Review on pool boiling heat transfer of carbon dioxide

Although application of carbon dioxide as working fluid in many fields of refrigeration technology has been recommended very often in the recent past, the data on nucleate boiling heat transfer of carbon dioxide in free convection are very scarce in the open literature and new investigations are almost entirely focussed on forced convective flow boiling. In the interpretation of the respective results, heat transfer to carbon dioxide is often characterized as being superior to other refrigerants due to the outstandingly favourable thermophysical properties of carbon dioxide for boiling heat transfer. Different from this view, the discussion of recent results on pool boiling heat transfer of carbon dioxide in this review demonstrates that the high heat transfer coefficients measured for carbon dioxide in comparison to hydrocarbon or halocarbon refrigerants are mainly due to the fact that application of carbon dioxide is mostly envisaged for conditions where reduced saturation pressure p*=p_s/p_c (p_c, critical pressure) is higher than for common refrigerants.

In the first part of the review, the three main influences—by heat flux, saturation pressure and fluid properties—on pool boiling of carbon dioxide are discussed using recent measurements for CO₂ by Kotthoff et al. [S. Kotthoff, U. Chandra, D. Gorenflo, A. Luke, New measurements of pool boiling heat transfer for carbon dioxide in a wide temperature range, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 2/A/3.30]; see also S. Kotthoff, U. Chandra, D. Gorenflo, Neue Messungen zum Behältersieden von Kohlendioxid in einem grösseren Temperaturbereich, DKV-Tagungsbericht 22 (2004) [Bd.II. 1] 233–256 and other organic substances (Gorenflo et al.) [D. Gorenflo, S. Kotthoff, U. Chandra, New measurements of pool boiling heat transfer with hydrocarbons and other organics for update of VDI—Heat Atlas calculation method, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 1/C/1.00]; Kotthoff and Gorenflo, [S. Kotthoff, D. Gorenflo, Influence of the fluid on pool boiling heat transfer of refrigerants and other organic substances, Proceedings of the IIR-Commission B1 Conference, Vicenza, 2005 [paper #TP-98]. In the second part, a comparison is given with the few former data available and with new results of Loebl and Kraus [S. Loebl, W.E. Kraus, Pool boiling heat transfer of carbon dioxide on a horizontal tube, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 1/A/1.20]; S. Loebl, W.E. Kraus, Zum Wärmeübergang bei der Verdampfung von Kohlendioxid am horizontalen Rohr, DKV-Tagungsbericht 22 (2004) [Bd.II.1] 219–232 on the influence of the heating wall (material and surface roughness). Finally, analogies between nucleate pool boiling and new flow boiling data are shown for those domains of flow boiling in which nucleation provides the dominant contribution to heat transfer and convective effects are of secondary importance.     

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.     

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%.     

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.     

Influence of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134A

The effect of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134a has been investigated in a wide range of heat flux and saturation pressure. The enhancement of the heat transfer coefficient, which can be achieved by ultrasound, is much more pronounced for mixtures than for pure substances. It is, however, limited to rather small heat fluxes ( ). Especially remarkable is the fact, that the maximum influence of ultrasound on the heat transfer coefficient of the mixtures occurs at medium saturation pressures (p/p_c ≈ 0.2); the effect is markedly less for higher and for lower saturation pressures. Obviously, the improvement of the heat transfer to mixtures is mainly caused by a decrease of the local saturation temperature near the heating wall, due to a better mixing in the liquid boundary. This explanation is supported by evaluating important parameters of bubble formation from high-speed photographs of the heating surface. It is further noticeable, that the well known hysteresis effect at the beginning of pool boiling is reduced to a great extent by exposure to ultrasound.     

Pool boiling heat transfer from a horizontal tube coated with oxide ceramics

Pool boiling heat transfer from a horizontal copper tube coated with 0.2 mm of aluminum oxide-titanium oxide ceramics has been investigated for several pure fluids (refrigerants and hydrocarbons) and three propane/n-butane mixtures. The heat transfer coefficient shows a similar dependence on heat flux and normalized saturation pressure as with a metallic heating tube. At normalized saturation pressures p/p_c0.1, the absolute values of the heat transfer coefficient are just as high as for a sandblasted copper tube of similar surface roughness and at lower saturation pressures even higher. The negative influence of the low thermal conductivity of the ceramics is completely compensated or even overcompensated by the positive influence of the microstructure which results in a higher nucleation site density. This is especially effective in pool boiling of mixtures.     

