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The air engine: Stirling cycle power for a sustainable future

A J Organ, mRT - Regenerative Thermal Machines, UK

 - contains previously unpublished insights into the pressure-wave and thermal-lag engines
 - deals with a technology offering scope for saving energy and reducing harmful emissions without compromising economic growth
 - identifies and discusses issues of design and their implementation

Two centuries after the original invention, the Stirling engine is now a commercial reality as the core component of domestic CHP (combined heat and power) – a technology offering substantial savings in raw energy utilization relative to centralized power generation. The threat of climate change requires a net reduction in hydrocarbon consumption and in emissions of 'greenhouse' gases whilst sustaining economic growth. Development of technologies such as CHP addresses both these needs.

Meeting the challenge involves addressing a range of issues: a long-standing mismatch between inherently favourable internal efficiency and wasteful external heating provision; a dearth of heat transfer and flow data appropriate to the task of first-principles design; the limited rpm capability when operating with air (and nitrogen) as working fluid. All of these matters are explored in depth in The air engine: Stirling cycle power for a sustainable future. The account includes previously unpublished insights into the personality and potential of two related regenerative prime movers - the pressure-wave and thermal-lag engines.

ISBN 1 84569 231 4
ISBN-13: 978 1 84569 231 5
August 2007
306 pages  234 x 156mm  hardback  
£155.00 / US$265.00 / €185.00

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About the author

Allan J Organ, PhD, DEng, ScD, FIMechE is known internationally for his work on Stirling engines. He is author of some 50 technical papers and four highly regarded texts on regenerative thermal cycles. The material and its treatment reflect experience accumulated over four-and-a-half decades of university research in the UK, Canada and South America.

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Contents

PART 1 A LONG-OVERDUE RE-APPRAISAL
PART 2 LIVING WITH INCOMPRESSIBLE FLOW DATA
PART 3 WORKING WITH THE REALITY OF COMPRESSIBLE FLOW
PART 4 SOME DESIGN CONSIDERATIONS
Appendices

PART 1 A LONG-OVERDUE RE-APPRAISAL

The famous engine that never was
 - Status quo
 - The legend
 - History reconstructed
 - Exploratory firing tests
 - Reassessment
 - Post script

What Carnot efficiency?
 - Ideal cycle – or perfect alibi? What Carnot efficiency? Old heat exchanger, new air pre-heater
 - Resources for first-principles gas path design
 - A further take on friction factor Cf – and not the last
 - Beyond the sound barrier
 - Geometric descriptors for the wire matrix
 - Inconsistencies uncovered
 - Résumé

The counter-flow spiral heat exchanger – Spirex
 - Heat provision as an integral part of the engine system
 - Thermal analysis
 - Heat transfer and flow friction correlations
 - Special case of high NTU
 - Numerical integration
 - Specimen solutions
 - Discussion

A high-recovery-ratio combustion chamber
 - The design problem
 - Principle
 - Operation
 - Materials
 - Preliminary operating experience
 - Second design interation

PART 2 LIVING WITH INCOMPRESSIBLE FLOW DATA

The regenerator problem brought down to size
 - Background
 - Assumptions
 - Defining equations
 - Boundary conditions
 - Flush ratio
 - Integration algorithm
 - Specimen temperature profiles
 - Design criterion for NTCR
 - Alternative formulation in corroboration
 - Conclusions

The regenerative annulus and shuttle heat transfer
 - Introduction
 - Background
 - Reformulation
 - Assumptions
 - Analysis
 - Cyclic shuttle heat transfer loss

The rotating-displacer air engine
 - Résumé
 - An alternative
 - Taylor parameter
 - A rotating-displacer air engine
 - Academic design exercise

The strange case of the self-regulating air engine
 - Background
 - Some realities
 - Constructional details
 - Exploratory power and torque measurement
 - 'Self-regulation'
 - Tentative explanation
 - Conclusions

10.1533/9781845693602.2.107
 - The concept
 - 'Thermal lag' engine
 - Ideal gas process sequence
 - A detailed model of the thermal processes
 - Limited heat transfer
 - Flow losses
 - A practical thermal lag engine
 - Preliminary operating experience
 - After-thought

PART 3 WORKING WITH THE REALITY OF COMPRESSIBLE FLOW

New correlations for old
 - Right data – wrong application
 - The misleading Cf – Re correlation
 - Flow data acknowledging Ma
 - Dynamic Similarity to the rescue
 - Farewell to friction factor
 - The new format
 - What the new format reveals about 'incompressible' flow data
 - Epitaph

Regenerator thermal analysis – un-finished business
 - Regenerator design in context
 - Assumptions
 - Modified diffusion law
 - Numerical solution
 - Parameters of operation
 - Pressure and velocity fields
 - Inevitable asymmetry of flow cycle
 - Anisotropic matrix
 - Discussion

Flow passage geometry
 - Scope
 - Symmetrical gauze – flow perpendicular to plane of weave
 - Flow parallel to plane of weave
 - Commercial availability
 - Specimen isotropic material – metal foam

Beyond the performance envelope
 - Introduction
 - Method of Characteristics
 - 'Unit process' of the integration sequence
 - High-speed operation – the pressure-wave engine
 - Discussion

For the sceptics
 - What does it all add up to? Flow in the isolated gauze aperture
 - Defining equations
 - Radial component of kinetic energy
 - The not-so-square-weave wire gauze
 - Kinetic energy of rotation
 - 'Real' (van der Waals) gas
 - Downstream pressure recovery
 - Simulated correlation p/p = p/p{Sg, Ma, y, dwmw.}
 - Implications for first-principles design
 - Résumé

PART 4 SOME DESIGN CONSIDERATIONS

Scaling - and the neglected art of back-of-the-envelope calculation
 - The overriding objective
 - Gas path scaling – update
 - Back-of-the-envelope Ma and Re in the regenerator
 - Limiting Ma
 - Compressibility vulnerability chart
 - Heat transfer
 - Implications for back-of-envelope design
 - A 'screening' test
 - The wider rôle of scaling

'How to make a business out of Stirling engines today'
 - Tribal wisdom
 - From alchemy to appropriate technology
 - What else has changed? The VDF-750(aS)
 - Drive mechanism/kinematics
 - General mechanical construction
 - Pressure balance seal
 - Beyond 2006

Appendices