Dimensionless Flow Rate Research Articles

Centrifugálkompresszor-karakterisztika regressziója a teljes működési tartományban nem lineáris matematikai modellhez

Gázturbinás berendezésekben – legyenek azok hajtóművek vagy ipari alkalmazásban tengelyteljesítményt leadó eszközök – a centrifugális kompresszorok széles körben elterjedtek. Jellemzőjük az egy fokozatban elérhető jelentős nyomásviszony, valamint a relatíve széles tömegáram-tartományban való stabil üzemelés. Ez utóbbi ellenére kis szállítás esetén – az axiális kompresszorokhoz hasonlóan – különféle instabil jelenségeket mutatnak (forgó leválás, pompázs), amelyek nyomás- és tömegáramlengések formájában jelentkeznek. Ezek elkerülése elsődleges fontosságú a berendezés és a kapcsolódó rendszerek megóvása érdekében, mert a lengésekben tárolt energia akár a lapátok kiszakításához is elegendő. A kompresszor matematikai modellezése kiemelkedő jelentőségű abból a szempontból, hogy egyrészt a különböző üzemállapotokban előre jelezhetővé válik a berendezés viselkedése, továbbá azon tartományokban, ahol az instabilitások fellépése várható, ott egy aktív beavatkozást biztosító rendszer a megfelelő reakciót kiváltva elháríthatja a szabályozás nélküli esetben bekövetkező rendellenes működést. Cikkünk célja, hogy egy centrifugális kompresszor teljes üzemi tartományában történő mérése alapján polinomos regresszióval közelítse a berendezés tömegáram-nyomásviszony, valamint tömegáram-hatásfok jelleggörbéit. Az eljárással megalkotott összefüggés a teljes üzemmódtartományt lefedi, előnyeit egy összehasonlítás mutatja be a korábban alkalmazott lineáris (például átszámított fordulatszám és dimenziótlan tömegáram szerint bilineáris) módszerek eredményeivel szemben. A kompresszor matematikai modelljét dinamikus szimulációnak vetettük alá, amelyet MATLAB Simulink környezetben hajtottunk végre, és az összehasonlítás érdekében tranziens mérések eredményeit használtuk fel. Az így megalkotott dinamikus modell pedig korszerű, aktív pompázsszabályozó rendszerek fejlesztését teszi lehetővé a későbbiekben.

To Interpret Rate Transient Analysis for the Determination of Reservoir Properties

Rate-time decline curve analysis is a major technique which is mostly used in petroleum engineering. Many methods are used for the determination of the decrease in the production rate within a given period of time. The main disadvantage of Arp’s decline type curve analysis is that it is only used for boundary dominated flow period; it is not used for transient flow period. The analysis of the Fetkovich is to determine the log-log type curve for both the transient flow period (early time period or infinite) and boundary-dominated flow period (late time period). Arps developed the type curve which shows the production rate decline with time for the finite reservoir or late time period. The exponential or constant flow decline, hyperbolic decline, and harmonic decline according to the value of decline curve exponent (b) is given by Arps. After that Fetkovich improved on earlier work done by Arps in predicting decline production rate of wells over a given period of time. The main objective of this study was to plotting the rate transient analysis curve. I will plot the Fetkovich type curve (combined early and late times region). The graph will be plotted between the dimensionless decline flow rate (qDd) and the dimensionless decline time” (tDd). This will be the objective of the study.

Numerical modelling for the solar driven bi-reforming of methane for the production of syngas in a solar thermochemical micro-packed bed reactor