Measurement of spray boiling refrigerant coefficients in an integral-fin tube bundle segment simulating a full bundle

Outside (refrigerant) boiling coefficients for a combination of spray and drip boiling for a low pressure refrigerant have been obtained from overall heat transfer coefficients in a 1024 fins per meter tube bundle segment. The tubes were heated by water on the inside; liquid refrigerant was sprayed and/or dripped on the outside. Also, refrigerant vapor was supplied at the bottom of the bundle segment. This configuration simulates an actual flooded evaporator under spray boiling conditions. The dripping corresponds to liquid film falling from upper rows while the inlet vapor is equivalent to the vaporized refrigerant rising from lower tubes; the refrigerant vapor can influence heat transfer performance by the combined effects of gas convection and liquid shear on the tubes. For a nominal heat flux of 23,975 W/m², a bundle average outside heat transfer coefficient of 8522 W/m² °C, based on nominal tube outer diameter, was found at an average bundle vapor mass flux equal to 12.4 kg/s m². The distributor plate below the bundle enhanced the heat transfer, especially at lower vapor mass fluxes, by providing a level of liquid hold-up just below the bottom tube row.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Use of fluorescence to measure the lubricant excess surface density during pool boiling

This paper presents what are believed to be the first measurements of the non-adiabatic lubricant excess surface density on a roughened, flat, plain horizontal pool-boiling surface. Pool boiling heat transfer data is given for pure R123 and a R123/lubricant mixture. Lubricant excess surface density data are given for the boiling R123/lubricant mixture. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent measurement technique was used to confirm the existence of the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the lubricant excess density on the heat transfer surface would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for both pure R123 and an R123/lubricant (1.8% lubricant mass fraction) mixture at 277.6 K. The heat transfer data shows that the lubricant excess causes an average degradation of 12% in the heat flux for a given superheat.

Résumé

This paper presents what are believed to be the first measurements of the non-adiabatic lubricant excess surface density on a roughened, flat, plain horizontal pool-boiling surface. Pool boiling heat transfer data is given for pure R123 and a R123/lubricant mixture. Lubricant excess surface density data are given for the boiling R123/lubricant mixture. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent measurement technique was used to confirm the existence of the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the lubricant excess density on the heat transfer surface would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for both pure R123 and an R123/lubricant (1.8% lubricant mass fraction) mixture at 277.6 K. The heat transfer data shows that the lubricant excess causes an average degradation of 12% in the heat flux for a given superheat.     

Dielectric and piezoelectric properties of sol–gel derived Ca doped PbTiO3

Synthesis of Ca doped PbTiO₃ powder by a chemically derived sol–gel process is described. Crystallization characteristics of different compositions Pb_1−xCa_xTiO₃ (PCT) with varying calcium (Ca) content in the range x = 0–0.45 has been investigated by DTA/TGA, X-ray diffraction and scanning electron microscopy. The crystallization temperature is found to decrease with increasing calcium content. X-ray diffraction reveals a tetragonal structure for PCT compositions with x ≤ 0.35, and a cubic structure for x = 0.45. Dielectric properties on sintered ceramics prepared with fine sol–gel derived powders have been measured. The dielectric constant is found to increase with increasing Ca content, and the dielectric loss decreases continuously. Sol–gel derived Pb_1−xCa_xTiO₃ ceramics with x = 0.45 after poling exhibit infinite electromechanical anisotropy (k_t/k_p) with a high d₃₃ = 80 pC/N, ′ = 298 and low dielectric loss (tan δ = 0.0041).     

Enhancement of pool boiling

This review considers the many techniques that have been developed to enhance boiling heat transfer. After introducing the techniques, the applications to pool boiling are described. Comments are offered regarding commercial introduction of this technology and the generations of heat transfer technology; advanced enhancement represents 3rd generation heat transfer technology.     

