Nucleate boiling heat transfer coefficients of HCFC22, HFC134a, HFC125, and HFC32 on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of HCFC22, HFC134a, HFC125, HFC32 were measured on a low fin, Turbo-B, and Thermoexcel-E tubes. All data were taken at the liquid pool temperature of 7 °C on horizontal tubes of 152 mm length and 18.6–18.8 mm outside diameter at heat fluxes of 10–80 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. For a plain and low fin tubes, refrigerants with higher vapor pressures showed higher nucleate boiling HTCs consistently. This was due to the fact that the wall superheat required to activate given size cavities became smaller as pressure increased. For Turbo-B and Thermoexcel-E tubes, HFC125 showed a peculiar behavior exhibiting much reduced HTCs due to its high reduced pressure. The heat transfer enhancement ratios of the low fin, Turbo-B, and Thermoexcel-E tubes were 1.09–1.68, 1.77–5.41, 1.64–8.77 respectively in the range of heat fluxes tested.     

Experimental correlation of pool boiling heat transfer for HFC134a on enhanced tubes: Turbo-E

The objectives of this paper are to study the heat transfer characteristics for enhanced surface tubes in the pool boiling and to provide a guideline for the design conditions for the evaporator using HFC134a. The shape of tube surfaces, the wall superheat, and the saturation temperature are considered as the key parameters. Copper tubes (d_o = 19.05 mm) are treated with different helix angles and the saturation temperatures are controlled from 3 to 16 °C. It is found that the pool boiling heat transfer coefficient decreases with increasing the wall superheat. It is also found that boiling heat transfer coefficients for Turbo-II and Turbo-III are 1.5–3.0 times and 1.2–2.0 times higher than that for Turbo-I without the helix angle, respectively. The higher heat transfer performance from Turbo-II and Turbo-III can be explained by the “bubble detention” phenomenon on the surface without the helix angle for the Turbo-I. The experimental correlations for the pool boiling heat transfer on the present enhanced tubes without (Type I) and with the helix angle (Type II and Type III) are developed with the error bands of ±30%, respectively.     

Experimental correlation of falling film condensation on enhanced tubes with HFC134a; low-fin and Turbo-C tubes

The objectives of this paper are to develop experimental correlations of heat transfer for enhanced tubes used in a falling film condenser, and to provide a guideline for optimum design of the falling film condenser with a high condensing temperature of 59.8 °C. Tests are performed for four different enhanced tubes; a low-fin and three Turbo-C tubes. The working fluid is HFC134a, and the system pressure is 16.0 bar. The results show that the heat transfer enhancement of low-fin tube, Turbo-C (1), Turbo-C (2) and Turbo-C (3) ranges 2.8–3.4 times, 3.5–3.8 times, 3.8–4.0 times and 3.6–3.9 times, respectively, compared with the theoretical Nusselt correlation. It was found that the condensation heat transfer coefficient decreased with increasing the falling film Reynolds number and the wall subcooling temperature. It was also found that the enhanced tubes became more effective in the high wall subcooling temperature region than in the low wall subcooling temperature region. This study developed an experimental correlation of the falling film condensation with an error band of ±5%.     

Boiling of ammonia/lubricant mixture on a horizontal enhanced tube in a flooded evaporator with inlet vapor quality

In a flooded evaporator of an ammonia vapor-compression refrigeration system, boiling commonly takes place with ammonia mixed with compressor lubricant and subjected to a vapor quality at the inlet of the evaporator. In the present study, flooded boiling tests of ammonia on an enhanced tube under simultaneous influence of a miscible lubricant and inlet quality were conducted. The results suggest that the boiling heat transfer coefficient increases with both saturation temperature and heat flux. The coefficient slightly increases or does not significantly vary with the inlet quality. The coefficient in general is decreased by adding lubricant to the refrigerant, but the coefficient does not necessarily decrease as the lubricant concentration increases. The lubricant effect is generally more significant than the inlet quality effect. A correlation was developed based on the present data for flooded boiling of lubricant/ammonia mixture on an enhanced horizontal tube under the influence of inlet quality.     

Prediction of heat transfer coefficient during condensation of water and R-134a on single horizontal integral-fin tubes

This paper presents a few salient features of an investigation carried out to study the heat transfer augmentation during condensation of water and R-134a vapor on horizontal integral-fin tubes. The experimental investigation was performed on two different experimental set-ups for water and R-134a. The test-sections were manufactured by machining fins over plain copper tubes of 24.4 ± 0.6 mm outside diameter. The performance of two types of finned tubes viz. circular integral-fin tubes (CIFTs) and spine integral-fin tubes (SIFTs) was studied for the condensation of water and R-134a. These tubes were positioned one by one inside the test-condenser to perform the experiments. All together the experiments were conducted for the condensation on 10 different test-section tubes. With the help of the experimental results, authors have developed an empirical equation. This equation predicts the condensing heat transfer coefficient from their own experimental data for the condensation over CIFTs and SIFTs within a range of ± 15% and experimental data of other thirteen investigators in a range of ± 35% for condensation of water and different refrigerants.     

