Browsing by Subject "Jets"

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Computations of strongly forced laminar cold-flow jet and methane-air diffusion flames
(2006) Barve, Vinayak Vidyadhar; Ezekoye, Ofodike A.
Previous work has shown that for sufficiently high periodic forcing amplitudes, laminar diffusion flames can burn in an effectively partially premixed mode. Experimental observations show that the luminosity and sooting properties of the forced flames are significantly modified by the presence of strong forcing. In this work, simulations are performed to study the effects of strong forcing on flow field development in strongly forced laminar isothermal jets and methane air diffusion flames. Unforced and strongly forced cold-flow jets are simulated using a higher order finite volume CFD code. The jet was forced by varying the jet exit velocity over a range of forcing amplitudes and frequencies and it was found that the jet Strouhal number (St) was the important parameter in characterizing flowfield development. Further, the forced jets showed increased entrainment and increased entrainment rates as compared to the non-forced jets. The computations are extended to laminar methane–air diffusion flames. The combustion reactions were modeled using detailed gas-phase chemistry and complex thermo-physical properties. The radiation heat transfer was modeled using the S-6 Discrete Ordinates Method. A 2 equation soot chemistry model for soot nucleation, surface growth and oxidation was used. First an unforced flickering methane–air diffusion flame was modeled and then the flame was forced by varying the amplitude and frequency of the fuel velocity in the nozzle. Cases where the peak velocity in the fuel stream reached 6 times the mean velocity are examined. The internal nozzle flow was also simulated since the near-nozzle region was of particular interest due to the strong mixing processes occurring there and the subsequent effect on the flame properties. Lifted forced flames were also examined, and it was found that the partial premixing in the near nozzle region and modified gas phase chemistry in the forced flames can explain the reduction in soot production for the strongly forced flames.
The effects of indoor jets on air distribution and human exposure to particles
(2014-12) Liu, Shichao, active 21st century; Novoselac, Atila
Indoor jets considerably dominate air movement and distribution of temperature and velocity, as well as transport of particles and other pollutants. Indoor air temperature and velocity distribution substantially impact occupants’ thermal comfort and productivity, heat and mass transfer on indoor surfaces. In addition, jets produced by human respiratory activities, such as coughing and sneezing, enhance the spread of particles that might carry bacteria or viruses. Understanding and characterizing indoor jets and their impacts on air distribution, temperature and velocity fields, and particle transport are crucial for advancing heating, ventilation, and air conditioning (HVAC) systems when considering thermal comfort and developing strategies for exposure mitigation. This dissertation contributes to the scientific understanding regarding to indoor air distribution and particle transport associated with indoor air jets. Current HVAC system design defines indoor air distribution related to the selection of diffusers/ grilles that distribute supply air jets, according to the specifics of the space and internal heating and cooling loads. However, current design guidance was developed over 40 years ago. It requires expansion of diffuser/ grille types and the update for air distribution by diffuser/ grille air jets supplying warm air at heating mode. Unlike jets from diffusers/ grilles, jets created by human activities are inherently transient in nature and might perform quite differently from steady-state ones. Understanding the dynamics of unsteady-state jets, such as coughs, enhances the current state of understanding of the mechanisms of respiratory disease transmission, which enables development of exposure reduction measures. The investigations presented in this dissertation extend the state-of-the-art knowledge on indoor jets and analyze the effect of steady-state and unsteady-state jets on particle transport in indoor environments. Figure 1 illustrates the two objectives and six investigations conducted in this dissertation. The first objective includes four investigations that address air distribution and particle transport associated with steady-state jets created by diffusers/ grilles, and the remaining two investigations relate to the second objective on unsteady-state cough jets. The first objective of this dissertation characterizes air distribution and particle transport in a space with steady-state jets created by diffusers/ grilles. One of the major contributions of this objective to the-state-of-the-art knowledge on indoor air distribution is the newly developed method for diffuser performance assessment and design when considering heating mode. It advances the current diffuser/ grille selection guide that was outdated decades ago. Furthermore, based on 650 experimental set-ups this objective provides a systematic analysis of indoor air velocity that can be further used in indoor heat transfer and pollutant emission and transport. The second objective investigates velocity fields in unsteady-state cough jets and transport of coughed particles. This objective provides a theoretical analysis of the dynamics of cough jets and examines how human thermal plume affects the exposure to coughed particles when considering different particle sizes. Ultimately, these investigations fill the knowledge gaps in indoor air distribution and particle transport associated with steady-state and unsteady-state jets in spaces using all-air HVAC systems. The newly developed diffuser guideline will improve HVAC design for both heating and cooling conditions when considering thermal discomfort or air stagnant zones caused by a wrong diffuser selection. In addition, the systematic analysis of indoor air velocity will improve the prediction of indoor heat transfer, mass transfer, particle resuspension rate, pollutant emission rate from the floor and other indoor surfaces. Finally, the theoretical analysis of unsteady-state jets contributes the knowledge for fluid dynamics of unobstructed human coughs and also transport of coughed particles, including the distribution in the vicinity of an exposed person.
