Bundle Heat Exchanger Research Articles

Contribution of transient heat transfer to overpressure protection in a passive Boiling Water Reactor

The BWRX-300 reactor is a small modular reactor with 300 MW nominal electric power output. As the abbreviation indicates, the reactor is a boiling type reactor using water as the coolant. The main operating principle of the reactor model is the use of natural circulation of the coolant for both steady-state operation and emergency cooling of the reactor. Isolation Condenser Systems (ICS) have been used in early BWRs and were reintroduced in the 1990s to implement passive safety.Modern technologies enable effective simulation of objects of various complexity. In this study, the thermal-hydraulic system code TRACE v.5 is used to simulate BWRX-300 reactor plant operation. The software enables calculation of a system consisting of the reactor pressure vessel and ICS in both steady-state and transient operation.The ICS is an emergency cooling system borrowed from the previous generation of GE Hitachi BWRs, the ESBWRs, and it consists of a vertical tube bundle heat exchanger immersed in a water tank. Although this system design remains unchanged, its purpose is different. The BWRX-300 design completely eliminates safety and relief valves. This is why the ICS must perform both overpressure protection and long term heat dissipation functions.Three reactor plant operation states were simulated: normal operation with nominal parameters; reactor isolation; and the transient process into the cooling down mode by reactor shutdown and ICS activation. The results demonstrate that the ICS is capable to mitigate overpressure transients. Upon ICS startup, its metallic structures absorb heat well in excess of its nominal steady-state rating. Initial thermal inertia of the system is important for overpressure mitigation in transients.

Numerical study of shell-side flow-induced noise characteristics of staggered straight tube bundle heat exchanger under different inflow conditions

This paper presents a numerical study on the shell-side flow noise characteristics of staggered straight tube bundle heat exchangers under varying inflow conditions. A three-dimensional numerical model of a compactly arranged staggered straight tube bundle heat exchanger was established. To validate the numerical model, a feasibility study using a cylindrical flow model was conducted. Subsequently, the large eddy simulation (LES) method was employed to investigate the unsteady shell-side flow field characteristics, and the vortex characteristics of the flow field were obtained based on the Q-criterion. Finally, the acoustic finite element method (FEM) was utilized to calculate and analyze the noise characteristics at the inlet and outlet positions of the heat exchanger shell side. The results show that the inflow angle has an influence on both the flow field and the acoustic field of the heat exchanger shell side. As the inflow angle increases, the flow stability decreases, horseshoe vortices replace boundary layer vortices, and the decrease in fluctuating pressure leads to a reduction in flow noise. Analysis of the sound pressure level spectrum indicates a line spectral characteristic at low frequencies, with high-frequency components experiencing rapid attenuation.

Experimental study on flow and heat transfer characteristics of three types of finned tube bundle heat exchangers applied in aero-engines

This study outlines the design and manufacture of three types of finned tube bundle heat exchangers (HX) for advanced aero-engines followed by a series of experimental evaluations on flow and heat transfer characteristics. Considering the compactness and reliability, the finned tube bundle HX is more suitable for aero-engines when the heat transfer unit is small-diameter serpentine tubes with the enhanced heat transfer area provided by fins. In this paper, the Logarithmic Mean Temperature Difference method (LMTD) was utilized for the design of a small-diameter (OD: 3.6 mm with 0.3 mm thickness) finned tube bundle HX. Mass reduction was achieved by perforating the fins and removing the support devices, respectively. The flow and heat transfer characteristics of HXs and the impact of the mass reduction schemes were evaluated based on a series of comparative experiments. Among them, the connectionless scheme can significantly improve the power-to-mass ratio, providing it certain application value despite causing considerable flow resistance. Subsequently, empirical correlations for the flow and heat transfer outside the tubes suitable for small-diameter finned tube bundle HXs were proposed based on the experimental results, with over 95 % of the data falling within a 10 % error margin. These correlations have substantially modified the previous one for large-diameter straight tubes, providing an important reference for the future design of HXs in aero-engine.

