Question

The air entering the impeller of a centrifugal compressor has an absolute axial velocity of 100m/s. At rotor exit the relative air angle measured from the radial direction is 26 degree 36', the radial component of velocity is 120m/s and the tip speed of the radial vanes is 500m/s. Determine the power required to drive the compressor when the air flow rate is 2.5 kg/s and the mechanical efficiency is 95%. If the radius ratio of the impeller eye is 0.3, calculate a suitable inlet diameter assuming the inlet flow is incompressible. Determine the overall total pressure ratio of the compressor when the total-to-total efficiency is 80%, assuming the velocity at exit from the diffuser is negligible.

Answer 1

To determine the power required to drive the compressor, we can use the following formula:

Power = (mass flow rate * change in total enthalpy) / mechanical efficiency

First, let's calculate the change in total enthalpy of the air at the compressor exit. The total enthalpy is given by:

h_total = h + (V^2)/2

where:

h_total = total enthalpy

h = specific enthalpy

V = velocity of the air

Given:

Absolute axial velocity at inlet (V1) = 100 m/s

Relative air angle at rotor exit = 26° 36' = 26.6° (converted to degrees)

Radial component of velocity at rotor exit (Vr2) = 120 m/s

Tip speed of the radial vanes (U2) = 500 m/s

Mass flow rate (m_dot) = 2.5 kg/s

Mechanical efficiency = 95%

First, we can calculate the velocity at rotor exit in the axial direction (V2a) using the relative air angle:

V2a = Vr2 * tan(θ)

V2a = 120 * tan(26.6°)

V2a = 62.5 m/s

Next, we can calculate the total enthalpy at rotor exit (h2_total) using the specific enthalpy and the velocity in the axial direction:

h2_total = h2 + (V2a^2)/2

where h2 is the specific enthalpy at rotor exit. Since the inlet flow is incompressible, the specific enthalpy remains constant:

h2 = h1

Now, we can calculate the change in total enthalpy:

change in h_total = h2_total - h1

change in h_total = h2 + (V2a^2)/2 - h1

Next, we can calculate the power required to drive the compressor using the mass flow rate, change in total enthalpy, and mechanical efficiency:

Power = (m_dot * change in h_total) / mechanical efficiency

Power = (2.5 * change in h_total) / 0.95

To calculate the suitable inlet diameter, we can use the following formula for incompressible flow:

A1 = m_dot / (ρ1 * V1)

where A1 is the inlet area, ρ1 is the density of the air at the inlet, and V1 is the axial velocity at the inlet. Since the flow is incompressible, the density remains constant:

ρ1 = ρ2

Now, we can calculate the overall total pressure ratio of the compressor using the total-to-total efficiency:

total pressure ratio = (total enthalpy at rotor exit - total enthalpy at inlet) / (total enthalpy at rotor exit - total enthalpy at diffuser exit)

where the total enthalpy at diffuser exit is assumed to be equal to the total enthalpy at rotor exit (h2_total). Given that the total-to-total efficiency is 80%, we can write:

total pressure ratio = (h2_total - h1) / (h2_total - h2_total)

Please provide values for specific enthalpy (h) and density (ρ) at the given conditions (inlet and rotor exit) in order to complete the calculations.

Step-by-step explanation: