Pin-fin Tubes Research Articles

The impact of drop-shaped pin-fins on the thermal and hydraulic characteristics of a finned tube

In this study, circular and drop-shaped pin-fins were employed to investigate the influence of pin-fins on the thermal behavior and flow characteristics of finned tubes using a combination of experimental and numerical methods. The configuration of in-line pin-fins was analyzed and compared with that of a smooth tube. The analysis covered Reynolds numbers spanning from Re = 7.03 × 103 to 35.17 × 103. Thermal and hydraulic contours were depicted. Two methodologies were utilized to assess the overall performance. The outcomes demonstrated that the average Nusselt number for the finned tubes equipped with drop and circular pin-fins rose by approximately 50.03%–93.1% and 59.59%–77.08%, respectively, in comparison to the smooth circular tube. Moreover, the drop-shaped pin-fins on the tube displayed a reduced friction factor, leading to a reduction of 1.36%–7.95% in comparison to the circular counterpart. Furthermore, both drop and circular pin-fins on the tubes exhibited approximately 2.93%–54.89% and 7.33%–37.1% higher efficiency, respectively, compared to the smooth tube. Generalized correlations were developed to compute the Nusselt number, friction factor, and effectiveness in relation to the Reynolds number, with the aim of providing guidance for future research and design efforts in heat exchangers incorporating pin-fin tubes. The utilization of tubes featuring drop-shaped pin-fins plays a significant role in energy conservation.

Optimization-Inspired Pin-Fin Array for Supercritical Carbon Dioxide Recuperator

Additively manufactured heat exchangers are one possible route to cost-effective sCO2 power cycles. In this paper, experimental results are obtained for two helical pin fin tubes that were designed following parametric optimization of the fin array. Neither the numerical optimization nor the experimental testing have been previously reported in the literature. The two tube designs were, (1) the optimization inspired design, and (2) the optimization-inspired design with a fin diameter increased by a factor of 2. To characterize the print, the designs were scanned using X-ray computed tomography to measure feature sizes and heat transfer area. The optimization was conducted in a commercial, computational fluid dynamics code. The code solved the Reynolds Averaged Navier-Stokes (RANS) and energy equations with turbulence closure provided by the shear stress transport (SST) k-ω model. In the experiments, the Nusselt number augmentation was measured using the Wilson plot technique and the friction factor was determined with mass flow and pressure drop measurements. The experimental testing indicated that the optimization-inspired design had a friction factor that was four times less than the baseline tube design at equal Nusselt number. Additionally, the optimization-inspired design had a 14% improvement in Nusselt number, at equal friction factor, relative to the best performing computational fluid dynamics (CFD) trial points. Tube design (2), with the larger diameter pin fins, had similar performance, within experimental error, as the tube with the smaller diameter pin fins (1). Both tubes achieved overall fin array efficiencies near 1. A performance factor, V/V0, equal to the volume of the enhanced heat exchanger divided by the volume of the baseline (no-fins) heat exchanger, is recommended to quantify internal cooling performance. The experimental shell and tube heat exchanger, using the additively manufactured tube, is competitive with printed circuit heat exchangers in its pressure drop class and could be further improved by optimizing a shell-and-tube heat exchanger utilizing this heat transfer enhancement feature.

Investigation of Condensate Retention on Horizontal Pin-Fin Tubes Using Water-Propanol Mixture

Condensers are an integral part of air conditioning systems. The thermal efficiency of condensers solely depends on the rate of heat transfer from the cooling medium. Fin tubes are extensively used for heat transfer applications due to their enhanced heat transfer capabilities. Fins provide appreciable drainage because surface tension produces pressure gradients. Much research, contributed by several scientists, has focused on adjusting parameters, such as fin design, flow rates and retention angles. In this study, a setup with an observing hole was used to inspect the influence on retention angle of adjusting the flow rates of the fluid. The increase in retention angle was examined using several velocities and concentration mixtures. Pin-fin tubes were used to obtain coherent results using a photographic method. The experimental setup was designed to monitor the movement of fluid through the apparatus. The velocity was varied using dampers and visibility was enhanced using dyes. Photographs were taken at 20 m/s velocities after every 20 s. and 0.1% concentration and the flooding point observed. The experimental results were verified by standard observation which showed little variation at lower velocity. For water/water-propanol mixtures, a vapor velocity of 12 m/s and concentration ratio of 0.04% was the optimal combination to achieve useful improvement in retention angle. With increase of propanol from 0% to 0.04%, the increase in retention angle was greater compared to 0.04% to 0.1%. For velocities ranging from 0 to 12 m/s, the increase in retention angle was significant. A sharp change was observed for concentration ratios ranging from 0.01% to 0.05% compared to 0.05% to 0.1%.

