Convective boiling pressure drop of refrigerant R-134a in horizontal smooth and microfin tubes

Present study deals with the pressure drop of refrigerant R-134a under convective boiling conditions in horizontal smooth and microfinned (‘grooved’) copper tubes. Experiments have been carried out in an experimental set up developed for change of phase studies with a test section made out of 7.0, 7.93, and 9.52 mm external diameter, 1.5 m long copper tubes, electrically heated by tape resistors wrapped on the external surface. Mass velocities and refrigerant qualities varied in the following ranges: 70–1100 kg s⁻¹ m⁻² and 5–95%. The annular flow pattern has been observed to occur over most of the operational conditions. For smooth tubes, the Jung and Radermacher correlation for the liquid two phase flow multiplier fits with reasonable precision the experimental data. As for grooved tubes, a correlation of the two phase flow multiplier in terms of the Martinelli's parameter has been developed which fits the data with an average absolute deviation of the order of 6.3%. The proposed correlation fits with good precision data obtained elsewhere for grooved tubes of different diameter and microfin geometry.     

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.     

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.     

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%.     

R134a在水平直齿外翅片管表面冷凝传热实验研究

在对R134a在水平直齿外翅片管表面冷凝传热理论研究的基础上,利用用计算机建立了传热数学模型,并在实验室中用5根紫铜外翅片铜管进行试验验证,结果表明该理论数学模型在一定范围内的预测值是准确的.     

Equations for the Thermal Conductivity of R-32, R-125, R-134a,and R-143a

At present hydrofluorocarbons (HFCs) such as R32, R-125, R-134a, and R-143a are widely used, and it is required to obtain accurate information of thermophysical properties, especially of the thermal conductivity of HFCs. In this paper new thermal conductivity equations for R-32, R-125, R134a, and R143a are proposed, applicable over a wide range of temperature and pressure including the critical region based on existing experimental data, and the reliability of the present equations is summarized. The problem that the thermal conductivity calculated from the thermal diffusivity in the critical region differs depending on the equation of state is also discussed. Paper presented at the Sixteenth European Conference for Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

R134a制冷剂在微通道中的相变传热特性实验

设计了一个可控制制冷剂流量、压力和温度等实验工况的微通道换热器相变流动与换热的可视化实验平台,对R134a制冷剂流经微通道换热器进行了冷凝换热实验研究.试验测量了小质量流率下的R134a制冷剂在多个饱和状态工况下的冷凝换热性能,涉及质量流量、进出口压力和温度等参数.实验分析了传热系数与雷诺数的关系,与Koyama的关联式预测比较接近.分析了摩擦系数随雷诺数的变化,与H L MO和Wu＆Little方程计算得到的数值相近.     

Thermodynamic Properties of Mixtures of R-32, R-125, R-134a, and R-152a

A mixture model explicit in Helmholtz energy has been developed that is capable of predicting thermodynamic properties of refrigerant mixtures containing R-32, R-125, R-134a, and R-152a. The Helmholtz energy of the mixture is the sum of the ideal gas contribution, the compressibility (or real gas) contribution, and the contribution from mixing. The contribution from mixing is given by a single equation that is applied to all mixtures used in this work. The independent variables are the density, temperature, and composition. The model may be used to calculate thermodynamic properties of mixtures, including dew and bubble point properties and critical points, generally within the experimental uncertainties of the available measured properties. It incorporates the most accurate published equation of state for each pure fluid. The estimated uncertainties of calculated properties are ±0.25% in density, ±0.5% in the speed of sound, and ±1% in heat capacities. Calculated bubble point pressures are generally accurate to within ±1%.     

Experimental study on the flow and heat transfer characteristics of a tetra-n-butyl ammonium bromide hydrate slurry (second report: Heat transfer characteristics)

The heat transfer characteristics of a tetra-n-butyl ammonium bromide hydrate slurry were investigated where the Reynolds number, tube diameters and solid fraction were varied as experimental parameters. For laminar flow, it was found that the ratio of Nusselt numbers increased with solid fraction. An approximation of Nusselt number could be derived using the Graetz number on the basis of the apparent Reynolds number, the solid fraction and the ratio of the average diameter of the hydrate particles to the test tube diameter. For turbulent flow conditions, the ratio of Nusselt numbers had a value of one for each condition at low solid fractions. The ratio of Nusselt numbers increased with solid fraction in the high solid fraction region. Moreover, the apparent Reynolds number, which can be derived by treating the hydrate slurry as a pseudoplastic fluid, can be used to determine the condition under which hydrate slurry heat transfer characteristics vary under turbulent flow.     

