Shin-Hyoung Kang

Tip Clearance Effect on Through-Flow and Performance of a Centrifugal Compressor

Numerical simulations have been performed to investigate tip clearance effect on through-flow and performance of a centrifugal compressor which has the same configuration of impeller with six different tip clearances. Secondary flow and loss distribution have been surveyed to understand the flow mechanism due to the tip clearance. Tip leakage flow strongly interacts with mainstream flow and considerably changes the secondary flow and the loss distribution inside the impeller passage. A method has been described to quantitatively estimate the tip clearance effect on the performance drop and the efficiency drop. The tip clearance has caused specific work reduction and additional entropy generation. The former, which is called inviscid loss, is independent of any internal loss and the latter, which is called viscous loss, is dependent on every loss in the flow passage. Two components equally affected the performance drop as the tip clearances were small, while the efficiency drop was influenced by the viscous component alone. The additional entropy generation was modeled with all the kinetic energy of the tip leakage flow. Therefore, the present paper can provide how to quantitatively estimate the tip clearance effect on the performance and efficiency.

Flow at the Centrifugal Pump Impeller Exit With Circumferential Distortion of the Outlet Static Pressure

The effects of circumferential outlet distortion of a centrifugal pump diffuser on the impeller exit flow were investigated. A fence with sinusoidal width variation was installed at the vaneless diffuser exit. The flow field was measured at the impeller exit with and without the fence, using a hot film probe and an unsteady pressure sensor. Flow parameters varied with the circumferential position and the mean flow parameters plotted against the local flow rate at each circumferential position showed loops along the quasi-steady curves, which were obtained from the result without the fence. Simple theoretical calculations were used to predict the velocity components at the impeller exit with the relative flow angle or total pressure assumed. Good result was obtained when the relative flow angle was assumed to vary quasi-steadily, not constant with the local flow rate. The radial velocity was also reasonably predicted when the total pressure was assumed to vary quasi-steadily. A simple method is proposed to predict the impeller exit flow with downstream blockage in two-step sequence

Three-dimensional analysis of heat transfer and thermophoretic particle deposition in OVD process

Heat transfer and particle deposition in the OVD process are simply modeled as a buoyant jet flow impinging on a circular target cylinder and numerically analyzed using a finite volume method on the flow, heat transfer and thermophoretic particle deposition over the cylinder. Effects of the three-dimensionality, conjugate heat transfer and longitudinal traversing of the jet on the particle deposition are investigated. The surface temperature, which is the most important value in the whole OVD process, is estimated considering the conjugate heat transfer. As the conductivity of the cylinder decreases, Nusselt number, particle deposition rate and deposition efficiency greatly decrease due to the reduced temperature gradient. Since the increase of the jet traversing speed keeps the surface temperature relatively low, Nusselt number and the particle deposition increase with the traversing speed.

Performance Evaluation of a Regenerative Blower for Hydrogen Recirculation Application in Fuel Cell Vehicles

Regenerative blowers are used for hydrogen gas recirculation application in fuel cell vehicles. In this paper we discuss the performance of theoretical models that describe the complex three dimensional flows in regenerative blowers. A one-dimensional performance prediction code is developed based on theoretical models and loss models. Numerical calculation is also performed using a commercial CFD code to analyze the three dimensional flows in a regenerative blower. The results of numerical analysis are used to evaluate the performance of the designed blower and improve the accuracy of performance prediction by correcting the loss models. The results of performance predictions are compared with measured data of a prototype regenerative blower to validate the one-dimensional performance prediction method.Copyright

Full scale Reynolds number effects for the viscous flow around the ship stern

We calculate the stern flow for the case of HSVA ship model at two different Reynolds number; one is Re=109 for full scale and the other is Re=5×106 for model scale. And we investigate the trend of scale effect on the stern flow by comparing the results of two calculations. The STERN/NS computer program, developed for solving the Navier-Stokes equations in a numerically generated general coordinate, is used with k−ɛ turbulence model for turbulent flows. At full scale, skin frictions are much reduced as expected from boundary layer concept, and the pressure in the thick boundary layer region around the stern and near wake is noticeably changed by decreasing of viscous-inviscid interaction in flow domain. At the propellar plane, full scale solutions show that the velocities in the shear layer increase and the wake region is substantially reduced by 20% compared with the model scale solutions.

