Heat Exchanger Core Research Articles

Numerical analysis of the effect of longitudinal and transverse pitch ratio on the flow and heat transfer of staggered drop-shaped tubes bundle

Several studies have been conducted to enhance the performance of heat exchangers. In the recent decades, shifting from the bluff bodies to more streamlined ones received more attention. The drop-shaped tube is one of the most propitious options for building a better heat exchanger core. This is because of the aerodynamic profile characteristics of the drop-shaped tube. The present work has been conducted to clarify fluid flow and heat transfer across staggered drop-shaped tubes bundle at various longitudinal and transverse pitch ratios. The investigation is performed at Reynolds numbers Re= (1.78–18.72) x103, longitudinal pitch ratios (PL =1.44, 1.54, 1.64, 1.74, 1.84, and 2.04), and transverse pitch ratios (PT =1.24, 1.44, 1.64, and 1.82). Ansys Fluent software package is used to predict the flow pattern around tubes. The RNG k−ε model is selected to evaluate the turbulence effect of the fluid flow. The results showed that an increase in PT significantly affects the heat transfer as compared to PL. An increase in the transverse pitch ratio from 1.24 to 1.82 enhances the heat transfer by 2.18 - 10.53% for the considered Re range. As the Re increases, the average Nusselt number Nu¯ increases and the friction factor f decreases. At a constant PL, the minimum f was varied with Re and PT. The maximum values of the effectiveness ε and the efficiency index η were achieved for (PT = 1.64; PL = 2.04) at Re=1.78 × 103, and (PT = PL = 1.64) at Re> 1.78 × 103. The drop-shaped tubes bundle showed its superiority over a circular one in terms of high thermal-hydraulic performance under the same operating conditions. Finally, generalized Nu¯, f, and ε correlations for the studied models were predicted, providing a useful reference for future studies or for the design of heat exchangers employing drop-shaped tubes.

Sound localization and quantification analysis of an automotive engine cooling module

Sound emissions of an automotive engine cooling system are studied using both single-microphone directivity measurements and a rotating beamforming technique. These measurements provide reference acoustic data on such a system and some new understanding of the effect that the radiator induces on the distribution of sound sources. Indeed, the beamforming results indicate that, above the frequency limit allowed by the Rayleigh criterion, it is possible to localize and quantify the noise sources even through the heat-exchanger core. Moreover, for the investigated operating points along the fan performance curve, the sources are always distributed at the tip of the blades and, in particular, at the leading edge. The present evidence, confirmed by the similar trends of the frequency spectra with and without the heat exchanger, leads to the conclusion that the dominant sound mechanism is the turbulence-interaction noise. Nevertheless, this turbulence is produced within the gap between the fan ring and its casing rather than generated by the radiator core. The latter appears to induce negligible acoustic transmission losses but, more significantly, is found to have a minimal influence on the aerodynamic modification of sound sources for all the analyzed operating conditions.

Configuration optimization of a membrane-based total heat exchanger with cross-corrugated triangular ducts considering thermal economy and entropy generation

Membrane-based total heat exchanger with cross-corrugated triangular ducts is a new structure, and the changing-section channels in the heat exchanger would make fluid undergo periodic contraction and expansion, which provides an effective approach for strengthening convective heat and mass transfer. Numerous studies proved that the geometric structure have great influence on performance of this type of heat exchanger core, whereas studies about structure optimization considering both thermal irreversibility and economic benefits are rare. In the present study, the total economic return and total entropy generation rate are analyzed as two objective functions simultaneously to evaluate thermal economic and thermal irreversibility performance respectively. A global sensitivity analysis was conducted to select the more sensitive paraments from the six key geometric design parameters: the channel height H, apex angle θ, channel number of each layer on fresh air side nf, channel number of each layer on exhaust air side ne, and the total number of plates nh. They are optimized by the multi-objective particle swarm optimization (MOPSO) algorithm by taking into account entropy generation minimization (EGM) and economic return maximization. The convergence and stability analysis of the algorithm is proposed and the result informs a good application. The optimal geometric design scheme is obtained from the Pareto set according to the normalization method. A parameters analysis was also conducted to study the influence of geometric parameters on the thermal irreversibility and thermal economy performance of the heat exchanger.

