Performance Investigation of Turning Diffusers at Various Geometrical and Operating Parameters

The aerodynamic losses in centrifugal compressors are mainly associated with the separated flow on the suction sides of impeller and diffuser vanes. The overall performance of such compressors can be improved by adding splitter vanes. The present work examines the effect of varying the geometrical location of the splitter vanes in the diffuser on the overall performance of a high-speed centrifugal compressor stage of a small gas turbine. To increase the pressure recovery through the diffuser, two radial sets of vanes are used. The first set of vanes (diffuser-1) is equipped with splitter vanes, placed mid-distance between the main vanes, while the vanes of the second set (diffuser-2) are conventional vanes. Flow through the compressor was simulated using the ANSYS 19 workbench program. Flow characteristics and compressor performance were obtained and analyzed for different circumferential positions of the splitting vanes relative to the main vanes of diffuser-1. The study covered seven positions of the splitter vanes including the original design of the diffuser where the splitter vanes were located at mid-distance between the main vanes. The analysis shows that, at design conditions, selecting the position of the splitter vanes to be nearer to the pressure side of the main vanes improves the stage performance. In the present study, locating the splitters at 33% of the angular distance between the main vanes leads to the best performance, and a significant improvement in the overall stage performance is recorded. The pressure recovery coefficient is raised by about 17%, the pressure ratio is increased by about 1.13%, and the stage efficiency is increased by about 2.01%, compared to the original splitter position. Performance improvement is related to the suppression of the flow separation and the more uniformity of flow. On the contrary, further moving the splitter closer to the main blade, the pressure recovery coefficient is decreased by about 2% than the position of 33% of the angular distance, but still higher than the original position by about 15% and a limited improvement in the compressor performance is noticed. Moving the splitter far out the main blade annihilates the static pressure recovery of the diffuser by about 2:7% compared with the original position. So, for the investigated compressor, the best position of the splitter blade in the circumferential direction, which provides the best stage performance in our parametric analysis, is not necessary to be at the mid-angular distance between the diffuser’s main blades, but it is achieved by moving the splitter to about 33% of the angular distance where the diminished loss from the suppressed flow separation is more prevailing and the instigated friction losses from splitter surfaces are less critical.

Curve diffuser is often used in HVAC and wind tunnel systems to provide pressure recovery and avoid excessive energy loss to the surrounding environment. Performance of curve diffuser is disturbed mainly by the presences of flow separation and secondary flow vortices occurred due to the effect of turning angle, in which scarce literature found. In this study, the effect of turning angle from 30° to 180° configured with an area ratio of 1.60 to 4.00 and inflow Reynolds number of 5.934x104 – 1.783x105 on loss characteristics and flow rectification of curve diffuser is investigated with optimum configuration is proposed. Performance of curve diffuser is evaluated in terms of pressure recovery and flow uniformity using ANSYS CFD equipped with validated Standard k-ɛ model (ske) and enhanced wall treatment of y+ = 1.2 - 1.7. Results show that performance of pressure recovery and flow uniformity decreases respectively by 85.71% and 45.84% as the angle of turn increases from 30° to 180°. Curve diffuser with minimum angle of turn 30o, optimum area ratio 2.16 and intermediate Rein 8.163x104 turns out to be the best configuration to provide pressure recovery of 0.399 and flow uniformity of 3.630 m/s.

Modeling of solid-liquid flow inside conical diverging sections using computational fluid dynamics approach

The flow diffuser characteristics of the St. Jude heart valve were investigated and compared to theoretical calculations based on the two dimensional geometry of the valve. Pressure recovery (Cp) and loss (K) coefficients were calculated for varying sized prostheses in both mitral and aortic valve configurations over a range of steady flow rates. In vitro experiments demonstrated that the pressure recovery (Cp = 0.36±0.04) and loss (K = 0.64±0.04) coefficients for flow through the central orifice were independent of the valve sizes and configurations examined. Comparison of ideal and experimentally determined pressure recovery coefficients yielded diffuser effectiveness (η = 0.62). We applied these flow diffuser characteristics to investigate the discrepancies between Doppler and catheter gradients across prosthetic St. Jude bileaflet valves. Transvalvular Doppler gradients can be reconciled with catheter measurements by multiplying the Doppler gradient by the pressure loss coefficient (K = 0.64) (r = 0.99, y = 1.11x-0.19).

