Falling film evaporation and pool boiling heat transfer of R1233zd(E) on thermal spray coated tube

Abstract

• Falling film evaporation heat transfer of R1233zd(E) was experimentally investigated. • Up to 4.2-fold heat transfer enhancement could be obtained by thermal spray coating. • R1233zd(E) achieved a lower heat transfer coefficient than R134a at 20 °C. Falling film evaporators have been expected as an alternative to flooded evaporators for reducing the refrigerant usage. It is important for the enhancement of the film evaporation heat transfer to keep a liquid film and promote nucleate boiling in the liquid film. This study focused on nucleate boiling heat transfer in the liquid film by thermal spray coating that can be applied regardless of the material of heat transfer tubes. The heat transfer performance of falling film evaporation on a horizontal copper tube was experimentally evaluated. Since nucleate boiling phenomena, such as activation of nucleation sites and bubble behaviors in the liquid film, strongly depend on the physical properties of the refrigerant, the experimental results for R1233zd(E), one of the alternative refrigerants with low GWP, were compared with those for R134a. Two types of tubes were used: the smooth tube and the thermal spray coated tube fabricated by an arc wire spraying method, with the outer diameter of 19.05 mm and the heating length of 50 mm. Pool boiling experiments were also conducted to confirm the wall superheat for nucleate boiling. The heat transfer coefficients were evaluated with a film mass flow rate range of 8.5 × 10 −3 to 6.3 × 10 −2 kg/(m·s), a heat flux range of 10 to 85 kW/m 2 , and a saturation temperature at 20 °C. The effects of the structure of the heating surface, heat flux, and thermophysical properties of the refrigerant on the heat transfer performance are discussed. Results show that the heat transfer of the falling film became the nucleate boiling dominant with a sufficient heat flux and liquid film flow rate. The thermal spray coated surface enhanced not only the nucleate boiling but also the liquid spreading by the vapor bubbles in the liquid film, and thus the coated tube produced higher heat transfer coefficients of 2.1 to 4.8 times those for the smooth tube. The wall superheat at the onset of nucleate boiling was higher for R1233zd(E) than for R134a, because R1233zd(E) has a higher surface tension and lower vapor density. Thus, the pool boiling and falling film evaporation heat transfer coefficients were lower for R1233zd(E) than for R134a.