M. Shoukri

Bubble Behavior and Mean Diameter in Subcooled Flow Boiling

Bubble behavior and mean bubble diameter in subcooled upward flow boiling in a vertical annular channel were investigated under low pressure and mass flux conditions. A high-speed video system was used to visualize the subcooled flow boiling phenomenon. The high-speed photographic results indicated that, contrary to the common understanding, bubbles tend to detach from the heating surface upstream of the net vapor generation point. Digital image processing technique was used to measure the mean bubble diameter along the subcooled flow boiling region. Data on the axial area-averaged void fraction distributions were also obtained using a single-beam gamma densitometer. Effects of the liquid subcooling, applied heat flux, and mass flux on the mean bubble size were investigated. A correlation for the mean bubble diameter as a function of the local subcooling, heat flux, and mass flux was obtained.

Interfacial Heat Transfer Between Steam Bubbles and Subcooled Water in Vertical Upward Flow

In two-fluid modeling, accurate prediction of the interfacial transport of mass, momentum, and energy is required. Experiments were carried out to obtain a data base for the development of interfacial transport models, or correlations, for subcooled water-steam bubbly flow in vertical conduits. The experimental data of interest included the interfacial area concentration, interfacial condensation heat transfer, and bubble relative velocity. In the present investigation, bubble condensation in subcooled water-steam flow in a vertical annulus at low flow rate and low pressure is investigated experimentally. A high-speed video system (up to 1000 frame/s) was used to visualize two orthogonal views of the flow simultaneously. A digital image processing technique was used to track and measure the velocity and size of the collapsing bubbles. The axial void fraction distribution was also measured by a single beam gamma densitometer.

Measurement of interfacial area concentration in subcooled liquid-vapour flow

Abstract In two-fluid modelling, accurate prediction of the interfacial transport of mass, momentum and energy is required. Experiments were carried out to obtain a database for the development of interfacial transport models, or correlations, for subcooled water-steam flow in vertical conduits. The experimental data of interest included the interfacial area concentration, interfacial condensation heat transfer and bubble relative velocity. This paper focuses on the interfacial area concentration. The interfacial area concentration was obtained by measuring the distributions of bubble volume and surface area as well as the area-averaged void fraction at various axial locations in subcooled water-steam condensing vertical upward flow under low flow rate and low pressure conditions. The bubble size and surface area were determined using high-speed photography and digital image processing techniques. The area-averaged void fraction was measured by a single-beam gamma densitometer. The results were compared with existing correlations, which were developed on the basis of data obtained for air-water adiabatic flows. Poor agreement between the present data and the existing correlations was obtained. Accordingly, new correlations suitable for subcooled liquid-vapour bubbly flow are proposed.

On the development of a model for predicting phase separation phenomena in dividing two-phase flow

Abstract A model to predict the dividing flow characteristics for annular flow in a T-junction is proposed consisting of mixture and vapour phase continuity equations, two pressure change correlations and a closure relationship. The pressure change from the inlet through the run of the T is modelled by way of a balance of axial momentum at the junction based on a separated flow assumption. The branch pressure change is modelled using a balance of mechanical energy for the branching flow consisting of reversible and irreversible components. The closure relationship links the phase separation characteristics with the junction pressure changes. It involves a balance between pressure and inertia forces within the junction volume defining a dividing surface for each phase between the run and branch flows. The branch quality is then determined using a well-defined inlet flow distribution. The model is capable of predicting the experimentally observed phase separation characteristics from three independent studies of annular/steam—water and air-water flow in dividing T-junctions.

Boiling Characteristics of Small Multitube Bundles

Boiling characteristics of multitube bundles have been investigated experimentally. Small bundles of up to nine rows were used. Void fraction profiles in the test vessel, tube surface temperatures, power input to individual tubes, and critical heat fluxes were measured for different bundle arrangements and boiling conditions. The data were used to study the system hydrodynamics, bundle heat transfer coefficients, and bundle critical heat flux. The data showed that for lower heat fluxes, the heat transfer characteristics are affected by the system hydrodynamics resulting in higher heat transfer coefficients, whereas at higher heat fluxes nucleate boiling is the dominant mechanism. The data also showed that within a tube bundle, the vapor rising from lower tubes enhances the CHF characteristics of the upper tubes.

On the thermal analysis of pressure tube/calandria tube contact in CANDU reactors

Abstract Under abnormal conditions contact between a pressure tube and the surrounding calandria tube in the core of a CANDU reactor may take place. The resulting temperature field may adversely affect the hydrogen diffusion characteristics in the pressure tube material. This paper is concerned with the thermal aspects of contacting pressure and calandria tubes. A critical review of existing thermal interfacial conductance correlations and their applicability to this problem was carried out. Experiments were also carried out to obtain detailed temperature distribution in the walls of typical pressure and calandria tubes in contact under simulated operating conditions. The thermal fields in both tubes were obtained as functions of the contact pressure and system temperatures. The results showed that the heat flow within the contact area is essentially one-dimensional. The data was used to calculate the interfacial thermal conductance as a function of contact pressure. The results were compared with available interfacial conductance correlations and an assessment of their applicability was accordingly made.

Rewetting of hot horizontal tubes

Abstract Experiments on the rewetting and refilling of hot horizontal tubes are presented. The experiments were carried out under a wide range of initial and boundary conditions. The effect of tube wall thickness, initial wall temperature, inlet mass flux and inlet subcooling were investigated. The results were used to identify the flow regimes associated with the rewetting and refilling of horizontal tubes. Although, stratified flow was commonly observed, inverted annular flow was observed under high mass flux conditions and skewed annular flow was also observed under limited conditions. The results were interpreted in terms of a new surface rewetting criterion which suggests that surface rewetting is initiated when the vapour film thickness in film boiling is reduced to a critical value irrespective of the surface temperature.

NEW VERSION OF SIMPLET AND ITS APPLICATION TO TURBULENT BUOYANCY-DRIVEN FLOWS

The essence of the SIMPLE method lies in its coupling between the momentum and continuity equations. One common feature of the SIMPLE family of methods (such as SIMPLER, SIMPLEC) is that the corrected velocity is obtained from the corrected pressure only. The SIMPLET algorithm was recently developed to take into consideration the effect of the temperature correction on the velocity correction based on the Boussinesq approximation. Because of this approximation, it is only valid for small temperature differences in the field. In the present article, a new version of SIMPLET is developed to remove this restriction and therefore make it applicable to general cases. When there are large temperature variations in flow fields, flows with appreciable length scales are nearly always turbulent, and turbulence model equations must be introduced into the governing equations. Computational examples are provided to illustrate the application of SIMPLET to turbulent buoyancy-driven flows using the renormalization group...