The test facility named as Middle East Technical University Condensation Test Facility (METU-CTF) was installed at the Mechanical Engineering Department of METU. The experimental set up consisting of an open steam or steam/gas system and open cooling water system is depicted in the flow diagram of Figure 1 [4].

Steam is generated in a boiler (1.6 m high, 0.45 m ID) by using four immersion type sheathed electrical heaters. Three of these heaters have a nominal power of 10 kW each and the fourth one has a power of 7.5 kW at 380 V. All the heaters can be individually controlled by switching on or off. The boiler tank was designed to withstand an internal pressure of 15 bar, at T = 20 °С, and was tested at this pressure. The maximum operating pressure of the tank is 10 bar. To ensure dry steam at the exit of the boiler, a mechanical separator directly connected to the exit nozzle was installed. The boiler tank was thermally insulated to reduce the environmental heat loss.

Compressed air can be supplied either to the boiler tank or to the steam line via nozzle after the orifice meter on the horizontal part of the pipe, which connects the boiler and the test section.

The pipe connecting the boiler tank and the test section has a length of approximately 2 m and an ID of 38.1 mm. The pipe was connected to the boiler tank via an isolation valve. This isolation valve is used to isolate the boiler until inside pressure of the tank is increased to a predetermined level.

The test section is a heat exchanger of countercurrent type, that is steam or steam/gas mixture flows downward inside the condenser tube and cooling water flows upward inside the jacket PiPe-

The condenser tube consists of a 2.15 m long seamless stainless steel tube with 33/39 mm ID/OD. The jacket pipe surrounding the condenser tube is made of sheet iron and has a length of 2.133 m and 81.2/89 mm ID/OD.

A total of 13 holes were drilled with an angle of 300 at different elevations along the condenser tube length to fix the thermocouples for inner wall temperature measurements. Similarly, 15 holes were drilled radially at different elevations for installation of the thermocouples to be used for cooling water temperature measurements. The jacket pipe was thermally insulated to reduce environmental heat loss. Ten thermocouples were fixed to a 2 mm diameter Inconel guide wire and installed at the central temperature measurements.

The experimental text matrix has been constituted by pure steam and steam/air mixture runs and the effect of NC gas has been analyzed by comparing the pure steam runs with mixture runs with respect the temperature, heat flux, air mass fraction, and film Reynolds number. The range of the measured parameters; Pn = 2-6 bars, Rev = 45,000-94,000, and Xi = 0 %-52%.

Relief Valve

Cooling Water Outlet

Isolating Valve

Jacket Pipe

(HI/89 ID/OD, L: 2133 mm)

the experimental heat transfer coefficient to a reference, pure steam, heat transfer coefficient. The reference heat transfer coefficient is calculated from Nusselt theory. Moreover, the enhancement of heat transfer coefficient due to the shear stress of the gas on liquid film is considered, fshear, and conveyed to the correlation. The other effects enhancing the condensation heat transfer coefficient are also taking into account, fothers, and are correlated in terms of liquid side Reynolds number, ReL. The suppression of the condensation heat transfer coefficient by the accumulation of the NC gas at the interface is clarified and denoted as f2. In this present study, both the enhancement and the suppression factors given in UCB formulation are modified by considering mixture side Reynolds number and the Sherwood number defining the radial concentration gradient of NC gas respectively.

f Type Correlation Modified by Sherwood Number

f = ff (1)

where

f is the degradation factor, f1 is the enhancement factor, and f2 is the suppression factor.

f1 = fshear, f others (2)

f shear =TL where; (3)

d 2

51: Film thickness without interfacial shear stress 5 2: Film thickness with interfacial shear stress.

The interfacial shear stress is influenced by both the interface velocity and the mixture side velocity. Moreover, the entrainment from a liquid film is associated with the onset of disturbance waves at the interface and, in general, depends on both the vapor and the liquid flow rates. In fully turbulent flow, above a film Reynolds number of 3000 the condition for the onset of entrainment depend mainly upon the vapor velocity [5]. For this reason, the fothers in Equation (2) is correlated as,

The build up of NC gases at the interface and its back diffusion into the core constitute a primary problem. The accumulation of NC gas at the interface is the principle reason for the mass diffusion resistance in radial direction, which causes lower condensation rates. This effect is encompassed into the correlations with the aid of air mass fraction in UCB correlation. However, air mass fraction is not defining the ongoing process, which is originally governed by concentration gradient formed between the interface and the core. Therefore, the Sherwood number is used instead of air mass fraction in the suppression factor, f2. When the variation of f2 is investigated with the Sherwood number, the segregation of individual runs from each other is observed and this situation is attributed to inlet pressure which is, therefore, superimposed into the correlation as air mole fraction. Under the light of these arrangements, the suppression factor is formulated as follows.

f2 = 1 — C3.Shrrz3 (5)

where

Shrr = yg. Sh (6)

yg: Mole Fraction of air Sh: Sherwood Number

C1-C3 and zi-z3 are the constants to be determined.

Results and the comparison of the correlations are given in section 4.2.