So what are the challenges of making a high efficiency Stirling engine? Stirling engine (1 GIF).

Less than a hundred years ago engines internal combustion tried to win their rightful place in the competition among other available machines and moving mechanisms. At the same time, in those days, the superiority of the gasoline engine was not so obvious. Existing machines on steam engines were distinguished by noiselessness, excellent power characteristics for that time, ease of maintenance, the possibility of using different kind fuel. In the further struggle for the market, internal combustion engines prevailed due to their efficiency, reliability and simplicity.

A further race for the improvement of aggregates and driving mechanisms, which was entered in the middle of the 20th century gas turbines and rotary engine varieties, led to the fact that, despite the supremacy of the gasoline engine, attempts were made to introduce a completely new type of engine onto the “playing field” - thermal, first invented back in 1861 by a Scottish priest named Robert Stirling. The engine was named after its creator.

Stirling engine: the physical side of the issue

To understand how it works desktop power station at Stirling, should be understood general information on the principles of operation of heat engines. Physically, the principle of operation is to use mechanical energy, which is obtained by expanding the gas during heating and its subsequent compression during cooling. To demonstrate the principle of operation, an example can be given based on an ordinary plastic bottle and two pots, one of which contains cold water, the other hot.

When lowering the bottle into cold water, the temperature of which is close to the temperature of ice formation, with sufficient cooling of the air inside the plastic container, it should be closed with a cork. Further, when the bottle is placed in boiling water, after a while the cork “shoots” with force, because in this case The work done by the heated air is many times greater than that done by the cooling air. When the experiment is repeated many times, the result does not change.

The first machines that were built using the Stirling engine faithfully reproduced the process demonstrated in the experiment. Naturally, the mechanism required improvement, consisting in the use of part of the heat that was lost by the gas during cooling for further heating, allowing heat to be returned to the gas to accelerate heating.

But even the application of this innovation could not save the situation, since the first Stirlings were large in size with low power output. In the future, more than once attempts were made to modernize the design to achieve a power of 250 hp. led to the fact that in the presence of a cylinder with a diameter of 4.2 meters, the real output power that the the 183kW Stirling plant was actually only 73kW.

All Stirling engines operate on the principle of the Stirling cycle, which includes four main phases and two intermediate ones. The main ones are heating, expansion, cooling and compression. As the transition stage, the transition to the cold generator and the transition to the heating element are considered. The useful work done by the engine is based solely on the temperature difference between the heating and cooling parts.

Modern Stirling configurations

Modern engineering distinguishes three main types of such engines:

• alpha stirling, the difference of which is in two active pistons located in independent cylinders. Of all three options this model differs most high power, having the highest temperature of the heating piston;
• beta stirling, based on one cylinder, one part of which is hot and the other is cold;
• gamma-stirling, which, in addition to the piston, also has a displacer.

The production of the Stirling power plant will depend on the choice of engine model, which will take into account all the positive and negative sides similar project.

Advantages and disadvantages

Thanks to their design features These engines have a number of advantages, but they are not without drawbacks.

Stirling's desktop power station, which is impossible in the store, but only from amateurs who independently collect such devices, include:

• big sizes, which are caused by the need for constant cooling of the working piston;
• the use of high pressure, which is required to improve engine performance and power;
• heat loss, which occurs due to the fact that the heat generated is transferred not to itself working body, but through a system of heat exchangers, whose heating leads to a loss of efficiency;
• a sharp decline power requires the application of special principles that differ from those traditional for gasoline engines.

Along with the disadvantages, power plants operating on Stirling units have undeniable advantages:

• any type of fuel, since like any engines using heat energy, this engine able to function at a temperature difference of any environment;
• economy. These devices can be an excellent replacement for steam units in cases where it is necessary to process solar energy, giving out an efficiency factor of 30% higher;
• environmental Safety. Since the kW tabletop power plant does not generate an exhaust moment, it does not produce noise and does not emit harmful substances into the atmosphere. Ordinary heat acts as a source of power, and the fuel burns out almost completely;
• constructive simplicity. For his work, Stirling will not require additional parts or fixtures. It is able to start independently without the use of a starter;
• increased resource of working capacity. Due to its simplicity, the engine can provide more than one hundred hours of continuous operation.

Applications for Stirling engines

The Stirling motor is most often used in situations where an apparatus for converting thermal energy is required, which is simple, while the efficiency of other types of thermal units is significantly lower under similar conditions. Very often, such units are used in the power supply of pumping equipment, refrigerators, submarines, batteries that store energy.

Video material: YouTube.com/watch?v=fRY6rkuw3LA

One of the promising areas for the use of Stirling engines is solar power plants, since this unit can be successfully used to convert the energy of sunlight into electrical energy. To carry out this process, the engine is placed in the focus of a mirror that accumulates the sun's rays, which provides permanent illumination of the area requiring heating. This allows you to focus solar energy on a small area. The fuel for the engine in this case is helium or hydrogen.

Modern automotive industry has reached a level of development in which, without fundamental scientific research it is almost impossible to achieve dramatic improvements in the design of traditional internal combustion engines. This situation forces designers to pay attention to alternative power plant designs. Some engineering centers have focused their efforts on the creation and adaptation to serial production of hybrid and electrical models, other automakers are investing in the development of engines powered by renewable sources (for example, biodiesel with rapeseed oil). There are other projects of power units, which in the future may become the new standard propulsion for Vehicle.

