Baoyu Guo

Simulation of Turbulent Swirl Flow in an Axisymmetric Sudden Expansion

The numerical simulation of the turbulent flows in axisymmetric sudden expansions with inlet swirl is addressed, with the focus being on the unsteady flow behavior. Numerical simulations using the Reynolds-averaged Navier-Stokes equations closed via the k-e turbulence model are presented. Results for an expansion ratio of 1.96 and a Reynolds number of 1 x 10 5 (at the inlet) are presented for a range of swirl numbers from 0 to 0.48. The precessing vortex core (PVC) and most of the features observed experimentally, including the precession direction, zero-frequency swirl number, and vortex breakdown have been predicted

Numerical Simulation of Unsteady Turbulent Flow in Axisymmetric Sudden Expansions

We are concerned with the numerical simulation of unsteady turbulent flows behind sudden expansions without inlet swirl. Time dependent simulations have been carried out using the VLES approach with the standard κ-e model. The expansion ratio investigated is in the range from 1.96-6.0. The flows in axisymmetric sudden expansions are inherently unstable when the expansion ratio is above a critical value. The precessing phenomenon, which features self-sustained precession of the global flowfield around the expansion centerline, is predicted successfully using CFD, with simulated oscillation frequencies that are in general agreement with reported data. For the case of expansion ratios from 3.5-6.0, a combination of a precession motion and a flapping motion in a rotating frame of reference is predicted in terms of the jet movement. Large-scale structures are identified in the downstream flowfield. Other important phenomena, such as the transition of the oscillation patterns, have also been predicted

Challenges of Simulating Droplet Coalescence within a Spray

Abstract This article reports various challenges that have been encountered in the process of developing validated Lagrangian and Eulerian models for simulating particle agglomeration within a spray dryer. These have included the challenges of accurately measuring droplet coalescence rates within a spray, and modeling properly the gas–droplet and droplet-droplet turbulence interactions. We have demonstrated the relative versatility and ease of implementation of the Lagrangian model compared with the Eulerian model, and the accuracy of both models for predicting turbulent dispersion of droplets and the turbulent flow-field within a simple jet system. The Lagrangian and Eulerian droplet coalescence predictions are consistent with each other, which implies that the numerical aspects of each simulation are handled properly, suggesting that either approach can be used with confidence for future spray modeling. However, it is clear that considerable research must be done in the area of particle turbulence modeling and accurate measurement of particle agglomeration rates before any Computational Fluid Dynamics tool can be employed to accurately predict particle agglomeration within a spray dryer.

CFD simulation of precession in sudden pipe expansion flows with low inlet swirl

Abstract In this paper, three-dimensional, time-dependent calculations are carried out using the finite volume CFD code CFX4 and the VLES approach with standard k – e model to simulate the turbulent swirl flow in an axisymmetric sudden expansion with an expansion ratio of 5.0 for a Reynolds number of 10 5 . This flow is unstable over the entire swirl number range considered between 0 and 0.48, and a large-scale coherent structure is found to precess about the centerline. Compared with the unswirled case, inclusion of a slight inlet swirl (swirl number below 0.23) can reduce the precession speed, cause the precession to be against the mean swirl and suppress the flapping motion. Several modes of precession are predicted as the swirl intensity increases, in which the precession, as well as the spiral structure, reverses direction. Accompanying the transition between different modes, abrupt changes in precession frequency are also experienced. Grid sensitivity and comparison with smaller expansion ratio data are also discussed.

Simulation of the agglomeration in a spray using Lagrangian particle tracking

Abstract This work aims to explore the possibility of simulating the agglomeration process in a spray using CFD methods. The model system consists of a spray nozzle within a uniform airflow in a square-section chamber. The CFD simulations are performed using a mixed Eulerian–Lagrangian approach. The flow is modelled by solving the usual Eulerian equations, and then representative droplets are tracked using the Lagrangian approach, with conventional gas–particle coupling. A number of representative particles are introduced at each time step, with each particle representing a group of real particles with the same properties, and are tracked in a transient flow. Due to turbulence, particles are dispersed and may coalesce when they are close. The inter-particle distance is used to calculate the collision probability from kinetic theory, and agglomeration is assumed to occur when the proximity function exceeds a critical value. This method is applied to the simulation of a round spray jet flow, and the results show some interesting insights regarding the role of particle size redistribution and agglomeration. The Sauter mean diameter is found to be the appropriate variable to quantify the agglomeration rate.

Simulation of Gas Flow Instability in a Spray Dryer

In this paper, turbulent flow behind a sudden pipe expansion followed by a contraction is simulated numerically, in order to investigate the effect of the downstream contraction on the flow instability in non-swirling flows, as well as in weakly swirling flows. Calculations are carried out using a commercial CFD code (CFX4.4), in which the transient Reynolds averaged N–S equation approach and the standard k–ɛ model are implemented. The first case investigated is the effect of a sudden contraction with varying length. The diameter ratio for both the expansion and the contraction is 5. The length of the large pipe (normalized by its diameter D) is varied in the range from 1.0 to 4.0, and the results are compared with those for the case without a contraction. The current results show that a sudden contraction tends to stabilize the non-swirling flow when the large pipe length is reduced. This stabilizing effect does not apply to swirling flow, although the precessing direction and frequency are affected significantly. The second case simulated is the gas flow in a simple short-form spray dryer configuration, for which some measured data are available in the literature. A self-sustained quasi-flapping oscillation is predicted, which behaves in a similar way to the phenomenon observed in experimental measurements.

An assessment of turbulence models applied to the simulation of a two-dimensional submerged jet

Abstract This paper is concerned with the investigation of the performance of different turbulence models in the numerical prediction of transient flow caused by a confined submerged jet. Several widely used models, i.e., the standard k – e , RNG k – e , low Reynolds number k – e models and the differential Reynolds stress model, as included in CFD codes, were compared with each other for a two-dimensional, incompressible, turbulent jet flow and with reported experimental data. A flapping oscillation was predicted regardless of the model used. A chosen Strouhal (St) number definition brought the fundamental frequencies from both the experiments and computations into close proximity. However, different turbulence models have exhibited quite different behaviours in terms of the frequency and regularity of the oscillation and in terms of the scale and duration of the vortices generated. All versions of the k – e model yielded regular oscillations, which agree with experimental observations. On the other hand, the Reynolds stress (RS) model produced a complex pattern but a slower dissipation of vortices. In addition, some aspects of gridding and inflow representation are also discussed.

VALIDATION OF THE LAGRANGIAN APPROACH FOR PREDICTING TURBULENT DISPERSION AND EVAPORATION OF DROPLETS WITHIN A SPRAY

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.