Analysis and experimental study of nucleation site densities in the boiling of mixed refrigerants
Introduction
Boiling of mixtures occurs in many chemical processes [1], [2], [3], [4], [5], [6], [7]. As a result, the heat transfer coefficient (HTC) is crucial for the design of heat exchangers. However, there can be large variations in the boiling processes of pure and mixed refrigerants, and it is impossible to guarantee thermal performance when mixed refrigerants are employed in heat exchangers which use pure refrigerants.
Boiling of mixtures has been investigated extensively in the past decades [8], [9], [10], [11], [12]. Many efforts have been made to study the deterioration of boiling heat transfer in mixtures [13]. Various explanations have been proposed to account for the differences in boiling heat transfer performance between pure refrigerants and their mixtures. A summary of the studied causes was provided by Celata et al. (1994) [13]:
- (1).
Increase in the local boiling point and mass transfer resistance.
- (2).
Changes in physical properties of the mixtures.
- (3).
The effect of mixtures on nucleation.
- (4).
Retardation of the main mechanisms of heat transfer.
In this paper, efforts have been made to study the reduction of nucleation sites during the boiling of binary mixtures.
During the boiling process, bubbles inside the liquid grow from active nucleation sites, whose density depends on the surface micro-roughness (size distribution of cavities, distribution of the half cone angles of the cavities, minimum cavity mouth radius, etc. [14]), the surface tension of the liquid, the physical properties of the heating surface and the liquid, and the wall superheat [15]. Due to the large uncertainty for finding the proper cavities and the pressure fluctuation by the bubble generated, the statistical method does not appear to predict the nucleation site density accurately [16]. As a result, based on the mechanisms and experimental studies of pure fluids (mostly water), many nucleation site models and correlations have been proposed[14], [15], [17], [18], [19], [20] and continually corrected [16], [21], [22], [23], [24], [25].
Compared to the large number of studies on the boiling nucleation site densities of pure refrigerants, similar studies are relatively rare for binary mixtures. Hui et al. (1984) [26] measured the boiling site density and the HTC of ethanol/water and ethanol/benzene mixtures at 0.101 MPa for a heated vertical brass disk. The mass diffusion effect was considered as the main reason for the strong effect of composition on the boiling site density. Benjamin et al. (1997) [27] investigated the nucleation site density of acetone/carbon tetrachloride and n-hexane/carbon tetrachloride under atmospheric pressure. They considered the effects of surface micro-roughness and the physical properties of the liquid, and proposed a correlation based on their previous study and experimental data for pure liquids. In their opinion, a part of the driving force provided by temperature was utilized to overcome the resistance caused by mass diffusion. Thus, an attenuation factor related to the diffusion coefficient was applied to the temperature difference term in their correlation. In these studies, the mass diffusion effect was regarded as the main reason for the reduction in the boiling nucleation site density of mixtures.
In the classical theory of homogeneous nucleation, nucleation sites are generated as a result of density fluctuations in the liquid (Katz, 1975 [28]). For the case of mixtures, this paper proposed that the concentration of the low-boiling point component should be aggregated in a localized region before the nucleation sites are generated. This concentration fluctuation effect is the main reason for the reduction in the number of boiling nucleation sites generated inside liquid mixtures.
This paper, firstly, provides evidence for the concentration fluctuation effect by analysing the experimental data for onset of boiling (ONB) and limits of superheat. Furthermore, by using the classical fluctuation theory [29], a model is developed to account for the reduction of boiling nucleation site density in binary mixtures. Secondly, a visualization study was carried out to investigate the boiling nucleation site densities of i-butane, propane, R134a/R32 and i-butane/propane mixtures under various pressures. A correlation was developed for pure refrigerants, based on the experimental data. Additionally, experiments on mixed refrigerants show that the boiling site densities first decrease and then increase as the fraction of the high-boiling point component increases. The attenuation factors for the reduction of boiling site densities were calculated for mixed refrigerants. Lastly, by using the proposed model, the qualitative results of calculations were compared with the experimental data.
Section snippets
Evidence of the concentration fluctuation effect
It is generally understood that under a static steady state, liquids do not vaporize until their temperature reaches the limit of superheat. To prove the attenuation effect of concentration fluctuation on the formation process of nucleation sites, the experimental data for ONB and the limits of superheat are studied here for binary mixtures.
Han et al. (2004) [30] studied the bubble behavior and heat transfer characteristics during saturated pool boiling of R11/R113 mixtures near the ONB
Experimental setup
An experimental test facility was constructed to investigate the boiling performance of different refrigerants. Fig. 2 shows the schematic diagram of the experimental apparatus used in the study. The experimental apparatus consists of two thermostatic baths, a water tank, a quartz tube welded on a quartz plate, a copper cylinder wound by a copper pipe, a high-speed camera and its light source. More specific information about the experimental set-up is given below:
- (1)
Thermostatic baths: Two
Results and discussion
To study the reduction in boiling site density in the binary mixtures, the boiling site density of pure refrigerants in this system should be determined. Experimental studies were conducted to determine the boiling site density of i-butane and propane.
Conclusion
In this paper, the boiling nucleation site densities for pure and mixed refrigerants were studied, and the following conclusions were obtained:
- (1)
The concentration fluctuation effect is proposed to account for the reduction of the boiling nucleation site density in mixtures. The experimental results of the limits of superheat and ONB are used as evidence of the concentration fluctuation effect.
- (2)
Experimental tests were conducted to investigate the boiling nucleation site density for pure
Acknowledgement
This work was supported by Natural Science Foundation of China (Grant number 51506057), the Fundamental Research Funds for the Central Universities (Grant number 2015ZM028), State Key Lab of Subtropical Building Science, South China University of Technology (Grant number 2016KA01, 2015ZC13).
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