COMPACT HEAT EXCHANGERS FOR SMALL GAS TURBINES 550142
Inasmuch as the future of small regenerative gas turbines depends on the compactness of its heat exchanger, the problem of selecting the engine design variables to facilitate the reduction of heat exchanger size is of vital importance and has been given detailed consideration. The effects of leakage and engine pressure loss factor, in relation to the conditions of minimum flow and minimum regeneration, and the corresponding pressure ratio requirements, have all been examined. In general, the minimum flow and regeneration required decrease with the engine pressure loss factor, which is, of course, favorable to heat exchanger size. However, in order to achieve compactness, a sufficient part of the engine pressure loss factor must be apportioned to the heat exchanger gas side pressure drop. Therefore, the importance of careful aerodynamic design to reduce pressure losses through the ducting cannot be over-stressed.
Two types of the more promising heat exchangers, the contra-flow rotary regenerator and the cross-flow recuperator, have been discussed in detail, and equations developed by which the heat exchanger dimensions and weight can be rapidly estimated from known performance requirements. An optimum gas to air side area ratio around 2 for regenerators has been found from the standpoint of minimum flow area and weight. A set of reference charts have been prepared to facilitate rotary regenerator calculations and method given with which the dimensions can be corrected for finite reduced rotor speed. The relative merits of the different matrix configurations have also been compared. It has been shown that for given heat transfer surface and Reynolds Number selection, the rotary regenerator dimensions are affected by the required air flow, w, efficiency, , and gas side pressure loss limit, Δp′,
and for the cross-flow recuperator,
For given performance requirements and heat transfer surface, the size of the heat exchanger has been shown to increase rapidly with the Reynolds Number. The latter, however, has little effect on flow area. Whereas the rotary regenerator can take full advantage of the low Reynolds Number design, the minimum value of the Reynolds Number that can be used for cross-flow recuperator design is limited by the length that is available in the no-flow direction. For a given heat transfer surface, Reynolds Number selection, and performance requirement, the necessary recuperator no-flow length also increases linearly with flow. Hence, whereas cross-flow recuperator merits serious consideration for small flow machines, rotary regenerators must be used when power requirement is above a certain level.
Finally, the problem of estimating the temperature distribution in the heat exchangers has been reviewed and an approximate method developed by which temperature distribution of cross-flow recuperators can be rapidly computed.