Showing papers on "Total pressure published in 2020"

Effects of Variable Total Pressures on Instability and Extinction of Rotating Detonation Combustion

Abstract: Instability, extinction and re-initiation of rotating detonation in a two-dimensional Rotating Detonation Engine (RDE) configuration are numerically investigated, and the emphasis is laid on the effects of variable total pressures on the above combustion dynamics. Two conditions, i.e. different constant pressures and time-varying pressures, are considered to investigate the combustion instability in RDE. It is seen that under constant pressure condition the rotating detonation is more prone to instability at low pressure, due to the instability from the deflagrative surface. The intrinsic frequency for the unstable cases with different pressures is close, and maybe related to the RDE configuration and/or fuel properties. For the time-varying pressures with various specified frequencies, the RDE shows the different levels of instability characterized by multiple frequencies. The dominant frequencies vary, depending on the competition between the forced external frequency and the intrinsic one induced by the RDE system. About the extinction of continuously rotating detonation, it can be found that when the total pressure is reduced to a relatively lower value the detonation is quenched and left with deflagrative combustion. Furthermore, when the total pressure is increased based on the extinguished flow fields, detonation fronts are re-initiated due to the local high pressure. The stochasticity in RDE re-initiation has been found, in terms of the location and number of initial detonation front, the propagation direction, their interaction and final stabilization.

Pressure balance in the multiphase ISM of cosmologically simulated disc galaxies

Abstract: Author(s): Gurvich, AB; Faucher-Giguere, CA; Richings, AJ; Hopkins, PF; Grudic, MY; Hafen, Z; Wellons, S; Stern, J; Quataert, E; Chan, TK; Orr, ME; Keres, D; Wetzel, A; Hayward, CC; Loebman, SR; Murray, N | Abstract: Pressure balance plays a central role in models of the interstellar medium (ISM), but whether and how pressure balance is realized in a realistic multiphase ISM is not yet well understood. We address this question by using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disc galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. We analyse how gravity determines the vertical pressure profile as well as how the total ISM pressure is partitioned between different phases and components (thermal, dispersion/turbulence, and bulk flows). We show that, on average and consistent with previous more idealized simulations, the total ISM pressure balances the weight of the overlying gas. Deviations from vertical pressure balance increase with increasing galactocentric radius and with decreasing averaging scale. The different phases are in rough total pressure equilibrium with one another, but with large deviations from thermal pressure equilibrium owing to kinetic support in the cold and warm phases, which dominate the total pressure near the mid-plane. Bulk flows (e.g. inflows and fountains) are important at a few disc scale heights, while thermal pressure from hot gas dominates at larger heights. Overall, the total mid-plane pressure is well-predicted by the weight of the disc gas and we show that it also scales linearly with the star formation rate surface density (ςSFR). These results support the notion that the Kennicutt-Schmidt relation arises because ςSFR and the gas surface density (ςg) are connected via the ISM mid-plane pressure.

On the interactions between a propagating shock wave and evaporating water droplets

Abstract: One-dimensional numerical simulations based on hybrid Eulerian-Lagrangian approach are performed to investigate the interactions between propagating shock waves and dispersed evaporating water droplets in two-phase gas-droplet flows. Two-way coupling for interphase exchanges of mass, momentum and energy is adopted. Parametric study on shock attenuation, droplet evaporation, motion and heating is conducted, through considering various initial droplet diameters (5-20 {\mu}m), number densities (2.5 x 1011 - 2 x 1012 1/m3) and incident shock Mach numbers (1.17-1.9). It is found that the leading shock may be attenuated to sonic wave and even subsonic wave when droplet volume fraction is large and/or incident shock Mach number is low. Attenuation in both strength and propagation speed of the leading shock is mainly caused by momentum transfer to the droplets that interact at the shock front. Total pressure recovery is observed in the evaporation region, whereas pressure loss results from shock compression, droplet drag and pressure gradient force behind the shock front. Recompression of the region between the leading shock and two-phase contact surface is observed when the following compression wave is supersonic. After a critical point, this region gets stable in width and interphase exchanges in mass, momentum, and energy. However, the recompression phenomenon is sensitive to droplet volume fraction and may vanish with high droplet loading. For an incident shock Mach number of 1.6, recompression only occurs when the initial droplet volume fraction is below 3.28 x 10-5.

Theoretical and Numerical Investigation on Total Pressure Gain in Rotating Detonation Engine

Abstract: The theoretical analysis and numerical simulations were presented in this study to explore the mechanism of total pressure gain in rotating detonation engines (RDEs). One-dimensional and two-dimens...

