Horizontal Microfin Tubes Research Articles

Optimum fin geometries on condensation heat transfer and pressure drop of R1234ze(E) in 4-mm outside diameter horizontal microfin tubes

This study experimentally investigated the condensation heat transfer and pressure drop characteristics of R1234ze(E) inside horizontal small-diameter (4.0-mm) microfin tubes with three different types of fin geometries. The three fin geometries were as follows: 40 fins with a fin height of 0.18 mm and a helix angle of 18°, 50 fins with a fin height of 0.15 mm and a helix angle of 12°, and 50 fins with a fin height of 0.12 mm and a helix angle of 25°. The experiments were carried out for a range of mass velocities from 50 to 400 kg m−2 s−1 and at a saturation temperature of 35 °C. The heat transfer coefficient of the microfin tubes was compared to those of smooth tubes and was evaluated in terms of the heat transfer enhancement factor. The effects of different fin geometry parameters, such as the number of fins, fin height, and helix angle, on the heat transfer and pressure drop were investigated. The heat transfer coefficient increased at 50 kg m−2 s−1 as the number of fins increased. Fin height exerted the greatest influence on heat transfer enhancement at 200 kg m−2 s−1. The measured heat transfer coefficient and pressure drop were compared with previous correlations, and the practical effectiveness of the results was verified in terms of the previous our correlations for small-diameter microfin tubes.

Experimental heat transfer coefficient during condensation of R-410A in horizontal micro-fin tubes

Condensation heat transfer coefficient has been calculated experimentally in microfin tubes of outer diameter 9.52mm with helix angles 15° and 18°. Condensation of refrigerant R-410A include mass fluxes from 200 to 600 kg/m2s, vapour qualities between 0.1 to 0.9 and saturation temperature at 35 °C and 40°C. The experimental results indicate that the average heat transfer coefficients of R-410A in micro-fin (MF) tubes were increased 1.15-1.47 times larger than those of smooth tube (ST). The experimental results are compared with the existing correlations of heat transfer coefficient proposed by other authors. The comparison indicated good agreement with these existing correlations within ±30%. A correlation has also been developed for predicting condensation heat transfer coefficient in micro-fin tubes.

Single-Phase Heat Transfer of R245fa + Lubricant Oil Mixtures Inside Horizontal Smooth and Microfin Tubes

The effect of lubricant oil on the single-phase heat transfer of R245fa inside horizontal smooth and microfin tubes was experimentally investigated. Tests were conducted using a horizontal smooth tube with an inner diameter of 8.32[Formula: see text]mm and two microfin tubes with equivalent diameters of 8.9 and 8.7[Formula: see text]mm, fin height of 0.12 and 0.18[Formula: see text]mm, and number of fins of 65 and 85, respectively. The heat transfer coefficients were measured for pure R245fa and R245fa + lubricant oil mixture in the Reynolds number range of 2000–10000, heat fluxes of 5 and 10[Formula: see text]kWm[Formula: see text], and refrigerant temperatures of 30∘C and 45∘C at the test section inlet. Oil concentration was varied at 1, 2 and 5[Formula: see text]wt.%. The results showed that the heat transfer of pure R245fa agreed well with previous correlations. However, the heat transfer coefficients of the R245fa + lubricant oil mixtures were lower than those of the pure R245fa, and decreased with an increase in oil concentration for both smooth and microfin tubes. The reduction in heat transfer was more marked for the HF tube than for the smooth and LF tubes. For the HF tube, the decrease in heat transfer was apparently more pronounced at higher oil concentrations from 2 to 5[Formula: see text]wt.% owing to immiscibility between the oil and refrigerant.

