Radial Vaned Impeller For A Centrifugal Compressor Engineering Essay

Impellers are the nucleus dynamic constituents found in assorted turbo machines such as pumps, compressors and turbines. In instance of a compressor, the map of the impeller is to make inactive force per unit area rise by spreading the comparative speed constituent of the consumption fluid by agencies of its revolving action. The cardinal issue that needs to be addressed when planing impellers is to command the rate of comparative speed diffusion along the impeller transition length. This paper aims at quickly planing an impeller for a centrifugal compressor to run into the operating design conditions by utilizing the basic regulating equations of fluid flow and CAD Modelling bundles. The design optimization was carried out in a qualitative mode. The design adhered to the natural philosophies of efficaciously spreading compressible flow.

Terminology

B = tip breadth

B = Blockage due to blade thickness

degree Celsiuss = absolute flow speed

cp = specific heat at changeless force per unit area

vitamin D = diameter

m = mass flow rate

M = Mach figure

N = rotational velocity

P = fluid force per unit area

R = radius

T = fluid temperature

U = peripheral speed

W = comparative speed

? = flow angle, angle between axial and meridional speed constituents.

? = relation flow angle

? = ratio of specific heat

? = efficiency

? = comparative angular co-ordinate

?s = faux pas factor

? = fluid denseness

? = angular velocity of rotary motion

Subscripts

0 = stagnancy conditions

1 = recess status

2 = issue status

P = force per unit area

R = radial

? = tangential

tip = with mention to inducer tip

hub = with mention to hub

acrylonitrile-butadiene-styrenes = with mention to the absolute speed coordinate system

rel = with mention to the comparative speed coordinate system

1. Introduction

Centrifugal compressors are widely used in aeronautical applications ; chiefly in small-scale gas turbine engines for choppers due to their concentration and ability to bring forth high force per unit area ratios in a individual compaction phase. All centrifugal compressors employ an impeller ; a revolving constituent that is used to speed up the entrance flow doing inactive force per unit area rise in the impeller channel. Although the flow is accelerated in the absolute speed coordinate system, the rise in inactive force per unit area is caused by diffusion in the comparative speed coordinate system. The balance of the accelerated flow enters the divergent transitions of the diffusor where the speed is farther reduced to bring forth force per unit area. In pattern, it is usual to plan the compressor so that about half the force per unit area rise occurs in the impeller channel and the other half in the diffusor.

The air mass flow through the compressor and the force per unit area rise depend on the rotational velocity of the impeller ; hence, impellers are designed to run at tip velocities of up to 500m/s ( infix a mention for this! ) . By runing at such high tip velocities, the air speed from the impeller is additions so that greater energy is available for transition to force per unit area by the diffusor. The recess air temperature is another factor act uponing the force per unit area rise, for the lower the temperature of the air come ining the impeller the greater the force per unit area rise for a given sum of work put into the air by the impeller. Besides, in order to keep the efficiency of the compressor, the clearance between the impeller and the shell are kept every bit little as possible to forestall inordinate air escape [ 1 ] .

2. Literature Survey

2.1 Centrifugal compressors

Centrifugal compressors belong to the category of uninterrupted flow, dynamic compressors that are used in a broad assortment of applications chiefly due to their smooth operation, big tolerance of procedure fluctuations and their higher dependability when compared to other types of compressors. Centrifugal compressors range in size from force per unit area ratios of 3:1 up to 12:1on experimental theoretical accounts. Proper choice of a compressor is a complex and of import determination. To guarantee the best choice and proper care of a centrifugal compressor, the applied scientist must hold equal cognition of legion technology subjects [ 2 ] .

Effectss of assorted parametric quantities on the compressor efficiency are good documented, for illustration Balje ; O.E. [ 3 ] studied the consequence of specific velocity ( shape factor ) and specific diameter on the efficiency of centrifugal compressors as shown in Figure 1. It was concluded from his research that the most efficient part for centrifugal compressor operation part is in the specific velocity scope 60 & A ; lt ; Ns & A ; lt ; 1500. For specific velocities greater than 3000, axial flow compressors have to be used.

When compared to axial flow compressors, centrifugal compressors are more robust, easy to fabricate and cheaper to bring forth. Although centrifugal compressors employ at most up to two phases of compaction, the force per unit area rise per phase is higher than that of axial flow compressors. Centrifugal compressors in general are used for higher pressure-ratios and lower flow-rates compared to lower-stage force per unit area ratios and higher flow-rates in axial compressors. While centrifugal compressors find their application in little graduated table gas turbines ( chopper engine ) and subsidiary power units, axial flow compressors are used in commercial and military aircraft engines ( Rolls Royce Pegasus 11-61 and EJ 200 ) .

