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    ber in the simulation, which was less than 350. Farmer, Pike, and
    Cheng (2005) outlined the numerical models available for analyz-
    ing complex processes having flow instabilities and complicated
    geometries such as spray combustion, acousticwaves and transient
    start-up. Breach and Ansari (2007) derived a numerical method
    to model dynamically varying vapor–liquid phase equilibrium for
    non-ideal binary systems by augmenting the existing constitu-
    tive equations of the CFD. Mass transfer during condensation and
    vaporizationwasmodeled via equivalently varying source and sink
    terms. The mass transfer was governed by chemical potential at
    the liquid–vapor interface. Equilibrium was assumed at the phase
    boundary during transient mass transfer prior to steady state con-
    ditions.Based on this survey, a reasonable number of CFD modeling
    studies have been undertaken to model the hydrodynamics and
    flow pattern in packed and sieve tray columns. However, a limited
    CFD modeling was done to model fluid hydrodynamics over valve
    trays. This is probably due to the complex gas–liquid contact upon
    the tray caused by this type of valve.
    2. Experimental aspects
    The experimental rig comprised a 30 cmdiameter, 120 cmheight
    transparent column equipped with a single valve tray, an 8 cm
    height weir and two downcomers as shown in Fig. 1. A novel
    method was employed to investigate the quality of gas–liquid con-
    tact. A very light fluctuating plate which was able to move in
    upward/downwarddirections,was installedat aheight of 8 cmfrom
    the tray. This plate was fabricated from polystyrene foam with a
    density of 10 kg/m3 with a total weight of 6.5 g. The air bubbles
    trapped by the liquid caused the fluid to hit the fluctuating plate and
    move it upward. The fluctuation frequency of this platewas used as
    a criterionfor identifying the gas–liquid contactperformance onthe
    tray. Three pairs of laser emitter/receiverswere placed at heights of
    8.5, 9.5 and 10.5 cmfromthe tray. Thefluctuating plate aswell as the
    laser emitter/receivers are also shown in Fig. 1. The red laser inter-
    cepted by the receiver is sent to a digital counter that records the
    number of disconnections between emitter and receivers caused by
    thefluctuating plate. Three valves all of the same shape but different
    weights of 10, 13.5 and 20 g were examined.
    In order to provide more quantitative results, a set of pho-
    tographs using a digital camera were taken from gas bubble
    distribution inside the liquid phase. Experimental photographs
    have been analyzed using “OlysiaM3” computer software (available
    with “Olympus GX71” microscope and “Olympus DP12” camera)
    to measure the bubble size distribution in the liquid for the three
    examined valves.
    2.1. Experimental design method
    Analysis of variance (ANOVA) is a reliable tool to analyze and
    determine the degree of certainty of experimental data. Nested or
    hierarchical design which is an arrangement of ANOVA method, is
    applied when the levels of one factor (e.g., factor B) are similar but
    not identical for different levels of another factor (e.g., A). In this
    design, the levels of factor B nested under the levels of factor A
    (Montgomery, 2001). In the present study, the repeatability of the
    measured values and the effect of valve weight on its performance
    were investigated using two-stage nested designs.2.2. Experimental results
    Three valves having the same shapewith differentweightswere
    used. The differences in their performances were analyzed pair to
    pair using two sets of two-stage nested designs. Each valve was
    examined at four different airflow rates and a constant liquid flow
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