Fluid Residence Time Research Articles

Investigation of the Pressure Drop of Random Packed Bed Adsorbers

Adsorption is one of the most often used unit operations in chemical engineering. However, due to computational limitations and time constraints simulations of adsorption processes are commonly done only with black box models or simplified porous media structures. In a new project an OpenFOAM® based CFD solver adsorpFoam, the required grid generation work flow using DEM methods for random packings and also post processing tools were created to carry out extended analyses of adsorption columns. Fully resolved packed beds were simulated to get more detailed information on gas side inhomogeneous distribution, bypass streams and near-wall effects (all these factors contribute to the total pressure drop of the packing), as well as fluid residence time distributions of random packings in columns.The current work will summarize the first results of adsorpFoam with respect to the validation using gas flow in laboratory scale random packings: The code and the procedure have been tested for spheres, mono- dispersed cylinders and cylindrical particles with size distribution. In all cases, a good agreement of the experimental results and the CFD simulations has been achieved.In a next stage of the project multi-component adsorption kinetics will be investigated, which are influenced by the local flow and temperature profile. Compared to common porous media CFD approaches a more realistic and reasonable residence time distribution can be achieved with this method.

Simultaneous 10 kHz TPIV, OH PLIF, and CH2O PLIF measurements of turbulent flame structure and dynamics

Simultaneous 10 kHz repetition-rate tomographic particle image velocimetry, hydroxyl planar laser-induced fluorescence (OH PLIF), and formaldehyde (CH $$_2$$ O) PLIF were used to study the structure and dynamics of turbulent premixed flames. The flames investigated span from the classically defined corrugated flamelet regime to conditions at which broadened and/or broken flamelets are expected. Methods are presented for determining 3D flame topologies from the Mie scattering tomography and for tracking features through space and time using theoretical Lagrangian particles. Substantial broadening of the CH $$_2$$ O region is observed with increasing turbulence intensity. However, OH production remains rapid, and the region of OH and CH $$_2$$ O overlap remains thin. Local flame speeds exceeding three times the laminar flame speed are observed in regions of flame–flame interaction. Furthermore, a method of tracking fluid residence time within the CH $$_2$$ O layer is presented and shows that residence time decreases at higher turbulence intensity despite the broader distribution of the CH $$_2$$ O, indicating an increase in local reaction rate.

Mathematical modeling of thrombus formation in idealized models of aortic dissection: initial findings and potential applications.

Aortic dissection is a major aortic catastrophe with a high morbidity and mortality risk caused by the formation of a tear in the aortic wall. The development of a second blood filled region defined as the “false lumen” causes highly disturbed flow patterns and creates local hemodynamic conditions likely to promote the formation of thrombus in the false lumen. Previous research has shown that patient prognosis is influenced by the level of thrombosis in the false lumen, with false lumen patency and partial thrombosis being associated with late complications and complete thrombosis of the false lumen having beneficial effects on patient outcomes. In this paper, a new hemodynamics-based model is proposed to predict the formation of thrombus in Type B dissection. Shear rates, fluid residence time, and platelet distribution are employed to evaluate the likelihood for thrombosis and to simulate the growth of thrombus and its effects on blood flow over time. The model is applied to different idealized aortic dissections to investigate the effect of geometric features on thrombus formation. Our results are in qualitative agreement with in-vivo observations, and show the potential applicability of such a modeling approach to predict the progression of aortic dissection in anatomically realistic geometries.

Groundwater flow, nutrient, and stable isotope dynamics in the parafluvial-hyporheic zone of the regulated Lower Colorado River (Texas, USA) over the course of a small flood

Periodic releases from an upstream dam cause rapid stage fluctuations in the Lower Colorado River near Austin, Texas, USA. These daily pulses modulate fluid exchange and residence times in the hyporheic zone where biogeochemical reactions are typically pronounced. The effects of a small flood pulse under low-flow conditions on surface-water/groundwater exchange and biogeochemical processes were studied by monitoring and sampling from two dense transects of wells perpendicular to the river. The first transect recorded water levels and the second transect was used for water sample collection at three depths. Samples were collected from 12 wells every 2 h over a 24-h period which had a 16-cm flood pulse. Analyses included nutrients, carbon, major ions, and stable isotopes of water. The relatively small flood pulse did not cause significant mixing in the parafluvial zone. Under these conditions, the river and groundwater were decoupled, showed potentially minimal mixing at the interface, and did not exhibit any discernible denitrification of river-borne nitrate. The chemical patterns observed in the parafluvial zone can be explained by evaporation of groundwater with little mixing with river water. Thus, large pulses may be necessary in order for substantial hyporheic mixing and exchange to occur. The large regulated river under a low-flow and small flood pulse regime functioned mainly as a gaining river with little hydrologic connectivity beyond a narrow hyporheic zone.

