Pipe Diffuser Research Articles

Aeration system design in integrated fixed-film activated sludge (IFAS) and moving bed biofilm reactors (MBBR) using stainless steel pipe diffusers, manifold, and down pipes

Aeration system design in integrated fixed-film activated sludge (IFAS) and moving bed biofilm reactors (MBBR) using stainless steel pipe diffusers, manifold, and down pipes

Experimental and Numerical Investigation of Tip Clearance and Bleed Effects in a Centrifugal Compressor Stage With Pipe Diffuser

The subject of this paper is the experimental and numerical investigation of a state-of-the-art high pressure centrifugal compressor stage with pipe diffuser for a jet engine application. This study shows the impact of impeller tip clearance- and bleed-variation on the centrifugal stage. The purpose of this paper is threefold. In the first place, it investigates the effects on the stage performance. Secondly, it seeks to explain local flow-phenomena, especially in the diffuser. Finally, it shows that steady CFD simulations are capable of predicting these phenomena. Experimental data were gathered using conventional pitot and three-hole-probes as well as particle-image-velocimetry. Numerical simulations with the CFD solver TRACE were conducted to get fundamental insight into the flow. Thus, this study contributes greatly towards understanding the principle of the flow phenomena in the pipe diffuser of a centrifugal compressor.

Constant flow rate charging characteristics of a full-scale stratified chilled water storage tank with double-ring slotted pipe diffusers

Temperature distributions formed during charging of a 14 m (46 ft) water depth, 7 m (23 ft) radius stratified chilled water storage tank with double ring octagonal slotted pipe diffusers were recorded and analyzed. Prior to the beginning of each test, the tank was fully discharged and at a relatively uniform temperature. To the extent possible, inlet flow rate and temperature were held constant during each test. Six tests were performed for flow rates varying from 50% to 95% of design. Thermal performance was quantified using thermocline thickness, half-cycle figure of merit (FoM 1/2) and equivalent lost tank height (ELH). The thermal performance for all tested charge flow rates was good by current standards (FoM 1/2 > 90%, ELH < 1 m in all but one case). Thermocline thickness for low and medium flow rates was relatively small, on the order of 1 m (3 ft). Erroneously large thermocline thickness measurements were obtained in the high flow rate case because of the effect of inlet temperature variation combined with the definition used. Significant bulk temperature increase observed below the thermocline in all cases was most likely due to large-scale convection cells driven by the inlet flow. Efforts to improve the performance of diffusers should focus on suppressing this circulation.

Formulation of semi-empirical models for predicting the illuminance of light pipes

There are now several ways of predicting the illuminance produced for different configurations of light pipe. This study is an overview of two such methods for two slightly different forms of light pipe diffuser. Both models are partly empirical in nature but have been formulated using significantly different techniques. Their outputs are also different in presentation, although both models deal with illuminance predictions (with one of the models offering luminous flux predictions also). The models have also used separately obtained measurements in their formulation and so can be seen as being two independent methods, using similar logic, to achieve similar goals. While the two sets of predictions cannot be compared to each other directly (due to the difference in diffuser in each case), the two models can be presented as an indication as to how modelling procedures of this type can be approached.

A design tool for predicting the performances of light pipes

Light pipes are simple means of directing daylight (diffuse and direct light) into interior spaces. Previous work by the authors described the initial work on a luminous flux and illuminance predictive model for straight light pipes, using a basic equation for illuminance distribution as a function of horizontal distance. Further work has now produced a model that uses the cosine law of illuminance to describe the distribution of light from the light pipe diffuser as well as takes into account pipe elbow pieces or bends. The resulting illuminance model can be described as a quartic cosine model. By producing a “luxplot” prediction for any given light pipe application, it is possible to maximise the potential of these daylight providers and design their configuration to suit any given need. As part of this study, wide-ranging illuminance and luminous flux data were collected both for the formulation of this model (as the formula is semi-empirical) and its validation.

Parametric Study of Single-Pipe Diffusers in Stratified Chilled Water Storage Tanks (RP-1185)

