General Aspects Of Design

The best compilation of design daia for the heat exchangers in Stirling engines is undoubtedly the classic work by Kays and London (1964) entitled Compact Heat Exchangers. The book is. quite simply, required reading for anyone wishing to undertake the design or analysis of the heat nature and wide availability of the book no effort will be made to reproduce the straightforward design procedures for compact tubular, finned and regenerative heat exchangers that are so well treated therein. Rather, the space available will be devoted to brief discussion of some of the aspects of design peculiar to Stirling engines that are not found or not stressed in Compact Heat Exchangers.

The principal consideration particular to Stirling engines is the compelling need to make effective use of the internal void volume of the heat exchangers and connecting ports. We have seen earlier how an increase in the dead volume results in a reduced volume compression ratio Vmux/ Vmln and in a reduced pressure ratio pnxaJpmin. The result is a small pressure-volume or work diagram for the engine and so the output declines progressively as the dead .space increases. The thermal efficiency on the other hand may be unaffected or may even increase if the increase in dead volume has been wisely applied to increasing the effectiveness of the regenerative or other heat exchangers.

If the dead space is simply increased by insertion of a spacer, an oversized header, or "wasted' in some other way the result will likely be decreases not only in the power but also in efficiency.

Overzealous pursuit of the minimum possible dead space is just as deleterious. It will result in insufficient area for heat transfer so the effectiveness of the heat exchangers will be reduced and, further, excessive frictional pressure drops will result with substantial decrease in output and efficiency. A proper balance of areas for heat transfer, with tolerable fluid-friction effects arising from flow, sudden changes in section. entrance effects, and flow reversals will be achieved by careful, thoughtful usage of the internal dead space. This is worthy of the most careful attention and instant, positive rewards await the judicious scrutiny of a design or ol the fabricated prototype. The insertion of instrumentation. particularly displacement or pressure transducers can result in a profound increase in the dead volume of a really Tight' engine design. One should always be alert to the unfortunate consequences that can result in the course of instrumenting a good engine.

HEATF.tt DESIGN

In general most heater designs can be divided into two classes: tubular or finned. An example of each is shown in I7ig. 7.6. Both systems combine three separate heat transfer processes:

(a) eonvective heat transfer from the external heating medium to the walls of the tube or tin.

(b) conductive heat transfer through the lube wall to the inner surface or to the root of the tin and hence to the internal fin.

(c) convective heat transfer from the internal walls of the lube or fin lo

Flo. 7.6. Types of heater head design

In most engines the working fluid is pressurized and so is relatively dense and moving with a relatively high velocity so the internal heat transfer process is well developed. Similarly most metals are relatively .good conductors of heat so that only a small temperature difference across the tube or cylinder wall is necessary to accomplish the desired heat transfer. In a combustion system at atmospheric pressure the limiting heat-transfer process is likely to be the external convectivc heat-transfer process. The products of combustion are not dense and may be moving relatively slowly so that a large temperature difference will be necessary to accomplish the transfer of heat.

Many designs feature a combustion space with the burner located along the axis of the cylinder. This should result in a radial distribution of heal with uniform Lemperatnre distribution around the circumference of the lins or tubes. However it is rarely possible to achieve a completely uniform temperature distribution and thermal distortion may occur. 'Hot spots' will exist and since, to prevent burnout, these must be at or less than the maximum sustainable operating temperature of the metal (the metallurgical limit) the mean temperature of the head will be substantially (up to 100 °C) less than the metallurgical limit. This low mean

The fin design shown in Fig. 7.6 lends itself well to application wilh an annular form of regenerator, li appears to be best suited for use with small engines (less than 1 kW) where the advantages (if simplicity and compact design are self-evident.

For larger engines tubular heat exchangers have been favoured and several interesting variations for tubular heaters are discussed in Chapters 12-16 dealing with engines manufactured by ihe Philips arid licensees of Philips companies.

Tubular beaters of the simple double hairpin tubes shown in Fig. 7.6 offer severe and relatively expensive production problems in brazing the tubes to the cylinder head. A 'dip brazing' or vacuum furnace brazing technique may be used for the stainless or high nickel steel tubes in concert with a stainless steel cylinder head. Tubular exchangers are well suited to designs where multiple regenerators can be used with groups of three to six tubes per regenerator. This provides a flexible design allowing for relatively unrestrained movement of the heater tubes during initial heating and subsequent cool down of the tubes after use. Cracking of heater tubes resulting from restraints imposed on thermal movements are a common feature of designs at an early stage.

The simple hairpin heaters of Fig. 7.6 have now largely been supplanted by more advanced designs using some form of manifold header at the upper end as shown in l ig. 7.7. This facilitates construction and permits the use of different tubes for the internal and external rows. It is not uncommon to find the outer row finned to compensate for the reduced heal transfer because of the temperature of the lower combustion products passing over the outer tubes.

The inequality in temperature between the inner and outer tubes, and also around the periphery of the tubes themselves, imposes a severe design limitation when combined with the very highest pressure stress levels found in advanced engines where thin wall tubes are used to minimize wall conduction effects. This is discussed briefly in Chapter 14 following an excellent presentation on the subject by Zacharias (1973).

Recent developments at Philips have resulted in tubular heaters having manifold headers at both the top and bottom of the lubes as shown in Fig. 7.8. This permits a single axial penetration to the expansion space. This, when coupled with the use of glass ceramic inserts acting as thermal insulators in the hot cylinder ends, allows the use of water-cooled expansion space cylinders of relatively low-cost steels. This interesting development at Philips (Meijer 1978) offers much hope for the future substantial reduction in cost that is necessary if Stirling engines are to move out into a wider stage of application.

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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