Microfin Tubes Research Articles

Flow condensation heat transfer correlation development of R32-oil mixture inside a micro-fin tube

The air-conditioners employ R32 as refrigerant because it has low GWP. The heat transfer performance of R32 in the heat exchangers would be influenced by the lubricating oil cycled with R32. In order to obtain the quantitative effect of oil on the flow condensation heat transfer characteristics, flow condensation heat transfer correlation of R32-oil mixture is needed. In this study, an experimental rig for testing the flow condensation heat transfer coefficients of R32-oil mixture is established. The test conditions covering the actual working conditions of air-conditioners are designed, including the condensation temperature ranging from 40 to 50 °C, mass flux ranging from 100 to 400 kg m − 2 s − 1, vapor quality ranging from 0.1 to 0.9 and oil concentration ranging from 0 % to 5 %. The results show that the flow condensation heat transfer coefficients decrease with the increasing of oil concentration because of the high viscosity of the oil. A new condensation heat transfer correlation is developed based on the enhanced structure of the micro-fin tube and physical properties of the working fluid, and it agrees with 82 % of the experimental results within a deviation of ± 20 %.

Two-phase flow condensation heat transfer characteristics of R-134a inside three-dimensional cylindrical micropillar enhanced tube

This paper presents an experimental investigation for evaluation of heat transfer coefficient, frictional pressure drops and flow regimes with their transitions during the condensation of R-134a inside a smooth, two microfins (MF1 and MF2) and a newly developed micropillar (MP) tubes. The experiments are carried out for a range of mass flux at an average saturation temperature of 35°C. Micropillar tube has produced the maximum heat transfer coefficient, while the microfin (MF1) tube resulted the highest frictional pressure gradient compared to other tubes. The experimental observations of microfin and micropillar tubes are plotted on the mass flux versus vapor quality flow map and Taitel and Duckler flow map to discuss the transitions within different flow patterns achieved. Visual inspection of the flow regimes with varying mass flux and vapor quality showed stratified-wavy, intermittent, and annular flow regimes. The occurrence of annular flow is found to be more in MF1 and MP tube. A performance evaluation factor (PEF) is defined to evaluate the thermal-hydraulic performance of the tubes. Using the same, it has been observed that the newly developed MP tube has a PEF value greater than unity which suggest augmentation of heat transfer coefficient at the expense of lesser pressure drop penalty.

A comprehensive study with high-speed camera assisted visualizations of HFO-1234ze(E) condensation inside an enhanced tube

Flow condensation of HFO-1234ze(E) inside a helical micro-finned tube of 4.28 mm ID underwent experimentation. Flow patterns were visualized with a high-speed camera while varying vapor qualities from 0.01 to 0.95 and mass fluxes from 100 to 400 kg m−2 s1, at saturation temperature of 30 °C. Intermittent, stratified-wavy, transitional, and annular flow pattern types were identified. The flow pattern map of Doretti et al. (validated by Cavallini et al.) accurately predicted the flow patterns. Heat transfer coefficients (HTCs) and frictional pressure drops (FPDs) were also measured for mass fluxes ranging from 100 to 600 kg m−2 s−1 at saturation temperatures of 30 °C and 40 °C. Both HTC and FPD elevated with higher mass fluxes and vapor qualities which stemmed from the presence of observed shear stress-driven annular flow patterns. HTC and FPD are generally higher at lower saturation temperatures. Experimental HTC and FPD have been compared against values estimated with empirical models. Furthermore, the average enhancement factor of the micro-finned tube was 2.94 with an area enhancement factor of 2.39. Plotting enhancement factor versus non-dimensional gas velocity provided detailed insight into the augmentation effect of micro-finned tubes under different operating conditions.

