Tube Rows Research Articles

Comparison of Local and Averaged Air-Side Heat Transfer Coefficients on Fin-and-Tube Heat Exchangers Obtained With Experimental and Numerical Methods

Abstract Numerical methods are often used to obtain two-dimensional air-side heat transfer coefficients (HTCs) on heat exchanger (HX) fin surfaces. The model's accuracy is usually verified through averaged HTCs by comparing with published experimental results. However, substantial disagreement is not uncommon and can hardly be explained by averaged HTCs. This study focuses on comparing experimental, local air-side HTCs to numerical ansys fluent results. A mass transfer experimental method was employed to obtain HTC distributions on the fin-and-tube HX fin surfaces. Therefore, disagreements between the experimental and numerical results can be explained in detail. There are several significant findings: inaccurate predictions of local HTCs are observed even though the averaged HTCs from the numerical method may agree with the averaged experimental results under some conditions. The models fail to capture horseshoe vortices, underestimate the HTCs in the wake region of the tubes, and overpredict row-by-row HTC degradation. Moreover, the accuracy of the numerical model decreases when the complexity of geometry increases. For the flat plate, numerically obtained HTCs agree with the experimental results within 10%. However, the error is more than 30% for the eight-row HX. Nonetheless, the model's accuracy becomes worse at higher airflow velocities. Oversized fin-and-tube HXs with multiple tube rows are often selected as a result of using the underpredicted averaged air-side HTCs from the numerical computational fluid dynamics (CFD) simulations. Thence, the authors have proposed a corrective method to improve the accuracy of the numerical model.

An experimental study of the air-side performance of a novel louver spiral fin-and-tube heat exchanger

We studied the effect of fin patterns on the air-side performance of a novel type of heat exchanger, namely louver spiral fin-and-tube heat exchangers (LSFTHXs). We investigated three types of fin patterns—radial, curved, and mixed-louver spiral fins—and compared their characteristics with plain spiral fin. We fabricated heat exchangers with a crimped aluminum fin base, two rows of steel tubes, and a fin pitch of 8.45 mm. All heat exchangers had both multipass parallel and counter cross-flow arrangements. The average heat transfer rate (Qave), heat transfer coefficient (ho), and pressure drop (ΔP) increased along with frontal air velocity. The louver-spiral fin provided greater Qave and ho than plain-spiral fin by about 9.7–15.6% and 13.4–27.1%, respectively. The j factors (Colburn factors) of louver-spiral fins were greater by approximately 10.4–13.1% (for mixed LSFTHX), 7.7–8.8% (for radial LSFTHX), and 2.1–5.1% (for curved LSFTHX) compared with the plain SFTHXs. Additionally, we found no significant difference in ΔP or f factor (friction factor) between louver-spiral and plain-spiral fins. LSFTHX configurations increased ho and j.

Improved Design and Thermo-Mechanical Verification of Deflection Magnet Beam Collimator of EAST-NBI System

The deflection magnet (DM) is the most important component of the Neutral Beam Injection (NBI) system of Experimental Advanced Superconducting Tokamak (EAST), which can magnetically deflect the un-neutralized charged particles after the neutralized process of the beam is extracted from the ion source, and then form a neutral beam injected into the tokamak. Under the operating conditions of the NBI system, by using the thermocouple monitoring system in the experiment, it can be found that the currently operating DM beam collimator has a quite high temperature rise. It is necessary to redesign the DM beam collimator to improve its heat transfer performance. The parallel arrangement of multiple rows of tubes is proposed as the basic method for the redesign of the beam collimator of DM, the thermal-fluid-structure analysis model of this redesign model is established and its temperature field, pressure field and stress field are analyzed. Taking the surface temperature of the beam collimator, the overall dimension after the total tube combination and the pressure drop of the whole structure of collimator as the optimization objectives, and setting the fluid velocity, the tube’s inner diameter and the number of tube rows as the design variables, the optimized design scheme of the beam collimator structure is obtained. From the results of simulation, the new structure can better meet the operation requirements of DM, and its maximum temperature rise is well controlled, which is expected to meet the long pulse operation requirements of the NBI system. The proposed simulation and design optimization method can provide a certain reference for the design and optimization of other high-heat-flux structures in complex large-scale neutral beam systems in the future.

