Sharp-edged Inlet Research Articles

Influencia de la geometría de entrada en tomas sumergidas sobre la sumersión crítica para la formación de vórtices que arrastran aire

RESUMEN Se realizó un estudio experimental para estudiar la influencia de la geometría de una toma horizontal sumergida en la formación de vórtices de superficie libre que arrastran aire. Se ensayaron dos tipos de perfiles de ingreso a la toma: perfil acampanado y de arista viva. En un gráfico adimensional, en el que la sumersión relativa se expresa en función del número de Froude, se delinearon para cada caso las regiones en las que no se formaron vórtices con arrastre de aire. Se determinó el tipo de vórtice para cada punto y se delinearon las regiones donde no se forma ningún vórtice. Se comprobó que, a medida que aumenta el número de Froude, es necesaria una mayor sumersión relativa para evitar el ingreso de aire hacia la toma sumergida. Además, la toma de arista viva requirió una mayor sumersión relativa que la toma de perfil acampanado, en el rango de número de Froude ensayado, para evitar la formación de vórtices que atrapen aire. Se comprobó, en los vórtices de tipo B, que los periodos de ingreso de aire eran más prolongados cuando se instalaba una toma de arista viva para las mismas condiciones hidráulicas que para la toma de perfil acampanado.

Experimental validation of RANS and DES modelling of pipe flow mixing

Velocity and concentration profiles in a pipe flow was measured downstream of injection of a tracer gas at the pipe centerline. The pipe had diameter D = 0.2 m and two Reynolds numbers, Re = 50000 and Re = 100000, were used. The profiles were measured at positions 5D and 10D downstream of the injection point. Three different industrial relevant geometrical configurations were used upstream of the injection point: a 10D straight pipe, two 10D pipes connected with a 90° bend or a straight 10D pipe with a mixer plate mounted 2D upstream the injection point. In all cases, air entered the pipe from the surroundings through a sharp-edged inlet. This represents many practical flow applications and is also a well-defined inlet condition that generates turbulence in the vena contracta in the inlet. The measurements were compared to predictions from three different computational models: two with Reynolds Averaged Navier-Stokes (RANS) and one with high-resolution Detached Eddy Simulation (DES). For RANS, the k-ω SST model had difficulty in predicting the turbulence created by the vena contracta. The k-ε model performed better, but gave completely wrong results for the inlet with a pipe bend. The DES was successful for all cases with only minor deviations from measurements.

On the Influence of the Entrance Section on the Rotordynamic Performance of a Pump Seal With Uniform Clearance: A Sharp Edge Versus A Round Inlet