Thermal and hydrodynamic analysis of a compact heat exchanger produced by additive manufacturing

The demand for higher heat transfer effectiveness has stimulated the combination of compact heat exchangers and additive manufacturing. The potential to fabricate complex geometries with different materials can optimize the trade-off between heat transfer and pressure drop. In this work, theoretical models for thermal and hydrodynamic performance were developed for a cross-flow compact heat exchanger manufactured with the SLM process, an alternative to Printed Circuits Heat Exchangers. The heat exchanger core has a cubic format with 100 mm edge and 2 mm channel diameter. The raw material used is AISI 316L stainless steel. Furthermore, the relative density of the prototype is 99.8% and the surface roughness measured was 12.21 µm. The one-dimensional steady-state with circular mini channels models are validated with experimental data. The experiments were evaluated in laminar, transition, and turbulent regimes, with different temperatures. The theoretical models have a good agreement, with 3.3% and 15.3% average errors for the thermal and hydrodynamic models. Besides, the impact of surface roughness in the pressure drop was negligible. The influence of replacing the core material in the thermal performance is significant in the turbulent regime.

Experimental investigation and ANN modelling on thermo-hydraulic efficacy of cross-flow three-fluid plate-fin heat exchanger

Compact cross-flow three-fluid heat exchanger with plain rectangular fins is experimentally investigated for both thermal and hydraulic efficacy under steady-state condition. The elaborative investigation is conducted for all the four viable flow arrangements of cross-flow configuration which comprises the effect of Colburn factor and effectiveness ratio of central and adjacent fluids, and friction factor with Reynolds number. It is learnt that thermal efficacy is sentient to the operating conditions, type of flow arrangement and variation in flow rate. The comparison of the efficacy between the plain duct (without fin core) and with fin heat exchanger core is presented to analyse the impact of fins in enthalpy exchange. The regression correlation is developed to estimate the thermal and hydraulic performance which are accurate within ±16% and ±5.8% respectively. The investigation is further assisted with the ANN modelling for the prediction of the thermo-hydraulic efficacy with two input (Reynolds number and flow arrangement type) and four output performance parameters (Colburn factor, friction factor and effectiveness ratios). The fraction of 77% and 23% of data is considered for training and testing the neural network respectively. It is learnt that ANN predictions are more promising and accurate than regression predictions and the magnitude of the value of an absolute fraction of variance equals unity.

Development of novel mathematical models of plate heat exchanger for the task of optimization main parameters of the aviation gas turbine engine with heat recovery

Nowadays, the task of increasing the economic and technical efficiency of the gas turbine engine and decrement of the specific weight and specific fuel consumption is one of the main targets in the development of the aviation gas turbine engine. A promising approach for decrement of specific fuel consumption and obtaining high thermal efficiency of the gas turbine engine (30% and above) is based on the concept of the recuperative cycle of the gas turbine engine. The article presents the computer-aided calculation for analysing the heat exchanger surface and developing novel mathematical models of heat exchanger in terms of weight goodness, flow passage goodness, and optimum weight and flow passage, furthermore, the study presents the novel mathematical models of the pressure drops in the air and gas channels of the heat exchanger. 18 different heat transfer surfaces for heat exchanger cores have been evaluated and the results are presented. The same hydraulic diameter assumed for all heat transfer surfaces and only their thermo-hydraulic performances evaluated during the calculation design. To assess the reliability of the obtained models, the results of the design calculations of the developed models compared with the data of other authors and with the data of the existing gas turbine engine with heat exchanger. The obtained models focus on mathematical design calculation for optimizing the main thermodynamic parameters of a gas turbine engine coupled with a heat exchanger at the stage of conceptual design of aviation gas turbine engine with heat recovery of exhaust gas.

Effect of brazing parameters on fillet size and microstructure of cladded fin-microchannel tube joints

Abstract This work concerns the optimization of furnace brazing conditions for joining micro-multiport aluminum tubes and fins made with AA4343/AA3003/AA4343 brazing sheet in mini-assemblies mimicking the core of an automotive heat exchanger. Taguchi method was used for design of experiment, considering five process parameters with two levels of values. The aim was to maximize the fillet size of the brazed joints, which has an important influence on the thermal integrity and mechanical properties. Fillet length measurements of brazed joints were performed with a metallographic microscope. The statistical analysis allowed to obtain the optimum values of process parameters (peak temperature, residence time, heating rate, microchannel tube type and flux). At a 95% confidence level, the variability of fillet length is most significantly affected by the peak brazing temperature (77%), residence time (15%) and heating rate (7%). The predicted maximum fillet length was (152±11) µm, which was corroborated by confirmation trials. The microstructural analysis of tube−fin joints showed that variations in peak temperature and residence time affect only the size of the eutectic zone of fillet formed, but not the nature or composition of the constituent phases.