A low frequency model for the aeroacoustic scattering of cylindrical tube rows in cross-flow

Heat exchanger tube rows can influence the thermoacoustic instability behaviour of combustion systems since they act as both acoustic scatterers and unsteady heat sinks. Therefore, with careful tuning of their thermoacoustic properties, heat exchangers have the potential to act as passive control devices. In this work, we focus on (only) the acoustic scattering behaviour of heat exchanger tubes. We present a comparison of existing acoustic models for tube rows and slits, models for the latter having the advantage of incorporating frequency dependence. We then propose a new model that enables the adaptation of slit models for tube rows. This model is validated against experiments and Linearised Navier–Stokes Equations (LNSE) predictions for the transmission and reflection coefficients, including phase information. The model predictions show very good agreement with the experimental and numerical validations, especially for low frequencies (Strouhal number <0.5, based on tube radius and excitation frequency), with mean differences less than 2% for the transmission coefficients (the reflection coefficient errors are somewhat larger since their magnitudes are very close to zero).

Experimental Investigation of the Heat Transfer between Finned Tubes and a Bubbling Fluidized Bed with Horizontal Sand Mass Flow

The sandTES technology utilizes a fluidized bed counter current heat exchanger for thermal energy storage applications. Its main feature is an imposed horizontal flow of sand (SiO2) particles fluidized by a vertical air flow across a heat exchanger consisting of several horizontal rows of tubes. Past international research on heat transfer in dense fluidized beds has focused on stationary (stirred tank) systems, and there is little to no information available on the impact of longitudinal or helical fins. Previous pilot plant scale experiments at TU Wien led to the conclusion that the currently available correlations for predicting the heat transfer coefficient between the tube surface and the surrounding fluidized bed are insufficient for the horizontal sand flow imposed by the sandTES technology. Therefore, several smaller test rigs were designed in this study to investigate the influence of different tube arrangements and flow conditions on the external convective heat transfer coefficient and possible improvements by using finned tubes. It could be shown that helically finned tubes in a transversal arrangement, where the horizontal sand flow is perpendicular to the tube axes, allows an increase in the heat transfer coefficient per tube length (i.e., the virtual heat transfer coefficient) by a factor of 3.5 to about 1250 W/m2K at ambient temperature. Based on the literature, this heat transfer coefficient is expected to increase at higher temperatures. The new design criteria allow the design of compact, low-cost heat exchangers for thermal energy storage applications, in particular electro-thermal energy storage.

Thermo-fluid performance enhancement of a fin-and-tube heat exchanger by rectangular winglets

In this study, numerical simulations have been performed to investigate the influence of rectangular winglets as Vortex generators for enhancement of thermo-fluid performance in a fin-and-tube type heat exchanger having five staggered rows of tubes. The influence of placement of winglets over different pair of tubes, attack angles of the winglet pair, and their location has been examined under laminar flow conditions for Re ranging 400–1500. For the present analysis rectangular winglet pairs having “common-flow-up” orientation were employed for vortex generation. The effect of placement of different number of rectangular winglet pairs (1–5) has been investigated and the maximum enhancement in thermo-fluid performance was obtained as 27.4% compared to the baseline model (without winglet model). The winglets attack angle (5°, 10°, 15°, 20°, and 25°) were also investigated for best thermo-fluid performance and the optimum attack angle resulted in enhancement up to 33% over the baseline model. The optimum stream-wise and span-wise locations were identified which resulted in enhancement up to 29% and 25%, respectively, over the baseline model. Correlations for the prediction of Nusselt number, friction factor, and enhancement factor have been developed for stream-wise distance, span-wise distance, and attack angle of the winglets.

