Void fraction in vertical intermediate and inlet headers of microchannel heat exchangers: Experiments and models

The void fraction in headers of microchannel heat exchangers (MCHEs) affects the refrigerant charge, distribution, pressure drop, and heat transfer coefficient of MCHEs. However, due to the complex geometries of headers, experimental measurements of void fraction in headers with refrigerant are still missing in the open literature. A model that predicts local void fraction between microchannel tubes in headers is also widely needed when predicting the refrigerant charge and the performance of MCHEs. This paper first presents experimentally measured void fraction between microchannel tubes in a intermediate (five inlets and five outlets) and inlet headers (ten or five outlets) with R134a by calibrated capacitive sensors. For the intermediate header, the ranges of inlet mass flow rate and quality are 3.2 g s(-1) to 5.2 g s(-1) and 0.2 to 0.8, respectively. The inlet mass flow rates vary from 3.2 g s(-1) to 7.2 g s(-1) and qualities range from 0.1 to 0.82 for the inlet headers. As the inlet quality increases, the average void fraction in headers becomes larger. Other local parameters in the headers, such as mass flux, quality, superficial velocity together with the visualization results are also obtained. Void fraction shows a strong relation to the local quality in headers and the refrigerant distribution in microchannel tubes. A drift-flux void fraction model based on the local variables is proposed in this paper. The local void fraction predicted by the model agrees well with the experimentally measured values, with most of the predictions falling into +/- 15% deviations. The proposed model has better accuracy when predicting void fraction in headers comparing to some widely used correlations.

Automated summary

This study experimentally measured the void fraction in headers of microchannel heat exchangers and found a strong correlation between void fraction, local quality, and refrigerant distribution in the tubes. A drift-flux void fraction model based on local variables was proposed for predicting void fraction in headers with better accuracy compared to widely used correlations.