Jehyun Baek

Tip Clearance Effects on Cavitation Evolution and Head Breakdown in Turbopump Inducer

The objectives of the present study were to investigate the effects of tip clearance on cavitation performance and flow characteristics in a turbopump inducer by using computational fluid dynamics. Three different tip clearances were analyzed under design (Qd) and off-design (0.8Qd and 1.2Qd) cavitating conditions. The Rayleigh–Plesset model was implemented in ANSYS CFX 13.0 by using rate equation controlling vapor generation and condensation in the context of two-phase one-fluid analysis to calculate the cavitating flows. Numerical results in this study were validated by comparison with experimental results for suction performance. Cavitation inception occurs at the leading edge of the blade tip. For high cavitation numbers, the static pressure under cavitating conditions is almost the same as that under noncavitating conditions because tip vortex cavitation and tip leakage vortex cavitation do not affect the flow significantly or deteriorate the overall performance. Tip vortex cavitation and tip leakage...

Effects of a Nonuniform Tip Clearance Profile on the Performance and Flow Field in a Centrifugal Compressor

This paper presents a numerical investigation of the effects of a nonuniform tip clearance profile on the performance and flow field in a centrifugal compressor with a vaneless diffuser. This study focuses in particular on the magnitude and location of the wake. Six impellers with different tip clearance profiles were tested in the flow simulations. The accuracy of the numerical simulations was assessed by comparing the experimental data with the computational results for a system characterized by the original tip clearance. Although the performance improved for low tip clearances, a low tip clearance at the trailing edge improved the compressor performance more significantly than a low tip clearance at the leading edge. The flow field calculated for a system characterized by a low tip clearance at the trailing edge produced a more uniform velocity distribution both in the circumferential and in the axial directions at the impeller exit because the wake magnitude was reduced. As a consequence, this impeller provided a better potential for diffusion processes inside a vaneless diffuser.

Effects of tip clearance on performance and characteristics of backflow in a turbopump inducer

The objectives of the present study were to investigate the effects of tip clearance on the performance and flow characteristics of a turbopump inducer by using computational fluid dynamics. Three different tip-clearance sizes were analyzed for design and off-design conditions. The numerical results were validated by comparison with experimental results. Our detailed investigation of the simulated flow fields shows that backflow penetrates farther upstream for large-tip clearances, which means that performance declines rapidly. On the other hand, for small-tip clearances, the extent of backflow is reduced but the development of hub separation in the middle of the passage causes performance degradation. Our results confirm that a particular value of the tip-clearance limits the development of secondary flow in the passage.

Tip Leakage Flow on the Transonic Compressor Rotor

It is known that tip clearance flows reduce the pressure rise, flow range and efficiency of the turbomachinery. So, the clear understanding about flow fields in the tip region is needed to efficiently design the turbomachinery. The Navier-Stokes code with the proper treatment of the boundary conditions has been developed to analyze the three-dimensional steady viscous flow fields in the transonic rotating blades and a numerical study has been conducted to investigate the detail flow physics in the tip region of transonic rotor, NASA Rotor 67. The computational results in the tip region of transonic rotors show the leakage vortices, leakage flow from pressure side to suction side and their interaction with a shock. Depen ding on the operating conditions, toad distributions and the position of shock-wave on the blade surface are very different close to the blade tip of the transonic compressor rotor. The load distribution and the shock-wave position close to the blade tip had the close relationship with the starting position of leakage vortex and the direction of leakage flow.

Effects of Axial Gap on Unsteady Secondary Flow in One-Stage Axial Turbine

Flow through turbomachinery under stage environment has a very complex structure and is intrinsically unsteady. Especially, recent design trends to the turbomachinery with a short axial gap make the flow extremely complex due to the interaction between stator and rotor. In this paper, effects of axia! gap on the unsteady secondary flow and performance in the one-stage turbine are investigated by three-dimensional unsteady flow analysis. The three-dimensional solver is parallelized using domain decomposition and Message Passing lnterface(MPI) standard to overcome the limitation of memory and save the CPU time in three-dimensional unsteady calculation. A sliding mesh interface approach has been implemented to exchange flow information between blade rows. It is confirmed in the present study that the upstream flow fields have great effects on the unsteady secondary flow fields and performance at the minimum efficiency instant but have relatively weak effects at the maximum efficiency instant according to the variation of axial gap.

