Mixtures In Horizontal Tubes Research Articles

A new heat transfer model based on non-equilibrium film theory for annular flow condensation of binary zeotropic mixtures

In recent years, zeotropic mixtures have been widely used as substitutes for pure fluids. Accordingly, their condensation properties have attracted increasing attention. However, it is a challenge to accurately describe the heat transfer performance of zeotropic mixtures in condensation, which is limited by the estimation of mixture effects, especially the mass transfer resistance therein. Focusing on this issue, the mass transfer due to the axial and radial concentration gradients in the vapor phase was considered, and a new heat transfer model for the annular flow of binary zeotropic mixtures in horizontal tubes was developed based on non-equilibrium film theory. In this model, the boiling point difference was introduced to approximate the effect of concentration gradients by combining with the vapor-liquid phase equilibrium properties of binary zeotropic mixtures. Secondly, equivalent specific heat capacities were defined for an analogy with single-phase heat transfer, which further simplifies the form of the model. The simplified model was evaluated with a consolidated database of 11 mixtures from 13 sources. The results show that the predictions of the model are in good agreement with the experimental results, with a mean absolute relative deviation (MARD) of 15.88 and 91.14% of points falling in the ±30% deviation band. Predictions were also performed for excluded data from other sources and satisfactory results were obtained with a MARD of 13.47%. Moreover, the mechanism of mixture effects is elaborated. It indicates that besides the commonly mentioned heat transfer degradation, there is also heat transfer enhancement for binary zeotropic mixtures condensation due to the opposite effects of the axial and radial concentration gradients.

Convective condensation of R600a, R290, R1270 and their zeotropic binary mixtures in horizontal tubes

The present paper concerns an experimental investigation about the heat transfer coefficient during convective condensation of hydrocarbons and their zeotropic mixtures. Experiments were performed in a horizontal smooth channel, with internal diameter of 9.43 mm for R600a, R290, R1270, R600a/R290 (70/30 molar fraction) and R600a/R1270 (75/25 molar fraction), for mass velocities ranging from 50 to 250 kg m −2 s −1 , vapor qualities from unity to zero, heat fluxes from 5 to 60 kW m −2 and a saturation temperature of 35°C. Experiments were also performed for R134a. In general, higher heat transfer coefficients were obtained for the hydrocarbons compared to R134a. The heat transfer process was dominated by shear effects for the hydrocarbons regardless of the mass velocity. At low mass velocities (below 100 kg m −2 s −1 ), both mixtures provided lower heat transfer coefficients than the pure fluids. At high mass velocities a reduction of the heat transfer coefficient compared to the pure fluid was noticed only for R600a/R1270 at high qualities. The experimental data is compared against prediction methods from literature and based on the pure fluids database, a new strictly empirical prediction method is proposed, which performs better for both pure fluids and mixtures database. For the mixtures, the performance of the methods was improved when considering a correction for the mass transfer effects associated to the concentration gradients near the liquid/vapor interface.

Improved General Correlation for Heat Transfer During Gas–Liquid Flow in Horizontal Tubes

Heat transfer to two-component gas–liquid mixtures is needed in many industries but there is lack of a well-verified predictive method. A correlation is presented for heat transfer during flow of gas–liquid nonboiling mixtures in horizontal tubes. It has been verified with a wide range of data that includes tube diameters of 4.3–57 mm, pressures from 1 to 4.1 bar, temperatures from 12 to 62 °C, gravity <0.1% to 100% earth gravity, liquid Reynolds number from 9 to 1.2 × 105, and ratio of gas and liquid velocities from 0.24 to 9298. The 946 data points from 18 sources are predicted with mean absolute deviation (MAD) of 19.2%. The same data were compared to five other correlations; they had much larger deviations. Therefore, the new correlation is likely to be helpful in more accurate designs.

