Kettle Reboiler Research Articles

An improved void fraction model for two-phase cross-flow in horizontal tube bundles

A physically based model has been developed to predict the void fraction for upward cross-flow through horizontal tube bundles. It agrees well with experimental void fraction measurements in refrigerant 11 and air–water mixtures for a sufficiently wide range of pitch mass flux, quality and P/ D ratios. It has also shown improved predictive capability for adiabatic air–water and diabatic R-113 cross-flows than the void fraction models developed by other researchers. The analysis performed also shows plausible applicability of the void fraction model to steam-water data. The present void fraction model will allow researchers and designers to obtain better estimates of void fraction in the shell side flow in applications such as kettle reboilers and the U-bend region of nuclear steam generators than with the customarily used homogeneous equilibrium model. This will result in better estimates of average fluid density and average velocity of two-phase upward cross-flow through horizontal tube bundles.

2-D kettle reboiler circulation model

Abstract A 2-D model is developed for sections of a kettle reboiler where liquid/vapour disengagement is not severe. Using a trial set of boundary conditions it is shown that the influence of flow around the periphery of the bundle has negligible effect on flow distribution over the tubes. Uniform heat flux solutions are obtained for q =20 and 50 kW/m 2 boiling pentane and R113 at 1 atm. Pronounced 2-D effects were obtained at the higher heat flux only. Use of typical superposition and asymptotic flow boiling correlations resulted in bundle average heat transfer coefficients for both heat fluxes no different from the values predicted by the 1-D model. However, it is suggested that the 2-D solution mass velocity and void fraction distribution are important for predicting the design maximum heat flux limitation.

Void fraction distributions in a kettle reboiler

The void fraction and pressure distribution in shell-side boiling have been experimentally investigated. These data were collected from a horizontal tube bundle test section capable of modeling a kettle reboiler and a submerged evaporator. Kettle reboiler void fraction and pressure distributions were investigated for heat fluxes of 10, 30, 50, and 70 kW/m 2. Void fractions in the kettle reboiler tend to increase rapidly with vertical position low in the tube bundle but may decrease somewhat in the upper regions. Relatively large vapor concentrations in the bundle's central regions were due to the strong recirculation of vapor from the upper portions of the bundles that reentered near the center, in addition to vertical vapor acceleration at the bundle exit. Horizontal void concentrations were found to be fairly uniform. Local pressure drops generally agreed with the flow patterns found by visual analysis.

An interfacial friction correlation for shell-side vertical two-phase cross-flow past horizontal in-line and staggered tube bundles

A correlation is presented for the interfacial friction factor between the gaseous and liquid phases in vertical two-phase flows past horizontal in-line and staggered tube bundles. The interfacial friction data were determined from pressure drop, void fraction, and mass flux data taken by Dowlati et al. (1990, 1992b) and Schrage et al. (1988). These data were correlated using two non-dimensional quantities: a Reynolds number based on the mixture density and relative velocity between the two phases, and the porosity of the tube bundle. These dependencies are explained in terms of the competing effects of viscosity and bouyancy and the influence of turbulence in the flow. The correlation is implemented in a numerical simulation of the recirculating shell-side flow encountered in kettle reboiler heat exchangers, the results of which are compared to the experimental observations of Cornwell et al. (1980).

A correlation for heat transfer during subcooled boiling on a single tube with forced crossflow

Many heat exchangers, such as shell and tube heat exchangers and kettle reboilers, involve boiling with flow across tubes. For rational design of such heat exchangers, it is desirable to be able to predict heat transfer on a single tube. The dimensionless correlation presented here agrees well with available data for subcooled boiling during crossflow on a single tube. The correlating parameters are the same as those used for boiling inside tubes 16. The data correlated include three fluids, four tube materials, tube diameters from 1.2 to 25.4 mm, subcooling from 0 to 80°C, and velocities from 0.02 to 7.8 m/s. The mean deviation of 334 data points is 9.5%. Hence the new correlation appears to be usable over a wide range of parameters.

Two-Phase Flow and Its Applications: Past, Present, and Future

Abstract This article presents the three main alternative approaches to the prediction of two-phase flow systems (empirical, analytical, and phenomenological) and explores their advantages and disadvantages. Empirical correlations are the most widely used but have considerable error whereas full analytical treatments are rarely applicable. By understanding the phenomena involved and representing them by simple models, better prediction methods are feasible. This approach is exemplified in its application to the prediction of annular two-phase flow and to the prediction of two-phase flow in equipment, for example a kettle reboiler. The article closes by indicating the possible lines that future development may follow in this area.

A THEORETICAL MODEL FOR RECIRCULATING FLOW IN A BOILER

The two-dimensional flow occurring in the cross-section of a kettle reboiler has been modelled using potential flow theory. The theory covers both the single-phase flow around the tube bundle and the two-phase flow within the bundle. Apart from geometrical and heat flux data, application of the theory requires a knowledge of circulation, and means of obtaining this are discussed. The model yields local flow velocities and directions for the whole area and qualities, voidages and slip-ratios for the two-phase flow in the bundle assuming separated flow. The results compare favourably with an earlier experimental study and lay the foundation for a two-dimensional analysis of heat transfer in a kettle reboiler.

Transient swelling of liquid level during pool boiling in an emergency condenser

A modified kettle reboiler is planned to be used as an emergency condenser for high pressure steam in nuclear power plants. The condensing is achieved by the boiling-off of water at atmospheric pressure. The bubbles released at the tubular surface have a finite residence time in the liquid column above the tubular surface. This ‘bubble holdup’ increases the level of the two-phase boiling system. It is important to know the level change in order to determine the free space available for disengagement of the entrapped droplets and the capacity of the condenser to condense the high pressure steam. A solution to this problem is presented below alongwith an actual case study.