Question 16.2: Determine the pressure ratio developed and the specific work...

Determine the pressure ratio developed and the specific work input to drive a centrifugal air compressor of an impeller diameter of 0.5 m and running at 7000 rpm. Assume zero whirl at the entry and T1t=T_{1 t}= 290 K. The slip factor and power input factor to be unity, the process of compression is isentropic and for air cp=c_{p}= 1005 J/kg K, γ=1.4\gamma=1.4.

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The impeller tip speed

 

U2=π×0.5×700060U_{2}=\frac{\pi \times 0.5 \times 7000}{60}

 

= 183.26 m/s

 

With the help of Eqs (16.6) and (16.7), we can write

 

T2tT1t=T3tT1t=1+ΨσU22cpT1t\frac{T_{2 t}}{T_{1 t}}=\frac{T_{3 t}}{T_{1 t}}=1+\frac{\Psi \sigma U_{2}^{2}}{c_{p} T_{1 t}} (16.6)

 

p3tp1t=(T3tT1t)γγ1=[1+ηc(T3tT1t)T1t]γγ1\begin{aligned}\frac{p_{3 t}}{p_{1 t}} &=\left(\frac{T_{3 t}^{\prime}}{T_{1 t}}\right)^{\frac{\gamma}{\gamma-1}} \\&=\left[1+\frac{\eta_{c}\left(T_{3 t}-T_{1 t}\right)}{T_{1 t}}\right]^{\frac{\gamma}{\gamma-1}}\end{aligned} (16.7)

 

Pressure ratio =[1+U22cpT1t]γγ1=\left[1+\frac{U_{2}^{2}}{c_{p} T_{1 t}}\right]^{\frac{\gamma}{\gamma-1}}

 

=[1+(183.26)21005×290]1.40.4=\left[1+\frac{(183.26)^{2}}{1005 \times 290}\right]^{\frac{1.4}{0.4}}

 

= 1.46

 

From Eq (16.3), specific work input =U22=(183.26)21000=33.58=U_{2}^{2}=\frac{(183.26)^{2}}{1000}=33.58 kJ/kg

 

w=ΨσU22w=\Psi \sigma U_{2}^{2} (16.3)

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