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.     

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.     

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.     

Experimental study on the heat transfer and flow characteristics of nanorefrigerants inside a corrugated tube

The heat transfer and flow characteristics of MWCNT-R141b nanorefrigerant with different mass fractions have been studied through experiments. Experimental results were compared with existing correlations. A two-step method was used to prepare the nanorefrigerants. Span-80 was used as surfactant with an average particle diameter of 20 nm. Transmittance method was used to evaluate the stability of nanorefrigerants. Results showed that the stability of MWCNT-R141b nanorefrigerant, which is the added dispersant, was good during the experiments. The 0.3 wt% MWCNT-R141b nanorefrigerants had optimal heat transfer enhancement effects compared with pure refrigerants. The maximum Nusselt number increased by 40%. The specific pressure drop of nanorefrigerant increased as the Reynolds number (Re) increased, and the specific pressure drop of the pure refrigerant was minimum, which is similar to R141b.     

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.     

Condensation of R-134a vapour over single horizontal integral-fin tubes: effect of fin height

An experimental investigation has been carried out to study the condensation of R-134 vapour over five single horizontal circular integral-fin tubes of 472 fpm fin density, 417 mm length and different fin heights of 0.45, 1.14, 1.47, 1.92 and 2.40 mm. The circular fins are rectangular in shape and the fin thickness of all tubes is 0.70 mm. The tube with the fin height of 0.45 mm has given the highest enhancement in heat transfer coefficient, h, of the order of 3.18 in comparison to that predicted by the Nusselt model for a plain tube.     

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.     

Vapor Pressure of the 1,1,1,2-Tetrafluoroethane (R-134a) + Polyalkylene Glycol System

Vapor pressures of the 1,1,1,2-tetrafluoroethane + polyalkylene glycol system were obtained at 72 points over the temperature range from 253.15 to 333.15 K at 10 K intervals and the composition range from 0 to 90 mass % polyalkylene glycol. It was found that below 273.15 K, the effect of the polyalkylene glycol on the vapor pressure was negligible up to 30 mass % polyalkylene glycol. The vapor pressure of the 1,1,1,2-tetrafluoroethane + polyalkylene glycol system decreased as the concentration of polyalkylene glycol increased. Raoults model and Flory–Huggins model were used for data reduction. Raoults model gave reasonable predictions for the vapor pressure of the system below 30 mass % polyalkylene glycol. The Flory–Huggins model gave reasonable predictions for the vapor pressure over the complete composition range. An empirical vapor pressure equation was obtained in terms of temperature and mass fraction polyalkylene glycol. The empirical equation was the most convenient way to calculate the vapor pressure.     

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.     

Investigation on the heat transfer characteristics during flow boiling of liquefied natural gas in a vertical micro-fin tube

This paper presents an experimental investigation on the heat transfer characteristics of liquefied natural gas flow boiling in a vertical micro-fin tube. The effect of heat flux, mass flux and inlet pressure on the flow boiling heat transfer coefficients was analyzed. The Kim, Koyama, and two kinds of Wellsandt correlations with different F_tp coefficients were used to predict the flow boiling heat transfer coefficients. The predicted results showed that the Koyama correlation was the most accurate over the range of experimental conditions.     

Experimental study on heat transfer enhancement of nanofluid flow through helical tubes

Fluid flow and heat transfer characteristics of nanofluids flowing through helically coiled tubes under uniform heat flux condition are studied experimentally. The turbulent flow of two different kinds of nanofluids, i.e. Ag-water and SiO₂-water, are examined. Three different helically coiled tubes along with straight ones are constructed to investigate the effects of geometrical parameters such as pitch circle diameter and helical pitch as well as nanoparticle volume concentration. The viscosity and thermal conductivity of nanofluids are determined experimentally in different volume fractions and temperatures. The range of Reynolds number is from 8900 to 11970. The experimental outcomes show that using nanoparticles in coiled tubes can be more effective in improving the heat transfer rate than the straight tube. Empirical correlations are extracted based on experimental data to predict the Nusselt number and friction factor of turbulent nanofluids flow through helically coiled tubes.