ASHRAE AB-10-001-2010 Influence of Test Section Entrance Conditions on Straight Flat Oval Duct Apparent Relative Roughness.pdf

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1、2010 ASHRAE 371ABSTRACTAn experimental program was initiated to study the influ-ence of test section entrance conditions on straight flat oval duct apparent relative roughness. The purpose of these tests was to assess whether deviations from test section entrance geometries as prescribed by ASHRAE S

2、tandard 120 have an impact on friction factor and relative roughness. The observed relative roughness values varied as the entrance length increased. The apparent relative roughness obtained using a test setup in compliance with Standard 120 was considerably lower than the value of relative roughnes

3、s obtained with a test setup that did not conform to the standard. It is recommended that all duct and fitting pressure loss tests be conducted in compliance with Standard 120, and that Standard 120 be modified so that the flow measuring chamber velocity does not exceed 1.5 m/s (300 ft/min). It is a

4、lso recommended that the entrance duct length be increased from 10 hydraulic diameters to 12 diameters.INTRODUCTIONThe straight flat oval duct pressure loss tests reported in and Idem et al. (2008) were performed with the same setups as depicted by Figures 2 and 3, where the larger ducts were tested

5、 with the plenum chamber/bellmouth. The nozzle chamber was (1) not large enough for the bellmouth connection to the test duct setup, and (2) the static pressure was not adequate to be measured downstream. It is noted that upstream conical tran-sitions into the straight duct test apparatus violated A

6、SHRAE Standard 120, which specifies that a bellmouth be located upstream of the test section. Furthermore, the transition angles were not in compliance with Figure 20 of ASHRAE Standard 120. Likewise, ASHRAE Standard 120 requires that all straight duct pressure loss tests be conducted on test setups

7、 having an entrance length exceeding ten hydraulic diameters. This study was initiated to determine the effects of varying the inlet conditions on measured pressure loss characteristics and to establish whether the data are repeatable by comparing to the data of Idem et al. (2008). EXPERIMENTAL PROG

8、RAMIn this project pressure loss characteristics of flat oval ducts were tested with different entrance lengths and condi-tions. The purpose of these tests was to assess whether devi-ations from test section entrance geometries as prescribed by ASHRAE Standard 120 have an impact on the measured fric

9、-tion factor and relative roughness. Referring to Figure 1 (excerpted from ASHRAE Standard 120), pressure loss tests on straight ducts and fittings must be performed with an entrance length exceeding ten hydraulic diameters upstream of the test section. In addition, the standard requires a bell-mout

10、h located upstream of the entrance duct. One entrance geometry employed in this study consisted of a plenum cham-ber with a bellmouth mounted upstream of the duct test section, as depicted schematically by Figure 2. Another entrance condition employed three transitions mounted between the airflow me

11、asuring nozzle chamber and the entrance duct to the test section, as sketched in Figure 3. For either upstream geometry condition the test section entrance length Liwas varied over a range of values from approxi-mately 6 to 12 hydraulic diameters. The flat oval duct cross sections considered in this

12、 study are summarized in Table 1. This table also details the upstream lengths that were employed in the experiments. The ducts were 24-gauge galvanized steel with 1.2 m (4 ft) sections Influence of Test Section Entrance Conditionson Straight Flat Oval Duct Apparent Relative RoughnessS. Khaire S. Id

13、em, PhDMember ASHRAES. Khaire is a research assistant and S. Idem is a professor in the Department of Mechanical Engineering, Tennessee Tech University, Cookeville, TN. AB-10-0012010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRA

14、E Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.372 ASHRAE Transactionsconnected by beaded slip couplings. In every case the ducts possesse

15、d a spiral seam having a pitch of 122 mm (4.75 in.). Wooden stands were used to support the duct sections, which were aligned visually and sealed using duct tape. The hydrau-lic diameter was determined by measuring the major and minor dimensions of three duct sections chosen at random and averaging

16、these values.For tests performed using the setup depicted in Figure 2 the plenum chamber served a dual purpose. The large cross section permitted the buildup of static pressure upstream of the test section, which was found to be vital when testing large cross section straight ducts and allowed for a

17、 measurable pres-sure loss. Moreover, the plenum chamber acted much like a muffler, in that any pressure fluctuations were effectively Figure 1 Straight duct test setup (ASHRAE 2008, Figure 16).Figure 2 Straight duct test setup with a plenum/bellmouth chamber.2010, American Society of Heating, Refri