Experimental study and general correlation for condensation heat transfer coefficient inside microfin tubes

This study investigated the condensation heat transfer characteristics of the pure refrigerants R1234yf and R32 in a microfin tube with a small diameter of 3.5 mm outside diameter and 3.18 mm inner mean diameter. Experiments were carried out at mass velocities between 50 and 200 kg m-2 s-1, saturation temperatures of 20 and 30 °C, and vapor quality between 0 and 1. The effects of vapor quality, mass velocity, saturation temperature, and refrigerant properties on condensation heat transfer characteristics were investigated. A new correlation is also developed to predict condensation heat transfer coefficients that can be applied to horizontal microfin tubes with inner diameters between 2.5 and 8.9 mm. The new correlation is validated with the database containing 1240 experimental data points from 10 independent research groups. The comparison results show that the three existing correlations overestimate the heat transfer coefficient data with mean deviation greater than 30.0 %. Meanwhile, the new correlation predicted the present experimental data and other researchers' data with mean deviation of 19.8 %.

Effect of micro-fin tube geometry parameters on evaporator material savings

A Performance Evaluation Criteria (PEC) analysis is presented that explores the influence of the micro-fin tube geometry on the required material volume. The analysis was done using a local flow boiling model, which can quantify the effect of fin parameters and the tube diameter. The PEC was done while requiring that the compared surfaces satisfy the same heat duty and pressure drop. The following tube parameters and ranges were explored: tube diameter (3 mm ≤ Do ≤ 6 mm), fin base thickness (0.12 mm ≤ tbf ≤ 0.29 mm), helix angle (6° ≤ α ≤ 30°), fin apex angle (11° ≤ β ≤ 22°), total number of fins (43 ≤ nf ≤ 70), and the fin height (0.11 mm ≤ e ≤ 0.3 mm). The tube that provided the smallest material requirements within the examined parameter ranges had: Do = 3 mm, tbf = 0.12 mm, α = 6°, β = 11°, nf = 43, and e = 0.3 mm. In general, small Do and tbf and large nf and e provided the best performance and work to minimize the required material volume to provide a given heat duty and pressure drop. The helix and the fin-tip apex angles were found to have very little influence on the performance for the examined range of parameters and angles associated with the lowest production cost should be chosen.

Augmentation of pool boiling heat transfer on tube bundles using metal foam

Pool boiling on a tube bundle is one of the most important heat transfer modes in several industrial applications, including steam generators, shell and tube heat exchangers for waste heat recovery, and desalination. Although several enhanced tubes (e.g., external micro-finned tubes) have been extensively studied and commercialized, the studies that pertain to the metal foam enhanced tube bundles are limited in the open literature. The objective of the present study is to perform an experimental study to analyze the pool boiling heat transfer characteristics of a metal foam tube bundle and compare its performance with that of a tube bundle with no enhancement. The performance of the metal foam tube bundles with different porosities (81%, 75%, and 62%) is compared against the conventional bare tube bundle. The results showed that the heat transfer coefficients of the metal foam tube bundles are 100–212% higher than those of the bare tube bundle. Among the different porosities, metal foam with 75% porosity showed a higher heat transfer coefficient. Furthermore, the wall temperature of the metal foam tubes is nearly 5–14⁰C lower than that of the bare tubes. When compared with a tube pitch of 25.4 mm, a tube pitch of 19.05 mm showed a maximum of 9% and 14% enhancement in bare and metal foam tube bundles, respectively.

Boiling heat transfer and pressure drop of R290 in a micro-fin tube

The present study focuses on the analysis of heat transfer and pressure drop of R290 in a horizontal micro-fin tube. In this regard, experiments were conducted, and flow patterns, heat transfer coefficient, and friction pressure drop were analyzed under various conditions. A wide range of conditions was studied, including saturation pressures ranging from 0.215 to 0.415 MPa, mass fluxes ranging from 70 to 190 kg m−2 s−1, heat fluxes in the range of 10.6–73.0 kW m−2, and vapor qualities from 0 to 0.96. The performed analyses revealed four flow patterns, including bubbly flow, plug flow, slug flow, and annular flow. It is found that the onset of dry-out occurs when the liquid film thickness is equal to the fin height and occurs earlier at high heat fluxes and low saturation pressures. The maximum heat transfer coefficient and friction pressure drop occur at the onset of dry-out, and the heat transfer coefficient exhibits a significant correlation with the heat flux. The obtained results were verified using the heat transfer correlation of Cavallini and the friction pressure drop correlation of Rollman, where the average relative deviation (ARD) was −24.01% and −8.65%, respectively.

