Effects of tube inclination on pool boiling heat transfer
Introduction
The mechanism of pool boiling heat transfer has been studied for a long time since it is closely related with the designs of more efficient heat exchanger and heat removal systems. Recently, it has been widely investigated in nuclear power plants for application to the design of new passive heat removal systems employed in the advanced light water reactors (ALWRs) designs (Corletti and Hochreiter, 1991, Kang, 1998). The passive heat exchangers can transfer the decay heat from the Reactor Coolant System (RCS) to the water tank even though the electric power becomes unavailable for heat removal.
To determine the required heat transfer surface area as well as to evaluate the system performance during postulated accidents, overall heat transfer coefficients applicable to the systems are needed. Since pool boiling heat transfer coefficient is usually the governing factor in determining overall heat transfer coefficients through the heat exchanging tubes of the passive heat exchangers, many researchers have studied about it. Through the review on the published results it can be concluded that one of the efficient ways to increase the heat transfer rate can be suggested to utilize the inclination angle (θ) of the heated surface. Although many researchers have investigated the effects of several design parameters on pool boiling heat transfer for the past several decades, results for the effects of inclination angle are relatively small. Two practical approaches have been employed to obtain effects of inclination angles on pool boiling heat transfer for the heated surface of wire, plate, and tube, as follows: (1) inclination angle itself and (2) combined effects with other design parameters (i.e. gap size, surface roughness, special shape, and flow area confinement). Previous researches are summarized and listed in Table 1.
Jakob and Hawkins (1957) recommended four empirical correlations for water boiling at standard atmospheric pressure under free convection conditions. He has set up the formulas for horizontal and the vertical heating surfaces in wide vessels. According to the results, the horizontal type (θ=0°) is more efficient than the vertical type (θ=90°) in the low heat flux region (i.e. q″<11.8 kW m−2) while the opposite is true in the high heat flux region.
Stralen and Sluyter (1969) performed a test to find out boiling curves for platinum wires in the horizontal and vertical position at atmospheric pressure. They concluded that the horizontal type was more effective than the vertical type both in the natural convection and boiling regions. The peak heat flux for θ=0° is 45% higher in comparison to the corresponding value for θ=90°. The major cause of the reduction in heat transfer for the vertical position is due to the formation of large vapor slugs. The coalesced bubbles are distributed over the entire heating surface for a vertical wire and this behavior differs from that for a horizontal wire, where bubble coalescence is generally restricted to nearby nuclei.
Nishikawa et al. (1984) studied heat flux (q″) and wall superheat (ΔT) on a flat plate oriented at an angle θ, that varied between 0 and 175° from a horizontal, upward-facing position in the water. According to them, boiling heat transfer coefficients are increased with an increase of inclination angle at the low heat flux region (less than 100 kW m−2) and inclination effects become negligible as the heat flux on the surface increases more than 100 kW m−2. The difference in the effect of surface configuration over the whole region of nucleate boiling is presumed as a change in heat transfer mechanisms between low heat fluxes and high heat fluxes. In addition, they explained the heat transfer mechanisms for the low heat fluxes and high heat fluxes as the inclination angle changes. One year later, Lienhard (1985) explained the loss of orientation dependence using the Moissis–Berenson Transition (MBT).
Jung et al. (1987) performed some experiments for inclined plates and R-11. Through the tests of two metal coated surfaces and a flat copper surface were subjected to heat fluxes up to 180 kW m−2 with surface orientations varying from horizontally facing upward (θ=0°), to vertical (θ=90°), to horizontally facing downward (θ=180°). They report results similar to the Nishikawa et al. (1984) result. For all surfaces investigated, the superheat decreases by 15–25% as the inclination angle changes from 0 to 165° in the relatively low heat flux range (i.e. 10–40 kW m−2). Beyond this heat flux range, however, the superheat remains constant regardless of the surface orientation.
Some more studies about plate are reported by Fujita et al. (1988). Fujita et al. studied the combined effects of inclination angle and gap size between two plates. According to outputs, the effect of inclination angle is closely related with the gap size. The general trend is similar to the Nishikawa et al. (1984) result. However, decreasing the gap size much narrower (0.15 mm for the case) the boiling behavior does not change with inclination angle.
Although some authors have studied effects of inclination angle on pool boiling heat transfer along with the effects of geometry, pressure, and surface conditions, no detailed studies have been performed for tubes until Chun and Kang (1998) studied the effect of tube orientation (θ=0 and 90°) on pool boiling heat transfer in combination with tube surface roughness. According to Chun and Kang (1998), the slope of q″ versus ΔT curve of the vertical tube becomes smaller than that of the horizontal tube as surface roughness decreases.
Most recently, Kang (2000a) carried out an experimental parametric study of a tubular heat exchanger to determine effects of the tube inclination angle on pool boiling heat transfer. The results obtained by Kang (2000a) at three angles of inclination (θ=0, 45, and 90°) have a large effect on pool boiling heat transfer. Moreover, he identified that the result for a tube is much different from those for a flat plate. The effect of inclination angle is more strongly observed in the smooth tube and if a tube is properly inclined (θ=45° for the case) enhanced heat transfer is expected in comparison with the horizontal and the vertical position. Some more detailed study for the inclination angle has performed by Kang (2000b) combining surface roughness to the analysis.
Summarizing the works, it can be said that the effect of inclination angle on pool boiling heat transfer closely depends on the heating surface geometry (i.e. wire, plate, or tube) and surface roughness. As Cornwell and Houston (1994) suggested nucleate boiling on a tube differs considerably from that on a flat plate. The same is true for the wire. Although many studies have been done for the flat plate, the results cannot apply to a tube. Moreover, Kang (2000a) performed experiments for the three inclination angles and one tube diameter (19.1 mm) only. Therefore, more detailed study of tube inclination angles and some other tube diameters are necessary. As such, the present study is aimed at the determination of effects of the tube inclination angle on pool boiling heat transfer (1) to improve Kang (2000a) result and (2) to investigate the potential areas for improvement of the thermal design of the passive heat exchangers.
Section snippets
Experiments
A schematic view of the present experimental apparatus and test sections is shown in Fig. 1. The water storage tank (Fig. 1(a)) is made of stainless steel and has a rectangular cross section (950×1300 mm) and a height of 1400 mm. This tank has a glass view port (1000×1000 mm) which permits viewing of the tubes and photographing. The tank has a double container system. The sizes of the inner tank are 800×1000×1100 mm (depth×width×height). The bottom side of the inner tank is situated 200 mm
Results and discussion
Fig. 2, Fig. 3 show changes of the boiling heat transfer coefficients for D=12.7 and 19.1 mm, respectively, as the tube inclination angle changes. According to the experimental results, the effect of tube inclination angle on pool boiling heat transfer is very large. When θ=15° (for D=12.7 mm) and θ=30° (for D=19.1 mm) the largest heat transfer coefficients are expected. In other words, as θ=75° the smallest heat transfer coefficients are expected regardless of the tube diameters. For the case
Conclusions
The major conclusions drawn from this experimental investigation of pool boiling in water at atmospheric pressure condition for the seven inclination angles (0, 15, 30, 45, 60, 75, and 90°) and two tube diameters (12.7 and 19.1 mm) may be stated as follows:
- 1
The inclination angle gives much change in heat transfer coefficients.
- 2
When a tube is near the horizontal and the vertical, the maximum and the minimum heat transfer coefficients are expected, respectively.
- 3
The maximum values are about five to
Acknowledgements
This work was supported by grant No. 2000-1-30400-012-3 from the Basic Research Program of the Korea Science and Engineering Foundation.
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