Enhancement of R1234ze(Z) pool boiling heat transfer on horizontal titanium tubes for high-temperature heat pumps

Abstract

R1234ze(Z), which has a global warming potential of less than 1, is a promising alternative refrigerant for hightemperature heat pumps designed for heat recovery in the industrial sector. The use of titanium as the material for heat exchangers exposed to acid exhaust is one solution to prevent oxidation. In this study, the pool boiling heat transfer characteristics outside of horizontal titanium tubes were experimentally investigated for R1234ze(Z). A plain tube and three enhanced titanium tubes were tested at saturation temperatures from 10 to 60 °C and heat fluxes from 0.55 to 79.8 kW m-2. Compared to the plain tube, the tested enhanced tube exhibited a 2.8 to 5.1 times higher heat transfer coefficient, on average, in the test range, which could compensate for the disadvantage in the thermal conductivity of titanium. The enhancement ratio predominantly depends on the saturation temperature and the wall heat flux. At conditions of higher saturation temperatures and lower heat fluxes, where smaller bubbles are observed, test tubes with smaller opening width exhibit higher heat transfer coefficients. The experimental results indicate the importance of fin geometry optimization to the operation conditions.

Recent advances on heat and mass transfer in refrigeration and air-conditioning systems

“…Nagata and colleagues (2017) also concluded that a correlation from the literature that showed good agreement with the experimental data of the plain copper tube, did not worked well for the enhanced titanium tubes and a second correlation from the literature was proposed for the former tubes. Finally, Nagata et al (2017) observed that at conditions of higher saturation temperatures and lower heat flux, where smaller bubbles were present, the titanium test tubes with smaller fin spaces exhibited higher heat transfer coefficients.…”

“…The active heat transfer length of the test tube was 400 mm. Nagata et al (2017) concluded that the thermal resistance of the titanium tubes was significantly affected by the lower thermal conductivity of the tube material. However, compared to the plain tube, the tested enhanced tube exhibited a 2.8 to 5.1 times higher heat transfer coefficient, on average, in the test range, and the authors indicated that this could compensate for the disadvantage in the thermal conductivity of titanium.…”

“…A unique characteristic of the work by Nagata et al (2017) is the investigation of pool boiling of the LGWP refrigerant R1234ze (Z), which has a GWP of less than one, in conjunction with titanium tubes for heat exchangers, which are used in heat recovery systems for preventing oxidation when exposed to exhaust containing acidic substances. One plain tube and three titanium tubes with enhanced outside surfaces were tested for pool boiling performance at saturation temperature ranging from 10°C to 60°C, and heat fluxes from 0.55 to 79.8 kW·m −2 , with the aim to determine the optimum tube geometry for R1234ze(Z) at relatively higher saturation temperatures for waste heat recovery.…”

Review of low-GWP refrigerant pool boiling heat transfer on enhanced surfaces

“…Nagata et al [70] measured the pool boiling heat transfer for R1234ze(Z) on a plain tube and three enhanced titanium tubes. The depth and the inner width of the reentrant cavities of the enhanced tubes ranged from 0.375 mm to 0.410 mm and from 0.270 mm to 0.310 mm, respectively.…”

Numerical energy and exergy analysis of cooling systems: low GWP alternative refrigerants (R450A and R1234ze) to R134a

“…Thanks to zero ODP and low GWP values, the availability of such refrigerants today and in the future is of great importance. The properties of R134a, R1234ze, and R450A are given in Table 1 [13,14].…”