High temperature heat transfer and thermochemical storage performances of the solar driven bi-reforming of methane (SDSCB-RM) in a solar thermochemical micro-packed bed (ST-μPB) reactor are numerically investigated under different operating conditions along ST-μPB reactor length. A pseudo-homogeneous mathematical model is developed to simulate the heat and mass transfer processes coupled with thermochemical reaction kinetics in ST-μPB reactor with radiative heat loss. The effect of several parameters including the gas flow rate (Qg), effective thermal conductivity (λs,eff), operating time (ti) and operating temperature (Top.) were investigated. The simulated results shown that the pressure drop increases with the increase of Qg. When the Qg is increased, the temperature profiles at the surface of the solid phase as well as the temperature profiles of the gas phase are remarkably decreasing. The consumption of reactants (CH4, H2O and CO2) is increased when the λs,eff is gradually increased. On the other hand, the production of products (H2, and CO) is remarkably increasing with the increase of the λs,eff. According to simulated results, the overall conversions of reactants (CH4 and CO2) and the dimensionless flow rate (DFR) of H2 reach the maximum values of 98.18%, 75.61% and 1.6278 at the operating time of 2.50 h. The thermochemical energy storage efficiency (ηChem) remarkably increases with the operating temperature and the maximum value of the ηChem can be as high as 74.21% at 1123 K. The overall conversions of reactants (CH4 and CO2), DFR of H2 and the energy stored as chemical enthalpy (QChem) were also evaluated in relation to the operating temperature and their maximum values of 99.43%, 89.03%, 1.6383 and 1.3745 kJ/s are obtained at 1225 K.

Numerical and analytical analysis of a robust flow regulator in electroosmotic microfluidic networks

This paper proposes a three-way electrokinetic flow regulator to control flow direction independent of the electrolyte solution and regulator material. In this two-dimensional regulator, the flow direction is simply controlled by varying the applied electrical voltage at microfluidic network ends. The performance of the regulator is discussed based on full detailed numerical simulations using the Nernst-Plank equation and a simplified semi-analytical approach. Numerical simulations revealed that the cut off voltage ratio required for switching flow direction is almost independent of the electrical double-layer (EDL) thickness and strongly depends on regulator geometry. A semi-analytical approach is introduced to determine the regulation voltage ratio and dimensionless flow rate during the regulation. It is showed that for thin EDLs, the presented approach can adequately predict the overall performance of the regulator with considerably lower computational efforts. Numerical and analytical approaches introduced in this paper, can be extended to more complex networks e.g. four-way networks.

Experimental startup characteristics of a prototype pump delivering tap water

As an important period of self-priming process, the startup process of self-priming pump is inevitable. In this paper, a small overhung self-priming centrifugal pump is tested to reveal the effect of startup time on startup performance. As a first step, the startup performance of the pump delivering water is tried firstly. Through testing, the transient characteristics of performance variables against time are emphatically analyzed in three lengths of startup time/duration and four typical steady flow rates. The test results show that with extension of the startup time, the steady rotational speeds present a slight rise trend. The flow rate curves overall rapidly rise and then grow slowly. The shorter the length of the startup time, the closer the head curves tend to be linear. Conversely, the head curve more likely can be best fitted by quadratic polynomial. At the beginning of rapid startup, there is a sudden-rise-and-sudden-drop oscillation phenomenon in the inlet static pressures curves, and a pressure impact phenomenon can be found in the outlet static pressure curves. However, the above two phenomena do not exist in the moderate and slow startups. The transient characteristics of dimensionless flow rate and head are associated with the length of startup time, while the dimensionless shaft power is not. There is no similarity in transient performance between rapid, moderate, and slow startup cases.