Condensing heat transfer of pure refrigerants and refrigerant mixtures flowing within horizontal microfin tubes: A new model

A new model is developed to predict the heat transfer coefficients (HTCs) of pure refrigerants and refrigerant mixtures condensing within horizontal microfin tubes. A 1312–point experimental database from 22 sources was compiled for this purpose. The data comprise CO2, R1234yf, R1234ze(E), R134a, R22, R407C, R404A, and R410A, 2.64–8.98 mm diameter tubes, −25 °C to 50 °C saturation temperatures, vapor qualities from 0.02 to 0.98, reduced pressures from 0.16 to 0.81, and heat and mass fluxes ranging from 1.79 to 98.1 kW/m2 and 49.0 to 872.5 kg/m2 s, respectively. To develop the model, one hundred twenty unique dimensionless parameters pertinent to condensing flows in microfin tubes were first selected. Multi-variable regression analysis was then applied to identify the most significant of these variables influencing the condensation Nusselt number. On an overall basis, the new model performs significantly better than any of six extant correlations. The new model was then assessed for different refrigerants, and many ranges of diameter, heat and mass flux, vapor quality, and reduced pressure. Generally, the new model is reasonably accurate, with mean absolute deviations generally smaller than 20–25%. The model also predicts annular flow HTCs fairly accurately. Parameter ranges in which Cavallini et al. (2009), the best of the extant correlations, gives better predictions than the new model and those in which more data would be useful for further analysis are also identified. In general, the new model can be recommended for a large variety of refrigerants under practically significant operating conditions.

An Experimental Investigation of Flow Patterns During Condensation of HFC Refrigerants in Horizontal Micro-Fin Tubes

Condensation heat transfer coefficients and flow regimes in two different horizontal micro-fin tubes are examined during the condensation of refrigerants R-134a and R-410A. The present investigation has focused on determination and prediction of condensation heat transfer coefficients and finding the interrelation between heat transfer coefficients and the prevailing flow regimes. During flow visualization, flow regimes have been captured using borosilicate glass tube at inlet and outlet of the test condenser using high speed digital camera. Stratified, stratified wavy, wavy annular, annular, slug and plug flows have been observed at different mass fluxes and vapor qualities of the refrigerants. The observed flow regimes are compared with the existing flow regime maps proposed by Breber et al. [Prediction of horizontal tube side condensation of pure components using flow regime criteria, J. Heat Transfer 102 (1980) 471–476], Tandon et al. [A new flow regime map for condensation inside horizontal tubes, J. Heat Transfer 104 (1982) 763–768.] and Thome et al. [Condensation in horizontal tubes, part 2: New heat transfer model based on flow regimes, Int. J. Heat Mass Transfer 46 (2003) 3365–3387.] Thome et al. [Condensation in horizontal tubes, part 2: New heat transfer model based on flow regimes, Int. J. Heat Mass Transfer 46 (2003) 3365–3387.] flow regime map shows good agreement with experimental data.

Experimental investigation of the heat transfer of supercritical R134a in a horizontal micro-fin tube

The heat transfer of supercritical R134a was investigated experimentally in a horizontal micro-fin tube for conditions related to the supercritical heater in trans-critical ORC systems, including the effects of heat flux, pressure, and mass flux. The results showed that Nutop was smaller than Nubottom due to the buoyancy. The effects of pressure and heat flux on Nu were found to differ in different bulk enthalpy regions and tube positions. Large mass fluxes increase Nu while buoyancy has a stronger effect at low mass fluxes. Previous correlations were evaluated with new correlations developed for Nu in a micro-fin tube. The Bishop correlation gave reasonable predictions of Nutop, but none of the correlations could accurately predict Nubottom. A pair of more accurate correlations were developed for the top and bottom Nusselt numbers using the dimensionless number πc to account for the effects of the drastic property variations. The new correlations have a mean deviation of 10.3% for Nutop, and 17.8% for Nubottom, with 96.1% of the experimental data for Nutop and 83.8% of the data for Nubottom well predicted with deviations smaller than 30%.