Effects of the Inlet Boundary Layer Thickness on the Flow and Loss Characteristics in an Axial Compressor

A three-dimensional computation was conducted to understand effects of the inlet boundary layer thickness on the internal flow and the loss characteristics in a low-speed axial compressor operating at the design condition (φ = 85%) and near stall condition (φ = 65%). At the design condition, independent of the inlet boundary layer thickness, flows in the axial compressor show similar characteristics such as the pressure distribution, size of hub corner-stall, tip leakage flow trajectory, limiting streamlines on the blade suction surface, etc. But, as the load is increased, for the thick inlet boundary layer at hub and casing, the hub corner stall grows to make a large separation region between the hub and suction surface, and the tip leakage flow is more vortical than that observed in the case with thin inlet boundary layer and has the critical point where the trajectory of the tip leakage flow is suddenly turned to the downstream. For the thin inlet boundary layer, the hub corner stall decays to form the thick boundary layer from hub to midspan on the suction surface owing to the blockage of the tip leakage flow and the tip leakage flow leans to the circumferential direction more than at the design condition. In addition to these, the severe reverse flow, induced by both boundary layers on the blade surface and the tip leakage flow, can be found to act as the blockage of flows near the casing, resulting in a heavy loss. As a result of these differences of the internal flow made by the different inlet boundary layer thickness, the spanwise distribution of the total loss is changed dramatically. At the design condition, total pressure losses for two different boundary layers are almost alike in the core flow region but the larger losses are generated at both hub and tip when the inlet boundary layer is thin. At the near stall condition, however, total loss for thick inlet boundary layer is found to be greater than that for thin inlet boundary layer on most of the span except the region near the hub and casing. In order to analyze effects of inlet boundary layer thickness on total loss in detail, total loss is scrutinized through three major loss categories available in a subsonic axial compressor such as profile loss, tip leakage loss and endwall loss.Copyright

A Three-Dimensional Numerical Simulation of Rotating Stall in an Axial Compressor

A three-dimensional computation is conducted to simulate a three-dimensional rotating stall in a low speed axial compressor. It is generally known that a tip leakage flow has an important role on a stall inception. However, almost of researchers have taken no interest in a role of the hub-comer-stall on the rotating stall even though it is a common feature of the flow in an axial compressor operating near stall and it has a large effect on the flows and loss characteristics. Using a time-accurate unsteady simulation, it is found that the hub-comer-stall may be a trigger to collapse the axisymmetric flows under high loads. An asymmetric disturbance is initially originated in the hub-comer-stall because separations are naturally unstable flow phenomena. Then this disturbance is transferred to the tip leakage flows from the hub-comer-stall and grows to be stationary stall cells, which adheres to blade passage and rotate at the same speed as the rotor. When stationary stall cells reach a critical size, these cells then move along the blade row and become a short-length-scale rotating stall. The rotational speed of stall cells quickly comes down to 79 percent of rotor so they rotate in the opposite direction to the rotor blades in the rotating frame.

Numerical Study About the Effect of the Low Reynolds Number on the Performance in an Axial Compressor

Abstract A three-dimensional computation was conducted to understand effects of the low Reynolds number on the performance in a low-speed axial compressor at the design condition. The low Reynolds number can originates from the change of the air density because it decreases along the altitude in the troposphere. The performance of the axial compressor such as the static pressure rise was diminished by the separation on the suction surface with full span and the boundary layer on the hub, which were caused by the low Reynolds number. The total pressure loss at the low Reynolds number was found to be greater than that at the reference Reynolds number at the region from the hub to 85% span. Total pressure loss was scrutinized through three major loss categories in a subsonic axial compressor such as the profile loss, the tip leakage loss and the endwall loss using Denton’s loss model, and the effects of the low Reynolds number on the performance were analyzed in detail. 기호설명 A w : 끝벽 넓이

