Numerical simulation of radial compressor stages with seals and technological holes

Article describes numerical simulations of an air flow in radial compressor stages in the NUMECA CFD software. Four different radial compressor stages were solved in this article. During the tasks evaluating the stepped and straight impeller seals and technological holes influence on working characteristics and the flow field was observed. Also the CFD results comparison with results from the empiric design tool is described.


Computed compressor stages
Results obtained from numerical simulations of following radial compressor stages will be presented in this article.We solved four different stages and they are marked from RTK 01 to RTK 04 due to clear arrangement.These stages are different in the blade number and type in corresponding areas of compressor channel, in the compressor channel shape or in working parameters.a The seals presence influence on working parameters was evaluated at all computed stages and the influence of the a tsyka@ntc.zcu.cztechnological hole in impeller blades was evaluated at stages RTK 02 and RTK 04.

RTK 01 stage
This stage is specific for the presence of an axial guidance device with regulatory blades in the compressor intake.Guidance device is used to ensure the proper impeller blades leading angle and to reduce compressor losses.In this case, simulations were solved only for one guidance blades location, when blades were located in the default position 0°.
The RTK 01 stage simulation was also done only for 7 460 rpm of the impeller, what corresponds to the relative small mass flow coefficient and the stage pressure ratio.

RTK 03 stage
The RTK 03 stage is very simmilar to th RTK 03 is the modified variant of the st the narrower channel.Due to this the lower mass flow coefficients and r pressure ratio values.
Basic construction parameters of th were kept -the number and the typ operational revolutions.

RTK 04 stage
The same as stages RTK 02 and RTK 0 04 has radial inlet into the stage model.

Impeller seals influe
Compressor stages numerical seals had the task to reveal the characteristics of whole stag model is shown mainly like th loss by seals, where the am returning gas through stepp important.The losses prese decreasing of the stages efficie

Geometry and mesh c
The computational grid was TurboGrid software.The grid cells and is block structured be time reduction.As a base for computation geometry, border curves of t impeller seals geometry are r 3D CAD stage model.This st software.ANSA is the suitab general surfaces and curves.F ANSA is being used in the creating and modifying of models.For the creation of blade from each row is needed After import of the NUMECA/AutoGrid environ border curves is required.I curves to the hub and the shro process is used for the definit up of several parameters like the number of grid element structured computational grid i

Mass flow outlet
For the obtaining of undistorted resu keep the grid fine enough and also is n certain grid cell properties, which a (> 20), expansion factor (< 3) or aspect r Pressurized medium is in our c compressible gas.The SST k-ω turbu used and cases were computed stationary ults is necessary to necessary to keep are orthogonality ratio (< 3000).rid.
ants without seals exaedral cells and ately of 5 000 000 el area is in both cal because of the nks to very small it was possible to d computations. d.
he pressure inlet and the mass flow t from the return functional model terface.Thanks to r of blades to one case air -ideal ulence model was y.

Efficiency and pressur
As was mentioned the o compressor stage is defined it most important parameters ar and the pressure ratio dependi pressurized gas through the sta shown working characterist depending on the mass flow co        From shown characteristics is obvious that the efficiency decrease is approximately 2 % at the most solved stages and it is in accordance with the pressure ratio decrease.The exception is the stage RTK 04, where efficiency and pressure ratio decrease is bigger.The efficiency decrease is between 4 to 5 % in both solved regimes with different revolutions, which is interesting because the stage RTK 04 has similar design, revolutions and working parameters as the stage RTK 01, where the efficiency decrease caused by presence of seals isn't so big.
In the area of the seals design these stages differs in the number of straight seal edges in certain domains.For now the question is if the big efficiency decrease is caused by the different seals design or different loss in the diffuser.The research of the stage RTK 04 is still running in order to find out what causes the decrease of working parameters.

Mass flow through impeller seals
Seals losses are dependent on the amount of the pressured gas, which flows through seals back against the direction of the main flow in the channel.For the correct design of the seals edges shape and their number is required to know amount of gas, which flows through seals.In the following graphs is shown the comparison of mass flow values obtained from CFD simulations in NUMECA and values from the 1D empiric design tool (KSTK).During evaluation of the RTK 04 sta that when the compressor stage was run revolutions (15 699 rpm) and in the pressure ratio, the mass flow in str approximately zero or with the negative

Technological hole influenc
Impellers can be manufactured by sever to achieve the most accurate impeller dim surface quality can be used electric disc but this method is very expensiv combination of milling and welding is b 04, 7 476 rpm.

