Tube Bank Research Articles

Numerical and experimental investigations on the thermal-hydraulic performance of heat exchangers with Schwarz-P and gyroid structures

The fluid flow and heat transfer are investigated in high-performance heat exchangers with Schwarz-P and Gyroid structures using numerical and experimental approaches. This study was carried out first for laminar flow (Re = 10–100, Pr = 3–6) numerically and second for turbulent flow (Re = 1850–5500, Pr = 3.5) numerically and experimentally. All simulations were performed with COMSOL software to investigate the flow patterns, thermal performance, and pressure drop in heat exchangers based on the Schwarz-P and Gyroid with various unit cell numbers between 1 and 64. For low Re simulations, it is observed that the heat transfer coefficient and performance evaluation coefficient (PEC) increase with increasing the number of cells (or decreasing the cell size) in a constant volume. By redefining the Reynolds number based on the hydraulic inlet diameter and the velocity at the inlet of the structure, the Colburn and friction coefficients become independent of the structure's density. Correlations are provided to predict these coefficients. The results show that the Gyroid with 64-unit cells provides an average of 24 % (maximum 33 %) enhancement in the PEC and 40 % (maximum 46 %) Nusselt number improvement over the tube banks. On the contrary, the Schwarz-P does not have an improvement in PEC or Nusselt number compared to the tube bank. For the high Re study, several prototype heat exchanger samples based on the Schwarz-P and Gyroid structures were fabricated utilizing the FDM 3D-printing technique with no support structures to show that these heat exchangers could be manufactured. The SLM method was also used to fabricate a Gyroid heat exchanger for use in the experimental studies. The performance of a Gyroid heat exchanger made of AlSi10Mg has been tested for input flow rates ranging from 1 to 3 L per minute. For numerical simulation, the k-epsilon and K–W-SST turbulence models are employed and their results are compared with the present experimental data. It is found that the results of the K–W-SST model provide better agreement with experimental ones.

Effect of the lengths of multiple splitter plates on the flow past a bank of cylindrical tubes

ABSTRACT A two-dimensional numerical study of the crossflow past a staggered cylindrical tube bank with multiple splitter plates attached behind the tube was performed. Simulations were conducted by keeping the central splitter plate’s length equal to the tube diameter while varying the lengths of the other two splitter plates positioned at the top and bottom of the central one. The results were validated by means of a grid independency study and via comparison with available experimental and numerical studies in the open literature. The Nusselt number, friction factor (f), and thermal performance factor (η) variations at length ratios to tube diameter ranging from 0.1 to 1 were investigated as a function of the Reynolds number between 5500 and 14,500. The friction factor was found to be around 0.25 at low length ratio case where it increased up to 2.4 at high length ratios on average within the Reynolds number range investigated. The Nusselt number was found to be higher for all the length ratio values when compared with the use of one splitter plate, and it showed an increasing trend along with the increasing lengths of the two additional splitters. It increased from 81 to 155 on average within the Reynolds number range investigated due to the additional splitter plates. The η for the tube with additional splitters that had a length ratio of 0.4 was calculated as 1.42, which was an average of 13% higher when compared with the one splitter plate case that showed a thermal performance factor equal to 1.26. This length ratio was determined to be optimum based on the highest thermal performance factor and the increase of 35% and 8% in the Nusselt number when compared with the bare tubes and the one plate case, respectively.

