Gravity Dependence of Pressure Drop and Heat Transfer in Straight Two-Phase Heat Transport System Condenser Ducts

Tutorial on Quantification of Differences between Single- and Two-Component Two-Phase Flow and Heat Transfer

“…-In the case of very lengthy lines in mechanically pumped two-phase loops the pressure (saturated temperature) gradient has to be kept small to guarantee a small end-to-end pressure (saturated temperature) difference to meet -6-NLR-TP-2003-037 the requested isothermality, and to keep the evaporator exit vapour quality below 0.15. The latter is because in flowing refrigerants the vapour quality usually increases with pressure decay (Delil, 1992). Ethane is an exception: This issue (called flashing) will be discussed later in this tutorial, since it one of the crucial differences between single-and two-component two-phase flow.…”

“…With reference to detailed discussions (Delil, 1991(Delil, , 1992(Delil, , 1998 it is remarked that, considering only the identity of Morton number and the identity of We/Fr for prototype and scale model, important conclusions can be drawn from figures 8 and 9, showing the temperature dependence of g.Mo l = l .σ 3 /µ l 4 and ( / l ) ½ = D.g ½ /(We/Fr) ½ : -First, scaling at the same gravity level means a fixed gMo = l σ 3 /µ l 4 -value for prototype and model. Figure 8 shows that the value l σ 3 /µ l 4 = 2*10 12 m/s 2 can be realised by 115°C ammonia, 115°C methanol, 35°C water, -10-NLR- TP-2003-037 180°C propanol, 235°C propanol, 250°C thermex and 350°C thermex.…”

“…An important quantity (to be measured during two-phase flow experiments) is the pressure drop in adiabatic sections and in condensers: sections, being considered crucial for two-phase system modelling and scaling. The equations for annular flow pressure gradients in straight tube condensers and adiabatic lines, extensively discussed in literature (Delil, 1991(Delil, , 1992(Delil, , 1998 are based on an elaborate journal article (Soliman, 1968). The total local (local position z-dependent) pressure gradient for annular flow is the sum of friction, momentum and gravity gradients.…”

“…This confirms the scaling statement, that room temperature low-gravity behaviour can be simulated by terrestrial tests at far lower temperatures. Figure 10 depicts the results of calculations of the vapour quality along the duct for three gravity levels (0, Earth and 2-g) and three duct diameters (8.05, 16.1, 24.15 mm) at 300 K, gave the ratio of duct lengths L c (m) needed for condensation under zero-g and one-g respectively (Delil, 1992). The ratio between full condensation lengths in zero-g and on Earth ranges from roughly 1.5 for the 8.05 mm duct, via 11 for the 16.1 mm duct, up to more than 30 for the 24.15 mm duct.…”

“…It is to be investigated if an annular flow assumption leads towards slightly or substantially overestimated full condensation lengths. To assess the impact of the saturation temperature on condensation, similar curves were calculated for two other temperatures, 243 K and 333 K (Delil, 1992). The calculations show that the full condensation length increases with the temperature for zero-g conditions, but decreases with temperature for the other gravity levels.…”