spray development and combustion characterization of oxygenated and Fischer-Tropsch fuels

Optical studies of combusting diesel sprays were done on three different alternative liquid fuels and compared to Swedish environmental class 1 diesel fuel (MK1). The alternative fuels were Rapeseed Oil Methyl Ester (RME), Palm Oil Methyl Ester (PME) and Fischer-Tropsch (FT) fuel. The studies were carried out in the Chalmers High Pressure High Temperature spray rig under conditions similar to those prevailing in a direct-injected diesel engine prior to injection. High speed shadowgraphs were acquired to measure the penetration of the continuous liquid phase, droplets and ligaments, and vapor penetration. Flame temperatures and relative soot concentrations were measured by emission based, lineof- sight, optical methods. A comparison between previous engine tests and spray rig experiments was conducted in order to provide a deeper explanation of the combustion phenomena in the engine tests. Results pertaining to spray behavior show that high viscosity fuels have wider spray cone angles, smaller discharge coefficients (Cd) and shorter vapor penetration than low viscosity fuels. Continuous liquid phase penetration is related to differences in surface tension, viscosity and density; while the penetration of droplets and ligaments is related to volatility, their penetration is short for highly volatile fuels and long for low-volatility fuels. Engine tests show that particle matter (PM) emissions are generally lower when these alternative fuels are used, but the use of RME leads to increased NOx emissions correlating with elevated flame temperatures.

Multi-species laser-based imaging measurements in a diesel spray

Multi-species laser based imaging measurements have been carried out in a reacting Diesel spray in order to provide a detailed data base for model development and validation. In a high-pressure high-temperature spray chamber the measurements addressed the fuel vapor concentration, ignition and flame development and the soot formation. The fuel vapor distribution was measured quantitatively by Rayleigh scattering and compared to measurements by tracer laser-induced fluorescence. Soot volume fractions were observed by laser-induced incandescence. Fuel vapor and soot distributions were measured simultaneously and provide insight in the ignition and pollutant formation process. Specific digital image processing algorithms were developed to correct for beam steering and laser attenuation.

Selecting an Expansion Machine for Vehicle Waste-Heat Recovery Systems Based on the Rankine Cycle

An important objective in combustion engine research is to develop strategies for recovering waste heat and thereby increasing the efficiency of the propulsion system. Waste-heat recovery systems based on the Rankine cycle are the most efficient tools for recovering energy from the exhaust gas and the Exhaust Gas Recirculation (EGR) system. The properties of the working fluid and the expansion machine have significant effects on Rankine cycle efficiency. The expansion machine is particularly important because it is the interface at which recovered heat energy is ultimately converted into power. Parameters such as the pressure, temperature and mass-flow conditions in the cycle can be derived for a given waste-heat source and expressed as dimensionless numbers that can be used to determine whether displacement expanders or turbo expanders would be preferable under the circumstances considered. The goal of this theoretical study was to use this approach to analyze waste-heat recovery systems for a heavy-duty diesel engine and a light-duty gasoline engine. Given the different waste-heat rates of these two engines, the relationships between Rankine cycle performance and design aspects such as the expansion ratio and the locations of pinch points in the heat exchanger were evaluated. The calculated values of these parameters were used as inputs in a dimensionless analysis to identify an optimal expansion machine for each case. The impact of varying the working fluid used was investigated, since it had a large impact on the results obtained and provided insights into design dependencies in these systems.

Numerical and Experimental Analysis of the Wall Film Thickness for Diesel Fuel Sprays Impinging on a Temperature-Controlled Wall

Analysis of spray-wall interaction is a major issue in the study of the combustion process in DI diesel engines. Along with spray characteristics, the investigation of impinging sprays and of liquid wall film development is fundamental for predicting the mixture formation. Simulations of these phenomena for diesel sprays need to be validated and improved; nevertheless they can extend and complement experimental measurements. In this paper the wall film thickness for impinging sprays was investigated by evaluating the heat transfer across a temperature-controlled wall. In fact, heat transfer is significantly affected by the wall film thickness, and both experiments and simulations were carried out to correlate the wall temperature variations and film height. The numerical simulations were carried out using the STAR-CD and the KIVA-3V, rel. 2, codes. Different wall impingement models and liquid film approaches were analyzed and numerical results were compared with measurements available in the literature and with experiments carried out at Chalmers. The conditions for which the simulations were performed correspond to those found during the compression stroke in a diesel engine. The fuel used was a 2-component model fuel (Idea, 70% n-decane and 30% α methylnaphthalene). The experiments were carried out in the Chalmers high-pressure/high-temperature spray rig for non-evaporating and evaporating conditions. The spray chamber was equipped with a temperature-controlled wall, including coaxial thermocouples for recording the surface temperature. The time histories of the surface temperatures were used to calculate the local heat fluxes applying a 1-dimensional transient heat conduction model.

Dimensionless expansion model for bubbling fluidized beds with and without internal heat exchanger tubes

A bed expansion model is presented for bubbling fluidized beds without tubes and for beds with staggered horizontal heat exchanger tubes. There are indications that the model also works for inline tube configurations. The bed expansion ratio is modelled as a function of a dimensionless drag force and, in the case of tubes present in the bed, a dimensionless horizontal and vertical tube pitch function. The data used to derive the model cover a wide range of operating conditions, with varying fluidization velocities, pressures, particle materials (Geldart groups B and D), bed geometries and tube configurations. The dimensionless drag force also takes temperature effects into account, and the model shows good agreement with data from a freely bubbling large-scale atmospheric fluidized bed boiler operating at 850 degrees C.

A Numerical and Experimental Study of Diesel Fuel Sprays Impinging on a Temperature Controlled Wall

Spray-wall as well as spray-spray interactions in direct injection diesel engines have been found to influence both the rate the heat release and the formation of emissions. Simulations of these phenomena for diesel sprays need to be validated and an issue is investigating what kind of fuels can be used in both experiments and spray calculations. The objective of this work is to compare numerical simulations with experimental data of sprays impinging on a temperature controlled wall, regarding spray characteristics and heat transfer. The numerical simulations were carried out using the STAR-CD and KIVA 3V codes. The CFD simulations accounted for the actual spray chamber geometry and operating conditions used in the experiments. Particular attention has been devoted to the fuel used for the simulations. Firstly a single component model fuel (n heptane) has been adopted; subsequently a 2 component model fuel (Idea, 70% n-decane and 30 % α methylnaphthalene) has been implemented into the code fuel libraries in order to account for the fuel used in the experiments. Finally, different break-up and wall impingement models were analyzed. The experiments were performed in the high pressure, high temperature spray rig at Chalmers with conditions corresponding to those found during the compression stroke in a heavy duty diesel engine. The temperature controlled wall was equipped with three coaxial thermocouples for recording the surface temperature. The time histories of the surface temperatures were used to calculate the local heat fluxes applying a 1 dimensional transient heat conduction model. The spray characteristics were measured using two different optical methods; Phase Doppler Anemometry and high speed imaging. Image analysis gave the characteristics of the general behavior of the axial and radial penetration. PDA-data gave the characteristics of droplet penetration before and after impinging the wall.