Secondary flows through annular seals in pumps must be minimized to improve their mechanical efficiency. Annular seals, in particular balance piston seals, also produce rotordynamic force coefficients, which easily control the placement of rotor critical speeds and determine system stability. A uniform clearance annular seal produces a direct (centering) static stiffness as a result of the sudden entrance pressure drop at its inlet plane when the fluid flow accelerates from an upstream (stagnant) flow region into a narrow film land. This so-called Lomakin effect equates the entrance pressure drop to the dynamic flow head through an empirical entrance pressure loss coefficient. Most seal designs regard the inlet as a sharp edge or square corner. In actuality, a customary manufacturing process could produce a rounded corner at the seal inlet. Furthermore, after a long period of operation, a sharp corner may wear out into a round section. Notice that to this date, bulk-flow model (BFM) analyses rely on a hitherto unknown entrance pressure coefficient to deliver accurate predictions for seal force coefficients. This paper establishes the ground to quantify the influence of an inlet round corner on the performance of a water lubricated seal reproducing a configuration tested by Marquette et al. (1997). The smooth surface seal has clearance Cr = 0.11 mm, length L = 35 mm, and diameter D = 76 mm (L/D = 0.46). The test case considers design operation at 10.2 krpm and 6.9 MPa pressure drop. Computational fluid dynamics (CFD) simulations apply to a seal with either a sharp edge or an inlet section with curvature rc varying from ¼Cr to 5Cr. Note the largest radius (rc) is just 1.6% of the overall seal length L. Going from a sharp edge inlet plane to one with a small curvature rc = ¼Cr produces a ∼20% decrease on the inlet pressure loss coefficient (ξ). A further reduction occurs with a larger circular corner; ξ drops from 0.43 to 0.17. That is, the entrance pressure loss will be lesser in a seal with a curved inlet. This can occur easily if the inlet edge wears due to solid particles eroding the seal inlet section. Further CFD simulations show that operating conditions in rotor speed and pressure drop do not affect the inlet loss coefficient, while the inlet circumferential swirl velocity does. In addition, further CFD results for a shorter (half) length seal produce a very similar entrance loss coefficient, whereas an enlarged (double) clearance seal leads to an increase in the entrance pressure loss parameter as the inlet section becomes less round. CFD predictions for most rotordynamic coefficients are within 10% relative to published test data, except for the direct damping coefficient C. For the seal with a rounded edge (rc = 5 Cr) at the inlet plane, both the direct stiffness K and direct damping C decrease about 10% compared against the coefficients for the seal with a sharp inlet edge. The other force coefficients, namely cross-coupled stiffness and added mass, are unaffected by the inlet edge geometry. The same result holds for seal leakage, as expected. A BFM incorporates the CFD derived entrance pressure loss coefficients and produces rotordynamic coefficients for the same operating conditions. The CFD and BFM predictions are in good agreement, though there is still ∼10% discrepancy for the direct stiffnesses delivered by the two methods. In the end, the analysis of the CFD results quantifies the pressure loss coefficient as a function of the inlet geometry for ready use in engineering BFM tools.

Experimental Study on Pressure Losses in Circular Orifices With Inlet Cross Flow

The ability to understand and predict the pressure losses of orifices is important in order to improve the air flow within the secondary air system. This experimental study investigates the behavior of the discharge coefficient for circular orifices with inlet cross flow which is a common flow case in gas turbines. Examples of this are at the inlet of a film cooling hole or the feeding of air to a blade through an orifice in a rotor disk. Measurements were conducted for a total number of 38 orifices, covering a wide range of length-to-diameter ratios, including short and long orifices with varying inlet geometries. Up to five different chamfer-to-diameter and radius-to-diameter ratios were tested per orifice length. Furthermore, the static pressure ratio across the orifice was varied between 1.05 and 1.6 for all examined orifices. The results of this comprehensive investigation demonstrate the beneficial influence of rounded inlet geometries and the ability to decrease pressure losses, which is especially true for higher cross flow ratios where the reduction of the pressure loss in comparison to sharp-edged holes can be as high as 54%. With some exceptions, the chamfered orifices show a similar behavior as the rounded ones but with generally lower discharge coefficients. Nevertheless, a chamfered inlet yields lower pressure losses than a sharp-edged inlet. The obtained experimental data were used to develop two correlations for the discharge coefficient as a function of geometrical as well as flow properties.

Comparison of flow patterns and droplet deformations of modified sharp-edged and conical orifices during high-pressure homogenisation using µPIV

Micro-particle image velocimetry was used as an optical, non-intrusive measurement method to measure the flow pattern and visualise droplet deformation in high-pressure homogenisation disruption units of two different inlet designs (sharp-edged and conical). The flow patterns were compared either at same Reynolds numbers $${\text{(}}Re=980)$$ or pressure differences ( $$\Delta p=52~{\text{bar}}$$ ) each, to describe the influence of inlet geometry on the droplet disruption efficiency. Therefore, the shear and elongation rates were calculated from the velocity profiles and discussed regarding the visualised deformation of the emulsion droplets. For this, the viscosity ratio between the droplet and continuous phase was varied. Afterwards, the droplet size distributions (DSD) of emulsions with corresponding viscosity ratio passing the sharp-edged and the conical orifice were characterised. The inlet geometry influenced the flow pattern, shear and elongation rate profile, droplet deformation and finally droplet size distributions during the high-pressure homogenisation. On the one hand, sharp-edged inlet design resulted in higher axial velocity profiles and smaller droplets with slightly bimodal character. On the other hand, conical inlet design resulted in perfectly monomodal DSD but comparatively bigger droplets.

