Mass Flow Pulsations Research Articles

Crank angle-resolved mass flow characterization of engine exhaust pulsations using a Pitot tube and thin-wire thermocouples

Characterizing pulsating flow in high-temperature, high-pressure engine exhaust gas is crucial for the development and optimization of exhaust energy recovery systems. However, the experimental investigation of engine exhaust pulses is challenging due to the difficulties in conducting crank angle-resolved measurements under these unsteady flow conditions. This study contributes to characterizing mass flow pulses from an isolated cylinder exhaust of a heavy-duty diesel engine using a single-pipe measurement system, developed for pulsating flow measurement. A Pitot tube-based approach is adopted to measure exhaust mass flow pulsations, complemented by fast temperature measurements obtained using customized unsheathed thin-wire thermocouples. The on-engine experiment is performed by isolating the in-cylinder trapped mass and the valve opening speed to produce different exhaust pulse waveforms. The adopted approach’s sensitivity in resolving instantaneous mass flows is evaluated analytically and experimentally, considering attenuated temperature measurement effects. Based on exhaust flow measurements, mass flow pulses are analyzed with regard to blow-down and scavenge phases. Under the load sweep, the main waveform change occurs during the blow-down phase, with pulse magnitude increasing with the load. In contrast, as the engine speeds up with a comparable trapped mass, the exhaust mass distribution in the blow-down phase decreases from 75.5% at 700 rpm to 41.9% at 1900 rpm. Additionally, it is observed that cycle-to-cycle variations in mass flow pulses align with combustion stability during the blow-down phase and are predominantly influenced by gas-exchange processes during the scavenge phase.

Investigation on the pressure fluctuation of hydrogen Roots pump with a novel reflow structure for fuel cell vehicles

The hydrogen pump is an important component in the hydrogen circulation system of fuel cells. Excessive pulsation at the outlet of the pump is challenging when the Roots hydrogen pump is used, which has a negative impact on the operation performance. To reduce the impact of excessive fluctuations in the hydrogen circulation system, a new structure of reflow grooves is proposed in this paper, which connects the different chambers and adjusts the pressure using a diversion method. The internal flow of the two pumps was studied for comparison, using a three-dimensional simulation model established by Computational Fluid Dynamics (CFD) method. Results showed that the reflow grooves reduce the exhaust pressure and mass flow pulsations, which has a positive effect on the pressure stabilization of the hydrogen circulation system and the anode inlet pressure of the fuel cell stack, contributing to the long durability and stability of the fuel cell stack.

Parametric Study of Residence Time Distributions and Granulation Kinetics as a Basis for Process Modeling of Twin-Screw Wet Granulation.

Twin-screw wet granulation is a crucial unit operation in shifting from pharmaceutical batch to continuous processes, but granulation kinetics as well as residence times are yet poorly understood. Experimental findings are highly dependent on screw configuration as well as formulation, and thus have limited universal validity. In this study, an experimental design with a repetitive screw setup was conducted to measure the effect of specific feed load (SFL), liquid-to-solid ratio (L/S), and inclusion of a distributive feed screw on particle size distribution (PSD) and shape as well as residence time distribution of a hydrophilic lactose/microcrystalline cellulose based formulation. An intermediate sampling point was obtained by changing inlet ports along the screw axis. Camera-based particle size analysis (QICPIC) indicated no significant change of PSD between the first and second kneading section, except for low L/S and low SFL where fines increase. Mean residence time was approximated as a bilinear fit of L/S and SFL. Moreover, large mass flow pulsations were observed by continuous camera measurements of residence time distribution and correlated to hold-up of the twin-screw granulator. These findings indicate fast granulation kinetics and process instabilities for high mean residence times, questioning current standards of two kneading compartments for wet granulation. The present study further underlines the necessity of developing a multiscale simulation approach including particle dynamics in the future.

Numerical study of disturbances generated by localized impulse action in a flat-plate boundary layer at Mach 2

The work is devoted to the numerical simulation of the excitation and development of localized disturbances in the supersonic boundary layer of a plate with a sharp leading edge at a Mach number of 2 in a two-dimensional formulation. Artificial disturbances are generated with the help of localized in time and space by changing the boundary conditions to the velocity component normal to the surface. The studies for two intensities of the impulse action were carried out. It was found that with an increase in the intensity of the action on the boundary layer, the amplitude of the generated disturbances increases. The development of disturbances of various amplitudes is similar. The ratio of the level of total temperature pulsations to the level of mass flow pulsations was estimated for the investigated disturbances. It was found that at small distances from the source of disturbances, this ratio increases downstream, at the same time in the far field, it reaches a constant value.

Экспериментальное исследование воздействия волны сжатия на течение в сверхзвуковом пограничном слое при числе Маха 2,5

Experimental study of the effect of a weak shock wave from the protuberance of two-dimensional roughness installed on the side wall of the test section of the wind tunnel on the supersonic boundary layer of the blunted flat plate at the Mach number 2.5 was carried out. The measurements were performed by a constant temperature hot-wire anemometer in the region of stream wise vortices generated by the shock wave from the protuberance during interaction with the flow in the vicinity of the leading edge of the model. The spectral and statistical analyses of the measured disturbances in the boundary layer were carried out. The amplitude-frequency spectra of mass flow pulsations and statistical diagrams of the measured disturbances in the supersonic part of the boundary layer were obtained.