Studying the solids and fluid flow behavior in rotary drums based on a multiphase CFD model

The objective of this study is to analyze the solid and fluid flow behaviors inside a rotary drum using computational fluid dynamics (CFD). The CFD model developed was based on an Eulerian–Eulerian multiphase flow approach. The capability of the multiphase CFD model to predict the transverse and axial solid flow patterns, the fluid flow profile, and particle residence time was assessed. The model was used to study the effects of drum rotational speed, filling degree, feed rate, and drum inclination angle. It was verified using experimental results of particle and fluid velocities and residence time. The experiments were conducted on pilot scale rotary drums. As could be proved by measured and simulated results, the particle flow near the bed surface exerted a strong entrainment effect on the transverse air flow in the proximity of the bed surface. The study demonstrated the capability of the multiphase CFD model to predict the particle and fluid flow behaviors inside a rotary drum simultaneously.

Use of mean residence time and flowrate of thermal waters to evaluate the volume of reservoir water contributing to the natural discharge and the related geothermal reservoir volume. Application to Northern Thailand hot springs

Tritium-based mean residence time, τw, and flowrate, Qw, of thermal waters from eleven different sites of Northern Thailand were used to evaluate the volume of reservoir waters contributing to the natural discharge, Vw, and the volume of the related geothermal reservoirs, Vres, assuming steady-state conditions and using simple relationships.Measured 3H contents of the thermal waters of interest, apart from San Kamphaeng, range from 0.4 to 1.3 TU. Based on the well-mixed reservoir model, mean fluid residence times between 500 and 1700 years are obtained. For most thermal waters of Northern Thailand, calculated Vw range from 0.13 to 0.68km3, whereas Vres vary from 13 to 68km3, assuming an effective porosity of 0.01. The only exception is San Kamphaeng, that has a high discharge leading to a calculated Vw of 2.2km3 and Vres of 220km3, although its mean fluid residence time of 955 years is within the range of other Northern Thailand thermal waters.Even though the uncertainties on both Vw and Vres are relatively high, the simple approach used here may be used to evaluate geothermal resources during the exploration stage. The obtained Vres may be used in the volume method of geothermal resource assessment (which is usually utilized to evaluate the productive potential of unexploited and minimally exploited geothermal fields) and may be compared with the Vres estimated by means of other approaches.

Influence of freeze–thaw dynamics on internal geochemical evolution of low sulfide waste rock

Continuous monitoring of a 15m high heavily instrumented experimental waste rock pile (0.053wt.% S) since 2006 at the Diavik diamond mine in northern Canada provided a unique opportunity to study the evolution of fresh run-of-mine waste rock as it evolved over annual freeze–thaw cycles. Samples were collected from soil water solution samplers to measure pore water properties, from twelve 4 to 16m2 basal collection lysimeters to measure basal leachate properties in the region underlying the crest of the pile (the core), and from basal drains to measure aggregate total pile leachate properties. By 2012, monitoring of pore water geochemistry within the core structure of the test pile revealed an apparent steady state with respect to weathering geochemistry, represented by (i) a flush of pre-existing blasting residuals and applied tracers, (ii) declining pH, (iii) a stepwise progression and subsequent equilibrium with acid-neutralizing phases (depletion of available carbonates; equilibrium with respect to aluminum hydroxide phases and subsequent iron (III) hydroxide phases), and (iv) concordant release of SO4, major cations (Ca, Mg, K, Na, Si), and trace metals (Al, Fe, Ni, Co, Cu, Zn). Distinct, high concentration ‘spring flushes’, characteristic of drainage in northern environments and primarily explained by a combination of fluid residence time and the build-up of oxidation products over the winter, were released from core drainage each season. Following the initial flush, the concentration of all dissolved constituents steadily declined, with distinct minimums prior to freeze-up. The opposite trend was observed in the cumulative pile drainage, in which early season leachate dominated by snowmelt and batter flow had low concentrations and late season leachate dominated by contributions from the core of the pile (indicated by season end merging of core and cumulative drainage geochemistry) had higher concentrations. Northern waste rock pile drainage geochemistry is strongly influenced by freeze–thaw cycling and varying core and batter subsystem contributions to total drainage. A comprehensive understanding of thermal cycling in waste rock piles is an important component of temporal predictions of drainage water composition based on up-scaling or reactive transport modeling.

