Keith Cornwell

Correlations for the prediction of boiling heat transfer in small-diameter channels

Abstract This paper describes aspects of the work relating to boiling in single, small-diameter tubes as part of a study of compact two-phase heat exchangers. In order to realise the energy-saving potential of compact heat exchangers for evaporating duties it is necessary to establish design procedures. A test facility was commissioned which was used to measure pressure drop and boiling heat transfer coefficients for R141b flowing through tubes 500 mm long with diameters of 1.39–3.69 mm. Established correlations predicted the heat transfer coefficients reasonably well for the largest tube but performed badly when applied to the smaller tubes. It would appear that simple nucleate pool boiling correlations, such as that of Cooper, best predict the data. While under some conditions increasing quality leads to an increasing heat transfer coefficient, it is suggested that intermittent dry-out occurs at very low quality in single narrow channels, thus reducing the average heat transfer coefficient below that expected from the pool boiling correlations.

Two-phase heat transfer to a refrigerant in a 1 mm diameter tube ☆

A study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels. R141b has been used as the working fluid. The boiling heat transfer in the small tube has been measured over a mass flux range of 300–2000 kg/m2 s and heat flux range of 10–1150 kW/m2. In this paper the boiling map for a mass velocity of 510 kg/m2 s and heat flux of 18–72 kW/m2 is discussed and the problems of determining heat transfer coefficients in small channels are highlighted.

BOILING IN SMALL PARALLEL CHANNELS

The need for intensification of process heat exchangers has led to the development of several types of compact heat exchangers suitable for evaporation heat transfer. These heat exchangers are characterised by small multi-channel passages operating in parallel. The various regimes of two-phase flow which occur within these passages determine the heat transfer and hence the heat exchanger effectiveness. Currently there are limited data available in the literature of use in the design and selection of compact heat exchangers for evaporation.

Nucleate pool boiling on horizontal tubes : a convection-based correlation

Abstract Pool boiling on horizontal tubes leads to a bubbly flow layer around the lower periphery of the tube in which bubbles slide around the surface. Heat transfer is by enhanced liquid convection at the surface around the bubble and by rapid evaporation of a thin layer under the sliding bubble. Both these mechanisms are a function of the vapour flow rate and a correlation of the following form is developed: Nu = AF(p) Re 0.67 b Pr 0.4 . A is a function of the critical pressure alone, F(p) is a function of the reduced pressure alone and Re b is based on the mean vapour mass flux from the surface and the tube diameter. The correlation applies to water, refrigerants and organics boiling on tubes of 8–50 mm in diameter. Bearing in mind the wide scatter found in pool boiling experiments the correlation fits well with the available world data and is particularly suitable for use in equipment design calculations.

A study of boiling outside a tube bundle using high speed photography

Abstract A high speed film of boiling outside tubes near the top of a horizontal reboiler tube bundle reveals a multitude of small bubbles which grow rapidly while sliding up the side of the tube. These bubbles originate from both the main stream and from nucleation sites at the base of the tube. The measured growth rate correlates well with theory based on an evaporating microlayer under the bubble. It is shown that sliding bubbles can account for the enhancement of heat transfer observed at the upper tubes of bundles.

Flow Boiling of Refrigerant R141B in Small Tubes

Small circular and non-circular tubes are widely encountered in compact evaporators and condensers. This paper presents an experimental study of two-phase flow and heat transfer of refrigerant R141b, in small tubes. Four circular tubes with diameters of 1.1, 1.8, 2.8, 3.6 mm and one square tube of 2×2mm 2 were used in the test programme. The parameter ranges were mass flux 50 ∼ 3500kg m −2 s −1 , heat flux 1 ∼ 300kWm −2 , and inlet pressure 1 ∼ 3 bar resulting in mean boiling heat transfer coefficients of 0.1 ∼ 10 kW m −2 C −1 . It was found that local heat transfer coefficients are not only a strong function of heat flux but also a function of vapour quality and a weaker function of mass flux for the all the small tubes tested, showing that both nucleate boiling and convective evaporation occur in small tubes. The mean heat transfer coefficient was found to be is primarily a function of the heat flux, rather than the mass flux. Comparison with data in the literature shows that a general flow map developed from adiabatic two-phase flow tests can provide guidance for prediction of flow regimes in heated tubes. This study provides useful design data for two-phase flow and heat transfer in small tubes and channels.

The influence of bubbly flow on boiling from a tube in a bundle

Abstract The forms of bubbly flow occurring within a tube bundle are discussed and the boiling process in the bundle is notionally divided into mechanisms due to liquid forced convection, sliding bubbles and nucleation. A novel experimental analysis of heat transfer from a tube in a bundle indicates the predominance of the sliding bubble part. There is a virtual absence of nucleation in a bundle except at the lowest tubes indicating that, once enough bubbles have been produced, the other mechanisms are sufficient to transfer the heat from the tubes.

Heat transfer to bubbles under a horizontal tube

Abstract The mechanisms of boiling heat transfer on the outside of a horizontal tube are multifarious and include contributions from site nucleation, micro-layer evaporation and liquid disturbance. Previous work has shown the strong influence of sliding bubbles which disturb the liquid and can leave a thin evaporating layer under the bubble. Results of experiments designed to determine the local temperature profiles around individual bubbles in water as they slide around the periphery of a thin downward-facing curved surface are presented. High-speed video recording of thermochromic paint on the surface followed by hue analysis and processing allowed detailed examination of a small section of the curved surface. The strong influence of liquid disturbance around the bubble on the local heat transfer is demonstrated. Under the low heat flux conditions of the experiments, the liquid layer under the bubble was found to dry out yielding a high-temperature spot which was quenched by the surrounding liquid when the bubble moved away.

Sliding and sticking vapour bubbles under inclined plane and curved surfaces

Abstract In flow boiling heat is transferred by the combined effects of nucleate boiling, with local generation of bubbles, and evaporative and convective cooling by the passage of bubbles generated elsewhere. In this study, nucleate boiling was eliminated by measuring the heat transfer near injected steam bubbles sliding under an inclined plate heated to low superheats, using liquid crystal thermography combined with high speed video recording and computerised image analysis. Heat was transferred by evaporation of the thin liquid film between the bubble and the wall and by enhanced convection in a wake region wider than the bubble and many bubble diameters long. Evaporation was the dominant mechanism for large, easily deformed, slow-moving bubbles. For small, faster-moving bubbles the reduction in evaporation was offset by an improvement in convection.

On boiling incipience due to contact angle hysteresis

Abstract The incipience of boiling on a metal surface is largely due to instability of the vapour-liquid interfaces which exist in minute natural cavities. The influences of surface roughness and contact angle on the equilibrium of the interface are studied and expressions relating roughness and advancing and retarding contact angles to the possibility of vapour trapping are developed. A model of boiling incipience based on contact angle hysteresis between the advancing and retarding angles of the interface within the cavity is presented. This hysteresis arises naturally from the roughness and heterogeneity of a surface on the microscopic scale and thus occurs on normal engineering boiling surfaces. The model predicts incipience at smaller radii than the cavity mouth radii and successfully explains the observed features of boiling incipience without postulating the existence of re-entrant cavities.