Development of boiling flow pattern map and heat transfer correlation of R32-oil mixture inside a horizontal micro-fin tube

R32 has been widely applied in room air-conditioners (ACs) because of its low GWP, and its heat transfer characteristics in refrigeration cycles may be affected by the lubricating oil mixed in R32. To quantitatively evaluate the effect of oil on the flow boiling heat transfer characteristics of R32, the flow pattern map and heat transfer correlation of R32-oil mixture are needed. In this study, an experimental rig for R32-oil mixture is established, in which the boiling flow patterns are observed and the heat transfer coefficients are measured. The experimental conditions cover the evaporation temperature ranging from -5 to 15 °C, mass flux ranging from 100 to 400 kg m−2s−1, vapor quality ranging from 0.1 to 0.9 and oil concentration ranging from 0% to 5%. A total of 80 flow patterns and 336 heat transfer coefficients have been obtained under the above experimental conditions. The results show that the flow patterns of R32-oil mixture contain stratified wavy flow, intermittent flow, annular flow and dryout flow, and the presence of the oil could promote the transformation from stratified wavy flow to annular flow; the heat transfer would be enhanced by the oil under lower vapor qualities and be deteriorated under higher vapor qualities. The boiling flow pattern map is developed based on observation results, which could well predict the flow patterns under the experimental conditions. The heat transfer correlation is developed based on the flow patterns and could predict 82% of the experimental results within a deviation of ± 20%.

A comparative study of plain and enhanced helical microfin evaporator tube with drift flux-based entropy generation model for R32 and R410A refrigerants

In the present paper, a comparative study is carried out to assess the relative thermodynamic performance of plain and helical microfin evaporator tubes with two new emergent refrigerants, R32 and R410A. An entropy generation analysis is performed based on the drift flux model for two-phase forced convective boiling flow, and the comparison of the enhanced tube to the plain one is done quantitatively. A critical value of relevant parameters, i.e., mass flux, tube diameter, length, heat flux, inlet quality, and operating pressure, are identified, below or above which the enhanced tube is observed to be thermodynamically more favourable than the plain tube. The study reveals that by increasing the tube diameter from 8 mm to 14 mm, the critical mass flux increases from 335 kg/m2s to 495 kg/m2s for R32 and 394 kg/m2s to 565 kg/m2s for R410A, respectively, below which the enhanced tube performs better. The helical microfin tube exhibits a better performance below a critical mass flux from 412 kg/m2s to 374 kg/m2s for R32 and 492 kg/m2s to 443 kg/m2s for R410A when tube length increases from 1 m to 5 m. The critical inlet quality, below which the enhanced tube is observed to be superior, shifts to a lower value (0.29 to 0.16 for R32 and 0.44 to 0.26 for R410A), increasing the mass flux from 400 kg/m2s to 500 kg/m2s. However, the enhanced tube shows a better performance above a critical operating pressure, which increases from 11.4 bar to 16.9 bar for R32 and 9.3 bar to 13.4 bar for R410A, with increasing the mass flux from 400 kg/m2s to 500 kg/m2s. The enhanced tube performs better below a critical mass flux which increases from 336 kg/m2s to 461 kg/m2s for R32 and 399 kg/m2s to 489 kg/m2s for R410A when the heat flux is increased from 8 kW/m2 to 15 kW/m2. The overall parametric study with two emergent refrigerants enables one for effective selection of the helical microfin evaporator tube for a suitable range of relevant parameters.

Parametric Evaluation of the Flow Boiling Heat Transfer Properties of R22 and R407c inside Horizontal Microfin Tube

The flow boiling heat transfer performance of R22 and R407c, in a microfin tube with a helix angle of 22° and an apex angle of 48°, was evaluated in a study. The study is aimed at investigating the impact of heat flux, mass flux, saturation temperature, and vapor quality on the heat transfer coefficient during flow boiling. Two different saturation temperatures, 293.15 K and 313.15 K, were used in the experiments, with heat fluxes ranging from 25 to 85 kW.m-2 and mass fluxes ranging from 150 to 350 kg.m-2.s-1. To validate the experimental data, the results were compared with existing correlations for microfin tubes. The calculated error margin among all correlations with the experimental dataset, which ±15% and ±30%, was also matched by 85% and 95% of the datasets, respectively. Findings reveal that at lower saturation temperatures, the average heat transfer coefficients augmented with increasing mass flux. The study also found that R22 has a higher heat transfer coefficient than R407c at low saturation temperatures due to its stronger thermal conductivity and lower viscosity.