Nucleate boiling heat transfer coefficients of pure halogenated refrigerants

Nuclate pool boiling heat transfer coefficients (HTCs) of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HCFC22, HFC125 and HFC32 on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accomodate high vapor pressure refrigerants such as HFC32 and HFC125 with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool of 7 °C. Test results showed that HTCs of HFC125 and HFC32 were 50–70% higher than those of HCFC22 while HTCs of HCFC123 and HFC134a were similar to those of CFC11 and CFC12 respectively. It was also found that nucleate boiling heat transfer correlations available in the literature were not good for certain alternative refrigerants such as HFC32 and HCFC142b. Hence, a new correlation was developed by a regression analysis taking into account the variation of the exponent to the heat flux term as a function of reduced pressure and some other properties. The new correlation showed a good agreement with all measured data including those of new refrigerants of significantly varying vapor pressures with a mean deviation of less than 7%.     

Nucleate boiling heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured at the liquid temperature of 7 °C on a low fin tube of 1023 fins per meter, Turbo-B, and Thermoexcel-E tubes. All data were taken from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. Flammable refrigerants' data showed a typical trend that nucleate boiling HTCs obtained on enhanced tubes also increase with the vapor pressure. Fluids with lower reduced pressure such as DME, isobutene, and butane took more advantage of the heat transfer enhancement mechanism of enhanced tubes than those with higher reduced pressure such as propylene and propane. Finally, Thermoexcel-E showed the highest heat transfer enhancement ratios of 2.3–9.4 among the tubes tested due to its sub-channels and re-entrant cavities.     

Nucleate boiling heat transfer coefficients of mixtures containing HFC32, HFC125, and HFC134a

Nucleate boiling heat transfer coefficients (HTCs) of binary and ternary mixtures composed of HFC32, HFC125, and HFC134a on a horizontal smooth tube of 19.0 mm outside diameter were measured. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature at 7 °C. HTCs of nonazeotropic mixtures of HFC32/HFC134a, HFC125/HFC134a, and HFC32/HFC125/HFC134a showed a reduction of HTCs as much as 40% from the ideal values while the near azeotropic mixture of HFC32/HFC125 did not show the reduction. Four of the well known correlations were compared against the present data for binary mixtures. Stephan and Körner's and Schlünder's correlations yielded a good agreement with a deviation of less than 10% but they can not be easily extended to multi-component mixtures of more than three components. A new correlation was developed utilizing only the phase equilibrium data and physical properties. A regression analysis was carried out to account for the reduction of HTCs and the final correlation, which can be easily extended to multi-component mixtures of more than three components, yielded a deviation of 7% for all binary and ternary mixtures.     

Condensation heat transfer coefficients of R22, R407C, and R410A on a horizontal plain, low fin, and turbo-C tubes

In this study, condensation heat transfer coefficients (HTCs) were measured on a horizontal plain tube, low fin tube, and Turbo-C tube at the saturated vapor temperature of 39 °C for R22, R407C, and R410A with the wall subcooling of 3–8 °C. R407C, a non-azeotropic refrigerant mixture, exhibited a quite different condensation phenomenon from those of R22 and R410A and its condensation HTCs were up to 50% lower than those of R22. For R407C, as the wall subcooling increased, condensation HTCs decreased on a plain tube while they increased on both low fin and turbo-C tubes. This was due to the lessening effect of the vapor diffusion film with a rapid increase in condensation rate on enhanced tubes. On the other hand, condensation HTCs of R410A, almost an azeotrope, were similar to those of R22. For all refrigerants tested, condensation HTCs of turbo-C tube were the highest among the tubes tested showing a 3–8 times increase as compared to those of a plain tube.     

Evaporation heat transfer characteristics of R-410A in 7 and 9.52 mm smooth/micro-fin tubes

In-tube evaporation heat transfer characteristics of R-410A were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux, and tube geometry. Evaporation heat transfer coefficients and pressure drops were measured for 3.0 m long smooth and micro-fin tubes with outer diameters of 9.52 and 7.0 mm, respectively. The test matrix in the present study included measurements for evaporation over a refrigerant mass flux range of 70–211 kg/m²s, a heat flux range of 5–15 kW/m² and an evaporating temperature range of −15 to 5. The objective of this study is to evaluate the heat transfer enhancement of the micro-fin tube with R-410A as a function of mass flux, heat flux, evaporating temperature and tube diameter.     

Bundle effect of ammonia/lubricant mixture boiling on a horizontal bundle with enhanced tubing and inlet quality

Shell-side heat transfer coefficients of individual tubes for ammonia/lubricant mixture boiling on a 3 × 5 enhanced tube bundle were measured, enabling a detailed study of tube bundle effect under the influences of inlet quality, concentration of miscible lubricant (co-polymer of polyalkylene glycol, PAG), saturation temperature, and heat flux. Tests were conducted in the range of heat flux from 3.2 to 32.0 kW/m², simulated inlet quality from 0.0 to 0.4, saturation temperature from −13.2 to +7.2 °C, and lubricant concentration from 0 to 10%. The data show that bundle effect is more significant at a higher saturation temperature. Most of the data in the bottom row are lower than the single-tube heat transfer coefficient data at a low saturation temperature. Lubricant renders the heat transfer coefficient lower in lower rows and higher in higher rows, therefore a larger range of data variation.     

