Daisaku Sakaguchi

Suppression of Unstable Flow at Small Flow Rates in a Centrifugal Blower by Controlling Tip Leakage Flow and Reverse Flow

The effects of the inlet recirculation arrangement on inducer stall and the diffuser width on diffuser stall in a high-specific-speed-type centrifugal impeller with inducer were analyzed by numerical simulation and also verified experimentally. It was found that the incipient unstable flow occurs due to a rolling-up vortex flow, resulting from an interaction between the tip leakage flow and the reverse flow accumulated at the pressure side immediately downstream of the inducer tip throat, in which a strong streamwise component of vorticity is included. By forming the inlet recirculation flow the tip leakage vortex is effectively sucked into the suction ring groove, and the flow incidence is decreased simultaneously. The unstable flow range of the test blower was reduced significantly by about 45% without deteriorating the impeller characteristics by implementing optimally both the ring groove arrangement and the narrowed diffuser width.

Measurement of temperature and velocity fluctuations in a turbulent diffusion flame by an optical fiber thermometer and a laser 2-focus velocimeter

Both fluctuations of local velocity and local temperature were measured in a steady turbulent diffusion flame of propane by using the semiconductor laser 2-focus velocimeter and the optical fiber thermometer and the optical fiber thermometer respectively. The flame temperature and the soot particle density were calculated by applying the IR two- color method to the measured radiant energy from the soot particles in the flame. In the analysis of the frequency power spectra of temperature and velocity fluctuations, the correlation-based slotting technique was adopted for those data with the nonuniform time interval. It is shown that the time mean value and the fluctuation of the flame temperature decrease gradually toward downstream in the luminous flame region, and those of the soot density increase due to decay of turbulence along the flame axis. On the other hand, both time mean and fluctuation of the flame temperature increase in the radial direction from the center to the periphery due to the effect of air entrainment marked in the peripheral region of the flame. Furthermore, the power spectrum of the velocity fluctuation is not always the same as that of the temperature fluctuation in the flame center.

Analysis of noise generated by low solidity cascade diffuser in a centrifugal blower

This paper deals with the effect of the blade leading edge location (RLSD ) of a low solidity cascade diffuser (LSD) on noise and diffuser performance in a centrifugal blower. The noise of the LSD was measured and analyzed comparing with that of vaneless diffuser (VLD) in view points of overall noise, discrete frequency noise and broadband noise. The numerical flow analysis was conducted in the impeller and the diffuser by using a Navier-Stokes solver. The noise of the VLD varied little in a wide flow rate range, on the other hand, that of the LSD increased remarkably in the small flow rate by about 7 dB. The noise of the LSD did not increase near the design flow and was almost equal to that of the VLD. It was found that the increase in noise due to LSD is dependent mainly on the broadband noise between 600∼1000Hz, which was closely correlated to the lift force of the LSD blade. The two kinds of discrete frequency noise appeared due to an interaction between the rotating impeller and the LSD blade and another interaction between the rotating impeller blades and the reverse flow toward the impeller exit, but their influence on the overall noise were relatively small. By shifting the LSD blade leading edge location downstream from RLSD = 1.1 to 1.2, the noise was reduced by about 3 dB at the maximum without deterioration of the diffuser performance. The maximum lift coefficient of the LSD blade was achieved as high as 1.5 at the high attack angle of 17 degrees even in the case of RLSD = 1.2, resulting in improvement of the diffuser performance by about 40% and in reduction of the unstable flow range by about 11%.Copyright

Suppression of Rotating Stall by Wall Roughness Control in Vaneless Diffusers of Centrifugal Blowers

By positioning the completely rough wall locally on the hub side diffuser wall alone in the vaneless diffuser, the flow rate of rotating stall inception was decreased by 42 percent at a small pressure drop of less than 1 percent. This is based on the fact that the local reverse flow occurs first in the hub side in most centrifugal blowers with a backswept blade impeller. The three-dimensional boundary layer calculation shows that the increase in wall shear component normal to the main-flow direction markedly decreases the skewed angle of the three-dimensional boundary layer, and results in suppression of the three-dimensional separation. It is also clarified theoretically that the diffuser pressure recovery is hardly deteriorated by the rough wall positioned downstream of R = l.2 because the increase in the radial momentum change, resulting from reduction in the skewed angle of the three-dimensional boundary layer, supports the adverse pressure gradient.

Effect of Pre-Whirl on Unstable Flow Suppression in a Centrifugal Impeller With Ring Groove Arrangement

In order to obtain a wider operating range in a centrifugal impeller with inducer, the effect of the pre-whirl induced by the inlet recirculation flow on the flow incidence and the impeller characteristics were analyzed numerically and compared with the experimental results. In order to control the swirl intensity of the recirculation flow, guide vanes were installed circumferentially in the annular bypass of the ring groove arrangement, and the setting angle of the guide vane was changed. The fundamental concept for surge suppression is to achieve the flow incidence less than or close to the critical one. A too large-positive flow incidence can be reduced by increasing the recirculation flow rate determined by the pressure difference between the two ring groove positions, on the other hand, a higher pressure rise in the inducer can be obtained at the flow incidence close to the critical one by suppressing the pre-whirl induced by the recirculation flow. It is clearly shown that the better impeller characteristics and the large recirculation flow rate can be achieved by giving a suitable setting angle of the guide vane. The unstable flow rate range of the tested impeller was reduced by about 53% almost without deterioration of the impeller efficiency in the whole flow rate range.Copyright

Effect of Guide Vane in Ring Groove Arrangement for a Small Turbocharger

A high‐pressure ratio and a wide operating range are highly required for a turbocharger in diesel engines. Ring groove arrangement is effective for flow range enhancement of centrifugal compressors. Two ring grooves on the suction pipe and the shroud casing wall are connected by means of the annular passage, and the stable recirculation flow is formed at small flow rates from the downstream groove toward the upstream groove through the annular bypass. It is well known that the ring groove arrangement shows the following two merits; (1) the formation of recirculation flow removes the low energy fluid around the downstream groove located near the splitter blade leading edge, (2) the flow incidence can be reduced because the recirculation flow merges again with the incoming main flow upstream of the impeller inlet. In the present study, the ring groove arrangement was applied to the high speed centrifugal compressor, in addition, in order to suppress too large pre‐whirl due to the recirculation flow, the eff...