THE EFFECT OF USE OF DIFFERENT DIAMETER ON IMPROVEMENT OF FLOW AND HEAT TRANSFER IN TUBE BANK HEAT EXCHANGERS

Boru demeti ısı değiştiricilerinde, boruların içindeki akışkana çapraz yönde etrafından geçirilen başka bir akışkan ile ısı transferi gerçekleştirilir. Boru dışından çapraz yönde geçirilen akışkan çoğunlukla gaz akışkanlardır. Art arda yerleştirilen borularda, en fazla ısı transferi çoğu kez birinci boruda elde edilmektedir. Birinci borudan sonra ısı ve akış karakteristiği birbirine benzer hal almaktadır. Bu durumda, hız ve sıcaklık konturlarının tekrarlanması ile periyodik akış elde edilmiş olur. Ardışık olarak gelen borularda sınır tabakanın sürekli olarak yenilenmesini sağlamak, diğer borularda birinci borudaki gibi yüksek ısı transferi elde edilmesine imkân tanıyacaktır. Isı transferi iyileştirilirken en önemli sorun basınç düşümünün de artmasıdır. Bu çalışmada düzgün sıralı dizilişe sahip boru demetinde, art arda yerleştirilen dört farklı çaptaki borular kullanılarak, ısı transferinin arttırılması ve basınç düşümünün azaltılması amaçlarıyla sayısal optimizasyon yapılmıştır. Karşılaştırmalar, ısı transferi yüzey alanının sabit, borular arasındaki boyuna ve enine mesafelerin aynı olduğu varsayılarak yapılmıştır. Düzgün sıralı dizilişe sahip boru demetinde ardışık olarak yerleştirilen dört adet borunun çaplarının D_1=5 mm,D_2=15 mm,D_3=6 mm,D_4=14 mm olması durumunda ısı transferi %14.5 oranında artarken, basınç düşümü de %377 oranında artmıştır.

Experimental study on gas–solid flow and heat transfer characteristics in downer moving bed

AbstractAs a special flow state of the downer, the downer moving bed has been applied in many fields. With the rapid development of the downer moving bed, experimental studies on the flow and heat transfer characteristics in the bed are urgently needed to promote the further development of it. Therefore, a complete set of downer moving bed experimental equipment is completed in this paper. Key parameters of flow characteristics such as void fraction and particle flow velocity are indirectly calculated through direct measurement of pressure and other data. The experimental results show that the factors affecting the solid circulation flow rate include bed inventory and inlet air velocity. Due to the high packing density of the particles, the void fraction of the downer moving bed has a small relative change. The particle flow velocity increases significantly with the increase of the inlet air velocity. Under the joint effect of particle velocity and void fraction, the variation of heat transfer coefficient along the height of the bed presents a single peak curve. And the heat transfer performance of the downer moving bed is obviously better than that of the downer fluidized bed at the same solid circulating flow rate. The differences between different tube bank arrangements and the single tube are compared. It is concluded that in practical applications, the requirements for heat transfer and heat transfer coefficient should be considered together and the best tube bank arrangement should be selected.

Investigation of a falling film tube bank heat exchanger with baffle design for water recovery applications

Shell and tube heat exchangers (STHE) are essential thermal equipment and widely used in daily life. A novel thermosyphon system called falling-film thermosyphon (FFTS) is introduced and integrated into STHE system, resulting in a better thermal performance. In this study, a rectangular solid tube bank of FFTS bundles with a baffle design is studied. The numerical simulation for heat and mass transfer of the FFTS heat exchanger is developed to predict the condensation rate of the vapor in the flue gas, and a lab-scale prototype is also built up in COMSOL. The prediction is validated with the experimental data from references, and the model's accuracy is verified within 10%-12% error. Also, the Non-dominated Sorting Genetic Algorithm, version 2 (NSGA-II) is implemented to improve the thermal performance of rectangular tube banks in this paper. Several parameters, e.g., baffle number, tube number, and tube space, are optimized. As a result, compact configurations with more baffles are preferred to enhance the performance associated with a high-pressure drop correspondingly. The optimized layout for the lab-scale prototype can increase by 18 to 32% condensation with a pressure loss of less than 200 Pa.

