Microfin Tubes Research Articles

Numerical Simulation Based on Heat Transfer Enhancement of Supercritical Fluid in Micro-Fin Tube

With the application of supercritical fluid heat transfer equipment in industrial fields such as solar thermal power generation, chemical industry, aerospace, etc., studying the heat transfer characteristics of supercritical fluid in micro-fin tubes has become a key theoretical basis for the development of micro-fin low-resistance heat transfer enhancement technology. In view of micro-fin tubes with different fin shapes, this paper took into account thermophysical properties of nitrogen under supercritical conditions and completed a numerical simulation study on the heat transfer process of nitrogen in 2 mm micro-fin tubes under supercritical pressure. The temperature field distribution of supercritical nitrogen in the micro-fin tube was analyzed, and the turbulent flow mechanism of the micro-fin was studied. It was found that micro-fin could increase the heat exchange area, destroy the boundary layer, and improve the heat transfer coefficient. This paper took comprehensive heat transfer performance evaluation factor PEC to compare the influence of different fin shapes on heat transfer enhancement performance of the heat exchange unit. It was found that the comprehensive heat transfer factor of the square straight micro-fin tube was about 1.22 times that of the smooth round tube, and PEC of the triangular straight micro tube was about 1.08 times that of the smooth tube. The results suggest that square straight micro-fin tube has significantly superior heat transfer performance than smooth round tube and triangular straight micro-fin tube.

A THEORETICAL INVESTIGATION ON THE EVAPORATIVE HEAT TRANSFER OF CO2 IN SMOOTH AND MICROFIN TUBE

Nowadays, global environmental events such as thinning of the ozone layer and climate changes are increasing. These types of events are not only affecting all creatures living on the Earth, but also decreasing the quality of life. For this reason, natural refrigerants which are not harmful to environment, have been preferred in cooling systems. In this study, heat transfer coefficient of CO2 was investigated during the evaporation process in smooth tube and designed microfin tube. Features of the tube used in evaporator of these cooling systems directly affect to the heat transfer coefficient. The geometric parameters of the microfin tubes are an outer diameter of 9.52 mm, number of fins 50, apex angle of 38°, helix angle of 20° and fin height of 0.12 mm. Theoretical model was created on MATLAB environment. The heat transfer coefficient was investigated based on vapor quality. When theoretical results obtained for microfin tube were compared with experimental results, 6% approach was seen. A theoretical model was created for smooth and microfin tube by selecting heat flux as 10 kw/m^2 and mass flux as 380 kg/m2s and heat transfer coefficients in different evaporation temperatures were compared based on vapor quality. It has been concluded that heat transfer coefficients of microfin tube at 5°C, 0°C and -8°C were 52%, 44% and 34% higher than that of smooth tube, respectively.

Theoretical analysis of annular film flow evaporation inside microfin tube

This study developed a theoretical model of film evaporation heat transfer in the microfin tube under a high vapour quality region. A liquid film flow model in the micro groove was constructed. The thickness and heat transfer characteristics in the thin-film region and meniscus region were analysed. As a result, the heat transfer coefficient increases as the vapour quality and the mass flux increase. Before the dryout occurs, the heat transfer coefficient reaches a peak due to the thinned meniscus film thickness. The theoretical predicted heat transfer coefficients were compared with available experimental data under actual tube geometries and operating conditions, and were well consistent with the experimental data. This result indicated the present theoretical analysis is beneficial for understanding the heat transfer enhancement mechanism in the microfin tubes.

Flow condensation of R-410A in multiport tubes having smooth or micro-finned internal channels

ABSTRACT Three micro-finned and two smooth multiport tubes having the same tube width and height with the microfin tubes were tested using R-410A. For the smooth multiport tube, the tube with smaller hydraulic diameter yielded both higher heat transfer coefficients and larger pressure drops. However, for the microfin tube, results suggested that, fin configuration (number of fins per channel, fin height, etc.) has to be considered in addition to hydraulic diameter. The trend of the pressure drop generally followed the hydraulic diameter – smaller hydraulic diameter tube yielded larger pressure drop. Data are compared with the predictions by existing correlations.

