In-tube condensation of mixture of R134a and ester oil: empirical correlations

This paper presents a comparative study of the condensation heat transfer coefficients in a smooth tube when operating with pure refrigerant R134a and its mixture with lubricant Castrol “icematic sw”. The lubricant is synthetic polyol ester based oil commonly used in lubricating the compressors. Two concentrations of R134a-oil mixtures of 2% and 5% oil (by mass) were analysed for a range of saturation temperatures of refrigerant R134a between 35 °C and 45 °C. The mass flow rate of the refrigerant and the mixtures was carefully maintained at 1 g/s, with a vapour quality varying between 1.0 and 0. The effects of vapour quality, flow rate, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. The experimental results were then compared with predictions from earlier models [Int J Heat Mass Transfer (1979), 185; 6th Int Heat Transfer Congress 3 (1974) 309; Int J Refrig 18 (1995) 524; Trans ASME 120 (1998) 193]. Finally two new empirical models were developed to predict the two-phase condensation heat transfer coefficient for pure refrigerant R134a and a mixture of refrigerant R134a with Castrol “icematic sw”.     

Refrigerant charge, pressure drop, and condensation heat transfer in flattened tubes

Horizontal smooth and microfinned copper tubes with an approximate diameter of 9 mm were successively flattened in order to determine changes in flow field characteristics as a round tube is altered into a flattened tube profile. Refrigerants R134a and R410A were investigated over a mass flux range from 75 to 400 kg m⁻² s⁻¹ and a quality range from approximately 10–80%. For a given refrigerant mass flow rate, the results show that a significant reduction in refrigerant charge is possible. Pressure drop results show increases of pressure drop at a given mass flux and quality as a tube profile is flattened. Heat transfer results indicate enhancement of the condensation heat transfer coefficient as a tube is flattened. Flattened tubes with an 18° helix angle displayed the highest heat transfer coefficients. Smooth tubes and axial microfin tubes displayed similar levels of heat transfer enhancement. Heat transfer enhancement is dependent on the mass flux, quality and tube profile.     

Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes

This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 °C, qualities from 5 to 90%, mass velocity from 100 to 500 kg s⁻¹ m⁻², and a heat flux of 5 kW m⁻². Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (G=100 kg s⁻¹ m⁻²) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg s⁻¹ m⁻². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient.     

Microchannel heat exchangers for charge minimization in air-cooled ammonia condensers and chillers

This paper presents experimental results from a prototype ammonia chiller with an air-cooled condenser and a plate evaporator. The main objectives were charge reduction and compactness of the system. The charge is reduced to 20 g/kW (2.5 oz/Ton). This is lower than any currently available air-cooled ammonia chiller on the market. The major contribution comes from use of microchannel aluminum tubes. Two aluminum condensers were evaluated in the chiller: one with a parallel tube arrangement between headers and “microchannel” tubes (hydraulic diameter D_h = 0.7 mm), and the other with a single serpentine “macrochannel” tube (D_h = 4.06 mm). The performances of the chiller and condensers are compared based on various criteria to other available ammonia chillers. This prototype was made and examined in the Air Conditioning and Refrigeration Center in 1998, at the University of Illinois at Urbana-Champaign.     

Evaporating heat transfer of R22 and R410A in horizontal smooth and microfin tubes

An experimental investigation of condensation heat transfer in 9.52 mm O.D. horizontal copper tubes was conducted using R22 and R410A. The test rig had a straight, horizontal test section with an active length of 0.92 m and was cooled by the heat transfer fluid (cold water) circulated in a surrounding annulus. Constant heat flux of 11.0 kW/m² was maintained throughout the experiment and refrigerant quality varied from 0.9 to 0.1. The condensation test results at 45 °C were reported for 40–80 kg/h mass flow rate. The local and average condensation coefficients for seven microfin tubes were presented compared to those for a smooth tube. The average condensation coefficients of R22 and R410A for the microfin tubes were 1.7–3.19 and 1.7–2.94 times larger than those in smooth tube, respectively.     

Spray absorbers in absorption systems using lithium nitrate–ammonia solution

In the present work, the spray absorption method is studied for the absorption of ammonia refrigerant vapour by lithium nitrate–ammonia solutions. Mass transfer coefficients attainable using the spray absorption method are estimated. In this study the low-pressure absorber of a double-stage absorption refrigeration system is considered. Results show that the mass transferred is maximum (about 60% of the total) during the deceleration period of the drops. This period represents about 13.4% of the time required to reach the equilibrium state at the end of the absorption chamber. The results show that a time-average mass transfer coefficient equal to k_m=18.6×10⁻⁵ m s⁻¹ may be attained.     

Flow condensation heat transfer coefficients of pure refrigerants

Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m⁻² s⁻¹ and heat flux of 7.3–7.7 kW m⁻². Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.     