Numerical simulation on the development of periodic three-dimensional disturbances in a supersonic boundary layer

The results of a numerical study of the development of periodic pulsations in a supersonic boundary layer on a flat plate are presented at a Mach number of 2.5 and a unit Reynolds number of 8×106 m–1. Using the software complex ANSYS, the complete Navier-Stokes equations were solved. Periodic mass flow disturbances with a frequency of 20 kHz were introduced into the boundary layer through a small-diameter hole on the surface of the model. Downstream the profiles of the longitudinal mass flow pulsations were recorded, and spectral analysis of the data was carried out. The main characteristics of the development of unstable disturbances in both physical and wave spaces are obtained.

Experimental study of shock-wave structure of supersonic underexpanded microjets

The structure of supersonic underexpanded room temperature air microjets is experimentaly studied. Microjets escaping from the nozzles with characteristic heights of 22.3 and 83.3 μm and widths of 2410 and 3900 μm respactacly with the Reynolds numbers of 640 and 2390. At these Reynolds numbers hot-wire anemometer measurements of mass flow pulsations are performed. It is shown that low value of the mass flow pulsations are corresponding with the high supersonic core length.

Experimental study of mean and pulsation flow characteristics in the 2D/3D supersonic boundary layer behind flat roughness elements

For the first time, detailed experiments aimed at evaluation of the influence of flat microroughness elements (adhesive tape stickers attached to the streamlined surface) on the mean and pulsation flow characteristics in flat-plate and swept-wing boundary layers were carried out under comparable dimensional conditions. At Mach number M = 2, the sticker shape was found to have almost no influence on the amplitude of mass-flow pulsations in the wake of sticker. The relative receptivity of 3D boundary layer to stationary disturbances was obtained to be approximately 1.5 times higher than the receptivity of 2D boundary layer. It was found that the presence of a sticker at the line of the source of controlled disturbance changes the pulsation spectra in 2D and 3D boundary layers in comparison with unperturbed flow. In the wake behind the center of a sticker, the flow can be most unstable.

Simulations of Plain-Orifice Injection of Gelled Propellants Under Manifold Crossflow Conditions

Unsteady Navier–Stokes simulations are used to study the static and dynamic discharge characteristics of shear-thinning fluids passing through plain-orifice atomizers fed by a crossflow. The crossflow velocity is varied to assess implications of feeding fluid in a multi-element injector fed from a common manifold. Large and distinct horseshoe vortices are periodically generated near the inlet lip and convected along the length of the orifice passage. As these and other vortical structures pass through the orifice exit plane, they create small and distinct mass-flow pulsations. These pulsations are characterized for a variety of orifice length-to-diameter ratios, Reynolds numbers, manifold crossflow velocities, and fluid rheologies. Using candidate gels as testing fluids, it is typically observed under the unsteady flow conditions that discharge coefficient ranges from 0.5 to 0.6, Strouhal numbers are around 0.3, and oscillation magnitudes are less than 5% of averaged discharge coefficient.

Numerical Simulations of Liquid Rocket Coaxial Injector Hydrodynamics

A series of unsteadyReynolds-averaged Navier–Stokes computations aremade for a range of conditions pertinent to coaxial rocket injectors used on large liquid rocket engines with the intent of investigating unsteadiness resulting from hydrodynamic instabilities within the confines of the injector and in the near field of the injection point in the chamber. Axisymmetric simulations are conducted using a real fluid treatment for liquid oxygen and gaseous hydrogen propellants. An in-depth look inside the injector recess region was performed in an effort to characterize the dynamic character of mass flow pulsations created from hydrodynamic instabilities. Wake instabilities downstream of the liquid oxygen post tip, the acoustic pulsations within the liquid oxygen tube, and the vortex shedding from the injector lip all contribute to natural pulsations, even in the event of purely steady upstream and downstream conditions. The effects of varying the oxidizer/fuel ratio, the liquid oxygen post thickness, the liquid oxygen post length, and the liquid oxygen post acoustic frequency on injector frequency response were investigated.

Warming automotive catalysts with pulsating flows

Temperatures of an automotive catalyst substrate warmed by convection pre-light-off have been measured. Direct comparison has been made of warm-up by steady and pulsating flow for a one-dimensional flow case. The 32 Hz pulsating mass flow did not feature flow reversal. Pulsations were achieved by interruption of the airflow by a rotating disc. Very small differences between steady and pulsating cases were observed because the effect of mass flow pulsations on heat transfer is minimal. Two different computational fluid dynamics methods were used to predict temperature. A one-dimensional porous medium model, which required input of an assumed heat transfer coefficient, was compared with a single-channel model. Predictions agreed closely and there was also qualitative agreement with measurements. Similar mass flow pulsations in the range 32-100 Hz have been studied for a case with a larger diameter automotive catalyst supplied via a conical diffuser. The radial flow distribution is controlled by pulsation frequency and the effect of frequency on temperature at different depths in the substrate was observed experimentally. Pulsations will affect catalyst warm-up in practical systems because of their effect on flow distribution, rather than on heat transfer.