A parametric study was performed of the charging thermal performance of a full-scale pipe diffuser in a single cylindrical stratified chilled water storage tank by applying factorial experimental theory to the results of simulations performed with a validated computational fluid dynamics (CFD) model. Dimensional parameters having the potential to influence charging inlet performance were identified and formed into dimensionless groups using the method of repeating variables. Parameters included: the inlet Richardson number based on inlet slot width (Ri l ), inlet Reynolds number (Re i ), ratio of inlet width to diffuser height (l/hi ), ratio of inlet diffuser height to tank radius (hi/RW ), and ratio of diffuser radius to tank radius (RD/RW ). Thermal performance was measured in terms of equivalent lost tank height (ELH). A full 2k factorial experiment of thirty-two simulations was performed and analyzed. Parameter ranges were: 0.05-2 for Ri l , 500–5000 for Re i , 0.1–1 for l/hi , 0.005–0.05 for hi/RW , and 0.707–0.866 for RD/RW . Within these ranges, Ri l , l/hi , and hi/RW were found to be of first-order significance, while Re i and RD/RW were not. Two-factor interactions involving Ri l , l/hi , and hi/RW were also significant. Regression models of equivalent lost tank height as functions of Ri l , l/hi , and hi/RW were developed. The predictive capabilities of the regression models were tested against the results of five additional CFD simulations having parameter values different from the 2 k factorial experiment cases. On average, regression models predicted factorial experiment data to within 10% with maximum error of 30% to 60%, depending on the model.

Measured and Modeled Charging of a Stratified Chilled Water Thermal Storage Tank with Slotted Pipe Diffusers

As part of ASHRAE Research Project 1185, field data from the constant flow rate charging of a stratified chilled water storage tank with double-ring octagonal slotted-pipe diffusers serving a university chilled water system were compared with results of a transient axisymmetric computational fluid dynamics (CFD) model. Charge processes at flows near design flow rate and 50% of design were modeled. Laminar and turbulent simulations were performed for a range of slot dimensions. Performance of the model was assessed by direct comparison of temperature profiles and by comparison of common thermal performance metrics, including thermocline thickness, lost capacity, and equivalent lost height. The inlet Richardson number based on slot hydraulic radius was found to have the strongest effect on model results. The model Richardson number giving best agreement with the lost height of the field data for near design flow rate was roughly an order of magnitude greater than the Richardson number of the actual diffuser. It is conjectured that this is due to inherent differences between the flow produced by multiple three-dimensional jets in the actual diffuser and the continuous slot of the axisymmetric model. The effect of turbulence varied from negligible to moderate as the Richardson number decreased.

The design and analysis of pipe diffusers for centrifugal compressors

The geometry of the region between the impeller tip and diffuser throat is accepted as being very influential, little understood and crucial to the performance of centrifugal compressors. This paper presents an experimental investigation of the flow in this region where the diffusing section of the compressor is of the ‘pipe diffuser’ configuration. The results are presented with an emphasis on design-type correlations. The principal features under investigation were the size of the diffuser throat and the influence of the number of diffuser passages.

Development of a Centrifugal Compressor With a Variable Geometry Split-Ring Pipe Diffuser

Higher noise levels resulted when a compressor was scaled to larger capacity. The machine’s sound pressure level was relieved by increasing the distance between the impeller blade tip and diffuser leading edge. However, the part-load surge line deteriorated severely as a consequence. A variable geometry pipe diffuser solved this problem, permitting operation at stringent off-design conditions. The addition of a variable diffuser permits compressor selection very near its most efficient full-load operating point, without regard for limitations normally imposed by part-load requirements. The principal lessons learned during aerodynamic design refinement include (a) how performance and surge depend upon positioning the variable inlet guide vanes and variable diffuser, and (b) how to define simultaneous variation of inlet guide vanes and diffuser for specific operational objectives. Generally, each operating point requires a unique setting of the variable components to achieve maximum efficiency. However, linked movement is shown to yield both a satisfactory surge line and improved performance for most applications when compared to a compressor without the variable geometry pipe diffuser.

Variable Geometry Pipe Diffusers

Theoretical analyses and experimental results are reported for two unique variable geometry techniques used with pipe diffusers to enhance off-design performance. One technique mechanically closes the diffuser throat in an unusual manner. The other allows flow recirculation to close the throat artificially while attempting to improve diffuser inlet flow characteristics. Results clearly show that surge margin may be significantly improved by either method and that flow recirculation may offer improved efficiency.

Flow Physics Leading to System Instability in a Centrifugal Pump

The off-design performance in a centrifugal pump is investigated experimentally. The objective is to identify flow features that lead to the onset of surge as the fundamental pumping system instability. Results show that there are primarily two reasons for the onset of surge as the flow is reduced in the pump studied: (a) adverse flow in the tongue region, and (b) destabilizing effect of the pipe diffuser. The former is due to premature diffusion of the flow entering the tongue region, which is manifested by increased flow recirculation through the tongue/impeller gap and flow separation on the volute outer side-wall opposite the tongue. These effects in the tongue region flow coupled with the destabilizing behavior of the pipe diffuser lead to the eventual unstable pump operation.