Condensation of an Azeotropic Mixture inside 2.5 mm ID Minitubes

The ongoing miniaturization of air conditioning and refrigeration systems, in order to limit, as much as possible, the refrigerant charge, calls for smaller and smaller heat exchangers. Besides, the new environmental regulations are calling for new pure refrigerants or refrigerants mixtures with lower values of global warming potentials (GWPs). In this context, this paper analyzes the possible implementation of minitubes during condensation of the azeotropic mixture R513A. Two minitubes are tested: a smooth tube with an inner diameter of 2.5 mm, and a microfin tube with an inner diameter at the fin tip of 2.4 mm. The effects of vapor quality (varied in the range 0.10–0.99), of mass velocity (varied in the range 200–1000 kg m−2 s−1), and of saturation temperature (30 °C and 40 °C) on the heat transfer coefficient are investigated. The experimental results indicate that the heat transfer coefficient increases as both vapor quality and mass velocity increase, both in the case of the smooth tube and of the microfin tube, but the slope of the heat transfer coefficient trend respect to vapor quality is higher in the case of the microfin tube. The microfin tube shows, on average, heat transfer coefficients are 79% higher than those of the smooth tube under the same working conditions. Since R513A is a possible substitute of R134a, some experimental data during condensation heat transfer are also compared against those for R134a. Finally, the experimental results are compared against values estimated by empirical correlations available in the open literature.

A numerical investigation of convective condensation in micro-fin tubes of different geometries

A numerical investigation was conducted to determine the effects of fin shape on heat transfer coefficients during in-tube condensation of R410A. Four micro-fin tubes of different geometries were selected for this study, including a straight-finned one and three helical tubes, and two different helical angles of 18° and 31° were included to explore the influence of helical angles. The numerical results were verified by previous experimental results. It was found that there was a positive relationship between heat transfer coefficients and mass velocities and vapor quality, while the helical angle seemed to improve the heat transfer coefficients especially for medium vapor quality. A nearly symmetrical liquid film distribution was found along the circumference for all three micro-fin tubes due to the centrifugal force, while the liquid tends to accumulate at the bottom of the straight-finned tube.

Condensation heat transfer characteristics of a mixture of R1234ze (E) and R152a flowing inside a horizontal micro-fin tube

ABSTRACT The condensation heat transfer characteristics of a new environmental refrigerant mixture R1234ze (E)/R152a (mass ratio 40:60, named NCUR01) in a horizontal micro-fin tube with an outer diameter of 9.52 mm were experimentally investigated. Experiments were conducted at saturation temperatures of 30°C, 35°C and 40°C, heat flux ranged from 3 to 8 kW·m−2, mass flux ranged from 25 to 75 kg·m−2·s−1, and vapor quality up to 1.0. Flow patterns of NCUR01 were estimated based on flow pattern maps. The effects of saturation temperature, heat flux, mass flux, and vapor quality on heat transfer coefficients and friction pressure drops were analyzed. The results demonstrate that the flow patterns of NCUR01 during condensation are predominantly stratified and stratified-wavy. The condensation heat transfer coefficients and friction pressure drops increase with mass flux and mean vapor quality, while decrease with saturation temperature. Heat flux has a positive correlation with heat transfer coefficients, but has little effect on friction pressure drops. In addition, experimental data were compared with predicted values of correlations in the literature, and the correlations with high accuracy were selected.

Condensation heat transfer of R1234yf in a small diameter smooth and microfin tube and development of correlation

Condensation heat transfer of R1234yf inside smooth and microfin tube with 2.5 mm outer diameter are studied experimentally. The experiments are carried out at a saturation temperature of 20°C and 30°C, mass velocity ranging from 50 to 200 kg m−2s−1 and vapor quality ranging from 0-1. The effects of mass velocity, vapor quality, and saturation temperature on condensation heat transfer coefficient are analyzed with R1234yf and R134a. Condensation heat transfer correlation for smooth tube is developed considering present experimental data and other researchers’ data. Correlation is developed using experimental data for 2.5 mm to 10 mm outer diameter tubes and refrigerants are R134a, R1234yf, R123 and R1234ze (E) by including other researchers’ data. Newly developed correlation is successfully predicted experimental data and other sources data within mean deviation of 15 %. Experimental data are also compared with previous correlations proposed for smooth and microfin tube.