Influence of upstream turbulence on the pressure drop inside a monolith

Abstract: This paper reports the pressure drop through a monolith for turbulent and laminar flow. A computational model of a monolith channel together with the open sections before and after it is used. Simulations at several channel Reynolds numbers, and assuming laminar and turbulent flow approaching the substrate are considered. Reynolds average Navier Stokes (RANS) and large eddy simulation (LES) are used as flow models in the cases with turbulence. The resulting pressure drop, power spectrum and flow regime are analysed. RANS predicts a decay of the turbulence as the flow approaches the substrate, a small fraction of the turbulence effectively entering the channels to decay rapidly, then steady flow from that point. On the other hand, LES predicts a total dissipation of the turbulence before the flow enters the channels; however, the flow remains unsteady along the entire substrate, in a sort of pulsating regime. Despite the significant differences in the flow regime, both models predict a marginal influence of the upstream turbulence on the total pressure drop. The dominating frequencies of the pulsating flow inside the channels were found to be comparable to the ratio of the channel velocity over the channel diameter, therefore, to the channel Reynolds number.

Vapor Pressure Assessment of Sulfolane-Based Eutectic Solvents: Experimental, PC-SAFT, and Molecular Dynamics.

Abstract: Since their discovery, deep eutectic solvents (DES) have been explored in multiple applications. However, the complete physicochemical characterization is still nonexistent for many of the proposed and used DES. In particular, vapor pressure, which is a crucial property for the application of DES as solvents, is very rarely available. In this work, the measurement of the total and partial pressures of two sulfolane-based DES, tetrabutylammonium bromide:sulfolane and tetrabutylphosphonium bromide:sulfolane, in several proportions, from 40 to 100 °C and atmospheric pressure, was performed using headspace gas chromatography mass spectrometry, HS-GC-MS. A large decrease on the total pressure was recorded which, together with the finding that total pressures showed negative deviations from Raoult's law, is indicative of the favorable, strong interactions between the two components within the DES. Additionally, the study of vapor pressure change with DES molar composition was carried out, and surprisingly, the existence of inflection points in the pressure curve was observed. Experimental results were modeled using the PC-SAFT equation of state, and in addition, MD simulations were performed to provide a molecular understanding of the pressure data. Considering the different results and insights obtained from the used strategies, it can be concluded that both DES systems have especially strong interactions between salt and sulfolane, at high sulfolane content, due to the different structural rearrangement of the liquid state.

On the interactions between a propagating shock wave and evaporating water droplets

Abstract: One-dimensional numerical simulations based on hybrid Eulerian-Lagrangian approach are performed to investigate the interactions between propagating shock waves and dispersed evaporating water droplets in two-phase gas-droplet flows. Two-way coupling for interphase exchanges of mass, momentum and energy is adopted. Parametric study on shock attenuation, droplet evaporation, motion and heating is conducted, through considering various initial droplet diameters (5-20 {\mu}m), number densities (2.5 x 1011 - 2 x 1012 1/m3) and incident shock Mach numbers (1.17-1.9). It is found that the leading shock may be attenuated to sonic wave and even subsonic wave when droplet volume fraction is large and/or incident shock Mach number is low. Attenuation in both strength and propagation speed of the leading shock is mainly caused by momentum transfer to the droplets that interact at the shock front. Total pressure recovery is observed in the evaporation region, whereas pressure loss results from shock compression, droplet drag and pressure gradient force behind the shock front. Recompression of the region between the leading shock and two-phase contact surface is observed when the following compression wave is supersonic. After a critical point, this region gets stable in width and interphase exchanges in mass, momentum, and energy. However, the recompression phenomenon is sensitive to droplet volume fraction and may vanish with high droplet loading. For an incident shock Mach number of 1.6, recompression only occurs when the initial droplet volume fraction is below 3.28 x 10-5.

Improvement of drainage structure and numerical investigation of droplets trajectories and separation efficiency for supersonic separators

Abstract: As a new type of separation device, supersonic separator has received more and more attention in many areas. However, the interaction between boundary layer and shock wave in supersonic section results in the reverse pressure gradient and makes the boundary layer thicken, which form a so-called "second throat" that reduces the actual flow area and causes the fluid to be chocked. A novel cylinder drainage structure with larger drainage area was developed. When shock wave occurs, the high pressure will make more fluid enter into the openings on cylinder drainage structure, which is equivalent to increase the flow capacity. Therefore, the shock wave is weakened and the total pressure loss is reduced. Moreover, the prediction model of discrete phase coupling verified by experiments was adopted in order to predict the trajectory of droplets and separation efficiency for supersonic separator. The results show that with the increase of droplet size, the separation efficiency increases gradually and the particle diameter of 2 - 4 µm is sensitive to the separation. The droplets trajectory is more clear and regular in the nozzle with reflux channel and cylindrical drainage structure.