A review of pilot-scale studies of bottom and surface velocities within air-agitated circular and rectangular vessels

The use of bottom or surface fluid velocity within air-agitated circular and rectangular vessels has been studied as a possible design parameter to achieve a specified scale of agitation. Experimental data are presented in terms of five dimensionless numbers involving the fluid velocity, the depth of fluid in the vessel, the elevation of the diffuser above the vessel floor, the air flow rate, and the compressor power required. Design equations are obtained for a total of 506 physical observations with a ring diffuser around the perimeter of a circular vessel, a pipe diffuser at the centre of a circular vessel, and a line diffuser at the centre line or end wall of a rectangular vessel. The applicable range of variables for each equation is provided. It is shown that both bottom and surface velocities increase with an increase in air flow rate or compressor power requirement for a specified fluid depth. For a constant air flow rate and fluid depth, the surface velocity always exceeded the bottom velocity. The surface and bottom velocities are related to operating conditions in different water and wastewater treatment units in which a specified degree of uniformity of the vessel contents has to be maintained in order to keep a specified particle in suspension. The sensitivity analysis of the model revealed that the fluid depth was the most important design parameter in controlling the velocities within air-agitated vessels. Key words: bottom velocity, surface velocity, velocity gradient, degree of uniformity, air-agitated rectangular vessels, air-agitated circular vessels.

Evaluation of a High Hub/Tip Ratio Centrifugal Compressor

A centrifugal compressor stage with an unusually high inlet hub/tip ratio of 0.87 was designed for a pressure ratio of 2.0 at a corrected mass flow of 2.45 lb per sec. The geometry was selected so that the centrifugal stage could replace several of the last stages of a multistage axial compressor. The stage was tested with two diffuser schemes. One diffuser consisted of a series of drilled conical pipes, whereas the other employed multirow vaned cascades. Sea level aerodynamic tests of the compressor stage with each diffuser showed a peak total-to-total efficiency at design speed of 83.8 percent for the pipe diffuser and 82.9 percent for the vaned cascade diffuser. Additional tests were conducted with a vaneless diffuser to determine effects of impeller discharge tip clearance and inlet prewhirl on impeller performance.

A Novel Low-Cost Diffuser for High-Performance Centrifugal Compressors

A novel, low-cost diffuser based upon discrete pipe drillings has been developed for high-performance centrifugal compressors. These discrete drillings are arranged in symmetrical array to mutually intersect in a radial plane so that a gradual transition from the open vaneless space at rotor tip to the individual throats of each pipe is implicitly defined. To draw a comparison, alternate cambered-vane and flat-plate diffusers were defined using the best available yet practical thinking. With a nominal 5:1 stage pressure ratio, rotor, 81.8 percent efficiency was obtained with the “pipe” diffuser at 5.42 pressure ratio. This efficiency level exceeded the best cambered-vane and flat-plate performance by 6.8 and 8.8 percent, respectively. Further testing with a second rotor yielded 81.3 percent efficiency at 6:1 pressure ratio for the pipe diffuser and 73 percent efficiency at 6:1 for the cambered-vane diffuser.

Some Preliminary Results for Conical Diffusers with high Subsonic entry mach Numbers

Some preliminary results are presented for the variation of total pressure loss coefficient with entry Mach number for conical diffusers. Included angles of 5, 8 and 12°, area ratios from 2 to 16 and entry lengths from 0 to 65 diameters are considered. In these initial tests the junction between the parallel entry pipe and the cone is sharp. The variation of Mach number and total pressure loss in the entry pipe is predicted simply using measured friction factors and typical velocity profiles. These calculations give the entry conditions to the diffusers in the absence of the diffusers. The effect of the junction of entry pipe and diffuser on the entry pipe flow is thus correctly attributed to the diffuser in the assessment of its total pressure loss. In addition a correctly specified mean entry Mach number is obtained. These considerations enable the losses in diffuser-pipe conjunctions of high Mach number to be logically analysed from separate pipe flow and diffuser data. The results obtained are unusual and indicate that previous assessments of diffuser performance at high Mach numbers have been confused by incorrect consideration of the effect of the diffuser entry pipe conjunction and incorrect specification of the effective mean entry Mach number. It is concluded that further experimental results are needed for developing compressible flows in constant area ducts in order that the present preliminary results may be made more precise. The momentum equation analysis in terms of suitable mean values is presented briefly. Previous diffuser results for low subsonic entry Mach numbers are considered briefly in comparison. Serious errors are shown to be present in these early results. Typically the errors originate in the definition of loss coefficient and static pressure efficiency and the use of the mass derived mean concept. In some cases the quoted static pressure efficiencies imply a decrease in entropy for the diffuser flow at low area ratios.