Heat transfer correlations for evaporation of refrigerant mixtures flowing inside horizontal microfin tubes
Introduction
During recent decades, for the need of substituting for HCFCs, there are many binary and ternary non-azeotropic or near-azeotropic refrigerant mixtures to be developed, and many aspects of evaporation heat transfer have been investigated and some correlations have been proposed for evaporation of refrigerant mixtures inside horizontal smooth tubes. Contrast to pure and azeotropic refrigerant mixtures, the heat transfer degradation occurs in non-azeotropic refrigerant mixtures during phase change. The degradation can be mended by heat transfer enhancement inside heat transfer tube. So the technology of heat transfer enhancement becomes the main measure to reform the heat transfer degradation of non-azeotropic refrigerant mixtures and to improve heat transfer effect.
Many kinds of heat tranrsfer tubes with enhanced surface have been developed to improve the performance of heat exchanger. The microfin tubes have outstanding performance from the point of a balance between two-phase heat transfer enhancement and pressure drop enlargement and have been widely used to save energy in refrigeration and air conditioning industries. However, the correlations for evaporation heat transfer of refrigerant mixtures in microfin tubes are very limited in the open literature due to lack of database, the complexity of two-phase heat transfer process, the influence of enhanced surface on heat transfer, etc., and it becomes one of the limitations in the design of efficient heat exchangers.
Based on the experimental results of evaporation heat transfer for R417A, which is a long-term substitute for R22, flowing inside two internally grooved tubes with different microfin geometrical parameters, the objective of this paper is to develop the prediction methods of evaporation heat transfer for non-azeotropic refrigerant mixtures flowing inside microfin tubes. Knowledge of new empirical correlations can provide the reference for design of evaporators, a key component that determines the performance of refrigerating and air conditioning systems.
Section snippets
Literature survey
The evaporation of refrigerant mixtures inside enhanced surface tubes is very complex due to the influencing of mixture properties and different enhanced surface on heat transfer, so there is only few models developed for mixtures evaporating in enhanced surface tubes. For pure refrigerants, Yun et al. [1] suggested a model considering the enhancement effect of turbulence generated by microfins, fin height and numbers of fin; Thome et al. [2], Cavallini et al. [3] and Koyama et al. [4]
Experimental work
Fig. 1 shows a schematic diagram of the test facility, a vapor compression system was chosen, which is consisted of three separate circuits, namely the refrigerant circuit, the heating circuit and the cooling circuit. The evaporating test section consists of three double-tube type heat exchangers, the inner tubes adopt a 3.0 m long smooth (after referred to as Tube I) and two 2.4 m long internally grooved tubes (after referred to as Tube II and Tube III) having different geometrical parameters
Development of microfin model for evaporation of refrigerant mixtures
Evaporation heat transfer correlations in microfin tubes were generally developed by introducing enhancement factors into smooth correlations, so it is a key for choosing right smooth correlations and enhancement factors. A model for smooth tube, developed by Kattan et al. [8] was found to express the local variations in heat transfer during the whole evaporation process since it takes into account the flow regimes, and is based on the refrigerant physical changes occurring in the tube, but it
Comparison of correlations with the experimental data
The experimental results of R417A inside Tube II and Tube III for vapor qualities up to 80% are compared with the microfin models developed at the present study for refrigerant mixtures and the microfin models suggested by Thome et al. [2] and Cavallini et al. [3]. The comparison is presented in Fig. 7, it reveals that the models by Thome et al. [2] and Cavallini et al. [3] overpredict the evaporation heat transfer coefficients for R417A inside internally grooved tube II and tube III rather
Conclusion
Based on the experimental results for evaporation of R417A flowing inside a smooth and two internally grooved tubes having different geometrical parameters, the present work deals with the development of prediction methods for evaporation heat transfer of refrigerant mixtures inside microfin tubes.
The Kattan model was modified using the experimental results of R417A inside smooth tube. The modified-Kattan model was tested against the present experimental data and was found to be in good
References (11)
- et al.
A generalized correlation for evaporation heat transfer of refrigerants in microfin tubes
Int J Heat Mass Transfer
(2002) - et al.
Two-phase heat transfer coefficients of three HFC refrigerants inside a horizontal smooth tube, part I: evaporation
Int J Refrig
(2000) - Thome JR, Kattan N, Favrat D. Evaporation in microfin tubes: a generalized prediction model. In: Proceedings of the...
- et al.
Refrigerant vaporization inside enhanced tubes: a heat transfer model
Int J Heat Technol
(1999) - et al.
Forced convective flow boiling heat transfer of pure refrigerants inside a horizontal microfin tube