Abstract
In air conditioning/ventilation systems that use a ceiling plenum as the route of airflow for air conditioning or ventilation, a beam inside the ceiling plenum can be the largest resisting substance for the airflow. In order to design the pressure resistance of the air flowing over beams inside a ceiling plenum, this report proposes simplified equations by comparing the calculated values with the experimental values for the local loss of the air flowing over the beams. In the former report, the equation to calculate the resistance coefficient of the air flowing over a one-step beam was presented. This report presents equations to calculate the resistance coefficient of the air flowing over consecutive 2-step beams. As a result of the experiment, the following were found. (1) In a range of height ratio (m_1=B_1/H_1) at 0.72≦m_1≦0.94, the resistance coefficient did not result in the value that doubles the ζ at one-step expansion. (2) As a result of visualized experiment and numerical analysis, the air flowing over the first beam adhered to the windward side of the second beam. 1) Resistance coefficient for one-step and two-step expansion can be expressed by two equations below respectively. 2) The adhering point (H_2) of the air flowing over the first beam on the second beam can be expressed by the equation below. H_2=-1.3B_1+1.34H_1(H_f=-1.3m_1+1.34) 3) Resistance coefficient obtained under the experimental condition this time can be expressed as follows; [numerical formula] Re in this experiment is 1.5×10^4≦Re≦2.6×10^4 The symbols used and the conceptual drawing of the air flowing over the beam are shown below. [figure] Legend of main symbols ζ: Fluid resistance coefficient of the air flowing over two-step beams V_1: Mean velocity inside ceiling plenum V_2: Mean velocity when H_2 represents the imaginary height of ceiling plenum H_1: Height of ceiling plenum H_2: Height of adhering point of the air flowing from the ceiling H_f: H_2/H_1 B_1: Height of beam B_2: Height of adhering point of the air flowing from the lowest edge of beam K_1: Velocity ratio (V_1/V_<c1>) K_2: Velocity ratio (V_2/V_<c2>) V_<c1>: Maximum velocity of the air flowing over the first beam V_<c2>: Maximum velocity of the air flowing over the second beam m_1: Height ratio (B_1/H_1) Re: Reynolds number of air flowing under beam