Among the possible sources of mechanical energy for cars of the future is the engine external combustion, which was invented in the middle of the XIX century by the Scot Robert Stirling as a thermal expansion machine.

Scheme of work

The Stirling engine converts thermal energy supplied from the outside into useful mechanical work at the expense changes in the temperature of the working fluid(gas or liquid) circulating in a closed volume.

IN general view the scheme of operation of the device is as follows: in the lower part of the engine, the working substance (for example, air) heats up and, increasing in volume, pushes the piston up. Hot air enters the top of the motor, where it is cooled by a radiator. The pressure of the working fluid is reduced, the piston is lowered for the next cycle. In this case, the system is sealed and the working substance is not consumed, but only moves inside the cylinder.

There are several design options for power units using the Stirling principle.

Stirling modification "Alpha"

The engine consists of two separate power pistons (hot and cold), each of which is located in its own cylinder. Heat is supplied to the cylinder with the hot piston, and the cold cylinder is located in the cooling heat exchanger.

Stirling modification "Beta"

The cylinder containing the piston is heated on one side and cooled on the opposite end. A power piston and a displacer move in the cylinder, designed to change the volume of the working gas. The return movement of the cooled working substance into the hot cavity of the engine is performed by the regenerator.

Stirling modification "Gamma"

The design consists of two cylinders. The first is completely cold, in which the power piston moves, and the second, hot on one side and cold on the other, serves to move the displacer. The regenerator for circulating cold gas can be common to both cylinders or be included in the design of the displacer.

Advantages of the Stirling engine

Like most external combustion engines, Stirling is inherent multi-fuel: the engine runs on a temperature difference, regardless of the reasons that caused it.

Interesting fact! Once, an installation was demonstrated that operated on twenty fuel options. Without stopping the engine, gasoline was supplied to the external combustion chamber, diesel fuel, methane, crude oil and vegetable oil - the power unit continued to work steadily.

The engine has simplicity of design and does not require additional systems And attachments(timing, starter, gearbox).

Features of the device guarantee a long service life: more than one hundred thousand hours continuous work.

The Stirling engine is silent, since detonation does not occur in the cylinders and there is no need to remove exhaust gases. Modification "Beta", equipped with a rhombic crank mechanism, is a perfectly balanced system that does not have vibrations during operation.

There are no processes in the engine cylinders that can have a negative impact on the environment. By choosing a suitable heat source (e.g. solar power), Stirling can be absolutely environmentally friendly power unit.

Disadvantages of the Stirling design

With all set positive properties immediate mass use of Stirling engines is impossible due to the following reasons:

The main problem lies in the material consumption of the structure. Cooling of the working fluid requires the presence of large volume radiators, which significantly increases the size and metal consumption of the installation.

The current technological level will allow the Stirling engine to compare in performance with modern gasoline engines only through the use of complex types of working fluid (helium or hydrogen) under pressure of more than one hundred atmospheres. This fact raises serious questions both in the field of materials science and user safety.

An important operational problem is related to the issues of thermal conductivity and temperature resistance of metals. Heat is supplied to the working volume through heat exchangers, which leads to inevitable losses. In addition, the heat exchanger must be made of heat-resistant metals that are resistant to high pressure. Suitable materials very expensive and difficult to process.

The principles of changing the modes of the Stirling engine are also fundamentally different from the traditional ones, which requires the development of special control devices. So, to change the power, it is necessary to change the pressure in the cylinders, the phase angle between the displacer and the power piston, or to affect the capacity of the cavity with the working fluid.

One way to control the shaft speed on a Stirling engine model can be seen in next video:

Efficiency

In theoretical calculations, the efficiency of the Stirling engine depends on the temperature difference of the working fluid and can reach 70% or more in accordance with the Carnot cycle.

However, the first samples realized in metal had an extremely weak high efficiency the following reasons:

• inefficient variants of the coolant (working fluid), limiting the maximum heating temperature;
• energy losses due to friction of parts and thermal conductivity of the engine housing;
• lack of structural materials resistant to high pressure.

Engineering solutions have constantly improved the device power unit. So, in the second half of the 20th century, a four-cylinder automobile Stirling engine with a rhombic drive showed an efficiency equal to 35% in tests on a water coolant with a temperature of 55 ° C. Careful study of the design, the use of new materials and fine-tuning of the working units ensured the efficiency of the experimental samples at 39%.

Note! Modern gasoline engines of similar power have a coefficient useful action at the level of 28-30%, and turbocharged diesels within 32-35%.

Modern examples of the Stirling engine, such as the one built by the American company Mechanical Technology Inc, show efficiency up to 43.5%. And with the development of the production of heat-resistant ceramics and similar innovative materials, it will be possible to significantly increase the temperature of the working environment and achieve an efficiency of 60%.

Examples of successful implementation of automotive Stirlings

Despite all the difficulties, there are many workable models of the Stirling engine applicable to the automotive industry.

Interest in Stirling, suitable for installation in a car, appeared in the 50s of the XX century. Work in this direction was carried out by such concerns as Ford motor company, Volkswagen Group and others.

UNITED STIRLING (Sweden) developed Stirling, which made maximum use of serial components and assemblies produced by automakers ( crankshaft, connecting rods). The resulting four-cylinder V-shaped engine had a specific gravity of 2.4 kg / kW, which is comparable to the characteristics of a compact diesel engine. This unit has been successfully tested as power plant seven ton cargo van.