Two phase heat transfer and flow regimes of R-134a and R-410A during condensation in horizontal micro-fin tubes

Two phase heat transfer and flow regimes of R-134a and R-410A during condensation in horizontal micro-fin tubes

Experimental comparison of the heat transfer of supercritical R134a in a micro-fin tube and a smooth tube

Measured heat transfer rates of supercritical R134a were compared for flows in a horizontal micro-fin tube and in a smooth tube for mass fluxes from 100 to 700 kg m−2 s−1, heat fluxes from 10 to 70 kW m−2, and pressures from 4.26 to 5 MPa. The results showed that the micro-fin tube wall temperatures had smaller changes as the heat flux was increased than on the smooth tube. The heat transfer coefficients along the top of the smooth tube are reduced more by the buoyancy effect than in the micro-fin tube. Then, the results were used to evaluate four buoyancy criteria for horizontal flows for both the smooth tube and micro-fin tube with the GrbReb2.0ρbρwxd criterion found to give the best prediction accuracy for both types of tubes. The threshold for the onset of the buoyancy effect in the smooth tube was around 100 but was around 2000 in the micro-fin tube, which indicates that the effect of buoyancy is greatly reduced in the micro-fin tube. Comparisons of the measured heat transfer coefficients for all 5520 experimental data points showed that the heat transfer coefficient in the micro-fin tube was 1.68 times that in the smooth tube on the top and 1.59 times that in the smooth tube on the bottom.

Experimental Heat Transfer Coefficient and Pressure Drop during Condensation of R-134a and R-410A in Horizontal Micro-fin Tubes

Condensation heat transfer coefficients and pressure drops of HFC refrigerants R-134a and R-410A have been investigated experimentally in smooth and micro-fin tubes (helix angles 18[Formula: see text] and 15[Formula: see text]) of outer diameter 9.52[Formula: see text]mm at mass fluxes from 200 to 600[Formula: see text]kg/m[Formula: see text]s, vapor qualities between 0.1 and 0.9 and at saturation temperatures of 35[Formula: see text]C and 40[Formula: see text]C. Results showed that the heat transfer coefficients of R-134a and R-410A inside micro-fin tubes were 1.21–1.82 and 1.15–1.47 times higher and frictional pressure drops were 2.11–2.56 and 1.62–2.12 times higher than those of smooth tubes. These experimental results are compared with the existing heat transfer and frictional pressure drop correlations proposed by different researchers. The comparison showed fairly good agreement with these existing correlations within [Formula: see text]30%. A new correlation has also been proposed for predicting heat transfer coefficient in micro-fin tubes. The oil concentrations measured for refrigerants R-134a and R-410A varied in the range of 1.3–1.5%, respectively.

Flow condensing heat transfer of R410A, R22, and R32 inside a micro-fin tube

ABSTRACT An experimental study of condensation heat transfer characteristics of flow inside horizontal micro-fin tubes is carried out using R410A, R22, and R32 as the test fluids. This study especially focuses on the influence of heat transfer area upon the condensation heat transfer coefficients. The test sections were made of double tubes using the counter-flow type; the refrigerants condensation inside the test tube enabled heat to exchange with cooling water that flows from the annular side. The saturation temperature and pressure of the refrigerants were measured at the inlet and outlet of the test sections to defined state of refrigerants, and the surface temperatures of the tube were measured. A differential pressure transducer directly measured the pressure drops in the test section. The heat transfer coefficients and pressure drops were calculated using the experimental data. The condensation heat transfer coefficient was measured at the saturation temperature of 48°C with mass fluxes of 50–380 kg/(m2s) and heat fluxes of 3–12 kW/m2. The values of experimental heat transfer coefficient results are compared with the predicted values from the existing correlations in the literature, and a new condensation heat transfer coefficient correlation is proposed.