Dependence of submerged jet heat transfer on nozzle length

In this work the influence of nozzle length in submerged jet impingement heat transfer was studied by validated direct numerical simulations, in the laminar flow regime. With the purpose of examining the entire range of nozzle lengths and 500⩽Re⩽2000, other effects were reduced by setting a low nozzle-to-heater spacing (H/D = 3) and ideal, undisturbed inlet conditions. While developing pipe-flow is well-known, this parametric study characterized in detail short and intermediate nozzle flow regimes, affected by a separation bubble at the sharp-edged inlet. It is found that the maximal (centerline) jet velocity first decreases with increasing effective nozzle length, Z = L/(D ⋅ Re), to a minimum at Z∗≈0.0015, beyond which it increases as in developing pipe-flow. For Z<Z∗ two physical regimes are found: while the plane of the vena-contracta is outside the nozzle (L/D⩽0.6) the maximal centerline velocity occurs there, rather than at the nozzle exit-plane; whereas in the intermediate range, 0.6⩽L/D and Z⩽0.0015, the centerline velocity scales with the effective nozzle length, Z, and a predictive correlation could be developed for it. As a clear linear dependence of heat transfer on centerline velocity was observed, this predictive correlation could easily be converted into a new stagnation point heat transfer correlation, found to give good agreement over the entire range of nozzle lengths. This correlation provides a practical design tool, especially applicable to micro-jet cooling where constraints correspond with the new model’s validity range.

Superheated liquid carbon dioxide jets: setting up and phenomena

We present an experimental investigation on liquid, superheated carbon dioxide jets. Our main goal is to identify the setting up requirements for generating coherent jets because these raise expectations on applications in the cleaning and cutting industry. The study leads us through a number of phenomena, which are described, categorized and explained. The experiments are based on compressed (350 MPa) and cooled carbon dioxide, which expands through a cylindrical nozzle into the atmosphere. The nozzle provokes hydraulic flip by a sharp-edge inlet leading to separation and constriction. Upstream-temperature and pressure are varied and the jet’s structure and phase state are monitored by a high-speed camera. We observe coherent, liquid jets far from equilibrium, which demands the solid or gaseous state. Therefore, these jets are superheated. Carbon dioxide jets, like water jets, below certain nozzle diameters are subject to fluid dynamic instabilities resulting in breakup. Above certain diameters flashing jet breakup appears, which is associated with nucleation.

Particle Inhalability at Low Wind Speeds

Accurate quantification of the dose delivered by aerosol exposures is essential for estimating the risk of potential adverse health effects. The fraction of airborne particles that can enter the nose or mouth during inhalation is referred to as the inspirable particulate mass fraction. This inhalable fraction is equivalent to delivered dose for particles greater than approximately 25 μm (aerodynamic particle diameter, dae), which deposit completely and almost exclusively in the extrathoracic airways. Particle inhalability at high wind speeds (1–9 m/s) has been well characterized. However, there is a paucity of data describing the inhalability of particles at low wind speeds (≤0.3 m/s), which are typical of indoor environments. High-wind-speed criteria poorly describe inhalability at low wind speeds. Based on the aspiration efficiencies of blunt and sharp-edged inlets, a function was developed for oral inhalability, P(IO), of particles at low wind speeds. This function predicts a slow decline in P(IO) from 0.95 at dae= 8 μ m, to 0.5 at dae = 74 μ m, and 0.1 at dae= 175 μm. Data available from the literature for inhalability at relatively low wind speeds during oral breathing are well described by this logistic function (r2= 0.69).

