David Rothamer

Turbulent mixing measurements in the Richtmyer-Meshkov instability

The Richtmyer-Meshkov instability is experimentally investigated in a vertical shock tube using a new type of broadband initial condition imposed on an interface between a helium-acetone mixture and argon (A = 0.7). The initial condition is created by first setting up a gravitationally stable stagnation plane between the gases and then injecting the same two gases horizontally at the interface to create a shear layer. The perturbations along the shear layer create a statistically repeatable broadband initial condition. The interface is accelerated by a M = 1.6 planar shock wave, and the development of the ensuing turbulent mixing layer is investigated using planar laser induced fluorescence. By the latest experimental time, 2.1 ms after shock acceleration, the layer is shown to be fully turbulent, surpassing both turbulent transition criteria based on the Reynolds number and shear layer scale. Mixing structures are nearly isotropic by the latest time, as seen by the probability density function of gradien...

Temperature and pressure imaging using infrared planar laser-induced fluorescence

A new diagnostic technique for measurements of temperature and pressure distribution in gaseous flows has been developed. The technique, based on infrared planar laser-induced fluorescence (IR-PLIF), is applicable to all IR-active species. A simple two-line excitation approach is used for measurements of temperature, while pressure measurements utilize online excitation on one rotational line and offline excitation on another. A demonstration of the technique in a supersonic underexpanded jet of 30% CO2 and 70% N2 was performed, and the results are, to the best of our knowledge, the first demonstration of temperature and pressure imaging using IR-PLIF. The developed diagnostic shows potential for single-shot two-dimensional measurements of temperature and pressure in gaseous flows.

Effect of Particle Size Distribution on the Deep-Bed Capture Efficiency of an Exhaust Particulate Filter

The exhaust filtration analysis system (EFA) developed at the University of Wisconsin – Madison was used to perform micro-scale filtration experiments on cordierite filter samples using particulate matter (PM) generated by a spark-ignition direct injection (SIDI) engine fueled with gasoline. A scanning mobility particle sizer (SMPS) was used to characterize running conditions with four distinct particle size distributions (PSDs). The distributions selected differed in the relative number of accumulation versus nucleation mode particles. The SMPS and an engine exhaust particle sizer (EEPS) were used to simultaneously measure the PSD downstream of the EFA and the real-time particulate emissions from the SIDI engine to determine the evolution of filtration efficiency during filter loading. Cordierite filter samples with properties representative of diesel particulate filters (DPFs) were loaded with PM from the different engine operating conditions. The results were compared to understand the impact of particle size distribution on filtration performance as well as the role of accumulation mode particles on the diffusion capture of PM. The most penetrating particle size (MPPS) was observed to decrease as a result of particle deposition within the filter substrate. In the absence of a soot cake, the penetration of particles smaller than 70 nm was seen to gradually increase with time, potentially due to increased velocities in the filter as flow area reduces during filter loading, or due to decreasing wall area for capture of particles by diffusion. Particle re-entrainment was not observed for any of the operating conditions.Copyright

Evolution of deep-bed filtration of engine exhaust particulates with trapped mass

Size-resolved particle mass and number concentrations were obtained from different operating conditions using a spark-ignition direct-injection engine and a heavy-duty diesel engine. Particle mass versus mobility diameter results obtained for the engines showed weak dependence on the operating condition. The particle mass–mobility data enabled the use of an integrated particle size distribution method to estimate the particulate matter mass concentration in the exhaust stream. Average mass concentrations determined with the integrated particle size distribution method were 77 − 32 + 47 % of the gravimetric measurements performed using Teflon filters. Despite the relatively low elemental carbon fraction (∼0.4 to 0.7), the integrated particle size distribution mass for stoichiometric spark-ignition direct-injection exhaust was 83% ± 38 % of the gravimetric measurement. Exhaust from the spark-ignition direct-injection engine was also used to perform wall-scale filtration experiments on identical cordierite filter samples with properties representative of diesel particulate filters. The filters were sequentially loaded with particulate matter from four spark-ignition direct-injection engine operating conditions, in order of increasing particulate matter mass concentration. Simultaneous particle size distribution measurements upstream and downstream of the filter sample were used to evaluate filter performance evolution and the instantaneous trapped mass within the filter for two different filter face velocities. The filtration experiments focused on the filter wall loading stage where the estimated trapped mass was < 0.3 g/m2. The evolution of filtration performance at a fixed filtration velocity was found to only be sensitive to the trapped mass, despite using particulate matter from different operating conditions. Higher filtration velocity resulted in a more rapid shift of the most penetrating particle size toward smaller mobility diameters.