Heat transfer, pressure drop, and flow pattern recognition during condensation inside smooth, helical micro-fin, and herringbone tubes

This paper presents a study of flow regimes, pressure drops, and heat transfer coefficients during refrigerant condensation inside a smooth, an 18° helical micro-fin, and a herringbone tubes. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 400 to 800 kg m⁻² s⁻¹, and with vapour qualities ranging from 0.85 to 0.95 at condenser inlet and from 0.05 to 0.15 at condenser outlet. These test conditions represent annular and intermittent (slug and plug) flow conditions. Results showed that transition from annular flow to intermittent flow, on average for the three refrigerants, occurred at a vapour quality of 0.49 for the smooth tube, 0.29 for the helical micro-fin tube, and 0.26 for the herringbone tube. These transition vapour qualities were also reflected in the pressure gradients, with the herringbone tube having the highest pressure gradient. The pressure gradients encountered in the herringbone tube were about 79% higher than that of the smooth tube and about 27% higher than that of the helical micro-fin tube. A widely used pressure drop correlation for condensation in helical micro-fin tubes was modified for the case of the herringbone tube. The modified correlation predicted the data within a 1% error with an absolute deviation of 7%. Heat transfer enhancement factors for the herringbone tube against the smooth tube were on average 70% higher while against the helical micro-fin tube it was 40% higher. A correlation for predicting heat transfer coefficients inside a helical micro-fin tube was modified for the herringbone tube. On average the correlation predicted the data to within 4% with an average standard deviation of 8%.     

Flow condensation heat transfer coefficients of R22, R134a, R407C, and R410A inside plain and microfin tubes

Flow condensation heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A inside horizontal plain and microfin tubes of 9.52 mm outside diameter and 1 m length were measured at the condensation temperature of 40 °C with mass fluxes of 100, 200, and 300 kg m⁻² s⁻¹ and a heat flux of 7.7–7.9 kW m⁻². For a plain tube, HTCs of R134a and R410A were similar to those of R22 while HTCs of R407C are 11–15% lower than those of R22. For a microfin tube, HTCs of R134a were similar to those of R22 while HTCs of R407C and R410A were 23–53% and 10–21% lower than those of R22. For a plain tube, our correlation agreed well with the present data for all refrigerants exhibiting a mean deviation of 11.6%. Finally, HTCs of a microfin tube were 2–3 times higher than those of a plain tube and the heat transfer enhancement factor decreased as the mass flux increased for all refrigerants tested.     

A generalized correlation for the characteristics of adiabatic capillary tubes

The capillary tube is often served as an expansion device in small refrigeration and air-conditioning systems. In this paper, a generalized correlation for predicting the refrigerant mass flow rate through the adiabatic capillary tube is developed with approximate analytic solutions based on the extensive data for R12, R22, R134a, R290, R600a, R410A, R407C, and R404A, in which a homogeneous equilibrium model for two-phase flow is employed, and there is a subcooled liquid or saturated two-phase mixture at the inlet of the capillary tubes. The collected database about capillary tubes covers the inner diameter from 0.5 mm to 2 mm, the tube length from 0.5 m to 5 m, the condensing temperature from 20 °C to 60 °C, the subcooling from 0 °C to 20 °C, and the quality from 0 to 0.3 at the inlet. Assessments for the correlation are made with some experimental data for R12, R22, R134a, R290, R407C, R410A, and R404A obtained from the open literature and some existing correlations based on the experimental database also. The present correlation yields an average deviation of −0.83% and a standard deviation of 9.02% from the database.     

Two-phase split of refrigerants at a T-junction

The present study experimentally investigated the two-phase flow split of refrigerants at a T-junction. As geometric parameters, the direction of the inlet or branch tube and the tube diameter ratio of branch to inlet tube were chosen. As inlet flow parameters, the inlet mass flux and quality were varied from 100 to 700 kg m⁻² s⁻¹ and from 0.1 to 0.9, respectively, for the condition of distribution header of a multi-pass evaporator in the general refrigeration system. All experiments were performed for R-22, R-134a, and R-410A. The measured data were compared with the values predicted by the models developed for air–water or steam–water mixture in the literature. We propose a modified model for application to the reduced T-junction and vertical orientation of tubes. Among the geometric parameters, the branch tube direction showed the largest sensitivity to the mass flow rate ratio for the gas phase, while the inlet quality showed the largest sensitivity to the mass flow rate ratio among the inlet flow parameters.     