The minimum flow rate of electrosprays in the cone-jet mode

Stable electrospraying in the cone-jet mode is restricted to flow rates above a minimum, and understanding the physics of this constraint is important to improve this atomization technique. We study this problem by measuring the minimum flow rate of electrosprays of tributyl phosphate and propylene carbonate at varying electrical conductivity $K$ (all other physical properties such as the density $\unicode[STIX]{x1D70C}$, surface tension $\unicode[STIX]{x1D6FE}$ and viscosity $\unicode[STIX]{x1D707}$ are kept constant and equal to those of the pure liquids), and through the analysis of numerical solutions. The experiments show that the dimensionless minimum flow rate is a function of both the dielectric constant $\unicode[STIX]{x1D700}$ of the liquid and its Reynolds number, $Re=(\unicode[STIX]{x1D70C}\unicode[STIX]{x1D700}_{o}\unicode[STIX]{x1D6FE}^{2}/\unicode[STIX]{x1D707}^{3}K)^{1/3}$. This result is unexpected in the light of existing theories which, for the conditions investigated, predict a minimum flow rate that depends only on $\unicode[STIX]{x1D700}$ and/or is marginally affected by $Re$. The experimental dependency on the Reynolds number requires the viscous stress to be a factor in the determination of the minimum flow rate. However, the numerical solutions suggest that a balance of opposing forces including the fixing viscous stress, which at decreasing flow rates may lower the acceleration of the flow to the point of making it unstable, is unlikely to be the cause. An alternative mechanism is the significant viscous dissipation taking place in the transition from cone to jet, and which at low flow rates cannot be supplied by the work done by the tangential electric stress in the same area. Instead, mechanical energy injected into the system farther downstream must be transferred upstream where dissipation predominantly takes place. This mechanism is supported by the balance between the energy dissipated and the work done by the electric stress in the transition from cone to jet, which yields a relationship between the minimum flow rate, the Reynolds number and the dielectric constant that compares well with experiments.

Joint Effect of Magnetic Field and Heat Transfer on Particulate Fluid Suspension in a Catheterized Wavy Tube

The effect of wall slip conditions, porous media, and heat transfer on peristaltic inflow of MHD Newtonian fluid with suspended particles in a catheterized tube has been studied with long-wavelength and low-Reynolds number approximations. The analytical solution has been derived for velocity and temperature. The amplitude ratio, particle concentration, catheter size, and the dimensionless flow rate were used to discuss the pressure gradient. The solutions for velocity and temperature derived in the analysis have been computed numerically and investigated. The tube surface is maintained at a fixed temperature. The variations of physical variables with the pertinent parameters were discussed graphically. The mathematical model presented corresponds to the flow in the annular space between two concentric tubes. It has been deduced that the thermal energy is reduced with particles’ concentration and with slip condition through the catheterized tube. The flow accelerates with the magnetic field and slip condition at the wall, whereas it decreases at the catheter. The catheter size has a different effect on both pressure drop and friction force.

Constructal entransy dissipation rate minimization for X-shaped vascular networks

Combining with entransy theory, constructal designs of the X-shaped vascular networks (XSVNs) are implemented with fixed total tube volumes of the XSVNs. The entransy dissipation rates (EDRs) of the XSVNs are minimized, and the optimal constructs of the XSVNs are derived. Comparison of the optimal constructs of the XSVNs with two optimization objectives (EDR minimization and entropy generation rate (EGR) minimization) is conducted. It is found that when the dimensionless mass flow rate (DMFR) is small, the optimal diameter ratio of the elemental XSVN derived by EDR minimization is different from that derived by EGR minimization. For the multilevel XSVN, when the DMFR is 100, compared the XSVN with the corresponding H-shaped vascular network (HSVN), the dimensionless EDRs of the elemental, second and fourth order XSVNs are reduced by 26.39%, 15.34% and 9.81%, respectively. Compared with the entransy dissipation number (EDN) of the second order XSVN before angle optimization, the EDN after optimization is reduced by 26.15%, which illustrates that it is significant to conduct angle optimization of the XSVN. Entransy theory is applied into the constructal design of the vasculature with heat transfer and fluid flow in this paper, which provides new directions for the vasculature designs.

Visualization of periodic emission of drops with micro-dripping mode in electrohydrodynamic (EHD) atomization