One of the successful examples is the four-cylinder Stirling engine of the Dutch production model "Philips 4-125DA", intended for installation on a car. The motor had a working power of 173 liters. With. in dimensions similar to the classic gasoline unit.

Engineers have achieved significant results General Motors, having built an eight-cylinder (4 working and 4 compression cylinders) in the 70s V-engine Stirling with a standard crank mechanism.

Similar power plant in 1972 equipped limited edition ford cars Torino, whose fuel consumption has decreased by 25% compared to the classic gasoline V-shaped eight.

Currently, more than fifty foreign companies are working to improve the design of the Stirling engine in order to adapt it to mass production for the needs of the automotive industry. And if you manage to eliminate the shortcomings of this type engines, while at the same time retaining its advantages, it is Stirling, and not turbines and electric motors, that will replace the gasoline internal combustion engine.

The Stirling cycle is considered an indispensable accessory of the Stirling engine. At the same time, a detailed study of the principles of operation of many designs created to date shows that a significant part of them have a duty cycle that is different from the Stirling cycle. For example, alpha stirling with pistons of different diameters has a cycle that is more similar to the Ericsson cycle. Beta- and gamma-configurations, which have a sufficiently large diameter of the piston-displacer rod, also occupy some intermediate position between the Stirling and Ericsson cycles.

When the displacer moves in the beta configuration, the change in the state of the working fluid occurs not along the isochore, but along an inclined line intermediate between the isochore and isobar. At a certain ratio of the diameter of the rod to the total diameter of the displacer, an isobar can be obtained (this ratio depends on the operating temperatures). In this case, the piston, which was previously a worker, plays only an auxiliary role, and the displacer rod becomes a real worker. The specific power of such an engine turns out to be approximately 2 times greater than in the usual stirlings, lower friction losses, since the pressure on the piston is more even. A similar picture is in alpha stirlings with different piston diameters. An engine with an intermediate diagram can have a load evenly distributed between the pistons, i.e. between the working piston and the displacer rod.

An important advantage engine operation according to the Ericsson cycle or close to it is that the isochore is replaced by the isobar or a process close to it. When the working fluid expands along the isobar, there are no pressure changes, no heat transfer, except for the transfer of heat from the recuperator to the working fluid. And this heating immediately performs useful work. During isobaric compression, heat is transferred to the heat exchanger.
In the Stirling cycle, when the working fluid is heated or cooled along the isochore, heat losses occur due to isothermal processes in the heater and cooler.

Configuration

Engineers divide Stirling engines into three various types:

• Alpha Stirling- contains two separate power pistons in separate cylinders. One piston is hot, the other is cold. The hot piston cylinder is in a heat exchanger with a higher temperature, while the cold piston cylinder is in a colder heat exchanger. This type of engine has a fairly high power to volume ratio, but unfortunately the high temperature of the "hot" piston creates certain technical problems.

The regenerator is located between the hot part of the connecting tube and the cold part.

• Beta Stirling- there is only one cylinder, hot at one end and cold at the other. A piston (from which power is removed) and a “displacer” move inside the cylinder, changing the volume of the hot cavity. The gas is pumped from the cold part of the cylinder to the hot part through the regenerator. The regenerator may be external, as part of a heat exchanger, or may be combined with a displacing piston.

• Gamma Stirling- there is also a piston and a “displacer”, but at the same time there are two cylinders - one cold (the piston moves there, from which power is removed), and the second is hot from one end and cold from the other (the “displacer” moves there). The regenerator can be external, in which case it connects hot part the second cylinder with cold and simultaneously with the first (cold) cylinder. The internal regenerator is part of the displacer.

There are also varieties of the Stirling engine that do not fall under the above three classic types:

• Stirling rotary engine- tightness problems solved (Mukhin's patent for hermetic rotation input (GVV), a silver medal for international exhibition in Brussels "Eureka-96") and bulkiness (there is no crank mechanism, because the engine is rotary).

Flaws

• Material consumption- the main drawback of the engine. For external combustion engines in general, and the Stirling engine in particular, the working fluid must be cooled, and this leads to a significant increase in the weight and dimensions of the power plant due to enlarged radiators.

• To obtain characteristics comparable to those of an internal combustion engine, it is necessary to apply high pressures (over 100 atm) and special types of working fluid- hydrogen, helium.

• Heat is not supplied directly to the working fluid but only through the walls of the heat exchangers. The walls have limited thermal conductivity, due to which the efficiency is lower than expected. The hot heat exchanger operates under very stressful heat transfer conditions and at very high pressures, which requires the use of high quality and expensive materials. Creating a heat exchanger that would satisfy conflicting requirements is very difficult. The higher the heat exchange area, the lower the heat loss. At the same time, the size of the heat exchanger and the volume of the working fluid that is not involved in the work increase. Since the heat source is located outside, the engine responds slowly to changes in the heat flux supplied to the cylinder, and may not immediately produce the desired power at start-up.

• To quickly change engine power, methods are used that are different from those used in internal combustion engines: buffer capacity of variable volume, change in the average pressure of the working fluid in the chambers, change in the phase angle between the working piston and the displacer. In the latter case, the reaction of the engine to the control action of the driver is almost instantaneous.