A new heat transfer model for flow boiling of refrigerants in micro-fin tubes

Experimental data points of heat transfer coefficient during flow boiling of refrigerants in horizontal micro-fin tubes were collected from literature and our previous experimental work to develop a new general heat transfer model. The database contains 2221 data points covering nine refrigerants, and the involved operation conditions are as follows: mass velocity 47–835 kg m−2 s, vapor quality 0.05–0.98, heat flux 3.9–85.2 kW m−2, and fin-root diameter 2.64–11.98 mm. Six existing general heat transfer correlations for micro-fin tubes were evaluated using the present database. It was found that these correlations were incapable of providing a satisfactory prediction. Among them, the correlation proposed by Mehendale shows the best predictive ability, predicting 70.3% and 91.3% of the database within ±30% and ±50% error bands, respectively. Then a new model based on the present database was generalized by modifying Cavallini et al. correlation, and a good agreement between the new model and the database was obtained. The new model can predict 82.1% of data points within ±30% error bands with a mean absolute deviation of 18.6%, and its parametric-trend predictive ability under varied operating conditions was also verified using several datasets.

Experimental study on heat transfer condensation of R1234ze(E) and R134a inside a 4.0 mm OD horizontal microfin tube

Small sized microfin tubes are a smart option to keep the refrigerant charge under the stricter and stricter values depicted by the new environmental laws. Despite of the large number of papers dedicated to phase change phenomena inside traditional microfin tubes with diameters larger than 5 mm, small sized microfin tubes are still a topic which deserves further studies. In this context, this paper proposes experimental results of heat transfer coefficients and frictional pressure drops collected during condensation of R1234ze(E) and R134a inside a small sized microfin tube with an outer diameter of 4.0 mm. Experimental tests were carried out for mass velocities from 100 to 1000 kg m−2 s−1, vapor qualities from 0.1 to 0.99, at saturation temperatures at the inlet of the test section of 30 °C and 40 °C. The range of working conditions permitted to better understand how each operative parameter affects the thermal and hydraulic behavior of the microfin tube during condensation heat transfer. The experimental results of heat transfer coefficient and frictional pressure drop were finally compared against values predicted by empirical correlations available in the open literature.

A new heat transfer coefficient correlation for pure refrigerants and near-azeotropic refrigerant mixtures flow boiling within horizontal microfin tubes

A new correlation is presented to predict the heat transfer coefficients (HTCs) of pure refrigerants and near-azeotropic refrigerant mixtures undergoing flow boiling within horizontal microfin tubes. This is accomplished by first putting together a 2622-point experimental database from 25 sources. The data includes CO2, R12, R1234yf, R1234ze(E), R134a, R22, R32, R404A, and R410A, 2.1–14.85 mm fin root diameter tubes, −20 °C to 39.9 °C saturation temperatures, vapor qualities from 0 to 1, reduced pressures from 0.03 to 0.78, and heat and mass fluxes ranging from 1 to 58.7 kW/m2 and 25 to 820 kg/s m2, respectively. The entire database was randomly divided into two subsets, 90% of the data being used to develop the new correlation, with the remaining 10% used to validate it. The correlation was developed in two steps. Thirty-eight unique dimensionless parameters pertinent to flow boiling in microfin tubes were first selected. Multi-variable regression analysis was then applied to identify the most significant variables influencing the flow boiling Nusselt number. The new correlation was evaluated and compared with six extant correlations on an overall basis as well as for several bins of parameters. Overall evaluation for the entire database, for the 10% test data, and for points with experimental uncertainties less than ±17% shows that the new correlation is significantly better than any of the extant correlations. For these three overall assessments, the new correlation predicts 71.5–80.5% of the data within ±30% error bands, with a mean absolute deviation of 21.5 to 25.2%. Additionally, the distribution of the entire database and the performance of the new correlation was examined relative to refrigerant, and for various bins of fin root diameter, Dr, mass flux, G, all-liquid Reynolds number, Real, reduced pressure, PR, saturation temperature, Tsat, heat flux q, and vapor quality, x. In general, the new correlation shows reasonably good predictions, which are better than those of the existing correlations for most parameter bins, with MAD values generally smaller than 35%. Based on the bin analysis, parameter ranges in which one of the extant correlations gives better predictions than the new correlation are also identified. With these exceptions in mind, the new correlation can be used as a reasonably reliable predictive tool for a large variety of refrigerants under different operating conditions of practical interest.