Characterization and Scaling of Forced Convective Swirl in Sinusoidal Wavy-Plate-Fin Cores of Compact Heat Exchangers

Abstract The characterization and scaling of the thermal-hydraulic performance in wavy plate-fin compact heat exchanger cores, based on the understanding of the physical phenomena and heat transfer enhancement mechanism is delineated. Experimental data are presented for forced convection in air (Pr = 0.71) with flow rates in the range 50 ≤ Re ≤ 4000. A variety of wavy-fin cores that span viable applications, with geometrical attributes described by the cross section aspect ratio α (ratio of fin spacing to height), fin corrugation aspect ratio γ (ratio of 2× corrugation amplitude to wave pitch), and fin spacing ratio ζ (ratio of fin spacing to wave pitch), are considered. To characterize and correlate the vortex-flow mixing in interfin spaces, a Swirl number Sw is introduced from the balance of viscous, inertial and centrifugal forces. It is shown that the laminar, transitional and turbulent flow regimes can be explicitly identified by this Swirl number. Based on the experimental results and extended analysis, new correlations for Fanning friction factor f and Colburn factor j are developed with Sw, α, γ, and ζ as scaling parameters. Requisite expressions are devised by a superposition of both enhancement components due to the corrugated surface area enlargement and induced swirl flow field, and they are combined to cover the laminar, transitional and turbulent regimes by the method of asymptotic matching. The resulting generalized correlations are further shown to predict all available experimental data for f and j factors to within ±20% and ±15%, respectively.

Reliability analysis of compact and high efficient heat exchanger for FLNG devices

Objectives So far, the compact and high-efficiency heat exchanger is the best choice for floating liquefied natural gas (FLNG) heat exchangers. Static strength analysis and reliability analysis are required to optimize the structural parameters of a heat exchanger and improve its performance. Methods First, the strength of the heat exchanger core is analyzed using the ANSYS Workbench static analysis module, and a simplified model is established which can represent the stress level of most channels. Next, the reliability analysis of the heat exchanger is conducted using the six sigma analysis of the ANSYS Workbench by choosing the cold channel thickness, cold channel pressure, hot channel pressure and elastic modulus as input random variables, and the effects of these random variables on the reliability of the heat exchanger are studied. Results The reliability analysis results show that the cold channel pressure loading is the greatest factor in structural strength, and the reliability of the heat exchanger is more than 99.9%, so the possibility of failure is very low. Conclusions The results of this study can serve as valuable references for the reliability analysis of heat exchangers based on strength.

Lateral dispersion in a lattice porous heater core for compact operations

Louvered fin-and-tube cores have been the most popular choice for enhancing the heat transfer performance of compact heat exchangers such as automotive climate control systems (ACCS), because of their moderate heat removal capacity with least pressure drop compared to other heat exchanger core elements. However, their performance in such systems is debilitated if the incident coolant stream is highly non-uniform due to lateral blockage of flow by the fins resulting from compactness of the overall system. This study demonstrates how such lateral blockage imposed by a conventional louvered fin-and-tube core can be relieved by a “lattice porous core” which induces three-dimensional momentum/thermal dispersion within the core under non-uniform incident coolant streams. Especially under the highly localized non-uniform distribution of coolant streams found in a typical ACCS, the lattice porous core thermally outperforms the conventional core by up to 35% within its typical operational range.