Advantages

However, the Stirling engine has advantages that force it to be developed.

• "Omnivorous" engine- like all external combustion engines (or rather, external heat supply), the Stirling engine can operate from almost any temperature difference: for example, between different layers of water in the ocean, from the sun, from a nuclear or isotope heater, a coal or wood stove, etc. .
• Simplicity of design- the design of the engine is very simple, it does not require additional systems, such as a gas distribution mechanism. It starts on its own and does not need a starter. Its characteristics allow you to get rid of the gearbox. However, as noted above, it has a greater material consumption.
• Increased resource- simplicity of design, the absence of many "delicate" units allows Stirling to provide an unprecedented resource for other engines of tens and hundreds of thousands of hours of continuous operation.
• economy- in the case of converting solar energy into electricity, stirlings sometimes give higher efficiency (up to 31.25%) than steam heat engines.
• Engine noiselessness- Stirling has no exhaust, which means it does not make noise. The beta stirling with a diamond mechanism is a perfectly balanced device and, with enough high quality manufacturing, does not even have vibrations (vibration amplitude is less than 0.0038 mm).
• Environmental friendliness- Stirling itself does not have any parts or processes that can contribute to contamination environment. It does not consume the working fluid. The environmental friendliness of the engine is primarily due to the environmental friendliness of the heat source. It should also be noted that it is easier to ensure the completeness of fuel combustion in an external combustion engine than in an internal combustion engine.

Application

Stirling engine with linear alternator

The Stirling engine is applicable in cases where a compact thermal energy converter is needed, which is simple in design, or when the efficiency of other heat engines is lower: for example, if the temperature difference is not enough to operate a steam or gas turbine.

Thermoacoustics is a branch of physics about the mutual transformation of thermal and acoustic energy. It was formed at the intersection of thermodynamics and acoustics. Hence the name. This science is very young. As an independent discipline, it arose in the late 70s of the last century, when the Swiss Niklaus Rott completed work on the mathematical foundations of linear thermoacoustics. And yet it didn't come out of nowhere. Its appearance was preceded by discoveries of interesting effects, which we simply must consider.

WHERE IT STARTED
Thermoacoustics has a long history dating back more than two centuries.

The first official record of heat-induced vibrations was made by Higgins in 1777. He experimented with an open glass tube in which acoustic vibrations were excited by a hydrogen burner positioned in a certain way. This experience has gone down in history as "the singing flame of Higgins".

Figure 1. Higgins Singing Flame

However, modern physicists are more familiar with another experiment called the Rijke tube. In the course of his experiments, Rijke created a new musical instrument from an organ pipe. He replaced the Higgins hydrogen flame with a heated wire screen and experimentally showed that the strongest sound is produced when the screen is located at a distance of a quarter of the tube from its lower end. The vibrations stopped when you cover upper end tubes. This proved that longitudinal convective draft was necessary to produce sound. The work of Higgins and Rijke later provided the basis for the science of combustion, which is now used everywhere that this phenomenon is used from

Figure 2. Rijke tube.

burning powder pellets to rocket engines. Thousands of dissertations all over the world are devoted to the phenomena occurring in the Rijke tube, but interest in this device has not weakened so far.

In 1850, Sondhauss turned to a strange phenomenon that glassblowers observe in their work. When the hot glass ball pushes air into the cold end of the blower's tube, a clear sound is generated. Analyzing the phenomenon, Sondhauss discovered that sound is generated by heating a spherical bulge at the end of a tube. In this case, the sound changes with a change in the length of the tube. Unlike the Rijke tube, the Sondhauss tube did not depend on convective draft.

Figure 3. Sondhaus tube.

A similar experiment was later carried out by Taconis. Unlike Sondhauss, he did not heat the end of the tube, but cooled it with a cryogenic liquid. This proved that it was not heating that was important for sound generation, but a temperature difference.
The first qualitative analysis of vibrations caused by heat was given in 1887 by Lord Rayleigh. Rayleigh's explanation of the phenomena listed above is now known to thermoacoustics as Rayleigh's principle. It sounds something like this: “If heat is transferred to the gas at the moment of greatest compression or heat is taken away at the moment of greatest rarefaction, then this stimulates oscillations. » Despite its simplicity, this formulation fully describes the direct thermoacoustic effect, that is, the conversion of thermal energy into sound energy.

swirl effect

swirl effect(Ranque-Hilsch effect) Ranque-Hilsch Effect) - the effect of separation of a gas or liquid when swirling in a cylindrical or conical chamber into two fractions. A swirling flow with a higher temperature is formed at the periphery, and a swirling cooled flow is formed in the center, and the rotation in the center occurs in the opposite direction than at the periphery. The effect was first discovered by the French engineer Joseph Rank in the late 1920s when measuring temperature in an industrial cyclone. At the end of 1931, J. Rank applied for an invented device, which he called the "Vortex tube" (in the literature it is found as the Ranke tube). It is possible to obtain a patent only in 1934 in America (US Patent No. 1952281). Currently, a number of devices have been implemented that use the vortex effect, vortex devices. These are "vortex chambers" for the chemical separation of substances under the action of centrifugal forces and "vortex tubes" used as a source of cold.

Since the 1960s, vortex motion has been the subject of many scientific studies. Specialized conferences on the vortex effect are regularly held, for example, at the Samara Aerospace University.

Vortex heat generators and microconditioners exist and are used.

