Power Engineering Test 1

Result

Power Engineering Test 1

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Weekly Quiz Competition

Question 1
1 / -0

The minimum and maximum temperature in a cycle are 280 K and 1080 K respectively. The pressure ratio is 6. If the medium is Helium, thermal efficiency of ideal Brayton cycle with regeneration is____%

A
46-48

B
464-481

C
467-486

D
469-482

Solution

Concept:
Helium is monoatomic gas,
\(\begin{array}{l}\therefore \gamma = \frac{5}{3}\\{\eta _{th}} = 1 - \frac{{{T_{min}}}}{{{T_{max}}}}{\left( {{r_p}} \right)^{\frac{{\gamma - 1}}{\gamma }}}\end{array}\)
Calculation:
Given:
T_min = 280 K, T_max = 1080 K, r_p = 6, \(\gamma = \frac{5}{3}\)
\({\eta _{nth}} = 1 - \frac{{280}}{{1080}}{\left( 6 \right)^{\left[ {\frac{{\frac{5}{3} - 1}}{{\frac{5}{3}}}} \right]}}\)
η_th = 0.4691 or 46.91%
Question 2
1 / -0

Volumetric efficiency of a reciprocating air compressor is lowered due to
i) Increase in pressure ratio
ii) Increase in clearance volume
iii) Increase in speed
Select the correct statements.

A
i only

B
i & ii only

C
i & iii only

D
All are true

Solution

Concept:
\({\rm{Volumetric\;efficiency}}\left( {{{\rm{\eta }}_{{\rm{vol}}}}} \right) = 1 + {\rm{c}} - {\rm{c}}{\left( {\frac{{{P_2}}}{{{P_1}}}} \right)^{\frac{1}{n}}}\)
i) If pressure ratio increases, then
\(C{\left( {\frac{{{P_2}}}{{{P_1}}}} \right)^{\frac{1}{n}}}{\rm{increases}}\)
Hence, η_vol decreases
\({\rm{ii}})\:C = \frac{{{V_c}}}{{{V_s}}}\)
If V_c increases, then C increases and η_vol decreases
iii) With increase in speed, the process of sucking air inside gets lesser time.
Hence, there is a decreases in mass of air induced and hence η_vol decreases.