A mathematical modelling of frost growth dynamics on a surface of fin-and-tube air cooler

A mathematical model was developed to predict the performance characteristics of fin-and-tube air coolers with plane fins, taking into account the temperature and humidity conditions, non-uniform thickness of the frost layer along the heat exchanger core, flow-pressure characteristics of the fans and the refrigerant superheat zone. The model was verified on the experimental data of air cooler performance in frosting conditions obtained by the authors in air temperature range from minus 6 to minus 20 ° C, and experimental data from independent researchers. The presented model predicts the cooling capacity and the overall heat transfer coefficient in the evaporator with error not exceeded 15%. It was found that during dry operating conditions of the air cooler, the refrigerant boiling temperature descent by 10 °C leads to a decrease in the overall heat transfer coefficient by 35%. The main reason of that is double decrease of the refrigerant mass velocity and the refrigerant-side heat transfer coefficient by 75%. The process of frost grow leads to an increase in overall thermal resistance in the studied air cooler by 30%, more than double increase in the aerodynamic resistance and decrease in cooling capacity by 15%.

Design and Development of a Trapezoidal Plate Fin Heat Exchanger for the Prediction of Heat Exchanger Effectiveness

A trapezoidal plate-fin exchanger has been designed and developed, and an experimental test rig fabricated to test the plate fin heat exchanger. The heat exchanger was constructed in a 5 layer cross-flow arrangement. The length of the trapezoidal fins between the layers were 380mm and its height, thickness, top width and bottom width were 40mm, 0.5mm, 20mm, and 80mm respectively. A hot fluid test has been conducted to determine the thermo- hydraulic performance of the given heat exchanger at different mass flow rates (4.975 kg/s to 9.751 kg/s) at a hot inlet temperature of 369K. The values of the effectiveness obtained were plotted against the corresponding values mass flow rate to compare and evaluate the variation of the results. Thus, the performance of a heat exchanger with trapezoidal fins has been studied experimentally and it has been determined that: the mass flow rate of the fluids is proportional to the temperature drop of the fluids after passing through the exchanger. Also Increase in mass flow rate, increases the effectiveness of the heat exchanger. Improper insulation influenced heat transfers in heat exchanger cores and caused energy imbalance in the heat exchanger. This study suggests that the calculated effectiveness of 0.98 using trapezoidal plate fin heat exchanger result provides benchmark data to evaluate and predicts the performance of a plate-fin heat exchanger with trapezoidal fins for energy recovery application. Keywords: Heat exchanger development; trapezoidal fin, effectiveness, heat exchanger analysis; hot and cold fluids DOI : 10.7176/JETP/9-8-03 Publication date: November 30 th 2019

Off-design performance of a louvered fin-tube heater core in an automotive climate control system

Louvered fin-and-tube heat exchanger cores, operating inside of an automotive climate control system (ACCS), are used to control temperature and humidity in a passenger cabin. The thermal performance of these cores is typically sensitive to flow features inside the core, which have been well studied for uniform flow fields as well as some non-uniform and inclined flow-fields upstream of the core. The compact geometry of an ACCS unit results in the flow-fields upstream of the core being different from those used in previous studies. When measuring the performance of a louvered fin-and-tube heater core inside a commercially manufactured ACCS unit, up to a 60% reduction in the core’s thermal performance compared to the performance when exposed to a uniform upstream flow is detected. Such a drastic performance reduction has not been reported in the literature. As such a systematic breakdown is made to elaborate how some key and specific ACCS unit features designed for the sake of compactness could play a detrimental role in reducing the overall thermal performance of a given louvered fin-and-tube heater core. By mimicking the actual ACCS features the performance deterioration in the commercial unit could be largely replicated. The individual contributing effects of the various fluidic features such as flow separation and recirculation are determined. Only 50% of the measured 60% deterioration, however, could be accounted for, suggesting that other ACCS unit properties not assessed in detail in this study may also adversely affect the core’s performance.

Numerical assessment of the performance of a heat exchanger for a low pressure ratio gas turbine

c) At the present work, a detailed analysis of the thermodynamic cycle of a gas turbine operating with low pressure ratio for heli engine configurations was performed. The conditions of the selected configuration were based on data found in open literature. A heat exchanger core of circular tubes was designed respecting the heli engine geometrical limitations, implementation constraints and operational conditions. The heat exchanger sizing utilized heat transfer related correlations from literature. The assessment of the heat exchanger performance was carried out with detailed CFD computations including integration aspects of the heat exchanger detail geometry in the heli engine configuration. The computational results were used for the calculation of pressure loss and heat transfer characteristics. Finally, an assessment of the heat exchanger thermal and hydraulic effect was performed on a thermodynamic model of a recuperative heli engine which included the derived performance characteristics of the heat exchanger. The results presented a thermal efficiency improvement of 10.6% and fuel consumption reduction of 9.3%. The investigated heli engine was compared to a non-recuperative one of similar technological level, showing the existing significant potential for optimization based on recuperation technology for designing more cost-efficient and environmentally friendly heli engines.