Impact of a Natural Gas Cooler Design on the Cooling Performance

This article describes a methodology for the identification of cooling performance of a natural gas cooler relative to the shape of its heat-transfer surface and presents the outputs of numerical solutions for four different shapes of heat-transfer surfaces in coolers designated as C_A, C_B, C_C and C_D. Calculations were carried out for a cooler with a single row of tubes, and for coolers with two through six rows of tubes that were positioned above one another with an alternating arrangement. In all of the surface shapes, the boundary conditions were respected in order to facilitate the identification of the shape of the heat-transfer surface which is the most appropriate for achieving maximum cooling performance. Out of these four shapes, the best results were observed with the heat-transfer surface of the cooler designated as C_A. The cooling performance of a 1 m long tube with such a surface was 1,650 W.

Numerical heat transfer in porous media heat exchangers of transport vehicles under unsteady flow

Due to the low metal consumption of heat exchangers with porous media, their use in cooling systems of transport vehicles is essential. This paper presents the results of a numerical study of heat transfer in a porous medium with a laminar pulsating airflow. The porous medium is represented as a two-dimensional channel with a row of square tubes and a porosity of 0.75. The pulsation amplitude corresponded to the values 1, 2, and 3; the pulsation frequency was 0.5, 0.75 and 1 Hz. A comparison of symmetric pulsations with asymmetric ones is made. It is shown that the intensification of heat transfer is increasing with symmetrical pulsations. Empirical correlations are given for predicting heat transfer with symmetric and asymmetric pulsations.

Vortex shedding from In-line tube banks on acoustic resonance

In the present paper the attention is focused on relation between acoustic resonance and vortex shedding from in-line tube banks. We have examined the characteristics of acoustic resonance generated from tube banks with tube pitch ratio of 4.0-1.4 in the flow direction. At the tube pitch ratio in the flow direction of 4.0-2.8, the acoustic resonance of low order mode in the transverse direction occurred. The resonance Strouhal number agreed well with the vortex shedding Strouhal number. At the tube pitch ratio in the flow direction of 2.0-1.4, the resonance Strouhal number did not agree with the vortex shedding Strouhal number. The acoustic resonance occurred at the lower frequencies of vortex shedding frequency. As the number of rows of tubes increased, the acoustic resonance of transverse mode occurred. We discussed the prediction formula of Strouhal number at the tube pitch ratio in the flow direction of 2.0-1.4.

Influence of Heat Exchanger Design on the Thermal Performance of a Domestic Wine Cooler Driven by a Magnetic Refrigeration System

Conventional and not-in-kind refrigerators require heat exchangers for their operation. Yet, most magnetic cooling studies do not take full account of those components despite their importance in defining the cooling capacity and temperature span. To investigate the influence of heat exchanger design parameters on the performance of magnetic refrigerators, a model was developed to integrate the heat exchangers, regenerators and thermal reservoirs. The results were compared with data generated in an apparatus that emulates the conditions of the thermal fluid supplied by the regenerators to a cold heat exchanger positioned inside the cabinet of a retrofitted 130-liter wine cooler. Six tube-fin heat exchangers were evaluated to identify the most suitable geometry (number of tube rows and fin density) for the compact magnetic refrigerator. Numerical simulations described the influence of the heat exchanger on the regenerator performance in terms of the liquid stream effectiveness. For a temperature span of 20°C between the external environment and the refrigerated compartment, the best heat exchanger/fan assembly resulted in a cooling capacity reduction of 37\% and a temperature span increase of 32\%, in comparison with an idealized system. The expected system coefficient of performance (COP) and second-law efficiency were 1.8\% and 13\%, respectively.

Study on Vibration and Heat Transfer Performances of a Modified Elastic Tube Bundle Heat Exchanger

Based on the bi-directional fluid-structure coupling method, the vibration and heat transfer performances of a modified elastic tube bundle heat exchanger are studied. The vibration frequency and amplitude of tube bundle are compared between different directions and tube row spacing. Furthermore, the Nusselt number of tube bundle are studied in vibrational and non-vibrational cases. The numerical results represent that the vibration amplitude of tube bundle is the largest in the z direction, which is 7.25 times that in the y direction (direction of the smallest amplitude). In the calculation scope, the vibration frequency and amplitude promote with tube row spacing. From 50 mm to 90 mm spacing, the vibration frequency and amplitude increase by 21.88% and 10.65%, respectively. The Nusselt number under the fluid-induced vibration increases up to 11.67% compared with non-vibration. Besides, the Nusselt number of tube bundle in the middle part is lower than that at the two ends. Within 50-90 mm interval, the enhanced heat transfer effect rises along with tube row spacing. The rising rate of the Nusselt number increases by 52.4%. in the range of computation.

