11/05/2024
By Danielle Fretwell

The Francis College of Engineering, Department of Mechanical Engineering, invites you to attend a Doctoral Dissertation defense by Aman Kumar on: Advancing Lagrangian-Eulerian Spray Model for Flashing Sprays: Effects of Near Nozzle Conditions & Parcel Initialization.

Candidate Name: Aman Kumar
Degree: Doctoral
Defense Date: Tuesday, Nov. 19, 2024
Time: 10:00 a.m. - Noon
Location: Perry 415

Committee:
Advisor: Noah Van Dam, Ph. D., Assistant Professor, Mechanical and Industrial Engineering, UMass Lowell

Committee Members*
J. Hunter Mack, Ph. D., Associate Professor, Mechanical and Industrial Engineering, UMass Lowell
Juan Pablo Trelles, Ph.D., Professor Mechanical and Industrial Engineering, UMass Lowell
David Schmidt, Ph.D., Professor, Mechanical and Industrial Engineering, UMass Amherst

Brief Abstract:
Parcel-based simulations are the preferred method to simulate fuel sprays, especially for reacting conditions, due to their lower computational cost compared to more highly resolved simulations. The three major areas are investigated where first flashing sprays are studied but also examine non-flashing sprays as a reference point. Then the sensitivity of Homogeneous Relaxation Model (HRM) was investigated to understand the accuracy of Volume of Fluid (VOF) simulations that were applied for Ammonia sprays which have stronger flash boiling tendencies than traditional hydrocarbon fuels. Traditionally, when parcel simulations are used, the injector tip geometry is either significantly simplified or removed altogether due to resolution limits. But the near nozzle region experiences rapid changes in ambient conditions due to the flashing liquid. So, it is crucial to understand the influence of spray boundary conditions on parcel-based simulations for improved predictions of fuel spray behavior in engine applications. Recent advances in computational power and numerical methods, however, have made it possible to resolve flow through these features.

First, the impact of detailed injector tip geometry in parcel simulations for non-flash boiling and flash conditions were investigated. Four key parameters varied individually from a baseline spray simulation model were injector tip geometry, injection rate shape, parcel initialization location and turbulence model. Macroscopic analysis of results such as penetration length exhibited limited sensitivity to these boundary conditions variations. However, local data such as the liquid volume fraction near the injector, displayed significant differences, which could affect mixing and combustion predictions in engine simulations. Similar trends were observed for the flash boiling condition, however the liquid parcels in flashing condition do not decrease after end of injection which indicates that parcels have reached local thermodynamic equilibrium. These findings emphasize the need to reassess simulation assumptions and refine guidelines for accurately predicting spray characteristics in advanced engine modeling.

Second, a high fidelity eulerian framework VOF simulation using HRM was performed to calculate vapor generated during phase change process. It was observed that different simulation time step giving different vapor generated, which is contrary to relaxation time constant impacting vapor rate. This time step model sensitivity was studied.

Finally, liquid ammonia sprays were simulated using VOF and parcel-based approach with increasing pressure ratios to better understand liquid ammonia spray behaviors, especially under extreme flash boiling conditions. The geometry for a single-hole research injector (Spray M) from the Engine Combustion Network (ECN) was used based on preliminary experimental data available for this injector. Results show experimental trends are captured. Liquid ammonia sprays under engine-like conditions also undergo extreme flash boiling, generating large amounts of vapor within the injector counterbore and creating under expanded jet-like conditions in the near nozzle region. Thus, a parcel and gas initialized together simulation was performed based on rate shape injection and compared with one way coupled parcel simulations. Such details of the conditions at the injector tip are explored to understand implications for the setting up of future
parcel-based simulations for full engine simulations.