A semi-empirical model for retained condensate on horizontal pin-fin tube including the effect of vapour velocity

During condensation process, the formation of condensate film is main resistance to retard heat transfer. This liquid condensate on horizontal pin-fin tube is known as condensate retention that generally blocks the lower part of tube and act as an insulation resulting in the form of resistance to heat transfer. Various, tube geometries experimentally tested with different pin-fin longitudinal and circumferential thickness and spacing, pin-fin height, internal and external tube diameters under the effect of vapour velocity are selected for model development. As it is extremely important to estimate the extent of condensate retention on such pin-fin tubes in order to make more effective horizontal pin-fin tubes in terms of heat transfer. The development of semi empirical correlations for condensate retention on pin-fin tubes as a function of vapour velocity (forced convection condensation) is proposed in this paper. Various geometric parameters and fluids are considered for model development. Accurate data are used for the model development. The proposed model which is based upon the dimensional analysis successfully predicts available data to within 5%. Relative standard deviation was about 2.44, 2.30 and 1.73 for water, ethylene glycol and refrigerant respectively. The development of such a semi empirical model is the first important step required for the development of full-scale heat transfer model of condensation heat transfer on horizontal pin-fin tubes during forced convection.

AN OVERVIEW OF RECENT PROGRESS IN CONDENSATION HEAT TRANSFER ENHANCEMENT ACROSS HORIZONTAL TUBES AND THE TUBE BUNDLE

The present paper presents a review of condensation heat transfer across smooth and enhanced horizontal surfaces due to its significance in refrigeration, air conditioning and heat pump applications. The emphasizes is on the recent understanding of experimental as well as the semi-empirical correlations to investigate the heat transfer phenomena during condensation associated with enhanced geometries. An effort has been made to submit free-convection condensation effects outside of single tubes and the tube bundle with the influence of tube geometries, condensate retention and gravity on film condensation; however, comparison of forced convection is also presented. Alternative of conventional refrigerants in condensation process by low-global warming potential (GWP) refrigerants is addressed as well due to increase in atmospheric burden affected by hydro-fluoro-carbons (HFCs). Although many researchers have reviewed the condensation impact across enhanced surfaces, a few of them revised its behavior across pin-fin tubes. The effects of geometry, surface wettability, and operating conditions on the location, amount and form of condensate film are discussed. Various theoretical models prediction with the new experimental data across pin-fin tubes is also revealed. This review is distributed into two main sections: the first section focuses on condensation across enhanced tubes, sub dividing the study into integral-fin and pin-fin tubes based on theoretical and experimental investigations. It covers the geometrical effects concerning three dimensional (3D) surfaces, fin density, fin spacing and fin thickness. The later part of the paper concentrates on condensation behavior across the tube bundle incorporating the effects of fin density and refrigerant mixtures highlighting both theoretical and experimental knowledge. Recent research shows an agreement between theoretical and experimental models in the defined area; though, a considerable amount of work on semi-empirical correlation formulation is visible in the literature. The strength of this paper is the latest findings on condensation against different geometrical parameters of extended surfaces specifically across pin- fin tubes and the tube bundle. Finally, theoretical enhancement factors along with many heat transfer correlations are presented and recommendations are suggested for the future work.

Experimental study on the heat transfer and resistance characteristics of pin-fin tube

In order to obtain the influence of the structural parameters of the pin-fin tube on the heat transfer and resistance characteristics, ten different structural parameters of the pin-fin tube bundle were studied in this paper. The influence of the transverse pitch and the longitudinal pitch of the tube bundles, the transverse fin spacing and the height of the fin on the heat transfer and resistance characteristics of the pin-fin tube bundle were obtained, and the corresponding correlations were proposed. The results showed that the convection heat transfer coefficient and the flow resistance increased with the longitudinal pitch of the tube. The transverse fin spacing decreased, the heat transfer coefficient and the flow resistance increased. The change of the transverse pitch and the height of the pin-fin had little effect on the heat transfer and resistance performance of the tube bundles.

