Control of the Separation Flow in a Sudden Enlargement

Experiments on heat transfer to transpired turbulent pipe flows

This study explored the three-dimensional flow characteristics in a recirculation zone near a permeable buttress in curved channels with varying curvatures. Understanding these characteristics is crucial for managing natural river bends, as rivers often meander, with backwater zones formed behind obstructions, such as mountains in the riverbed. The direct comparison of the recirculation zones across different bend types revealed the correlation between the flow characteristics and bend curvature. However, previous studies have focused on flow velocities and turbulent kinetic energy without a probability density analysis. This analysis provided a more comprehensive understanding of the flow characteristics. Gaussian kernel density estimation was applied in this study to observe the distribution of the flow velocities, turbulent kinetic energy, and turbulent kinetic energy dissipation rate. The results indicated that the longitudinal time-averaged flow velocity in the recirculation zone typically ranged from −0.2 m/s to −0.8 m/s, with all the skewness coefficients exceeding 0. The horizontal time-averaged flow velocity in the recirculation zone fell between −0.175 m/s and −0.1 m/s. The skewness coefficients were negative at water depths of 16%, 33%, and 50% within the 90° and 180° bends, indicating a non-normal distribution. The probability density distribution of turbulent kinetic energy in the recirculation zone was skewed, ranging from 0 to 0.02 m2·s−2, with the skewness coefficient almost always greater than 0. The plot demonstrated multiple peaks, indicating a broad distribution of turbulent kinetic energy rather than a concentration within a specific interval. This distribution included both the high and low regions of turbulent kinetic energy. Although the overall rate of turbulent kinetic energy dissipation in the recirculation zone was relatively low, there were multiple peaks, suggesting the localized areas with higher dissipation rates alongside the regions with lower rates. These findings were significant for managing the meandering river channels, restoring the subaqueous ecosystems, understanding the pollutant diffusion mechanisms in backwater areas, the sedimentation of nutrient-laden sediments, and optimizing the parameters for spur dike design.

Interaction of fires with water have been a focus of study for human kind from the times immemorial. With the recent advances in water-based suppression, and restrictions put on halon usage, sprinklers and water-mist based fire suppression systems have become popular for industrial usage. Of this water mist is an interesting candidate, for it has advantages and to name a few would be less water requirement, non-wetting nature, gas phase action on fire. Water mist, as an effective fire suppressant, works on the principle of oxygen dilution and flame cooling.In this work, we are studying the action of water mist on a diffusion flame. The water mist was generated from a Delavan nozzle, fed with high pressure water, which is then directed downward into the raising flame. A circular burner fed with Methane in a controlled manner generated the diffusion flame. The reason to prefer a gas burner is to have a control over heat release rate which will help in simulating various fuel sources of different burning rate. A glass cubicle was used to isolate the experiments from ambient disturbances. A Digital SLR camera was used to record the flame and subsequently the images were processed to obtain flame height. Particle Image Velocimetry was used to study the effect of the flame on the water mist., by imaging the Mie scattered laser light from the droplets. Parameters that were varied were the injection pressure and flow rate of the Methane into the burner.Visual imaging from the camera indicated that there were occasional flame tip flattening during the suppression mechanism. The flame luminosity is diminished and the diffusion flame height is reduced due to the action of the water mist. Moreover, this effect is more pronounced for smaller flame than larger flame, indicating the role of the buoyancy in fire suppression effectiveness. The PIV results show that the droplets are slowed down very close to the nozzle due to the flame. Also, a recirculation zone is observed at the periphery of the spray cone, very close to the nozzle. It is a toroidal vortex that is generated due to the interaction, and the droplets thrown away from the spray cone gets trapped into this flow. The flow structure is present in a highly turbulent region and is identified in the mean field. Another finding is that close to the burner, fine droplets that could make it to the base, gets entrained into the flame. It is expected that the droplets tend to be entrained into the larger flame, as the entrainment is stronger for large flame, compared to the smaller flame. But the PIV results indicate otherwise, which is attributed to the larger size of droplets making its way close to the burner.We believe that these results will enlarge our understanding of the suppression mechanism of water mist and could form a basis for validating various simulation models.

