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Measurements of flat oval elbow loss coefficients.

Article Excerpt
INTRODUCTION

This paper presents results of an experimental program to determine loss coefficients for several flat oval elbows, i.e., 90[degrees] easy bend elbows, mitered 90[degrees] easy bend elbows with and without vanes, and mitered 90[degrees] hard bend elbows with and without vanes. A complete description of the test program is provided in Idem et al. (2008). In a hard bend flat oval elbow the fluid turns about an axis which is parallel to the minor axis of the fitting while in an easy bend elbow the fluid turns about an axis which is parallel to the major axis of the fitting. This research was motivated by the need to increase the population of flat oval fittings in the ASHRAE Duct Fitting Database (DFDB) (2006). Presently only straight flat oval duct and the 5-gore hard bend elbow are in the DFDB.

EXPERIMENTAL PROGRAM

Refer to Figure 1 for a sketch of the various elbows considered in this project. The dimensions of the flat oval and round elbows are listed in Tables 1 through 3. The test specimens were constructed using 20 gage galvanized steel. The ratio of major-to-minor dimensions ranged from 1.0 to 5.5. For the 5-gore easy bend elbow the dimensionless turning radius ratio R/a equaled 1.5.

[FIGURE 1 OMITTED]

Table 1. Elbows Tested Fitting Geometry Fitting Sizes Test Conditions CF3-2 Elbow, Flat Oval, 5 See Table 2 R/a = 1.5 Gore, 90[degrees], Easy Bend CF3-3 Elbow, Flat Oval, See Table 2 Mitered, Easy Bend, without Vanes CF3-4 Elbow, Flat Oval, See Table 2 Mitered, Hard Bend, without Vanes CF3-5 Elbow, Flat Oval, See Table 2 See Table 3 for Mitered, Easy Bend, Number of Vanes with Vanes CF3-6 Elbow, Flat Oval, See Table 2 See Table 3 for Mitered, Hard Bend, Number of Vanes with Vanes Table 2. Nominal Elbow Dimensions Nominal Aspect Ratio A x a, mm x mm (in. x in.) 1.0 762 x 762 (30 x 30) 2.2 356 x 152 559 x 254 787 x 356 (14 x 6) (22 x 10) (31 x 14) 3.7 381 x 102 559 x 152 940 x 254 (15 x 4) (22 x 6) (37 x 10) 3.8 584 x 152 965 x 254 (23 x 6) (38 x 10) 5.5 838 x 152 (33 x 6) Table 3. Number of Turning Vanes Easy Bend with Vanes Hard Bend with Vanes Minor Axis, mm (in.) Number of Vanes Major Axis, mm (in.) Number of Vanes 102 (4) 2 356 (14) 4 381 (15) 4 152 (6) 2 559 (22) 5 584 (23) 5 254 (10) 3 787 (31) 5 838 (33) 5 356 (14) 4 940 (37) 5 965 (38) 5

The elbow test setups shown in Figure 2 included a 30 hp centrifugal fan. A cylindrical nozzle chamber was used for flow measurement. A variable frequency drive (VFD) was used to control the fan speed, and hence the air flow through the system. Flow control screens were mounted upstream and downstream of the nozzle board to settle the flow. The system was blow through in nature. The nozzle board contained four long-radius spun aluminum flow nozzles having throat diameters of 51 mm (2 in.), 102 mm (4 in.), 152 mm (6 in.) and 203 mm (8 in.). The nozzles were mounted on a 25 mm (1 in.) thick plywood board. Various combinations of flow nozzles were selected to obtain a desired flow rate; unused nozzles were blocked by means of smooth vinyl balls. The nozzle pressure drop was measured by two piezometer rings located 38 mm (1.5 in.) on each side of the nozzle board, with both sides connected to a manometer. The nozzle chamber was constructed in...