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03 November 2006
Metastatic Systems: Thermodynamic Economies Follow-on.
A better name for this post would probably be ‘homeostatic systems.’ The only problem is that you can’t associate the word ‘homo’ or any of its derivatives with airplanes. Really. It’s a pilot thing. We’re juvenile, I know. I plan on growing up after I’m senile. That way, I’ll keep forgetting that I was supposed to grow up at some point, and I‘ll never have to. Anyways, here’s the post. Its boring… consider yourself warned.
This is a continuation of the Thermodynamic Economies post, in a series of three posts. They work from theoretical to practical, beginning with systems engineering methodologies and ending with a single-fluid aircraft. One of the implications of the first post was that passive energy reclamation would create a more efficient overall system. Building reclamation systems as a finishing touch on an extant design would result in some savings, but tying together multiple systems from the outset would yield a far more efficient overall result. Fusing systems ensures that the summed absolute sub-system heat flow is closer to net thermodynamic flows, hence the system as a whole would be more elegant and economical. In order to do so, the now-fused systems would need some means of regulating themselves to an equilibrium. Let’s steal a page from God (and biology.)
Stealing From Biology. It’s cold outside. You start shivering. Your body burns energy to produce heat, bringing your core temperature back up to where it’s supposed to be. It’s hot outside. You start sweating. Your body burns energy to dissipate heat, bringing your core temperature back down to where it’s supposed to be. Both of these are active heat pumping systems, something along the lines of the Carnot cycle. Nothing cosmic here. We do this in human engineered systems all the time. The oil temperature is high, so the oil cooler flaps open wider, increasing drag. We burn energy to pump out heat. We need anti-ice on the leading edges of the wings, so we steal late stage compressor bleed air to heat it. We burn energy to make heat. We shiver and we sweat. At the same time. And here is the problem.
Bodies generally don’t shiver and sweat at the same time (unless you‘re snowboarding.) In fact, most of the time bodies are neither shivering nor sweating. Usually, our bodies are well able to keep core temperature near nominals merely by varying the rate at which the heat produced through normal processes is lost to the environment. We open or close pores, allowing more or less heat to escape into the environment. There will be times where energy production is too high or the environment is either too hot to allow heat to escape or too cold to allow heat to be effectively retained. We’ll call this surge capacity. During surge states, the body must resort to active means of pumping heat. This active pumping only serves to pick up the slack that the passive systems are unable to provide. This results in a much more efficient system that is still able to deal with a variety of environments.
System Basics. In order to appropriate this design, we must understand two things: energy flows and system architecture. Energy flows will allow the system to interact with the outside world; System architecture will allow the system to interact with itself. So quick thermo lesson. Exothermic systems give off heat to their environment. Endothermic systems take in heat from their environment. Set a fire, it’s exothermic (once you get past the activation energy thing.) Melt an ice cube, its endothermic. Because I’m lazy and I don’t feel like using the ‘cut and paste’ function of my word processor, I’ll call these ‘down’ and ‘up’ respectively. A ‘down’ transfer will reduce system temperature by moving heat to the outside. (We’re assuming the outside world is a giant heat sink.) A ‘up’ transfer will do the same. No, just kidding. It will do the opposite, of course.
So the basics of our system. In the course of normal ops, our system creates a degree of heat. In burning food, the body’s core temperature goes pretty high. In burning gas, an engine’s core temperature gets pretty hot. Same deal. At thirty grand, the air is pretty cold, and so is an aircraft’s skin. In normal weather, skin is touching air much cooler than 98.6, hence the skin is going to be the coolest part of the body. All the stuff in between is going to be somewhere between the core temperature and the skin temperature.
Energy Flows. The core of our system is the passive up and down transfer mechanisms. Assuming that during normal ops, heat is being created by the system, we can passively regulate the temperature of the system by changing how much of that heat is dissipated by the system. Using our biological example, closing pores is a passive up, opening them is a passive down. We can also increase or decrease how much of the ’normal ops’ heat is incorporated into the system, regulating both the inflow and the outflow of heat. For example, if we put a heat exchanger next to a propulsion system to capture additional heat, as well as an outflow heat exchanger open to the outside world, we could regulate how much fluid went through those heat exchangers and hence control system temperature using only ambient energy.