Effect of bulk lubricant concentration on the excess surface density during R123 pool boiling

This paper investigates the effect that bulk lubricant concentration has on the non-adiabatic lubricant excess surface density on a roughened, horizontal flat (plain) pool-boiling surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for pure R123 and three different mixtures of R123 and a naphthenic mineral oil. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent technique was used to measure the effect of bulk lubricant concentration on the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the effect of bulk lubricant composition on the lubricant excess density and in turn the effect on the heat transfer would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for pure R123 and three R123/lubricant mixtures at 277.6 K. For heat fluxes between approximately 25 to 45 kW/m², an average enhancement of the heat flux of 9 and 5% was achieved for the 0.5 and 1% lubricant mass fractions, respectively, and an average degradation of 5% in the heat flux was obtained for the 1.8% lubricant mass fraction mixture.

Résumé

This paper investigates the effect that bulk lubricant concentration has on the non-adiabatic lubricant excess surface density on a roughened, horizontal flat (plain) pool-boiling surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for pure R123 and three different mixtures of R123 and a naphthenic mineral oil. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent technique was used to measure the effect of bulk lubricant concentration on the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the effect of bulk lubricant composition on the lubricant excess density and in turn the effect on the heat transfer would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for pure R123 and three R123/lubricant mixtures at 277.6 K. For heat fluxes between approximately 25 kW/m² to 45 kW/m², an average enhancement of the heat flux of 9% and 5% was achieved for the 0.5% and 1% lubricant mass fractions, respectively, and an average degradation of 5% in the heat flux was obtained for the 1.8% lubricant mass fraction mixture.     

Heat transfer in nucleate boiling of different low boiling substances

Heat transfer coefficients for nucleate boiling of methane, ethane, ethylene, argon and carbon dioxide were determined using an apparatus for the precise investigation of pool boiling heat transfer in the low temperature range. The apparatus used a horizontal cylinder as the heating element. The influence of the thermophysical properties of the boiling liquid was established by comparing the absolute values of the heat transfer coefficients in a normalized boiling state, i.e. a saturation pressure equal to 10% of the critical pressure and a heat flux density equal to 2 × 10⁴ W m⁻². By including the results for a number of higher boiling liquids, which were investigated previously under similar experimental conditions, and using literature data for three very low boiling liquids, an empirical correlation is established which allows an approximate prediction of the absolute value of the heat transfer coefficient at nucleate boiling for substances of different molecular structure.     

Heat transfer coefficient during two-phase flow boiling of HFC-134a

This paper reports a study of the evaporation of HFC-134a inside smooth, horizontal tubes. Tests were performed with the pure refrigerant and with oil-refrigerant mixtures. The heat flux was varied from 2 to 10 kW m⁻². The inner diameter of the tubes was 12 mm. Two evaporators were used, 4 and 10 m long, and the oil content was varied from 0 to 2.5 mass percentage (synthetic oil, EXP-0275). Oil-free HFC-134a had a higher heat transfer coefficient than HCFC-22 at the same heat and mass fluxes. The effect of oil in the refrigerant is dependent on the heat flux. At 2 and 4 kW m⁻² the heat transfer coefficient had a maximum value for an oil content of around 0.5 mass percentage; no increase is registered for a heat flux of 6 kW m⁻². The heat transfer coefficients for the pure refrigerant were also compared with two existing correlations. The measured heat transfer coefficients averaged over the evaporator deviate less than 40% from the correlation according to Pierre. The heat transfer coefficients at the short evaporator lie within 20%. The correlation given by Jung overestimates the heat transfer coefficient by approximately 50%.     

Heat transfer coefficient of a snow bag

In snow shooting, pressurised liquid carbon dioxide is injected via a lance into a permeable snow bag mounted near the ceiling of an insulated transport container. The decrease in pressure causes the liquid carbon dioxide to convert to “snow” and vapour inside the snow bag. The snow bag acts as a phase separator, allowing the sublimated snow to keep the preserving temperature inside a container. In this paper the heat transfer coefficient of such a snow bag was determined experimentally. The average heat transfer coefficient was found to be 3.80 Wm⁻² K⁻¹.     

