Recuperator Design

From a heat transfer standpoint, the best recuperator design is usually one in which the flue gas is pulled though relatively large passages while the air is pushed through smaller passages at high velocity. This also assures that any leaks (and there will eventually be some leaks) will not dilute the combustion air and upset control of the combustion process. 

Maintain a minimum airflow of 10% of maximum recuperation design through the recuperator during all operating conditions to assure some coolant flow through all tubes to prevent them from being heated to flue gas temperature. 

Recuperator External Area (1-1 10 ) (k recuperator external area) Recuperator area derived from code (recuperator design team) Section 9. External area for recuperator is 14000 cm 18000 cm for 100 kWe and 200 kWe recuperators, respectively Approximately 97% - 99% total for internal and external areas oombined-Commeicial data from Hamilton Sundstrand and Garrett for similar type heat exchangers (Section 9) Leak to space 99.63% lOOlWVe 99.53% 200 kWe... 

Sundstrand recuperator design (from Reference 9- 54) to the mass models being... 

Another approach in assessing the effects of pressure drop on system performance Is to express the pressure drop in terms of system entropy as discussed in Reference 9- 44. This Reference also compares various recuperator designs and evaluates their performance from both a pressure drop and entropy increase perspective. 

Figure 9 18 Simplified temperature entropy diagram illustrating the effects of increased compressor pressure ratio on recuperated energy (assumes 100% effective recuperator designs).

One technique to help balance the flow maldistribution effects is to maintain symmetry in the manifold designs for the high pressure and low pressure sides of the recuperator. In a recuperator design which is primarily counter flow, this symmetry helps to maintain similar flows for a given region of the recuperator for the low pressure and high pressure side flow paths. 

Recuperator designs in the past have incorporated plate overlays which are brazed to the sides of the recuperator and welded to the top/bottom plates and the manifold structures to minimize the number of exposed brazed joints to space. One example of this design approach is provided in Reference 9- 49. NRPCT recommends that a similar all-welded external structure (vs exposed brazed joints) be incorporated to minimize the likelihood of a recuperator missionending leak to space. 

In Reference 9- 67 Garrett Corporation (now part of Honeywell) estimated the failure probabilities of various Brayton system components based on a variety of open cycle engine data. Table 9-15 summarizes the projected failure probabilities for the various Brayton system components. For the recuperator a failure probability rate of 3 x 10 failures/year was estimated. This estimate is based on open cycle systems and is not directly applicable to a recuperator designed for a Prometheus based mission. However, the estimates provide confidence that a reliable recuperator can be constructed for a 15 to 20 year operational life. 

Perform further flow maldistribution studies on the recuperator designs proposed for Prometheus. In conjunction with these studies, perform component design trades on recuperator mass, pressure drop and effectiveness to determine if the low pressure drop goals can be met with the current mass and effectiveness goals. 

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