Discussion: “Design Improvements to a Biomass Stirling Engine Using Mathematical Analysis and 3D CFD Modeling” (Mahkamov, K., 2006, ASME J. Energy Resour. Technol., 128, pp. 203–215)

For readers wanting to know how computational fluid dynamics ͑CFD͒ modeling might help the Stirling engine realize its full potential as a source of sustainable energy, the " closure " from Mahkamov in the December 2007 issue ͓1͔, may have proved somewhat disappointing. The author has chosen for the most part not to answer the specific questions raised by the three discussion contributors ͑Bur-ton, Burton, and Kinnersly͒ regarding the large differences between test data and CFD computation ͓2͔, the detailed evaluation of Figs. 13 and 14 with regard to entrapment ͓3͔, and indeed the confusion arising from some of the figure captions of the original paper ͓4͔. Instead Mahkamov has preferred to concentrate on supposed " shortcomings " in the original ␥ engine, which I designed and which he apparently converted to ␣ mode. Before discussing in detail these issues, it would be useful to dispel one major misconception. Mahkamov stated ͓1͔ that his " calculation ͑MM͒ demonstrated that the ␣ machine had higher power output ͑than the original ␥ engine͒ because of its smaller overall dead volume and lower hydraulic losses. " In converting the original ͑as designed͒ gamma engine, with its 180 mm diameter power piston, into an alpha engine, the original 220 mm displacer piston now becomes the power piston, and so the power swept volume increases some 40% ͓3͔. As West ͓5͔ so clearly explained, " it is the expansion piston that is pushed out during the high pressure phase and can do work. " One does not need CFD modeling to know this will happen; a back-of-envelope calculation ͑which Mahkamov seemed not to like͒ will suffice. That the increase in power output in ␣ mode found in both mod-eling and test work exceeded 40% was due almost entirely to the " gas entrapment problem, " which restricted speed and power in ␥ mode. The author has not addressed the most important question raised in discussions of his original paper—the large difference in test-bed power outputs compared with CFD predictions. As I have pointed out ͓2͔, the CFD power predictions for the engine in ␥ mode are around 3.5 times smaller than those observed during test-bed work carried out in my presence, yet Mahkamov's original claim ͓6͔ was that " experimental data, which is available of the mechanical brake power output and speed, indicates that ͓these are͔ reasonably close to those predicted by using the 3D …