Optimization of Inlet Ring Groove Arrangement for Suppression of Unstable Flow in a Centrifugal Impeller

An optimization of the inlet ring groove arrangement has been pursued in the present study for obtaining better impeller characteristics and a wider operation range at both small and large flow rates in a high specific speed type centrifugal impeller with inducer. The effects of the shape parameters with respect to the inlet ring groove on the impeller characteristic and the flow incidence were analyzed mainly based on numerical simulations, but also compared to the experimental results. At small flow rates, a significant improvement in the impeller characteristic is achieved due to reduction in the excessive-positive flow incidence by optimizing both location and width of the rear groove near the inducer tip throat. On the other hand, the impeller characteristic is improved at large flow rates by implementing the corner radius at the rear groove edge and by placing another front ring groove in the suction pipe. As a result, by the optimized configuration of the front and rear ring grooves, the unstable flow range of the test impeller can be reduced by about 50% without deterioration of the impeller characteristic even at the 125% flow rate.Copyright

Evaluation of Mass Flow Rate Distribution in Diesel Fuel Spray by L2F

A laser 2-focus velocimeter (L2F) has been applied for measurements of velocity and size of droplets of diesel spray and an evaluation method of mass flow rate has been proposed. The L2F has a micro-scale probe which consists of two foci. The distance between two foci is 17μm. The data acquisition rate of the L2F has been increased to 15MHz in order to capture every droplet which appears in the measurement volume. The diesel fuel spray injected intermittently into the atmosphere was investigated. The orifice diameter of the injector nozzle was 0.113mm. The injection pressure was set at 40MPa by using a common rail system. Measurements were conducted on ten planes 5 to 25mm downstream from the nozzle exit. It was clearly shown that the velocity of droplet was the highest at the spray center. The size of droplet at the spray center decreased downstream within 15mm from the nozzle exit. The mass flow rate near the spray center was found to be larger than that in the spray periphery region. It was confirmed that the fuel mass per injection evaluated by the proposed method based on the L2F measurement was near to the injected mass in a plane further than 15mm from the nozzle exit. However, fuel mass was underestimated in a plane closer to the nozzle exit. The probability density of infinitesimal distance between surfaces of adjacent droplets increased remarkably near the spray center 5 and 12mm downstream from the nozzle exit. As infinitesimal distance can be thought as an indicator of a highly dense region, it is understood that underestimation of fuel mass near the nozzle exit is due to the highly dense region. The diameter of the region, where the highly dense region was observed, was estimated as an order of 0.2mm in a plane 5mm downstream from the nozzle.Copyright

Behavior of Secondary Flow in a Low Solidity Tandem Cascade Diffuser

Low solidity circular cascade diffuser abbreviated by LSD was proposed by Senoo et al. showing a high blade loading or a high lift coefficient without stall even under small flow rate conditions. These high performances were achieved by that the flow separation on the suction surface of the LSD blade was successfully suppressed by the secondary flow formed along the side walls. The higher performance of the LSD was achieved in both pressure recovery and operating range by adopting the tandem cascade because the front blade of the tandem cascade was designed suitably for small flow rates while the rear blade of the tandem cascade was designed suitably for large flow rates.In order to clarify the reason why the tandem cascade could achieve a high pressure recovery in a wide range of flow rate, the flow in the LSD with the tandem cascade is analyzed numerically in the present study by using the commercial CFD code of ANSYS-CFX 13.0. The behavior of the secondary flow is compared between the cases with the single cascade and the tandem one. It is found that the high blade loading of the front blade is achieved at the small flow rate by formation of the favorable secondary flow which suppresses the flow separation on suction surface of the front blade, and the flow separation on pressure surface of the front blade appeared at the design flow rate can be suppressed by the accelerated flow in the gap between the trailing edge of the front blade and the leading edge of the rear blade, resulting in the positive lift coefficient in spite of a large negative angle of attack.Copyright

Multi-Point Optimization of Recirculation Flow Type Casing Treatment in Centrifugal Compressors

High-pressure ratio and wide operating range are highly required for a turbocharger in diesel engines. A recirculation flow type casing treatment is effective for flow range enhancement of centrifugal compressors. Two ring grooves on a suction pipe and a shroud casing wall are connected by means of an annular passage and stable recirculation flow is formed at small flow rates from the downstream groove toward the upstream groove through the annular bypass. The shape of baseline recirculation flow type casing is modified and optimized by using a multi-point optimization code with a metamodel assisted evolutionary algorithm embedding a commercial CFD code CFX from ANSYS. The numerical optimization results give the optimized design of casing with improving adiabatic efficiency in wide operating flow rate range. Sensitivity analysis of design parameters as a function of efficiency has been performed. It is found that the optimized casing design provides optimized recirculation flow rate, in which an increment of entropy rise is minimized at grooves and passages of the rotating impeller.