Figure 1: Centrifugal compressor map [ 3 ]

2.2 Components of a centrifugal compressor

The radial flow ( centrifugal ) compressor consists of four basic constituents or subdivisions:

a. A stationary recess, besides known as recess shell

B. A revolving impeller

c. A stationary ; vaned or vane less diffusor

d. The aggregator or spiral shell

The part of each constituent of the compressor in bring forthing the force per unit area rise in the phase is illustrated in Figure 2.

Figure 2: Pressure rise across centrifugal compressor [ 4 ]

2.2.1 Inlet shell

The map of the recess shell is to present the air to the impeller oculus with minimal loss and to supply a unvarying speed profile at the oculus. Design of the recess shell of a centrifugal compressor with inboard mounted bearings is simple and presents no restraints. The recess rim is axisymmetric and the recess canal takes the signifier of a simple convergent nose. The stagnancy heat content remains changeless as there is no energy transportation in the recess shell.

2.2.2 Impeller

Energy transportation occurs in the impeller of the compressor, hence there is an addition in stagnancy heat content and force per unit area. To achieve peak compressor efficiency, great attention must be taken to accomplish really efficient diffusion processes in the impeller and the diffusor. Since the diffusion procedures are related to the flow Mach figure, it is desirable to set up conditions which lead to a minimal comparative Mach figure at the impeller recess and a minimal absolute Mach figure at the impeller mercantile establishment.

2.2.3 Diffuser

The diffusor is a stationary constituent fitted straight around the impeller. The map of the diffusor is to change over the kinetic energy of the fluid go forthing the impeller tip expeditiously into inactive force per unit area. The influence of the diffusor on compressor efficiency is important, since about half of the fluid energy at the impeller tip is kinetic energy. Centrifugal compressors are normally fitted with either a vane less or a fledged diffusor. Virtually all vaned diffusors besides use a little vane less ‘gap ‘ to cut down unstable noise degrees and diminish the Mach figure at the entry to the vanes.

2.2.4 Collector or spiral shell

The map of the aggregator or spiral ( coil ) is merely to roll up the diffusor issue flow and to steer it every bit expeditiously as possible to the issue pipe or manifold ( sometimes via a conelike diffusor to supply extra diffusion ) , without hindering the effectivity of the diffusor. Merely really mild diffusion can be achieved in the spiral if circumferential force per unit area deformations at the diffusor issue are to be avoided. If clash in the spiral is neglected, so the design may be based on the premise that the angular impulse of the flow remains changeless.

2.3 Impeller design

For a start, when planing high velocity turbo machinery constituents such as impellers, the interior decorator must hold concrete background cognition on the flow natural philosophies in order to carry through a successful design of the impeller. From theory, we infer that the consumption flow is rotated at high velocity by the impeller. As a consequence, the comparative speed of the fluid in the impeller channels undergoes a rapid slowing from the impeller oculus through towards the impeller tip. In add-on to the map of reassigning energy to the air, the impeller should move as an efficient diffusor. A severely shaped channel will interfere with the diffusion processes, doing flow separation at the impeller walls, taking to high impeller losingss. The cardinal design point here is to plan the impeller channel in such a manner so as to detain the oncoming of flow separation, i.e. to obtain gradual slowing of the flow in the channel by optimizing the blade form, hub and shroud profiles. The optimum design will let for a smooth gradual alteration in the comparative Mach figure along the mean flow way.

Figure 3: The two major planes of an impeller [ 2 ]

Did you refer to the above figure in your text?

The impeller inducer plays an of import function in finding the degree of efficient diffusion which will be attained. The taking border of the inducer requires really careful design consideration. The flow arrives at the taking border with a comparative Mach figure in the high subsonic or, in the instance of force per unit area ratios above 4:1 or 5:1, the supersonic scope at the impeller tip. Further, the comparative Mach figure and incidence flow angle vary along the inducer taking border from high values at the oculus tip to much lower values at the oculus hub. Consideration must be given to avoiding daze formation which can take to boundary bed separation. Therefore, attack Mach Numberss transcending 1.2 should be avoided at any subdivision of the inducer. An effort to visualise the flow was carried out by Boyce, M.P. , [ 5 ] which detailed the flow separation points in the impeller channel. To accomplish maximal diffusion, one scheme in planing the inducer is to spread the flow to the separation bound before the impeller transition turns towards the radial way. This leads to hanker axial inducers.