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].     

Condensation inside and outside smooth and enhanced tubes — a review of recent research

Condensation heat transfer, both inside and outside horizontal tubes, plays a key role in refrigeration, air conditioning and heat pump applications. In the recent years the science of condensation heat transfer has been severely challenged by the adoption of substitute working fluids and new enhanced surfaces for heat exchangers. Well-known and widely established semiempirical correlations to predict heat transfer during condensation may show to be quite inaccurate in some new applications, and consequently a renewed effort is now being dedicated to the characterisation of flow conditions and associated predictive procedures for heat transfer and pressure drop of condensing vapours, even in the form of zeotropic mixtures. This paper critically reviews the most recent results appeared in the open literature and pertinent to thermal design of condensers for the air conditioning and refrigeration industry; both in-tube and bundle condensation are considered, related to the use of plain and enhanced surfaces.     

Effects of fin shape on condensation in herringbone microfin tubes

Effects of fin height and helix angle on condensation inside a herringbone microfin tube have been experimentally investigated with five types of herringbone microfin tubes. Heat transfer coefficients are about 2–4 times higher than that of the helical microfin tube under high mass velocity conditions. In the low mass velocity, they are equal to that of the helical microfin tube. The heat transfer enhancement increases with fin height up to 0.18 mm; higher fin heights show enhancement values similar to the 0.18 mm results. Pressure drop increases with the fin height. Larger helix angle yields higher heat transfer and higher pressure drop. For the lowest fin and/or smallest helix angle, the pressure drop is comparable with that of the helical microfin tube, while the heat transfer enhancement is higher. The enhancement mechanism is discussed from flow pattern observations. Effect of mass transfer resistance for R410A is estimated and negligible effects have been proved.     

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.     

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.     

An experimental study on condensation of refrigerant R134a in a multi-port extruded tube

In the present study, the local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of pure refrigerant R134a in two kinds of 865 mm long multi-port extruded tubes having eight channels in 1.11 mm hydraulic diameter and 19 channels in 0.80 mm hydraulic diameter. The pressure drop is measured at an interval of 191 mm through small pressure measuring ports. The local heat transfer rate is measured in every subsection of 75 mm in effective cooling length using heat flux sensors. It is found that the experimental data of frictional pressure drop agree with the correlation of Mishima and Hibiki [Trans. JMSE (B) 61 (1995) 99], while the correlations of Chisholm and Laird [Trans. ASME 80 (1958) 227], Soliman et al. [Trans. ASME, Ser. C 90 (1998) 267], and Haraguchi et al. [Trans. JSME (B) 60 (1994) 239], overpredict. As a trial, the data of local heat transfer coefficient are also compared with correlations of Moser et al. [J. Heat Transfer 120 (1998) 410] and Haraguchi et al. [Trans. JSME (B) 60 (1994) 245]. The data of high mass velocity agree with the correlation of Moser et al., while those of low mass velocity show different trends. The correlation of Haraguchi et al. shows the trend similar to the data when the shear stress in their correlation is estimated using the correlation of Mishima and Hibiki.     

Flow boiling heat transfer characteristics of CO2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO₂ at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO₂ at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO₂ is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO₂ in good agreement with the experimental data, suggesting the need for further research in this area.     

Two-phase flow regimes in round, square and rectangular tubes during condensation of refrigerant R134a

An experimental investigation of two-phase flow mechanisms during condensation of refrigerant R134a in six small diameter round (4.91 mm), square (D_h=4 mm, α=1), and rectangular (4×6 and 6×4 mm: D_h=4.8 mm, α=0.67 and 1.5; 2×4 and 4×2 mm: D_h=2.67 mm, α =0.5 and 2) was conducted. Unique experimental techniques and test sections were developed to enable the documentation of the flow mechanisms during phase change. For each tube under consideration, flow mechanisms were recorded over the entire range of qualities for five different refrigerant mass fluxes between 150 and 750 kg m⁻² s⁻¹. The flow mechanisms were categorized into four different flow regimes: intermittent flow, wavy flow, annular flow, and dispersed flow. In addition, the large amount of data enabled the delineation of several different flow patterns within each flow regime, which provides a clearer understanding of the different modes of two-phase flow. Transition lines between the respective flow patterns and regimes on these maps were established based on the experimental data. It was found that for similar hydraulic diameters, flow regime transitions are not very strongly dependent on tube shape or aspect ratio. These maps and the transition lines can be used to predict the particular flow pattern or regime that will be established for a given mass flux, quality and tube geometry.     

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.