Abstract
This paper presents the R245fa flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter at the fin tip of 4.2 mm, having 40 fins, 0.15 mm high with a helix angle of 18°. The tube was brazed inside a copper plate and electrically heated from the bottom. Sixteen T-type thermocouples are located in the copper plate to monitor the wall temperature. The experimental measurements were carried out at constant mean saturation temperature of 30 °C, by varying the refrigerant mass velocity between 100 kg m-2 s-1 and 300 kg m-2 s-1, the vapour quality from 0.15 to 0.95, at two different heat fluxes: 30 and 60 kW m-2. The experimental results are presented in terms of two-phase heat transfer coefficient, onset dryout vapour quality, and frictional pressure drop. Moreover, the experimental measurements are compared against the most updated models for boiling heat transfer coefficient and frictional pressure drop estimations available in the open literature for microfin tubes.
Highlights
Since the invention of Fujie et al (1977), microfin tubes have received a lot of attention because they can assure higher heat transfer coefficients compared to smooth tubes, with a relatively small increase of pressure drop
The results are given in terms of heat transfer coefficient and frictional pressure gradients, as a function of the operative test conditions, i.e. mean vapor quality, mass velocity, and heat flux
The paper presents experimental heat transfer coefficients and pressure drops measured during flow boiling inside a mini microfin tube with an inner diameter at the fin tip of 4.3 mm
Summary
Since the invention of Fujie et al (1977), microfin tubes have received a lot of attention because they can assure higher heat transfer coefficients compared to smooth tubes, with a relatively small increase of pressure drop. Experimental results regarding heat transfer coefficient, pressure drop, and vapor quality at the onset of the dryout are available, as well as flow pattern maps and empirical equations for the estimation of those parameters, which can be used to design evaporators and condensers commonly used in traditional air conditioning and refrigeration systems. The use of these mini microfin tubes may imply a large reduction of the refrigerant charge of the system, facing with the new stricter environmental regulations. For these reasons, large manufacturers are exploring the possible use of mini microfin tubes and there is a strong interest in understanding the heat transfer and pressure drop behaviors of these enhanced tubes. Among the most recent works, Kondou et al (2013) investigated R32, R1234ze(E), and two R32/R1234ze(E) mixtures (20:80 and 50:50 by wt%) flow boiling inside a water heated microfin tube (inner diameter at the fin tip of 4.94 mm) at a saturation temperature of 10 °C, heat fluxes of 10 and 15 kW m-2, and mass velocities from 150 to 400 kg m-2 s-1
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