Parameters

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Parameters

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On page Parameters one has to put in or to modify parameters resulting from approximation functions in dependence on specific speed nq (see Approximation functions).

For details of how to handle the parameter edit fields please see Edit fields with empirical functions.

 

Parameter and efficiency values can be handled manually or can be switched to automatic update by the checkbox on top of the page. Then the default values are used always, even after design point modifications (see Global setup).

If the automatic mode is not selected the current default values can be specified by one of the following options:

globally by the button on top of the page

regionally by the default button within the Parameters or Efficiency region

individually by the default button within the input field when selected

Parameters

The panel Parameters allows defining alternative parameters in each case for the calculation of the following impeller diameters:

inlet

outlet

dS1, dH1

dS2, dH2

With the help of the following parameters the inlet of the rotor can be calculated. If the Diameter ratio is chosen the inlet is determined independent of the upstream swirl of an upstream component. If Degree of reaction is chosen, the upstream swirl is either determined by the upstream component (check box "Consider upstream swirl" checked, see setup) or by the absolute inlet flow angle α1.

Isentropic velocity ratio νis

Mean inlet diameter 0.5(dS1+dH1)

Degree of reaction R

Inlet diameter hub and tip, dH1 & dS1

Absolute inlet flow angle α1

Inlet diameter hub and tip, dH1 & dS1

Diameter ratio dH/dS

Inlet hub diameter dH1

The outlet section can be calculated with:

Meridional velocity ratio
cm2/cm1

0.9..1.1

strictly coaxial        dH2 = dH1 and dS2 = dS1
coaxial @Hub        dH2 = dH1
coaxial @Mid-spandM2 = dM1
coaxial @Shroud        dS2 = dS1

Efficiency

In the group Efficiency the following efficiencies need to be given:

Design relevant

Rotor efficiency ηts (total-static)

Information only

Mechanical efficiency ηm

Internal and mechanical efficiency form the overall efficiency (coupling efficiency):

PQ: (isentropic) Rotor power

PD: Power output (coupling/ driving power)

The rotor efficiency (or blade efficiency) ηtt describes the energy losses within the turbine caused by friction and vorticity. Friction losses mainly originate from shear stresses in boundary layers. Vorticity losses are caused by turbulence and on the other hand by changes of flow cross section and flow direction which may lead to secondary flow, flow separation, wake behind blades etc.. The rotor efficiency is the ratio between the actual specific enthalpy difference and the ideal (isentropic) specific enthalpy difference at loss less transmission:

The mechanical efficiency mainly includes the friction losses in bearings and seals:

rising with impeller size.

Information

In the right panel of the tab sheet Parameter some variables are displayed for Information:

actual Power PD

PD = PQ·ηttSt

Power loss PL

PL = PQ - PD

Flow Qt

calculated with total density in the outlet:

Pressure ratio total-total

Pressure ratio total-static

Efficiency total-total

ηtt

Efficiency total-static

ηts

Polytropic efficiency

(n .. polytropic exponent

κ .. isentropic exponent)

In general for cost reasons single-stage & single-intake machines are preferred covering a range of about 10 < nq < 400. If especially high specific speed values (nq > 400) do occur one can reduce rotational speed n or mass flow rate ṁ if feasible. Another option would be to operate several single-stage turbines - having a lower nq - in parallel.