Thermal-hydraulic investigation of liquid metal cross flow different arrangements of tube bundles under pulsating

The topic of this paper is to couple the passive and active approaches to enhance the heat transfer performance in the inline tube bundle heat exchanger. Firstly, the k-kl-ω turbulence model and Kays turbulent Prandtl number model are validated by liquid metal cross flow tube bundle experiments and rectangular groove pulsating flow experimental results. Then the numerical studies were carried out for pulsating velocity amplitude of 0.5, frequency of 10, and Re of 20,000 with transverse and streamwise pitch-to-diameter ratios of 1.5, 1.65, and 1.8. The introduction of pulsating flow effectively improves the turbulence characteristics between the tube bundles, especially at the first three rows, where the velocity fluctuation is enhanced by about 1.5–2 times. The tube heat transfer performances are all improved. The performance evaluation criterion (PEC) variation ranges from 1.04 to 1.32, which signifies that the introduction of pulsating flow can enhance the comprehensive heat transfer performance. The per unit flow rate thermal entropy generation decreased by about 9.7–33.3 %, in addition, a phase difference between the transient thermal entropy production and pulsating velocity was found. The correlation equations about Nu and f factor within the scope of this paper are presented.

Numerical investigation of the influence of the heat exchange structure on the gas-liquid dispersion in a large-scale fermenter

A combination of computational fluid dynamics (CFD) and population balance model (PBM) was used to study the gas-liquid mixing conditions in a fermenter. The effects of the heat exchange structure on the flow structure, gas holdup, agitator power, bubble size distribution and interfacial area were investigated by numerical simulations. The simulation results showed that radial gas dispersion and liquid penetration are strongly hindered by the spiral heat exchanger, as there is no gap between the annular tubes at the circumference. An unstable flow structure, gas accumulation around the shaft and gas leakage, as well as large bubbles around the heat exchange tubes are due to the poor gas distribution. By increasing the spacing of heat exchange tube bundles in circumferential, the gas-liquid mixing in the fermenter is gradually improved with the uniform row heat exchanger, the compact row heat exchanger and the bundle type heat exchanger. The bundle heat exchanger acts as a baffle to suppress the large bubbles and effectively reduce the bubbles sizes and promote gas dispersion, which exhibits the maximum interfacial area and interfacial area per unit power. Compared to the fermenter with a spiral heat exchanger, the interfacial area and interfacial area per unit power of the fermenter with a bundle heat exchanger increase by 36.5 % and 9.1 %, respectively.

Modelling and fault analysis of APROS-based moisture separation reheater

A modular modelling approach was adopted to build a dynamic simulation model of the moisture separation reheater on a high-precision APROS real-time platform to investigate the dynamic operating characteristics of the moisture separation reheater. The model was subjected to steady-state and step dynamic tests at different loads. The results show that the established model can correctly reflect the steam flow and heat transfer properties and can meet the requirements of the simulation. Using the one-dimensional distribution of the model, the effect of a small breakage failure in the heat exchanger bundle was further investigated. The results showed that the heat transfer tube bundle had leaks, reducing the heat transfer capacity and affecting the quality of the steam used to work in the low-pressure turbine. Simultaneously, the effect on steam outlet parameters increased with the degree of leakage in the heat exchanger.

Numerical investigation of thermal-hydraulic characteristics in cross-flow heat exchangers with different twisted oval tubes

This study presents a new strategy to strengthen the flow and thermal performance of gas-liquid tube bundle heat exchangers by combining oval tubes with variable-direction twist method. The effects of twist direction, twist angle and flow velocity on the overall performance were investigated. The variable-direction twisted oval tube enhances the mixing degree of the main stream and the tail vortex, thereby improving the thermal performance and reducing the pressure drop. Both heat transfer coefficient and pressure drop increase with the increase of twist angle, and the overall performance is negatively correlated. Twist period has little effect on overall performance. Convective heat transfer coefficient of the variable-direction twisted oval tube with a twist angle of 120° is increased by 5.8%–10.2%, pressure drop is reduced by 5.0%–7.8% and overall performance is increased by 7.4%–10.7%, as compared circular tube bundle. The proposed variable-direction twisted oval tube realizes the synergistic enhancement of thermal-hydraulic performance, and can directly replace the traditional circular tube. The work of this paper provides technical guidance for improving the processing technology of twisted oval tube and developing new heat exchangers.