Evaluation of flow boiling heat transfer in horizontal circular trapezoidal-shaped microfin tube

Evaluation of flow boiling heat transfer in horizontal circular trapezoidal-shaped microfin tube

Prediction of Friction Factor and Heat Transfer Coefficient for Single-Phase Forced Convection Inside Microfin Tubes

Microfin tubes are widely used to enhance heat transfer in heat exchangers in order to reduce volumes, costs and refrigerant charge. Much experimental work has been published for the flow of liquids, while some experimental work is available for the flow of gases for the chemical, refrigeration and air conditioning industry. This work reviews the literature and presents new experimental friction factors for the flow of the superheated vapor of R1234ze(E) in a 5 mm outside diameter microfin tube. The authors have also collected an extensive data bank of heat transfer coefficients (around 648 points from different research laboratories) and friction factors (around 536 points), covering 45 different geometries of inner finned tubes. After comparing the predictions from available correlations with experimental data, the present paper suggests the best performing equations for the calculation of the friction factor and of the Nusselt number during forced convection flow of liquids and gases. The suggested model for friction factor estimates the experimental values with a relative and absolute deviation of −0.3% and 7.9%, respectively, whereas the suggested model for the heat transfer coefficient predicts the experimental data bank with a relative and absolute deviation of −3.3% and 13.9%, respectively. The validity range of the two correlations is extremely wide, covering microfin tubes with diameters from 2.6 mm to 24.4 mm, and Reynolds number from about approximately 1000 to 300,000 for the friction factor, and from 3000 to 1,000,000 for the heat transfer coefficient.

Convective condensation of R134a and R1234ze(E) inside microfin tube

Environmental concerns are forcing the replacement of the commonly used refrigerants and finding new fluids is a top priority. The hydro-fluoro-olefin (HFO) R1234ze(E), because of its smaller global warming potential (GWP) and shorter atmospheric lifetime, replaced R134a. Accordingly, for HVAC systems design, a detailed knowledge of the thermo-fluid-dynamic characteristics of the fluids and reliable predictive models are required. To improve the understanding, R134a and R1234ze(E) were employed in convective condensation experiments (saturation temperature Tsat = 35°C, mean quality xm = 0.1~0.9, quality changes Δx = 0.05~0.6, mass flux G = 43~444 kg·m−2s−1) inside a microfin tube (outer diameter D = 9.52 mm, fin number n = 60, fin height H = 0.2 mm). The results were used for two goals: the former is the comparison of the heat transfer features of the two fluids, while the latter aims at testing the performance of prediction models available in the open literature. At the saturation temperature T = 35°C, the two fluids show small differences in the thermal properties so that, as expected, the experiments highlighted a very similar behavior in the typical operating conditions of HVAC systems. In fact, for all the operating conditions marginal differences were observed in the pressure drop, the heat transfer coefficient and the flow pattern maps. The issue of prediction reliability, however, is still open. Actually, not all the models achieving good results for R134a show the same performance for R1234ze(E), especially for the pressure drop.

Intensified convective energy tapping in modified tubes using Azadirachta indica brewed zinc oxide as potential thermo-fluids