Measured and predicted heat transfer coefficients for boiling zeotropic mixed refrigerants in horizontal tubes

The use of mixed gas working fluids has become common in Joule–Thomson type cryocoolers for a variety of applications. However, there is a scarcity of data currently available with supporting theory capable of predicting the heat transfer coefficients associated with two-phase, multi-component mixtures at cryogenic temperatures. This paper aims to fill this void by providing experimental data for the heat transfer coefficient associated with of multicomponent zeotropic mixtures boiling in small channels over temperatures ranging from 100K to room temperature. The sensitivity of the measured heat transfer coefficient to parameters such as heat flux, mass flux, pressure, tube diameter, and mixture composition is also presented. The results indicate that the heat transfer process is driven, principally, by convective boiling; however, composition, diameter, and surface roughness affect the measured heat transfer coefficient. Evaporating pressure has less relevance compared to the other parameters.The actual experimental data collected as part of this effort and additional data from Nellis et al. (2005) are used to evaluate models to characterize the heat transfer process for boiling zeotropic mixtures in horizontal tubes. The heat transfer coefficient data is predicted well using correlations described by Granryd (1991) and Little (2008). The Granryd correlation is the recommended correlation to predict heat transfer coefficient of zeotropic mixtures because it shows the best accuracy with an Absolute Average Deviation (AAD) of 16% and predicts 83% of the data with a relative error lower than 25%. The Little correlation exhibits greater accuracy at high Reynolds number, high thermodynamic quality, or both; however, its accuracy is substantially reduced for low qualities and low Reynolds number.

Description and validation of the Little correlation for boiling zeotropic mixtures in horizontal tubes from cryogenic to room temperature

The use of mixed gas working fluids has become common in Joule-Thomson (JT) type cryocoolers for a variety of applications operating in temperatures ranging from 80 to 230 K. The thermal efficiency of mixed gas JT cryocoolers is dependent on the optimization of the gas mixture composition. Most optimization methodologies focus on thermodynamic criteria of the cycle because there are very little data or theory currently available regarding the heat transfer coefficients associated with these multi-component mixtures in two-phase regimes at cryogenic temperatures. A generally accepted correlation to predict the local heat transfer coefficient (htc) for mixtures during the boiling process does not exist. Little [1] proposed a correlation to be used on horizontal tubes that shows good agreement with Nellis et al. [2] experimental data of nitrogen-hydrocarbon mixtures. However, it is not clearly shown how the correlation is obtained and how it should be applied. This paper provides a more complete description of the Little correlation and also expands its validation using the experimental data provided by Barraza [3]. The new experimental data include measurements of the local heat transfer coefficient for mixtures comprising 2 component (binary) up to 5 components in the temperature range between 100 K and room temperature. These mixtures are formed from nitrogen-hydrocarbon and argon-fluorocarbon mixtures and evaporate in horizontal tubes with diameters from 0.5 to 3.0 mm for different heat flux, mass flux, evaporating pressure, and composition.

Nucleate boiling at the forced flow of binary non-azeotropic mixtures in horizontal tubes

Analysis of experimental values of heat transfer coefficients obtained through investigation of nucleate boiling of the two-component non-azeotropic mixtures inside the horizontal smooth tubes by various authors is presented. In the zone of nucleate boiling, the experimental data are in good agreement with the calculation dependence.

Local heat transfer measurements of steam/air mixtures in horizontal condenser tubes

Condensation of vapor/noncondensable gas mixtures in horizontal tubes is not well understood because condensate stratification and the multidimensional nature add to the complexity of the phenomena. The heat transfer and fluid flow phenomena in a horizontal condenser tube were experimentally studied, along with the heat transfer reduction effect of a noncondensable gas. The temperature gradient across the condenser tube wall was measured locally to investigate the asymmetrical heat transfer characteristics around the condenser tube periphery. The condensation heat transfer coefficients on the tube top were much greater than the values at the bottom and the noncondensable gas significantly reduced the heat transfer rate.

Flow pattern and pressure drop of pure refrigerants and their mixture in horizontal tube

Two-Phase flow pattern and pressure drop data were obtained for pure refrigerants R134a and R123 and their mixtures as test fluids in a horizontal tube. The flow pattern is observed through tubular sight glasses located at inlet and outlet of the test section. The flow map of Baker developed for air-water two-phase flow at atmospheric pressure failed to predict the observed flow patterns at the higher value of the mass velocity used in the present study. The map of Kattan et al. predicted the data well over the entire region of mass velocity selected in the present study. The measured pressure drop increased with an increase in vapor quality and mass velocity. A new two-phase multiplier was developed from a dimensional analysis of the frictional pressure drop data measured in the present experiment. This new multiplier was found successfully to correlate the frictional pressure drop.