Effects of the Inlet Boundary Layer Thickness on the Loss Characteristics in an Axial Compressor

Λ th ree -d imens iona l computa t ion w a s conduc ted to unders tand e f fec t s of the inlet bounda ry layer th ickness on the loss charac te r i s t ics in a low-speed axial compres so r ope ra t i ng at the design condi t ion ( φ = 8 5 % ) and near stall cond i t ion ( φ = 6 5 % ) . At the des ign cond i t ion , f lows in the axial compres so r s h o w , independen t of the inlet b o u n d a r y layer th ickness on the hub and cas ing , s imilar charac te r i s t i cs such as the pressure d is t r ibut ion , s ize of hub corner stall , tip leakage f low t ra jec tory and l imi t ing s t reaml ines on the b lade suct ion su r face and hub . H o w e v e r , as the load is increased, the d i f f e r ence in internal f lows is deve loped by inlet cond i t i ons of d i f fe ren t bounda ry layer th ickness . For the thick inlet bounda ry layer, the hub corner stall g r o w s to make a large separa t ion region in the junc t ion of hub and suct ion surfaces . T h e t ip leakage flow is more vort ical than that obse rved for the thin inlet boundary layer and has a par t icular point where the t ra jec tory of the tip leakage f low is abrup t ly turned into the d o w n s t r e a m . For the thin inlet b o u n d a r y layer, the h u b corner stall r ema ins barely unchanged . The tip leakage f low leans to the c i rcumferent ia l d i rec t ion more than that at the des ign condi t ion but has no such par t icular poin t . Moreove r , a large separa t ion region is fo rmed upon the suct ion sur face near the cas ing and ac ts as the b lockage of f lows , resu l t ing in a heavy loss. A s a result of these d i f f e r ences in the internal f low induced by d i f fe ren t inlet condi t ions , the spanwise d is t r ibut ion of the total loss is c h a n g e d dramat ica l ly . At the des ign cond i t ion , total p ressure losses in two cases are a lmost alike in the core How region but the larger losses are genera ted both at hub and t ip w h e n the inlet boundary layer is thin. At the near stall cond i t ion , however , total loss for the thick inlet boundary layer is found to be greater than that for the thin inlet bounda ry layer on mos t of the span except r eg ions near the hub and cas ing . In order to analyze e f fec t s of the inlet b o u n d a r y layer th ickness on the total loss in detai l , the total loss is scrut in ized th rough three ma jo r loss ca tegor ies ava i lab le in a s u b s o n i c axial c o m p r e s s o r such as p rof i l e loss, tip leakage loss and endwal l loss.

Effects of the Clocking on the Internal Flow in a 1.5 Stage Axial Turbine

A three-dimensional unsteady flow simulation is conducted to investigate clocking effects of a row of stators on the performance and internal flow in a 1.5 stage axial turbine. Although the original turbine has 22 blades of the first stator, 28 blades of the rotor and 28 blades of the second stator, the first stator is reduced by a factor of 22/28 to fit the blade ratio 1:1:1. The unsteady flow solver is implemented using the second order time marching and sliding mesh scheme between blade rows. And then, this flow solver is parallelized using MPI (Message Passing Interface) libraries to overcome the limitation of memories and to save the calculation time. Six relative positions of two rows of stators are investigated by positioning the second stator being clocked in a step of 1/6 pitch. The relative efficiency benefit of about 1% is obtained depending on clocking positions. At mid-span, the first stator wake is mixed up with the rotor wake before arriving at the leading edge of the second stator. The time-averaged local efficiency along the span at the maximum efficiency shows more uniform distribution than that at the minimum efficiency. Moreover, the variation of local efficiency at the mid-span does not coincide with that of overall efficiency. Therefore, it is found in this case that the only wake trajectory of the first stator is not a proper means of predicting the best and worst efficiency positions. This is why the relative efficiency benefit depending on the clocking position is obtained near the hub and casing in this study. So, it is necessary to find a general cause of the clocking effect which is applicable to every test case. The difference between maximum and minimum instantaneous efficiencies during one period is found to be smaller at the maximum efficiency than at the minimum efficiency.Copyright