Prediction of the Nonuniform Tip Clearance Effect on the Axial Compressor Flow Field

It is well-known that nonuniform tip clearance in an axial compressor induces pressure and velocity perturbations along the circumferential direction. This study develops a numerical modeling to predict perturbed flows in an axial compressor with a nonuniform tip clearance and presents a mechanism of the flow redistribution in the axial compressor at design and off-design conditions. The modeling results are compared with CFD results (2006, “Prediction of the Fluid Induced Instability Force of an Axial Compressor,” ASME FEDSM 2006, Miami, FL) not only to validate the present modeling, but also to investigate more detailed flow fields. In an axial compressor, nonuniform tip clearance varies local flow passage area and resultant axial velocity along the circumferential direction. There are small axial velocity differences between maximum and minimum clearances near the design condition, while large pressure differences are investigated according to local locations. However, contribution of the main flow region overrides the tip clearance effect as the flow coefficient deviates from the design condition. Moreover, the flow field redistribution becomes noticeably strong when the off-design effects are incorporated. In case of high flow coefficients, the low relative flow angle near the minimum clearance regions results in a large negative incidence angle and forms a large flow recirculation region and a corresponding large amount of loss occurs near the blade pressure surface. It further promotes strong flow field perturbations at the off-design conditions. The integration of these pressure and blade loading perturbations with a control volume analysis leads to the well-known Alford’s force. Alford’s force is always negative near the design condition; however, it reverses its sign to positive at the high flow coefficients. At the high flow coefficients, tip leakage flow effects lessen, while increased off-design effects amplify blade loading perturbations and a steep increase in Alford’s force. This study enables that nonuniform flow field, and the resultant Alford’s force, which may result in an unstable rotor-dynamic behavior, can be easily evaluated and assessed during the compressor, fan, or blower design process.

Reynolds Number Effects on the Performance Characteristic of a Small Regenerative Pump

This paper studies the effect of the Reynolds number on the performance characteristics of a small regenerative pump. Since regenerative pumps have low specific speeds, they are usually applicable to small devices such as micropumps. As the operating Reynolds number decreases, nondimensional similarity parameters such as flow and head coefficients and efficiency become dependent on the Reynolds number. In this study, the Reynolds number based on the impeller diameter and rotating speed varied between 5.52 × 10 3 and 1.33 × 10 6 . Complex three-dimensional flow structures of internal flow vary with the Reynolds numbers. The coefficients of the loss models are obtained by using the calculated through flows in the impeller. The estimated performances obtained by using one-dimensional modeling agreed reasonably well with the numerically calculated performances. The maximum values of flow and head coefficients depended on the Reynolds number when it is smaller than 2.65 × 10 5 . The critical value of the Reynolds number for loss coefficient and maximum efficiency variations with Reynolds number was 1.0×10 5 .

Performance Characteristics of Side Channel Type Regenerative Pumps

The performance of a regenerative pump is affected by many parameters, especially blade shape of impeller, leakage flow in the clearance and head losses at the inlet & outlet. An impeller with J-shape blade was designed and 5 times scale up model was tested at similarity conditions to evaluate the performance. Performance variations with clearance change were executed. The amounts of leakage flow through the clearance were estimated using the one-dimensional leakage flow models and analysis. Main leakage flow is generated through the gap between the impeller and casing. The inlet & outlet head losses were also estimated. Such corrections are very important to evaluate the final performance of the impeller and pump. Cavitation test was also performed at 1,200 rpm. NPSH of the regenerative pump was obtained and growth of cavity within blades was visualized.

Cavitation Mode Analysis of Pump Inducer

The onset of cavitation causes head and efficiency of a main pump to be reduced significantly and generates vibration and noise. In order to avoid these phenomena, the inlet of the pump is fitted with a special rotor called an inducer, which can operate satisfactorily with extensive cavitation. The motivation of this study is to find out cavitation modes from the inducer inlet pressure signals and event characteristics from outlet ones at various operating conditions. The cavitation modes are analyzed by using a cross-spectral density of fluctuating pressures at the inducer inlet. The time-frequency characteristics of wall pressures downstream of the inducer are presented in terms of event frequency, its duration time, and number of events by using the Choi-Williams distribution.

Stall Inception in a High-Speed Centrifugal Compressor

Stall inception in a high-speed centrifugal compressor has been examined. The main objective was to find stall precursor and to develop a reliable stall warning method. Eight equally spaced fast-response pressure transducers in the inducer detected the spatial structure of small amplitude perturbations, via spatial Fourier transform, as stall is approached. Near the stall inception point, the phase of spatial Fourier coefficients increased linearly with the speed of impeller rotation for several impeller revolutions at all test speeds, and the spectrum at impeller frequency increased as stall is approached. This is the clear evidence that the impeller frequency participates in the stalling process.For stall warning, a method which uses the spectrum at impeller frequency is suggested. The use of spectrum at impeller frequency as a stall warning method showed a warning time of about two hundreds impeller revolutions. This method uses only one sensor that it has made the stall warning method more useful. And the well-known traveling wave energy method proved to be a good method for stall warning also in a high-speed centrifugal compressor. The warning time was about one hundred impeller revolutions at lower speeds, and about one thousand impeller revolutions at higher speeds. The stall warning methods used here were found to be robust and reliable. Therefore, it seems to be promising to set up a reliable stall avoidance control based on this analysis.Copyright