System Mean Void Fraction Research Articles

An Appraisal of System Mean Void Fraction and Its Application for the Moving Boundary Simulation of Phase-Change Heat Exchangers

In gas–liquid two-phase flow, void fraction is the most unique parameter which influences all the transport processes. In the most general case, though the void fraction varies nonlinearly with the channel length, many practical simulations make use of the “system mean void fraction.” The present investigation makes a critical assessment of different system mean void fraction models for a wide range of slip velocity and density difference between the phases. To this end, different correlations for slip ratio have been considered and, for all the cases, closed form expression for the system mean void fraction has been presented. The local as well as the system mean void fractions have also been estimated numerically from a heat transfer based model. Predictions from the heat transfer based model and the slip ratio based model have been compared. As an application, the slip ratio based system mean void fraction is used in to build the moving boundary model for phase-change heat exchangers. The prediction of startup transients for both an evaporator and a condenser of an automotive air conditioning system (AACS) agrees well with the experimental results.

An Investigation Into the Effect of Subcooled Liquid Inertia on Flow-Change-Induced Transient Flow Surges in Horizontal Condensing Flow Systems

The objective of this research is to investigate large-scale transient flow surges of the condensate leaving in-tube condensing flow systems because of perturbations in the inlet vapor flow rate, and the influence of the subcooled liquid inertia of the condensate on these transient responses. Small changes in the inlet vapor flow rate momentarily cause large transient flow surges in the outlet liquid flow rate. Condensate inertia is seen to destabilize the system into an underdamped behavior where the flow rate can overshoot the final steady-state position several times. A one-dimensional, two-fluid, distributed parameter system mean void fraction (SMVF) model of the time-dependent distribution of liquid and vapor within the two-phase region is developed for predicting these transient characteristics, which it is seen to do quite well, especially when consideration is given to the complex nature of the problem.

Modeling the Characteristics of Thermally Governed Transient Flow Surges in Multitube Two-Phase Condensing Flow Systems With Compressibility and Thermal and Flow Distribution Asymmetry

In a tube-type condenser involving complete condensation, small changes in the inlet vapor flow rate momentarily cause very large transient surges in the outlet mass flow rate. An Equivalent Single-Tube Model (ESTM), based on the System Mean Void Fraction Model, is developed that predicts these transient flow surges for a multitube system; including the effects of compressibility as well as thermal and flow distribution asymmetry. The model is verified theoretical and experimentally. From a design perspective, the significant value of the ESTM is that it includes the primary physical mechanisms involved in such complex flow transients, yet is simple enough to be solved on typical “spreadsheet” software.

Uniqueness of System Response Time for Transient Condensing Flows

The unique characteristics under consideration in this paper are encountered in condensing flows, and have to do with a system's response time for various degrees of outlet flow quality. Specifically, the system response time for condensing flows appears to increase monotonically with decreasing outlet flow quality, reaching a maximum for systems having an outlet flow quality of between 10 and 20%. The system response time then decreases for outlet flow qualities that are less than that value. These unique characteristics are predicted theoretically by system mean void fraction model. The purpose of this paper is to develop analytically the characteristics, explain the physics of the phenomena responsible, and discuss the experimental verification efforts that have thus far been carried out.

Modeling the Thermally Governed Transient Flow Surges in Multitube Condensing Flow Systems With Thermal and Flow Distribution Asymmetry

In a tube-type condenser involving complete condensation, small changes in the inlet vapor flow rate momentarily cause very large transient surges in the outlet liquid flow rate. An equivalent single-tube model is proposed that predicts these transient flow surges for a multitube system. The model, based upon a system mean void fraction model developed earlier, includes the effects of thermal and flow distribution asymmetry associated with each individual condenser tube in the multitube system. Theoretical and experimental verification for a two-tube system is presented.

An Experimental and Theoretical Study of Transient Pressure Drop in Two-Phase Condensing Flows

This paper presents the results of an experimental and theoretical investigation of the pressure drop associated with transient two-phase condensing flows involving complete condensation. Utilizing the system mean void fraction model, and the similarity relationships associated with it, an analytical prediction of the transient pressure drop is possible, including a simplified closed-form version. The capability of the proposed theory is demonstrated by comparison with experimental measurements of the transient pressure drop in a horizontal tube condenser following an exponential-type change in the inlet mass flow rate. Good agreement is shown to exist between the predicted and experimental results.