Heat recovery optimization of a shell and tube bundle heat exchanger with continuous helical baffles for air ventilation systems

We report a numerical evaluation of the impact of continuous helical baffle on the heat recovery efficiency of counterflow tube bundle heat exchangers. The baffle inclination angle has been varied from 11∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$11^{\\circ }$$\\end{document} to 22∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$22^{\\circ }$$\\end{document}. Since the fluid flows over the tube bundle at an angle due to helical flow inside the shell, the heat exchanger operates in cross counter mode. Fluent simulations with the k-ω\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega$$\\end{document} transition shear stress transport turbulence model have been performed to investigate the thermal-hydraulic parameters of the system in terms of heat recovery efficiency, pressure loss, and overall heat transfer rate. Outside air temperature has been varied to mimic cold and warm weather. Pressure loss has been constrained to be less than 250 Pa, conforming to EU guidelines for energy labeling of residential ventilation units. At the maximum volume flow rate of 40 m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}/h, the device performed with over 80%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$80\\%$$\\end{document} heat recovery efficiency for the considered temperature difference. Continuous helical baffles helped to improve convective heat transfer by reducing cross flow area and increasing velocity. Smaller angles result in greater pressure loss while having no discernible effect on heat recovery efficiency for the considered geometry. The analysis demonstrates the potential of a compact counterflowing recuperative heat exchanger with continuous helical baffles for decentralized ventilation systems and serves as a basis for further optimization.

Numerical study on the effect of baffle structure on the heat transfer performance of elastic tube bundle heat exchanger

To make up for the research gaps in the existing literature on the effect of baffle structural parameters on the vibration and heat transfer of elastic tube bundle (ETB) heat exchangers, and improve the comprehensive heat transfer performance (HTP) of heat exchangers, based on an improved elastic tube bundle (I-ETB) heat exchanger used in previous studies, using the bi-directional fluid-structure interaction calculation method, the effects of the baffle structure improvement (height and curvature) on the comprehensive HTP of the I-ETB heat exchanger at different inlet velocities (Uin) were numerically studied. The results show that: while the height factor increases from 0.70 to 0.80, the vibration amplitude of I-ETBs decreases maximum by 10.05 %, and the heat transfer coefficient (h) of I-ETB increases maximum by 5.50 %. Increasing the baffle height improves the comprehensive HTP while suppressing the vibration of the I-ETBs. While the baffle curvature is 10°, the vibration of I-ETBs is the most intense, with the amplitude of the I-ETBs improving maximum by 10.79 %. The Nusselt number per unit pressure drop Nu/ΔP and performance evaluation criteria PEC and JF (a dimensionless number related to Colburn factor j and Fanning friction factor f) values of the heat exchanger all reach the maximum while the baffle curvature is 10°. The increase of baffle curvature has a positive effect on the vibration-enhanced HTP of the I-ETBs, and when the curvature is 10°, the comprehensive HTP of the heat exchanger is better.

Effect of pulsation parameters on the spatial and temporal variation of flow and heat transfer characteristics in liquid metal cross flow the in-line tube bundle

Liquid metal has a broad application prospect in tube bundle heat exchangers because of its excellent thermal conductivity. However, conventional in-line tube bundle arrangements often exhibit limited secondary flow, so new methods are urgently needed to improve heat transfer. The topic of this paper is to combine two methods, liquid metal cross flow tube bundle and pulsating flow, to improve the heat transfer performance in-line tube bundles. Firstly, the applicability of the model was validated through experimental data from existing literature, encompassing overall flow dynamics, heat transfer performance, and circumferential pressure distribution around the tubes. Subsequently, numerical simulations are conducted involving liquid metal cross flow in-line tube bundles at varying pulsating frequencies, amplitudes, and Reynolds numbers. Circumferential pressure distributions and temperature profiles for individual tubes were analyzed and compared. The amplitude of reduction in thermal resistance compared to the no-oscillation condition increases gradually from a minimum of 8 % in the first row to a maximum of 40 % in the third row, while the reduction in the rear row of tubes is similar to that of the third row. Remarkably, a reversal in the trend of thermal resistance for the first three rows of tubes under pulsating flow were observed, contrasting conventional flow except for the case with an amplitude of 0.1. By analyzing the local transient flow field distribution near single tube and the corresponding circumferential heat transfer performance, the influence of the vortex evolution characteristics on the heat transfer performance at different phases of the pulsating velocity is revealed, which transient Nu is up to three times that of flow with no pulsating. Finally, a series of coherence analyses reveal that as frequency rises, turbulence exhibits a richer spectrum of small-scale vortex structures, intricately linked to enhanced energy cascade efficiency. Amplitude augmentation intensifies velocity fluctuations, particularly at high amplitudes, resulting in substantial energy peak amplification. The integrated heat transfer performance PEC factor varies in the range of 1.25 to 1.51 for all the conditions simulated in this paper, which indicates that the liquid metal coupled pulsating flow approach can significantly increase the comprehensive performance of the tube bundle heat exchangers. These findings hold significant promise for advancing heat exchanger design and enhancing thermal efficiency, with implications for various engineering applications.

