Thermodynamics: An Engineering Approach 6th Edition,Yunus A. Cengel, Michael A.

Question

Thermodynamics: An Engineering Approach 6th Edition,Yunus A. Cengel, Michael A. Boles, problem 4-39;
An insulated piston-cylinder device contains 5 L of saturated liquid water at a constant pressure of 175 kPa. Water is stirred by a paddle wheel while a current of 8 A flows for 45 minutes through a resistor placed in the water. If one-half of the liquid is evaporated during this constant-pressure process and the paddle-wheel work amounts to 400 kJ, determine the voltage of the source. Also, show the process on a P-v diagram with respect to saturation lines.
Answer: 224 V

Explanation / Answer

V = 5 L (saturated liquid) P = 175 kPa (constant) I = 8 A t = 45 min = 2700 s mf = mg W = 400 kJ initial mass, vf = V/m m = V/vf, vf = vf(175 kPa) mass of vapor and liquid m_liq = m_vap = 0.5m quality, x = m_vap / m x = m_vap/m enthalpy of final condition, h = hf + x*(hg-hf) heat transfer, ?U = Q - W, W = p?v ?H = Q Q = m*(h - hf) (from energy balance) Voltage, Q = VIt + W_paddle V = (Q - W_paddle) / It

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