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of working fluid. There is every reason to believe that the compound working fluid would be similarly dependent but there is insufficient data presently available for rigorous analysis.

R efrigera i ion appl ic at ions

All the work outlined above was carried out with reference to prime movers, power systems converting heat to work. The expansion space was heated and the compression space was cooled.

The applicability of the concept of compound working fluids to refrigeration applications was investigated by Walker and Agbi (1973). It was found that a compound working fluid had the same benelicial effect resulting in an increase in the effective volume compression ratio. This in turn caused an increase in the range of the pressure excursion and hence an increase in the refrigerating capacity of the machine with no increase in engine size, weight, or cost compared with a gaseous working fluid. The degree of improvement of the compound working fluid was significantly greater at relatively high refrigeration temperatures and declined progressively until at cryogenic temperature levels it was not markedly ditferent to the conventional gaseous cycle. Moreover, at the lower temperatures it was necessary to resort to a hypothetical fluid as the phase change component was required to have properties not possessed by any known real fluid.

Potential applications for compound working fluids

The preliminary investigations outlined above indicated that the specific output of Stirling engines, acting as prime movers or cooling engines, may be significantly improved by the use of compound working fluids. The degree of improvement (twice as much) was sufficient to justify the next level of investigation.

With a compound working fluid and consequent high specific output it may be possible to decrease the very high pressure levels of gaseous working fluids that are characteristic of automotive engines. Moreover, the boiling and condensing processes occurring to the phase change component during the cycle are associated with very high rates of heat transfer, and the phase change component may have a high enthalpy of evaporation (water has a very high value). Consequently, the processes of compression and expansion in the engine are likely to approximate to isothermal conditions more closely than in the gaseous machine.

The presence of liquid as droplets in suspension or as a film, could have profoundly beneficial effects on the effectiveness and life of fluid seals. The presence of water or light hydrocarbons on PTFF. filled plastic (Rulon) seals operating in helium has been found by the author to greatly enhance both the operation and life. The effects are extremely variable and diflicult to reproduce, and insufficient experimental work has been done to arrive at any other than the most general conclusions. Because of the difficulty in reproducing results, no account of the work, has been published.

Improvement to seal life, friction, and wear is significant in double-acting engines for the dry seal on the piston separating the two Stirling systems (at different pressure levels) found in each cylinder. The seal is invariably located at the colu (ambient temperature) end of the piston and thus adjacent to a compression space where the phase change component would exist as a liquid.

In engines with a mechanical drive, crank, rhombic, or swash-plate, another pressure seal is found where the piston or displacer-rod passes through the cylinder. This can either be a rolling seal (Philips) or a Rulon rubbing seal. Invariably the seal is located at the ambient-temperature end of the cylinder where the phase change component would exist as a liquid. It would be a simple matter indeed to design the seal locations as shown in Fig. 8.13 such thai the liquid phase change component accumulates in the seal well. This transforms the seal from a gas seal to a liquid seal and thereby relaxes the seal requirements by several orders of magnitude.

In the studies of compound working fluids described above the

Fig. h. 13. Sliding liquid seal iu engine cylinder with compound working fluids.

greatest degree of improvement iri specific output was found at the lower expansion-space temperatures (super-ambient) in the ease of prime-mover engines and at the higher refrigeration temperatures (sub-ambient) for cooling engines. This suggests that compound working fluids might well be applied in engines operating on low grade thermal sources, i.e. exhaust-heat b.Ottommg-cycle systems for gas turbine or internal combustion engines, or relatively unsophisticated, and therefore low-cost, solar collectors. For cooling engines the best applications, at high refrigeration temperatures, may be air conditioning units, heat pumps, and food preservation or processing.

Intriguing combinations of Stirling-cycle prime movers driving Stirling-cycle cooling engines are at an outline design stage. Both units use the same compound working fluid, one of the Freons in combination with hydrogen, as thermally-activated cooling engines driven by solar energy, exhaust heat, natural gas. fossil fuel, municipal waste, and biomass combustion.

