Experimental Study on Condensation of Pure Refrigerants in Horizontal Micro-Fin Tube – Proposal of Correlations for Heat Transfer Coefficient and Frictional Pressure Drop– (original) (raw)
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Horizontal Convective Condensation of Alternative Refrigerants Within a Micro-Fin Tube
Journal of Enhanced Heat Transfer, 1999
This paper presents local convective condensation measurements for four refrigerants: R134a, R410A(R32/R125, 50/50 % mass), R125, and R32 in a micro-fm tube^Both heat transfer and pressure drop measurements are provided. The heat transfer degradation associated with R410A was shown to be relatively small and believed to be mostly due to nonlinear property effects. The measured convective condensation Nusselt numbers for all of the test refrigerants were correlated to a single expression consisting of a product of dimensionless properties. The correlation was shown to predict some existing data from the literature within acceptable limits. The correlation poorly predicted the heat transfer performance of cross-grooved, micro-fm tubes. The pressure drop measurements for the micro-fm mbe were predicted satisfactorily by an existing correlation for flow boiling pressure drop in a smooth mbe. Correlation of the pressure drop measurements suggested that the heat transfer enhancement was due to the fins behaving as a surface roughness.
A generalized correlation for evaporation heat transfer of refrigerants in micro-fin tubes
A generalized correlation for flow boiling heat transfer in horizontal micro-fin tubes was developed by implementing non-dimensional parameters accounting for heat transfer enhancement over smooth tubes and physical phenomena into the basic form of a smooth tube correlation. The enhancement factor in nucleate boiling consists of surface tension and turbulence effects generated by the liquid flow. A modified Reynolds number having some similarities with the roughness Reynolds number is introduced into the model to estimate heat transfer enhancement in convective boiling. The ratio of the liquid film thickness to the fin height is also employed in the correlation. The database of the present correlation includes 749 data points for five different refrigerants. The present correlation has a relatively simpler form to employ and yields closer fit to the experimental data with a mean deviation of 20.5% as compared to the existing correlations in the literature. Ó
In the present study the local condensation heat transfer characteristics in a horizontal microfin tube are found to be about 2 times higher than those of a smooth tube with the same inner diameter. This enhancement effect on heat transfer coefficient seems mainly caused by the enlargement ratio of heat transfer area. From this point of view, a correlation, which is modified from the correlation of Haraguchi et al. for smooth tubes, is developed for the condensation heat transfer in microfm tubes with pure refrigerants. With this correlation, the condensation heat transfer characteristics in some kinds of microfin tubes can be predicted very well.
1999
This paper presents a pressure drop correlation for evaporation and condensation in smooth and micro-fin tubes for lubricant-free refrigerants and refi-igerantllubricant mixtures. The form of the generalized correlation was taken from the Pierre pressure drop model. NIST micro-fin tube pressure drop data for R134a, R22, R125, R32, R407C, R41OA, and R32R134a (25175 % mass) were regressed to a modified Pierre correlation. The NIST database was post-predicted with an average absolute residual of 10.8 %. Further validations performed with extensive data from the literature for lubricant-free refhgerants in smooth and micro-fin tubes showed an average absolute residual between measurements and predictions not to exceed 17.6 % for the various data sets. The condensation and evaporation pressure drops for different refrigerantllubricant mixtures were predicted with average absolute residuals not exceeding 19.6 % and 28.0 %, respectively.
A flow regime map for refrigerant condensation in herringbone micro-fin tubes (Part II).
Experiments were conducted with refrigerants R-22, R-134a and R-407C. Their thermodynamic properties were obtained from a reference database. 19 In these experiments, mass fluxes varied between 300 and 800 kg m -2 s -1 ; pressures between 1500 kPa and 1800 kPa; saturation temperatures between 39°C and 41°C; inlet vapour qualities between 0.85 and 0.98; and outlet vapour qualities between 0.02 and 0.15. Data were captured only once energy balance errors were below 1%. Uncertainties in the experimental work, which were calculated by Coetzee 20 and re-evaluated on the basis of present experimental results, 13 are presented in .
Evaporation Heat Transfer and Pressure Drop in Micro-Fin Tubes Before and After Tube Expansion
Journal of Enhanced Heat Transfer, 2005
The ozone-depleting effect of chlorofluorocarbons (CFC's) has caused an intensive search for alternative fluids to be used in vapor-compression refrigeration and air-conditioning machinery. A promising replacement for CFC-12 has been identified, hydro-fluorocarbon (HFC) 134a. This new refrigerant is expected to have different heat transfer and pressure drop characteristics than CFC-12 when changing phase in the evaporator component. The presence of oil for lubrication purposes may also have an effect. Heat transfer coefficients and pressure drops of HFC-134a and CFC-12 boiling in horizontal tubes were measured. Polyalkylene Glycol (PAG) and Ester oils were mixed with HFC-134a to determine effects of various concentrations of oil. Tests were run in the mobile airconditioning range of temperature (5°C), mass fluxes (100 to 500 kg/m 2 s), and heat fluxes (5 to 30 kW/m 2 ). Various inlet qualities to the test section were studied, from 20 to 60 percent. Comparison of data with previous correlations from the literature is made, and a new correlation for pure refrigerant heat transfer is proposed.
Heat Transfer Performance During Condensation Inside Spiralled Micro-Fin Tubes
Journal of Heat Transfer-transactions of The Asme, 2004
An experimental investigation was conducted into the heat transfer characteristics during in-tube condensation of horizontal smooth, micro-fin, and herringbone tubes. The study focused on the heat transfer coefficients of refrigerants R-22, R-134a, and R-407C inside a series of typical horizontal smooth, micro-fin, and herringbone tubes at a representative average saturation temperature of 40°C. Mass fluxes ranged from 300 to 800 kg/ m 2 s, and vapor qualities ranged from 0.85 to 0.95 at condenser inlet, to 0.05 to 0.15 at condenser outlet. The herringbone tube results were compared with the smooth and micro-fin tube results. The average increase in the heat transfer coefficient of the herringbone tube, when compared with the smooth tube at comparable conditions, was found to be 322%, with maximum values reaching 336%. When compared with the micro-fin tube, the average increase in heat transfer coefficient was found to be 196%, with maximum values reaching 215%. Moreover, a new correlation was developed to predict the heat transfer coefficients in a herringbone and micro-fin tube. Semi-local heat transfer coefficients were calculated from the modified Wilson plot technique, using measurements of condenser subsection inlets and outlets, and from knowledge of the temperature gradient on the annulus side. The correlation predicted the semi-local heat transfer coefficients accurately, with 96% and 89% of the data points falling in the ±20% region for the herringbone tube and the micro-fin tube, respectively. The average heat transfer coefficients were accurately predicted, too, with all the data points for the herringbone tube and 83% of the data points for the micro-fin tube falling in the ±20% region. The derived heat transfer correlations can be used for design, especially for reversible heat pumps. This research proves that predicting the flow pattern during intermittent and annular flow is not a prerequisite for predicting the heat transfer accurately to within 20% of the measurements.