Boiling heat transfer of HFO-1234yf flowing in a smooth small-diameter horizontal tube

The flow boiling heat transfer coefficient of the low-GWP (global warming potential) refrigerant HFO-1234yf inside a smooth small-diameter horizontal tube (inner diameter: 2 mm) was experimentally investigated. The local heat transfer coefficient was measured at heat fluxes of 6-24 kW m⁻², mass fluxes of 100-400 kg m⁻² s⁻¹, an evaporating temperature of 288.15 K, and an inlet vapor quality of 0-0.25. The results show that the effect of heat flux on the heat transfer was large at low vapor quality, while the effect of mass flux was large at high vapor quality. The heat transfer coefficient of HFO-1234yf was almost the same as that of R-134a. The heat transfer coefficients calculated based on correlations with Saitoh et al. agreed well with the measured values compared to other correlations. The measured pressure drop agreed well with that predicted by the Lockhart-Martinelli correlation.     

CO2 and propane blends: Experiments and assessment of predictive methods for flow boiling in horizontal tubes

The CO₂ and propane blends are an interesting alternative to solve technical and safety issues related to the use of pure CO₂ or pure hydrocarbons. These mixtures of pure fluids are environmentally friendly and have a large glide, that affects remarkably heat transfer.In this paper a review of works and predictive methods on flow boiling of wide-boiling mixtures is first presented. Experiments during flow boiling in a smooth horizontal tube with an internal diameter equal to 6.00 mm of CO₂ and propane mixtures (with 83.2/16.8% and 70.0/30.0% in mass concentrations) are reported. The experiments are related to the following operating ranges: mass fluxes from 200 to 350 kg m⁻² s⁻¹, heat fluxes from 10 to 20.2 kW m⁻², temperatures of the mixture from 6.9 to 14.0 °C in the whole range of vapor qualities.An assessment of predictive methods based on the present and independent databases is reported.     

Water temperature and heat flux at the base of river ice covers

The heat flux from flowing water to river ice covers can have a significant effect on decay of ice. To calculate the heat transfer coefficient and heat flux to the Liard River ice cover, detailed measurements of water temperature and velocity were made prior to, and during, breakup. Comparison of four different techniques for calculating heat transfer coefficients showed that it is essential to consider ice roughness and to measure water temperature to a high degree of accuracy. Heat transfer coefficients calculated from the Colburn analogy method were in closest agreement to those using a temperature decay approach. Those estimated using a standard empirical technique derived from laboratory data seriously underpredicted the heat flux.The water temperature measurements showed large spatial and temporal variations which had important implications to the heat flux. Prior to breakup, water temperatures were generally close to 0° C, never rising above 0.022° C, yet resulted in heat fluxes as high as 100 W/m². The highest temperatures and heat fluxes tended to occur in the high velocity sections of the channel which had a thin ice cover. As breakup progressed, the water temperature increased to 1.6° C. During breakup, water temperature decayed rapidly below ice jams. Typically, water temperature decreased from over 1° C to near 0° C within 4 km. Over this distance the heat flux decreased from 10,000 W/m² to near zero with an average of 4000 W/m².     

Coefficients d'échange locaux au cours de l'ébullition du R22 et du R407C dans des tubes horizontaux, lisse ou micro-ailetéLocal heat transfer coefficients during boiling of R22 and R407C in horizontal smooth and microfin tubes

The purpose of this study is to experimentally investigate forced convective boiling. The heat transfer coefficients of pure refrigerant R22 and non azeotropic refrigerant mixture R407C were measured in both a smooth tube and a microfin tube. The tests have been carried out with a uniform heat flux all along the tube length. The refrigerant mass flux was varied from 100 to 300 kg m⁻² s⁻¹ and heat fluxes from 10 to 30 kW m⁻². Local heat transfer coefficients depend strongly on heat flux at a low quality and on mass fluxes at a high quality. When compared to smooth tube, the microfin tubes exhibit a significant heat transfer enhancement, up to 180%. In comparison to R22, the R407C heat transfer coefficients of smooth and microfin tubes are 15 to 35% lower, respectively. The best heat transfer enhancement is obtained at low heat flux and mass flow rate.     

Experimental study on thermal characteristics of ammonia flow boiling in a plate evaporator at low mass flux

Thermal characteristics of a plate evaporator using ammonia are experimentally investigated. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient of the evaporator are discussed. The experiments are conducted for mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is obtained as a function of quality for all experimental conditions. The characteristics of heat transfer coefficient are discussed and compared with those of earlier works. All experimental results are compiled by using Lockhart–Martinelli parameter. The developed empirical correlation predicts 85% of the experimental data within ±30% range.     

Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

An experimental investigation and modelling of flow boiling heat transfer of isobutane-compressor oil solution in a horizontal smooth tube

Experimental results of local heat transfer coefficients for the boiling of working fluids (solutions of R600a with mineral naphthenic oil ISO VG 15) in a smooth tube with a small diameter (5.4 mm) are presented. The experiments have been performed in the following ranges: for the inlet pressure from 65.7 kPa to 82.2 kPa, for the heat flux from 2500 to 3300 W m⁻², and for the mass velocity of the working fluid from 11.90 to 15.99 kg m⁻² s⁻¹). The quantitative estimation in reduction of the heat transfer coefficient of the wetted surface in the evaporator at a high oil concentration in the mixture is examined. The influence of heat flux and mass velocities on the values of the local heat transfer coefficients is analyzed. The equation for the modelling of the local heat transfer coefficient for boiling of an isobutane/compressor oil solution flow in the tube is suggested.     

Characterisation of the hydraulic maldistribution in a heat exchanger by local measurement of convective heat transfer coefficients using infrared thermography

A methodology was developed to characterise the heat exchangers' performance decrease due to two-phase flow maldistribution. It consists in measuring the spatial distribution of the local heat transfer coefficients with a rapid, non-invasive and fluid independent method. The method is based on the infrared (IR) thermography measurement of the temperature response to an oscillating heat flux. The amplitude of the measured temperatures is compared to the solution of an analytical model. The problem is solved iteratively to obtain the heat transfer coefficients. This method has been applied to evaluate the uneven phase distribution of an air–water mixture in a compact heat exchanger. The exchanger is composed of seven multiport flat tubes, a vertical downward header and horizontal channels. Experiments were performed for mass flux from 29 kg m⁻² s⁻¹ to 116 kg m⁻² s⁻¹ and for quality from 0.10 to 0.70.     

Evaporation heat transfer for R-22 and R-407C refrigerant–oil mixture in a microfin tube with a U-bendTransfert de chaleur lors de l'évaporation de R-22 et de R-407C mélangés avec de l'huile dans un tube à micro-ailettes avec un coude en U

Evaporation heat transfer experiments for two refrigerants, R-407C and R-22, mixed with polyol ester and mineral oils were performed in straight and U-bend sections of a microfin tube. Experimental parameters include an oil concentration varied from 0 to 5%, an inlet quality varied from 0.1 to 0.5, two mass fluxes of 219 and 400 kg m⁻²s⁻¹ and two heat fluxes of 10 and 20 kW m⁻². Pressure drop in the test section increased by approximately 20% as the oil concentration increased from 0 to 5%. Enhancement factors decreased as oil concentration increased under inlet quality of 0.5, mass flux of 219 kg m⁻² s⁻¹, and heat flux of 10 kW m⁻², whereas they increased under inlet quality of 0.1, mass flux of 400 kg m⁻² s⁻¹, and heat flux of 20 kW m⁻². The local heat transfer coefficient at the outside curvature of an U-bend was larger than that at the inside curvature of a U-bend, and the maximum value occurred at the 90° position of the U-bend. The heat transfer coefficient was larger in a region of 30 tube diameter length at the second straight section than that at the first straight section.     

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.     

Study on the distillation rates of LiCl-KCl eutectic salt under different vacuum conditions

A study on the distillation rate of LiCl-KCl eutectic salt under different vacuums from 0.5 to 50 Torr was performed by using thermogravimetric (TG) method. A distillation rate of the order of 10⁻⁴-10⁻⁵ mol cm⁻² s⁻¹ was obtainable at temperatures of 1200-1300 K and vacuums of 5-50 Torr. Based on the non-isothermal TG data, model distillation flux equations could be derived as a function of temperature. Pure gas-phase and gas-liquid interfacial resistances at different vacuum conditions were evaluated from the comparison of experimental vaporization fluxes with the maximum flux obtained from the kinetic theory of gas. The difference between interfacial mass transfer coefficients and gas-phase ones increases with the temperature. Gas-phase resistance is much greater than that of the phase transition between condensed and gas phases at tested vacuum conditions of 0.5-50 Torr.     

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.     

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.     

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.     

Experimental study on forced convective boiling heat transfer of pure refrigerants and refrigerant mixtures in a horizontal tube

Convective boiling heat transfer coefficients of pure refrigerants (R22, R32, R134A, R290, and R600a) and refrigerant mixtures (R32/R134a, R290/R600a, and R32/R125) are measured experimentally and compared with Gungor and Winterton correlation. The test section is made of a seamless stainless steel tube with an inner diameter of 7.7 mm and is uniformly heated by applying electric current directly to the tube. The exit temperature of the test section was kept at 12°C ± 0.5°C for all refrigerants in this study. Heat fluxes are varied from 10 to 30 kW m⁻² and mass fluxes are set to the discrete values in the range of 424–742 kg m⁻² s⁻¹ for R22, R32, R134a, R32/R134a, and R32/R125; 265–583 kg m⁻² s⁻¹ for R290, R600a, and R290/R600a. Heat transfer coefficients depend strongly on heat flux at a low quality region and become independent as quality increases. The Gungor and Winterton correlation for pure substances and the Thome-Shakil modification of this correlation for refrigerant mixtures overpredicts the heat transfer coefficients measured in this study.     