18、gerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHR

19、AE 373damped out upstream of the test sections. The plenum cham-ber did not possess flow settling screens. The plenum chamber cross section was 1.2 m 1.2 m (4 ft 4 ft), and the overall length was 2.4 m (8 ft). The cross section was chosen so that the maximum air velocity in the plenum was on the ord

20、er of 1.5 m/s (300 ft/min). The walls were constructed from 16-mm (5/8 in.) thick plywood sheets. All joints were reinforced inter-nally by 51 mm 102 mm (2 in. 4 in.) wooden boards nailed directly to the plywood. In addition steel corner brackets were placed along the interior of the chamber to incr

21、ease its strength and ensure the cross section remained square. Referring to Figure 2, the plenum chamber was connected to the nozzle chamber by a 508-mm (20 in.) long 610-mm (24 in.) to 406-mm (16 in.) round transition made of galvanized steel. A commercial-grade flat oval bellmouth having an appro

22、ximate 32 mm (1.3 in.) radius of curvature was mounted on the down-stream end of the plenum chamber to facilitate connection to the test duct. The radius ratio r/Dhin each case violated the requirements in ASHRAE Standard 120 (r/Dh 0.25). All plenum chamber joints and connections were further secure

23、d and sealed using a combination of sheet metal screws and sili-cone caulking.For tests performed using the setup sketched in Figure 3 the transition between the nozzle chamber and the test duct consisted of a 610-mm (24 in.) to 406-mm (16 in.) round tran-sition, a 229 mm (9 in.) long straight duct,

24、 and a 406-mm (16 in.) to 305-mm (12 in.) round transition. These transitions were followed by a round to flat oval transition piece, as shown in Figure 4. The round to flat oval transitions (Table 2) had diverging angles of 22.6 and 4.6 in the flow direction, thus exceeding the maximum of 3.5 permi

25、tted by ASHRAE Stan-dard 120.All straight duct pressure loss tests employed static wall pressure taps soldered onto the duct surface at axial locations as prescribed by ASHRAE Standard 120. The pressure taps were fashioned into a piezometer ring using flexible plastic tubing. The piezometer rings we

26、re connected to a single micromanometer by means of flexible tubing so as to measure the pressure drop across the test section. Static gage pressure was measured at each location by inserting tees into the pres-sure tubing. In every test setup the system was blow-through. Airflow was generated by a

27、30 hp centrifugal fan with a variable frequency drive used to control the system airflow. A 1143 mm (45 in.) diameter nozzle chamber was used for flow measure-ment, and screens mounted upstream and downstream of the nozzle board inside the chamber were used to settle the flow. The nozzle board conta

28、ined 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 Table 1. Test Duct Entrance LengthsFlat Oval DuctCross Section(A a)Dhmm (in.)Limm (in.)Li/Dh356 152 mm(14 6 in.)222 (8.8)1346 (53.1) 6.061827

29、 (72.1) 8.232183 (86.1) 9.832689 (106.0) 12.11559 152 mm(22 6 in.)248 (9.8)1469 (58.0) 5.931950 (76.9) 7.872560 (101.0) 10.332907 (114.7) 11.73Figure 3 Straight duct test setup with a round to flat oval transition.2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

30、(www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.374 ASHRAE Transactionsmounted on a 25-mm (1 in.) thick

31、 plywood board. Various combinations of flow nozzles were employed, depending on the desired flow rate. Nozzles that were not used were blocked using smooth vinyl balls. The pressure drop was measured by two piezometer rings located 38 mm (1.5 in.) on each side of the nozzle board, with both sides c

32、onnected to a manometer. For every test the fan was allowed to run for a few minutes before taking pressure and temperature readings so as to allow the readings to stabilize.For each case pressure drop measurements over the test section and across the nozzle board were performed using liquid-filled

33、micromanometers having a measurement accu-racy of 0.025 mm (0.001-in.). Likewise, the pressure upstream and downstream of the test section was measured by means of inclined liquid-filled manometers having a readabil-ity of 0.25 mm (0.01-in.). Static pressure in the nozzle cham-ber was measured using