Heat transfer characteristic of particle flow around the out-wall of different geometries

In this study, large-scale simulations were carried out for different tube banks to explore the effect of geometrical factors. In addition, the heat transfer experiment of dense particles was carried out to validate the heat transfer model used in discrete element method. The stagnation zone and void zone with nearly triangular shape are found in staggered tubes. The stagnation zone with rectangular shape is formed between vertical adjacent tubes of the aligned tube, and the width of stagnation zone is one third of the tube diameter. A triangular particle blockade zone is formed in the rectangular stagnation zone. The heat transfer rate of staggered tubes is increased by 26% compared with that of the aligned tubes. The staggered elliptical-like combined tubes bank with chamfers significantly increase the heat transfer rate by 54.2% with the same heat transfer coefficient, compared with the staggered circular tubes bank. The local heat transfer performance can be enhanced by the addition of chamfers at the junction area of tubes. When penetration thermal resistance is dominant compared with the contact thermal resistance, the circular tube has higher heat transfer coefficient compared with elliptical-like combined tube with chamfers. The thermal resistance models for different tubes are developed.

Optimizing the tube geometry to enhance thermohydraulic performance of tube bank heat exchanger

In this study, optimum design parameters of circular, diamond and enhanced shaped geometries for staggered tube banks are explored utilizing a multi-objective genetic algorithm (MOGA-II). For given three geometries, two step optimization studies are compared: (1) circular and diamond shaped geometries, (2) diamond and enhanced shaped geometries. Steady, incompressible, and turbulent flow around three-dimensional numerical models is solved using finite volume method. Initially, using literature experimental data numerical results are validated for circular and diamond geometries. In the optimization studies, maximization of heat transfer and minimization of required pumping power are preferred as objective functions. The results of optimization studies are demonstrated graphically. Furthermore, velocity and temperature distribution for the optimum cases are illustrated. The results of first step optimization studies exhibited that diamond shaped tube bank shows higher heat transfer rate than circular one for the same given volume (156%). Besides, as a result of second step optimization studies, optimal enhanced shaped geometry showed 0.59% higher heat transfer rate, 21.73% lower pumping power requirement and 9.65% lower total tube bank volume than optimal diamond shaped one.

Flow Characteristics and Vortex Structure of Flow Across Micro Cylinders by Micro-PIV Measurement and CFD Simulation

The micro tube bank model is a commonly used structure in microelectro mechanical cooling systems. Flow across a micro tube bank is a basic benchmark for analyzing flow resistance and heat transfer capability. In this paper, the flow around a micro tube bank model was studied both experimentally and numerically. Micro-Particle Image Velocimetry technology was used to measure the flow field at different inlet flow rates. The laminar flow model, S-A model and standard k-ε model were used to calculate the flow field inside the micro tube bank model. By comparing the results from the three numerical models with the experimental results, there is a certain gap between them can be noted. However, it was found that the standard k-ε model is better than laminar model and S-A model in the comparison between numerical results and experimental results. In conclusion, the standard k-ε model is more suitable for the numerical calculation of the flow field around a micro scale tube bank model.

Study of natural convection heat transfer for a cylindrical and semicylindrical heat transfer system geometry

The paper presents the experimental and numerical study of the heat transfer parameters for the heated tube bank and arc-shaped plate made up of four tubes and four arc protrusions arranged one above the other. To evaluate the local heat transfer coefficients, the holographic interferometry is used as the experimental method, while the numerical simulations are performed by the Ansys Fluent software. The tube bank supplemented by the fin of different thickness and the double arc-shaped plate is further considered to improve the heat transfer parameters and the numerical results are compared with the basic arrangements. Considering the natural convection, the Rayleigh numbers are specified in the range of 16,460 to 46,340. It is observed that the tube bank achieves a higher average Nusselt number in the range of 37%–82% compared with the other arrangements depending on the Rayleigh number. The tube bank with fins does not increase the heat transfer parameters; however, there is an increase in average heat flux compared to the tube bank without fins. The double arc-shaped plate arrangement slightly increases the heat transfer and markedly increases the average heat flux compared to the single one. The highest average heat flux is achieved for the double arc-shaped plate and tube bank with fins in the range of 27.430 Wm-2 to 126.139 Wm-2.

