Abstract
To protect the environment, a new low-GWP refrigerant R1234ze(E) was created to substitute R134a. However, its flow boiling performances have not received sufficient attention so far, which hinders its popularization to some extent. In view of this, an experimental investigation was carried out in a 1.88 mm horizontal circular minichannel. The saturation pressures were maintained at 0.6 and 0.7 MPa, accompanied by mass flux within 540–870 kg/m2 s and heat flux within 25–65 kW/m2. For nucleate boiling, a larger heat flux brings about a larger heat transfer coefficient (HTC), while for convective boiling, the mass flux and vapor quality appear to take the lead role. The threshold vapor quality of different heat transfer mechanisms is around 0.4. Additionally, larger saturation pressure results in large HTC. As for the frictional pressure drop (FPD), it is positively influenced by mass flux and vapor quality, while negatively affected by saturation pressure, and the influence of heat flux is negligible. Furthermore, with the measured data, several existing correlations are compared. The results indicate that the correlations of Saitoh et al. (2007) and Müller-Steinhagen and Heck (1986) perform best on flow boiling HTC and FPD with mean absolute deviations of 5.4% and 10.9%.
Highlights
With the increasing concerns on environment protection, the demand for environmentfriendly refrigerants in many fields is becoming more and more urgent
The flow boiling heat transfer coefficient (HTC) and frictional pressure drop (FPD) were measured with psat = 0.6 and 0.7 MPa, G = 540–870 kg/m2 s, and q = 25–65 kW/m2
The saturation pressure, mass flux, and heat flux fell in the scopes of 0.6–0.7 MPa, 540–870 kg/m2 s, and 25–65 kW/m2, respectively
Summary
With the increasing concerns on environment protection, the demand for environmentfriendly refrigerants in many fields is becoming more and more urgent. The heat transfer was mainly regulated by convective boiling, nucleate boiling, and thin liquid film evaporation Based on these mechanisms and flow patterns, a new HTC correlation was deduced, which performed much better than the correlations of Gungor and Winterton [14], Saitoh et al [15], and Kim and Mudawar [16]. Mancin et al [20] and Diani et al [21] examined FPD variations of R1234ze(E), R1234yf, and R134a in a copper foam with tsat = 30 ◦ C, G = 50–200 kg/m2 s and q = 50–100 kW/m2 They found that for larger mass flux and vapor quality, the corresponding pressure drop was larger. Since understanding the flow boiling characteristics of R1234ze(E) more clearly and choosing suitable predictive correlations is essential to practical applications, an experimental investigation is conducted in a horizontal circular minichannel in this paper. With the experimentally acquired data, several well-known predictive correlations of HTC and FPD are compared, and the best ones are revealed
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