Bamorobat R.   Yun E.   Yoon S.   Kim K.  

Flow boiling visualization of zeotropic mixture of R134a and R245fa in a vertical tube.

Reporter: Kim K.

While pure refrigerants have a constant temperature during phase change in constant pressure, some of their mixtures do not fall in this definition. Specifically, zeotropic mixtures experience a temperature glide while changing their phase from liquid to vapor. This phenomena might be a disadvantage in many fields but in power systems like ORC (organic Rankine cycle) systems it is an advantage indeed. Although the literature is full of myriad of papers about zeotropic mixtures and their application in ORC systems, little is known about their behavior when their phase changes in heat exchangers and expanders. This calls for a thorough investigation of zeotropic mixtures, mainly visualizing their phase-change and studying the heat transfer involved in the phase change.
In this experiment the zeotropic mixture of R134a and R245fa has been chosen because of its high performance in power production systems, ORCs. Different mixtures with different molar concentrations have been made and in each case the heat transfer in various mass and heat fluxes has been investigated. Also the phase-change of each mixture in different conditions has been visualized by means of a high speed camera, capable of taking 15000 images per second, to provide an optic tool for deeper understanding mixtures behaviors and identifying the flow regimes in each flow condition.
The test section used for visualization is a glass tube with 6 mm outer diameter and 3 mm inner diameter and 20 cm length. This glass tube is preceded by an identical tube made from steel used for heat transfer measurement. Ten thermocouples are placed on the surface of the steel tube to measure the temperatures. The test loop used for the experiment which is consisted of a reservoir tank, a pump, a flowmeter, test section and a condenser connected to a chiller. Also an orifice has been used to provide a small pressure difference in the system to ease the startup.
The result shows a bubble moving in a slug flow captured from pure R245fa while its pressure is 90 kPa. The fluid experiences a 5kPa pressure drop in the test section while the inlet temperature is fixed on 10 °C. The mass and heat fluxes are 80 kg/m2s and 100 kW/m2, respectively. It is shown that the bubbly flow of the mixture of 25% molar concentration of R134a and 75% molar concentration of R245fa in 180 kPa pressure, mass flux of 800 kg/m2s and heat flux of 100 kW/m2. The churn flow appears at the mixture of 33%-67% molar concentration of R134a-R245fa in 270 kPa pressure . It is shown that the annular flow of the mixture of 50%-50% molar concentration of R134a-R245fa 400 kPa pressure.

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