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%.     

State-of-the-art of two-phase flow and flow boiling heat transfer and pressure drop of CO2 in macro- and micro-channels

A comprehensive review of flow boiling heat transfer and two-phase flow of CO₂ is presented that covers both macro-channel tests (diameters greater than about 3 mm) and micro-channel investigations (diameters less than about 3 mm). The review addresses flow boiling heat transfer experimental studies, macro- and micro-scale heat transfer prediction methods for CO₂ and comparisons of these methods to the experimental database, highlighting the various limitations of current approaches and the divergence of some data sets from others. In addition, two-phase flow pattern results available in the literature are summarized and compared to some of the leading flow pattern maps, showing significant deviations for CO₂ from the maps prepared for other fluids at lower pressures. Available two-phase pressure drop data for CO₂ are also compared to leading prediction methods.     

Two-fluid model of refrigerant two-phase flow through short tube orifice

Pronounced hydrodynamic and thermodynamic non-equilibrium exist in the flow of refrigerant through a short tube orifice under typical operating conditions. A non-equilibrium two-fluid model (TFM) for refrigerant two-phase critical flow inside the short tube orifice is developed. Both inter-phase velocity slip and inter-phase temperature difference are taken into account in the model. The mass flow rate, the two-phase velocity and temperature distributions in a short tube orifice are simulated. Comparisons among the experimental data of refrigerants R134a, R12, R22, R410A and R407C flowing through short tubes, the predictions by the TFM and by the homogeneous equilibrium model (HEM) show that the TFM gives acceptable predictions with the deviations of ±20%, while the HEM underestimates the flow rate by 20% or so.     

A general steady state mathematical model for fin-and-tube heat exchanger based on graph theory

Fin-and-tube heat exchangers are widely used in air conditioners, chillers, etc. A lot of factors, including arrangement of refrigerant circuits, configure specification of fins and tubes, and operating conditions, have significant influence on the performance of fin-and-tube heat exchangers. For the purpose of fast design of high performance heat exchangers, a simulator reflecting the influence of these factors is necessary. In this paper, a general steady state mathematic model based on the graph theory is presented. With the help of the directed graph and graph-based traversal methods (Breadth-first search and Depth-first search), this model is capable to describe any flexible refrigerant circuit arrangement, and quantify the refrigerant distribution in the refrigerant circuit and heat conduction through fins. An alternative iteration method is also developed to solve the conservation equations, which can shorten the simulating time effectively. The model is verified with the experimental results, and the maximum error is within ±10.0%. A simulator based on this model has been used for designing practical fin-and-tube heat exchangers.     

Boiling heat transfer of R-22, R-134a, and CO2 in horizontal smooth minichannels

This study examined convective boiling heat transfer in horizontal minichannels using R-22, R-134a, and CO₂. The local heat transfer coefficients were obtained for heat fluxes ranging from 10 to 40 kW m⁻², mass fluxes ranging from 200 to 600 kg m⁻² s⁻¹, a saturation temperature of 10 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm. The section was heated uniformly by applying an electric current to the tubes directly. Nucleate boiling heat transfer was the main contribution, particularly at the low quality region. An increasing and decreasing heat transfer coefficient occurred at the lower vapor quality with increasing heat flux and mass flux. The mean heat transfer coefficient ratio of R-22:R-134a:CO₂ was approximately 1.0:0.8:2.0. Laminar flow was observed in the minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in minichannels was developed with a mean deviation of 11.21%.     

Evaporation of R407C and R410A in a horizontal herringbone microfin tube: heat transfer and pressure drop

Based on experimental data for R134a, the present work deals with the development of a prediction method for heat transfer in herringbone microfin tubes. As is shown in earlier works, heat transfer coefficients for the investigated herringbone microfin tube tend to peak at lower vapour qualities than in helical microfin tubes. Correlations developed for other tube types fail to describe this behaviour. A hypothesis that the position of the peak is related to the point where the average film thickness becomes smaller than the fin height is tested and found to be consistent with observed behaviour. The proposed method accounts for this hypothesis and incorporates the well-known Steiner and Taborek correlation for the calculation of flow boiling heat transfer coefficients. The correlation is modified by introducing a surface enhancement factor and adjusting the two-phase multiplier. Experimental data for R134a are predicted with an average residual of 1.5% and a standard deviation of 21%. Tested against experimental data for mixtures R410A and R407C, the proposed method overpredicts experimental data by around 60%. An alternative adjustment of the two-phase multiplier, in order to better predict mixture data, is discussed.     