Artificial Neural Network (ANN) For Evaluating Permeability Decline in Permeable Reactive Barrier (PRB)

Artificial neural networks (ANNs) were developed which enable evaluation of long-term permeability losses that occur in permeable reactive barriers (PRBs) used in groundwater remediation. The network architectures consist of non-changing input and output layer(s) while the optimal hidden layer types and structures were determined through trial-and-error. Fluid residence time within the PRB, pressure drop, inlet volumetric flow rate, dynamic viscosity of fluid, average porosity, average particle size and the length of the reactor were selected as the input parameters to estimate the output parameter, namely, permeability. Of all experimental data available for each ANN structure, 70 % was used for training, 15 % for validation and the remaining 15 % for testing the ANN. The ANN structures were developed using a combination of soft computing techniques and mathematical association of varying physical parameters. Predictions obtained from the optimized ANN structures were compared with linear and non-linear regression models to assess their performance. The results indicate that ANN performs significantly better than the regression models and ANN modelling is a promising tool for the simulation and assessment of the permeability decline in PRBs.

Three‐dimensional versus two‐dimensional bed form‐induced hyporheic exchange

AbstractThe hyporheic zone is often a critical component of river systems. Hyporheic exchange is generally forced by variation in riverbed topography such as due to bed forms. Most previous research on bed form‐driven hyporheic flow has focused on two‐dimensional (2‐D) dunes and ripples, while little has been done on their three‐dimensional (3‐D) counterparts. Here we compared hyporheic exchange and associated metrics for a previously studied pair of corresponding 2‐D and 3‐D bed forms. To accomplish this, a series of multiphysics computational fluid dynamics models were conducted both in 2‐D and 3‐D with similar open channel Reynolds numbers (Re). Results show that the pressure gradient along the sediment‐water interface is highly sensitive to the spatial structure of bed forms, which consequently determines hyporheic flow dynamics. Hyporheic flux is a function of Re for both 2‐D and 3‐D dunes via a power law; however, the equivalent 3‐D dunes have a higher flux since the 3‐D form induces more drag. The hyporheic zone depths and volumes are only slightly different with the 3‐D case having a larger volume. The mean fluid residence times for both cases are related to Re by an inverse power law relationship, with the 3‐D dune having smaller residence times at moderate to high Re. The effects of increasing flux on residence time in 3‐D dunes are partly modulated by a slightly increasing hyporheic volume. Our results suggest that a 2‐D idealization is a reasonable approximation for the more complex 3‐D situation if local details are unimportant but that development of predictive models for mean fluxes and residence times, which are critical for biogeochemical processes, based on 2‐D models may be insufficient.

Groundwater age, brine migration, and large‐scale solute transport in the Alberta Basin, Canada

AbstractThere is a great contrast in geochemical and hydrogeologic estimates of the residence times of pore fluids in sedimentary basins. This contrast is particularly evident in the Alberta Basin, Canada, which has served as the study area for important studies of long‐term fluid flow and transport. To address these differences, we developed two‐dimensional simulations of groundwater age, constrained by both hydrogeologic and geochemical observations, to estimate the residence time of fluids and the amount and timing of flushing by meteoric waters in the Alberta Basin. Results suggest that old, residual brines have been retained in the deepest parts of the basin since their formation ca. 400 Ma, but significant dilution by younger waters has reduced the age of these pore waters to no more than approximately 200 My. Shallower formations have been flushed extensively by fresh, young waters. Loss of brines and dilution of older pore waters occurred primarily after the uplift of the Rockies with the introduction of the gravity‐driven flow regime. Despite these large changes in flow regime, solute exchange between deep saline aquifers and the overlying vigorous freshwater flow system was found to be consistent with long‐term dispersive mixing across subhorizontal concentration gradients rather than by direct flushing. Sensitivity studies using an analytic solution supported the use of 100 m for transverse dispersivity in large‐scale numerical models. These simulations confirm that the age and origin of brines are in many cases poor indicators of long‐term solute transport rates in sedimentary basins, but the geochemical indicators that are used to determine the origin of brines can provide useful constraints for calculating groundwater age and are far more commonly available than isotopic groundwater age tracers.