Experimental Investigation of Condensation Heat Transfer and Adiabatic Pressure Drop Characteristics Inside a Microfin and Smooth Tube

Experiments on condensation heat transfer and adiabatic pressure drop characteristics of R134a were performed inside smooth and microfin horizontal tubes. The tests were conducted in the mass flux range of 50[Formula: see text]kg/m2s to 200[Formula: see text]kg/m2s, vapor quality range of 0 to 1 and saturation temperature range of 20[Formula: see text]C to 35[Formula: see text]C. The effects of mass velocity, vapor quality, saturation temperature, and microfin on the condensation heat transfer and frictional pressure drop were analyzed. It was discovered that the local heat transfer coefficients and frictional pressure drop increases with increasing mass flux and vapor quality and decreasing with increasing saturation temperature. Higher heat transfer coefficient and frictional pressure drop in microfin tube were observed. The present experimental data were compared with the existing well-known condensation heat transfer and frictional pressure drop models available in the open literature. The condensation heat transfer coefficient and frictional pressure drop of R134a in horizontal microfin tube was predicted within an acceptable range by the existing correlation.

Experimental investigation of refrigerant condensation heat transfer characteristics in the horizontal microfin tubes

The condensation heat transfer characteristics for refrigerant R22 and near-azeotropic mixture, R410A are investigated in the horizontal smooth tube and microfin tube, respectively. The 9.52-mm-OD tube with active length 3.4m and 5-mm-OD tube with 5m are selected for condensation test cooled by the heat transfer fluid (water) circulated in a surrounding annulus. A refrigerant superheating 4–7°C is set at the test section inlet and the mass flux range is 100–400kgm−2s−1 for 9.52-mm-OD tube, and 300–550kgm−2s−1 for the 5-mm-OD tube. The heat transfer coefficient (HTC) of the microfin tube is around 1.65–2.55 times of that of the smooth one. The microfin one yields ∼30% higher pressure drop than the smooth one. A lower condensation saturation temperature can generate a higher HTC and a higher pressure drop. 5-mm-OD tube produces both the higher HTC and pressure drop than the 9.52-mm-OD one due to the increased effect of the surface tension and the sheer stress at the gas-liquid interface. R22 performs marginally better for HTC than R410A and the former has a ∼40% higher pressure drop. In addition, it is necessary to collect more data source and make a generally-accepted and more accurate correlation in the near future.

A new general correlation for frictional pressure drop during condensation inside horizontal micro-fin tubes

Experimental pressure drop data of condensation from present study and literature were collected to develop a new frictional pressure drop correlation for horizontal micro-fin tubes. The collected database contained 481 data points, covering nine refrigerants including CO2 at average saturated condensing temperatures ranging between 14 and 65°C, with mass velocities ranging from 50 to 800kg/m2s, and average vapor qualities from 0.11 to 0.91. The hydraulic diameter of micro-fin tubes varied from 2.16 to 5.67mm and was employed in the calculation of Reynolds number. The Fanning frictional factor was calculated by adopting the Churchill model with the empirically fitted relative roughness. Four existing pressure drop correlations developed for micro-fin tubes were evaluated by the database for condensation in micro-fin tubes. The correlation proposed by Cavallini et al. was the best prediction model among them, predicting 85.6% of the collected data points within the 30% error band. In addition, a new correlation based on the Martnelli parameter Xtt modified by incorporating the reduced pressure was proposed to predict the present database, which showed a good agreement. Finally, some corresponding experimental work was conducted for evaluating the reliability of the new prediction model.