Chemistry and combustion of fit-for-purpose biofuels

From the inception of internal combustion engines, biologically derived fuels (biofuels) have played a role. Nicolaus Otto ran a predecessor to todays spark-ignition engine with an ethanol fuel blend in 1860. At the 1900 Paris worlds fair, Rudolf Diesel ran his engine on peanut oil. Over 100 years of petroleum production has led to consistency and reliability of engines that demand standardized fuels. New biofuels can displace petroleum-based fuels and produce positive impacts on the environment, the economy, and the use of local energy sources. This review discusses the combustion, performance and other requirements of biofuels that will impact their near-term and long-term ability to replace petroleum fuels in transportation applications.

Comparisons of particle size distribution from conventional and advanced compression ignition combustion strategies

Particulate size distribution measurements are of importance in engine research as stricter regulations on particulate matter emissions (both mass and number based) are being implemented. Particulate size distribution measurements can be very sensitive to the laboratory environment or experimental setup, making it difficult to compare results for different combustion strategies acquired in different labs. In this study, a comparison of particulate size distribution measurements over a wide variety of conventional and advanced combustion strategies was conducted using a four-stroke single-cylinder diesel engine test setup to eliminate lab-to-lab variations and enable direct comparison of particulate size distribution results for different combustion strategies. Eight combustion strategies are included in the comparison: conventional diesel combustion, diesel/gasoline reactivity controlled compression ignition, homogeneous charge compression ignition, two types of gasoline compression ignition (early injection and late injection), diesel low temperature combustion, natural gas combustion with diesel pilot injection, and diesel/natural gas reactivity controlled compression ignition. Measurements were performed at four different load-speed points with matched combustion phasing when possible; for several strategies, it was necessary to operate with slightly different combustion phasing. Particle size distributions were measured using a scanning mobility particle sizer. To study the influence of volatile particles, measurements were performed with and without a volatile particle remover (thermodenuder) at low and high dilution ratios. The results show that non-uniformity in the fuel distribution caused by direct injection results in increased accumulation-mode particle concentrations compared to premixed strategies even for low particulate mass advanced combustion strategies. Premixed combustion strategies (homogeneous charge compression ignition) and early injection gasoline compression ignition show higher nuclei-mode particle concentrations. Overall particle number and mass concentrations vary significantly between engine operating conditions and between combustion strategies.

Dynamic Heterogeneous Multiscale Filtration Model: Probing Micro- and Macroscopic Filtration Characteristics of Gasoline Particulate Filters

Motivated by high filtration efficiency (mass- and number-based) and low pressure drop requirements for gasoline particulate filters (GPFs), a previously developed heterogeneous multiscale filtration (HMF) model is extended to simulate dynamic filtration characteristics of GPFs. This dynamic HMF model is based on a probability density function (PDF) description of the pore size distribution and classical filtration theory. The microstructure of the porous substrate in a GPF is resolved and included in the model. Fundamental particulate filtration experiments were conducted using an exhaust filtration analysis (EFA) system for model validation. The particulate in the filtration experiments was sampled from a spark-ignition direct-injection (SIDI) gasoline engine. With the dynamic HMF model, evolution of the microscopic characteristics of the substrate (pore size distribution, porosity, permeability, and deposited particulate inside the porous substrate) during filtration can be probed. Also, predicted macroscopic filtration characteristics including particle number concentration and normalized pressure drop show good agreement with the experimental data. The resulting dynamic HMF model can be used to study the dynamic particulate filtration process in GPFs with distinct microstructures, serving as a powerful tool for GPF design and optimization.