Electrohydrodynamic (EHD) atomization could produce mono-dispersed drops in different spraying modes. The generation of drops of controllable size in the micrometer range in micro-dripping mode has been focused because of the important and broad practical applications. In present study, the periodic emission of drops from an electrified meniscus attached to the end of a capillary nozzle in micro-dripping mode and the dynamics of the ligament formation were investigated. The charge-to-mass ratio, the frequency of drops generation, dimensionless equivalent diameter, ligament length and width could be obtained under different dimensionless flow rates (q) and electric Bond numbers (BE), in which q is small but could be varied by a factor of around one hundred and BE is bound to a narrow range of order unity. The charge carried by each drop produced in present work is smaller than the fifty percent of critical value of the Rayleigh limit. Drop generation frequency with rang of a few to several hundred Hz obtained from offline analysis of time-resolved images captured by a high-speed digital camera with zoomed lens highly depends on q and BE, in which the drops produced are usually the smaller than one to ten times the diameter of the nozzle. The micro-dripping regime may be sustained and occur periodically within a certain ranges of flow rate and voltage, where the meniscus would be stretched by a tangential electric stress and further elongated by electric polarization stress and gravity, finally develops a drop ejected from the meniscus. The dimensionless equivalent diameter (dd) would increase as powers of dimensionless flow rate q, in which the correlation gives dd ∼ q0.32 for very small q and dd ∼ q0.45 for larger q. The dimensionless length ls and width ds would increase as powers of dimensionless flow rate q, in which the relation roughly could be given as ls ∼ q0.30–0.34 and ds ∼ q0.33–0.46 for very small q, while ls ∼ q0.43–0.45 and ds ∼ q0.52–0.62 for larger q. The results in present work were similar to previous studies and also existed some deviation, the possible reasons including electric field, liquid properties and viscosity and air resistance were discussed.

Experimental and theoretical investigation of a low-Reynolds-number flow through deformable shallow microchannels with ultra-low height-to-width aspect ratios

The emerging field of deformable microfluidics widely employed in the Lab-on-a-Chip and MEMS communities offers an opportunity to study a relatively under-examined physics. The main objective of this work is to provide a deeper insight into the underlying coupled fluid–solid interactions of a low-Reynolds-number, i.e. $$Re \sim O(10^{-2}$$ – $$10^{+1})$$ , fluid flow through a shallow deformable microchannel with ultra-low height-to-width-ratios, i.e. $$O(10^{-3})$$ . The fabricated deformable microchannels of several microns in height and few millimeters in width, whose aspect ratio is about two orders of magnitude smaller than that of the previous reports, allow us to investigate the fluid flow characteristics spanning a variety of distinct regimes from small wall deflections, where the deformable microchannel resembles its corresponding rigid one, to wall deflections much larger than the original height, where the height-independent characteristic behavior emerges. The effects of the microchannel geometry, membrane properties, and pressure difference across the channel are represented by a lumped variable called flexibility parameter. Under the same pressure drop across different channels, any difference in their geometries is reflected into the flexibility parameter of the channels, which can potentially cause the devices to operate under distinct regimes of the fluid–solid characteristics. For a fabricated microchannel with given membrane properties and channel geometry, on the other hand, a sufficiently large change in the applied pressure difference can alter the flow-structure behavior from one characteristic regime to another. By appropriately introducing the flexibility parameter and the dimensionless volumetric flow rate, a master curve is found for the fluid flow through any long and shallow deformable microchannel. A criterion is also suggested for determining whether the coupled or decoupled fluid–solid mechanics should be considered.

Cross-ventilation in a generic isolated building equipped with louvers: Wind-tunnel experiments and CFD simulations

Natural ventilation can be used to improve indoor air quality, remove contaminants from spaces and to remove heat from a building during the day, or during the night. In some cases, openings for natural ventilation are equipped with shading devices – such as louvers – to reduce solar heat gains while allowing natural ventilation. This study presents wind-tunnel experiments and computational fluid dynamics (CFD) simulations of a cross-ventilated building equipped with louvers. Four opening positions are studied: (i) openings in the center, (ii) upper or (iii) lower part of the windward and leeward facades or (iv) one opening in the upper part of the windward facade and one opening in the lower part of the leeward facade. The 3D steady Reynolds-averaged Navier-Stokes (RANS) simulations are performed with three turbulence models (RNG k-ε, SST k-ω, RSM) and validated with the wind-tunnel experiments. The experimental results show that the largest velocities occur in a building with openings in the upper part of the facade. The best agreement with experimental data is provided by RSM. In addition, CFD simulations for buildings without louvers are conducted for the same opening positions to evaluate the effect of louvers on the dimensionless volume flow rate, age of air and air exchange efficiency. The highest dimensionless volume flow rate at reduced scale (0.69) is obtained in the building with louvered openings in the upper part of the facade and the highest air exchange efficiency is achieved for a building with louvered openings in the center of the facade (45%).

