Experimental study of the thermal hydraulic performance of sub-cooled refrigerants flowing in smooth, micro-fin and herringbone tubes

• Thermo-hydraulic performance of different microfin tubes used for heat transfer enhancement.
• Evaluation of number of fins and working fluid effects.
• New results for herringbone tubes.

The present paper reports an investigation of the thermo-hydraulic performance of two standard microfin tubes with different number of fins and a herringbone microfin tube. As a reference for comparison purposes a smooth tube of equal external diameter and wall thickness has been tested for similar operational conditions. Copper tubes of 9.52 mm external diameter, electrically heated, have been used in the investigation. Most of the reported data has been gathered with refrigerant R134a though refrigerant R22 was also used in some of the tests with both the smooth and one of the standard microfin tubes. The test tube entrance state of the working fluid was kept constant at a temperature of the order of −2.5 °C and a pressure of 500 kPa, whereas its mass velocity varied in the range between 100 kg/s m2 and 1350 kg/s m2. It has been found that the thermal performance of the herringbone tube is superior to that of the two standard microfin tubes though the pressure drop obtained with the former is clearly higher. All the microfin tubes present significant heat transfer enhancement with respect to the smooth tube. According to the present results, the enhancement ratio of the microfin tubes attains an asymptotic value for Reynolds numbers of the order of 20,000, the herringbone tube presenting the higher asymptotic value. A figure of merit parameter has been introduced to qualify each of the microfin tubes tested, with the both standard microfin tubes being the best qualified with respect to the herringbone one, especially for applications that operate at the intermediate Reynolds numbers range.