Parametric analysis of the influence of geometric variables of vortex generators on compact louver fin heat exchangers

This paper studies the performance of vortex generators (VG) in compact louvered fins and tubes heat exchangers. CFD simulations and genetic algorithm optimization were used to create surrogate models for the Colburn and friction factor of the heat exchanger as a function of VG positioning, size, and angle of attack. The models were used to study how the design parameters affects heat transfer enhancement and pressure losses. The study was conducted with a Reynolds number of 208 based in the hydraulic diameter of the flow channel. It was found that the performance of the VG in the first row of tubes is coupled to the aspect ratio of the VG of the second row of tubes. Further that vortex generators with large angles of attack produce recirculation areas behind them diminishing the heat transfer. Also, that best performing vortex generators correspond to small aspect ratios. Yet, these produce a low impact on the heat transfer enhancement when their angles of attack are greater than 50°. Vortex generators with large aspect ratios produce no significant effects on the heat exchanger performance. Larger vortex generators produce stronger and more intense vortices, greater drag, and frictional losses. This work shows how the different parameters of the VG affect heat transfer and pressure losses. The optimal model presents a PEC value of 1.055, in correspondence with a j/j0 ratio of 10.43 and a f/f0 ratio of 14,38.

Experimental quantification of air-side row-by-row heat transfer coefficients on fin-and-tube heat exchangers

• Mass transfer experimental method to visualize and quantify local air-side HTCs. • Local and averaged HTCs on real-scale HX aluminum fin surfaces. • HTC comparison between fin-and-tube HXs with inline and staggered tube arrangement. • Row-by-row HTC degradation of plain fin-and-tube HXs at various Re. • Correlations of averaged HTC and pressure drop on measured HX fin geometry. An absorption-based mass transfer method has been employed in the current study to obtain two-dimensional HTC distributions on two different 8-row fin-and-tube heat exchangers. The method utilizes a color change coating material and a tracer gas to quantify local mass transfer on the fin surfaces and then employs the heat and mass transfer analogy to calculate local air-side HTCs. The accuracy and robustness of the method were validated in previous studies. The current study focuses on investigating the row-by-row HTC degradation by increasing the number of tube rows. The local HTC measurements of the current study ranged from 0.5-4.8 ms −1 face airflow velocity using dry fin surfaces. The two 8-row fin-and-tube heat exchangers have the same tube diameter, tube pitch, fin pitch, and fin shape. The only difference is the staggered vs. inline arrangement of the tube bundle. Averaged HTCs of each row are quantified and compared. The results show HTC degradation is related to the tube arrangement, airflow velocity, and row number. The heat exchanger with inline tube arrangement shows a more substantial HTC degradation than the heat exchanger with the staggered tube arrangement. The maximum HTC decline is observed on the second row with 40% compared to the first row while flow velocity is below 1 ms −1 . The row-by-row HTC degradation becomes less significant for higher flow velocities. HTC degradation appears to be negligible past the 5 th row. The current study provides a method that can be used to predict air-side HTCs of fin-and-tube heat exchangers with multiple rows.

Numerical Simulation of Heat Transfer Tube Characteristics of Refrigeration and Air Conditioning Based on Mixed Fractional Model

The main parameters affecting the heat transfer performance of heat transfer tube heat exchanges include fin shape, fin spacing, fin thickness, tube row arrangement, tube diameter, dry and wet bulb temperature and flow rate. The air side heat transfer performance of heat transfer tube heat exchange and the influence of velocity field and temperature field distribution on heat transfer effect have been the focus of domestic and foreign scholars. In this paper, based on the mixed fraction model, CFD software is used to simulate the absorption process of gravity falling film outside the heat transfer tubes of refrigeration and air conditioning, and to study the flow and heat transfer characteristics of the process. The results show that, for the heat transfer tubes with the selected structure, the heat transfer capacity increases with the increase of water flow velocity, and the heat transfer enhancement effect of turbulence is enhanced. The heat transfer tubes have better comprehensive heat transfer performance than smooth tubes with the same diameter.

