A Fluid Resistance Coefficient of a Beam inside a Plenum above Suspended Ceiling : Part 1-Air Flowing over a Beam

In air conditioning/ventilation systems which use a ceiling plenum as the route of airflow for air conditioning or ventilation, a beam inside the ceiling plenum can be regarded as the largest resisting substance for the airflow. Various airflow resisting substances inside a ceiling plenum can increase the initial and running cost. Moreover, the uneven pressure distribution generated can cause to make the supply and return airflow rate not uniform and deteriorate the indoor air quality and thermal environment. In order to assist the pressure resistance designing of a ceiling plenum, this report verified the effectiveness of equations by comparing the calculated and experimental values for the local loss of air flowing over a beam inside the ceiling plenum. The analysis was applied to the resistance loss of the beam with the lowest edge of right angle and with that of 1/5 gradient angle. As a result of the analysis, the following were found. (1) For the beam with lowest edge of right angle (The height of the beam is represented by B in Figure-1); Resistance coefficient can be expressed approximately by the equation for the one-step expansion of air flowing over the beam. ζ_1=(1/K-1)^2 Re in this case is 9.6×10^3<Re<7.9×10^4 (2) For the beam with lowest edge of 1/5 gradient angle (The height of the beam is represented by B' in Figure -2); 1) Resistance coefficient for one-step and two-step expansion can be expressed by two equations below respectively. [figure] Fig. -1 Conceptual drawing of airflow at one-stage expansion (in case of right angle) ζ_1'=(1/K'-1)^2 ζ 2'=(1/K'-1/(1-m'))^2+(1/(1-m')-1)^2 2) In a range of the height ratio at 0.68<m<0.85, it can be expressed approximately by the equation for one-step expansion. 3) In a range of the height ratio at 0.85≦m≦0.94, the experimental value stays between the values of the expressions for one-step and two-step expansion. Re in this case is 1.6×10^4<Re<6×10^4 The symbols used and the conceptual drawing of air flowing over the beam are shown below. Legend of main symbols ζ_1: Fluid resistance coefficient of one-step expan sion (in case of right angle) ζ_1: Fluid resistance coefficient of one-step expan sion (in case of 1/5 gradient angle) [figure] Fig. -2 Conceptual drawing of airflow at one-stage and two-stage expansion (in case of 1/5 gradient angle) ζ_2: Fluid resistance coefficient of two-step ex-pansion (in case of 1/5 gradient angle) V_1: Mean velocity inside ceiling plenum V_c: Maximum velocity of air flowing over beam (in case of right angle) V_c': Maximum velocity of air flowing over beam (in case of 1/5 gradient angle) H: Height of ceiling plenum B: Height of beam (in case of right angle) B': Height of beam (in case of 1/5 gradient angle) K: Velocity ratio (V_1/V_c) (in case of right angle) K': Velocity ratio (V_1/V_c') (in case of 1/5 gradi ent angle) m: Height ratio (B/H) (in case of right angle) m': Height ratio (B'/H) (in case of 1/5 gradient angle) Re: Reynolds number of air flowing under beam