Forced convective boiling heat transfer of R-410A in horizontal minichannels

Convective boiling heat transfer experiments were performed in horizontal minichannels with binary mixture refrigerant, R-410A. The test section is made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and with lengths of 1500 mm and 3000 mm, respectively, and is uniformly heated by applying electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 10–30 kW m⁻², a mass flux range of 300–600 kg m⁻² s⁻¹, and quality ranges of up to 1.0. The experimental results were mapped on Wang et al.'s (C.C. Wang, C.S. Chiang, D.C. Lu, Visual observation of two-phase flow pattern of R-22, R-134a, and R-407C in a 6.5-mm smooth tube, Experimental, Thermal and Fluid Science 15 (1997) 395–405) and Wojtan et al.'s (L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: part I – a new diabatic two-phase flow pattern map, International Journal of Heat and Mass Transfer 48 (2005) 2955–2969) flow pattern maps to observe the flow regimes. Laminar flow appears in flow with minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for R-410A was developed with 11.20% mean deviation; it showed a good agreement between the measured data and the calculated heat transfer coefficients.     

Experimental studies on the evaporative heat transfer and pressure drop of CO2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm

Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. The tests were conducted at mass fluxes of from 212 to 656 kg m⁻² s⁻¹, saturation temperatures of from 0 to 20 °C and heat fluxes of from 6 to 20 kW m⁻². The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52 mm OD tube and 170–210% for 5 mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement.     

CO2 and R410A flow boiling heat transfer, pressure drop, and flow pattern at low temperatures in a horizontal smooth tube

Flow boiling heat transfer coefficient, pressure drop, and flow pattern are investigated in the horizontal smooth tube of 6.1 mm inner diameter for CO₂, R410A, and R22. Flow boiling heat transfer coefficients are measured at the constant wall temperature conditions, while pressure drop measurement and flow visualization are carried out at adiabatic conditions. This research is performed at evaporation temperatures of −15 and −30 °C, mass flux from 100 to 400 kg m⁻² s⁻¹, and heat flux from 5 to 15 kW m⁻² for vapor qualities ranging from 0.1 to 0.8. The measured R410A heat transfer coefficients are compared to other published data. The comparison of heat transfer coefficients for CO₂, R410A, and R22 is presented at various heat fluxes, mass fluxes, and evaporation temperatures. The difference of coefficients for each refrigerant is explained with the Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlation based on the thermophysical properties of refrigerants. The Wattelet et al. [J.P. Wattelet, J.C. Chato, B.R. Christoffersen, J.A. Gaibel, M. Ponchner, P.J. Kenny, R.L. Shimon, T.C. Villaneuva, N.L. Rhines, K.A. Sweeney, D.G. Allen, T.T. Heshberger, Heat Transfer Flow Regimes of Refrigerants in a Horizontal-tube Evaporator, ACRC TR-55, University of Illinois at Urbana-Champaign, 1994], and Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlations give the best agreement with the measured heat transfer coefficients for CO₂ and R410A. Pressure drop for CO₂, R410A, and R22 at various mass fluxes, evaporation temperatures and qualities is presented in this paper. The Müller-Steinhagen and Heck [H. Müller-Steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. 20 (1986) 297–308], and Friedel [L. Friedel, Improved friction pressure correlations for horizontal and vertical two-phase pipe flow, in: The European Two-Phase Flow Group Meeting, Ispra, Italy, 1979 (paper E2)] correlation can predict most of the measured pressure drop within the range of ±30%. The relation between pressure drop and properties for each refrigerant is described by applying the Müller-Steinhagen and Heck correlation. The observed two-phase flow patterns for CO₂ and R410A are presented and compared with flow pattern maps. Most of the flow patterns can be determined by the Weisman et al. [J. Weisman, D. Duncan, J. Gibson, T. Crawford, Effect of fluid properties and pipe diameter on two-phase flow patterns in horizontal lines, Int. J. Multiphase Flow 5 (1979) 437–462] flow pattern map.     

An experimental comparison of evaporation and condensation heat transfer coefficients for HFC-134a and CFC-12

Experimental heat transfer coefficients are reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation. These heat transfer coefficients were measured in a horizontal, smooth tube with an inner diameter of 8.0 mm and a length of 3.67 m. The refrigerant in the test-tube was heated or cooled by using water flowing through an annulus surrounding the tube. Evaporation tests were performed for a refrigerant temperature range of 5–15°C with inlet and exit qualities of 10 and 90%, respectively. For condensation tests, the refrigerant temperature ranged from 30 to 50°C, with et and exit qualities of 90 and 10%, respectively. The mass flux was varied from 125 to 400 kg m⁻² s⁻¹ for all tests. For similar mass fluxes, the evaporation and condensation heat transfer coefficients for HFC-134a were significantly higher than those of CFC-12. Specifically, HFC-134a showed a 35–45% increase over CFC-12 for evaporation and a 25–35% increase over CFC-12 for condensation.     