In this world there are things of genius, incomprehensible and completely unreal. So unrealistic that they seem to be artifacts from some parallel universe. Among such artifacts, along with the Stirling engine, vacuum radio tube and Malevich's black square, the so-called. Tesla Turbine.
Generally speaking distinguishing feature of all such things - absolute simplicity. Not oversimplification, but simplicity. That is, as in the works of Michelangelo - there is no everything superfluous, some technical or semantic "props", pure consciousness, embodied "in iron" or splashed onto the canvas. And with all this, absolute non-circulation. The Black Square is a kind of "ort" of art. The second such written by another artist can not be.

All this fully applies to the Tesla turbine. Structurally, it consists of several (10-15) thin disks mounted on the turbine axis at a small distance from each other and placed in a casing resembling a police whistle.

It is not worth explaining that the disc rotor is much more technologically advanced and reliable than even the "Laval wheel", I am silent about the rotors conventional turbines. This is the first advantage of the system. The second is that, unlike other types of turbines, where special measures must be taken to laminarize the flow of the working fluid. In the Tesla turbine, the working fluid (which can be air, steam or even liquid) flows strictly laminar. Therefore, the losses due to gas-dynamic friction in it are reduced to zero: the efficiency of the turbine is 95%.

True, it should be borne in mind that the efficiency of a turbine and the efficiency of a thermodynamic cycle are somewhat different things. Turbine efficiency can be characterized as the ratio of the energy converted into mechanical energy on the turbine rotor shaft to the energy of the working cycle (that is, the difference between the initial and final energies of the working fluid). So the efficiency of modern steam turbines is also very high - 95-98%, however, the efficiency of the thermodynamic cycle due to a number of restrictions does not exceed 40-50%.

The principle of operation of the turbine is based on the fact that the working fluid (let's say - gas), spinning in the casing, due to friction "entrains" the rotor. At the same time, giving part of the energy to the rotor, the gas slows down, and due to the Coriolis force that arises when interacting with the rotor, like tea leaves, it "rolls" to the rotor axis, where there are special holes through which the "waste" working fluid is removed.
The Tesla turbine, like the Laval turbine, converts the kinetic energy of the working fluid. That is, the transformation of potential energy (for example compressed air or superheated steam) into the kinetic one must be made before it is fed to the turbine rotor using a nozzle. However, the Laval turbine, having a fairly high overall efficiency, turned out to be extremely inefficient at low speeds, which forced the design of gearboxes, the dimensions and mass of which were many times greater than the dimensions and masses of the turbine itself. The fundamental difference between the Tesla turbine is the fact that it operates quite efficiently in a wide range of rotational speeds, which allows its shaft to be connected directly to the generator. In addition, the Tesla turbine is easily reversible.

Interestingly, Nikola Tesla himself positioned his invention as a way of highly efficient use of geothermal energy, which he considered the energy of the future. In addition, the turbine, without any modifications, can turn into a highly efficient Vacuum pump- it is enough to unwind its shaft from another turbine or electric motor.

The manufacturability of the Tesla turbine allows you to make its variants literally from anything: a disk rotor can be made from old CDs or "pancakes" from a failed computer "hard drive". At the same time, the power of such an engine, despite the "toy" materials and dimensions, is very impressive. Speaking of dimensions: 110 hp engine. was no larger than the system unit of the current personal computer.

Rank effect devices

From the very beginning, the Rank effect attracted inventors with the seeming simplicity of its technical implementation - in fact, the simplest implementation vortex tube is a piece of the most common pipe, where on one side the original flow is tangentially fed in, and an annular diaphragm is installed on the opposite end, and the cooled part of the flow comes out of its inner hole, and its hot part comes out of the gap between the outer edge of the diaphragm and the inner surface of the pipe . However, in reality, not everything is so simple - it is far from always possible to achieve effective separation, and the efficiency of such installations is usually noticeably inferior to widespread compressor heat pumps. In addition, the parameters of the Ranque effect plant are usually calculated for a specific power, determined by the velocity and flow rate of the initial flow, and when the parameters of the inlet flow deviate from the optimal values, the efficiency of the vortex tube deteriorates significantly. Nevertheless, it should be noted that the capabilities of some installations based on the Ranque effect inspire respect - for example, the record cooling, which was achieved at one stage, is more than 200 ° C!

However, taking into account our climate, the use of the Ranque effect for heating is of much greater interest, and at the same time I would also like not to go beyond the “improvised means”.

The essence of the Rank effect

When a gas or liquid flow moves along a smoothly turning surface of a pipe, an area is formed near its outer wall high blood pressure and temperature, and near the inside (or in the center of the cavity, if the gas is swirled over the surface of a cylindrical vessel) - a region of low temperature and pressure. This well-known phenomenon is called Rank effect by the name of the French engineer Joseph Ranque (G.J. Ranque, sometimes spelled “Ranke”), who discovered it in 1931, or Ranque-Hilsch effect(German Robert Hilsh continued to study this effect in the second half of the 1940s and improved the efficiency of the Rank vortex tube). Designs using the Rank effect are a kind of heat pump, the energy for which is taken from the supercharger, which creates a flow of the working fluid at the pipe inlet.

The paradox of the Rank effect is that centrifugal forces in a rotating flow are directed outward. As is known, warmer layers of a gas or liquid have a lower density and must rise upwards, and in the case of centrifugal forces, tend to the center, colder ones have a higher density and, accordingly, must tend to the periphery. Meanwhile, at high speed rotating flow everything happens exactly the opposite!