An additively manufactured metallic manifold-microchannel heat exchanger for high temperature applications

This work presents an additively manufactured manifold-microchannel heat exchanger made of Inconel 718 and experimentally tested for high temperature aerospace applications. The heat exchanger core with a size of 66 mm × 74 mm × 27 mm was fabricated as a single piece through the direct metal laser sintering process. A minimum fin thickness of 180 μm was achieved. Successful welding of additively manufactured headers with the heat exchanger core and conventionally manufactured flanges was demonstrated through the fabrication of the unit. The heat exchanger was tested using nitrogen (N2) on the hot-side and air on the cold-side as the working fluids. The experimental tests were conducted at 600 °C on the hot-side and 38 °C on the cold-side. A maximum heat duty of 2.78 kW and a maximum overall heat transfer coefficient of 1000 W/m2K were achieved during the experiments. The decent agreement between the experimental and the numerical results demonstrates the validity of the numerical analysis model used for heat transfer and pressure drop prediction of the additively manufactured manifold-microchannel heat exchanger. Compared to conventional plate fin heat exchangers, nearly 25% improvement in heat transfer density— the ratio between heat duty and mass (Q/m)—was noted at a coefficient of performance (COP) of 62.

Conceptual design study of a geared turbofan and an open rotor aero engine with intercooled recuperated core

The development of more efficient aero engines is becoming a matter of great importance due to the need for pollutant emissions reduction (e.g. CO2, NOx). Toward this direction, two of the most promising aero engine architectures that have been proposed are the ultrahigh by-pass geared turbofan and the open rotor configurations, both of which are targeting the low thrust-specific fuel consumption and reduced NOx production. In the current study, investigations are performed in order to determine the improvements in thrust-specific fuel consumption for these configurations. More specifically, on the basic geared turbofan and open rotor configurations an intercooler and a recuperator are implemented between the compressors and the exhaust nozzle, respectively. The intercooler is installed in order to reduce the high pressure compressor work demand, while the recuperator is used in order to preheat the compressor discharge air by exploiting the otherwise wasted increased enthalpy content of the exhaust hot gas. The recuperator consists of elliptically profiled tubes and its design is based on the innovative tubular heat exchanger core arrangement that has been invented and developed by MTU Aero engines AG. The intercooled recuperative geared turbofan is evaluated against a nonintercooled recuperative geared turbofan, while the intercooled recuperative open rotor is evaluated against a nonintercooled recuperative open rotor. The results showed that the implementation of intercoolers and recuperators can further improve specific fuel consumption and can also lead to NOx emission reduction.

A mathematical model for flow maldistribution study in a parallel plate-fin heat exchanger

In this paper, a mathematical model is developed to quantitatively evaluate the effect of flow maldistribution in a multi-channel heat exchanger. A computational fluid dynamics (CFD) technology is used to obtain the fluid distribution in the core of heat exchanger, taking into account the effects of gross passages, headers and distributors, etc. Based on the CFD simulation profile, a heat exchanger model is then developed. A parallel plate-fin heat exchanger with N flow passages is divided into N sub-exchangers when only one fluid is in maldistribution mode or 2N-1 sub-exchangers when both fluids are in nonuniformity modes to guarantee the uniform flow patterns in each sub-exchanger. Based on ε-NTU theory, the effectiveness of the heat exchanger is calculated by modeling the heat exchanger as a parallel coupling of the sub-exchangers. The core pressure drop of the whole heat exchanger is taken as the average pressure drop of the sub-exchangers. The mathematical model combining with the achieved flow distribution was utilized to investigate the performance of the heat exchanger with conventional header configurations, a punched baffle header configuration and a quasi-S header configuration, respectively. Results indicated that a good synergy of flow rates for both cold and hot fluids in the adjacent sub-exchangers can effectively reduce effectiveness deterioration. The performance of the heat exchanger is improved by using the improved header configurations. And the performance with a quasi-S header configuration is the best. The proposed mathematical model provides a theoretical basis for multi-channel heat exchangers in solving the problem of flow maldistribution.

