Entrained fluid heat capacity is shown to have a significant and positive effect on the performance of a passive regenerator. The ineffectiveness of the regenerator is presented as a function of three dimensionless parameters: the number of transfer units, the utilization, and the entrained fluid to matrix heat capacity ratio. Three different behaviors are observed for a regenerator with entrained fluid heat capacity. The effect of the entrained fluid can be accounted for over a large range of conditions using the concept of an augmented-NTU which can be substituted for the actual NTU in analyses that neglect entrained fluid capacity. (c) 2005 Elsevier Ltd. All rights reserved.