Experimental investigation in the pressure drop characteristics of supercritical carbon dioxide in the uniformly heated horizontal miniature tubes

Abstract: This study investigates experimentally the pressure drop characteristics of supercritical carbon dioxide uniformly heated in horizontal circular smooth tubes. The results illustrate that channel pressure drop would be enlarged with increases in mass flux and inlet fluid temperature, while it would decrease with increases in outlet pressure and tube diameter. Frictional pressure drop is the most dominant contributor (51%–85%) for total pressure drop, while accelerational pressure drop, up to 28%, and form loss, up to 24%, would contribute substantially with the increase of tube diameter due to significant reduction of fluid density at the channel outlet. Several classic correlations for turbulent flow and non-isothermal correlations for supercritical fluids are examined in smooth tubes. In overall, the existing empirical correlations could reasonably predict the frictional pressure drop within a relative error of about 30% against the present data set under uniform heating conditions.

Flow pressure evaluation on generic surfaces by robotic volumetric PTV

Abstract: An experimental approach for the measurement of the time-average fluid flow pressure over the surface of generic three-dimensional objects is presented. The method is based on robotic volumetric PTV measurements followed by the integration of the pressure gradient. The domain for pressure evaluation is subdivided in two parts: in the irrotational region the static pressure is obtained following Bernoulli relation; in the turbulent wake and close to the object the pressure gradient is integrated. An approach based on the total pressure distribution is proposed to estimate the boundary between these two regions. The method is first assessed with experiments around a sphere equipped with pressure taps. A criterion for minimum spatial resolution is formulated in terms of maximum ratio between bin size and local radius of curvature of the object. An experimental database from a three-dimensional problem of higher geometrical complexity is considered: the time-averaged flow field around a full-scale cyclist. The surface pressure distribution is discussed in connection to the topological features of near-surface streamlines and streamwise vortices.

Experimental analysis and modeling of the losses in the tip leakage flow of an isolated, non-rotating blade setup

Abstract: Improving pressure rise capabilities of axial compressors requires an in-depth understanding of the losses produced in the tip leakage region. Here, a generic setup that magnifies the tip region of an isolated, non-rotating blade is used with the objectives of describing the main flow components and evaluating the related sources of loss. The flow at the tip is structured by the jet flow out of the gap which, under the effect of the main stream, rolls-up into a tip-leakage vortex. The current setup is characterized by the tip gap height and the thickness of the incoming boundary layer at the casing, here a flat plate, for a given incidence of the blade. Measurements are performed using LDV and a multi-port pressure probe. Variations in the tip-leakage flow are found to be mainly driven by gap height. A small, intermediate and large gap regimes are more specifically found, with threshold around 4% and 8% of gap to chord ratio for the present setting. The incoming boundary layer thickness is shown to provoke a notable effect on the vortex lateral position and total pressure losses. The local entropy creation rate is computed from LDV data and used to identify the sources of loss in the flow. A decomposition into wake and vortex losses is further proposed, allowing to relate the contributions of the various flow components to the overall losses. An empirical model of the formation of the tip vortex is developed to account for the increased losses as a function of gap height. The model provides a useful mean for the practical approximation of the gap sensitivity of pressure losses.

Modelling and Analysis of a Magnetorheological Damper with Nonmagnetized Passages in Piston and Minor Losses

Abstract: This work aims to establish the mathematical model with the high effectiveness in predicting the damping force of an MR damper with nonmagnetized passages in piston. The pressure drops due to viscous loss, MR effect, and the minor losses at the inlet and outlet of passages are considered in the mathematical model. The widely reported Bingham model is adopted to describe the mechanical property of MR fluid. The mechanical behaviours of the MR damper are experimentally evaluated under different excitations and current. The yield stress of MR fluid with respect to the current applied to piston coil is obtained by finite element analysis in Ansoft Maxwell 14.0. The proposed model is validated by comparing the simulated damping characteristics with the measured data under various currents applied to the piston coil. The simulated results are also compared with those obtained from the mathematical model without the pressure drop due to the minor losses at the inlet and outlet of passages. The comparisons show that the proposed mathematical model can yield more accurate predictions of damping force. This indicates that the pressure drop due to the minor losses is significant and nonnegligible. The nonlinearity of force-velocity characteristics is discussed. In order to quantitatively explain the necessity of taking the minor losses into account for modelling the MR damper, the proportion of pressure drop due to the minor losses to the total pressure drop is investigated and discussed. Pressure drops due to the minor losses and viscous loss are also investigated and discussed. At last, the proposed mathematical model is used to analyse the working principle of nonmagnetized passages.