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RADIO CONTROL BY BOB YOUNG Jet engines in model aircraft; Pt.3 Last month we looked at the input of air to the jet engine via the compressor and we established that the radial or centrifugal compressor was the best choice for model jet engines. This month we look at the diffuser, combustion chamber and tur­bine. Having accelerated the air to the outside edge of the com­ p ressor wheel, it is now time to collect the air, achieve the full measure of com­ pression possible and remove as much tur­bulence in preparation for the entry of the air to the combustion chamber. To do this we use a diffuser (stator or collector ring). Here we are faced with some difficult choices. Essentially, diffusers may be divid­ ed into two categories, bladeless and bladed diffusers. Professional engine men refer to the bladeless diffuser as a bladeless annular space. They are easy to man­ ufacture and can be quite efficient if designed correctly. As there are no blades there is no flow breakaway. The main advantage of the bladeless diffuser is that the compressor as a whole has excellent self-regulatory characteristics. The disadvantages are that they must be of a larger diamet­er than the bladed diffuser and that they cannot smooth out the twisting motion im­ parted to the airflow by the compres­ sor. As the overall diameter of the finished engine is very important to the modeller, the increased diameter is a serious drawback. Therefore, the best solution for a model turbine is the bladed diffuser with the blades set back from the com­ pressor, leaving a clear annular space where the airspeeds are the highest 84  Silicon Chip and most unevenly distributed. The wedge-shaped diffuser blade can be useful for mounting the diffuser to the outer casing. Screws can be fair­ ed into the wedge with a minimum disturbance to the airflow. The action whereby the air is slowed and compressed in the diffus­ er is a complex mix of subtle factors. If you are interested in model aircraft jet engines, this book by Thomas Kamps and entitled Model Jet Engines is a good one to have. It’s published by Traplet Publications UK (ISBN 0 9510589 9 1). The spiral law governing action of a fluid in a centrifugal compressor states that the product of the radius (r) in the diffuser system and the speed (cu) in the peripheral direction is constant (spiral law: r x cu = con­ stant). Thus, as the radius increases, the speed is re­duced. This basic law plays an important role in model jet engines and an interesting analog is found in the common tea cup. As we stir a cup of tea we speed up the centre of the mixture but at the edge the speed is the slowest. This causes an increase in the fluid pressure and the level of the fluid at the wall of the cup rises, leaving a dish or well at the centre; the faster the rotational speed, the deeper the well. According to Bernoulli’s theorem, the total energy in the flow must al­ ways remain constant. Therefore, if the speed de­creases as the air moves out, away from the compressor into the larger diameter diffuser, the pres­ sure goes up, as in the tea cup. So contrary to what one might expect, compression is due largely to the centrifugal force applied to the air leaving the compressor and moving outwards into a space of a larger di­ ameter, and not due to the change in volume between the compressor and the diffuser. True, this change in volume will also cause a slowing of the airflow with the subsequent increase in pres­ sure but not of the order required. In fact, if the size of the annular non-bladed duct is increased, there is a danger of the airflow break­ing up into turbulence with a severe loss of efficiency. For this reason, some bladeless diffusers have a cross sec­ tion which tapers to a more narrow section as the diameter increases, This close-up view shows the JPX-T-240 turbine engine fitted to Kevin Dodds’ (Tingalpa, Qld) A-10 “wart hog”. The maximum engine speed is 122,000rpm! Note the discoloration on the rear of the fuselage from the exhaust. forc­ing the air to move more quickly to the outer edge. Here we come across another dis­ advantage of the centrifugal or radial compressor. To get a worthwhile increase in diameter in the diffuser we need to have a diffuser of ap­ proximately twice the diameter of the compressor. Practical experience has estab­lished that this figure may be cribbed somewhat but a minimum ra­ tio is around 1.6 times the diameter of the compressor wheel if we introduce vanes or guides into the diffuser. Thus it is the diffuser that most contributes to the dumpy appearance of the model jet engine. With blades in the diffuser, we now have real cause