Boiling and Condensation of Alternative Refrigerants in a Horizontal Smooth Tube (original) (raw)
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Applied Thermal Engineering, 2004
This paper presents experimental heat transfer coefficients and pressure drop results obtained during the evaporation of pure R22 and the azeotropic mixture R507 (R125-R143a 50%/50% in weight). The test section was a smooth, horizontal, stainless steel tube (6 mm ID, 6 m length) uniformly heated by Joule effect. The effects of heat flux, mass flux and evaporation pressure on the heat transfer coefficients have been investigated. Each working parameter was bound within the range: evaporating pressure 3-12 bar, refrigerant mass-flux 250-286 kg/m 2 s, heat flux 10.6-17.0 kW/m 2 , respectively. Additionally the experimental results have been compared with existing correlations which characterize the evaporative heat transfer coefficient to assess the validity of these models for refrigerant mixtures.
Heat Transfer Engineering, 2008
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International Journal of Heat and Mass Transfer, 2012
HFO1234yf has been proposed for mobile air-conditioners due to its low global warming potential (GWP) and performance comparable to that of R134a. However, its performance is inferior to that of R410A. This makes it difficult to be applied to residential air-conditioners. In order to apply the low-GWP refrigerant to residential air-conditioners, refrigerant mixtures of HFO1234yf and R32 are proposed, and their flow boiling heat transfer performances were investigated at two mass fractions (80/20 and 50/50 by mass%) in a smooth horizontal tube with an inner diameter of 2 mm. The experiments were conducted under heat fluxes ranging from 6 to 24 kW/m 2 and mass fluxes ranging from 100 to 400 kg/m 2 s at the evaporation temperature of 15 °C. The measured heat transfer coefficients were compared with those of pure HFO1234yf and R32. The results showed that the heat transfer coefficients of the mixture with an R32 mass fraction of 20% were 10-30% less than those of pure HFO1234yf for various mass and heat fluxes. When the mass fraction of R32 increased to 50%, the heat transfer coefficients of the mixture were 10-20% greater than those of pure HFO1234yf under conditions of large mass and heat fluxes. Moreover, the heat transfer coefficients of the mixtures were about 20-50% less than that of pure R32. The performances of the mixtures were examined at different boiling numbers. For refrigerant mixture HFO1234yf and R32 (80/20 by mass%), the nucleate boiling heat transfer was noticeably suppressed at low vapor quality for small boiling numbers, whereas the forced convective heat transfer was significantly suppressed at high vapor quality for large boiling numbers. This indicates that the heat transfer is greatly influenced by the mass diffusion resistance and temperature glide of the mixture.
Refrigerant forced-convection condensation inside horizontal tubes
1969
Condensing heat transfer rates inside a horizontal tube were investigated-for large quality changes across the tube. The proposed correlation is a modification of the work of Rohsenow, Webber and Ling [29]. The result of the investigation is modified through new variables which include the effect of the true axial pressure gradient in a tube. Experimental data are presented for a range of flow conditions. A 0.493 in. ID, 19.75 ft. long nickel tube was used for condensing Refrigerant-12. The saturation temperature was varied from 84.6*F to 118*F and flow rates of vapor-liquid mixture ranged from 151,000 lbm/ft 2hr to 555,000 lbm/ft 2hr. The inlet quality was essentially 100% at saturation and exit qualities ranged from 50% to zero and subcooled liquid. The test results for average heat transfer coefficient ware correlated by the analysis within 15%. NOMENCLATURE A Cross section area ft 2 c Specific heat Btu/lbm *F D Tube inner diameter ft D. Tube outer diameter ft f Friction factor F. Pressure Gradient in the Tube lbf/ft 2/ft g Gravity ft/sec 2 2 G Mass velocity of the liquid lbm/hr ft G vMass velocity of the vapor ibm/hr ft 2 h fg Latent heat of the evaporation Btu/lbm h Local heat transfer coefficient Btu/hr ft 2 *F z k Conductivity of the liquid Btu/hr ft *F L Length of the cooling water jacket ft Nu Nusselt Number Pr Prandtl Number (q/A) Heat flux Btu/ft 2hr Re Reynolds Number T Inner wall temperature *F T Outer wall temperature *F AT Temperature difference between vapor and condensing wall AT Cooling water temperature rise *F Vz Velocity of the condensate flow ft/sec W Flow rate of the fluid lbm/hr W Flow rate of the cooling water lbm/hr w z Distance from condensation starting point ft 0075 62.3 73 62.32 a [ I p MNIMIIIMIIIIMMMINII M ON110111101111111i
International Journal of Heat and Mass Transfer, 2011
An experimental investigation on two-phase flow boiling heat transfer with refrigerants of R-22, R-134a, R-410A, C 3 H 8 and CO 2 in horizontal circular small tubes is presented. The experimental data were obtained over a heat flux range of 5-40 kW m À2 , mass flux range of 50-600 kg m À2 s À1 , saturation temperature range of 0-15°C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5, 1.5 and 3.0 mm, and lengths of 330, 1000, 1500, 2000 and 3000 mm. The experimental data were mapped on [5] and [6] flow pattern maps. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the heat transfer coefficient are reported. The experimental heat transfer coefficients were compared with some existing correlations. A new boiling heat transfer coefficient correlation that is based on a superposition model for refrigerants in small tubes is presented with 15.28% mean deviation and À0.48% average deviation.