Experimental Study of Turbulent Mixing in the Richtmyer-Meshkov Instability

An interface that separates two fluids of different densities is unstable to acceleration by a shock wave. In this interaction, called the Richtmyer-Meshkov instability (RMI) [1, 2], vorticity is baroclinically deposited on interfacial perturbations. The vorticity drives the growth of perturbation amplitudes and can lead to turbulentmixing between the two fluids. This instability is relevant to inertial confinement fusion, where mixing between different layers of the capsule can degrade performance by cooling the central hot-spot or diluting the fuel with ablator material.

Evolving one-dimensional transient jet modeling by integrating jet breakup physics:

High-speed optical measurements of unsteady liquid fuel jets under engine-like conditions have shown that the initial penetration of the jets does not follow the behavior predicted by previously introduced one-dimensional jet models based on gas-jet principles. The experimental data indicate that the transient jet penetration velocity is initially controlled by the jet exit velocity, transitioning to gas-jet like mixing-dominated penetration further downstream. This behavior is consistent with the common description of high-pressure fuel jets as containing a liquid core surrounded by entrained gas and fuel droplets. In this paper, a new one-dimensional modeling methodology is introduced that couples the transport equations for the evolution of the liquid core of the jet and the surrounding sheath of droplets resulting from breakup. This allows for the penetration of the jet to be initially governed by the liquid core, which is relatively unaffected by the ambient gas, transitioning to spray penetration dominated by the entrained ambient gas. The model also provides a defined jet centerline velocity, which allows for the shape of the radial profiles of fuel velocity and fuel volume fraction to be solved for directly, without the need for a steady-jet assumption, as was used in previous one-dimensional models. This change removes the need for a constant momentum flux assumption, improving the transient nature of the model. The results of the model are validated against the aforementioned optical transient jet measurements. The model and all associated experimental data have been made available for use at rothamer.erc.wisc.edu/dlp .

Experimental Shock-Initiated Combustion of a Spherical Density Inhomogeneity

A planar shock wave that impulsively accelerates a spherical density inhomogeneity baroclinically deposits vorticity and enhances the mixing between the two fluids resulting in a complex, turbulent flow field. This is known as the classical shockbubble interaction (SBI) and has been a topic of study for several decades [1,2,3,4, 5,6,7,8,9,10,11,12], and closely related the Richtmyer-Meshkov instability (RMI) [13, 14]. While the classical SBI problem concerns a reactively neutral bubble, the present experimental study is the first of its kind in which a spherical bubble filled with a stoichiometric mixture of H2 and O2 diluted with Xe is accelerated by a planar shock wave (1.35 < M < 2.85) in ambient N2, and will be referred to as reactive shock-bubble interaction (RSBI).Experimental results for an inert spherical density inhomogeneity accelerated by a strong incident shock wave (M = 2.8) are compared with a reactive mixture of similar density. When a heavy bubble is shock accelerated in a lighter ambient gas corresponding to a large Atwood number (A > 0), the shock wave at the exterior periphery of the bubble travels faster than the interior transmitted wave, resulting in shock-focusing at the downstream pole of the bubble. The shock wave convergence results in localized temperatures and pressures an order of magnitude higher than the conditions behind the shock wave. If the bubble is composed of a reactive mixture, these localized conditions allow for a controlled, point-source ignition for the combustible mixture within the bubble. The chemical and hydrodynamic coupling is investigated. The reactive mixture is composed of a stoichiometric mixture of H2 and O2 diluted with Xe (30%, 15% and 55% by molar fraction, respectively), corresponding to A = 0.5. For the purpose of comparison, experiments are performed on an inert mixture, where the Atwood number is matched using a combination of Xe and He (58% and 42% by molar fraction, respectively). The experiments are performed at the Wisconsin Shock Tube Laboratory in a 9 m vertical shock tube with a 25.4 £ 25.4 cm 2 cross-section. A pneumatic injector is used to generate a 5 cm diameter soap bubble fllled with the gas mixture. The injector retracts ∞ushly into the side of the tube releasing the bubble into a state of free fall (Ranjan 2005, 2007). Diagnostics are performed using chemiluminescence of the OH i molecule present during the combustion process and planar Mie scattering with a frequency doubled Nd:Yag. Due to an inherently weak signal, the chemiluminescence is captured with an intensifled CCD camera, while the initial conditions are captured with a front-lit, high speed camera.