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TECHNICAL PAPERS: Evaporation, Boiling, and Condensation

Internal Condensing Flows inside a Vertical Pipe: Experimental/Computational Investigations of the Effects of Specified and Unspecified (Free) Conditions at Exit

[+] Author and Article Information
A. Narain1

Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, MI 49931narain@mtu.edu

J. H. Kurita, M. Kivisalu, A. Siemionko, S. Kulkarni, T. W. Ng, N. Kim, L. Phan

Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, MI 49931

1

Corresponding author.

J. Heat Transfer 129(10), 1352-1372 (Feb 10, 2007) (21 pages) doi:10.1115/1.2755063 History: Received October 18, 2005; Revised February 10, 2007

Reported experimental and computational results confirm that both the flow features and heat-transfer rates inside a condenser depend on the specification of inlet, wall, and exit conditions. The results show that the commonly occurring condensing flows’ special sensitivity to changes in exit conditions (i.e., changes in exit pressure) arises from the ease with which these changes alter the vapor flow field in the interior. When, at a fixed steady mass flow rate, the exit pressure is changed from one steady value to another, the changes required of the interior vapor flow toward achieving a new steady duct flow are such that they do not demand a removal of the new exit pressure imposition back to the original steady value—as is the case for incompressible single phase duct flows with an original and “required” exit pressure. Instead, new steady flows may be achieved through appropriate changes in the vapor/liquid interfacial configurations and associated changes in interfacial mass, heat-transfer rates (both local and overall), and other flow variables. This special feature of these flows has been investigated here for the commonly occurring large heat sink situations, for which the condensing surface temperature (not heat flux) remains approximately the same for any given set of inlet conditions while the exit-condition changes. In this paper’s context of flows of a pure vapor that experience film condensation on the inside walls of a vertical tube, the reported results provide an important quantitative and qualitative understanding and support an exit-condition-based categorization of the flows. Experimental results and selected relevant computational results that are presented here reinforce the fact that there exist multiple steady solutions (with different heat-transfer rates) for multiple steady prescriptions of the exit condition—even though the other boundary conditions do not change. However, for some situations that do not fix any specific value for the exit condition (say, exit pressure) but allow the flow the freedom to choose any exit pressure value within a certain range, experiments confirm the computational results that, given enough time, there typically exists, under normal gravity conditions, a self-selected “natural” steady flow with a natural exit condition. This happens if the vapor flow is seeking (or is attracted to) a specific exit condition and the conditions downstream of the condenser allow the vapor flow a range of exit conditions that includes the specific natural exit condition of choice. However, for some unspecified exit-condition cases involving partial condensation, even if computations predict that a natural exit-condition choice exists, the experimental arrangement employed here does not allow the flow to approach its steady natural exit-condition value. Instead, it only allows oscillatory exit conditions leading to an oscillatory flow. For the reported experiments, these oscillatory pressures are induced and imposed by the instabilities in the system components downstream of the condenser.

FIGURES IN THIS ARTICLE
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Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

The schematic for the flow through test-section and exit-condition issues

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Figure 2

For tube flow situations specified as in Phan (3) (see their Table 1 and Fig. 2), the figure depicts three steady film thickness profiles for three different exit conditions. The figure also indicates time trends for two sets of δ(x,t) predictions for t>0; one curve starts at Ze=0.3 at t=0, and tends, as t→∞, to the solution for Ze∣Na=0.215. The other curve starts at Ze=0.15 at t=0 and tends, as t→∞, to the same Ze∣Na=0.215 solution.

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Figure 3

The schematic of the flow loop for achieving specified exit-condition category I flows for partial condensation cases

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Figure 4

The schematic of the flow loop for achieving unspecified exit-condition category II flows for partial or FC cases

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Figure 5

(a) The photograph of condenser test section. (b) The test-section schematic (diameters in (a) and (b) are not to the same scale). The condensing surface covers the zone x0⩽x⩽x10.

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Figure 6

(a) Time history depiction of Ṁin, ṀV, and ΔT¯ values for multiple steady states of partial condensation cases, viz., Natural-1 (run 1 from Table 1), Specified-1 (run 1 from Table 2), Specified-2 (run 18 from Table 2), and Specified-1 Approx. (b). Time history of pressures (along the test section) and Δp values (across the test section) for the cases shown in (a) (c) Time history of temperature values along the test section (subsystem B) and Tsat(pin) for the cases shown in (a).

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Figure 7

Two-dimensional test data matrix for category II (unspecified exit condition) partial condensation flow cases and different bounding curves represented on the Ṁin−ΔT¯ plane

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Figure 8

For the Natural-1 and Specified-2 flow cases in Figs.  666, this figure shows the computationally obtained representative film thickness and wall heat-flux variation along the test section. The film thickness and heat-flux values shown have been obtained for smooth interface conditions. In reality, they are modulated by waves due to presence of noise (see Ref. 3).

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Figure 9

The schematic of FC showing point of FC and its downstream region with zero interfacial mass and heat transfer.

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Figure 10

Two-dimensional test data matrix for category II (unspecified exit-condition) FC case points and different bounding curves represented on the Ṁin−ΔT¯ plane

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Figure 11

(a) Time history depiction of Ṁin and ΔT¯ values for multiple steady states of FC category II cases, viz., Natural-1 (run 1 from Table 3), Natural-2 (run 18 from Table 3), and Natural-1 Repeated. (b) Time history of pressures (along and outside the test section) and Δp values for the cases shown in (c) Time history of temperature values along the test section (subsystem B) for the cases shown in (a).

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Figure 12

Time history (t3⩽tmin⩽t3+224) of mass flow rate and inlet vapor pressure for an attempted unspecified exit-condition (category II) FC case that resulted in instability or transients as the boundary curve X in Fig. 1 is crossed

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Figure 13

For a typical category II FC case, this figure shows the computationally obtained representative film thickness and wall heat-flux variation along the test section over 0⩽x⩽XFC. The film thickness and heat-flux value shown have been obtained for smooth interface conditions. In fact, they are modulated by waves in the presence of noise (see Ref. 3). The figure also shows that the length of FC XFC=0.2m is sufficiently shorter than the test-section length L(=0.7m).

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Figure 14

(a) Time history depiction of Ṁin, ṀV, ṀL, and Δp values for an oscillatory partial unspecified exit-condition case (category II). In Fig. 7, this flow’s appearance is indicated by the crossing of the dotted curve A. (b) For the case in (a), this figure shows the time history depiction of the rotameter temperature TR and the following pressures: inlet pressure pin, pressure at location 6 in Fig. 5(px6), and exit pressure pexit.

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