Numerical simulation of vortex-induced vibration of a full-scale flexible riser at supercritical Reynolds number in uniform and linear shear flow

The design of the radial exhaust hood of a low pressure (LP) steam turbine has a strong impact on the overall performance of the steam turbine. A higher pressure recovery of the diffuser will lead to a substantial higher power output of the turbine. One of the most critical aspects in the design of such devices is the steam guide, which guides the flow near the shroud from axial to radial direction and has a high impact on the pressure recovery. This paper presents an extension of a design activity previously reported for the design optimization of the steam guide of a steam turbine for industrial power generation and mechanical drive of centrifugal compressors. Whereas this previous work only focused at peak efficiency, this paper will look into the off-design aspect. Peak performance, as usually used as design criteria, will now be replaced by proper off-design criteria guaranteeing a high performance level at both design and off-design conditions. On the basis of these considerations a multi-objective optimization of the steam guide has been performed keeping the exhaust outer casing unchanged. The maximization of the objective functions is achieved by means of a numerical optimization method that uses a metamodel assisted differential evolution algorithm in combination with a CFD solver. The profile of the steam guide is parameterized by a Bezier curve. This allows for a wide variety of shapes respecting the manufacturability constraints of the design. The pressure recovery coefficient is computed over a wide operating range through several RANS computations including the last stage but introducing a mixing plane between the rotating blade and the diffuser inlet to reduce the computational burden. Steam tables are used for the accurate prediction of the steam properties. Finally, the optimized design is analyzed by a frozen rotor computation to validate the approach. Also off-design characteristics of the optimized diffuser are shown.

The paper experimentally investigates the individual and combined effects of periodic unsteady wake flows and freestream turbulence intensity (FSTI) on flow separation along the suction surface of a low pressure turbine blade. The experiments were carried out at a Reynolds number of 110,000 based on the suction surface length and the cascade exit velocity. The experimental matrix includes freestream turbulence intensities of 1.9%, 3.0%, 8.0%, 13.0% and three different unsteady wake frequencies with the steady inlet flow as the reference configuration. Detailed boundary layer measurements are performed along the suction surface of a highly loaded turbine blade with a separation zone. Particular attention is paid to the aerodynamic behavior of the separation zone at different FSTIs at steady and periodic unsteady flow conditions. The objective of the research is (a) to quantify the effect of FSTIs on the dynamics of the separation bubble at steady inlet flow condition, and (b) to investigate the combined effects of FSTI and the unsteady wake flow on the behavior of the separation bubble. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University.

The flow patterns and pressure recoveries in rotating, conical diffusers were investigated experimentally. The conical diffusers have straight walls and opening angles of 7°, 10°, 15°, respectively. Even in a diffuser having no separated zone in a stationary state such as in the case of diffusers with an opening angle equal to or less than 10°, separations occur on the lower pressure wall when the diffusers are rotated at a speed exceeding a certain limit. Velocity profiles and pressure distributions as well as the flow separation in diffuser sections are governed by a parameter defined by the ratio of a rotational Reynolds number to a flow Reynolds number. As the rotation rate increases, the point of separation approaches the inlet section of the diffuser and wakes formed by this separation grow in size, resulting in a poor pressure recovery in the diffuser. At the same rate of rotation, the pressure recovery in the diffuser with an opening angle 10° was the best of the diffusers tested.

Serpentine air intake system is an essential requirement in modern combat aircraft. Flow control techniques are used to achieve the desired performance of flow uniformity and pressure recovery. Mechanical vane type vortex generator designed in the previous work is considered as a flow control technique. The performance of vortex generator at various off-design conditions of different subsonic throat Mach numbers and three different altitudes are of interest to this present study. Numerical simulations were performed for throat Mach numbers from 0.45 to 0.7 in the increment of 0.05, with and without vortex generator at sea level static inflow condition. Flow non-uniformity and pressure losses were found to increase with increase in Mach number. However, flow uniformity is better with vortex generators for all Mach numbers with slight reduction in total pressure recovery at sea level condition. In order to check the effect of increasing altitude, duct with VG was simulated for 6 km and 11 km altitudes. All the simulations are conducted with reducing the inlet total pressure from sea level to represent the altitude and varying the exit static pressure to arrive at the desired throat Mach number. In general, Reynolds number decreases with increasing altitudes and promotes the possibility of flow separation. While reduced pressure recovery was seen, DC-60 was lower at 6 km altitude for all the Mach numbers. Higher value of DC-60 at sea level condition is primarily due to the dynamic pressure at AIP. Generally, a threshold value of DC-60 is to be given as a design requirement. Present design of vortex generator has reduced DC-60 value from 29.7 to 20.7 at 0.65 Mach number and sea level condition with a maximum of 25 from all the other conditions, thereby proving its benefits.