To improve the heat recovery of hot dry rock (HDR) and realize the efficient and sustainable development of geothermal energy, it is necessary to analyse the factors affecting the heat recovery performance of HDR fractured reservoir. In this paper, a numerical model of heat recovery in fractured reservoirs with HDR is established. The effects of injection parameters on the heat recovery performance of the Enhanced Geothermal System (EGS) are compared and analysed by means of the average temperature, heat extraction rate, production temperature and heat recovery power of matrix rocks at different locations. The influence degree of injection flow rate and injection temperature on production temperature was analysed by grey correlation analysis method. The results showed that reducing the injection temperature is beneficial to fully extract the heat energy in the rock mass. The injection flow rate is positively correlated with the heat extraction rate and negatively correlated with the extraction temperature. In this simulation scheme, the optimal results are as follows: injection temperature is 298.15K, injection flow is 12 kg/s, and the influence of injection flow on production temperature is greater than that of injection temperature. This study provides a theoretical basis for the development and utilization of geothermal energy in HDR and provides a reference for the design of engineering operation parameters.

Impact of confinement on flowfield of swirl flow burners

The vortex structure formed in front of and behind a hemispheric surface hump has been studied in a water recirculating flume. In the recirculation zone behind the hemisphere, arched vortex tubes were formed. Up to a certain critical Reynolds number, each tube was shed individually from the recirculation zone. However, when the Reynolds number exceeded a critical value, several of the tubes coalesced in the recirculation zone before shedding. In front of the hemisphere, some parabolic vortex tubes formed. Their number increased with Reynolds number below its critical value and decreased above this value. The dimensionless height of the parabolic vortex tube decreased with Reynolds number.

Flow dynamics in low aspect ratio dump combustor

A mass transfer between recirculation zone and outside flow of an annular premixed propane flame was investigated experimentally using MgO powder tracing method and NaCl flame emission. The loacation of the stagnation point in the recirculation zone of the burning flow moved gradually from the nozzle exit with an increase of the gas exit velocity and became constant independently of the gas exit velocity in the very high velocity region. In the region of lower gas exit velocity, the average residence time of the fluid in the recirculation zone of the burning flow was evidently smaller than the time of the cold flow (air) at the same exit velocity, and both residence times decrease gradually with an increase of the gas exit velocity and became a same constant value in the region of very high velocity. Thus, the rate of mass transfer through the surface of the recirculation zone of a high velocity burning flow issuing from the annular nozzle was estimated by the data on the annular air jet.

In modern high pressure turbine, a certain amount of cooling air is bled off from compressor and then directly injected into the inter-stage gap between the stationary and rotating components. This paper presents a computational study of the interaction between mainstream flow and purge flow, with the objectives of evaluating the impacts of purge flow on turbine aerodynamic design parameters, tracing the loss sources involved with the injection of purge flow and describing the secondary flows near the hub region. The purge flow through the rim seal has been varied between 0–2% of the main flow and the axial position of rim seal has been also changed. Steady-state simulation using a 3D RANS solver is presented with particular emphasis on the mechanisms of loss production. It is found that purge flow has a primary effect on the spanwise distribution of turbine aerodynamic design parameters, especially near the hub region. The losses brought about by the injection of purge flow can be divided into four parts: reaction redistribution between vane and blade in one stage, a shear layer between purge flow and mainstream flow due to different circumferential momentum, hub passage vortex interaction and decrease of output work near the end wall. However these four loss sources are not independent of each other. Shear induced vortex (SIV) and slot leakage vortex (SLV) appear near the hub region after purge flow is introduced. The shear induced vortex is formed due to the shear interaction between mainstream flow and purge flow which develop into hub passage vortex. The slot leakage vortex is formed due to the relative motion of the cavity disks and its strength is relatively weak compared with the shear induce vortex. The results gained from this paper may give some useful guidelines for turbine aerodynamic design and end wall profile optimization.

