Effects of Gravity and Surface Tension and Interfacial-Waves and Heat-Transfer Rates in Internal Condensing Flows

2003 ◽  
Author(s):  
Q. Liang ◽  
X. Wang ◽  
A. S. Barve ◽  
A. Narain
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Numerical Solutions ◽  
Film Condensation ◽  
Vapor Flow ◽  
Transfer Rates ◽  
Condensing Flows ◽  
Steady Solutions ◽  
Internal Condensing Flows ◽  
Good Agreement

The paper presents accurate numerical solutions of the full 2D governing equations for steady and unsteady laminar/laminar internal condensing flows. The chosen geometry allows for film condensation on the bottom wall of a tilted (from vertical to horizontal) channel. It is found that it is important to know whether the exit conditions are constrained or unconstrained because incompressible vapor flows occur only for exit conditions that are unconstrained. For the incompressible vapor flow situations, a method for computationally obtaining the stable steady/quasi-steady solutions is given here and the resulting solutions are shown to be in good agreement with some relevant experimental data for horizontal channels. These solutions are shown to be sensitive to the frequency-content and strength of ever-present minuscule transverse vibrations of the condensing surface. The effects of noise-sensitivity, gravity (terrestrial to zero-gravity), and surface tension on the attainability of stable steady/quasi-steady solutions, structure of superposed waves, and heat-transfer rates are discussed. It is shown that significant enhancement in wave-energy and heat-transfer rates are possible by designing the condensing surface noise to be in resonance with the intrinsic waves.

Effects of Gravity, Shear and Surface Tension in Internal Condensing Flows: Results From Direct Computational Simulations

2004 ◽  
Vol 126 (5) ◽  
pp. 676-686 ◽  
Author(s):  
Q. Liang ◽  
X. Wang ◽  
A. Narain
Keyword(s):  
Surface Tension ◽  
Normal Gravity ◽  
Numerical Solutions ◽  
Vertical Channel ◽  
Film Condensation ◽  
Vapor Flow ◽  
Zero Gravity ◽  
Pressure Drops ◽  
Condensing Flows ◽  
Internal Condensing Flows

The paper presents accurate numerical solutions of the full two-dimensional governing equations for steady and unsteady laminar/laminar internal condensing flows. The results relate to issues of better design and integration of condenser-sections in thermal management systems (looped heat pipes, etc.). The flow geometry, in normal or zero gravity, is chosen to be the inside of a channel with film condensation on one of the walls. In normal gravity, film condensation is on the bottom wall of a tilted (from vertical to horizontal) channel. It is found that it is important to know whether the exit conditions are constrained or unconstrained because nearly incompressible vapor flows occur only for exit conditions that are unconstrained. For the incompressible vapor flow situations, a method for computationally obtaining the requisite exit condition and associated stable steady/quasi-steady solutions is given here and the resulting solutions are shown to be in good agreement with some relevant experimental data for horizontal channels. These solutions are shown to be sensitive to the frequency and amplitude of the various Fourier components that represent the ever-present and minuscule transverse vibrations (standing waves) of the condensing surface. Compared to a vertical channel in normal gravity, shear driven zero gravity cases have much larger pressure drops, much slower wave speeds, much larger noise sensitive wave amplitudes that are controlled by surface tension, and narrower flow regime boundaries within which vapor flow can be considered incompressible. It is shown that significant enhancement in wave-energy and/or heat-transfer rates, if desired, are possible by designing the condensing surface noise to be in resonance with the intrinsic waves.

Internal Condensing Flows Inside a Vertical Pipe: Experimental/Computational Investigations of Effects of Constrained and Natural Exit Conditions

2005 ◽  
Author(s):  
A. Narain ◽  
A. Siemionko ◽  
J. H. Kurita ◽  
T. W. Ng ◽  
N. Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Power Plants ◽  
Computational Simulation ◽  
Vertical Tube ◽  
Film Condensation ◽  
Flow Loop ◽  
Transfer Rates ◽  
Application Systems ◽  
Condensing Flows ◽  
Internal Condensing Flows

The flow and heat transfer rates inside a condenser depend on the specification of inlet, wall, and exit conditions. For steady/quasi-steady internal condensing flows (that involve compressible vapor at low Mach Numbers), the vapor’s ability to change its density — and hence interfacial mass transfer rates and associated locations of the interface — allows the flow to have a rather significant dependence on exit conditions. Both experimental and direct computational simulation results presented here show that this is indeed the case for flows of pure vapor experiencing film condensation on the inside walls of a vertical tube. In applications, the totality of boundary conditions are determined not only by the condenser; but also by the flow-loop (or the system) — of which the condenser is only a part. Therefore, the results outlined here should contribute towards a better understanding of the behavior (particularly the extent to which vapor compressibility effects affect the flow regimes of operation — i.e. annular, plug/churn, etc.) and response (transients due to start-up, system instabilities, etc.) of condensers in application systems (e.g. Rankine Cycle power plants, Capillary Pumped Loops, Looped Heat Pipes, etc.). In this connection, an experimental example of a relevant system instability is presented here. In summary, the experimental results presented here, and computational results presented elsewhere, reinforce the fact that there exist multiple steady solutions (with different heat transfer rates) for different exit conditions and that there also exists a “natural” steady solution for straight vertical condensers (circular and rectangular cross-sections).