Experimental investigation of the effect of rotor tip gaps on 3D separating flows inside the stator of a highly loaded compressor stage

It is well understood that a volute, depending on its operating point, acts either as a nozzle or diffuser. However, the resulting static pressure distortion at diffuser exit is rather considered as side effect and has rarely been investigated systematically in detail. Evidence whether this distortion is amplified or alleviated towards impeller exit is even contradictory in literature. In this work a thorough investigation of aerodynamic volute-impeller interaction is presented. First, a 1D-analysis is carried out in order to understand the fundamental relation between volute matching and pressure distortion. Then, one large external volute, coupled with different impellers, is investigated by means of CFD. These configurations feature different diffuser ratios and blade exit angles, while identical matching ensures comparable conditions. The results reveal that pressure distortion is directly related to the volute’s pressure recovery coefficient. Consequently, at diffuser exit, a local pressure recovery coefficient around circumference can be defined, which is widely independent from the impeller but rather depends on volute geometry and matching. It is this local pressure recovery factor that defines the static pressure distortion at diffuser exit, together with dynamic pressure. In a next step it is analysed, how the resulting pressure distortion evolves towards the impeller. Therefore, stand-alone CFD calculations of impellers around the entire circumference are performed, applying defined pressure distortions at diffuser exit and varying operational conditions carefully. In doing so, fundamental understanding of when distortion is amplified or alleviated towards the impeller is gained.

Solar water heating is one of the most efficient solar technologies in the domestic sector. The most important component of the solar thermal systems is the solar collector, which converts solar radiation to useful thermal energy. There are many types of solar collectors, which are categorized based on the operating temperature (low, medium and high temperatures) or the working fluid (gas or liquid). One of the newest types of solar collectors is direct absorption solar collector in which solar radiation is absorbed through the working fluid, unlike other collectors that use the surface absorber or indirectly absorb the solar radiation. The common working fluid for DASC is the suspension of metal, metal oxide or carbon nanomaterials in solar common fluids (water, EG, PG and Therminol VP-1). In this review paper, the effect of design and operating parameters on the thermal performance of low-temperature direct absorption solar collector is summarized. Using the numerous studies done in this field, the efficiency enhancement of DASC by variation of the collector geometric properties, the flow properties (the flow rate and Reynolds number), the working fluid properties (the base fluid, the nanoparticle material and size, and the nanofluid concentration), and the collector design is identified. This paper also identifies the current challenges facing the direct absorption solar collectors and the future recommendations for developing and commercializing these collectors.

In gravity casting, the quality of an aluminum alloy casting relies on, among other things, the design of the runner system in which the ingate velocity into the mold cavity should be controlled to stay under a critical velocity (close to 0.5 m/s). In this study, a diffuser was proposed to reduce the velocity of liquid metal to below this critical value, while the flow rate remained almost unchanged. Flow separation and dead zones in the diffuser design were avoided. A computational modeling package and a real casting experiment (water analogy method) were employed for exploring and verifying the new design. The efficiency of the diffuser was quantified by the measurement of coefficient of discharge Cd. For this new diffuser, the pressure recovery coefficient C p and the loss coefficient K L were also estimated.

This paper reports the results of a numerical study of the cold flow field in a dump combustor with a tapered exit. The numerical model was benchmarked against the data available in the literature. The flow field inside the combustor was investigated by numerical visualization of different regions in the flow field, and the effect of the combustor length was studied. It was seen that the presence of shear layer between the potential core and the recirculation region, as well as the pressure and velocity variation inside the combustor, alters the recirculation region and hence the flow field. It was also observed that the extent of the recirculation region increases, while the shear layer becomes thinner as the length of the chamber decreases. The effect of the variation in the flow Reynolds number and the inlet turbulence intensity on the flow field of a low aspect ratio (2.3) dump combustor was studied in detail. It was observed that the extent of recirculating flow increases with a decrease in the Reynolds number, and the increase in turbulence intensity results in higher turbulence energy generation in the shear layer. The pressure recovery was found less, but the recirculation was stronger in the low aspect ratio combustor. The results of this study can help optimize the combustor chamber to achieve better mixing of fuel with air and stabilization of the flame.

A study has been performed to characterize the effects of steady boundary-layer injection on the separated flow of a hump diffuser. Results of parametrically varying discrete hole blowing characteristics have identified two separate regimes where the injection momentum coefficient and velocity ratio, respectively, are the primary scaling parameters for pressure recovery. Both discrete hole and slot injection have been investigated for varying degrees of adverse pressure gradient in the streamwise direction, indicating optimal discrete hole injection to be more efficient than slot injection in terms of necessary injection momentum coefficient to achieve maximum levels of increased pressure recovery. The effects of discrete injection parameters such as hole diameter, hole spacing, and streamwise injection (yaw) angle have been studied. Angled injection (45-deg yaw) has been shown to be most effective in removing separated flow and increasing the pressure recovery. The angled injection enhances shear layer mixing through large-scale corotating vortical motion induced by the yawed jet-main flow interaction. A computational fluid dynamics/data comparison study has been performed on the results of the discrete injection tests, capturing overall data trends and representing the net effect of streamwise and angled injection on pressure recovery in the hump diffuser.