Figure 4: Flow in the meridional plane ( hub-to-shroud plane ) [ 5 ]

Figure 3: Flow in the meridional flow plane ( hub-to-shroud plane ) Another scheme that has been used in the past regarding take downing the comparative Mach figure at recess is to suit the recess casing with guide vanes ; more normally known as recess usher vanes or IGV ‘s. The intent of suiting IGV ‘s into a centrifugal compressor system is to let certain control of the flow features, one of them being to take down the Mach figure. IGV ‘s induce pre-whirl or rotary motion to the incoming fluid as a consequence of which the work done by the impeller to alter the way of the fluid from the axial to the radial way is significantly reduced. Rodgers, C et al [ 6 ] studied the consequence of pre-whirl on the efficiency of the centrifugal compressor as shown in Figure 5. It was concluded that for a given force per unit area ratio, increasing the vane angle increases the efficiency of the compressor. By take downing the recess Mach figure, daze formation is avoided thereby detaining the oncoming of boundary bed separation in the impeller channels.

Figure 5: Consequence of recess pre-whirl on compressor efficiency [ 6 ]

3. Design methodological analysis

To sum up the design procedure, the interior decorator begins by handling the flow to be 1-dimensional to find the rule dimensions of the impeller utilizing the basic regulating equations of fluid flow. Based on the dimensions, legion impeller designs are produced and analysed in footings of efficient spreading capableness with minimum losingss. The optimal geometry is so produced utilizing advanced CAD package bundles which can so be subsequently simulated for comprehensive 3-D flow and stress analysis. The geometry is besides analysed for feasibleness in fabrication by developing accurate NC information to be fed into the 5-axis machining Centre, which, nevertheless, is beyond the range of this undertaking survey. The followers is a flow diagram bespeaking the elements of the design procedure that autumn good under the range of this undertaking:

Figure 6: Design flow procedure

3.1 Premises

In add-on to the flow theoretical account premises, compressor operating conditions need to be assumed based on the operating scope of centrifugal compressors, particularly those found in chopper engines viz. the Turbomeca Arrius 2K1 and Boeing 502-6 Turbo shaft gas turbine.

Table 1: Operating conditions of chopper engines

Operating conditions

Turbomeca Arrius 2K1

Boeing 502-6 Turbo shaft

Mass flow rate, a?? ( kg/s )

3.00

1.60

Pressure ratio, P02/P01

9:1

3.5:1

Rotational velocity, N ( revolutions per minute )

100,000

36,500

Efficiency, ?zc

90 %

80 %

Hence, the design will be carried out for a centrifugal compressor assumed to run at the undermentioned conditions:

Mass flow-rate, a?? = 1.00 kg/s

Pressure ratio, P02/P01 = 6:1

Rotational velocity, N = 66,000 revolutions per minute

Efficiency, ?zc = 85 %

3.2 Calculation of rule impeller dimensions

The impeller dimensions that are to be determined are illustrated in the Figure as shown below:

Figure 7: Side profile of the impeller

The speed distribution at entry and issue to the impeller can be approximated to speed trigons more normally known as vector diagrams. The undermentioned figure illustrates the speed vector diagrams at entry and at issue:

Figure 8: Velocity vector diagrams at recess and mercantile establishment ( shown along the radial axial profile of the impeller )

Euler ‘s equation is used to depict the energy/work transportation procedure to the fluid by the rotor. Therefore, work transportation can be given as:

Since C1 is strictly axial at entry, . Therefore, the equation now becomes ;

For a radial fledged impeller, the commotion or digressive constituent of the absolute fluid speed at issue ; should be equal to the impeller tip velocity, U2 ( in ideal instances ) . However, in world non all the air passes swimmingly through the transition formed between the impeller blades and therefore the digressive constituent of the absolute fluid speed at issue tends to be less than the impeller tip velocity. Therefore, a rectification factor or more normally known as the faux pas factor which is the ratio between the digressive constituent of the absolute air speed at issue to the impeller tip velocity has to be multiplied to the energy equation. For maximal work/energy transportation, implies that faux pas factor, = 1, but in world the faux pas factor has a lesser value than that of one. Therefore, the faux pas factor can be calculated based on the figure of blades by utilizing the undermentioned empirical expression proposed by Stanitz for a radial vaned impeller which is:

Where, Z = figure of blades

Solving the above equation iteratively yields the undermentioned consequences:

Omega

0.80

10

0.85

13

0.90

20

0.95

40

By and large higher the figure of blades outputs better flow counsel taking to increased rate of work transportation by the impeller. However, it leads to loss in efficiency due to increase in frictional losingss, blade obstruction ( country inaccessible to the flow ) and tip losingss. Besides, a lesser figure of blades might take down frictional and tip losingss but there will be hapless flow counsel in add-on to a decrease in work transportation from the impeller. Consequently, the interior decorator must be able to strike a balance in order to obtain the right figure of blades in order to function the proposed design standard. In world, it is good design pattern to take the figure of blades in the scope of 15-20. Therefore, the figure of blades chosen for this case is 18 which yields a slip factor, = 0.89.