Direct Computational Simulations for Condensation Inside Tubes and Channels

2005 ◽  
Author(s):  
L. Phan ◽  
X. Wang ◽  
S. Kulkarni ◽  
A. Narain
Keyword(s):  
Numerical Solutions ◽  
Vertical Tube ◽  
Film Condensation ◽  
Vapor Flow ◽  
Steady Flows ◽  
Channel Geometry ◽  
Pressure Drops ◽  
Condensing Flows ◽  
Gravity Driven ◽  
Internal Condensing Flows

The paper presents accurate numerical solutions of the full 2D governing equations for steady and unsteady laminar/laminar internal condensing flows of pure vapor (R-113 and FC-72) inside a vertical tube and a channel. The film condensation is on the inside wall of a tube or one of the walls of a channel (the lower wall in case of a downward sloping channel). The new geometry in this paper is the cylindrical in-tube geometry with axisymmetric flows (vertical 1g or 0g flows). The new results encompass both the cylindrical and the earlier studied channel geometry. Exit condition specifications are again found to be important. The computations are able to predict whether or not a steady flow exists under a natural exit condition (selected from a range of choices available at the exit). If natural steady/quasi-steady flows exist — as is shown to be the case for gravity dominated or strong shear dominated condensate flows — the computations are able to predict both the natural exit condition and the associated condensate flow’s point of transition from stable to unstable behavior. Compared to gravity driven, shear driven cases (zero gravity or horizontal cases) tend to destabilize easier and generally have much larger pressure drops, much slower wave speeds, much larger role of surface tension, and much narrower flow regime boundaries within which the vapor flow can be modelled incompressible. It is found that only in gravity driven cases, be it vertical in-tube or inclined channel geometry, interfacial waves are able to cause a concurrent enhancement in heat transfer rates along with an enhancement in interfacial shear. Also it is found that this enhancement is significant if the condensing surface noise is in resonance with the intrinsic waves.

Direct Computational Simulations for Internal Condensing Flows and Results on Attainability/Stability of Steady Solutions, Their Intrinsic Waviness, and Their Noise Sensitivity

2004 ◽  
Vol 71 (1) ◽  
pp. 69-88 ◽  
Author(s):  
A. Narain ◽  
Q. Liang ◽  
G. Yu ◽  
X. Wang
Keyword(s):  
Numerical Solutions ◽  
Resonance Condition ◽  
Vertical Channel ◽  
High Growth ◽  
Film Condensation ◽  
Noise Sensitivity ◽  
Reported Study ◽  
Condensing Flows ◽  
Steady Solutions ◽  
Internal Condensing Flows

The paper presents a new two-dimensional computational approach and results for laminar/laminar internal condensing flows. Accurate numerical solutions of the full governing equations are presented for steady and unsteady film condensation flows on a sidewall inside a vertical channel. It is found that exit conditions and noise sensitivity are important. Even for stable steady solutions obtained for nearly incompressible vapor phase flows associated with unconstrained exit conditions, the noise sensitivity to the condensing surface’s minuscule transverse vibrations is high. The structure of waves, the underlying characteristics, and the “growth/damping rates” for the disturbances are discussed. A resonance condition for high “growth rates” is proposed and its efficacy in significantly enhancing wave motion and heat transfer rates is computationally demonstrated. For the unconstrained exit cases, the results make possible a separately reported study of the effects of shear, gravity, and surface tension on noise sensitive stable solutions.

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

2007 ◽  
Author(s):  
J. H. Kurita ◽  
A. Narain ◽  
M. Kivisalu ◽  
A. Siemionko ◽  
S. Kulkarni
Keyword(s):  
Heat Transfer ◽  
Vertical Tube ◽  
Film Condensation ◽  
Vapor Flow ◽  
Steady Flows ◽  
Duct Flow ◽  
Transfer Rates ◽  
Exit Pressure ◽  
Condensing Flows ◽  
Inlet Conditions

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 occuring condensing flows’ special sensitivity to changes in exit conditions (i.e. changes in exit pressure) arise from the ease with which these changes alter the vapor flow field in the interior. When exit pressure is changed from one steady value to another, the changes required of the interior vapor flow towards achieving a new steady duct flow are such that they do not demand 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 is 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 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 important quantitative and qualitative understanding as well as allow us to propose important exit-condition based categorization (viz. Categories I – III) of these flows.