Such a system would rely on a large number of bypasses to increase or decrease fluid flow through the heat exchanger. In fact, these bypasses could turn many things into heat exchangers, for instance an aircraft’s wing. Run a bunch of lines in parallel across the wing’s surface area, starting at the leading edge and moving back. When you need more cooling, open up more lines. This increases the volume of the fluid required in the system, requiring either surplus capacity on the primary fluid pump or reserve fluid in a variable-sized accumulator (or both.) This parallel construction would allow an inherent anti-hemorrhage capability. If the bypass valve detected a massive uncommanded pressure drop in the line, it would close itself (and the other valve connected to the parallel circuit,) hence isolating the leak. The system would then automatically reroute itself through the remaining lines due to pressure gradients.
For surge capacity, such as running for a person or takeoff for an aircraft, we will need to augment our passive systems with active heat pumps. Most likely, we will need an active up and an active down. For the person, the active up is shivering, while the active down is sweating. For our system, some sort of a heater would be our active up, and some sort of refrigerator would be our active down. These supplemental systems sustain us until we can get back into normal ops conditions.
It is critical to understand that surge systems are for transient conditions. During takeoff, the ambient air temperature is not low enough to be an effective heat sink, and the engines are at max power. While running outside in 100 degree weather, ambient air which would normally be a heat sink actually begins to transfer heat into the body. Surge capacity is for times like this. If you need more cooling or heating during normal ops, you should increase the size of the passive system, rather then relying on the active system. Similarly, it is inefficient to design a passive system to accommodate transient conditions: you will have a lot of extra weight for capacity that you’re not going to use much. The active system will typically cost far more energy to move heat, although it will have a much higher capacity. This is similar to an afterburner, but remember that you don’t use ‘speed of heat’ for stable conditions (or you‘d be bingo fuel in 10 minutes.)
For an overall view, imagine a line. Here, Ill draw one.
---------------------------------------------------------------------------------------------------------------------
Unsustainable | Lo Surge Rng| Normal Governing Range |Hi Surge Rng| Unsustainable
----(Active up engaged)---- -----(Passive transfers only)----- ----(Active down engaged)----
[Sorry... looks better on Microsoft Word.]
System Architecture. Energy flows give us the means by which our system can seek an equilibrium with the outside world. External flows are only one half of the puzzle: we also need to figure out internal thermodynamic flows. We have to tie our system together thermodynamically. In order to do this, we need an internal heat transfer mechanism, and a partitioning of internal temperature zones.
Thermo gives us three options for heat transfer: conduction, convection and radiation. Conduction moves heat the fastest, and generally involves the least amount of losses. Guess what… we’re going to plagiarize God again here. Blood. The human body’s primary internal heat sink is blood. Blood moves heat from the core to the extremities, and ties the body together as one thermodynamic entity. So we’ll use fluid as our primary internal thermodynamic medium. Blood doesn’t just sit there and wait for conduction to take its course, it speeds up the process by being pumped through the body. The blood that was just next to the bodies core is taking that heat to the body’s extremities due to its motion from the heart. So our fluid must also be pumped. Accordingly, as a heat sink, we will use a relatively high volume of low pressure fluid. This will also make our design more survivable and maintainable.
The core temperature will always be higher than the temperature of the extremities, even if we increase the amount of fluid and the flow rate. Our design must reflect this. Now that we have tied the system together, we must decide how to partition the internal thermodynamic zones of the system. There are two basic options: continuous or discrete. Continuous designs, where temperatures move gradually down as you move away from the core, are very efficient and elegant, but they require very advanced adaptive processes, such as those found in biology or Nanotechnology (which are really the same thing.) So since we’re not cool enough to make a continuous system, we’re stuck with a discrete system with a number of zones of set temperature.