The Ranque effect manifests itself both for a gas flow and for a liquid flow, which, as is known, is practically incompressible and, therefore, the adiabatic compression / expansion factor is not applicable to it. However, in the case of a liquid, the Ranque effect is usually much less pronounced - perhaps for this very reason, and the very small mean free path of particles makes it difficult to manifest itself. But this is true, if we remain within the framework of the traditional molecular kinetic theory, and the effect may have completely different reasons.

In my opinion, on this moment the most complete and reliable scientific description of the Rank effect is presented in the article by A.F. Gutsol (in pdf format). Surprisingly, at its core, his conclusions about the essence of the phenomenon coincide with those obtained by us “on the fingers”. Unfortunately, he ignores the first factor (adiabatic compression of the gas at the outer radius and expansion at the inner one), which, in my opinion, is very significant when using compressible gases, although it only acts inside the device. And A.F. Gutsol calls the second factor “separation of fast and slow microvolumes”.

Stirling's engine– engine with external heat supply. External heat supply is very convenient when there is a need to use non-fossil fuels as a heat source. For example, you can use solar energy, geothermal energy, waste heat from various enterprises.

A nice feature of the Stirling cycle is that its efficiency is equal to the efficiency of the Carnot cycle. Naturally, real Stirling engines have lower efficiency and often much more. The Stirling engine began its existence with a device that has many moving parts, such as pistons, connecting rods, crankshaft, bearings. In addition, the generator rotor was also spinning (Figure 1).

Figure 1 - Alpha type Stirling engine

Look at the Stirling Alpha type engine. When the shaft rotates, the pistons begin to distill the gas either from a cold to a hot cylinder, or vice versa, from a hot to a cold one. But they do not just distill, but also compress and expand. The thermodynamic cycle takes place. You can mentally imagine in the picture that when the shaft turns so that the axis on which the connecting rods are attached is at the top, then this will be the moment of greatest compression of the gas, and when it is at the bottom, then expansion. True, this is not entirely true due to thermal expansions and compressions of the gas, but approximately all this is true.

The heart of the engine is the so-called core, which consists of two heat exchangers - hot and cold, and between them there is a regenerator. Heat exchangers are usually made of plate, and the regenerator is most often a stack made of metal mesh. It is clear why heat exchangers are needed - to heat and cool the gas, but why do we need a regenerator? And the regenerator is a real heat accumulator. When the hot gas moves to the cold side, it heats the regenerator and the regenerator stores thermal energy. When the gas moves from the cold to the hot side, the cold gas is heated in the regenerator, and thus this heat, which without a regenerator would have been irretrievably spent on heating the environment, is saved. So, the regenerator is extremely necessary thing. A good regenerator increases the efficiency of the engine by about 3.6 times.

For fans who dream of building such an engine on their own, I want to tell you more about heat exchangers. Majority homemade engines Stirling, of those that I have seen, do not have heat exchangers at all (I'm talking about alpha type engines). The heat exchangers are the pistons and cylinders themselves. One cylinder heats up, the other cools down. At the same time, the area of ​​the heat exchange surface in contact with the gas is very small. So, it is possible to significantly increase engine power by installing heat exchangers at the inlet to the cylinders. And even in Figure 1, the flame is directed straight at the cylinder, which is not entirely true in factory engines.

Let's return to the history of the development of Stirling engines. So, let the engine be good in many ways, but the presence oil scraper rings and bearings reduced the life of the engine, and the engineers thought hard about how to improve it, and came up with.

In 1969, William Bale investigated resonant effects in engine operation and was later able to make an engine that needed neither connecting rods nor a crankshaft. The synchronization of the pistons arose due to resonant effects. This type of engine came to be called a free piston engine (Figure 2).

Figure 2 - Free-piston Stirling engine

Figure 2 shows a beta type free piston engine. Here, the gas passes from the hot region to the cold region, and vice versa, thanks to the displacer (which moves freely), and the working piston does useful work. The displacer and piston oscillate on helical springs, which can be seen on the right side of the figure. The difficulty is that their oscillations must be at the same frequency and with a phase difference of 90 degrees, and all this is due to resonant effects. It is rather difficult to do this.

Thus, the number of parts was reduced, but at the same time, the requirements for the accuracy of calculations and manufacturing were tightened. But the reliability of the engine has undoubtedly increased, especially in designs where flexible membranes are used as a displacer and piston. In this case, there are no rubbing parts in the engine at all. Electricity, if desired, can be removed from such an engine using a linear generator.

But even this was not enough for the engineers, and they began to look for ways to get rid of not just rubbing parts, but generally moving parts. And they found a way.

In the 1970s, Peter Zeperli realized that the sinusoidal fluctuations in gas pressure and velocity in a Stirling engine, and the fact that these fluctuations are in phase, are remarkably similar to gas pressure and velocity fluctuations in a traveling sound wave (Fig. 3 ).

Figure 3 - Plot of pressure and velocity of a traveling acoustic wave as a function of time. It is shown that pressure and velocity oscillations are in phase.

This idea came to Zeperli not by chance, since before him there were many studies in the field of thermoacoustics, for example, Lord Rayleigh himself in 1884 qualitatively described this phenomenon.