Combustion characteristics of diesel sprays from equivalent nozzles with sharp and rounded inlet geometries

The ignition delay, flame structure, temperature, and soot distribution in a diesel spray injected at 80 MPa in a high-temperature (830 K) and high-pressure (6 MPa) quiescent air was studied for two nozzles, one with 0% hydrogrinding (HG) and another with 20% HG. HG in diesel nozzles is the process of forcing an abrasive fluid through the nozzles with sharp inlets; the abrasive fluid wears the sharp inlet edge of the spray holes until a prescribed flow rate is achieved. The percentage of HG used in this article is a measure of an increase in the volume flow rate after the HG process in a low-pressure flow test. The difference is substantial. For convenience, at some instances 0% HG is referred to as the sharp inlet and 20% HG as the rounded inlet. Spray impingement studies were made to evaluate the time-resolved spray momentum, nozzle discharge coefficient, and turbulence kinetic energy to characterize the nozzle internal flow effects on spray combustion. Equivalent nozzles were selected such that the momentum rates of the spray from both nozzles, as determined by the spray impingement, were the same. This was obtained by increasing the orifice diameter of the nozzle with 0% HG to compensate for the higher friction losses and lower discharge coefficient of the nozzle. The differences in discharge coefficient indicate that the flows inside the nozzles have different turbulence and cavitation levels. Inspite of the strong differences in internal flow, the sprays, which had the same momentum rate, behaved identically. In particular, the spray dispersion, penetration, ignition delay, combustion temperatures, flame volumes, soot concentration, and liftoff distances were almost the same for both sprays. Also, the use of noncircular injection orifices was shown not to change the combustion and emission performance of a diesel engine when the momentum of the fuel jets is the same. The work thus shows that diesel spray combustion is fully controlled by the spray momentum and that for realistic injection and combustion conditions the internal nozzle flow structure does not matter as long as it does not change the momentum.

Fuel Atomization Using Silicon Plate Nozzle

Fuel atomization is known as one of the most effective way to reduce both exhaust emission and fuel consumption of spark ignition engines. Authors have studied an influence of turbulence upstream of a nozzle on the fuel atomization, which is effective for improving the fuel atomization. This paper discusses an experimental investigation of the turbulence on the fuel atomization using a silicon plate nozzle (SPN) with a sharp edge inlet and aperture size of 100 μm or less. Here, the basic configurations of the SPN, that is, its aperture size and sharpness of the edge were changed so as to investigate characteristics of the fuel atomization for the respective cases. The characteristics of the fuel atomization were also investigated in case of superposing two SPNs and combining the SPN with a needle valve. As a result, it is clarified that the fuel atomization cannot be improved by merely reducing the orifice aperture size, and the turbulence upstream of the nozzle is also effective for the SPN with a small aperture.

Local heat transfer on the entrance segment of a tube with a sharp inlet edge. 2. Correction for the entrance segment under turbulent boundary layer conditions

A correction relation eX=f(Red;X/d) is proposed for the entrance segment, where turbulent flow occurs in a surface boundary layer developing upon completing the transient process. An elevated local heat transfer intensity in a separated flow region beyond a sharp inlet edge is considered to be a consequence of the M-shaped velocity field profile that exists over a substantial length of the entrance segment.

Local heat transfer on the entrance segment of a tube with a sharp inlet edge. 1. Nature and behavior of heat transfer intensity extrema

Experimental results on local heat transfer on the entrance segment of a round smooth tube with a sharp leading edge when Red=(13–110)·103, X/d=0.1–13, which support the previously proposed model for separated flow, are examined. It is shown that under separated flow conditions the usual sequence of flow transitions in a boundary layer for mixed nonseparated flows and Red>104 is still retained. Formulas are obtained for calculating local heat transfer coefficients on a segment where a laminar boundary layer occurs, including cross sections where these coefficients acquire extreme values.

