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dudleybenton

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  1. We need more details to address this question. Do you mean an air powered siphon? Do you mean air bubbles rising in a vertical pipe producing an upward movement of the surrounding water? A picture of the device would help.
  2. Two months before my third birthday I changed all the light bulbs on the Christmas tree to be red yellow green red yellow green... I dismantled the radio at age seven and the television at eight. These were the old kind with vacuum tubes and did not survive the process. I rebuilt my first engine at eleven and was working as a professional mechanic by fourteen—before I could drive a car. I have a grandson who is three and destined for the same path. My father didn't know which end of a screwdriver to hold. The engineer gene came from my maternal grandfather.
  3. Fluids above the critical temperature are called "super critical." We sometimes call a liquid above the critical pressure "super critical." For example, in most large coal-fired steam power plants, the feedwater entering the boiler is about 4400 psia (30 MPa). The critical point is where the saturated liquid and vapor are indistinguishable; that is, a distinction is physically meaningless. There are no bubbles formed when boiling a liquid above the critical pressure. This is why you must have special equipment to clean (i.e., "polish") the feedwater in a supercritical coal-fired plant, as the boiling process doesn't naturally separate out impurities and they build up over time. The pressure at the bottom of the sea is above critical. There are "vents" in sea floor where the water is spewing out at near 700ºF (371ºC), yet there are no bubbles. You can find videos of these sea floor vents online. Refrigerants have a much lower critical temperature and pressure. You can also find online old black-and-white videos of a supercritical refrigerant flowing in a clear pipe undergoing a transition from "liquid" to "vapor" but you can't see any difference or tell where it happens, which is completely different from what this process looks like at subcritical pressures. Interesting things happen around the critical point, which is why we study this region and some designs focus on this. For example, there is considerable current interest in supercritical CO2 systems.
  4. The seawater probably flowed on the inside of the 90/10 Cu-Ni tubes at about 7 ft/sec (2 m/s), so multiply nD²π/4 to get volumetric flow then by density to get mass flow. You get surface area the same way. The overall heat transfer coefficient, U, is probably about 5 BTU/hr/ft²/ºF (25 W/m²/ºC). The delta-T is probably about 15ºF to 25ºF (8ºC to 14ºC). The specific heat of natural gas is about 0.5 BTU/lbm/ºF (2 kJ/kg/ºC). From that you can calculate the flow rate of natural gas and estimate the heat transfer capacity.
  5. There are several excellent texts on this subject. I always recommend Lindon Thomas' Heat Transfer because he's been a friend for many years. This is a large topic. To get a meaningful answer, you must further qualify your question. What type of heat exchanger? What do you expect it to do? What fluids? What temperatures? What flow rates? Heat exchangers are used for everything from natural gas to peanut butter.
  6. You will find many projects on ResearchGate where graduate students are investigating heat transfer within enclosures with a variety of conditions and fluids. You should be able to find something informative to your particular problem there. The complexity of the solutions varies considerably, so look for something on the level you're interested in pursuing.
  7. While pressure is clearly absolute (i.e., zero has a clear definition), temperature is most often with respect to some state (e.g., minimal crystalline structure). We can readily measure conditions and calibrate pressure instruments on an absolute basis. We don't have the same flexibility with temperature and so we recognize this in our measurements, calculations, and formulations. Also, heat is defined as that form of transient energy that crosses a system boundary by virtue of a temperature *difference*, not an *absolute* temperature.
  8. Hi there. Im an undergrad working on a supercritical carbon dioxide simple brayton cycle. Are there methods to calculate the efficiency of the compressor and turbine based on their characteristics.

  9. We know theoretically how pressure and density must interact at the critical point. Specifically, the first two partials (dP/drho and d²P/drho²) must be zero at the critical point for it to exist. This is mathematically equivalent to having three roots at the critical point. Simple equations of state (e.g., van der Waals) can reproduce this behavior, but may not fit data well anywhere else. In fact, the critical compressibility for a van der Waals fluid is Zc=3/8, which is larger than any known substance. The liquid densities are also off by a factor of 1/3 to 1/2 for the van der Waals EoS, as are the saturation pressures. What this means is that, in order to be accurate in all of these details, quite complex approximations are necessary. For example, see the properties of steam (1967, 1969, 1984, 1995, 1997, and 2020). This can require a lot of work, a lot of data, and a lot of software. Bringing all this together for any single substance is difficult. Steam is worth the effort because of the many industrial applications. It may not be worth the effort for other less-used fluids. You can get good and accurate properties for many fluids from NIST with their REFPROP software. If they don't have what you need, I'd be glad to pull the properties together for you. I've done it so many times, it isn't hard for me and I have all the software ready to go. I recently did this for NOVEC-649, which has the distinction of the lowest greenhouse potential of any refrigerant. Was there some particular fluid you needed or were you interested theoretically? If your interest is theoretical, steam is an excellent example. For example, there's lots of data, but most of it is clustered. This naturally arises from the experimental apparatus used to acquire the data.
  10. I cover this at some length in my book, "Thermodynamic and Transport Properties of Fluids." The Ebook is free on these days: 4/16,4/24,5/2 at this link https://www.amazon.com/dp/B07Q5L1CHT The software is always free at this link http://dudleybenton.altervista.org/software/index.html and one of the Excel AddIns (also free) may be helpful.
