U. Renz

The detailed flame structure of highly stretched turbulent premixed methane-air flames

Abstract The premixed stoichiometric turbulent methane flames are investigated on a piloted Bunsen burner with a nozzle diameter of 12 mm and mean nozzle exit velocities of 65, 50, and 30 m/s. Advanced laser diagnostics of the flow field using two-component and two-point laser Doppler anenometer (LDA), as well as of the scalar fields with 2-D Rayleigh thermometry and line Raman/Rayleigh laser-induced predissociation fluorescence (LIPF)-OH techniques, are applied to obtain both the instantaneous and mean flame structure in terms of velocity, temperature, and major species concentrations, as well as turbulent kinetic energy and length scales. In terms of their location on the combustion diagram, the three flames cover the entire range of the distributed-reaction-zones regime from the borderline to the well-stirred reactor regime to the flamelet regime. Measurements were from X/D = 2.5 above the nozzle exit plane to X/D = 12.5 downstream. Thus, a complete database is established for comparison with the numerical predictions. Within the mixing layer between the unburnt gas and the pilot flame, the instantaneous temperatures are much lower than the adiabatic flame temperature due to the short residence time and heat loss to the burner. With increasing residence time the mean flame temperature increases in the axial direction. The radial mixing of the turbulence generated with the shear layers between the nozzle jet stream and surrounding pilot stream is surpressed, such that the turbulence kinetic energy remains nearly constant on the centerline. From the two-dimensional (2D) temperature fields instantaneous iso-temperature contours are plotted showing broad regions where burnt and unburnt gas are partially mixed. These regions are interpreted in terms of the quench scale lq = (eτc3)1/2. The measured values of the flame brush thickness are proportional to the quench scale for the two high-velocity flames, whereas the low-velocity flame exhibits essential flamelet behavior.

Eulerian simulation of bubble formation at a jet in a two-dimensional fluidized bed

A promising method to describe the fluid dynamics of highly loaded particle flows is the Eulerian representation. In this approach, the solid phase is treated as a continuum, although physically it consists of individual particles. This is possible by using physical models derived from the kinetic theory of granular flow. Central to these models is the so-called granular temperature, representing the specific fluctuating kinetic energy of the particles. In this paper, several forms proposed for these models are summarized and different descriptions of granular temperature are investigated. Furthermore, an alternative approach derived from soil mechanics is tested. Measurements of a two-dimensional bubbling bed found in literature are finally used to verify the results.

Hydrodynamics of three-dimensional waves in laminar falling films

Experiments were performed to investigate the flow and surface structure in laminar wavy films over a Reynolds number range from Ref=ρūδf/η=27–200. Measurements of the velocity distribution by particle image velocimetry and film thickness by a fluorescence technique enabled to gain detailed information on the transient conditions within the three-dimensional wavy flow. In the entire range of Reynolds numbers, the flow in the wave crest is in a decelerated state, as its momentum is partially transferred into the near wall region, which results in acceleration of the wave back above the equilibrium state. This also affects the residual film behind the waves and causes subsequent waves to collide with their predecessors. The three-dimensional effects and the wave collision frequency increase with increasing flow rate. Transitions from streak-like to surge-like waves and the development of turbulent spots are first observed to occur at Ref≈75. The wave shapes at Ref≈200 become completely unsteady and approximately every second wave collision causes the formation of a turbulent spot.

Development of a fast fiber-optic two-color pyrometer for the temperature measurement of surfaces with varying emissivities

A two-color pyrometer has been developed to measure the temperature of surfaces with unknown emissivities during high speed turning processes. Quartz fibers enable measurements at locations with limited optical access. The sensitivity of the pyrometer has to be high enough to measure temperatures down to 300 °C of an aluminum alloy with an emissivity as low as 0.2. The accuracy of the two-color pyrometer has been compared with the accuracy of monochromatic pyrometers for different metallic surfaces. The different arguments for the choice of the two pyrometer wavelengths 1.7 and 2.0 μm are explained. The influences of the surface emissivities, the digitization, and the noise on the absolute and relative measurement error have been determined. Fast amplifiers and data acquisition allow a maximum time resolution of a few microseconds and a local resolution of ∼0.5 mm2. Some test measurements of an aluminum alloy surface are presented.