Characteristics of R513A evaporation heat transfer inside small-diameter smooth and microfin tubes

This paper investigates the characteristics of R513A during flow boiling. R513A is an azeotropic mixture made of R1234yf and R134a (0.56/0.44 by mass), and it has been proposed as a direct drop in of the common R134a. Experimental tests were run in a wide range of operative conditions: mass velocity in the range 200-800 kg m−2 s−1, heat flux in the range 12-60 kW m−2, for saturation temperatures of 15, 20 and 25°C. Two different mini tubes were tested: a smooth tube with an inner diameter of 2.5 mm, and a microfin tube with an inner diameter at the fin tip of 2.4 mm. Heat transfer coefficients and frictional pressure drops were evaluated from the experimental measurements. The approximately same diameter of the two tubes permitted to highlight the effect of the microfins on the thermal and hydraulic behavior during R513A flow boiling. Finally, experimental heat transfer coefficients and frictional pressure drops were compared against values predicted by correlations available in the literature.

Characteristics of evaporation of propane (R290) in compact smooth and microfinned tubes

Evaporation of flowing propane and the effect of enhanced geometry on heat transfer coefficient and pressure drop is experimentally investigated. One smooth tube and two microfinned tubes with an outer diameter of 5 mm were tested. Heat transfer coefficient and pressure drop were determined for saturation temperatures of 0, 5 and 10 °C for smooth tube, and the effect of using tubes with enhanced geometries was evaluated at heat fluxes ranging between 15 and 33 kW m−2 and mass fluxes between 250 and 500 kW m−2 s−1. The increase of heat transfer coefficient for microfinned tubes relative to smooth tube diminishes with increasing mass flux, while the increase of pressure drop remains unaffected. Comparison of the experimental results with correlations for prediction of pressure drop and heat transfer coefficient demonstrates availability of reliable correlations. Nevertheless, correlations show considerable divergence in accuracy for different microfinned tubes.

Development of a correlation for pressure drop of two-phase flow inside horizontal small diameter smooth and microfin tubes

Abstract This study investigated the pressure drop of adiabatic vapor-liquid two-phase flow inside horizontal smooth and microfin tubes with 2.5 mm outside diameter. R134a and R1234yf were used as working fluid and the experiments were carried out at saturation temperatures of 20 and 30°C, mass velocities ranging from 50 to 200 kg m−2s−1 and vapor qualities ranging from 0.1 to 0.9. The effects of mass velocity, vapor quality, refrigerant properties and tube diameter on the pressure drop were analyzed. Some typical previous pressure drop correlations were used to predict the experimental data. A new two-phase pressure drop correlation for small diameter tubes was proposed based on the experimental data. The new correlation agrees with the available other researchers’ data and it can be broadly applied to the small diameter smooth and microfin tubes, wide range of mass velocity and many kinds of refrigerants.

Enhancing the thermal performance of TiO2/water nanofluids flowing in a helical microfin tube

A hybrid method with a microfin tube and TiO2/water nanofluid with various nanoparticles concentration has been investigated experimentally. The objectives of this work are to characterize thermohydraulic including friction factor and thermal performance, and to develop correlation based on the experimental result. In general, adding nanoparticles to the base fluid causes increased thermal performance. Pressure drop inside the microfin tube at TiO2/water concentrations of 0.05, 0.15, and 0.30 vol% were respectively 73%, 77%, and 80% higher than that in the smooth tube. Furthermore, the pressure drop increased with an increment in Re number. In this work, the developed correlation was proposed to predict practically Nu number. The present findings contribute to our understanding of how to enhance heat transfer using helical microfins inside tubes and nanofluids.

Effect of Refrigerant Properties Estimation on the Prediction Capabilities of Well-Established Two-Phase Heat Transfer and Pressure Drop Models for New Refrigerants

R1233zd(E) and R1224yd(Z) are two HCFO (Hydro Chloro Fluoro Olefin) low-GWP refrigerants recently proposed as substitutes of the HFC (Hydro Fluoro Carbon) fluids commonly used in air conditioning, high temperature heat pumps and ORC applications. A few experimental works regarding the two phase heat transfer in small diameter smooth and microfin tubes have already been published in the open literature. The estimations of thermophysical properties may have a remarkable impact on the prediction of the heat transfer coefficients and pressure drops, especially during two-phase heat transfer. The thermophysical properties databases and predictive tools for these refrigerants are continuously being updated and improved as additional, accurate measurements are published in the open literature. The present paper aims to highlight the impacts of thermophysical properties prediction models on the consistency of estimations of heat transfer coefficient and pressure drop using correlations published in the open literature. As a benchmark, some experimental heat transfer and pressure drop data collected for two new HCFOs, R1233zd(E) and R1224yd(Z) are used. The experimental database has been collected at the Department of Management and Engineering of the University of Padova. For the thermophysical properties estimations, different Equations of States contained in REFPROP as well as a more “simplified” approach based on group contribution methods coupled with a Peng-Robinson Equation of State, are considered.

