Flow boiling of R245fa in 1.1Â mm diameter stainless steel, brass and copper tubes

An experimental study of flow boiling heat transfer and pressure drop was conducted using R245fa in stainless steel, brass and copper tubes of 1.1 mm internal diameter. Experimental conditions include: mass flux range 100-400 kg/m2 s, heat flux range 10-60 kW/m2, pressure of 1.8 bar and exit vapour quality range 0-0.95. The tube surfaces were compared using scanning electron microscopy (SEM) and surface data acquired from confocal laser microscopy (CFLM), both showing differences between the surfaces. The heat transfer coefficient is similar in magnitude for all three materials but with a slight variation in trend. The heat transfer coefficient is seen to peak at high vapour qualities for stainless steel and brass, which is less evident with copper. The results were compared with past heat transfer correlations. These results showed better agreement with stainless steel compared to copper and brass. The pressure drop was shown to differ with surface characteristics, with the pressure drop for brass having a much steeper increase with heat flux. The pressure drop correlations tested did not show good agreement with the experimental results.