A critical review of filmwise natural and forced convection condensation on enhanced surfaces

The use of enhanced surfaces is an efficient method to improve filmwise condensation. Due to their immense potential, many enhanced structures were developed and investigated with the aim of improving natural and forced convection condensation heat transfer coefficients. This has resulted in large collections of predictive models and experimental data being reported. In this review, the developments in this field of research in the past few decades and the recent advances are collated and examined. This paper focuses on the review of natural convection condensation on the external surfaces of enhanced flat plates and tubes and forced convection condensation in enhanced tubes. The various models predicting the heat transfer coefficients on plain and enhanced surfaces are evaluated. For natural convection condensation, the liquid film-based models, semi-empirical models and numerical modes are reviewed whereas, for forced convection condensation, the gravity-dominated and vapor shear-dominated models are discussed. The effects of these enhanced structures on the liquid film and two-phase flow characteristics are analyzed and the various types of enhanced tubes and flat plates investigated are categorized. The manufacturing techniques employed to fabricate these surfaces are identified. A detailed evaluation of the heat transfer and pressure drop performances of the various enhanced surfaces is performed. In addition, their thermal performances are summarized and compared, and their associated heat transfer mechanisms are elucidated. For external condensation on a single tube row, three-dimensional fin structures were found to provide better thermal performance than two-dimensional structures, with some three-dimensional fin structures exhibiting more than 6 times the heat transfer coefficients of a plain surface. However, in a tube bundle, the heat transfer coefficient of three-dimensional fin tubes decreases more significantly with increasing tube row as compared to two-dimensional fin tubes. For convective condensation in circular tubes, the herringbone and pin fin tubes demonstrated better thermal and pressure drop performances than other internally enhanced tubes. Their efficiency indices were between 1.25 and 1.28. Based on the literature surveyed, the various experimental results are compared, existing research gaps are identified and frameworks for future research work are provided.

Experimental investigation of condensate retention on horizontal pin fin tube with varying pin angle

This paper presents the effect of pin fin rotational angle on the condensate retention as function of vapour velocity (0–20 m/s) on four different pin fin tubes, by systematically changing the pin fin angle from 0° – 45° with horizontal axis. While other parameters of pin fin tubes like tooth height, tooth thickness, longitudinal spacing inner and outer diameter remain constant for all 4 tubes tested. Three fluids (2-propanol, distilled water and ethylene glycol) with varying surface tension to density ratio are tested. Experimentation is conducted in a vertical wind tunnel by downward supply of air (to simulate vapour) with velocity from 0 - 20 m/s. By accurate small holes on upper side of the tube a constant flow of condensate was assured on a tube to be tested. At almost all vapour velocities; it is observed that retention angle increases with increase in pin fin angle for all fluids tested. As the vapour velocity increased from 0 to 20 m/s, it is observed that the retention angle for tube A1 increased for all test fluids, whereas with increasing velocity, a decrease in retention angle is observed for tubes A2, A3 and A4 for all fluids tested.

Condensation Heat Transfer Characteristics of Moist Air Outside Hydrophilic and Super-hydrophobic 3D Pin Fin Tube

Three dimensional pin fin tube is an effective heat transfer element. It has been widely used in industry process. However, condensation heat transfer characteristic of moist air outside it has not been experimentally studied. The wettability of tube surface has an important effect on this process. In this study, experiments were carried out, in which moist air condensing outside three dimensional pin fin tube with different wettability. Smooth tube is also tested to make comparison. Effects of steam mole fraction, moist air temperature, and wall subcooling are analysed in detail. Super-hydrophobic surface is obtained using oxidation and self-assembling monolayer method. The results show that heat transfer coefficient increases with the increase of steam mole fraction, wall subcooling and the decrease of moist air temperature. Heat transfer coefficient ranges from 55 to 2,035 W/m2K within the study of this paper, and hydrophilic three dimensional finned tube can achieve 2 times heat transfer coefficient compared with smooth tube and superhydrophobic three dimensional pin fin tube.

Forced convection condensation of R134a in three-dimensional conical pin fin tubes

This paper investigates the forced convection condensation heat transfer performance of R134a in circular tubes with three-dimensional conical pin fin structures. Five conical pin fin tubes of different circumferential fin pitch (pc) and longitudinal fin pitch (pl) were fabricated by Selective Laser Melting (SLM) with the aim of enhancing the internal forced convection condensation of R134a. Experiments were performed to characterize the condensation heat transfer coefficients (href) and pressure drops (ΔP) across these enhanced tubes. These experiments were conducted at the refrigerant mass fluxes (mref) of 50 kg/m2·s to 200 kg/m2·s, average vapor qualities (xave) from 0.2 to 0.8 and saturation pressure (Psat) of 13.4 bar. The effects of xave, mref, pc and pl on href and ΔP were determined and the results were compared against a commercial Al tube, a plain tube fabricated by SLM and two SLM fabricated enhanced tubes with dome-shaped fins. It was found that href of the conical pin fin tubes increases with increasing xave and mref and these values are also significantly higher than those of the plain tubes. Both pl and pc were found to significantly affect the href values of the conical pin fin tubes whereas the ΔP values were affected only by the change in pc. An efficiency index (η1) is defined to evaluate the thermal-hydraulic performances of the enhanced tubes. The experimental results show that all the conical pin fin tubes demonstrated higher η1 than the dome-shaped fin tubes. Based on the boundary layer approach, a semi-empirical model is developed to predict the Nusselt numbers of the conical pin fin tubes. Reasonably accurate predictions were achieved with an overall mean absolute error (MAE) of 10.5%.