Steady Two-Dimensional Free-Surface Flow Past Disturbances in an Open Channel: Solutions of the Korteweg–De Vries Equation and Analysis of the Weakly Nonlinear Phase Space

Two-dimensional free-surface flow past disturbances in an open channel is a classical problem in hydrodynamics—a problem that has received considerable attention over the last two centuries (e.g., see Lamb’s Treatise, 1879). With traces back to Russell’s experimental observations of the Great Wave of Translation in 1834, Korteweg and de Vries (1895), and others, derived an unforced equation to describe the balance between nonlinearity and dispersion required to model the solitary wave. More recently, Akylas (1984) derived a forced KdV equation to model a pressure distribution on the free-surface (e.g., a ship). Since then, the forced KdV equation has been shown to be a useful model approximation for two-dimensional flow past disturbances in an open channel. In this paper, we review the stationary solutions of the forced KdV equation for four types of localised disturbances: (i) a flat plate separating two free surfaces; (ii) a compact bump, or dip in the channel bottom topography; (iii) a compact distribution of pressure on the free surface and (iv) a step-wise change in the otherwise constant horizontal level of the channel bottom topography. Moreover, Dias and Vanden-Broeck (2002) developed a phase plane method to analyse flow over a bump, and this general approach has also been applied to the three other types of forcing (see Binder et al., 2005–2015, and others). In this study, we use eleven basic flow types to classify the steady solutions of the forced KdV equation using the phase plane method. Additionally, considering solutions that are wave-free both far upstream and far downstream, we compare KdV model approximations of the uniform flow conditions in the far-field with exact solutions of the full problem. In particular, we derive a new KdV model approximation for the upstream dimensionless flow-rate which is conveniently given in terms of the known downstream dimensionless flow-rate.

Electroviscous effect on pressure driven flow and related heat transfer in microchannels with surface chemical reaction

This study investigates the fluidic flow and related heat transfer in a microchannel formed by two parallel plates. Uniform heat flux is applied at the walls with considering the surface chemical reaction, which determines the surface charge on the wall of the microchannel. Our results show that the pH and ionic concentration apparently affect the surface chemical reaction and the subsequent fluidic behavior and heat transfer in the microchannel, especially with the boundary slip. The dimensionless velocity and temperature increase with the ionic concentration. The difference of the dimensionless flow rate and the Nusselt number is obvious under three different electrical boundary conditions: constant surface charge density, constant zeta potential and MI model. Under the boundary slip, the dimensionless flow rate and Nusselt number decrease by 50% and 12%, respectively, with increasing pH under the MI model, but keep unchanged under constant zeta potential or surface charge model. The dimensionless flow rate and Nusselt number also increase with the ionic concentration due to weakened electroviscous effect. Meanwhile, the slip increases the influence of ionic concentration on the dimensionless flow rate and Nusselt number. The results of this work may guide the design and optimization of microfluidic devices.

Flow behaviour of the exchange flow through a ceiling vent in natural convection: A numerical approach using CALIF3S-ISIS CFD software

In the fire safety context, the exchange flow through a horizontal opening connecting two superposed zones receives a particular interest for developing predictive models and validation of computational codes. Its random behaviour over time could influence the fire source and the vertical spread of the fire smoke. This work is part of a research program developed at the French “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN), and its main objective is to bring new knowledge on this subject by carrying out a numerical study with the computational open-source software named CALIF3S-ISIS developed by IRSN. The configuration of the simulations is that of an experimental reduced-scale set-up, on which data have been obtained and could be used for validation. Large Eddy Simulation (LES) were performed (variation of wall boundary conditions, air injection temperatures, vent diameter and vent mesh refinement), and the bidirectional flow is properly estimated in comparison with the experiment. Particularly, it was determined that the increment of the geometric ratio L/D of the vent of diameter D and thickness L, also leads to an increment of a dimensionless exchange flow rate Froude number at the vent. However, a discrepancy was observed with the model proposed by Epstein based on his experiments in natural convection. The disagreement between the simulation results and the experiments is attributed principally to differences on the vertical temperature stratification of the lower room. Finally, it was investigated the influence of the vertical location within the vent over the mean flow behaviour through different levels of the vent, specifically an exchange of the downward and upward flow organization within the opening.