Thermal Calculations of Four-Row Plate-Fin and Tube Heat Exchanger Taking into Account Different Air-Side Correlations on Individual Rows of Tubes for Low Reynold Numbers

Currently, when designing plate-fin and tube heat exchangers, only the average value of the heat transfer coefficient (HTC) is considered. However, each row of the heat exchanger (HEX) has different hydraulic–thermal characteristics. When the air velocity upstream of the HEX is lower than approximately 3 m/s, the exchanged heat flow rates at the first rows of tubes are higher than the average value for the entire HEX. The heat flow rate transferred in the first rows of tubes can reach up to 65% of the heat output of the entire exchanger. This article presents the method of determination of the individual correlations for the air-side Nusselt numbers on each row of tubes for a four-row finned HEX with continuous flat fins and round tubes in a staggered tube layout. The method was built based on CFD modelling using the numerical model of the designed HEX. Mass average temperatures for each row were simulated for over a dozen different airflow velocities from 0.3 m/s to 2.5 m/s. The correlations for the air-side Nusselt number on individual rows of tubes were determined using the least-squares method with a 95% confidence interval. The obtained correlations for the air-side Nusselt number on individual rows of tubes will enable the selection of the optimum number of tube rows for a given heat output of the HEX. The investment costs of the HEX can be reduced by decreasing the tube row number. Moreover, the operating costs of the HEX can also be lowered, as the air pressure losses on the HEX will be lower, which in turn enables the reduction in the air fan power.

Multivariate Design and Analysis of Aircraft Heat Exchanger Under Multiple Working Conditions Within Flight Envelope

Abstract This paper presents a multi-condition design method for the aircraft heat exchanger (HEX), marking with lightweight, compactness, and wide range of working conditions. The quasi-traversal genetic algorithm (QT-GA) method is introduced to obtain the optimal values of five structural parameters including the height, the tube diameter, the tube pitch, and the tube rows. The QT-GA method solves the deficiency of the conventional GA in the convergence, and gives a clear correlation between design variables and outputs. Pressure drops, heat transfer, and the weight of the HEX are combined in a single objective function of GA in the HEX design, thus the optimal structure of the HEX suitable for all the working conditions can be directly obtained. After optimization, the weight of the HEX is reduced to 2.250 kg, more than 20% lower than a common weight of around 3 kg. Based on the optimal structure, the off-design performance of the HEX is further analyzed. Results show that the extreme working conditions for the heat transfer and the pressure drops are not consistent. It proves the advance of the multi-condition design method over the traditional single-condition design method. In general, the proposed QT-GA design method is an efficient way to solve the multi-condition problems related to the aircraft HEX or other energy systems.

Proposal, Robustness Analysis and Equivalent Implementation of Optimization Method for Row-by-Row Fin Distribution in Multi-Row Frosting Evaporator

The evaporator in a frost-free refrigerator typically has more tube rows, but frost deposition reduces along the airflow direction. Correspondingly, the evaporator fin distribution is thinner in the upstream rows but denser downstream, and a good match between frost and fin distribution is achieved to recover evaporator capacity loss. However, quantitative design principles of non-uniform fin distribution are lacking. A quasi-static frosting evaporator model is established and experimentally verified considering a three-dimensional (3D) evaporator, 1D frost growth and 1D non-uniform fin distribution. An optimization method for row-by-row fin distribution of a multi-row frosting evaporator is proposed based on the air pressure drop’s increase rate. When the increase rate in the air pressure drop of each row is almost equal, the smallest overall evaporator pressure drop is obtained, leading to the highest air flowrate and the greatest evaporator capacity. By applying the method, the air flowrate and the evaporator capacity increase by 5.5% and 4.6%, respectively, compared to the original fin distribution scheme. Moreover, the robustness of the optimization method is validated under wide temperature and humidity operating conditions. An equivalent implementation under an initial no-frost condition is also proposed to facilitate the optimization method without calculating the whole frosting process.