Force Based Measurement Method for Cooling Flow Quantification

AbstractQuantification of heat exchanger performance in its operative environment is in many engineering applications an essential task, and the air flow rate through the heat exchanger core is an important optimizing parameter. This paper explores an alternative method for quantifying the air flow rate through compact heat exchangers positioned in the underhood of a passenger car. Unlike conventional methods, typically relying on measurements of direct flow characteristics at discrete probe locations, the proposed method is based on the use of load-cells for direct measurement of the total force acting on the heat exchanger. The air flow rate is then calculated from the force measurement. A direct comparison with a conventional pressure based method is presented as both methods are applied on a passenger car’s radiator tested in a full scale wind tunnel using six different grill configurations. The measured air flow rates are presented and discussed over a wide range of test velocities. The advantages and draw backs of both approaches are compared and discussed in detail. The proposed method is non-intrusive, leaving the heat exchanger core intact, with no need for integration of measurement points over the core region. Due to the measuring principle, the load-cell method will inherently over-predict the air-flow rate. This error is quantified and an empirical correction function is investigated. This paper shows that the corrected force based method determines the air flow rate through a heat exchanger with an accuracy similar to that of traditional pressure/velocity methods while offering a considerable number of advantages.

Numerical investigation on various heat exchanger performances to determine an optimum configuration for charge air cooler, oil and water radiators in F1 sidepods

Abstract The present work focuses on a three-dimensional CFD approach to predict the performance of various heat exchangers in conjunction with non-isothermal transitional flows for motorsport applications. The objective of this study is to determine the heat transfer, pressure drop and inhomogeneous flow behaviour for distinct heat exchangers to identify an optimum configuration for the charge air cooler, water and oil radiators placed in the sidepods of a formula one (F1) car. Therefore, a comprehensive analysis of various heat exchanger configurations has been carried out in this work. In order to assess the reliability of the obtained results, a mesh sensitivity study along with additional parametric investigations have been performed to provide numerical parameters predicting accurately (a) the heat transfer rate at the fluid-solid interface and (b) the sporadic separation. As a result of the performed validation procedure in this study, the aerodynamic- and thermal boundary layer development along with the convective characteristics of the air flow have been captured accurately near to the heated surface. The characterization of a heat exchanger core and a core configuration in a closed domain is also possible with this procedure. The presented three-dimensional CFD approach could overcome the difficulties of macroscopic heat exchanger and porous media methods for F1 applications, because it can be used to predict the heat transfer and pressure drop related to the mass flow rate correlation curves. The contribution of fins to the total heat transfer rate has been predicted theoretically, and application benchmark test cases have been presented to analyze five different heat exchanger configurations in accordance with the 2014 formula one technical regulations. The numerical data extracted directly from three-dimensional CFD simulations can be used in the sidepod design process of the external cooling system of F1 engines.

Pressure drop and heat transfer characteristics of a high-temperature printed circuit heat exchanger

Printed circuit heat exchanger (PCHE) is one of the leading intermediate heat exchanger (IHX) candidates to be employed in the very-high-temperature gas-cooled reactors (VHTRs) due to its capability for high-temperature, high-pressure applications. In the current study, a reduced-scale zigzag-channel PCHE was fabricated using Alloy 617 plates for the heat exchanger core and Alloy 800H pipes for the headers. The pressure drop and heat transfer characteristics of the PCHE were investigated experimentally in a high-temperature helium test facility (HTHF) at The Ohio State University. The PCHE helium inlet temperatures and pressures were varied up to 464°C/2.7MPa for the cold side and 802°C/2.7MPa for the hot side, respectively, while the maximum helium mass flow rates on both sides of the PCHE reached 39kg/h. The corresponding maximum channel Reynolds number was approximately 3558, covering the laminar flow and laminar-to-turbulent flow transition regimes. New pressure drop and heat transfer correlations for the current zigzag channels with rounded bends were developed based on the experimental data. Comparisons between the experimental data and the results obtained from the available PCHE and straight circular pipe correlations were conducted. Compared to the heat transfer performance in straight circular pipes, the zigzag channels provided little advantage in the laminar flow regime but significant advantage near the transition flow regime.