Wavelet decomposition method decoupled boiling/evaporation oscillation mechanisms over two to three timescales: A study for a microchannel with pin fin structure

Boiling/evaporation heat transfer in a microchannel with pin fin structure was performed with water as the working fluid. Simultaneous measurements of various parameters were performed. The chip wall temperatures were measured by a high spatial-time resolution IR image system, having a sensitivity of 0.02°C. The flow pattern variations synchronously changed wall temperatures due to ultra-small Bi number. The wavelet decomposition method successfully identified the noise signal and decoupled various temperature oscillations with different amplitudes and frequencies. Three types of temperature oscillations were identified according to heat flux q and mass flux G. The first type of oscillation occurred at q/G<0.62kJ/kg. The approximation coefficient of wavelet decomposition decided the dominant cycle period which was ∼3 times of the fluid residence time in the microchannel, behaving the density wave oscillation characteristic. The detail coefficients of wavelet decomposition decided the dominant cycle period, which matched the flow pattern transition determined value well, representing the flow pattern transition induced oscillation. For the second type of oscillation, the wavelet decomposition decoupled the three oscillation mechanisms. The pressure drop oscillation caused the temperature oscillation amplitudes of 5–10°C and cycle periods of 10–15s. The density wave oscillation and flow pattern transition induced oscillation are embedded with both the pressure rise and decrease stages of the pressure drop oscillation. The third type of oscillation happened at q/G>1.13kJ/kg, having the density wave oscillation coupled with the varied liquid film evaporation induced oscillation. The liquid island, retention bubble induced nucleation sites and cone-shape two-phase developing region are unique features of microchannel boiling with pin fin structure. This study illustrated that pressure drop oscillation and density wave oscillation, usually happened in large size channels, also take place in microchannels. The flow pattern transition and varied liquid film evaporation induced oscillations are specific to microchannel boiling/evaporation flow.

Modelling Three-dimensional Interfacial Flow with Sand Dunes

Hyporheic zone is considered as a dynamic hydrologic ecotone that critical for maintaining the health of river systems. The hyporheic flux occurs generally in response to variations in river bedforms. In this study, we first generated several typical bedforms. The turbulent flow over the 3D dunes and hyporheic flow in the sediment are simulated through a computational fluid dynamics (CFD) approach. Turbulent flow in the water column is simulated by solving the Reynolds-averaged Navier-Stokes (RANS) equations with the k-ω turbulence closure model, and a steady state groundwater flow model is applied for the underlying porous media. These two sets of equations are coupled through the pressure distribution at the sediment-water interface (SWI).Each case was subjected to different hydraulic conditions, i.e., increasing open channel Reynolds Numbers (Re). Results show that the pressure gradient along the SWI is highly controlled by the spatial structure of bedform, which consequently determines flow dynamics in the porous media. The interfacial flux is dominated by the pressure configuration over the SWI which is a function of Re via a power-law trend. The mean fluid residence time is related to Re by an inverse-power law relationship. This study has led to the basic understanding of hyporheic flow induced by more natural 3D dunes.

Investigation on a Soft Tubular Model Reactor Based on Bionics of Small Intestine – Residence Time Distribution

Abstract The human small intestine is responsible for virtually all nutrient uptake and more than 95% of the water absorption in digestion, which is attributed to the vast mucosal surface area and the peristalsis of small intestine. Under the broad conceptual framework of bio-inspired chemical process engineering, by mimicking the structure and functions of small intestine, a flexible tubular reactor with villous protrusions distributed evenly on the inner wall was designed and constructed in this study. In order to understand the flow behavior in the reactor, the residence time distribution (RTD) of fluid particles in the reactor was measured by introducing electrochemical active tracer. Also, a simple mechanism of peristalsis was introduced, and its effects on the RTD in the reactor were investigated. The experimental results showed that the tailing of RTD function curve in the small intestine model reactor was extended significantly compared to a normal tubular reactors. The residence time and mixing of fluid (particles) in the reactor can be regulated efficiently by controlling the peristaltic actions (frequency and location).