Heat transfer and flow pattern during two-phase flow boiling of R-134a in horizontal smooth and microfin tubes

Flow pattern and heat transfer during evaporation in a 10.7 mm diameter smooth tube and a micro-fin tube are presented. The tubes were tested in the ranges of mass flux between 163 and 408 kg m⁻² s⁻¹, and heat flux between 2200 and 56 000 W m⁻². The evaporation temperature was 6 °C. Flow maps for both the tubes are plotted in the coordinates of mass flux and vapor quality. The relations of flow pattern and local heat transfer coefficient are discussed. The heat transfer coefficients for intermittent and annular flows in both the smooth tube and the micro-fin tube are shown to agree well with Gungor and Winterton's correlation with modified constants.     

Experimental and theoretical study on flow condensation with non-azeotropic refrigerant mixtures of R32/R134a

Experiments on flow condensation have been conducted with both pure R32, R134a and their mixtures inside a tube (10 m long, 6 mm ID), with a mass flux of 131–369 kg m⁻²s⁻¹ and average condensation temperature of 23–40°C. The experimental heat transfer coefficients are compared with those predicted from correlations. The maximum mean heat transfer coefficient reduction (from a linear interpolation of the single component values) occurs at a concentration of roughly 30% R32 for the same mass flux basis, and is approximately 20% at Gr = 190 kg m⁻²s⁻¹, 16% at Gr = 300 kg m⁻²s⁻¹. Non-ideal properties of the mixture have a certain, but relatively small, influence on the degradation. Among others, temperature and concentration gradients, slip, etc. are also causes of heat transfer degradation.     

Prediction of two-phase pressure gradients of refrigerants in horizontal tubes

Two-phase pressure drop data were obtained for evaporation in two horizontal test sections of 10.92 and 12.00 mm diameter for five refrigerants (R-134a, R-123, R-402A, R-404A and R-502) over mass velocities from 100 to 500 kg/m² s and vapor qualities from 0.04 to 1.0. These data have then been compared against seven two-phase frictional pressure drop prediction methods. Overall, the method by Müller-Steinhagen and Heck (Müller-Steinhagen H, Heck K. A simple friction pressure drop correlation for two-phase flow in pipes. Chem. Eng. Process 1986;20:297–308) and that by Grönnerud (Grönnerud R. Investigation of liquid hold-up, flow-resistance and heat transfer in circulation type evaporators, part IV: two-phase flow resistance in boiling refrigerants. Annexe 1972-1, Bull. de l'Inst. du Froid, 1979) were found to provide the most accurate predictions while the widely quoted method of Friedel (Friedel L. Improved friction drop correlations for horizontal and vertical two-phase pipe flow. European Two-phase Flow Group Meeting, paper E2; June 1979; Ispra, Italy) gave the third best results. The data were also classified by two-phase flow pattern using the Kattan-Thome-Favrat (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes. Part 1: development of a diabatic two-phase flow pattern map. J. Heat Transfer 1998;120:140–7; Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes. Part 2; new heat transfer data for five refrigerants. J Heat Transfer 1998;120:148–55; Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes. Part 3: development of a new heat transfer model based on flow patterns. J. Heat Transfer 1998;120:156–65) flow pattern map. The best available method for annular flow was that of Müller-Steinhagen and Heck. For intermittent flow and stratified-wavy flow, the best method in both cases was that of Grönnerud. It was observed that the peak in the two-phase frictional pressure gradient at high vapor qualities coincided with the onset of dryout in the annular flow regime.     

Flow-boiling of R22, R134a, R507, R404A and R410A inside a smooth horizontal tube

Flow boiling heat transfer coefficients of R22, R134a, R507, R404A and R410A inside a smooth horizontal tube (6 mm I.D., 6 m length) were measured at a refrigerant mass flux of about 360 kg/m² s varying the evaporating pressure within the range 3–12 bar, with heat fluxes within the range 11–21 kW/m². The experimental data are discussed in terms of the heat transfer coefficients as a function of the vapour quality. The experimental results clearly show that the heat transfer coefficients of R134a are always higher than those pertaining to R22 (from a minimum of +6 to a maximum of +45%).     

Local and global heat transfer coefficients of a stabilised ice slurry in laminar and transitional flows

The thermal behaviour of a new two-phase secondary refrigerant has been analysed. The “stabilised ice slurry” is a suspension in a low viscosity oil of ultraporous polymeric particles filled with water. In order to determine the convective heat transfer coefficient of this secondary refrigerant with water–ice phase change, an experimental set-up was built. It allows determining the local heat transfer coefficients inside two heat exchangers, having rectangular sections (80 × 8 mm²) of 1 m length, by mean of fluxmeters located along the working section. The slurry is first cooled and frozen in one of the exchangers, then heated and melted in the other exchanger. The results obtained for laminar or transitional flows shows that the heat transfer coefficients of the ice slurry are obviously higher than the heat transfer coefficients obtained with the single-phase fluid (oil). Correlations giving the local and global Nusselt numbers, depending on the Graetz or Reynolds numbers and on the particle mass fraction, have been established.