Numerical Study of Unsteady Pulsed Suction through Endwall Bleed Holes in a Highly Loaded Compressor Cascade

Abstract Unsteady pulsed suction (UPS) was applied as an unsteady flow control (UFC) technique in a certain highly loaded compressor cascade to control the flow separations. Only two bleed holes symmetrically mounted on the endwalls (one on the upper endwall and another on the lower endwall) were set up to achieve steady constant suction (SCS) and UPS. The improvements in aerodynamic performance by SCS and UPS under different time-averaged suction flow rates are firstly investigated and compared. The related unsteady aerodynamic parameters of UPS such as excitation frequency, excitation location, pitch angle, and skew angle are discussed and analyzed in detail. The results show that UPS can provide a better flow control effect than SCS with the same time-averaged dimensionless suction flow rate in the control of flow separation. The aerodynamic performance of compressor cascades can be significantly enhanced by UPS when unsteady aerodynamic parameters are in their optimum ranges. Based on the optimum parameters for UPS, the total pressure loss coefficient is reduced by 19 % only with the time-averaged dimensionless suction flow rate ms =0.4 %.

Whole wellbore liquid loading recognition model for gas wells

Liquid loading is a serious issue in the production of gas wells. In order to recognize liquid loading, the flow regime and the transition criteria of the gas-liquid two-phase in the wellbore were obtained by theoretical analysis. The prediction correlation for the size of the entrained droplet was given for every flow pattern. Based on the force balance condition of the largest droplet in the gas core, the dimensionless critical gas mass flow rate was defined to recognize liquid loading along the whole wellbore. The influence of different factors on liquid loading was analyzed by an example. The results show that there are three kinds of flow regime in the wellbore, which are co-current annular flow with disturbance wave torn off, co-current annular flow with bag breakup and churn-annular flow with wave under-cut. The maximum size of the droplet in the gas core depends on the droplet breakup process under co-current annular flow and droplet entrainment process under churn-annular flow. The dimensionless critical gas mass flow rate decreases with the increase of gas flow rate and the decrease of tube diameter, which reduces the possibility of liquid loading. Liquid loading can be avoided by the decrease of surface tension. The viscosities of the gas and liquid phases have slight influence on the dimensionless critical gas mass flow rate.

Pressure-rate convolution and deconvolution response for fractured conventional and unconventional reservoirs using new decline rate model

Abstract This paper introduces new approach for pressure-rate convolution and deconvolution analysis of multi-stages hydraulically fractured conventional and unconventional reservoirs. This approach demonstrates the impact of variable Sand face flow rate on reservoir performance. A new model for P/R deconvolution is used to convert pressure pulse from variable flow rate to single and constant rate response. The target of this study is fractal reservoirs with and without stimulated and unstimulated reservoir volume. Multi-linear flow regimes approach is used to describe pressure behavior in the reservoirs while decline flow rate behavior is described by newly proposed model in this study. This model depicts, instead of van Everdingen model, indirectly the declining rate with time by using pressure responses with production time. Decline flow rate behavior simulated by linear and bi-linear flow models are also studied and compared with the one obtained by the new model. Several analytical models are used in this study by applying P/R convolution and deconvolution technique and solved for constant and variable flow rate considering different reservoir configurations and operating conditions. The results are interpreted and analyzed for better understanding pressure behaviors, flow regime types, and productivity index trends for continuously changing flow rate especially at early production time. Estimating stimulated reservoir volume ( V s r v ) is considered one of the applications of convolved pressure since it is calculated from pseudo-steady state flow when late time boundary dominated flow regime is reached. The outcomes of this study can be summarized as: 1) Introducing new approach for pressure-rate convolution and deconvolution technique for multi-stages hydraulically fractured reservoirs by applying new decline flow rate model that indirectly simulates variable flow rate with time. 2) Generating analytical models for dimensionless pressure and flow rate for constant and variable flow rate using the concept of P/R convolution and deconvolution. 3) Comparing the result of newly proposed models with the results obtained by applying van Everdingen model for decline rate behavior. 4) Studying the applicability of linear and bi-linear flow models in converting variable flow rate pressure response to single and constant flow rate pressure response. 5) Applying the deconvolution technique to simulate pressure response at late production time to estimate stimulated reservoir volume. The most interesting points are: 1) The main difference in wellbore pressure behavior between variable and constant flow rate can be seen at early production time, however intermediate production time could also show very limited changes for the case of variable rate wellbore pressure. 2) A unit slope line flow regime could be developed for varied flow rate pressure response at very early production time similar to the wellbore storage dominated flow regime. 3) Productivity index calculated by the proposed models for variable flow rate is greater than the index for constant flow rate. 4) The impact of petrophysical properties of porous media and hydraulic fracture characteristics on pressure response are similar in the two cases of variable and constant flow rate. 5) The decline rate models for linear and bi-linear flow are not applicable in pressure deconvolution technique.

