Experiments and global linear stability analysis are used to obtain the critical flow rate below which the highly stretched capillary jet generated when a Newtonian liquid issues from a vertically oriented tube, is no longer steady. The theoretical description, based on the one-dimensional mass and momentum equations retaining the exact expression of the interfacial curvature, accurately predicts the onset of jet self-excited oscillations experimentally observed for wide ranges of liquid viscosity and nozzle diameter. Our analysis, which extends the work by Sauter & Buggisch (2005), reveals the essential stabilizing role played by the axial curvature of the jet, being the latter effect especially relevant for injectors with a large enough diameter. Our findings allow us to conclude that, surprisingly, the size of the steady threads produced at a given distance from the exit can be reduced by increasing the nozzle diameter.
Combining the lumped capacitance method and the simplified distributed activation energy model to describe the pyrolysis of thermally small biomass particles, Energy Conversion and Management 175 (2018) 164-172. The original publication is available at www.elsevier.com Abstract The pyrolysis process of thermally small biomass particles was modeled combining the Lumped Capacitance Method (LCM) to describe the transient heat transfer and the Distributed Activation Energy Model (DAEM) to account for the chemical kinetics. The inverse exponential temperature increase predicted by the LCM was considered in the mathematical derivation of the DAEM, resulting in an Arrhenius equation valid to describe the evolution of the pyrolysis process under inverse exponential temperature profiles. The Arrhenius equation on which the simple LCM-DAEM model proposed is based was c Heating parameter [min -1 ].c s Specific heat of the solid particle [J kg -1 K -1 ]. d Particle diameter [mm]. E Activation energy [kJ mol -1 ]. E 0 Mean value of gaussian distribution of activation energy [kJ mol -1 ]. E a Value of activation energy for which the step function changes [kJ mol -1 ].
We report an experimental and theoretical study of the global stability and nonlinear dynamics of vertical jets of viscous liquid confined in the axial direction due to their impact on a bath of the same liquid. Previous works demonstrated that in the absence of axial confinement the steady liquid thread becomes unstable due to an axisymmetric global mode for values of the flow rate, Q, below a certain critical value, Q c , giving rise to oscillations of increasing amplitude that finally lead to a dripping regime (Sauter & Buggisch 2005;Rubio-Rubio et al. 2013). Here we focus on the effect of the jet length, L, on the transitions that take place for decreasing values of Q. The linear stability analysis shows good agreement with our experiments, revealing that Q c increases monotonically with L, reaching the semi-infinite jet asymptote for sufficiently large values of L. Moreover, as L decreases a quasi-static limit is reached, whereby Q c → 0 and the neutral conditions are given by a critical length determined by hydrostatics. Our experiments have also revealed the existence of a new regime intermediate between steady jetting and dripping, in which the thread reaches a limit-cycle state without breakup. We thus show that there exist three possible states depending on the values of the control parameters, namely steady jetting, oscillatory jetting and dripping. For two different combinations of liquid viscosity, ν, and injector radius, R, the boundaries separating these regimes have been determined in the Q-L parameter plane, showing that steady jetting exists for small enough values of L or large enough values of Q, dripping prevails for small enough values of Q or sufficiently large values of L, and oscillatory jetting takes place in an intermediate region whose size increases with ν and decreases with R.
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