Pool boiling heat transfer of propane, isobutane and their mixtures on enhanced tubes with reentrant channels
Introduction
The role of chlorofluorocarbons (CFCs) in the process of ozone depletion and their contributions to global warming is now widely accepted. Hydrocarbons offer the possibility of a cheap, readily available and environmentally acceptable alternative to CFCs and CFC substances [2]. Propane and isobutane are widely used as a refrigerants [3]. Mixtures of propane and isobutane can be composed to obtain a saturation temperature/pressure curve as well as a cooling capacity very similar to those of R12, and have therefore been investigated as servicing or retrofitting refrigerants [2]. Investigations show a better COP for hydrocarbons than for R12 in refrigeration systems e.g. [4]. Though there are some published boiling heat transfer data for propane, the data for isobutane and propane/isobutane mixtures are scarce (refer Section 3.6), and it seems that no such data are available for enhanced surfaces.
The commercial enhanced boiling tubes may be categorized into three groups: low-finned tubes (e.g. Gewa-K), modified finned tubes (e.g. Gewa-T, Thermoexcel-E and Turbo-B) and porous layer coated tubes (e.g. High Flux) [5]. Generally, for pure fluids, the heat transfer performances of the High Flux, Thermoexcel-E and Turbo-B surfaces are better than those of the Gewa-T and Gewa-K surfaces, at least at low and moderate heat fluxes [5], [6], [7], [8], [9]. Modified finned tubes can be further divided into three groups: tubes with narrow gaps (e.g. Gewa-T), tubes with surface pores (e.g. Thermoexcel-E) and tubes with surface pores connected by gaps (e.g. Turbo-B and the PB-tubes used in this work (Fig. 2)) [10].
For mixtures, the heat transfer performance can differ substantially depending on the kind of enhanced surface and on the mixture used. For the High Flux porous tubes, the boiling heat transfer coefficient of ethanol-water mixtures can be greater than the ideal value [11], [12]. The reduced degradation suffered by the High Flux tube compared with the smooth tube, according to Thome [11], is the result of a liquid Prandtl number augmentation of the liquid-phase convection process inside the porous matrix, which partially counterbalances the negative mass transfer effects. For acetone/water mixtures, the High Flux tube has a sharp drop in performance with the addition of small concentrations of acetone, this drop is much greater than that for the smooth tube [5]. At higher concentrations, the performance of the High Flux tube increases substantially, while the smooth tube values remain low [5]. However, for R113–R11 mixtures, the High Flux tube suffers a much severer degradation than the smooth tube [13]. For finned tubes (e.g. Gewa-K), the reported data show a similar mixture effect as that for the smooth tubes [14]. The plain tube mixture boiling correlation predicted the finned tube mixture data fairly well [5].
Very limited data has been reported for boiling of mixtures on reentrant tubes. For boiling of R113–R11 mixtures, the Turbo-B surface suffers a much higher degree of heat transfer degradation than the smooth surface [13]. This is also true for boiling of R114–oil mixtures on the Turbo-B, Thermoexcel-HE and High Flux surfaces [8]. A surmise for the large degradation of mixture boiling heat transfer on the reentrant tubes is that there is a large buildup of the less volatile component next to the heated surface due to the inability of the free stream to affect the heat transfer process in the sub-surface channels [13].
The main objective of this paper is to provide a comprehensive pool boiling database for propane, isobutane and their mixtures on smooth and reentrant enhanced tubes. In the accompanying paper [1], the boiling mechanisms, especially the mixture effects on the enhanced boiling, will be studied by means of visualization and by evaluating the bubble dynamics data, which provides partial explanations for the different heat transfer performances as will be shown in the present paper.
Section snippets
Introduction to experiments
The experimental setup is mainly composed of test vessel, test section, power supply unit, PC based data acquisition system and visualization equipment (Fig. 1). The test section consists of an outer tube with the test surface and a copper inner tube (16.02 mm in diameter) shrunk into the outer tube. The surface is heated by a heater cartridge. In the copper inner tube wall, four platinum resistance thermometers (PT100) are guided in four axial grooves, 90° apart, to different axial locations.
Boiling hysteresis
Fig. 3 show the boiling curves obtained by first increasing and then decreasing the heat flux with the start point indicated by an arrow. Boiling hysteresis was found for all surfaces and it is much more pronounced during boiling of the 50–50% mixture than the pure fluids for each surface. For the various tubes used, the smooth tube has the highest temperature overshoot (short: T-overshoot). Almost no T-overshoot is seen for boiling of isobutane on PB3 and also for boiling of propane on PB4.
Conclusions
- (1)
Boiling hysteresis is found for all surfaces. The temperature overshoot for the smooth tube is generally higher than that for the enhanced tubes. The mixtures show much stronger hysteresis than the pure components. The data generally show that, the lower the temperature overshoot, the higher the heat transfer performance.
- (2)
Compared with the smooth tube, the enhanced tubes generally show different heat transfer improvements depending on the specific enhanced surface, working fluid and experimental
Acknowledgement
This work has been partially funded by the Commission of the European Community. The financial support from Friedrich-Ebert-Stiftung for the doctoral study of one of the authors, Yuming Chen, is greatly appreciated.
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