Eulerian computation of heat transfer in fluidized beds

Basically two different methods exist for the numerical simulation of particle laden multiphase flows. Whereas the Lagrangian approach for dilute flows handles each particle separately, the Eulerian formulation of the governing equations treats the particulate phase as a continuous phase. The Eulerian formulation can be realized using the kinetic theory of granular flows, which describes the physical properties of the particle phase. In the past, promising results of the fluid dynamics in a bubbling fluidized bed (Boemer, A. (1996) Euler/Euler-Simulation der Fluiddynamik blasenbildender Wirbelschichten, Dissertation, at RWTH Aachen, Germany) were achieved by estimating the solids pressure and the solid viscosity based on the kinetic theory. One parameter of this theory is the so-called granular temperature, which is a quantity of the macroscopic particle fluctuating energy. In addition to the fluid dynamics the numerical simulation of the heat transfer between a fluidized bed and immersed heat transfer surfaces is of particular interest. This work shows different physical models which are needed to give an Eulerian formulation of the particulate enthalpy equation. Besides the heat transfer between the phases, the effective thermal conductivity of the particle phase has to be specified. The combination of these models has been tested in a simple two-dimensional test case consisting of a heated horizontal tube immersed in a fluidized bed. To examine the influence of rising bubbles on the heat transfer, which usually leads to enhanced heat transfer coefficients as a consequence of the turbulent mixing of the particles, bubbles are generated periodically with the help of a pulsed jet.

Numerical prediction of heat transfer in fluidized beds by a kinetic theory of granular flows

Abstract In dense gas–solid two-phase flows of bubbling fluidized beds the particle-to-particle interactions cannot be neglected and an Eulerian approach has been used to predict the fluid dynamics as well as the heat transfer. The physical properties of the solid phase can be modeled with the kinetic theory of granular medias and the governing equations are solved numerically. The present work compares different physical models for the thermal transport coefficients of the solid phase for a lab-scaled two-dimensional fluidized bed filled with mono-disperse glass beads. The numerical results show a strong correlation between fluid dynamics and the instantaneous heat transfer similar to the so-called packet theory by Mickley and Fairbanks [1].

Numerical simulation of fuel sprays at high ambient pressure: the influence of real gas effects and gas solubility on droplet vaporisation

This paper deals with the numerical simulation of the vaporisation of an unsteady fuel spray at high ambient temperature and pressure solving the appropriate conservation equations. The extended droplet vaporisation model accounts for the effects of non-ideal droplet evaporation and gas solubility including the diffusion of heat and species within fuel droplets. To account for high-temperature and high-pressure conditions, the fuel properties and the phase boundary conditions are calculated by an equation of state and the liquid/vapour equilibrium is estimated from fugacities. Calculations for an unsteady diesel-like spray were performed for a gas temperature of 800 K and a pressure of 5 MPa and compared to experimental results for droplet velocities and diameter distribution. The spray model is based on an Eulerian/Lagrangian approach. The comparison shows that the differences between the various spray models are pronounced for single droplets. For droplet sprays the droplet diameter distribution is more influenced by secondary break-up and droplet coagulation.

Heat transfer and film thickness during condensation of steam flowing at high velocity in a vertical pipe

Abstract In the present study the heat transfer of pure steam flowing downward in a vertical condenser pipe is determined and the thickness of the wavy film is measured using a laser absorption method. The measured data are compared to numerical predictions based on the solution of the conservation equations for mass, momentum and energy for the vapor and liquid phases. The k-e turbulence model of Jones and Launder is used for both phases. It will be shown that the transition from laminar to turbulent film flow can be predicted reasonably well if the turbulent kinetic energy in the film is conserved to a minimum value.

Deposition of fine particles from a turbulent liquid flow: Experiments and numerical predictions

A theoretical model is developed to predict the deposition rate of fine particles from turbulent, non-isothermal liquid flow. The model accounts for the relevant transport mechanisms, describing particle adhesion by the interaction forces calculated from the DLVO-theory. Predictions reasonably agree with the experimental data obtained in a simple plate heat exchanger. The influence of chemical and thermal conditions on adhesion is adequately described by the equilibrium and reaction enthalpy data of the surface ionisation reactions. Transport is found to be dominated by hydrodynamic lift which is often referred to as a secondary effect. The hydrodynamic lift suppresses deposition of 1.2 μm-particles even under laminar flow conditions. The experimental results show that thermophoretic transport is important, but overpredicted by theories developed for stagnant fluids. Apparently particle rotation induced by flow shear diminishes temperature gradients within the particle and in the surrounding fluid and leads to a considerable decrease of the thermophoretic migration velocity.

Verification of Eulerian simulation of spontaneous bubble formation in a fluidized bed

A computer code to simulate the fluid dynamics of fluidized beds with the Eulerian approach is being developed. To verify the results, experimental investigations were carried out at a two-dimensional lab-scale fluidized bed. The local-time-dependent solid volume fractions were measured with a video system. Comparison of the simulation with these experiments and various empiricalmodels found in literature are presented.