THERMO REV QUIZ
SAMPLE KNEC QUIZ AND SOLUTION FOR THERMODYNAMICS In a steam engine the steam at the beginning of the expansion process is at 7 bar, dryness fraction 0.98 and expansion follows the law � � 1.1 p v 1 . 1 = constant, down to a pressure of 0.34 bar. Calculate per kg of steam : (i) The work done during expansion ; (ii) The heat flow to or from the cylinder walls during the expansion SOLUTION Initial pressure of steam, � 1 = 7 p 1 = 7 bar = 7 × 1 0 5 � / � 2 = 7 × 1 0 5 N / m 2 Dryness fraction, � 1 = 0.98 x 1 = 0 . 9 8 Law of expansion, � � 1.1 p v 1 . 1 = constant Final pressure of steam, � 2 = 0.34 p 2 = 0 . 3 4 bar = 0.34 × 1 0 5 � / � 2 0 . 3 4 × 1 0 5 N / m 2 . At 7 bar : � � = 0.273 � 3 / � � v g = 0 . 2 7 3 m 3 / k g ∴ ∴ � 1 = � 1 � � = 0.98 × 0.273 = 0.267 � 3 / � � v 1 = x 1 v g = 0 . 9 8 × 0 . 2 7 3 = 0 . 2 6 7 m 3 / k g Also, � 1 � 1 � = � 2 � 2 � p 1 v 1 n = p 2 v 2 n i.e., � 2 � 1 = ( � 1 