Flow Boiling Heat Transfer in Two-Phase Micro Channel Heat Sink at Low Water Mass Flux

Boiling heat transfer at water flow with low mass flux in heat sink which contained rectangular microchannels was studied. The stainless steel heat sink contained ten parallel microchannels with a size of 640 × 2050 μm in cross-section with typical wall roughness of 10–15 μm. The local flow boiling heat transfer coefficients were measured at mass velocity of 17 and 51 kg/m²s, heat flux on 30 to 150 kW/m² and vapor quality of up to 0.8 at pressure in the channels closed to atmospheric one. It was observed that Kandlikar nucleate boiling correlation is in good agreement with the experimental data at mass flow velocity of 85 kg/m²s. At smaller mass flux the Kandlikar model and Zhang, Hibiki and Mishima model demonstrate incorrect trend of heat transfer coefficients variation with vapor quality.     

Investigations on the heat transfer of boiling binary refrigerant mixtures

Heat transfer during nucleate pool boiling was experimentally determined for the mixtures R-12/R-113, R-22/R-12, R-13/R-12, R-13/R-22 and R-23/R-13. For purposes of comparison, the respective five pure refrigerants were also investigated. Dependent upon the mixture, the measurements were made at boiling pressures of p = 0.1 to 2 MPa within the temperature region of t = 198 to 333 (−75° + 60°C) and at heat fluxes of Q = 4 × 10³ to 10⁵ W m⁻². A horizontal, electronically heated copper plate with A = 3 cm² was used. The following quantities were measured: pressure; temperature difference between the heating surface and the boiling liquid; composition and temperature in the liquid and vapour phases; and heat flow rate. The mean error of the heat transfer coefficients found was ± 5%.The results clearly show that the heat transfer for an evaporating mixture deteriorates as compared to the pure components. Essential parameters influencing this reduction are pressure, difference between vapour and liquid composition and heat flux. The fundamental relations and characteristic differences between the individual mixtures are illustrated by figures. The heat transfer coefficients measured can be represented within the whole region studied by a modified relation according to Körner.Observation of the process of evaporation has shown that by agitation (increase of convection) the heat transfer in mixtures can be improved. Additional experiments with evaporation during fluid flow in a pipe are presently in progress.     

Refrigerant R134a condensation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental tests on HFC-134a condensation inside a small brazed plate heat exchanger: the effects of refrigerant mass flux, saturation temperature and vapour super-heating are investigated.A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m² s. For refrigerant mass flux lower than 20 kg/m² s, the saturated vapour heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [Nusselt, W., 1916. Die oberflachenkondensation des wasserdampfes. Z. Ver. Dt. Ing. 60, 541–546, 569–575] analysis for vertical surface. For refrigerant mass flux higher than 20 kg/m² s, the saturated vapour heat transfer coefficients depend on mass flux and are well predicted by the Akers et al. [Akers, W.W., Deans, H.A., Crosser, O.K., 1959. Condensing heat transfer within horizontal tubes. Chem. Eng. Prog. Symp. Ser. 55, 171–176] equation. In the forced convection condensation region, the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by the Webb [Webb, R.L., 1998. Convective condensation of superheated vapour. ASME J. Heat Transfer 120, 418–421] model. The heat transfer coefficients show weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux.     

Experimental investigation on downward flow boiling heat transfer characteristics of propane in shell side of LNG spiral wound heat exchanger

For designing LNG spiral wound heat exchangers (SWHE), the boiling heat transfer mechanism of two-phase hydrocarbon refrigerant flowing downward in shell side should be known. In this study, an explosion-proof experimental rig was established for measuring heat transfer coefficients (HTC) and observing flow patterns. The test section contains three-layer tube bundles to emulate the actual structure and flow conditions of an SWHE. Propane as one main component of shell-side refrigerant is used as the tested fluid. The experimental conditions cover heat fluxes of 4~10 kW⋅m⁻², mass fluxes of 40~80 kg (m²⋅s)⁻¹ and vapor qualities of 0.2~1.0. The results indicate that HTC initially increases and then decreases with the increment of vapor quality, representing a maximum at a vapor quality of 0.8~0.9; the effect of heat flux on HTC increases with the increment of heat flux. A correlation of HTC was developed covering 98% of the experimental data within a deviation of ±20%.     

CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO₂ flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m⁻² s⁻¹, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO₂ two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.