Three-dimensional flow field of a rectangular array of practical air jets

A number of recent investigators have examined the flow fields of practical nozzles used in industry. Past investigators considered only single nozzles. This present paper concerns the velocity field below an array of practical nozzles. A common industrial use of such an array is the production of fine polymeric fibers by the process known as melt blowing. The velocity field was measured below a rectangular array of 165 air nozzles. The array was arranged in a rectangular pattern with three side-by-side columns. All nozzles were identical. Each nozzle had a sharp-edged inlet, and each nozzle discharged air through an area that approximated an annulus. Based on momentum conservation considerations, a single equation was developed that fits the data well and can be used to predict the velocity at any position below the nozzle array. At large distances from the array, the velocity field can be approximated as the field of a two-dimensional jet.

Aerosol sampling at small forward-facing angles: differentiation of yaw from pitch

Abstract When sampling aerosols in ambient or industrial air environments with an inlet, small changes in wind direction or physical constraints in positioning the inlet in the system necessitate the assessment of sampling efficiency in both the vertical and horizontal planes at small angles. In this study we have experimentally investigated the overall sampling efficiency of tubular sharp-edged inlets in yaw (horizontal plane) and pitch (vertival plane) orientations at 0–20° from horizontal aerosol flows. We have developed a model for the overall sampling efficiency at yaw and pitch by extending our previously developed model for pitch to yaw based on our new wind tunnel data. In our model, the difference between yaw and pitch is expressed by the effect of gravity on the wall impaction process inside the inlet described by a gravity effect angle. At yaw, the gravity effect on the wall impaction process does not change with sampling angle. At pitch, the gravity effect on the impaction process results in particle loss increase for upward and decrease for downward sampling. A new relationship has been developed between the gravity effect and physical sampling angles. Using our model, we have developed graphical representations for aerosol sampling at small angles. These can be used in the field to determine the overall sampling efficiency of inlets at several operating conditions and the operating conditions that results in an acceptable sampling error. Pitch and diameter factors have been introduced for relating the efficiency values over a wide range of operating conditions to those of a reference condition. The pitch factor determines the overall sampling efficiency at pitch from yaw values, and the diameter factor determines the overall sampling efficiency at different inlet diameters.

Structure of the static pressure field on the entrance length of a pipe with a sharp inlet edge

The three-dimensional static pressure field on the entrance length of a circular pipe with a sharp inlet edge is measured. The field structure, which is characterized by its nonuniformity in the longitudinal and radial directions, is in fairly good agreement with the previously proposed separated flow model and indicates the existence of M-shaped velocity profiles in cross sections of the entrance length.

Pressure drop characteristics for turbulent flow in a straight circular tube situated downstream of a bend

Experiments are performed to quantify the effects of a bend on the pressure distributions and friction factors in a straight tube situated downstream of a bend. Results are reported for parametric values of the Reynolds number (5000–80 000), bend turn angle (0, 90, and 180 deg), and the flow delivery configuration at the inlet of the bend (either a hydrodynamic development tube or a sharp-edged inlet). The 0 deg turn angle corresponds to the no-bend case, which serves as a baseline against which the with-bend cases are compared. It is found that the substantial circumferential pressure variations which exist in a bend dissipate almost immediately when the flow passes from the bend into the downstream-positioned straight tube. With the bend in place, the axial pressure gradients in the entrance region of the tube are smaller than the fully developed gradients at the lower Reynolds numbers and larger at the higher Reynolds numbers. A lengthwise-linear pressure variation is attained for all cases at an axial distance of 20 diameters or less from the tube inlet. Compared with the measured no-bend baseline values, the fully developed with-bend friction factors display deviations no greater than 9%, but with larger deviations (up to 18%) when compared to straight-tube correlations from the literature.