Condensing heat transfer characteristics of hydrocarbon refrigerants in 9.52 and 12.7 mm smooth tube
Heat and Mass Transfer, 2005
Experimental results for heat transfer characteristic and pressure gradients of hydrocarbon (HC) refrigerants and R-22 during condensing inside horizontal double pipe heat exchangers are presented. The test sections which have one tube diameter of 12.70 mm with 0.86 mm wall thickness, another tube diameter of 9.52 mm with 0.76 mm wall thickness are used for this investigation. The local condensing heat transfer coefficients of HC refrigerants were higher than that of R-22. The average condensing heat transfer coefficient increased with the increase of the mass flux. It showed the higher values in HC refrigerants than R-22. Comparing the heat transfer coefficient of experimental results with that of other correlations, the presented results had a good agreement with most of the Cavallini’s correlations.
Convective boiling pressure drop of refrigerant R-134a in horizontal smooth and microfin tubes
International Journal of Refrigeration, 2004
Present study deals with the pressure drop of refrigerant R-134a under convective boiling conditions in horizontal smooth and microfinned ('grooved') copper tubes. Experiments have been carried out in an experimental set up developed for change of phase studies with a test section made out of 7.0, 7.93, and 9.52 mm external diameter, 1.5 m long copper tubes, electrically heated by tape resistors wrapped on the external surface. Mass velocities and refrigerant qualities varied in the following ranges: 70 -1100 kg s 21 m 22 and 5 -95%. The annular flow pattern has been observed to occur over most of the operational conditions. For smooth tubes, the Jung and Radermacher correlation for the liquid two phase flow multiplier fits with reasonable precision the experimental data. As for grooved tubes, a correlation of the two phase flow multiplier in terms of the Martinelli's parameter has been developed which fits the data with an average absolute deviation of the order of 6.3%. The proposed correlation fits with good precision data obtained elsewhere for grooved tubes of different diameter and microfin geometry. q
Horizontal Convective Boiling of Pure and Mixed Refrigerants within a Micro-Fin Tube
Journal of Enhanced Heat Transfer, 2008
This paper presents local convective boiling measurements in a micro-fin tube ** for four pure refrigerants: R22, R32, R125, and R134a; and four refrigerant mixtures: R410B (R32/125, 45/55% mass), R32/R134a (27/73% and 30/70% mass), and R407C (R32/125/134a, 25/23/52% mass). All testing was conducted using a counterflow water-heated horizontal 9.5 mm (D o) U-tube with helical micro-fins. Saturation temperatures ranged from 274.5 K to 293.6 K. Flow boiling heat transfer coefficients for the mixtures' pure components and R22 were measured to establish a baseline for the heat transfer degradation calculations. The measured convective boiling Nusselt numbers for all of the test refrigerants were correlated to a single expression consisting of a product of dimensionless properties valid for mass velocities ranging from 70 kg/m 2 ⋅s to 370 kg/m 2 ⋅s and for vapor qualities between 0 and 0.7. These measurements were within ±20% of the new correlation predictions for 94.6% and 87.3% of the pure refrigerant and mixed refrigerant measurements, respectively. The correlation was shown to predict some existing data from the literature to within 20%. The degradation in heat transfer performance of the mixtures was found to range from 1% to 50% for all refrigerants tested.
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.