Owing to the persistent interest in the approximate methods, the order of magnitude analysis is revisited in the present work and a method is proposed for the quick estimation of drop in temperature of a fluid flowing through a circular duct. The principle involves judicious guessing of temperature drop for one of the investigated cases, while the temperature estimates for rest of the cases are deduced from the initial guess. A problem of duct flow with widely varying inlet temperature and mass flow rate conditions, typical of a high altitude simulation ground test facilities for scramjet combustors, re-entry vehicles, etc., are considered which vary from 400 K to 1200 K with corresponding mass flow rates of 300 kg/s to 0.5 kg/s. The results of this quick estimation method portray remarkable agreement with the exit temperatures of the fluid as predicted by solving the problem using transient quasi-one dimensional codes and two dimensional CFD techniques over the entire operating regime and hence provide a conservative estimate of vital parameters for any design exercise as well as to rate an existing system at off-design conditions. The significant advantages of the proposed method over the others are viz. (a) simplicity (b) lesser computational effort and (c) reasonable accuracy, will be elucidated in detail in this article.

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