The mesh of the techn
The technological hole grid cr and difficult than the creation because the manual work is environment it is possible to automatically but this tool is d Due to the block structured g the inserting of technological connection definition with the There are two ways to con main one.Both methods have connection but the connect Connection of the hole m "interface" boundary condition way the "interface" is used suction side of the impeller bla are in the easy geometry and m insignificant intervention to th main disadvantage can be the density in the area of the conne blades is in this case the ation used during welding ellers consist of cover and ion is made in the specific the hole is created during s the prevention from the e in the impeller blade.

ple of the technological hole
The technological hole is middle of the blade height in ocated at the blades edge in e and the cover disc too.a question how significant cal hole on the compressor pressure ratio.The basic ency and the pressure ratio wing of some amount of the sure side to the suction side followed by the certain etween impeller blades.e influencing of working ld be considered during the nological hole reation is more complicated n of the main channel mesh required.In the AutoGrid create technological holes determined to turbine stages.grid which NUMECA uses hole grid blocks and their main channel is required.nnect the hole mesh to the e the same principle of the tion location is different.mesh is provided by the n in both cases.In the first on the pressure and the ade.Benefits of this method mesh preparation and in the he main channel mesh.The e big difference of the cell ection.
In the second method of the mesh connection "interface" boundary conditions were moved to the greater distance from the technological hole area.The main benefit is the precise hole edge definition in the model but there are large interventions to the main stage mesh which increase the amount of work involved and the is also local higher cell density around the technological hole area.
Both ways of the mesh creation were tested on numerical simulations of the compressor stage RTK 04.The stage efficiency and pressure ratio decrease was important to us.Simulations were done on geometric variants with and without the technological hole.In figures 27 and 28 are shown efficiency and pressure ratio characteristics depending on the RTK 04 stage mass flow coefficient for 7 476 rpm.From results it is obvious that the technological hole influence on stage working characteristics is in this case insignificant.Moreover it is shown that the hole mesh design influence is the same for both methods.Also small decrease of values due to local higher cell density is presented.The same differences depending on the mesh design were found in simulations with 15 699 rpm.

Working characteristics
Following characteristics of stages RTK 02 and RTK 04 show the impeller blades technological hole influence.On the first sight it is obvious that in the case of 3D shaped impeller blades of the RTK 02 stage is the hole influence great and variable but in the case of 2D shaped impeller blades of the RTK 04 the influence on the working characteristics is very small and constant.On the other hand the RTK 04 stage behavior is stable and consistent in all characteristics working points and the efficiency decrease is up to 1% in whole operating range.For the better evaluation of the situation in compressor stages streamlines from technological holes were drawn.Pictures show that flow character between impeller blades is totally different.In the RTK 02 stage is the massive flow separation from the blade surface.The flow separation influences velocity and pressure field up to the middle of the channel high on the impeller outlet.
The flow situation in the RTK 04 stage is much better.The separation is also presented but it isn't so large and copies the impeller blades suction side surface.

Conclusions
In this article was mainly investigated the influence of impeller seals and technological holes presence.It was found out that both design features have insignificant influence on the stage behavior and it is important to know their functional rules.
In the case of stepped and straight impeller seals is the influence on working characteristics more or less constant in all working points, but still the design of some stages has deficiencies which can cause unexpected results.
The situation with the technological holes design is similar.This area of interest is relatively unexplored and numerical simulations show that the inappropriate design of technological hole dimensions, location or orientation can cause the dramatically working parameters decrease in some specific situations.

Figure 3 .
Figure 3.The RTK 04 stage mod impeller e seals influence on working ges.Seals presence in the he impeller friction loss and mount of the heated and ped and straight seal is ence consequence is the ency and the pressure ratio.reation created in the NUMECA d consists only of hexaedral ecause of the computational nal grid creation the blades the stage channel and the required from the complete tep was done in the ANSA ble tool for the work with For these and other benefits e automotive industry for very complex geometry the channel grid only one d.preparedgeometry into nment the definition of It means assigning certain oud of the channel.Similar tion of blades.After setting the number of blades and s generating of the block is possible.

Figure 4 .Figure 5 .
Figure 4.The detail of the impeller blades gr Base computational grids of all varia consisted approximately of 3 000 000 he variants with seals consisted approxima cells.The grid structure in the channe variants (with and without seals) identic best possible results comparison.Than height of cells at the walls (0.005 mm) reach maximal y+ around 10 in all solve

ce
ral ways.In order mensions and fine charge machining, ve therefore the being used in most cases.The hole in impeller technological blades modifica manufacturing.Welded impe carrier disc and their connecti impeller blades height.So t impeller assembling and it i local tension concentration.

Figure 26 .
Figure 26.The technological hole In the figure 26 the examp design is shown (RTK 04).located approximately in the m this case but the hole can be lo the crossing between the blade In this context there is a influence has this technologic stage efficiency and the p assumption is that the efficie should decrease due to the flow pressurized gas from the press of impeller blades what is influencing of the flow field be If it turns out that the characteristics is large it shoul compressor stage design.