A parametric investigation of the heat transfer enhancement of tube bank heat exchanger with reversed trapezoidal profile fins

In this study, a reversed trapezoidal profile fins attachment to the tube bank heat exchanger was proposed to enhance the heat transfer performance. A validated computational fluid dynamics (CFD) model was used to investigate the flow behaviour and thermal performance of the staggered tube bank heat exchanger with reversed trapezoidal profile fins. The simulation employed the RNG k-ε model with enhanced wall treatment. The effects of fins’ angle (2°≤αtf ≤ 10°), length (0.5D ≤ Ltf ≤ 2D) and thickness (1 mm ≤ Htf ≤ 5 mm) within a range of Reynolds number (5500 ≤ Re ≤ 14500) were discussed. Nusselt number (Nu), pressure drop (ΔP) and performance evaluation criterion (PEC) were used to evaluate the heat exchanger’s performance. The results showed that both Nu and ΔP followed an increasing trend as αtf increased, which indicates a heat transfer enhancement and a larger airflow resistance. As Ltf increased from 0.5D to 2D, Nu showed a continuous increase with the maximum value obtained at Ltf = 2D. However, ΔP showed a tendency to increase and then decrease as Ltf increased, with the maximum value observed at Ltf = 1.25D. A larger thickness resulted in a higher airflow resistance penalty compared to the improvement in Nu. The thickness was recommended to be as small as possible, provided that the manufacturing process and mechanical strength requirements were met. If the adverse effect of pressure drop was not of concern, the optimal thermal–hydraulic performance (PEC) could be obtained at a larger angle, length and smaller thickness.

Secondary Flow and Heat Transfer Enhancement Characteristics of a Channel Formed by Circular Tube Bank Fins Stamped with Convex/Concave Pyramids

Thermal and flow characteristics of a channel formed by circular tube bank fins stamped with convex/concave pyramids were numerically investigated by ANSYS Fluent software, and the parametric studies on the thermal performance of the pyramid fin are examined for varying pyramid height (H p), pyramid length (L p), and pyramid width (W p) as Reynolds number (Re) ranged from 1297 to 4541. The pyramid fin provides superior thermal performance to the counterpart plain fin, which is ascribed to the punched pyramids effectively expanding heat transfer area and inducing the secondary flow to better fluid mixing, and the action of the convex/concave pyramids lead to the area of undesirable heat transfer dead zone behind the circular tube being significantly reduced. Under identical pumping power constraint, the thermal performance factor of 1.41 is achieved for H p = 1 mm, L p = 5.74 mm, and W p = 3 mm at Re = 3892, and the optimal pyramid height and width fin respectively are H p = 1 mm and W p = 3 mm for studied cases, while the optimal pyramid length is associated closely with Re. The correlations of average Nusselt number and friction factor are proposed to estimate thermal performance of the pyramid fin used in a finned circular tube heat exchanger.

Numerical Investigation for Single-Phase and Two-Phase Flow in Duct Banks with Multi Types of Vortex Generators

This paper examines the numerical work of the thermal convection over tube banks with winglets through the heat exchanger in two parts under turbulent conditions. The first section investigates the influence of two-phase (water and air) flow on the performance of two kinds of vortex generators (Delta and Rectangular) winglets across the oval tube banks. The second section studied the performance of four types of winglets (Delta, Rectangular, Zikzak, and Sinusoidal wavy), circular and oval tubes, forward and downward configurations, and various angles of attack (15°

Numerical investigation of combined effects of radiation and convection heat transfer from tube banks placed in a participating medium

In the present study, combined convective and radiative heat transfer from in-line tube banks placed in an absorbing, emitting and isotropically scattering participating medium was numerically investigated. The governing flow and radiative transfer equations were solved numerically by utilizing the finite volume methodology and the P1 approximation. The flow is laminar, and the tube walls are black and isothermal. The parameters affecting the heat transfer in such media are the tube pitch, scattering albedo, Reynolds number and conduction-to-radiation parameter. Numerical simulations were carried out for Reynolds numbers between 100 and 300, and the transversal and longitudinal pitches ranging from 1.5D to 3-D, respectively. Also, the medium related parameters such as scattering albedo and conduction-to-radiation parameter were ranged between 0 to 1 and 0.0367 to 0.5080, respectively. The effects of the change in scattering albedo, conduction-to-radiation parameter as well as flow characteristic and tube pitch on temperature distribution and heat transfer were examined. The convective and radiative heat transfer from tubes is analyzed via the surface averaged mean Nusselt number over the whole tube wall. The lowest and the highest total-radiative Nusselt number values were achieved as 3.66-0.091 in the purely scattering medium and as 32.95-25.14 in the non-scattering medium, respectively. Also, a novel correlation (with a coefficient of determination of 0.99804) for tube banks subjected to convection and radiation in a participating media was developed.