for concern because the path of the air leaving the com­ pressor rim is a very complex func­ tion. It requires careful calculation to get the diffuser blades set at the correct angle so as to minimise flow breakaway on the guide vanes. What must be kept in mind with these engines are the very high air­ speeds involved. In an engine using a shaft speed of 100,000 rpm and a 66mm diameter compressor, the rim speed (Rs) of the compressor is given by the formula Rs = n x d x π/60, where n is the shaft speed and d is the diam­ eter. This works out to 345.5 metres per second or 1243.44km/h. But wait, I hear you cry, that is in excess of the speed of sound! Not so, for we are working with air at higher temperatures and pressures, so the speed of sound in the medium is much high­er. Even at rim speeds as high as 450m/s, the sound barrier can­ not be exceeded inside these engines. However these are phenomenally high airspeeds and if the diffuser blades are set incorrectly then there are serious rami­ fications. Unfortu­ nately these are the sorts of speeds required if the Reynolds numbers are to be moved up into a reasonably efficient range. Yet model jet engines can be throt­ tled down successfully to much lower speeds, such is the amount develop­ ment work that has been poured into this the most difficult of all modelling dreams. If any reader is interested in a full mathematical analysis of the model jet engine then there is a very good book on the sub­ject written by Thomas Kamps and entitled Model Jet Engines, available from Traplet Publications UK (ISBN 0 9510589 9 1). If we can slow the air by 50% in the diffuser, we will convert about 75% of the speed energy to pressure energy, as the energy in the gas is proportional to the square of its speed. Thus, re­ turning to our motor using a 66mm compressor at 100,000 rpm, we find that typical throughput of air will be about 1.35 - 1.75kg. As the thrust of the motor rises in proportion to the throughput, the higher figure is the more acceptable. At this point the compressor will deliver a compres­ sion ratio of about 1.9:1. Another important factor in the compressor/diffuser design is the expansion angle of the diffuser blades. The blades start off more close­ ly spaced and gradually move apart as they move out to the rim of the dif­ fuser. This divergence angle is known as the expansion angle and it plays a large part in the compression of the incom­ing air. Too shallow an angle will mean more losses as the air will stay in the duct longer and boundary layer losses will rise. Typical expansion angles are around 15 degrees which calls for 24 blades in the diffuser. Smaller angles will call for more blades and more friction losses. For this reason it is better to use blades which are curved forward slightly, forming gently wid­ ening ducts. This type of diffuser and a radial compressor with retro curved blades will result in an engine capable of rapid throttle response and which will be quite resistant to surging. The compressed and stabilised air now passes to the combus­tion cham­ ber and we haven’t even got to the March 1998  85 Table 1: Model Jet Engine Fuels Densi ty (kg/l ) H0u (MJ/kg) Boi l i ng Range (oC) Diesel Petrol JP1/Jet A JP4 Propane Methanol 0.85 0.76 0.804 0.76 0.5(1) 0.79 42.8 42.5 43.3 >42 46.3 19.5 190-334 80-130 160-260 60-240 -42 65 Fuel tank Capaci ty (ml ) 880 990 920 990 1380 (5 mi nutes, 30N thrust) Fl ammabi l i ty/Fi re Low H i gh Low H i gh Very Hi gh Hazard (1) Li qui d under pressure; (2) Suffi ci ent for 5 mi nutes of powered fl i ght at a thrust of 30 Newtons (speci fi c consumpti on = 0.3kg/N/h) 2080 H i gh Source: Model Jet Engi nes, by Thomas Kamps hard part yet. Is it any wonder that the model jet engine took so long to develop? The combustion chamber Single stage turbines and compres­ sors take up little space but not so the combustion chamber. This is why model jet engines do not look at all like their full-size cousins from the outside. Actually, the proportions are almost reversed. In the full-size motor, the combustion chamber is a short section between the compressor and turbine, whereas in the model engine the combus­tion chamber is the largest component. There are other difference between model and full-size turbines in terms of specific power. Model size com­ pressors and turbines are less efficient than industrial aircraft engines. If the engine is to run at all, the turbine must extract most of the available energy from the exhaust flow at the turbine. As a result, there is little left in the residual exhaust flow to produce thrust. For this reason, the shape of the tail-cone is vitally important; a correctly shaped tail-cone can increase