The effect of heat conduction through fins on the performance of finned-tube CO2 supercritical gas coolers

Air cooled CO2 finned-tube gas coolers are widely used in CO2 trans-critical refrigeration systems due to simplified structural designs. However, the longitudinal heat conduction along fins can lead to inverse heat transfer between adjacent tubes and thus capacity degradation of the heat exchanger. To cope with this adverse effect, the heat exchanger design with split fins between tube rows can be applied although further verification and analysis are expected. Subsequently, as explained in this paper, detailed CFD models have been purposely developed and simulated for the CO2 gas coolers with both continuous and split fins to quantify the effect of the heat conduction through fins. At various operating conditions of both air and refrigerant sides, totally 36 cases were simulated to study the influence of air inlet velocity, air inlet temperature, CO2 pressure and CO2 mass flow rate as well as the heat conduction along fins on the performance of the finned-tube gas coolers with and without split fins. The simulation results show that the approach temperature can be reduced by 0.6–7.5 K for the heat exchanger with split fins and a specific tube circuitry arrangement. Subsequently, the maximum and average heating capacities can be increased up to 22% and 10% respectively when the finned-tube CO2 gas cooler with split fins is applied. Furthermore, the simulation results show that the application of split fins should be designed and implemented appropriately based on different designs of tube circuitry arrangement in the heat exchanger.

The Dynamic Behavior of Once-Through Cooling Water Systems under Fouling Phenomena

Fouling in condensers of thermal power stations can reduce their heat load, which increases the backpressure of the steam turbine and cause deleterious effects to the energy output. A complete analysis of the condenser behavior must handle the exchanger as an integrated part of an entire cooling system, which also encompasses hydraulic equipment such as the pumping system. This paper presents a computer simulation analysis of the behavior of a condenser affected by inorganic precipitation fouling in the water side. Instead of using a fixed ‘fouling factor’, a dynamic fouling model is incorporated in the overall thermal-hydraulic calculations in order to simulate fouling resistances over time during long term condenser operation, and their dependence on operating parameters such as water velocity and temperature. The mathematical model employed shows how the system performance is affected by the reduction of the overall heat transfer coefficient due to the deposit accumulation and also predicts the hydraulic impact of fouling, which increases the hydraulic resistance for the cooling water flow, resulting in different fouling rates in different tube rows and along each tube length. The condenser model is validated through its comparison with operating data of real power plants.

Influence of different categories of condensates on film condensation of R-134a on in-line arranged 3D finned tubes

The condensates generated in the process of film condensation, such as condensing, falling, and acting condensates, are identified and defined to obtain precise inundation effect model of 3D finned tube bundle. Furthermore, the influence of three categories of condensates on the film condensation under different heat fluxes and condensing temperatures is experimentally investigated via the homologous method. A test bench with a test section contains two arrays of 8 horizontal tubes is initially constructed. Experimental results show that the inundation effect coefficient decreases from 1.00 to 0.78 as acting condensate Re increases from 75 to 1860. The influence of heat flux on three categories of condensates is more significant than that of condensing temperature. As tube row depth increases from 1 to 6, the proportion of condensing condensate Re to acting condensate Re decreases from 100% to 15.0%, whereas the proportion of falling condensate Re to acting condensate Re increases from 0 to 85.0%. Furthermore, the condensation heat transfer model of single 3D finned tube and inundation effect model are established, and the deviations of results between experiments and models are almost within ±3.0%. The deviations of inundation effect coefficients between experiment and model proposed in this study are 92.5% and 50.0% less than those between experiment and models using routine method and considering single condensate with homologous method, respectively. Moreover, the inundation effect model proposed in this study reveals that different proportions of condensing or falling condensates to acting condensate lead to different inundation effect coefficient at the same acting condensate Re. The research results will lay the foundation for further revealing the mechanism of film condensation on 3D finned tube bundle and guide the thermal design of 3D finned tubes applied in shell-and-tube condensers.