Experimental work with compound working fluids

Little experimental work with compound working fluids has been reported. William Beale added a small quantity or water to the cylinder of a small demonstration free-piston engine (see Fig. 8.1 1) to improve sealing in the working space. He found the resultant increase in the cyclic-pressure excursion was sufficient for the output to improve dramatically until, after a few cycles, the displacer collapsed with the increased pressure.

This experience prompted the author to undertake the studies described above and to construct the apparatus shown in Fig. 8.14. It consists of a long cylinder closed at both ends and containing a hollow displacer. The displacer was actuated by a crank connecting-rod mechanism driven by a motor. Various speeds and strokes of the displacer may be obtained. One end ol (he cylinder was heated (by internal electric resistance heaters) and the other end was cooled by a water jacket. The cylinder was fitted with a Kistler quartz piezoelectric pressure transducer and thermocouples in the hot and cold spaces in the cylinder to measure mean temperatures; The apparatus was mounted within a structural steel frame so that ¡1 could be operated with the cylinder axis at any orientation. In the cylinder, a gas charging valve was provided that may be coupled to a compressed gas bottle. Similarly, a liquid charging valve was provided so that a measured volume of liquid could be injected into the cylinder by hypodermic syringe (before the cylinder was pressurized with gas).

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Electrical resistance tubular heater

Regenerator

Water coolcr

Motor driven adjustable crank/connecting rod mechanism

Electrical resistance tubular heater

Regenerator

Water coolcr

Motor driven adjustable crank/connecting rod mechanism

Cylinder

Displacer

"Structural steel frame

Fig. 8.14. Experimental apparatus for the study of compound working fluids.

Cylinder

Displacer

"Structural steel frame

Fig. 8.14. Experimental apparatus for the study of compound working fluids.

hot end of the cylinder. The other end operated always at the cold end so that along the length of the displacer there was an appreciable temperature difference. The displacer and cylinder walls were made of thin wall stainless steel to reduce the conduction losses.

A family of displacers with a range of length to diameter ratios has been made. Similarly, the cylinder is made up from several elements of a length to suit the various displacers and the several displacer strokes that may be used. The cylinder elements are constructed in several internal diameters to provide for a range of clearances in the regenerative annulus. The 'gap' or diametral clearance has a critical effect on regenerative annulus performance. For a given pressure drop the mass-flow rate is a function of the cube of the gap dimension. 1l is vital that the displacer be maintained central (to provide a uniform gap) and that the gap be minimized.

An alternative arrangement, as yet unused, is available where a seal is provided at the cold end of the displacer and a flow path for the working fluid is provided front the hot to cold end through:

lb) a regenerator case, which, in turn, is connected to (e) a water-cooled tubular heat exchanger connected to the cold end of the cylinder.

The apparatus, in either form, corresponds to a single-cylinder Stirling engine of the pjston-displacer type in which the piston is held stationary, so the volume of the system remains constant. As the displacer moves iri the cylinder, fluid is displaced from the hoi space to the cold space and vice versa. This causes a cyclic variation in the pressure of the working fluid that is, of course, exactly in phase with the motion of the displacer. or the volume variation in the hot 'expansion' space. Maintaining the total volume constant by, in effect, holding the piston fixed, introduces considerable simplification in the analysis of a Stirling engine system. The intent of the apparatus was. first, to support the development of a procedure for predicting the range of pressure variation in the constant total volume system that conformed well with the measured values for different compound fluids. It was anticipated that a similar procedure could be adapted for real Stirling engines with a variable total volume system. The same apparatus was intended for subsequent studies of regenerative heat exchangers in which the fluid experienced a phase change. Virtually no information in the literature has been located about the operation of regenerative heat exchangers operating with a phase change of the fluid flow.