Condensate retention of water-ethanol mixture on horizontal enhanced condensing tubes

Condensate retention has been found an important parameter for heat transfer on horizontal enhanced condensing tubes. In this research, five pin-fin (varying in circumferential pin spacing) and three integral-fin horizontal tubes are investigated for condensate retention angle (which is measured from the top of tooth to the fully flooded flank) using water-ethanol mixture. An attempt was made to find the optimum concentration of ethanol in water for maximum retention angle. Concentration of ethanol was varied in between 0.5% to 1.5% by weight. Data revealed the importance of integral-fin tube over pin-fin tube as significant increase in retention angle was observed for integral-fin tube ((having same longitudinal spacing, tooth thickness, tooth height, inner and outer diameter) while retention angle for pin-fin tubes remain unchanged. Optimum ethanol concentration was observed to be 0.75%.

Condensate retention as a function of condensate flow rate on horizontal enhanced pin-fin tubes

The extent of condensate flooding as a function of condensate flow rate is measured on six horizontal pin-fin tubes (varying in circumferential pin-spacing) via simulated experimentation. Surface tension to density ratio is tested using three fluids namely water, ethylene glycol and R-141b. Results show that flooding was strongly effected by changing the condensate flow rate. An increase in flow rate caused a marginal decrease in flooding angle (an angle extracted from top of the test tube to the fully flooded flank). Similarly, circumferential pin-spacing also effected the retention angle and the effect goes on increasing by decreasing the surface tension to density ratio.

Design and investigation of hydriding alloy based hydrogen storage reactor integrated with a pin fin tube heat exchanger

The reaction between metal hydride (MH) and hydrogen gas generates substantial amount of heat. It must be removed rapidly to sustain the reaction in the metal hydride hydrogen storage reactor. Previous studies indicate that the performance of the reactor can be improved by inserting an efficient heat exchanger design inside the metal hydride bed. In the present study, a cylindrical shaped metal hydride system containing LaNi5, integrated with a finned tube heat exchanger assembly made of copper pin fins and tubes, is presented. A 3-D numerical model is formulated in COMSOL Multiphysics 4.4 to study the transient behavior of sorption process inside the reactor. Experimental data obtained from the literature is used to approve the legitimacy of the proposed model. Influence of various operating and geometric parameters on the total absorption time of the reactor has been investigated. It is found that hydrogen supply pressure is the most influencing factor to increase the absorption rate of hydrogen. Total absorption time of the reactor is found to be 636 s with maximum storage capacity of 1.4 wt% at the operating conditions of 15 bar H2 gas supply pressure, heat transfer fluid temperature of 298 K and flow rate of 6.75 l/min.

An analytical model for prediction of condensate flooding on horizontal pin-fin tubes

In this paper, a generalized model has been developed to predict the condensate flooding (i.e. the portion of the tube covered with condensate) on the horizontal pin-fin tubes during free convection condensation. The model is based on the balance of forces act up on the condensate in upward and downward directions due to the effects of surface tension of the fluid and gravity. The model has been compared with the wide range of experimental data already reported by researchers for a wide range of fluid and tube combinations. The model predicts the condensate flooding on horizontal pin-fin tubes to within ±15% over most of the experimental data.

An experimental investigation of pure steam and steam–air mixtures condensation outside a vertical pin-fin tube

A set of condensation experiments in the steam–air and pure steam conditions has been conducted to evaluate the heat removal capacity of a vertically pin-fin tube under free convection. Condensation heat transfer coefficient and heat transfer rate have been obtained. The bulk temperature, wall temperature and temperature near tube surface are measured in detail. As a reference, a smooth tube is tested under the same condition. For pure steam case, the pin-fin tube generates restrictions on the condensation heat transfer process in low wall subcooling region. With a higher wall subcooling, the pin-fin tube shows a larger condensation heat transfer coefficient than smooth tube does. For steam–air mixtures case, the result of the temperature measurement demonstrates that there is an air rich region near the pin-fin tube. Compared with the smooth tube, the pin-fin tube generates reduction effect on the condensation heat transfer process under low air mass fraction situation. As air mass fraction increasing, the condensation heat transfer coefficient of the pin-fin tube becomes larger than that of the smooth tube.