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

2007 ◽  
Vol 129 (10) ◽  
pp. 1352-1372 ◽  
Author(s):  
A. Narain ◽  
J. H. Kurita ◽  
M. Kivisalu ◽  
A. Siemionko ◽  
S. Kulkarni ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Vertical Tube ◽  
Film Condensation ◽  
Vapor Flow ◽  
Computational Results ◽  
Duct Flow ◽  
Transfer Rates ◽  
Exit Pressure ◽  
Condensing Flows ◽  
Inlet Conditions

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.

Modeling of Interfacial Shear for Gas Liquid Flows in Annular Film Condensation

1996 ◽  
Vol 63 (2) ◽  
pp. 529-538 ◽  
Author(s):  
A. Narain
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Internal Flow ◽  
Admissible Solution ◽  
Interfacial Shear ◽  
Film Condensation ◽  
Vapor Flow ◽  
Pressure Drops ◽  
Transfer Rates ◽  
Stability Type

Internal flow of pure vapor experiencing film condensation on the walls of a straight horizontal duct is studied. The commonly occurring annular case of turbulent (or laminar) vapor flow in the core and laminar flow of the liquid condensate—with or without waves on the interface—is emphasized. We present a new methodology which models interfacial shear with the help of theory, computations, and reliable experimental data on heat transfer rates. The theory—at the point of onset of condensation—deals with issues of asymptotic form of interfacial shear, nonuniqueness of solutions, and selection of the physically admissible solution by a stability type criteria. Other details of the flow are predicted with the help of the proposed modeling approach. These predictions are shown to be in agreement with relevant experimental data. The trends for film thickness, heat transfer rates, and pressure drops are also made available in the form of power-law correlations.

Analysis of Laminar Falling Film Condensation Over a Vertical Plate With an Accelerating Vapor Flow

2009 ◽  
Vol 131 (7) ◽  
Author(s):  
A.-R. A. Khaled ◽  
Abdulhaiy M. Radhwan ◽  
S. A. Al-Muaikel
Keyword(s):  
Heat Transfer ◽  
Vertical Plate ◽  
Finite Difference Methods ◽  
Saturation Temperature ◽  
Film Condensation ◽  
Vapor Flow ◽  
Falling Film ◽  
Suction Parameter ◽  
Transfer Rates ◽  
Mass And Heat Transfer

Laminar falling film condensations over a vertical plate with an accelerating vapor flow is analyzed in this work in the presence of condensate suction or slip effects at the plate surface. The following assumptions are made: (i) laminar condensate flow having constant properties, (ii) pure vapor with a uniform saturation temperature in the vapor region, and (iii) the shear stress at the liquid/vapor interface is negligible. The appropriate fundamental governing partial differential equations for the condensate and vapor flows (continuity, momentum, and energy equations) for the above case are identified, nondimensionalized, and transformed using nonsimilarity transformation. The transformed equations were solved using numerical, iterative, and implicit finite-difference methods. It is shown that the freestream striking angle has insignificant influence on the condensation mass and heat transfer rates, except when slip condition is present and at relatively small Grl/Re2 values. Moreover, it is shown that increasing the values of the dimensionless suction parameter (VS) results to an increase in dimensionless mass of condensate (Γ(L)/(μl Re)) and Nusselt number (Nu(L)/Re1/2). Thus, it results in an increase in condensation mass and heat transfer rates. Finally, it is found that the condensation and heat transfer rates increase as Jakob number, slip parameter, and saturation temperature increase. Finally, the results of this work not only enrich the literature of condensation but also provide additional methods for saving thermal energy.

Free Surface Tracking and Steady/Unsteady Computational Simulations for Internal Condensing Flows

2001 ◽  
Author(s):  
A. Narain ◽  
Q. Liang ◽  
A. S. Barve
Keyword(s):  
Film Condensation ◽  
Navier Stokes ◽  
Steady Flows ◽  
Time Step ◽  
Navier Stokes Equation ◽  
Stability And Instability ◽  
Flow Mechanisms ◽  
Condensing Flows ◽  
Steady Solutions ◽  
Internal Condensing Flows

Abstract The computational investigation in this paper explores a new technique for steady and unsteady internal condensing flows in the annular/stratified regime. Simulation capabilities for both steady and unsteady interface laminar/laminar situations are presented for film condensation on the bottom wall of a small-to-moderate gap channel. The unsteady simulations employ a suitable adaptive grid and the solution of an interface tracking hyperbolic equation (of the type used in level-set or VOF methods). At each time-step, the scheme locates the interface, solves the Navier-Stokes equation in each phase, satisfies the full non-linear conditions at the phase change interface, and satisfies the necessary inlet, outlet, and wall conditions. The stability and instability of various exit condition dependent steady solutions are inferred from direct unsteady simulations and this leads to insights into the flow mechanisms that determine the eventual quasi-steady flows. For some cases it is found that waviness and heat transfer rates are enhanced significantly by the flow’s sensitivity to the frequency content of persistent noise in the inlet data.

Sign in / Sign up

Export Citation Format

Share Document