Effects of fire source position on smoke extraction efficiency by natural ventilation through a board-coupled shaft during tunnel fires

Experimental studies on the influences of fire positions on the smoke extraction efficiency of a board-coupled shaft (BCS) are conducted in a 1:15 scale model tunnel, and the general condition that the first shaft cannot extract the entirety of the smoke is considered in this study. The results indicate that the BCS with proper board dimensions and installation positions can achieve better smoke extraction performance than the traditional shaft, whether the fire is located beneath the shaft or not. Besides, the advantages of BCS will gradually decrease as the distance between the fire source and the shaft increases. Flow visualisations captured beneath the shaft illustrate different flow patterns as the fire location changes. Based on the one-dimensional theory, an analytical model is developed to analyze the flow process in and around the vertical shaft, and a general formula is derived for predicting the dimensionless volume flow rate in the BCS, which may contribute positive significance in the tunnel smoke control and extraction design.

Two-dimensional numerical simulation of wind driven ventilation across a building enclosure with two free apertures on the rear side: Vortex shedding and “pumping flow mechanism”

This paper investigates the single-sided wind driven natural ventilation with two apertures on the rear wall, regarding different horizontal aperture separations, magnitudes of incoming wind speed and various side ratios (L/D, length/width). In the present work, CFD simulations are performed with Reynolds-Averaged Navier-Stokes (RANS) approach, and pressure correction linked equation is adopted to provide closure. The simulation results indicate that wind flow behind the rear wall is essentially driven by the pumping mechanism due to vortex shedding, in which wind flow direction alternates at a mean rate. The frequency of the “pumping” flow increases dramatically with the wind flow velocity at a wide range of Reynolds numbers. The decrease of free aperture separation reduces the enclosure flow rate whilst the frequency of the wind flow almost maintains at a constant value. The increase of sidewall length has similar impacts on the ventilation frequency and the dimensionless ventilation flow rate. Numerical investigations further demonstrate that non-dimensional ventilation rate can be promoted by increasing incoming wind velocities when the wind speed is less than 5 m/s. This promotion will be restrained when the wind speed is further increased. The simulation results of built enclosure flow rate, frequency of enclosure flow can be applied to improve the ventilation performance of buildings merely with rear wall openings.

Experimental investigation of coaxial impinging air jets

This study experimentally investigates hydrodynamical and thermal characteristics of coaxial confined impinging turbulent air jets. Various values of the dimensionless nozzle-to-plate distances and dimensionless flowrate ratio are tested for a constant total flowrate. It is shown that flowrate ratio enhances significantly the uniformity of the heat transfer and increases the average Nusselt number while increasing nozzle-to-plate distance decreases the intensity of convective heat transfer over the impingement plate. For pressure distribution on the impingement plate, subatmospheric regions are observed. The results obtained for coaxial case are also compared with those for the single circular impingement jet.