Experimental and numerical investigation of soot deposition on exhaust heat exchanger tube banks

The exhaust heat exchanger is widely used in waste heat recovery systems (WHR) to recover energy from exhaust gas. However, fouling of the exhaust heat exchanger due to the deposition of soot particles reduces the heat transfer efficiency. In this investigation, a deposition performance experimental system and an improved CFD-based soot deposition prediction model were proposed to investigate the deposition characteristics on tube banks in exhaust heat exchangers. Firstly, the test bench is introduced in detail, and the combination of air heater and soot generator was applied in the test bench to provide the particle flow with specified parameters. Then, considering the effect of thermophoresis, diffusion, inertial impaction, and removal mechanisms, an improved prediction model was proposed. Compared with the traditional two-fluid model (TFM), the thermal-hydraulic performance of exhaust heat exchangers is calculated by CFD software in this model, so it is suitable for all dividing wall type heat exchangers. Afterwards, the influence of particle diameter and tube spacing on the tube bank during the 4 h deposition process was experimentally or numerically investigated. It was found that the comprehensive performance of the tube bank is deteriorated by about 0.45%-1.85% after 4 hours of soot deposition. Besides, the deposition amount on the tube bank with smaller tube spacing (1.25D) is lower.

Dropwise and liquid-jet laminar flow of subcooled water falling over horizontal tube banks

Falling-film heat exchangers are widely used to promote heat and mass transport within industrial environments. In contemporary condensers, for example, liquid films continuously form over a tube row and impact multiple tubes beneath, resulting in a rich variety of two-phase flow phenomena, including the classical Rayleigh-Plateau instability. It is widely known that altering the contact angle of the tubes influences the heat exchangers’ heat and mass transfer characteristics by changing the falling films’ flow behaviour (i.e. dropwise or jet). Therefore, resolving interfacial flow characteristics, such as the Rayleigh-Plateau instability, is crucial for falling film studies as this instability may change the state of the falling film from being jet flow to dropwise/jet flow or even just dropwise. To explore this aspect, three-dimensional direct numerical simulations are used in the present work to replicate the jet instability behaviour. By employing certain mesh refinement strategies demonstrated from various case studies, the interfacial flow behaviour and jet instability have been captured, showing good agreement with experimental and theoretical data previously published. This is determined from the increased jet bifurcation and a maximum error of 2.2% of the film thickness in comparison to Nusselt's result within the bulk region of the tube. In addition, the average and local heat transfer coefficients across the first tube were compared with numerical solutions to give an average error of 3.2 and 14 %. Jet instability is found to be directly related to the amplitude of the axial pressure variations which is augmented by the growth rate of the capillary wave that travels upstream. Specifically, for contact angles of 30 and 60°, the flow is in a dropwise and dropwise/jet state due to the emergence of the Rayleigh-Plateau instability. Notably, the additional satellite droplets that materialize from the unstable jet lead to an increase in the local heat transfer coefficient of the film compared to without the Rayleigh instability improvement.