Chemical weathering processes in the Great Artesian Basin: Evidence from lithium and silicon isotopes

Variations in lithium and silicon isotope ratios in groundwaters of the Great Artesian Basin in Australia, and the causes of these variations, have been explored. The chemistries of Li and Si in groundwater are influenced by the dissolution of primary phases, the formation of secondary minerals, and the reaction of solid phases with dissolved constituents, while isotopic variations are generated by uptake into clays, which preferentially incorporate the light isotopes. The lithium isotopic composition (expressed as δLi7) of the groundwaters ranges from +9 to +16‰, and clearly reflects changes in aquifer conditions. Reaction-transport modelling indicates that changes in Li concentrations are principally controlled by the ratio of the weathering rate of primary minerals to the precipitation rate of secondary minerals, whereas δLi7 is affected by the extent of isotope fractionation during secondary mineral formation (which is dependent on mineralogy). The patterns of groundwater Si concentrations and δSi30 values versus flow distance suggest that Si is at steady state in the aquifer. The δSi30 value of most of the groundwater samples is close to −1‰, which is significantly lower than the δSi30 value of the reservoir rocks (∼0‰). Since precipitation of clays preferentially removes the light Si isotopes from solution, the most plausible explanation for these low groundwater δSi30 values is addition of Si by dissolution of isotopically light secondary minerals. These data, together with model calculations, show that Li isotopes are extremely sensitive to changes in the chemical and physical conditions in the aquifer, whereas Si is not. Importantly, the model suggests that even in large aquifers with long fluid residence times, where steady-state would be expected to be reached, the concentrations and isotopic fractionation of trace elements are not controlled by Li adsorption. The model developed here provides a basis for using Li isotopes measured in groundwaters and surface waters to constrain weathering processes.

Can deep boreholes solve America׳s nuclear waste problem?

The United States is in need of a new and more adaptive long-term strategy for spent nuclear fuel. In this communication, we outline the fundamental reasons why deep borehole disposal should receive more detailed investigation, alongside traditional shallow mined repositories. This potential solution is supported by advancing drilling technologies and an improving understanding of extremely long fluid residence times in deep bedrock. Radionuclide isolation is supported by verifiable and stable geologic barriers such as long transport distances to aquifers, low permeability, and reducing chemical conditions. The modular nature of implementing deep borehole disposal could offer unique programmatic and economic advantages. Experience with a pilot borehole program will be required to confirm the feasibility of drilling and emplacement operations, and key chemical and hydraulic conditions.

A Model Linking Stable Isotope Fractionation to Water Flux and Transit Times in Heterogeneous Porous Media

Hydrogeochemical processes governing the composition of groundwater are often inferred from water samples that are the flux-weighted average of a heterogeneous system. The stable isotope ratios of these samples are commonly employed to evaluate biogeochemical cycling or the intensity of weathering. However, these data are often interpreted using simplified relationships that assume the chemical composition and fluid residence times are homogeneous. This disparity results in part from the difficulty in obtaining appropriate observations needed to quantitatively link the effects of variable fluid residence time to stable isotope fractionation. Here, we present a synthetic dataset of steady state stable isotope ratios using the aqueous, irreversible reduction of hexavalent chromium (Cr(VI)) and precipitation of chromium hydroxide (Cr(OH)3(s)) in structurally correlated heterogeneous porous media using the CrunchTope reactive transport code. Our results demonstrate that flux-weighted average Cr(VI) values collected across a fixed control plane for multiple heterogeneous permeability fields consistently produce higher reactant concentrations and less enriched stable isotope ratios than a comparable homogeneous domain for average flow rates ranging across several orders of magnitude. This variability is thus directly attributable to the physical heterogeneity of the porous media. Furthermore, the isotopic signature of accumulated Cr(OH)3(s) is highly contingent upon the pH-dependent precipitation rate and the structure of the flow field, resulting in a wide range of bulk isotope ratios. These results provide a first step in the development of new frameworks for linking the effects of subsurface heterogeneity to the observed stable isotope ratios and fractionation factors in a wide variety of systems.