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%).     

Refrigeration system performance using liquid-suction heat exchangersPerformance d''un système frigorifique utilisant des échangeurs liquide-vapeur à l''aspiration

Heat transfer devices are provided in many refrigeration systems to exchange energy between the cool gaseous refrigerant leaving the evaporator and warm liquid refrigerant exiting the condenser. These liquid-suction or suction-line heat exchangers can, in some cases, yield improved system performance while in other cases they degrade system performance. Although previous researchers have investigated performance of liquid-suction heat exchangers, this study can be distinguished from the previous studies in three ways. First, this paper identifies a new dimensionless group to correlate performance impacts attributable to liquid-suction heat exchangers. Second, the paper extends previous analyses to include new refrigerants. Third, the analysis includes the impact of pressure drops through the liquid-suction heat exchanger on system performance. It is shown that reliance on simplified analysis techniques can lead to inaccurate conclusions regarding the impact of liquid-suction heat exchangers on refrigeration system performance. From detailed analyses, it can be concluded that liquid-suction heat exchangers that have a minimal pressure loss on the low pressure side are useful for systems using R507A, R134a, R12, R404A, R290, R407C, R600, and R410A. The liquid-suction heat exchanger is detrimental to system performance in systems using R22, R32, and R717.     

Flow pattern and heat transfer characteristics during flow boiling of CO2 in a horizontal micro fin tube and comparison with smooth tube data

Flow pattern observations and measurements of the heat transfer in a helical grooved micro fin tube are presented and compared with results for a smooth tube. The micro fin tube used (OD of 9.52 mm) was a Wieland Cuprofin EDX tube with 60 fins (height 0.25 mm) and a helix angle of 18°. The flow pattern observations at 39.7 bar (T_s=+5 °C, p_r=0.54) and 26.4 bar (T_s=−10 °C, p_r=0.36) show a wide range of the annular flow region. The transition from slug to annular flow does not occur, as expected, at constant vapour quality for all mass fluxes but there is an interdependence between transition vapour quality and mass flux. For the heat transfer in the micro fin tube, measurements at 39.7 bar are presented for heat fluxes up to 120 kW m⁻², mass fluxes between 75 and 250 kg m⁻² s⁻¹ and vapour qualities between 0.1 and 0.9.     

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.     

Pseudo-Pure Fluid Equations of State for the Refrigerant Blends R-410A, R-404A, R-507A, and R-407C

Pseudo-pure fluid equations of state explicit in Helmholtz energy have been developed to permit rapid calculation of the thermodynamic properties of the refrigerant blends R-410A, R-404A, R-507A, and R-407C. The equations were fitted to values calculated from a mixture model developed in previous work for mixtures of R-32, R-125, R-134a, and R-143a. The equations may be used to calculate the single-phase thermodynamic properties of the blends; dew and bubble point properties are calculated with the aid of additional ancillary equations for the saturation pressures. Differences between calculations from the pseudo-pure fluid equations and the full mixture model are on average 0.01%, with all calculations less than 0.1% in density except in the critical region. For the heat capacity and speed of sound, differences are on average 0.1% with maximum differences of 0.5%. Generally, these differences are consistent with the accuracy of available experimental data for the mixtures, and comparisons are given to selected experimental values to verify accuracy estimates. The equations are valid from 200 to 450 K and can be extrapolated to higher temperatures. Computations from the new equations are up to 100 times faster for phase equilibria at a given temperature and 5 times faster for single-phase state points given input conditions of temperature and pressure.     

