First Law - Open Systems: Interactive Simulations

These simulations were prepared using Mathematica. Download the free Wolfram player, and then download the simulation CDF file (link given below or click on figure to download). Try to predict the behavior when a parameter changes before using a slider to change that parameter. Screencasts below explain how to use these simulations.

A gas flows through a pipe into an insulated tank (0.1 m3) that is initially at a lower pressure and at a different temperature. Click the “valve (open)” play button to start gas flow into the tank; the valve closes when the pressures equalize. The tank initially contains either an ideal gas (N2), and more N2 is added, or it contains a vapor-liquid mixture of water, and superheated steam is added. Sliders change the pipe pressure and temperature and the initial tank pressure. The initial mass of water in the tank is 1 kg and that does not change when the initial tank pressure is changed with the slider. When the tank contains water, it is initially at saturation temperature at the initial pressure. Use the “valve (reset)” button to go back to the initial conditions before the valve was opened.

Try to answer these questions before determining the answers with the simulation. We suggest that you write down the reasons for your answers.

  1. When the line pressure (P2) of superheated steam increases, does the final amount of water vapor in the tank increase or decrease? Why?
  2. If the feed pressure of steam in the pipe is 9 bar, what will be the final temperature in the tank?
  3. For N2 feed at 150°C to a tank that contains N2 at 200°C, will the temperature be higher or lower than 200°C?
  4. As the initial pressure of N2 in the tank increases (at the same initial temperature), will the final temperature in the tank increase or decrease? Why?

This simulation models the behavior of a well- insulated tank that initially contains 10 kg of liquid water at 40°C and water vapor in equilibrium. When the pump is turned on by clicking the play button, it removes water vapor so the water pressure is below its saturation pressure. As a result, liquid water evaporates in an attempt to reach equilibrium. The slider is used to select the mass of water to evaporate before the pump is turned off. The energy to evaporate water is obtained by cooling the remaining liquid water, and then freezing some of the liquid when the temperature reaches 0°C. The bar graph on the right shows the amounts of liquid and solid water in the tank and the amount of vapor that evaporated. In the tank, the remaining liquid is blue and the solid is green. The amounts of each phase are determined using unsteady-state mass and energy balances.

Try to answer this question before determining the answer with the simulation. We suggest that you write down the reason for your answer.

1. Once the water is at 0°C, when 0.1 kg of water evaporates, does more or less than 0.1 kg of ice form?