Thermodynamics Review Problems for Mechanical Engineering Students | Series 5

josephace135 (69)in #steempress • 8 years ago

Hi folks!

This is the 5^th series of my "Thermodynamics Review Problems for Mechanical Engineering Students". This series features review problems for the thermodynamic process - isobaric process for which it is known as the constant pressure process. This time around the review problems are a bit easy and requires less maneuvers or simplification of formulas which are usually dependent to what is provided in the problem. And once again, without further ado, let us now proceed to the most exciting part of this series - the computations. Lastly, don't forget to pull out your scientific calculators, as for me, again I've misplaced my CASIO FX-991 ES PLUS scientific calculator but I'm really thankful to Google's scientific calculator which has been helping me for the last 3 series.

Review Problem 1

Oxygen at 200 kilo-pascal and 27 degree Celsius is contained in a piston-cylinder device that is arranged to maintain a constant pressure. How much work in kilojoule per kilogram (kJ/kg) is produced by this system when it is heated to 227 degree Celsius?

Solution

For this review problem the way to obtain the work that is being produced by the system is simply using the universal gas constant formula for which we need to express the formula for work as a function of temperature. You might be wondering why only temperature, well it is simply because from the formula of work which is W = P∆V, the volume (V) can be expressed as V = mRT/P, and thus W = P x mR∆T/P wherein pressure P gets cancelled out. And with that, for the gas constant of oxygen gas is equal to R = 0.25981 kilojoule per kilogram per degree Kelvin, which is also used in Series 2 – Review Problem 1. Lastly, the work that is being done by the system is equal to 52 kilojoule per kilogram.

Review Problem 2

Air at 1 mega-pascal and 27 degree Celsius is contained in a piston-cylinder device that is arranged to maintain a constant pressure. How much heat in kilojoule per kilogram (kJ/kg) is required to raise the temperature of this air to 527 degree Celsius?

Solution

For this review problem the way to obtaining the heat Q that is being produced when raising the temperature of an air having an initial temperature of 27 degree Celsius to 527 degree Celsius is just by remembering that the heat added/rejected in a system that is undergoing an isobaric or constant pressure process is always equal to the change in enthalpy (∆H). For the change in enthalpy, it is expressed as a function of the specific heat capacity at constant pressure (c_p) of the substance/medium that is used by the system, which is mathematically expressed in this formula, ∆H = mc_p∆T = Q. And with that we found out that the heat that is required by the system that experience a 500 degree Celsius temperature rise is equal to Q = 500 kilojoule per kilogram, and its just numerically equal to the temperature change since the specific heat capacity of air at constant pressure is equal to c_p = 1.00 kilojoule per kilogram per degree Kelvin.

Curriculum

_{Computations and screenshots are made by the author.}
_{Special thanks to @jbeguna04 for designing the GIF photos.}

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