SpinOffs

The previous blog, Entropy Happens… Deal with It! ended with the statement, “That’s how Concepts NREC deals with the constraints of the Second Law of Thermodynamics when designing high-efficiency turbomachinery – we literally put entropy to work!” That blog not only lead to an increase in sales of bumper sticker sales emblazoned with “Working on it, STAY TUNED!”, it led to a request to explore this statement in a little more detail – particularly as the idea might apply to the analysis, design and manufacturing of turbomachinery.

If there’s one thing good about sitting in snarled traffic in Boston, it’s that you get to see some very original bumper stickers. The most recent bumper sticker I saw was probably the strangest one, no doubt created by some engineering professor who doesn’t see that the glass is half full, but that it has a safety factor of 2! Nevertheless, the bumper sticker stated the obvious when one thinks about it: ENTROPY HAPPENS! And then, to emphasize the point, the artist has the letters slowly “evaporating,” demonstrating graphically that entropy proceeds from order to chaos.

When discussing the efficiency of transforming one form of energy to another, circularity is the way to go. Anyone who has spent even a little time studying engineering thermodynamics knows that the continuous transformation of energy from a heat energy source to produce mechanical or electrical power must contend with components that operate in a cycle. The key word here being “continuous”. The combustion of any carbon-hydrogen bond material (i.e., fossil fuels), or the liberation of heat energy from any number of materials when placed in a piston-cylinder, would not be very useful if the piston is not returned to its initial “precombustion” position. It is literally the difference between the one-time launching of an object from the cylinder or the continuous production of rotary shaft power; power that can be used to propel a vehicle forward or turn an electric generator. It is the cyclic operation of the fluid in the thermodynamic cycle that enables heat engines and refrigeration cycles to provide continuous power, or cooling, that is needed for the safety, security, comfort and all the other “hierarchy of needs” that was so well formulated by the renowned humanist psychologist, Dr. Abraham Maslow.

In this blog series, I covered a lot of thermo-fluid options in engineering analysis, from the simplest perfect gas (When Perfect is Good Enough – Perfect Gas Models) and ideal liquid, (Fluid Modeling: Liquified ) to much more complex approaches (Going Through a Phase – Modeling Phase Change with Cubics) and (Getting Real – Advanced Real Gas Models). In this blog, I’ll cover the ultimate in thermo-fluid modeling: non-equilibrium modeling. It's rare and expensive, sort of like the Schorschbrau’s Schorschbock 57, a beer that sells for $275/bottle.

When fluids undergo a phase change (see Phase Change - Make Mine a Double), it typically has a very significant effect of the flow behavior and energy level of the system.  Some examples of this are: cavitation in a pump, condensing near the exit of a steam turbine, even the everyday phenomenon of the weather is basically a never-ending phase change process of water, and its interaction with air. 

It sounds like an opening to a joke, but I wanted to share some creative thinking sparked during a field trip to a local gas turbine cogeneration plant with some undergraduate students I teach. The students were learning about basic thermodynamics, and visiting a power plant was a good way to give them perspective of what a kW is, and how much hardware and floor space is needed to generate 21,000 of them. I scheduled the trip for when the Gas Turbine system was “down for routine maintenance”, so the students could really crawl in and around the “beast” and be able to hear the explanations of the tour guide. 

In a previous blog, Fluid Phenomena Primer: Energy Versus Temperature, Specific Heat, I explained the behavior for gas phase fluids and how the temperature is affected at high energy levels.  In another blog, When Perfect is Good Enough - Perfect Gas Models, we looked at the simple perfect gas model.  In this blog, we’ll explore the next step up in the hierarchy of gas thermodynamic modeling: semi-perfect gas.

As I’ve always said, there’s as much thermodynamics in a glass of beer, as there is in a power plant. Don't believe me? Read on. Phase change is common phenomena that we see all the time.  We’re most familiar with H₂O, of course, in its various forms: ice, water, and steam. This is partly because it’s a very common substance (on Earth anyway) but also because it’s one of the rare fluid types that readily changes phase at temperatures and pressures humans can typically dwell in. 

Back to Beer...

What is a perfect gas?

A perfect gas is one that has a linear variation in energy with respect to temperature and a linear variation in pressure with respect to temperature at constant volume. The perfect gas model is the simplest of all models for gas phase fluids. For a perfect gas, we need only two unique properties of the substance to determine the relationships between pressure, density, energy, and temperature. 

As one might expect, the temperature of a substance typically increases as energy is added to it. This is the case with most substances in all phases. The exception is when a substance crosses to a different phase, which usually involves no temperature change. The energy difference between these phases is called the “energy of formation”.