Thus, he proposed to abandon the pistons and displacers altogether, and use only an acoustic wave to control the pressure and movement of the gas. This results in an engine with no moving parts and theoretically capable of reaching the efficiency of the Stirling cycle, and hence Carnot. In reality, the best performance is 40-50% of the efficiency of the Carnot cycle (Figure 4).

Figure 4 - Scheme of a thermoacoustic engine with a traveling wave

It can be seen that a traveling wave thermoacoustic engine is exactly the same core, consisting of heat exchangers and a regenerator, but instead of pistons and connecting rods, there is simply a looped pipe, which is called a resonator. But how does this engine work if there are no moving parts in it? How is this possible?

First, let's answer the question, where does the sound come from? And the answer is that it occurs by itself when there is a sufficient temperature difference between the two heat exchangers for this. The temperature gradient in the regenerator allows you to amplify sound vibrations, but only of a certain wavelength, equal to the length resonator. From the very beginning, the process looks like this: when a hot heat exchanger is heated, micro rustles appear, perhaps even crackles from thermal deformations, this is inevitable. These rustles are noise that has a wide frequency spectrum. From all this rich spectrum of sound frequencies, the engine begins to amplify that sound vibration, the wavelength of which is equal to the length of the tube - the resonator. And no matter how small the initial swing, it will be amplified to the maximum possible value. The maximum sound volume inside the engine occurs when the power of sound amplification with the help of heat exchangers is equal to the power of losses, that is, the power of attenuation of sound vibrations. And this maximum value sometimes reaches huge values ​​​​of 160 dB. So what's inside similar engine really loud. Fortunately, the sound cannot come out, since the resonator is sealed and therefore, standing next to a running engine, it is barely audible.

Amplification of a certain sound frequency occurs due to the same thermodynamic cycle - the Stirling cycle, which is carried out in the regenerator.

Figure 5 - Stages of the cycle roughly and simplified.

As I already wrote, in a thermoacoustic engine there are no moving parts at all, it generates only an acoustic wave inside itself, but, unfortunately, it is impossible to remove electricity from the engine without moving parts.

Usually energy is extracted from thermoacoustic engines using linear generators. The elastic membrane oscillates under the pressure of a high-intensity sound wave. Inside copper coil with a core, magnets fixed on the membrane vibrate. Electricity is generated.

In 2014, Kees de Blok, Pawel Owczarek, and Maurice Francois of Aster Thermoacoustics showed that a bidirectional impulse turbine connected to a generator is suitable for converting sound wave energy into electricity.

Figure 6 - Diagram of an impulse turbine

The impulse turbine rotates in the same direction, regardless of the direction of flow. Figure 6 schematically shows the stator blades on the sides and the rotor blades in the middle.
And this is what the turbine looks like in reality:

Figure 7 - Appearance bidirectional impulse turbine

It is expected that the use of a turbine instead of a linear generator will greatly reduce the cost of the design and will increase the power of the device up to the capacities of typical CHP plants, which is impossible with linear generators.

Well, we will continue to closely monitor the development of thermoacoustic engines.

List of sources used

M.G. Kruglov. Stirling engines. Moscow "Engineering", 1977.
G. Reeder, C. Hooper. Stirling engines. Moscow "Mir", 1986.
Kees de Blok, Pawel Owczarek. Acoustic to electric power conversion, 2014.

- a heat engine in which a liquid or gaseous working fluid moves in a closed volume, a kind of external combustion engine. It is based on periodic heating and cooling of the working fluid with the extraction of energy from the resulting change in the volume of the working fluid. It can work not only from fuel combustion, but also from any heat source.

You can observe the chronology of events associated with the development of engines of the 18th century in an interesting article - "The History of the Invention of Steam Engines". And this article is dedicated to the great inventor Robert Stirling and his brainchild.

History of creation...

The patent for the invention of the Stirling engine, oddly enough, belongs to the Scottish priest Robert Stirling. He received it on September 27, 1816. The first "hot air engines" became known to the world at the end of the 17th century, long before Stirling. One of the important achievements of Stirling is the addition of a purifier, nicknamed by him "housekeeper".

In modern scientific literature, this cleaner has a completely different name - "recuperator". Thanks to him, the performance of the engine increases, since the cleaner retains heat in the warm part of the engine, and at the same time the working fluid is cooled. Through this process, the efficiency of the system is greatly increased. The recuperator is a chamber filled with wire, granules, corrugated foil (corrugations go along the direction of the gas flow). The gas passes through the recuperator filler in one direction, gives (or acquires) heat, and when moving in the other direction, takes (gives) it away. The recuperator can be external in relation to the cylinders and can be placed on the displacer piston in beta and gamma configurations. The dimensions and weight of the machine in this case are less. To some extent, the role of the recuperator is played by the gap between the displacer and the cylinder walls (if the cylinder is long, then there is no need for such a device at all, but significant losses appear due to the viscosity of the gas). In alpha stirling, the heat exchanger can only be external. It is mounted in series with a heat exchanger, in which the working fluid is heated from the side of the cold piston.

In 1843, James Stirling used this engine in a factory where he worked as an engineer at the time. In 1938, Philips invested in a Stirling engine with a capacity of more than two hundred horsepower and a return of more than 30%. Because the Stirling's engine has many advantages, then in the era steam engines it was widespread.

Flaws.

