Question

Helium expands in a nozzle from 0.8 MPa,500 K0.8\ \mathrm{MPa}, 500 \mathrm{~K}, and negligible velocity to 0.1 MPa0.1\ \mathrm{MPa}. Find the throat and exit areas for a mass flow rate of 0.34 kg/s0.34 \mathrm{~kg} / \mathrm{s}, assuming the nozzle is isentropic. Why must this nozzle be convergingdiverging?

Solution

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The stagnation temperature of helium at inlet is:

T01=T1+V122cp\begin{equation*} T_{01}=T_1+\frac{V_1^2}{2c_p} \end{equation*}

where T1T_1 is static temperature at inlet, V1V_1 is helium velocity at inlet and cpc_p is constant-pressure heat capacity of the helium.

If the velocity at inlet is negligible (V1=0V_1=0), then we get:

T01=T1=500  K\begin{equation*} T_{01}=T_1=500\;\text{K} \end{equation*}

The stagnation pressure at inlet is given by:

P01=P1(T01T1)k/(k1)\begin{equation*} P_{01}=P_1\bigg(\frac{T_{01}}{T_1}\bigg)^ {k/(k-1)} \end{equation*}

With P1=0.8  MPaP_1=0.8\;\text{MPa}, T01=T1=500  KT_{01}=T_1=500\;\text{K} and k=1.667k=1.667, it is obtained:

P01=0.8  MPa(500  K500  K)1.667/(1.6671)=0.8  MPa\begin{equation*} P_{01}=0.8\;\text{MPa}\cdot\bigg(\frac{500\;\text{K}}{500\;\text{K}}\bigg)^{1.667/(1.667-1)}=0.8\;\text{MPa} \end{equation*}

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