Laminar Flow Theory as a Guide to Compact Recuperator Design

A small gas turbine needs a good recuperator to achieve high efficiency. It is desirable that this recuperator be small in size, light in weight, and low cost. Laminar flow theory is used to show the effect of heat transfer surface selection, passage size, and flow length. A comparative study was then made of counterflow recuperators using heat transfer test data from published sources to show how these variables apply to a typical design. One group of surfaces with simple flow passages, smooth, straight, and constant cross-section, followed laminar flow theory quite closely. A second group of more complicated surfaces deviated considerably from laminar flow theory. For a given performance, both groups show that as flow length is reduced, along with the corresponding passage size, the flow volume and surface area decreases. The corresponding material weight and cost also decrease. There are practical limitations on how far this can go.