The use of computational fluid dynamics to estimate fluid residence time and flow hydrodynamics in open digesters of wastewater treatment plants: a case study

Residence time is an important parameter in mixing systems. Particularly, the performance of the processes in the digesters of wastewater treatment plants is influenced by the residence time distribution. This can be obtained experimentally by injecting inert chemicals to measure their concentration at the outlet. An alternative approach is to simulate the flow field with computational fluid dynamics (CFD) software. This paper explores a methodology to compute fluid residence time and investigate flow hydrodynamics in open digesters of full-scale wastewater treatment plants by CFD calculations through a case study. The methodology is based on the resolution of an additional transport equation for a separate species. The concentration of this species at the outlet of the digesters is monitored and can be related with the residence time of the fluid. Furthermore, the resolution of the unsteady flow field provides a complete set of data that was used to detect stagnation or bypass regions.

Dynamic flow modelling in precipitator vessels – A study of turbulence modelling approaches

Precipitation is a fundamental unit process within the Bayer circuit for the production of smelter-grade alumina, with the predominant technology used in this unit process being the mechanically-agitated draft-tube precipitator. Previous physical modelling studies have identified dynamic behaviour in these vessels which could potentially impact fluid residence time and the ability to obtain adequate solids suspension. In the current study the commercial CFD code ANSYS CFX14 is used to simulate the dynamic, single phase flow behaviour in a laboratory-scale replica of a full-scale precipitator. Simulations are conducted to investigate the impact of mesh refinement and turbulence model selection, with two-equation, Reynolds Stress and the Scale Adaptive Simulation (SST–SAS) models being investigated. The impact of the different modelling choices on the accuracy of the simulations is assessed through comparison of the CFD results with high-quality Laser Doppler Velocimetry (LDV) data obtained in a laboratory-scale vessel.

CO2-induced shift in microbial activity affects carbon trapping and water quality in anoxic bioreactors

Microbial activity is a potentially important yet poorly understood control on the fate and environmental impact of CO2 that leaks into aquifers from deep storage reservoirs. In this study we examine how variation in CO2 abundance affected competition between Fe(III) and SO42--reducers in anoxic bioreactors inoculated with a mixed-microbial community from a freshwater aquifer. We performed two sets of experiments: one with low CO2 partial pressure (∼0.02atm) in the headspace of the reactors and one with high CO2 partial pressure (∼1atm). A fluid residence time of 35days was maintained in the reactors by replacing one-fifth of the aqueous volume with fresh medium every seven days. The aqueous medium was composed of groundwater amended with small amounts of acetate (250μM), phosphate (1μM), and ammonium (50μM) to stimulate microbial activity. Synthetic goethite (1mmol) and SO42- (500μM influent concentration) were also available in each reactor to serve as electron acceptors. Results of this study show that higher CO2 abundance increased the ability of Fe(III) reducers to compete with SO42- reducers, leading to significant shifts in CO2 trapping and water quality. Mass-balance calculations and pyrosequencing results demonstrate that SO42- reducers were dominant in reactors with low CO2 content. They consumed 85% of the acetate after acetate consumption reached steady state while Fe(III) reducers consumed only 15% on average. In contrast, Fe(III) reducers were dominant during that same interval in reactors with high CO2 content, consuming at least 90% of the acetate while SO42- reducers consumed a negligible amount (<1%). The higher rate of Fe(III) reduction in the high-CO2 bioreactors enhanced CO2 solubility trapping relative to the low-CO2 bioreactors by increasing alkalinity generation (6X). Hence, the shift in microbial activity we observed was a positive feedback on CO2 trapping. More rapid Fe(III) reduction degraded water quality, however, by leading to high Fe(II) concentration.

Large eddy simulation based studies of jet–cavity interactions in a supersonic flow

Interactions of a cavity flameholder with an upstream injected jet in a Ma 2.52 supersonic flow are investigated numerically. A hybrid RANS/LES (Reynolds-Averaged Navier–Stokes/Large Eddy Simulation) method acting as wall-modeled LES is adopted, for which the recycling/rescaling method is introduced to treat the unsteady turbulent inflow. Patterns of the fluid entrainment into the cavity and escape from the cavity are identified using a scalar-tracing method. It is found that the jet–cavity interactions remarkably enhanced the mass exchange between the fluids in and out of the cavity, resulting in reduced residence time of the cavity fluids. Increasing the distance between the fuel injection and the cavity leading edge tends to attenuate the jet–cavity interactions, leading to weaker mass exchange. Raising the injection pressure appears to enhance the jet–cavity interactions, resulting in a shorter residence time of the cavity fluids. Moreover, the mass decay processes for the fuel and air within the cavity are basically the same while the entrainment processes for the fuel and air into the cavity seem quite different.