Two-phase pressure drop of R-410A in horizontal smooth minichannels

Convective boiling pressure drop experiments were performed in horizontal minichannels with a binary mixture refrigerant, R-410A. The test section was 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. This test section was uniformly heated by applying electric current directly to the tubes. Experiments were performed at inlet saturation temperature of 10 °C, mass flux ranges from 300 to 600 kg m⁻² s⁻¹ and heat flux ranges from 10 to 40 kW m⁻². The current study showed the significant effect of mass flux and tube diameter on pressure drop. The experimental results were compared against 15 two-phase pressure drop prediction methods. The homogeneous model predicted well the experimental pressure drop, generally. A new pressure drop prediction method based on the Lockhart–Martinelli method was developed with 4.02% mean deviation.     

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.     

Comparison of parallel- and counter-flow circuiting in an industrial evaporator under frosting conditions

This paper describes a theoretical model of a large-scale, ammonia-fed evaporator coil used in an industrial refrigeration system and operating under low temperature air and refrigerant conditions that are typically encountered in refrigerated storage spaces. The model is used to simulate the performance of counter-flow and parallel-flow circuited evaporator coil designs under frosting conditions. The counter-flow frost model is validated using in situ data obtained from a field-installed evaporator coil. The performance of an evaporator in a parallel-flow circuit arrangement is simulated and compared to counter-flow circuiting. The effects of coil circuiting are evaluated in terms of the frost distribution across the evaporator coil and the associated reduction in cooling capacity during operation.     

Numerical analysis of evaporation performance in a finned-tube heat exchanger

This study discusses the effects of the heat exchanger type, refrigerant, inner tube configuration, and fin geometry on evaporator performance by adopting updated correlations of EVSIM, a numerical analysis model based on the tube-by-tube method developed by Domanski. The heat exchanger types considered are the cross-counter flow type and cross-parallel flow type. The refrigerants considered for the numerical test as a working fluid are R-134a, R-410A and R-22. For inner tube configuration, enhanced tube and smooth tube cases are considered. For the air side evaporation performance, heat exchangers using plate fins, wavy fins and slit fins are analyzed. Results show that the heat transfer rate of the cross-counter flow type heat exchanger is 3% higher than that of the cross-parallel flow type with R-22. The total heat transfer rate of the evaporator using R-410A is higher than those using R-22 and R-134a, while the total pressure drop of R-410A is lower than those of R-22 and R-134a. The heat transfer rate of the evaporator using enhanced tubes is two times higher than that using smooth tubes, but the pressure drop of the enhanced tube is 45–50% higher than that of the smooth tubes. The evaporation performance of slit fins is superior to that of plate fins by 54%.     

Condensation of refrigerants in horizontal microfin tubes: comparison of correlations for frictional pressure drop

This paper presents a comprehensive comparison of eight previously proposed correlations with available experimental data for the frictional pressure drop during condensation of refrigerants in helically grooved, horizontal microfin tubes. Calculated values are compared with experimental data for seven refrigerants (R11, R123, R134a, R22, R32, R125 and R410A) and eight tubes and with mass velocity from 78 to 459 kg/m² s. The tubes had inside diameter at the fin root between 6.41 and 8.91 mm; the fin height varied between 0.15 and 0.24 mm; the fin pitch varied between 0.34 and 0.53 mm and helix angle between 13 and 20°. The results show that the overall r.m.s. deviations of relative residuals of frictional pressure gradient for all tubes and all refrigerants taking together decreased in the order of the correlations of Nozu et al. [Exp. Therm. Fluid Sci. 18 (1998) 82], Newell and Shah [Refrigerant heat transfer, pressure drop, and void fraction effects in microfin tubes. In: Proc. 2nd Int. Symp. on Two-Phase Flow and Experimentation, vol. 3. Italy: Edizioni ETS; 1999. p. 1623–39], Kedzierski and Goncalves [J. Enhanced Heat Transfer 6 (1999) 161], Cavallini et al. [Heat Technol. 15 (1997) 3], Goto et al. (b) [Int. J. Refrigeration 24 (2001) 628], Choi et al. [Generalized pressure drop correlation for evaporation and condensation in smooth and microfin tubes. In: Proc. of IIF-IIR Commision B1, Paderborn, Germany, B4, 2001. p. 9–16], Haraguchi et al. [Condensation heat transfer of refrigerants HCFC134a, HCFC123 and HCFC22 in a horizontal smooth tube and a horizontal microfin tube. In: Proc. 30th National Symp. of Japan, Yokohama, 1993. p. 343–5], and Goto et al. (a) [Int. J. Refrigeration 24 (2001) 628], i.e., this final correlation (Goto et al. (a)) gives the best overall representation of the data.     