Reusing certain types of decomposed tree leaves is beneficial for biosynthesizing nanoparticles for nanofluids to meet energy sustainability and conservation. Encapsulating and reductant agents in Azadirachta indica (neem) help with fluid stability besides providing higher thermal transport properties. Sol-gel technique was adopted to prepare zinc oxide (ZnO) nanoparticles with neem solution to obtain neem-doped zinc oxide (ZnOneem) particles. According to the Debye-Scherrer relation, the size of ZnOneem was 36 nm. Optimal fluid stirring and sonication parameters were identified to synthesize and stabilize 0.05, 0.2, and 0.6% concentrated nanofluids with aqua-antifreeze (50:50) without surfactant. While the optimal stirring speed and time of 0.05, 0.2, and 0.6% nanofluids were 322 rpm and 36 min, 329 rpm and 33 min, and 334 rpm and 29 min, respectively, the optimal sonication time were 39 min, 34 min, and 32 min, respectively. However, sodium dodecyl sulphate (SDS) at a dispersion fraction of 0.6 ensured 35 months of good suspension. Pumping these turbulent fluids (Re = 5000–10000) through a microfin tube snug-fit with twisted tape inserts (twist pitch, y = 63, 84, and 105 mm) ensured the best outcomes with all the fluid samples. The Nusselt number ratio of 2.114 is the best at a penalty of 2.228 friction factor ratio with 0.6% concentrated nanofluid at Re = 10,000 circulating within the tube fit with Y = 6 tape. For this case, the Bejan number was 0.72, with the maximum Performance Evaluation Factor (PEF) and the Entropy Performance Evaluation Factor (EPEF) being 1.619 and 154.02, respectively.

Experimental analysis of photovoltaic thermal collector (PVT) with nano PCM and micro-fins tube counterclockwise twisted tape nanofluid

A hybrid PVT (photovoltaic thermal system) nanofluid circulation system was tested, which included a micro-fin tube (inner grooved) and twisted tape with nano PCM (phase change material). The use of nanoparticles allowed the nano PCM and nanofluid to have higher thermal conductivities, improving their performance. Experiments were carried out within a controlled environment by utilizing an indoor solar simulator with a solar irradiance level of 800W/m2. The PVT system used water/SiC (0.6, 0.3) vol% nanofluid as a working fluid. The nanoPCM comprises 1% SiC nanoparticles, which enhance thermal, electrical (photovoltaic), and photovoltaic thermal efficiency (total efficiency). The mass flow rate range is 0.008–0.058 kg/s. The energy analysis showed that the PVT had an electrical efficiency of 10.8%. The system's maximum thermal efficacy was 83.3% for the PVT with the T (twisted tape), MF (micro-fins), NF (nanofluids), and NPCM (Nano PCM). The percentage of improved electrical power compared between the bare PV and the PVT NF T NF NPCM was 44.5%. The micro fin tube with twisted tape improved the heat transfer properties for enhanced thermal performance.

Flow boiling heat transfer characteristics over horizontal smooth and microfin tubes: An empirical investigation utilizing R407c

The flow boiling heat transfer performance of horizontal smooth and microfin tubes was investigated using the refrigerant R407c. The evaporator test tube contains an outside, inner diameter and length of 9.52, 8.52, and 1000 mm, respectively. The investigations were performed out at 283.15 and 303.15 K saturation temperatures with hot water circulation heat flux of 75 kW.m-2, refrigerant heat flux varying from 5 to 85 kW.m-2, and mass flux varied from 15 to 250 kg.m-2.s-1, keeping the area ratio of microfin tube 1.68%. The experimental results are compared to several previously recognised correlations. Experimental outcomes demonstrated that the heat transfer rate of the microfin tube is superior to the smooth tube. The percentage of heat transfer enhancement was considered to exceed in microfin tube by 28–280%. The pressure drop in regard to R407c was found to be augmented than smooth tube according to the experimental results and enhancement was achieved in microfin tube by 68–110%. Finally, a sensitivity analysis is carried out in flow boiling heat transfer phenomenon via the smooth tube.

Anomalous Adverse Effect of Mass Velocity on Convective Flow Boiling in Small-Diameter Microfin Tubes

Abstract Recent studies report an anomalous phenomenon, particularly for small diameter microfin tubes, where the flow boiling heat transfer coefficient decreases with increasing mass velocity in the convective regime, which is contrary to that found for smooth tubes and larger diameter microfin tubes. This paper presents a critical literature review and mechanistic analysis of the anomalous phenomenon. Our analysis suggests that the anomalous phenomenon is a result of the transition of annular flow pattern from “flooded-groove” (film thickness > fin height) to “in-groove” (film thickness < fin height). The latter is associated with lower degree of turbulence, smaller wetted area, and therefore smaller heat transfer coefficient than the former. We speculated that the in-groove annular flow only occurs for small diameter tubes where the liquid film is relatively uniform, while larger tubes tend to remain flooded at the tube-bottom until dryout. This may explain the tendency of the anomalous phenomenon to occur in small diameter tubes.