Initial experience with the apparatus using air and water has been most disappointing for no way has been found to maintain a uniform distribution of the two components in the cylinder. The water either collects in the cold space, or if the apparatus is operated upside down, with the hot space below the cold space, there are intermittent, massive pressure excursions when large droplets of water are suddenly vaporized. The operation is reminiscent of those unfortunate early aviators whose spectacular catastrophes are regularly screened on television to the huge amusement of one's children.

Notwithstanding the author's own lack of success, the principle of the apparatus is commended to other potential researchers as a good place to start the study of this exciting field of research.

CHEMICALLY P. FACT I'VE WORKING M UIDS Introduction

Improvement in the power density of a Stirling engine can be obtained by the use of a chemically-reactive working fluid operating as a condensing. dissociating gas. It may be used alone or in combination with the inert gaseous carrier component.

The effect of a condensing, dissociating working fluid is similar to the two-phase two-component working fluid discussed above. The chemically reactive working fluid, liquid in the low temperature region of the cycle, evaporates to a vapour as the temperature increases with an accompanying increase in the specific volume. At higher temperatures the vapour or gas may dissociate into less complex compounds or to elemental species in the gaseous state. Dissociation may be accompanied by a stoichiometric mole change and the reaction may be endothermic or exothermic.

Nitrogen tetroxide

The fluid that has received most attention is nitrogen tetroxide. The dissociation reactions of interest are:

Both reactions are rapid and equilibrium conditions are swiftly approached at all temperatures (Russer and Wise 1956). The further reaction involving dissociation of NO to elemental species is. by comparison, a slow reaction that may be disregarded.

Nitrogen tetroxide is of interest because of the large increase in the number of moles as the equilibrium is shifted from left to right, and because the reactions are fast and are endothermic from left to right. This has the elfect of increasing the apparent heat capacity of the fluid and assists in a closer approach to isothermal expansion. Finally, and most importantly, the reaction has been studied extensively and tables of thermodynamic and transport data are available (Baker et (d. 1964. Krasin and Nes-terenko 1967. Krasin 1971).

Nitrogen tetroxide is highly corrosive and very toxic, an unpleasant fluid altogether to contemplate for use in Stirling engines. Such considerations are not important in theoretical investigations of the mole change elfect. though for practical engines or for laboratory investigations they are all important.

Walker and Metwally (1977) studied the use of nitrogen tetroxide as the working fluid in a Stirling engine. The study was an elementary theoretical analysis, based on the Schmidt cycle, with nitrogen tetroxide in combination with an inert gaseous carrier component. The study was designed to complement the earlier work of Walker and Agbi (1974) with a two-phase two-component condensing, non-reactive working fluid (water and air). The results of the two studies were presented in the literature in exactly similar format for direct comparison. With the partially-reactive, condensing, working fluid there was a very substantial increase in the specific output compared with the use of a gaseous working tluid. Comparisons were made on the basis of similar maximum pressures, maximum volumes, and temperature ratios so that, at least to first order approximation, the engines were the same weight, size, and cost. The comparison was made for the standard design case for

Flo. 8.15. Effect of system variables in tlie power parameter of a Schrnidt-cyclc Stirling engine system with a partially reactive, condensing working fluid; 0 is the mass ratio (ntjmm) oi reactive component to inert gaseous carrier (after Walker and Mctwally 1977).

Flo. 8.15. Effect of system variables in tlie power parameter of a Schrnidt-cyclc Stirling engine system with a partially reactive, condensing working fluid; 0 is the mass ratio (ntjmm) oi reactive component to inert gaseous carrier (after Walker and Mctwally 1977).

maximum power output with a gaseous working fluid given by Walker (1962), i.e. temperature ratio r^O.3, dead volume ratio A'= 1.0, swept volume ratio k =0.74 and the phase angle a ^0.54-xr (97°).

The results of a parametric study about the standard design case are presented in Fig. 8.15. In this figure, the mass ratio fi is m,/ma where m, and ma are the masses of reactive component and inert gaseous carrier respectively in the working space. This figure may be compared with Fig. 8.9, a similar diagram for the two-phase two-component working fluid.