Effect of circumferential pin thickness on condensate retention as a function of vapor velocity on horizontal pin-fin tubes

This paper reports the effect of circumferential pin thickness on retention angle as a function of vapor velocity on four pin-fin tubes (varying in circumferential pin thickness from 0.5 mm to 2.0 mm). Three fluids (namely water, ethylene glycol and R-141b) with high, intermediate and low surface tension to density ratios were tested. Experimentation was performed by providing downward air (to simulate vapor) through a vertical wind tunnel with velocities from 0 to 18 m/s. By providing small holes on the upper side of tube a continuous flow of condensate was ensured along the circumference of the test tubes. At low approaching zero vapor velocity; an increase in circumferential pin thickness caused a decrease in retention angle (an angle measured from top of test tube to the point of flooded flank in circumferential direction) in case of pin-fin tubes for all fluids tested. At high vapor velocity; the role of vapor shear was less effective on the upper half of pin-fin tubes when compared to the equivalent integral-fin tube (i.e. with same longitudinal fin spacing, tooth thickness, tooth height, inner and outer diameter as that of pin-fin tubes). This less effective role of vapor shear on the upper half of pin-fin tubes, when compared with integral-fin tube is thought to be due to the presence of trapped condensate in between the horizontal cuts (circumferential pin spacing) available on pin-fin tubes, which tends to resist the downward motion of the condensate. On the lower half of test tubes (as in case of test fluid R-141b) vapor velocity showed no effect on retention angle for all pin-fin tubes while for the case of equivalent integral-fin tube, retention angle was decreased with the increase of vapor velocity.

Hydrodynamics in bubble columns with pin-fin tube internals

Hydrodynamics in a 0.8m diameter bubble column with pin-fin tube internals covering 9.2% total cross-sectional area of the column is experimentally investigated. The experimental results indicated that pin-fin tube internals significantly affect gas holdup and liquid velocity. It can be inferred from the measured data that the pin fins on the tubes obviously reduce the distributor region in the column. Furthermore, non-uniform arrangement of pin-fin tube internals through symmetrically removing two tubes near column wall gives rise to severe gas short-circuiting and even no downward liquid flow there, while there is imperceptible influence for bubble column with non-uniform plain tube internals.

A semi-empirical model for free-convection condensation on horizontal pin–fin tubes

Abstract A simple semi-empirical correlation accounting for the combined effect of gravity and surface tension has been developed for condensation on horizontal pin–fin tubes. The model divides the heat transfer surface into five regions, i.e. two types of pin flank, two types of pin root and the pin tip. Data for three fluids (i.e. steam, ethylene glycol and R113) condensing on eleven tubes with different geometries were used in a minimization process to find three empirical constants in the final expression. The model gives good overall agreement (within ±20%) with the experimental data, as well as correctly predicting the dependence of heat-transfer enhancement on the various geometric parameters and fluid types.

Effect of vapour velocity on condensate retention on horizontal pin-fin tubes

New experimental data for condensate retention angle as a function of vapour velocity (0–19m/s) are reported on six horizontal pin-fin tubes and an equivalent integral-fin tube (i.e. with same longitudinal fin spacing, tooth thickness, tooth height, inner and outer diameter as that of pin-fin tubes) using water, ethylene glycol and R-141b. Only geometric parameter varied was the circumferential pin spacing. For all tubes tested, an increase in vapour velocity causes an increase in condensate retention angle for the cases when retention angle was less than 90° at low approaching zero vapour velocity. For the cases when the retention angle was greater than 90° at low approaching zero vapour velocity, vapour velocity shows negligible effect on retention angle for all pin-fin tubes, while for the case of integral-fin tube (i.e. using R-141b as a test fluid where retention angle is greater than 90° at low approaching zero vapour velocity) retention angle decreased with increasing vapour velocity.

An investigation of condensate retention on pin-fin tubes

Retention angle measurements on 15 rectangular pin-fin tubes are made under static conditions (without condensation) using water, ethylene glycol and R-113. Condensate retention angles found to be considerably larger than the equivalent integral-fin tubes (i.e. with the same fin height, root diameter and longitudinal pin thickness and spacing) in a range of 5%–60%. An expression for condensate retention angle on pin-fin tubes was proposed and found to agree with the measured retention angles to within 15%.