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A discharge coefficient equation was incorporated into a mass balancing procedure to compute the elevation of the neutral pressure axis (NPA) for a general distribution of openings. An equation was developed to compute the discharge coefficient of an arbitrary opening as a function of the three dimensional geometry, the pressure difference, the total minor loss coefficient, and the air properties.
A two cell environmental chamber was constructed to simulate the temperature gradients across a wall and ceiling section of a two story residence. Idealized openings could be mounted in the wall at different elevations and one mounting location was provided in the ceiling section.
An experiment was designed to investigate the factors which influence the location of the NPA and to test the validity of the mass balancing procedure. A total of eight opening distributions were defined. Four of these distributions included an opening placed in the ceiling section. The parameters varied were: the total leakage area mounted in the test sections; the size of the individual openings, the geometry of the openings; the vertical placement and the mean temperature difference.
The results indicated that the location of the NPA depends on the relative size of the openings in a distribution, a variable discharge coefficient, and the vertical placement of the openings. The location of the NPA was not a function of the mean temperature difference and the observed degree of temperature stratification had no effect on the NPA. The location of the NPA was predicted within ±2.22 percent of the eave height for each case using the mass balancing procedure.
Application of the mass balancing procedure to an actual structure would require a method to model the air flow through components of envelope leakage as an equivalent opening. A modelling equation was developed which could be used to determine the cross-sectional area and the three dimensional geometry of an equivalent straight rectangular opening which would provide the same air flow as the modeled component.
An experiment was performed to develop the concept of modelling components of envelope leakage as an equivalent straight rectangular opening. Differential pressure measurements were obtained for a group of straight openings which ranged in cross-sectional geometry from a near infinite rectangular slot to a cylinder. The dimensionless flow length, z/Dh, of the openings was varied from 2.0 to 15.9.
It was apparent from the results that the equivalent opening areas were in close agreement with the actual areas of the defined openings. Also the observed flow rates were predicted within the uncertainties of the measurements using a single mean total minor loss coefficient with the discharge coefficient equation and the equivalent opening parameters obtained by application of the modelling equation.