A Comprehensive Assessment of Two-Phase Flow Boiling Heat Transfer in Micro-Fin Tubes Using Pure and Blended Eco-Friendly Refrigerants

This review study examines flow boiling heat transfer in micro-fin tubes using mixed and pure refrigerants with zero ozone-depleting potential (ODP) and minimal global warming potential (GWP). This investigation focuses on the extraordinary relationship between heat transfer coefficients (HTCs) and vapor quality. Since the introduction of micro-fin heat exchanger tubes over 30 years ago, refrigerant-based cooling has improved significantly. Air conditioning and refrigeration companies are replacing widely used refrigerants, with substantial global warming impacts. When space, weight, or efficiency are limited, micro-fin heat exchangers with improved dependability are preferred. This review article discusses flow boiling concepts. The researchers used several refrigerants under different testing conditions and with varying micro-fin tube parameters. Micro-fin tubes are promising for improved heat transfer techniques. This tube increases the heat transfer area, fluid disturbance, flow speed, and direction owing to centrifugal force and HTC. As the focus shifts to improving heat transfer, pressure drop, mean vapor quality, and practical devices, this subject will grow more intriguing. A radical shift will reduce equipment size for certain traditional heat transfer systems and bring new products using micro-scale technologies. This suggested review effort helps comprehend saturation flow boiling through micro-fin tubes and find the right correlation for a given application. This domain’s challenges and future relevance are also discussed.

Experimental investigation on flow condensation pressure drop of R134a in microfin tube at high mass flux

Abstract To clarify the influencing mechanisms of the experimental condition and the tube structure on the pressure drop of heat transfer fluid, the two-phase flow condensation pressure drop of R134a inside the microfin tube was experimentally studied at high mass flux, which is different from other conventional researches. The experimental result shows the pressure drop is proportional to mass flux and fin helical angle, and is inversely proportional to condensation temperature and coolant Reynolds number. Moreover, the experimental data of pressure drop was compared with the predicted value of some existing correlations for the microfin tube. It can be found that the correlations of Cavallini et al., Han et al. and Haraguchi et al. show a good prediction effect with mean relative deviation (MRD) of 13.89, 16.08 and −2.19%, respectively. The correlations of Pierre/0.053, Kedzierski et al. and Choi et al. all underestimate most of the experimental data of the pressure drop inside the tube, and their prediction deviations are >10%. That is, the application effect of the separated flow model is better than that of the homogeneous flow model. Finally, the Kedzierski et al. correlation was improved to realize a high-precision prediction of the fluid flow mechanism inside the tube. Because the prediction deviation of the improved correlation between the experimental value and the predicted value was greatly reduced, its prediction deviation is <10% for R134a and R410A; therefore, it can be said the improved correlation has a good predictive result for the pressure drop.

A Comprehensive Review on the Nucleate/Convective Boiling of Low-GWP Refrigerants: Alternatives to HFC Refrigerants

This review presents the nucleate/convective boiling performance for a variety of important low global warming potential (LGWP) alternatives to commonly used high-global warming potential (GWP) refrigerants (such as R-134a, R404A, and R-410A, etc.). Efforts are stressed on the assessment of their evaporation pressure drop and heat transfer coefficient (HTC) characteristics. These alternatives include R-1234ze(Z), R-1234ze(E), R-1233zd (E), R-1234ze(E), R-410A, R-1234yf, and R-513A. The authors investigated the thermo-fluid properties within and outside a tube, mini-channel, micro-fin tube, and plate heat exchanger. The investigation of the numerical, experimental, and simulated results revealed that the evaporation pressure drop and HTC characteristics were dependent on a variety of variables. These factors include the working fluid’s thermodynamics and transport properties, the refrigerant’s mass flux, heat flux, saturation temperature, the vapor quality, the conditions and flow patterns, the orientation of the heating surface, and the geometry (shape, size, and surface area smooth/enhanced) of the heating surface. An expanded LGWP refrigerants, surfaces, and conditions database is needed. Mechanistic models may assist. These models can optimize boiling, anticipate heat transfer, and develop high-performance geometries.