Condensation heat transfer of a pure fluid and binary mixture outside a bundle of smooth horizontal tubes. Comparison of experimental results and a classical model

The condensation of pure HFC134a and different zeotropic mixtures with pure HFC134a and HFC23 on the outside of a bundle of smooth tubes was studied. The local heat transfer coefficient for each row was experimentally determined using a test section composed by a 13×3 staggered bundle of smooth copper tubes, measuring cooling water temperature in the inlet and the outlet of each tube, and measuring the vapour temperature along the bundle. All data were taken at the inlet vapour temperature of 40°C with a wall subcooling ranging from 4 to 26 K. The heat flux was varied from 5 to 30 kW/m² and the cooling water flow rate from 120 to 300 l/h for each tube. The visualisation of the HFC134a condensate flow by means of transparent glass tubes reveals specific flow patterns and explains the difference between the measured values of the heat transfer coefficient and the calculated values from Nusselt's theory. On the other hand, the experimental heat transfer data with the binary mixtures HFC23-HFC134a show the important effects of temperature glide and the strong decrease of the heat transfer coefficient in comparison with the pure HFC134a data. The measured values with the different zeotropic mixtures were compared with the data calculated with the classical condensation model based on the equilibrium model. An improvement of this model is proposed.     

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.     

Condensation of refrigerants in horizontal microfin tubes: comparison of prediction methods for heat transfer

A comparison was made between the predictions of previously proposed empirical correlations and theoretical model and available experimental data for the heat transfer coefficient during condensation of refrigerants in horizontal microfin tubes. The refrigerants tested were R11, R123, R134a, R22 and R410A. Experimental data for six tubes with the tube inside diameter at fin root of 6.49–8.88 mm, the fin height of 0.16–0.24 mm, fin pitch of 0.34–0.53 mm and helix angle of groove of 12–20° were adopted. The r.m.s. error of the predictions for all tubes and all refrigerants decreased in the order of the correlations proposed by Luu and Bergles [ASHRAE Trans. 86 (1980) 293], Cavallini et al. [Cavallini A, Doretti L, Klammsteiner N, Longo L G, Rossetto L. Condensation of new refrigerants inside smooth and enhanced tubes. In: Proc. 19th Int. Cong. Refrigeration, vol. IV, Hague, The Netherlands, 1995. p. 105–14], Shikazono et al. [Trans. Jap. Sco. Mech. Engrs. 64 (1995) 196], Kedzierski and Goncalves [J. Enhanced Heat Transfer 6 (1999) 16], Yu and Koyama [Yu J, Koyama S. Condensation heat transfer of pure refrigerants in microfin tubes. In: Proc. Int. Refrigeration Conference at Purdue Univ., West Lafayette, USA, 1998. p. 325–30], and the theoretical model proposed by Wang et al. [Int. J. Heat Mass Transfer 45 (2002) 1513].     

Mass flow characteristics and empirical modeling of R22 and R410A flowing through electronic expansion valves

The applications of electronic expansion valves (EEVs) into multi-type heat pumps and inverter heat pumps in building air-conditioning systems have increased for comfort environmental control and energy conservation. However, test data and mass flow models of EEVs are very limited in open literature. The objectives of this study are to investigate the mass flow characteristics of R22 and R410A through EEVs and to develop an empirical correlation for the prediction of mass flow rates of R22 and R410A through EEVs. Mass flow rates through six EEVs were measured by varying the EEV opening, inlet and outlet pressures, and the subcooling. Mass flow rates of R410A were compared with those of R22 at the same test conditions. Based on the experimental data, an empirical correlation for mass flow predictions in EEVs was developed by modifying the orifice equation. The predictions of the present correlation showed good agreement with the measured data with average and standard deviations of 0.76% and 5.9%, respectively. Approximately 92% of the measured data were within ±10% of the predictions.