A numerical investigation on the utilization of a depleted natural gas field for seasonal hydrogen storage: A case study for Değirmenköy gas field

An Outer Planets Probe which retains the charred heatshield during atmospheric descent must deploy a sampling tube through the heatshield to extract atmospheric samples for analysis. Once the sampling tube is deployed, the atmospheric samples ingested must be free of contaminant gases generated by the heatshield. Outgassing products such as methane and water vapor are present in planetary atmospheres and hence, ingestion of such species would result in gas analyzer measurement uncertainties. This paper evaluates the potential for, and design impact of, the extracted atmospheric samples being contaminated by heatshield outgassing products. Flight trajectory data for Jupiter, Saturn and Uranus entries are analyzed to define the conditions resulting in the greatest potential for outgassing products being ingested into the probe's sampling system. An experimental program is defined and described which simulates the key flow field features for a planetary flight in a ground-based test facility. The primary parameters varied in the test include: sampling tube length, injectant mass flow rate and angle of attack. Measured contaminant levels predict the critical sampling tube length for contamination avoidance. Thus, the study demonstrates the compatibility of a retained heatshield concept and high quality atmospheric trace species measurements.

Numerical investigation of the local extinction and re-ignition mechanisms of premixed flame in a micro combustor with a flame holder and preheating channels

A method is described of measuring the bulk mass-flow rate in steady and pulsating air streams with the aid of a suitable tracer gas such as carbon dioxide. The method briefly involves controlled injection of the tracer into the main flow, mixing of the two flow media, and analysis of the mixture to determine the concentration of the tracer. The main flow is derived from the injection flow rate and from the concentration of the tracer in the sample mixture. An inexpensive infrared analyzer was constructed for determining mass concentrations of carbon dioxide in air ranging from 1/2 to 3 per cent. Results obtained by the gas-tracer method in both steady and pulsating streams were compared with those obtained by other methods of mass-flow measurement. A few possible practical applications of the measuring technique are discussed.

There is a significant opportunity to expand the understanding of subcutaneous injection mechanics with an aim to increase injectable volume while controlling tissue strain and associated subject pain. Computational modeling can evaluate the mechanics of subcutaneous injections as a supplement to experimental, animal and clinical studies. The objectives of this study are to (1) develop a computational model for subcutaneous injection in tissue, (2) investigate the influence anisotropic tissue permeability has on bolus formation, and (3) explore the effects that injection flow rate and viscosity have on injection flow and tissue strain. Poroelastic models with subsurface flow were implemented in finite element software (COMSOL, ABAQUS). Pore pressure and injectate distribution showed excellent agreement with experimental results when evaluated at multiple injection rates (20 ml/hr, 120 ml/hr and 360 ml/hr). Including the anisotropy of tissue permeability causes the injectate to preferentially spread horizontally, similar to experimentally observed bolus distributions. Cases are presented to provide additional insight into injection mechanics, including variations on the delivery rate, the injection volume, viscosity and the thickness of the subcutaneous layer. The results support the use of computational modeling as a valid tool for understanding tissue strains and injectate distributions for large volume injections.

Deep geothermal energy is a renewable energy source with broad distribution, vast resource potential, and promising development prospects. The current main extraction methods include enhanced geothermal systems（EGS）, annular heat exchange well systems（AGS）, fault zone fluid circulation extraction, and coaxial casing extraction methods. However, challenges such as unstable heat extraction power, high seismic risks, and low heat extraction efficiency persist. Addressing the bottlenecks in current deep geothermal extraction technology, we adheres to the principle of energy exchange without material exchange during the extraction process and aims for large-scale, sustainable, and stable development of deep dry hot rock geothermal resources. We propose the clustered multi-branch U-shaped well heat extraction method（UMW-DGS） and its key technologies. On this basis, an axisymmetric thermal conduction model for the wellbore is established. We calculated the spatiotemporal evolution of the temperature field and heat extraction power around the well under constant wellbore diameter conditions and analyzed the effects of three sensitive factors—temperature difference, thermal conductivity, and wellbore diameter—on heat extraction power. In addition, to address the boundary value problem of the UMW-DGS, a three-dimensional thermo-hydro-mechanical coupling numerical algorithm based on the finite volume method（FVM） was developed. This algorithm was used to study the heat exchange efficiency of a single horizontal well section of the UMW-DGS and the spatiotemporal evolution of the temperature field under different injection flow conditions. By analyzing the effective heat exchange amount, duration, and power at different flow rates, we found that increasing the injection flow rate decreases the effective heat exchange energy and duration while causing the effective heat exchange power to first increase and then decrease. The research results indicate that deep geothermal energy development requires designing injection flow rates under the condition of balancing heat exchange temperature and power to achieve optimal heat exchange efficiency.