Effects of Two-Phase Pressure Drop on the Self-Sustained Oscillatory Instability in Condensing Flows

This paper presents the results of an extension of an experimental and theoretical investigation of an unstable flow phenomenon that leads to self-sustained limit-cycle-type oscillations of large amplitude, and which, under certain conditions, can involve flow reversals. The influence of two-phase pressure drop is examined and shown to have a stabilizing effect on the instability. Inclusion of the two-phase pressure drop as part of the downstream throttling allows the utilization of a previously developed linearized analysis, based on the system mean void fraction model, to predict successfully the experimentally observed stability boundary.

A Generalization of the System Mean Void Fraction Model for Transient Two-Phase Evaporating Flows

The system mean void fraction model has been successful in the prediction of a variety of transient evaporating and condensing flow phenomena; however, applications of the model have been restricted to physical situations involving complete vaporization or condensation. The major contribution of this paper is the development of a generalization of the existing system mean void fraction model, applicable to the broader class of transient two-phase flow problems involving incomplete vaporization. Present applications of the generalized system mean void fraction model to transient evaporating flows indicate good agreement with experimental void fraction and mass flux response data available in the literature. These data represent a variety of different flow geometries, types of fluids, and a wide range of operating conditions.

A Self-Sustained Oscillatory Flow Phenomenon in Two-Phase Condensing Flow Systems

This paper presents the results of an experimental and theoretical investigation of an unstable flow phenomenon that leads to sustained limit-cycle type of oscillations of large amplitude, and which under certain conditions, can involve flow reversals. This unstable behavior normally exists for conditions of low outlet throttling. Upstream compressible volume and downstream inertia appear to be the dominant energy storage mechanisms for the self-sustained oscillations. A linearized analysis based on the system mean void fraction model successfully predicts the experimentally observed stability boundary.

Transient and Frequency Response Characteristics of Two-Phase Condensing Flows: With and without Compressibility

In a tube-type condenser, involving complete condensation, small changes in the inlet vapor flowrate momentarily cause very large transient surges in the outlet liquid flowrate. Experimental data are presented which indicate that compressibility effects tend to attenuate the amplitude of these flow surges. The system mean void fraction model was extended to include compressibility effects and its predictions are shown to agree well with experimental data. The model is further extended to predict the response characteristics to an oscillatory inlet flowrate and compared with predictions based upon the drift-flux model.

A system mean void fraction model for predicting various transient phenomena associated with two-phase evaporating and condensing flows

The system mean void fraction model's principle virtue is its simplicity. The model converts the two-phase evaporating or condensing flow system into a type of lumped parameter system, generally yielding simple, closed form solutions in terms of the important system parameters. The particular applications of the model which are demonstrated in this paper are for a class of transient flow problems where complete vaporization or condensation takes place, and where the system mean void fraction can be considered to be time-invariant. This assumption uncouples the problem from the transient form of the momentum principle, an analytical simplification of considerable magnitude. The specific transients under consideration are caused by changes in the inlet flowrate. For evaporating flows, these transients are the effective liquid dry-out point, and the outlet flowrate of superheated vapor. For condensing flows, they are the effective point of complete condensation, and the outlet flowrate of subcooled liquid.

An Experimental and Theoretical Investigation Into Thermally Governed Transient Flow Surges in Two-Phase Condensing Flow

The specific transient phenomenon under consideration is the outlet flowrate of subcooled liquid from a tube-type condenser where complete condensation takes place. Experimental data are presented which indicates that a small change in the inlet vapor flow-rate will momentarily cause a very large transient surge in the outlet flowrate of subcooled liquid. These experimentally measured flow transients are predicted quite accurately using a system mean void fraction model. Also, some preliminary results are presented which indicate the influence of throttling at the condenser outlet as a means of attenuating the transient overshoot characteristics.