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0.8 m3 of saturated liquid water at 200°C is expanded isothermally in a closed system until its quality is 61%. Determine the total work produced by this expansion in MJ. IMPORTANT INSTRUCTION: Put your answer in the text box up to two decimal places or as a whole number.Please note you don't need to write the unit for your answer. Just write the numerical value up to two decimal places.

Question

0.8 m3 of saturated liquid water at 200°C is expanded isothermally in a closed system until its quality is 61%. Determine the total work produced by this expansion in MJ. IMPORTANT INSTRUCTION: Put your answer in the text box up to two decimal places or as a whole number.Please note you don't need to write the unit for your answer. Just write the numerical value up to two decimal places.

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Solution

To solve this problem, we need to use the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. In this case, we are dealing with an isothermal process, which means the temperature is constant and thus the change in internal energy is zero. Therefore, the work done by the system is equal to the heat added to the system.

  1. First, we need to find the initial and final specific volumes (v) of the water. We can find these values from the steam tables for water at 200°C. The specific volume of saturated liquid water (v_f) at 200°C is approximately 0.001156 m3/kg, and the specific volume of saturated steam (v_g) at 200°C is approximately 0.8857 m3/kg.

  2. Next, we calculate the initial and final masses (m) of the water using the given volume and the specific volumes we just found. The initial mass (m_i) is the initial volume divided by the initial specific volume, and the final mass (m_f) is the final volume divided by the final specific volume. In this case, the initial and final volumes are the same (0.8 m3), so m_i = m_f = 0.8 m3 / 0.001156 m3/kg = 692.307 kg.

  3. Now we can calculate the quality (x) of the steam, which is the mass of the steam divided by the total mass. The final quality is given as 61%, so x_f = 0.61.

  4. The specific volume at any quality can be found using the equation v = v_f + x(v_g - v_f). Plugging in the values we found, the final specific volume (v_f) is 0.001156 m3/kg + 0.61(0.8857 m3/kg - 0.001156 m3/kg) = 0.541 m3/kg.

  5. Finally, we can calculate the work done by the system. The work (W) done in an isothermal process is given by the equation W = m(P_fv_f - P_iv_i), where P is the pressure. However, since this is a closed system and the process is isothermal, the initial and final pressures are the same, so the equation simplifies to W = m(P(v_f - v_i)). We don't have the values for the pressure, but we know that the work done is equal to the heat added to the system, so we can use the specific enthalpy values (h) from the steam tables instead. The specific enthalpy of saturated liquid water (h_f) at 200°C is approximately 908.79 kJ/kg, and the specific enthalpy of saturated steam (h_g) at 200°C is approximately 2799.5 kJ/kg. The specific enthalpy at any quality is given by the equation h = h_f + x(h_g - h_f), so the final specific enthalpy (h_f) is 908.79 kJ/kg + 0.61(2799.5 kJ/kg - 908.79 kJ/kg) = 2056.5 kJ/kg. The work done by the system is then W = m(h_f - h_i) = 692.307 kg * (2056.5 kJ/kg - 908.79 kJ/kg) = 793.5 MJ.

Therefore, the total work produced by this expansion is approximately 793.5 MJ.

This problem has been solved

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