Two is the minimum number of zones for our discrete system: one hot and one cold. Three is a better design, though. You have one hot zone, which is the exothermic reservoir or ‘hot sink’ for the system. This hot zone can increase its temperature through passive means, capturing more of the heat from the core area, or through active means with an incorporated heat pump. It decreases its temperature by transferring fluid to the cold zone. The cold zone, the endothermic reservoir or ‘cold sink’ of the system, does the opposite. It decreases its temperature passively by opening more of itself to the outside and hence dumping heat, or actively through an incorporated heat pump. (You could make both heat pumps the same pump if you were really slick. I don’t know how to do that, so I won’t try.) It increases its heat by siphoning off more fluid from the hot zone. The mixed zone regulates its temperature with fluid from the hot and the cold zones, and completes the circuit by sending back the now warm fluid to the two zones in the proportion it came in at, thereby maintaining constant system pressure. Just as in the passive transfer systems, we rely on bypasses to regulate temperature.
We can increase the number of zones if we require different temperatures in different places (avionics at one temperature, passengers at another, etc.) Note, though, as we increase the number of zones past a certain point, we need to be more creative with the routing. It starts to look more like a continuous system. Note also that the fused system actually simplifies our design process. Instead of thermodynamically engineering every single subsystem, we only measure that subsystem’s thermodynamic impact on the total system during different phases of flight. We then adjust pump flow rate, total fluid capacity, passive transfer and active transfer capacity accordingly (and zoning if required.)
Example: ExecJet. This is what the whole thing was really about, in case you haven’t guessed it. Airplanes. Tragically, not Herks. ExecJets typically have more than enough runway, so takeoff performance is not quite as critical as with other aircraft. Also, they typically cruise straight and level at thirty grand, where the air is cold (and a great heat sink.) They don’t do anything really cool or exciting (sorry C-21 guys,) so they make a pretty good test case.
We’ll start with system architecture. We’ll fuse the endothermic leading edge anti-ice system, the exothermic engine oil system, and the variable environmental control system (excluding the pressurization function.) We’ll use oil as a medium, as we require the viscosity in order for it to work in the engine, and oil works as an effective heat sink. Neither of the other two systems have any viscosity requirements, only heat capacity requirements. We’ll run the fluid through a high-capacity, low-pressure pump attached to the engine shaft. (Next post, we’ll describe the specifics of that pump along with many other things.)
Now that we have a means of fusing the system, let’s define discrete thermal zones. There will be three: engine section, wing section and cabin section. The engine section will incorporate the active and passive up systems, and will also lubricate the engine. This lubrication process generates a significant amount of heat, and forms our core section of our system. This system will be regulated to 200 deg F (subject to change.) The wing section will incorporate the active and passive down systems. The leading edge needs to be heated to at least 32 deg F, so this system will be regulated to 50 deg F. The cabin section is built to meet the passenger’s comfort needs, and is regulated to approx. 70 deg F (variable.) This zoning follows the three zone discrete architecture previously discussed.
The passive down system incorporated into the wing system is composed of multiple fluid lines running in parallel from the leading edge, working back along the wing lengthwise. Bypasses govern the amount of fluid flowing through the virtual heat exchanger. The active down is a traditional ram-air oil cooler, possibly augmented by Venturis (an air conditioner run off the engine would supplement cooling requirements during ground ops.) The passive up system incorporated into the engine system is comprised of multiple parallel coils running around the turbine’s exhaust. If inadequate heat is being drawn from the lubrication process, oil will be routed through these tubes, costing some power but far less than bleed air currently does. The active up system is an electrical resistance heater.
Concept of operations is that the air conditioner runs during hot temperature ground ops. The cooling system is at surge capacity during takeoff, until the aircraft gets to altitude, and active systems are no longer required. The aircraft cruises at altitude, using ambient air and engine heat to regulate its temperature. Then the aircraft returns for landing, retaining surge capacity for a go-around.
Next time, we’ll go all the way with our plagiarism of the circulatory system. We’ll try to get rid of the pesky bleed air system, and create a more efficient, more survivable, and simpler aircraft: a single-fluid system. (Well, sort of two-fluid system. We’d have to use organic chemistry to get to a true single-fluid system. I’m not smart enough for that.) The more we look like nature, the better off we are. After all, if we could design an aircraft to the specs of a cockroach, that thing would never die (of course, the passengers and crew might turn to mush well beforehand. So you still can‘t crash unless you can undo the laws of deceleration. You need a pretty big crumple zone for 500 knots.) Anyways, see ya next time.
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