Overall efficiency of tubular inlets sampling at 0–90 degrees from horizontal aerosol flows

We have developed a unified and comprehensive model for the overall efficiency of tubular inlets sampling from horizontal aerosol flows at 0–90° relative to the wind direction. In our model, derived from experimental data obtained in our wind tunnel, the transmission efficiency is separated into two components: one due to gravitational settling in the boundary layer and the other due to impaction. The gravitational settling component is determined by extending our previously developed isoaxial sampling model to nonisoaxial sampling. The impaction component is determined by a new model that quantifies the particle losses caused by direct wall impaction. The model also quantifies the additional particle losses resulting from the turbulent motion in the vena contracta which is formed in the inlet when the inlet velocity is higher than the wind velocity. The equation for the gravitational settling component considers the inertial behaviour of the particles by including the Stokes number, the flow development in the boundary layer by including the Reynolds number and the gravitational settling in the boundary layer by including a modified gravitational settling parameter. The equation for the impaction component considers direct wall impaction and the losses due to vena contracta formation through the Stokes number, velocity ration and sampling angle. Direct wall impaction is further differentiated by the gravity effect on the impaction process which distinguishes upward from downward sampling. Our equations for transmission efficiency in combination with aspiration efficiency equations determine the overall efficiency of tubular, sharp-edged inlets at all orientations of the inlet relative to horizontal aerosol flows.

Flow in the initial segment of a tube with a sharp inlet edge. Comparative analysis

When flow becomes detached in the inlet segments of tubes and channels with sharp leading edges and where the Reynolds numbers exceed 75·103, the displacement of the maximum of the local heat transfer is in agreement with the onset of secondary laminar flow.

Heat transfer in a straight tube situated downstream of a bend

The axial variation of the local heat transfer coefficient in a straight tube situated downstream of a bend is measured by a mass transfer technique for parametric values of the Reynolds number (5000–85000), bend turn angle (0, 30, 60, and 90°), and flow condition at the bend inlet. For one flow condition, the bend inlet is fed by a hydrodynamically developed tube flow. In a second case, the bend is fed from a large upstream plenum through a sharp-edged inlet. The Schmidt number for the mass transfer is 2.5 (naphthalene sublimation in air). It is found that the presence of a bend fed from a plenum through a sharp-edged inlet gives rise to a decrease in the mass (heat) transfer coefficients in the downstream tube compared with those for the no-bend case. On the other hand, a bend fed with a developed flow generally tends to decrease the coefficients in the downstream tube at lower Reynolds numbers while increasing the coefficients at higher Reynolds numbers. For both bend inlet configurations, there is clear evidence of laminarization of the flow at the lower Reynolds numbers and larger bend turn angles.

Comparison of Turbulent Thermal Entrance Regions for Pipe Flows With Developed Velocity and Velocity Developing From a Sharp-Edged Inlet

The thermal entrance region for turbulent flow in circular tube has been analytically investigated for two types of velocity distributions. In one case, the flow is hydrodynamically developed throughout the entire length of the thermal entrance region. In the other, the velocity is uniform at the inlet of the heated section, so that there is a hydrodynamic development of the flow which occurs simultaneously with the thermal development. The latter situation is rarely, if ever, encountered in practice and is, in fact, difficult to achieve in a laboratory experiment. The effect of various types of velocity distributions on the turbulent heat transfer coefficients in the thermal entrance region of an isothermal-walled tube was investigated experimentally by Boelter et al. The different velocity distributions were attained by use of different configurations of the inlet of the heated section. As noted in Kays and Crawford, the insufficiencies of the experimental technique used by Boelter et al. suggest that the results be regarded as qualitative rather than quantitative. The objective of the present research is to provide definitive thermal entrance region heat transfer results for an isothermal-walled tube for two types of turbulent velocity distributions that are of practical relevance. The two setsmore » of results will be compared to yield the quantitative response of the thermal entrance region to the nature of the velocity condition at the tube inlet. The work to be reported here is experimental and covers the Reynolds number range from approximately 5,000 to 85,000.« less