Abstract
This is one of the most common fittings of a return air duct system. In spite of this, there have been relatively few theoretical investigation of the pressure loss in these fittings and leading reference books contain little information on this subject. Some efforts already have been made in answering these problems. In 1959, Niitsu et al. made tests on this type fittings and offered empirical formulas from their results. But these formulas have no wide application range. This paper attempts to give a survey of this field by setting up the theoretical formulas for this type of flow and fittings. These theoretical formulas are gained on several assumptions as a result of momentum theory applied to confluent air flow in this type fittings. The charactor of resistance of this type fittings may be explained qualitatively by these theoretical formulas. A few laboratory tests have been made on the fitting models of near full scall at Osawa Memorial Laboratry, Department of Architectural Environmental Engineering, Kanto-Gakuin University by the auther. The theoretical formulas are modified with the auther's test results and the available experimental data by Niitsu et al. The modified formulas are given a name "Modification of the applied theoretical formulas". Within the limits of this test results and the experimental data, these modification formulas would be more extensively useful in the application range than the restrictive empirical formulas by Niitsu et al. These modification formulas may be more exactly agreeable with their experimental data than their empitical formulas.