Flow Condensing Heat Transfer of R410A inside a Micro-fin Tube

An experimental study of condensation heat transfer and pressure drop inside a horizontal micro-fin tubes are conducted using the azeotropic mixture refrigerant R410A as a test. This study was especially focused on influence of heat transfer area on the condensation heat transfer coefficient and pressure drop. The test sections were double tube of counter-flow type; the refrigerant was flowing condensed inside of test tube by heat exchange with cooling water flowing in the annular side. The temperature and pressure of refrigerant were measured at inlet and outlet of test section, and the temperature surface of tubes were measured. The pressure drop in the test section were directly measured by a differential pressure transducer. The heat transfer coefficient and frictional pressure gradient were calculated using the experimental data. The heat transfer coefficient and pressure drop were measured at the saturation temperature of 48°C within mass fluxes of 50-380kg/ (m2s) and heat fluxes of 6-12kW/m2. The pressure drop and heat transfer coefficient characteristics of the R410A were examined and compared with those of the R22 at the same test conditions. The experiment results are compared with the existing heat transfer coefficient correlations, and a new correlation is developed with good prediction.

Numerical simulation of R410A condensation in horizontal microfin tubes

ABSTRACTThe heat transfer characteristics of condensation for R410A inside horizontal microfin tubes with 0° and 18° helical angles were investigated numerically. The numerical data fit well with the experimental results and with the empirical correlations. The results indicate that local heat transfer coefficients increase with increasing mass flux, vapor quality, and helical angle. The heat transfer enhancement in the helical microfin tubes is more pronounced at higher mass flux and vapor quality. The centrifugal force induced by the microfin with a 18° helical angle tends to spread the liquid from the bottom to the top, leading to a nearly symmetrical liquid–vapor interface during condensation. Swirling flows in the liquid phase are observed in the tube with the 18° helical angle, but the liquid phase tends to flow to the bottom due to gravity in the tube with the 0° helical angle.

Study on flow boiling heat transfer characteristics of R161/oil mixture inside horizontal micro-fin tube

The research on alternative refrigerants has been one of the urgent issues in the refrigeration and air conditioning industry for the sake of reducing ozone depletion and global warming effect. R161, as a refrigerant with good environmental acceptability (GWP=12, ODP=0), good thermo-physical properties, good heat transfer and flow characteristics, and highly similar thermo-physical properties to R22, has attracted broad attention as a substitute of R22 and R407C. In this paper, the boiling heat transfer and pressure drop characteristics of R161/POE mixture in a 7mm O.D. horizontal micro-fin tube were investigated by experimental methods. The local heat transfer coefficients and pressure drops were measured at mass flux range of (100∼250) kg/(m2·s), heat flux range of (11.76∼52.94)kW/m2, bubble point temperature range of (−5∼8)°C, and oil concentration range of (0∼5)%. The results showed that: (1) at low vapor quality region, the local heat transfer coefficient increased with oil concentration, while at the high vapor quality region, it decreased rapidly with the increase of oil concentration; (2) the pressure drop increased with the increase of oil concentration, mass flux and heat flux; (3) the influence of oil on heat transfer coefficient and pressure drop decreased with the increase of mass flux. A new correlation to predict the local boiling heat transfer of R161/oil mixture inside small micro-fin tubes was developed on the basis of the local properties of refrigerant/oil mixture and structure of enhanced tube, and the predicted data agreed with the experimental data within the deviations of (−15∼+20)%.

New models for heat transfer and pressure drop during flow boiling of R407C and R410A in a horizontal microfin tube

The heat transfer of R407C and the pressure drop of R407C and R410A during flow boiling in a horizontal microfin tube were investigated. The measurements were conducted at saturation temperatures between −30 and +10 °C. The heat flux was varied between 1000 and 20,000 W/m2. The mass flux amounted to 25–300 kg/s m2 and the vapor quality was between 0.1 and 1.0.The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long. It is heated electrically.A total number of 1614 heat transfer measurements and 952 pressure drop measurements was conducted.A new method for deriving correlations is shown. With this new method a new correlation for the Nusselt number is being derived. The mean deviation of the calculated from the measured values is 9.8%. Within the range of ±30% there are 94.2% of the measured values.A new correlation for the total pressure drop is derived. The mean deviation of the calculated from the measured values is 18.1%. Within the range of ±30% there are 83.1% of the measured values.