the thrust dramatically. The low residual thrust combined with the low compression ratios avail­ able in the model engine means that only 3 - 8% of the energy contained in the fuel is turned into thrust. Howev­ er, due to the low mass of the model engine, thrust to mass ratios are much the same as full-size engines. The drawback in the model engine is fuel consumption. Modellers wishing to use a jet in their new model should leave plenty of space for the fuel tank. Table 1 shows the most common fuels suitable for use in model turbines. Early model jets used propane gas but there was some risk with this fuel. Theoretically, the jet engine is not re­ 86  Silicon Chip stricted to one type of fuel, the main requirement being that the maximum energy is released during combustion. In practice, most jets are designed to run on one of the many mineral oil products commer­c ially available. Alcohol fuels such as methanol are not suitable due to their low energy densities. These days, most model en­ gine manufacturers choose kerosene. The design of the combustion cham­ ber is critical. If this component falls short in any way, there are serious conse­quences, the most drastic being the destruction of the motor. If combustion is uneven, then the incoming air will not be heated to full temperature in parts of the combustion chamber. The enthalpy of this portion of the air rises only slightly and con­ sequently does little work on its way through the engine. Worse still, to compensate, the rest of the air must become that much hotter to keep the engine running. The result is uneven speed distribution in the turbine and lower overall efficiency. In the worst case, the engine will not run at all. The purpose of the combustion chamber is to heat the air in order that it can do more work when it is decom­ pressed than was required to heat it. If the air is heated during decompression then this effect is largely nullified. For this reason, combus­ tion must be contained inside the combustion chamber as much as possible. If the flames are too long, they will extend into the turbine area and the turbine will overheat. The clue for this prob­ lem is high exhaust gas temperature. Mixture considerations Stable combustion can only be achieved if a stoichiometric mixture is present. This is referred to as an air surplus of one. A mixture is said to be rich if the air surplus is less than one and lean if it is greater than one. A lean mixture can result in the flame being blown out if the throttle is closed suddenly because the compressor is still delivering a large quantity of air to a weak flame. If the mixture is too rich, the flame burns yellow due to glowing carbon particles. These cannot be fully burnt because the necessary oxygen is ab­ sent. The result is a layer of soot depos­ ited on the combustion chamber walls. When using kerosene or diesel, sto­ ichiometric combustion occurs with a fuel/air ratio of 14.7:1 and results in burn tem­peratures of about 2000°C, even in model engines. To reduce this temperature to the desirable 650850°C, cool air must be introduced by dividing the combustion chamber into two parts. These are called the primary and secondary zones. In full-size engines, high combus­ tion chamber temperatures (900°C) are a real problem and nickel based alloys (Nimonic or Inconel) are the usual solution. In a model engine, this heating is not so severe due to the low­ er compression ra­tios. This means the air temperature is not as high from the compressor and therefore the cooling effect is much greater. A neat trick is to drill small holes (1-1.5mm dia.) in the primary zone to introduce cooling air. This air forms a thin, cool boundary layer and protects the combustion chamber primary zone from the stoichiometric temperatures. As a result, V2A sheet steel is OK for the combustion chambers. On the other hand, it is desirable that the cooling air in the secondary zone penetrates deeper into the com­ bustion chamber and this calls for larger holes. The number, size and location of the holes has a large effect on the overall exhaust gas tempera­ture and temperature distribution and is a key factor in the engine design. The aim is to obtain perfectly even heating with as short a flame length as possible and with all fuel completely burnt. The exhaust gas heat must be directed away from the root of the tur­ bine blades, as the stresses are highest at this point. Most heat is directed towards the centre of the turbine disc, towards the shaft and bearings. By now, the reader should be aware of why it took so long to make one of these engines run at all. Did Mr Ball ever have those engines running in SC 1947? I really doubt it.