Flow boiling of ammonia in a plain and a low finned horizontal tubeEbullition en éncoulement d'ammoniac dans un tube lisse ou un tube ailetté

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40 < T_sat < 4°C, 0 < x< 0.9, 50 < < 150 kg/m² s and 2 < ΔT_w < 15 K with resulting heat fluxes of 17 < < 75 kW/m². The vapor quality is denoted as x, is the mass flux and ΔT_w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zürcher (Zürcher, O., Thome, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89–101), and with the correlations of Steiner (Steiner D. Strömungssieden gesättigter Flüssigkeiten. VDI-Wärmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes: part 3 — development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation.     

Transition from nucleate boiling to convective boiling in compact heat exchangers

The boiling of pure fluids has been experimentally studied in several types of compact heat exchanger channels. Plate fin and corrugated heat exchangers have been studied (seven geometries). Controlling the flow parameters (mass flux and vapour quality), the heat flux and measuring the wall temperature, have allowed characterization of the local heat transfer coefficient.

The results clearly show that the dominant mechanisms occurring could be nucleate or convective boiling. The transition between these two mechanisms depends on the flow characteristics and also on the channel geometry. Based on these measurements, an objective criteria can be established to identify the flow boiling regime. The knowledge of such a criteria is useful if we want to extend the use of compact heat exchanger to boiling of mixtures.     

Enhancement of R123 pool boiling by the addition of hydrocarbonsAmélioration de l'ébullition libre du R123 par l'ajout d'hydrocarbures

This paper presents pool boiling heat transfer data for 10 different R123/hydrocarbon mixtures. The data consisted of pool boiling performance of a GEWA-T^™ surface for pure R123 and for 10 dilute solutions of five different hydrocarbons: (1) pentane, (2) isopentane, (3) hexane, (4) cyclohexane, and (5) heptane with R123. The heat flux and the wall superheat were measured for each fluid at 277.6 K. A maximum (19±3.5)% increase over the pure R123 heat flux was achieved with the addition of 0.5% mass isopentane to R123. Other mixtures of isopentane, pentane, hexane, and cyclohexane with R123 exhibited smaller maximums than that of the R123/isopentane (99.5/0.5) mixture. Presumably, a layer enriched in hydrocarbon at the heat transfer surface caused the heat transfer enhancement. Conversely, an R123/heptane (99.5/0.5) mixture and an R123/cylcohexane (99.5/0.5) mixture exhibited only degradations with respect to the pure component performance for all test conditions. Several characteristics of the hydrocarbons were examined to determine their influence on the boiling heat transfer performance: molecular weight, molecular structure, composition, surface tension, and vapor pressure.     

Two-phase heat transfer to a refrigerant in a 1 mm diameter tubeTransfert de chaleur vers un frigorigène circulant dans un tube d'un diamètre d'un mm

A study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels. R141b has been used as the working fluid. The boiling heat transfer in the small tube has been measured over a mass flux range of 300–2000 kg/m² s and heat flux range of 10–1150 kW/m². In this paper the boiling map for a mass velocity of 510 kg/m² s and heat flux of 18–72 kW/m² is discussed and the problems of determining heat transfer coefficients in small channels are highlighted.     

Characteristics of evaporative heat transfer and pressure drop of carbon dioxide and correlation development

Carbon dioxide among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In this study, transcritical refrigeration cycle using carbon dioxide is of great interest, and the evaporation process is investigated by experiment and analysis. This paper presents the measured heat transfer coefficients and pressure drop during evaporation process of carbon dioxide in a horizontal smooth tube. The test section was made of a seamless stainless steel tube with the inner diameter of 7.53 mm, and length of 5 m. Heat is provided by a direct heating method to the test section. Experiments were conducted at saturation temperatures of −4 to 20 °C, heat fluxes of 12 to 20 kWm⁻² and mass fluxes of 200 to 530 kgm⁻² s⁻¹. A comparison of different heat transfer correlations applicable to evaporation of carbon dioxide has been made. Based on the experiments for the evaporation heat transfer, useful correlation is developed.