In-plane fluidelastic instability study of a tube bundle with a rotated triangular layout and small pitch ratio

Since the shutdown of the San Onofre Nuclear Generating Station (SONGS), the question of the possibility of in-plane fluidelastic instability in the steam generators was raised. In a step to analyse the stability of the APR1400 steam generator, this paper reports on an experimental fluidelastic instability study in single phase (water) and two-phase (air–water) cross flow of the vertical tube bank in the steam generator. The study concerns the vibration behaviour of single flexible tube with an otherwise rigid array as well as fully flexible array. The tube pattern analysed is the rotated triangular array (60°) having a pitch spacing (P/D) of 1.33. This spacing is generally more confined than what is usually seen in the literature. Adjustable tube mounts allow the tubes to vibrate only in one direction, whether in the streamwise or transverse to the flow. The stability threshold in the transverse direction of such a tightly spaced rotated triangular array is somewhat lower than the other layouts. Damping of this rotated triangle array reaches its maximum value at a void fraction near 80%. Fluidelastic instability critical velocity in the streamwise direction was found to be much higher than the range of velocities implied by the ASME design guideline, which implies relatively high Connors constants. This was observed in the case of fully flexible array only, where cross-coupling between the tubes is significant. However, A single flexible tube did not undergo instability, which indicates that the streamwise instability is governed by the fluid stiffness mechanism.

Effect of Pitch Ratio of Tube Banks on Passive Acoustic Properties

Tube banks are common design elements in heat exchangers. The most common tube bank patterns are In-line tube banks and Staggered tube banks. These are characterized by their longitudinal and transverse pitch ratios (pitches to diameters). The tube bank with a pitch ratio of less than 1.25 is considered a compact tube bank and a pitch ratio greater than 2 is considered as widely spaced. In this paper, the attention is focused on the effects of pitch ratio in tube banks on its passive acoustics properties which are experimentally investigated. The flow duct experimentation is performed with and without flow to analyse the passive acoustic properties of tube banks. The Experiment is conducted for both the In-line and Staggered tube banks with pitch ratios from 1.2 to 2.5. Experimental results consist of scattering matrix coefficients and power balance. The results are compared to see the pitch ratio effect.

Effect of surfaces roughness of a staggered tube bank in cross flow with air on heat transfer and pressure drop

Many options are devoted to enhancing heat transfer alongside lowering the pressure drop inside the tube banks in cross flow with air. Among these, is attaching splitter plates (SPs) to the trailing edges of the tubes as extended surfaces. That already proved from literature its effective use. Another proposed option is to increase the surface's roughness, with limited available data from literature. Thus, the present research was established to study the effect of using different surface roughness of a staggered tube-bank in cross flow with air on heat transferred and pressure drop besides the comparison with installing SPs. To this aim, a full 3D CFD model is developed to study the two mentioned options without any symmetrical boundary conditions. The study extended to include the effect of Remax changes from 5000 to 100,000. While three surface relative roughness; ks/D = 0 (smooth), ks/D = 0.01, and ks/D = 0.02 are investigated. The results confirmed that increasing heat transfer surface roughness helps to augment the total heat transferred. While a limited pressure drop increase is reported compared to installing SPs. Combining both; increasing surface roughness, and, adding SPs, led to multiplying the total heat transfer rate.

Heat Transfer Characteristics for Natural Convection through Inline and Staggered Horizontal Banks of Tubes

Heat Transfer Characteristics for Natural Convection through Inline and Staggered Horizontal Banks of Tubes

Heat Transfer Enhancement of Partially Serrated Twisted Finned Tube Bank

Due to the large thermal resistance on the airside of the air-cooled heat exchangers, it is necessary to improve heat transfer performance and reduce flow resistance by improving the fin structure. In this paper, partially serrated finned tube bank was proposed and numerically investigated. The experimental system of finned tube bank was built to verify the correctness of the numerical simulation. The finned tube bank is twisted and serrated on the windward side to enhance heat transfer. Moreover, the non-grooved sector region on the leeward side reduces the flow resistance and improves the temperature uniformity of the fins. The effects of fin twisting of the windward side and non-grooving angle of the leeward side were explored. Nusselt number and friction factor decrease as the leeward non-grooving sector region increases. The correlations of Nusselt number and friction factor were derived based on the simulation results of partially serrated twisted finned tube banks with 120° non-grooving region on the leeward side. Nusselt number, friction factor and comprehensive heat transfer coefficient of partially serrated twisted finned tube bank are 13.3%, 13.0% and 10.5% higher than that of annular finned tube bank with Reynolds number from 6200 to 20,600, respectively.