Jet characterization in Au + Au collisions at STAR
(2013-05) Dávila Leyva, Alán; Markert, Christina
The present study combines modern jet reconstruction algorithms and particle identification (PID) techniques in order to study the enhancement of proton/pion ratio at mid transverse momentum ([mathematical symbols] 1.5 - 4.0 GeV/c) observed in central Au + Au collisions at [mathematical symbols] = 200 GeV. The ratio enhancement is thought to be caused by recombination processes and/or parton fragmentation modification of jets in relativistic heavy ion collisions. The fragmentation modification hypothesis is tested in this analysis by reconstructing and selecting energetic jets presumably biased to fragment outside of the medium created in Au + Au collisions and comparing their particle composition to the recoiling (medium-traversing) jets. The bias assumption is confirmed by comparing jets in central collisions, where the effect of proton/pion enhancement is present, with peripheral ones where no medium effects are expected. The selected jets are reconstructed by using the anti-k[subscript T] algorithm from the modern FASTJET package. The PID in the p[subscript T] region of interest is possible by combining measurements of the particles' energy deposition and velocity from the Time Projection Chamber and the recently installed (2009-2010) Time of Flight detectors at STAR. The acceptance of these detectors, [eta] < 1.0 and full azimuth, make them extraordinary tools for correlation studies. These features allow for the measurement of relative azimuth ([phi] [subscript jet] - [phi] [subscript pion,proton]) distributions by using the selected jet axis in order to disentangle the uncorrelated background present in the high multiplicity heavy ion collisions. The proton/pion ratios in two different centrality bins and p[subscript T] = 1.2 - 3.0 GeV/c are presented for biased (vacuum fragmenting) jets and their recoiling counterparts
Stability analysis of a single three dimensional rock block: effect of dilatancy and high-velocity water jet impact
(2009-08) Asadollahi, Pooyan; Tonon, Fulvio
In simulation of closely- or separately-joined rock masses, stability of rock blocks is of primary concern. However, there seems to be no approach that can handle general modes of simultaneous sliding and truly large rotation under general forces, including non-conservative forces such as water forces. General causes of failure for rock blocks, such as limit points, bifurcation points, and dynamic instability (divergence and flutter), have never been addressed. This research implements a formulation, called BS3D(an incremental-iterative algorithm introduced by Tonon), for analyzing general failure modes of rock blocks under conservative and non-conservative forces. Among the constitutive models for rock fractures developed over the years, Barton's empirical model has been widely used because it is easy to apply and includes several important factors associated with fracture characteristics. Although Barton's failure criterion predicts peak shear strength of rock fractures with acceptable precision, it has some weaknesses in estimating the peak shear displacement, post-peak shear strength, dilation, and surface degradation in unloading and reloading. In this dissertation, modifications are made to Barton's original model in order to address these weaknesses. The modified Barton’s model is validated by a series of direct shear tests on rock fractures and implemented in BS3D to consider the dilatant behavior of fractures. The mechanical behavior of a rock block formed in the roof of a tunnel is governed by its geometry, the mechanical characteristics and the deformability of the fractures forming the block, the deformability of the block and that of the surrounding rock mass, and the stresses within the rock. BS3D, after verification and validation, is used to investigate the effect of dilatancy on stability of rock blocks formed in the roof of a circular tunnel. High-velocity plunging jets, issuing from hydraulic artificial or natural structures, can result in scouring of the rock riverbed or the dam toe foundation. Assessment of the extent of scour is necessary to ensure the safety of the dam and to guarantee the stability of its abutments. BS3D is used to investigate effect of high-velocity jet impact on stability of rock blocks in plunge pools.