Material consumption is the main drawback of the engine. For external combustion engines in general, and the Stirling engine in particular, the working fluid must be cooled, and this leads to a significant increase in the weight and dimensions of the power plant due to enlarged radiators.

To obtain characteristics comparable to those of an internal combustion engine, it is necessary to use high pressures (over 100 atm) and special types of working fluid - hydrogen, helium.

Heat is not supplied to the working fluid directly, but only through the walls of the heat exchangers. The walls have limited thermal conductivity, due to which the efficiency is lower than expected. The hot heat exchanger operates under very stressful heat transfer conditions and at very high pressures, which requires the use of high quality and expensive materials. Creating a heat exchanger that would satisfy conflicting requirements is very difficult. The higher the heat exchange area, the lower the heat loss. At the same time, the size of the heat exchanger and the volume of the working fluid that is not involved in the work increase. Since the heat source is located outside, the engine responds slowly to changes in the heat flux supplied to the cylinder, and may not immediately produce the desired power at start-up.

To quickly change the engine power, methods are used that are different from those used in internal combustion engines: a variable volume buffer tank, a change in the average pressure of the working fluid in the chambers, a change in the phase angle between the working piston and the displacer. In the latter case, the reaction of the engine to the control action of the driver is almost instantaneous.

Advantages.

However, the Stirling engine has advantages that force it to be developed.

The “omnivorousness” of the engine - like all external combustion engines (or rather, external heat supply), the Stirling engine can operate from almost any temperature difference: for example, between different layers in the ocean, from the sun, from a nuclear or isotope heater, coal or wood stove and etc.

Simplicity of design - the design of the engine is very simple, it does not require additional systems, such as a gas distribution mechanism. It starts on its own and does not need a starter. Its characteristics allow you to get rid of the gearbox. However, as noted above, it has a greater material consumption.

Increased resource - simplicity of design, the absence of many "delicate" units allows Stirling to provide an unprecedented resource for other engines of tens and hundreds of thousands of hours of continuous operation.

Profitability - in the case of converting solar energy into electricity, stirlings sometimes give greater efficiency (up to 31.25%) than steam heat engines.

Noiselessness of the engine - Stirling has no exhaust, which means it does not make noise. Beta Stirling with a rhombic mechanism is a perfectly balanced device and, with a fairly high quality of workmanship, does not even have vibrations (vibration amplitude is less than 0.0038 mm).

Environmentally friendly - Stirling itself does not have any parts or processes that can contribute to environmental pollution. It does not consume the working fluid. The environmental friendliness of the engine is primarily due to the environmental friendliness of the heat source. It should also be noted that it is easier to ensure the completeness of fuel combustion in an external combustion engine than in an internal combustion engine.

Alternative to steam engines.

In the 19th century, engineers tried to create a safer alternative steam engines of that time, due to the fact that the boilers of already invented engines often exploded, unable to withstand the high pressure of steam and materials that were not at all suitable for their manufacture and construction. Stirling's engine became a good alternative because it could convert any temperature difference into work. This is the basic principle of the Stirling engine. The constant alternation of heating and cooling of the working fluid in a closed cylinder sets the piston in motion. Usually air acts as a working fluid, but hydrogen and helium are also used. But experiments were also carried out with water. main feature Stirling engine with a liquid working fluid is small size, high operating pressure and high power density. There is also a Stirling with a two-phase working fluid. The specific power and working pressure in it are also quite high.

Perhaps you remember from a physics course that when a gas is heated, its volume increases, and when it is cooled, it decreases. It is this property of gases that underlies the operation of the Stirling engine. Stirling's engine uses the Stirling cycle, which is not inferior to the Carnot cycle in terms of thermodynamic efficiency, and in some way even has an advantage. The Carnot cycle consists of slightly different isotherms and adiabats. The practical implementation of such a cycle is complex and unpromising. The Stirling cycle made it possible to obtain a practically working engine in acceptable dimensions.

In total, there are four phases in the Stirling cycle, separated by two transitional phases: heating, expansion, transition to a cold source, cooling, compression, and transition to a heat source. When moving from a warm source to a cold source, the gas in the cylinder expands and contracts. During this process, the pressure changes and useful work can be obtained. Useful work is produced only by the processes taking place with constant temperature, that is, it depends on the temperature difference between the heater and the cooler, as in the Carnot cycle.

Configurations.

Engineers classify Stirling engines into three different types:

Preview - Click to enlarge.

Contains two separate power pistons in separate cylinders. One piston is hot, the other is cold. The cylinder with the hot piston is in the heat exchanger with a higher temperature, and the cylinder with the cold piston is in the colder heat exchanger. The ratio of power to volume is quite large, but the high temperature of the "hot" piston creates certain technical problems.

Beta Stirling- one cylinder, hot at one end and cold at the other. A piston (from which power is removed) and a “displacer” move inside the cylinder, changing the volume of the hot cavity. The gas is pumped from the cold part of the cylinder to the hot part through the regenerator. The regenerator may be external, as part of a heat exchanger, or may be combined with a displacing piston.

There is a piston and a “displacer”, but at the same time there are two cylinders - one cold (the piston moves there, from which power is removed), and the second is hot from one end and cold from the other (the “displacer” moves there). The regenerator can be external, in which case it connects the hot part of the second cylinder with the cold one and simultaneously with the first (cold) cylinder. The internal regenerator is part of the displacer.