  11. You misunderstand the principle. Whether some working fluid is hot or cold compared to what humans consider normal atmospheric conditions isn't what determines whether or not a device is efficient. When I taught thermodynamics at university, I would always give a test question regarding a cup of coffee cooling to room temperature or a can of beer warming to room temperature. Both generate entropy (dS>0). I work out this example in my book, Thermodynamics. There are several reasons we build power plants using steam instead of air as the working fluid. The latent heat is a very important part of this. Water and ammonia have two of the largest latent heats of any substance known. This is why ammonia was used in refrigeration for decades, even into the 1990s and beyond at some skating rinks and commercial facilities. Another reason is the HUGE difference in specific volume (V=1/density). For a flowing device, dW=VdP. The work required to pump liquid water up to 200 or more atmospheres is nothing compared to what it would cost to pump gaseous air up to that same pressure because of the difference in V. Expansion works in our favor too. The specific volume of steam at atmospheric temperature is significantly larger than air (compare molecular weights of 18 vs 29 and the ideal gas constant R/MW). With a large V, we get more power out of expanding steam than air.
  12. Some of these items are more important than others. The basic design of heat exchangers is driven by the application. Consider a fuel gas heater and an oil cooler on a typical stationary gas turbine, such as you might find at a combined cycle power plant. You will not likely ever need to clean the fuel gas heater. The operating pressure will exceed 40 atm. You wouldn't use a plate design. The oil cooler might need to be cleaned several times per year. A plate design held together with threaded rods is often used. It's easy to dismantle and clean. The working pressure isn't too high, it doesn't matter if it leaks a bit (the oil isn't going to explode), and it can stay out in the weather. Consider a feed water heater in a supercritical coal-fired power plant. It has a working pressure of 350 atm and is completely welded together. You would have quite a time cutting it apart with an acetylene torch. It matters if the fluids are clean, dirty, or corrosive. You don't use expensive alloys unless it's necessary. If you inspect any industrial plant or manufacturing facility, you will see a variety of heat exchangers. These diverse solutions illustrate human creativity and ingenuity. We're always looking for a better way to solve problems.
  13. I have always stressed both theory and practice. It has served me well throughout my long career. I've literally crawled in and out of power plants, paper mills, and various industrial facilities all over the world. I can solve a differential equation while replacing a CV joint. For inspiration and real-life examples, please read my book entitled, "Living Math," available on Amazon http://www.amazon.com/dp/B01LXZYLVX The eBook will be free on March 17 and 25, then April 4, 10, and 18.
  14. The answers you get to this question may not be what you're looking for. "Extraction" has a definite meaning in the context of steam turbines. In a Rankine cycle with regenerative heating, steam is bled off the turbine at various stages and directed to feedwater heaters, which raise the temperature of the compressed liquid upward toward the inlet of the boiler, as with an economizer. This actually decreases the power output of the steam turbine, but it increases the efficiency of the overall process. This is often illustrated in textbooks, showing that the regenerative Rankine cycle is more rectangular (closer to Carnot in shape) and generates less entropy for the same work output; thus, the increase in efficiency. You must distinguish between extracting steam (i.e., a mass flow rate) and extracting power (i.e., an energy flow rate). I suspect you mean to ask, "is is possible to get more power out of a steam turbine? (with the same input)" While the answer to this question is technically, "yes," that doesn't mean it is practical to do so or that anyone has figured out how. I have been asked many times why we don't just build more efficient machines or just increase the efficiency to 100% so that there will be no waste heat? Many smart and resourceful people have been trying to do just that for a very long time. The big bad oil companies don't have a secret carburetor that would get 500 miles per gallon locked away in a safe in Switzerland so that they can gouge motorists at the pump. Humanity has accomplished many remarkable things, which is good reason to keep on trying to do better. Before we can do better, we must understand what has already been done--then try to improve upon it. This has always been the challenge for the next generation. The expression "standing on the shoulders of giants" is applicable in this case. You need to read Ken Cotton's book, "Evaluating and Improving the Performance of Steam Turbines." It's expensive, but well worth it. Ken Cotton was very influential in the development of the modern steam turbine and one such giant.
  15. You are quite right. The true uncertainty of the process you're describing is much more complicated than it may seem from reading the literature. I have a particular interest in this subject, work for a company that is intimately concerned with such things, and am currently collaborating with a group retired professors to adequately address this gap in the literature. Not only does the sensor itself present multiple uncertainties, at the very least both random and systematic, but the sampling process also contains uncertainty. So does the analog-to-digital conversion. Thermocouples have greater uncertainty than platinum RTDs, due to consistency and sampling. It is very difficult to accurately measure small DC voltages. I once set up a test to prove this to a colleague, using everything from a cheap analog multimeter from Radio Shack to a digital one costing many thousands of dollars--all connected to a single nominal 1.5V C size Duracell battery. There is also the matter of how long do you sample a moving target? Most systems of practical interest vary over time. There are far too many people who think that, if you sample long enough, you will know a quantity with certainty; but this is not the case. There are also far too many people who think you're supposed to divide the uncertainty or the standard deviation by sqrt(n), which magically makes all the uncertainty vanish. This is just ignorance and wishful thinking. There are glaring errors in prestigious test codes, including ASME PTC-19.1 (Test Uncertainty) and even ISO JCGM 100 (Evaluation of measurement data--Guide to the expression of uncertainty in measurement), which can be demonstrated with actual data and also Monte Carlo simulations. In recent years, NIST has begun using different words, including "repeatability," which means, "We keep getting the same number, but we have no idea if it's right." History is littered with examples of people who were absolutely sure beyond a shadow of a doubt that they were obtaining accurate measurements of something we now know doesn't exist or isn't what they thought it was. I encourage you to diligently pursue this matter and consider all of the complicating circumstances you can think of in doing so. I don't mean to be overly negative. Just because we may not know something to the level of precision that we might like, doesn't mean that what we do know is worthless or that attention to detail is without reward.
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