Condensation of Zeotropic Mixtures in Horizontal Tubes: New Simplified Heat Transfer Model Based on Flow Regimes

The need for optimal design of heat exchangers with in-tube condensation of zeotropic refrigerant mixtures has pushed the research of predictive methods in the last years. Some procedures have been developed, based on the Colburn and Drew (1937, Trans. AIChemE 33, pp. 197–215) analysis, that require significant numerical effort and the diffusivity properties of the mixture to calculate the mass transfer resistance in the process and, hence, are rarely used for heat exchanger design. Proposing a modified version of the well-known simplified approach of Silver (1947, Trans. Inst. Chem. Eng. 25, pp. 30–42) and Bell and Ghaly (1973, AIChE Symp. Ser. 69, pp. 72–79) to include the effects of interfacial roughness and nonequilibrium effects, the present study extends the flow-pattern-based model of Thome, El Hajal, and Cavallini (2003, Int. J. Heat Mass Transfer 46, pp. 3365–3387) for condensation of pure fluids and azeotropic mixtures to zeotropic mixtures. By implementing this within the above flow-pattern-based heat transfer model, it leads to an improved method for accurately predicting local mixture heat transfer coefficients, maintaining a clear relationship between flow regime and heat transfer, and achieving both the goals of higher prediction accuracy and low calculation effort. The method has been verified for refrigerant mixtures (both halogenated and hydrocarbon) having temperature glides of 3.5–22°C, that is temperature differences between the dew point and bubble point temperatures (at a fixed pressure and bulk composition).

An Experimental Investigation of Pressure Drop in Flow Boiling of Pure Refrigerants and Their Mixture in Horizontal Tube

Experimental investigation of pressure drop for pure refrigerants R134a and R123 and their mixtures as test fluids was carried out for both an adiabatic and a diabatic two-phase flow in a horizontal tube. The measured frictional pressure drop in the adiabatic experiments increased in the S-shaped curve as equilibrium vapor quality was increased. These data were compared to various correlations proposed in the past for the frictional pressure drop. The homogeneous model considerally underpredicted the present data both for pure fluids and their mixtures in the entire mass flux range 150 to 600kg/m2s covered in the measurements, while Friedel correlation was found rather well to correlate the frictional pressure drop data among compared correlations. However a detailed examination showed Friedel correlation underpredicted the present data in the stratified and stratified-wavy flow regions at low vapor quality and overpredicted in the annular flow region at high quality. A new two-phase multiplier was developed from a dimensional analysis of the frictional pressure drop data measured in the adiabatic experiment. This new multiplier was found successfully to correlate the frictional pressure drop measured in the diabatic flow boiling experiments of pure refrigerants and their mixtures almost with a mean deviation of 20%.

Heat transfer in horizontal tubes during two phase natural circulation with presence of noncondensing gas

The condensation process of steam inside horizontal tubes during natural circulation gains in importance regarding the reactor safety research for existing and future nuclear power plants. Experimental investigations due to the condensation process were realized with the rig HORUS to study the behaviour of water-steam-gas mixtures in horizontal tubes. The paper includes statements regarding the flow and heat transfer conditions inside the tube and the temperature distribution inside the small tube wall. The experiments showed a blockade of the heat transfer area with Nitrogen which is connected with an increasing primary pressure followed by a compression of the Nitrogen and a reentry of steam into the tube. The experiments serve for the creation of an experimental data base. A model development for calculation of the heat transfer is described. The model was implemented in the German thermal-hydraulic code ATHLET. The comparison of calculated data and the measured parameters of HORUS rig show the code improvement for the simulation of noncondensing gases.

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures in horizontal tubes

A study on the prediction of heat transfer coefficient and pressure drop of refrigerant mixtures is reported. Heat transfer coefficients and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis assuming evaporation in horizontal tubes. Results indicate that nucleate boiling is suppressed at qualities greater than 20% for all mixtures, and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8–10% increase in heat transfer coefficients. Some mixtures with large volatility difference exhibit as much as 55% reduction compared to R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32%) and reduced mass flow rates (23%). Other mixtures with moderate volatility difference exhibit 20–30% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall heat transfer coefficient of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10% in transport properties causes a change of less than 6% in heat transfer prediction.