Peter Arthur Kew

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.

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.

Examination and visualisation of heat transfer processes during evaporation in capillary porous structures

Abstract This paper summarises the progress and the preliminary results of an investigation into the heat transfer process in wire mesh wicks. Particular emphasis is placed on the boiling and wicking limits in capillary porous structures made up of one or more layers of wire mesh. The two limits are shown to be interrelated: vapour bubbles trapped in the structure of a mesh wick block the wick and reduce the effective cross-sectional area, thus reducing the capillary flow. The presence of local dryout due to trapped bubbles leads to a decrease in the heat transfer coefficient. The experimental facility allowed measurement of the fluid and heater surface temperature and recording of the heat transfer process by using a high-speed video. Different wicks (mesh 50, mesh 100, mesh 150, mesh 200) with a thickness of one, three and five layers have been tested in submerged pool boiling and in wicking mode with distilled water as a working fluid. The parameters investigated included mesh size, number of layers and submergence of wick. Results confirm that the use of multiple layers of mesh wick increases the maximum attainable heat flux (qDHF) in wicking mode, but results in a lower heat transfer coefficient when compared with a bare surface or a single mesh. A linear relationship between wicking height h and qDHF is suggested by the results to date. Visualisation results provide an insight into the heat transfer mechanisms in mesh wicks during steady state conditions and as dryout occurs.

Development of the liquid film above a sliding bubble

Sliding bubbles contribute to the boiling heat transfer from an inclined surface. The results of a detailed investigation of steam bubbles sliding under a heated plate have recently been reported by Kenning et al . 1 . This gives an opportunity to compare an estimate of the liquid film thickness, made by thermal analysis of the experimental results, with a prediction based on the mechanics of the flow around the bubble. The fact that the estimates were of similar order (50–100 μm) confirms that, unless evaporation rates from the film are high, the film thickness is determined by flow phenomena rather than by heat transfer. The theory for predicting the film thickness is developed to include the effect of drainage and results are given for a range of slopes and bubble sizes.

The effect of diameter on boiling heat transfer from the outside of small horizontal tubes

Recent studies have suggested that tube diameter is a relevant parameter which should be included when correlating heat transfer data for pool boiling on horizontal tubes. However, there are few data for tubes of diameter less than 8 mm, and the diameter effect for small tubes has not been determined. This paper reports the results of an experimental investigation using tubes of 1–6 mm diameter. Tests have been carried out with water and refrigerant R141b at heat fluxes ranging from 1 to 90% of the critical heat flux. The results indicate that correlations which show an inverse relationship between heat transfer coefficient and diameter should not be applied for tubes of less than 6 mm diameter. Conventional pool boiling correlations derived from data acquired from boiling on wires and flat plates yielded acceptable results when applied to the test data from this study.

Enhanced Evaporation Heat Transfer Surfaces

Within the frame of the “JOULE” r & d programme of the Commission of the European Communities a project is carried out to develop and investigate novel enhanced evaporation heat transfer surfaces with the aim to employ them in improved industrial two-phase heat exchangers. Main surfaces which are investigated are structured and covered planar and tubular surfaces and planar surfaces with narrow channels. Typical results obtained so far are presented which demonstrate the excellent evaporation heat transfer characteristics of these surfaces.

Boiling Heat Transfer on Small Diameter Tube Bundle

An experimental study has been carried out using a tube bank representing a section of a tube bundle. The bank comprised 3 columns each of 10 stainless steel electrically heated tubes of 3mm outside diameter with pitch to diameter ratio of 1.5 in an in-line arrangement. Flow rate through the test section was controlled. Each tube in the central column was instrumented to permit determination of the tube temperature and heat flux, hence permitting calculation of the heat transfer coefficient. These tests were carried out using distilled water at nominal atmospheric pressure over a range of heat fluxes between 6 - 21 kW/m2. Results of the heat transfer tests are presented and compared with correlations used for conventionally sized bundles. Correlations developed for large tube bundle overestimate the experimental results.

Comparison of the Three–Zone Evaporation Model with Boiling Heat Transfer in a Compact Tube Bundle

The recent interest in boiling heat transfer in small diameter tubes has led to the study of boiling heat transfer outside a compact tube bundle of diameter 3mm. The bank comprised 3 columns each of 10 stainless steel electrically heated tubes of 3mm outside diameter, with pitch to diameter ratio of 1.5 in an in-line arrangement. These tests were carried out using distilled water and R113 at nominal atmospheric pressure over a range of heat fluxes between 4-21 kW/m2 for mass fluxes from G=5.6 - 32.8 kg/m2s. The recent three-zone evaporation model developed by Thome, Dupont and Jacobi for boiling inside micro channels was used to compare with experimental results as photographic study showed that bubbles confined within the bundle were responsible for the heat transfer enhancement observed. It was observed that the three state model was promising in its application to the bundle arrangement as the confinement number Co for bundle has been shown to be in the order of 0.63