The Euler ‘s equation can besides be expressed in footings of the compressor runing conditions as shown in the followers:

The above equation can hence be expressed in footings of a dimensionless public presentation parametric quantity as shown below:

The look on the left-hand-side of the equation is known as the velocity parametric quantity which can be used to foretell the efficiency of the compressor. Re-arranging and replacing the false values for the operating conditions, outputs the preliminary dimension of the impeller which is the impeller tip diameter,

d2 = 149mm ( r2 = 74.5 millimeter )

The absolute Mach figure, Maabs, 1 corresponding to the absolute speed at recess, C1 is normally in the scope so in this case, the absolute Mach figure at recess is assumed to be 0.4

Using isentropic flow equations for a compressible fluid,

So, Tabs, 1 = 288.76K and Pabs, 1 = 0.90 saloon

Hence,

Besides, from theory we infer that the comparative Mach figure at recess is normally the highest at the inducer tip

In order to forestall the happening of shockwaves, the inducer must be designed for a subsonic comparative speed. Hence, the comparative Mach figure at recess, Marel, 1 is assumed to be 0.8. Again, utilizing the isentropic flow equations, Trel, 1 = 264.20K and Prel, 1 = 0.656 saloon. This gives

an recess comparative speed,

Remembering, the recess speed trigon, utilizing values of C1 and W1 ; the average inducer tip velocity U1 and blade angle ?1 can be calculated utilizing trigonometric ratios

The average inducer diameter is given by,

Therefore,

d1 is related to the tip and hub diameters by the undermentioned relation:

Re-arranging, gives us the undermentioned look:

From the continuity equation, we have

Thereby, work outing equations. Simultaneously, Inducer tip and hub diameters are obtained

dtip = 95mm ( rtip = 47.5mm )

dhub = 18mm ( rhub = 9mm )

Remembering the mercantile establishment speed trigon, the undermentioned speeds and blade angles can be obtained.

Using the thermodynamic equations of province, the inactive force per unit area, temperature and denseness at the impeller issue can be found out. This when combined with the continuity equation yields the outlet country. The obstruction factor must besides be accounted for, which indicates the country that is unaccessible to the flow. Finally, replacing these values yields a tip width value of 6mm.

b2 = 6mm

3.3 Impeller blade optimisation

From the dimensions obtained by utilizing the flow analytical equations, four impeller designs were produced with changing axial lengths. The ground for this was that, the length of the impeller channel would find the degree of efficient diffusion that will take topographic point in order to forestall pre-mature flow separation at entry to the inducer in bend minimizing losingss. The undermentioned side-view profiles of the blade were generated for choice of the best suitable design to run into the demands:

3.4 CAD Modelling

3-D Modelling of the impeller is carried out after the design choice phase. The intent of transporting out 3-D Modelling is to look into for how good the impeller has been proportioned. Normally after the CAD Modelling phase, many interior decorators try to optimise the design farther by running FEA and CFD codifications which are eventually checked for proof. Although, these codifications can better upon bing designs, it can turn out to be arduous, time-consuming and besides carry certain restrictions that can non be wholly eliminated. Manufacturing feasibleness can besides be carried out either by rapid prototyping or 5 axes machining both of which make usage of the generated CAD geometry file.

4. Consequences

Impeller Dimensions

Impeller tip diameter, d2 = 149mm

Inducer tip diameter, dtip = 95mm

Hub diameter, dhub = 18mm

Tip breadth, b2 = 6mm

Impeller axial length = 37.25mm ( 50 % of r2 )

5. Discussions

For a successful blade choice, cognition of the flow natural philosophies is highly important. With mention to Figure 9, it can be noted that the impeller channel is highly narrow. This does non turn out to be a good design as a narrow channel indicates immediate flow separation upon entry to the inducer. Besides, the rate of diffusion is highly high increasing diffusion losingss.

Figures 10 and 11 both autumn under the optimum design class. Rate of diffusion is controlled bespeaking delayed separation of the entrance flow. This leads to take down losingss maximising work transferred by the impeller to the fluid.

With mention to Figure 12, the impeller blade channel has the highest axial length. Although long axial inducers control the growth/onset of flow separation, a really long axial inducer implies greater losingss due to clash. Since frictional losingss account for the highest per centum among all the other associated losingss in the impeller, this design falls under the unacceptable part.

Therefore, in this case, Figure 10 was chosen as the optimal blade design as it possesses the ability to detain the oncoming of flow separation, i.e. command the rate of diffusion as the flow turns radial from its primary axial way of entry.

6. Decisions

Principle impeller dimensions were obtained from the regulating equations for a given set of operating conditions and premises ( Mach Numberss )

Four sample designs were analyzed and the best design was selected based on flow natural philosophies and flow nature.

The selected best design was so modeled in SolidEdge V20 as shown in the undermentioned figure. The impeller is good proportioned and can be deemed suited for industry.

Figure 13: Front and isometric positions of the chosen impeller design