34、 an electronic manometer having the scale readability of 0.25 mm (0.01-in.). Because of observed pressure fluctuations associated with static pressure measure-ments in the nozzle chamber and test section, these measure-ments were presumed to exhibit an accuracy of 0.63 mm (0.025-in.). The air temper

35、ature in the nozzle chamber was measured using a mercury thermometer having a scale read-ability of 0.5C (1.0F). The dry-bulb and wet-bulb temper-atures of the ambient air were measured using an aspirated psychrometer, with an accuracy of 0.5C (1.0F). The test section temperature was not measured di

36、rectly, but was assumed to be the same as the temperature of the air inside the nozzle chamber. Ambient pressure was measured with a Fortin-type barometer, with an accuracy of 0.25 mm (0.01-in.) of mercury. All measurements of temperature and pressure in this project were in compliance with ASHRAE S

37、tandard 120. All dimensional measurements in these experiments were assumed to have an accuracy of 1%.Each time a new setup was completed, a leakage test was performed on the duct sections downstream of the flow measuring station per ASHRAE Standard 120. This was done to verify that measured leakage

38、 from ducts did not exceed 0.5% of the minimum test flow rate at the maximum test pres-sure, and that the pressure taps and tubing were free of leaks. In every instance the leakage was well below acceptable limits. For brevity the leakage data are not shown.DATA REDUCTIONIn this study all data reduc

39、tion complied strictly with ASHRAE Standard 120. For the straight duct tests the Darcy friction factor was calculated by Equation 1; the plane loca-tions are depicted in Figure 1.(1 SI)(1 I-P)For flat oval ducts the hydraulic diameter is defined such that(2)The flow rate for each test point was calc

40、ulated by Equa-tion 3, where 5 denotes the section upstream of the nozzle and 6 indicates the nozzle throatTable 2. Round to Flat Oval Transition Dimensions (Figure 4)A amm mm(in. in.)L mm (in.)D mm (in.)356 152 (14 6) 305 (12) 305 (12)559 152 (22 6) 305 (12) 305 (12)Figure 4 Round to flat oval tran

41、sition.fpf 12,L1212-1V12Dh1000()-=fpf 12,()L121V11097()2Dh12()-=Dh2 Aa()aa24-+Aa()a+-=2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, dis

42、tribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 375(3 SI)(3 I-P)Additional equations necessary to support the flow calcu-lation per Equation 3 can be found in ASHRAE Standard 120. The Reynolds number in the test section

43、 was determined by Equation 4.(4 SI)(4 I-P)The average air velocity in the duct V was defined by the continuity equation using Equation 5.(5 SI)(5 I-P)The measured pressure loss data were plotted on a Moody diagram in terms of friction factor f as a function of relative roughness /Dh and Reynolds nu

44、mber. These quantities are related by the Colebrook equation.(6 SI)(6 I-P)The relative roughness was determined iteratively by fitting the experimentally determined friction factors to the Colebrook equation using the least squares method; this approach is described in more detail in Idem et al. (20

45、08).The measurements were subjected to an uncertainty anal-ysis based on the method of Kline and McClintock (1953), as prescribed by ASHRAE Standard 120 for random variations of the measurands. In every instance the measurement uncer-tainty estimates were performed with a 95% confidence level.RESULT

46、SFigures 5 through 12 portray the pressure loss data, plotted on Moody diagrams, obtained for straight flat oval ducts having cross sections of 356 mm 152-mm (14 in. 6 in.). The 559 mm 152 mm (22 in. 6 in.) data are not shown, but are avail-able in Idem et al. (2008). The horizontal bars through the

47、 data points represent the expected uncertainty in the measured Reynolds numbers, with a 95% confidence limit. Similarly the vertical bars through each point depict the expected uncertainty in the measured friction factor, with a confidence limit of 95%. Figures 5 through 8 present friction factor d

48、ata for 356 mm 152 mm (14 in. 6 in.) straight flat oval ducts with a bellmouth mounted on a plenum chamber/bellmouth located at the entrance to the test section. The entrance length ratio Li/Dhwas varied over a range from 6 to 12. Similarly Figures 9 through 12 depict friction factor data measured o

49、n 356 mm 152 mm (14 in. 6 in.) straight flat oval ducts where conical transitions were mounted on a nozzle chamber at the entrance to the test section. The entrance length was varied over the same range as the plenum chamber tests. For the Moody diagrams shown in Figures 5 through 12, where friction factors are plotted as a function o