Saturation temperature effect on heat transfer coefficient during convective boiling in microfin tubes

The growing attention on environmental aspects puts severe constraints on HVAC technology, mainly involving the working fluids and energy efficiency. Both are related to the main limiting factor of HVAC system: the heat transfer process, which, frequently, involves boiling and condensation. To provide suitable tools for the HVAC system design, it is necessary to gather information on the heat transfer characteristics of the new refrigerants. Particular interest is focused on R1234ze(e), because it is one of the viable options to face the R134a phase out. Something similar can be repeated for the low temperature ORC systems using a refrigerant as working fluid. Using a specifically designed test rig, heat transfer coefficient and pressure drop measures were performed during flow boiling of R1234ze(e). The operating conditions were defined by four parameters: the evaporation temperature (5 °C, 35 °C and 45 °C), the mass flux (two values were considered: 160 kg/m2s and 220 kg/m2s), the average thermodynamic quality (which varies between 0.25 and 0.75) and the quality change (which was fixed to 0.2). The uncertainty affecting the pressure drop and the heat transfer coefficient resulted lower than 1% and 5% respectively. The results highlighted that the saturation temperature strongly affects the heat transfer coefficient and the pressure drop: as a consequence of the saturation temperature increase from 5 °C to 45 °C, it was observed up to 40% heat transfer coefficient increase and 90% pressure drop reduction.

Frictional pressure drop during flow boiling in micro-fin tubes: A new general correlation

Due to the wide commercial application of micro-fin tube and eco-friendly refrigerants, a more general frictional pressure drop correlation is required for a better prediction, and this study is aimed at compared existing correlations and provide guides for the furthermore improvement. Experimental data points for frictional pressure gradients during flow boiling of different working fluids in horizontal helical micro-fin tubes were extracted from reported experimental investigations in the literature to evaluate numerous existing frictional pressure drop correlations and specify the applicability of each correlation to meet the urgent demand of extensive application of eco-friendly refrigerants. The database consists of 1014 data points covering eleven refrigerants (R1233zd(Z), R410A, R1234ze(E), R410A, R22, R32, R1234ze(Z), R22, R134a, R245fa and R1234yf included), and the covered operating conditions are as follows: mass velocity 94-888 kg m−2s−1, vapor quality 0.04-0.99, heat flux 3.9-85.2 kW m−2, and equivalent diameter 2.12-11.84 mm. Eight general frictional pressure drop models, including Cavallini et al., Kuo and Wang, Wongsangam et al., modified Friedel, Diani et al., Choi et al., Goto et al., and Rollman and Spindler correlation, were compared against the collected data points. The results indicated that none of these correlations could provide a satisfactory prediction for a general operating condition, though the Diani et al. correlation exhibits the highest predictive accuracy with 73.0% of the data points in an error band of ±30%. A new empirical correlation was developed by introducing the Churchill model for the effect of fins and optimizing the relationship between vapor quality and frictional pressure gradients.

Experimental study on flow boiling heat transfer and pressure drop of R245fa/R141b mixture in a horizontal microfin tube

• The HTC and frictional pressure drop of R245fa/R141b in a micro fin tube are tested. • Effects of mass fraction of R245fa on HTC and frictional pressure drop were investigated. • Experimental data of HTC and frictional pressure drop are compared against predicted values. The heat transfer and pressure drop characteristics of the mixture are the design basis of evaporators and optimization of Organic Rankine Cycle (ORC). As a promising mixture for ORC systems, R245fa/R141b with different mass fractions (1/0, 0.75/0.25, 0.5/0.5, 0.25/0.75 and 0/1) were selected as the working fluid. The heat transfer coefficient (HTC) and frictional pressure drop in a horizontal microfin tube with 8.6 mm equivalent diameter were measured. During the experimental tests, the saturation pressure is vary from 0.15 to 0.25 MPa (saturation temperature from 25.3 to 60.5 ℃), mass fluxes from 203.19 to 306.64 kg•m -2 s -1 , and heat fluxes from 7 to 15 kW m -2 . Additionally, based on the empirical correlations in the literature, we developed a correlation with higher prediction accuracy. The results indicate that the HTC and pressure drop of R245fa/R141b increase with mass flux, while decrease with saturation pressure. The increase of the mass fraction of R245fa results in an increase of the HTC and friction pressure drop for R245fa/R141b. The correlation of Cavallini–Kattan are consistent with the experimental data of heat transfer with mean absolute deviations of 16.68%. When mass flux is less than 200 kg m -2 s -1 and vapor quality is less than 0.2, modified Dittus–Boelter correlation can predict 87.3% data points in the error range of ±30% with a MAD of 6.75%. As to frictional pressure drop, the deviations of ±10.34% experimental data are obtained by the modified Filho correlation.

