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International Heat Transfer Conference 16

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

A NEW CORRELATION FOR PREDICTING THE HEAT TRANSFER COEFFICIENTS OF R407C UNDERGOING FLOW BOILING WITHIN HORIZONTAL MICROFIN TUBES

Sunil S. Mehendale
Michigan Technological University, Houghton, Michigan 49931, USA

DOI: 10.1615/IHTC16.bae.022945
pages 889-905


KEY WORDS: Boiling and evaporation, Heat transfer enhancement, microfin, refrigerant, mixture, heat transfer coefficient, correlation

Abstract

A new correlation is presented to predict the heat transfer coefficients (HTCs) of the non-azeotropic refrigerant mixture (NARM) R407C undergoing flow boiling within horizontal microfin tubes. This is accomplished by first putting together a 735−point experimental database from 6 sources. The data covers 6.5−12 mm fin root diameter tubes, -20°C to 15.6°C saturation temperatures, vapor qualities from 0.05 to 1, reduced pressures from 0.05 to 0.17, and heat and mass fluxes ranging from 0.98 to 30 kW/m2 and 25 to 422.3 kg/s.m2 respectively. The correlation was developed in two steps. Forty-one unique dimensionless parameters pertinent to the flow boiling of NARMs in microfin tubes were first selected. Some of these variables were specifically chosen to account for the fact that R407C displays a significant temperature glide as it boils at constant pressure. Multi-variable regression analysis was then applied to identify the most significant variables influencing the flow boiling Nusselt number. The new correlation was evaluated and compared with four extant correlations on an overall basis as well as by sources of data. Overall evaluation for the entire database shows that the new correlation is significantly better than any of the extant correlations. For this overall assessment, the new correlation predicts 91.4% of the data within ±30% error bands, with a mean absolute deviation of 13.3%. Apart from these overall assessments, the distribution of the entire 735−point database and the MAD of the new correlation was examined relative to the source from which data was collected. The new NARM correlation shows reasonably good predictions which are better than those of the four existing correlations for each of the six data sources, with MAD values in the 10.1% to 24.8% range. Based on these comparative assessments, the new correlation can be used as a reliable tool to predict the microfin flow boiling HTCs of R407C under different operating conditions of practical interest.

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