Work diagrams are given in Fig. 8.16 for the standard-design case with a gaseous working fluid, ft = 0, and with a partially reactive condensing working fluid. This figure may be compared directly with Fig. 8.7 for the two-phase two-component working fluid.

The work ratio C, or degree of improvement compared with a gaseous working fluid, as a function of the mass ratio (i is as shown in Fig. 8.17. This fit?tire for a nnrtiidlv-w»nrtiv*» u«/>ri*!nn n«.;ri m««» j:—

Fig. 8.16. Work diagrams foi a Schmidt-cycle Stirling engine system with a partially reactive condensing working fluid. Diagrams arc drawn foi the standard design cat.c r = 0.3. X - 1. a = 0,54tt r«ids, k = 0.72: ft is the mass ratio {ni,/j»,) of reactive component to inert gaseous carrier (after Walker and Metwally |977>.

Expansion space

Fig. 8.16. Work diagrams foi a Schmidt-cycle Stirling engine system with a partially reactive condensing working fluid. Diagrams arc drawn foi the standard design cat.c r = 0.3. X - 1. a = 0,54tt r«ids, k = 0.72: ft is the mass ratio {ni,/j»,) of reactive component to inert gaseous carrier (after Walker and Metwally |977>.

Fig. 8.17 Work ratio for a Schmidt-cvcle Stirlinc enttinc svstcm with a nartiallv rmrtivr

Fig. 8.17 Work ratio for a Schmidt-cvcle Stirlinc enttinc svstcm with a nartiallv rmrtivr

The general conclusion drawn from this very limited and highly idealized study was that the chemically reactive working fluid offered no significant advantages over the more simple two-phase, two-component working fluid. However, it is not satisfactory for the matter to be simply left there, for the work done so far is not of sufficient depth for definitive conclusions to be drawn. Others are therefore encouraged to extend these studies.

Single component multi-phase systems

There is the possibility that fluids could be found with the appropriate properties to allow their use as the sole component in a condensing, dissociating system. Removal of the inert gaseous carrier would obviate the problem of ensuring a uniform mass ratio (i at the various tempera-lure levels in the engine. Continuous uniform mixing has proved so far to be a particular difficulty in experimental work with two-phase two-component working fluids. There is no reason to believe the problem would be any less with condensing, dissociating fluids in combination with an inert carrier. There is much to be gained by the use of a single component working fluid, liquid in the cold space, and vapour, perhaps dissociated, in the hot region. Water was used in this wav as the working fluid for the Thermoelectron 'tidal regenerator engine* of an artificial heart system described in Chapter 17.

Wolgemuth (1969a) studied the equilibrium performance of the theoretical Stirling cycle with nitrogen telroxide and later Kovtun et al (1967) did similar work. Wolgemuth concluded that with a condensing chemically-reactive working fluid, a substantial gain in power density could be achieved. However, the gain was somewhat offset by a loss in thermal efliciency depending on the cycle pressure ratio and the regenerator effectiveness. On the other hand, Kovtun et al. concluded that use of a dissociating gas in the ideal Stirling engine would increase the efliciency. The example was given for an efficiency of 13 per cent writh a non-dissocialing gas, increasing to 24 per cent with a dissociating gas.

The reason for the discrepancy in the lindings of Wolgemuth and Kovtun et al. lies simply in the fact that both assumed deviations from the ideal Stirling cycle and made different assumptions. In the simple idealized Stirling cycle <as in every thermodynamic cycle where all the external heat is added or rejected at constant temperatures) the thermal efficiency has to be the Carnot efficiency, whatever the working fluid.

The availability of large computers and the increasing level of sophistication in Stirling engine analysis combine to suggest that interesting studies in the field of complex working fluids lie ahead. There is an urgent need for fundamental experimental work in this area.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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