Experimental Study of Lubricating Oil Effect on R600a Condensation inside Micro-Fin Tubes

In this study, the effect of lubricating oil on the heat transfer characteristics of R600a condensing flow inside a horizontal micro-fin tube is experimentally investigated. The results are presented in three parts, including the characteristics of pure R600a condensation inside a smooth tube, the impact of using a micro-fin tube, and the effect of adding lubricating oil on the condensing heat transfer inside a micro-fin tube. The test condensers are smooth and micro-fin copper tubes of 14.18 mm inner diameter which are installed in a 6 cm polyethylene pipe to form double-pipe heat exchangers. The test conditions include mass velocity of 54 to 90 kg/m2s and vapor quality of 0.03 to 0.78, while condensation pressure is between 550 to 700 kPa. The results show that in the case of pure R600a, using the micro-fin tube may enhance the local heat transfer coefficients up to 79%. In addition, for the refrigerant-oil mixture condensing flow, by increasing the vapor quality, the heat transfer coefficient increases up to a critical value after that it starts to decrease due to the effect of oil on the condensation liquid film.

Diabatic visualization study of R245fa two phase flow pattern characteristics in horizontal smooth and microfin tube

This study investigates the flow pattern characteristics of R245fa in the smooth and microfin tubes under diabatic conditions. A novel electromagnetic induction system was designed to obtain visualization data in the transparent test section with a controlled heat flux. The experiments were conducted with mass fluxes of 50–240 kg/m2 s, heat fluxes of 5.65–14.87 kW/m2 and a saturation pressure Psat = 0.2 MPa. Six flow patterns were identified in both smooth and microfin tubes. The effects of the vapor quality, mass flux and heat flux on the flow patterns were analyzed. Compared with the smooth tube, the transition from intermittent to wavy-annular flow occurred at lower vapor quality in the microfin tube. Moreover, the liquid film was spread, and the shrinkage of the heated thin film was restrained in the microfin tube, so it can significantly improve the evaporation heat transfer performance. Six predictive models were compared with the present flow pattern map. The Frl = cXttn format criterion was suitable for the intermittent/wavy-annular transition of R245fa in horizontal tubes.

R513A condensation heat transfer inside tubes: Microfin tube vs. smooth tube

The imminent phase-down of the common refrigerant R134a is calling for lower GWP alternatives. Real alternatives must have a lower global warming impact and they should be not flammable. In this context, R513A (azeotropic mixture made of R1234yf and R134a at 56% and 44% by mass) has been proposed as alternative to R134a due to its similar thermodynamic and transport properties and due to its lower GWP.This paper proposes a direct comparison between the thermal performances of a 3.5 mm ID smooth tube and those of a 3.4 mm ID microfin tube, during R513A condensation under the same working conditions of vapor quality (from 0.10 to 0.99), of mass velocity (from 100 to 1000 kg m−2 s−1), and of saturation temperature (30 °C and 40 °C). The comparison permits to highlight in which working conditions the microfin tube leads to a real heat transfer augmentation which is higher than the mere increase of heat transfer area. In the end, the experimental heat transfer coefficients, both for the smooth tube and for the microfin tube, are compared against values calculated with empirical correlations from the open literature.

Condensation heat transfer of R290 in micro-fin tube with inside diameter of 6.3 mm

ABSTRACT An experimental investigation on condensation heat transfer of R290 with inside diameter of 6.3 mm in horizontal micro-fin tube was presented. Effects of each case on the condensation heat transfer coefficient of R290 were analyzed at saturation temperatures of 40°C, 50°C, and 55°C with heat flux ranging of 3–8 kW/m2, mass flux ranging of 100–250 kg/(m2·s) and vapor quality up to 1.0. The results show that the condensation heat transfer coefficients increase with mass flux and heat flux, however decrease with saturation temperature. In addition, compared with the smooth tube, the condensation heat transfer coefficients of R290 in the micro-fin tube increase by about 25%. Comparing the experimental data, the condensation heat transfer correlations of Yu et al. has high accuracy.