Large Eddy Simulation of the Inlet Cross-Flow in the CiADS Heat Exchanger Using the Lattice Boltzmann Method

The liquid lead-bismuth eutectic (LBE) is the coolant of the heat exchanger in China initiative Accelerator Driven System, which may have a risk of structural failure due to the washout of the coolant in the inlet of the heat exchanger. It is necessary to study the mechanical properties of the heat exchanger bundles of CiADS, especially the fatigue life of the bundle structure in the transverse flow of the LBE. The numerical simulation is the Lattice Boltzmann method combined with the large eddy simulation by Python codes. The velocity distribution of the flow field and the time domain characteristics of the heat exchanger bundles’ force are calculated, and the frequency domain characteristics of the heat exchanger bundles’ vibration are obtained by Fourier transform. The bundles vibrate at high cycle fatigue in turbulent flow at high Reynolds number. The transverse flow of LBE does not affect the fatigue life of the bundle. No structural failure occurs in the CiADS heat exchanger due to the transverse flow of LBE.

Investigation of Outdoor Air Temperature Effects on Heat Recovery in Cross Flow Heat Exchanger

A heat recovery device was designed and manufactured to provide air-to-air heat transfer by using a cross-flow heat exchanger on the pipe bundles. For this purpose, different cold air inlet temperature values and the temperature values of high temperature hot exhaust gases in the tube bundle heat exchanger were determined as operating parameters. According to the temperature increase of the cold air entering the heat exchanger; The kinematic viscosity values of the air increased and the maximum Reynolds number in the flow state of the air passing through the test heat exchanger was decreased. The average Nusselt numbers and average air convection coefficients in the flow of air decreased according to the temperature increase of the cold air. According to the temperature increase of the cold air entering the heat exchanger, both the analytical and experimental outlet temperature values of the cold air leaving the heat exchanger increase very close to each other. With this increase, the amount of heat transferred from the fluid to the fluid in the heat exchanger decreases. While the temperature value of the cold air entering the heat exchanger increases, the temperature difference value of the advancing air from the beginning to the outlet (ΔT= Ts – Ti) decreases. Due to this decrease, a decrease was observed in the amount of heat transfer.

A feasibility study of a tube bundle exchanger with phase change materials: A case study

Latent heat thermal storage offers a flexible service to members of a heat district grid as the stored heat can be used to reduce the morning peak demand, limiting the consequences for the production facilities. This work investigates the optimized design of a latent heat thermal storage reactor, integrated in an existing building supplied by district heating, for demand-side management applications. The storage reactor was designed as a tube bundle heat exchanger in which a commercial-grade paraffin was used as the phase change material. The optimized design variables that maximized the use of the Phase Change Material were the tube pitches and the fin heights. The integration of the storage reactor with the energy system required the control and prediction of its state of charge and the power output. These parameters are usually evaluated through 3D numerical dynamic simulations that require a large computational effort, which is beyond the capability of basic microcontroller systems. A simplified parametric model has been used to overcome this issue. A load-tracking algorithm was embedded in an existing building program. The algorithm was able to reduce the thermal power peak by 62 kWh from 6:00 to 9:00 a.m. Decreasing the pitch from 95 mm to 82 mm led to a +34% increase in the use of the PCM. Similar results were obtained when the fin height was increased from 20 mm to 32 mm. The results of this investigation suggest that the proposed model could be applied to similar buildings fed by district heating network systems.

Mathematical Investigation of Heat Transfer Characteristics and Parameter Optimization of Integral Rolled Spiral Finned Tube Bundle Heat Exchangers

In this study, the effects of fin tip thickness and fin root thickness of integral rolled spiral finned tube bundles on flow resistance, heat transfer performance and heat transfer and flow exergy destruction were investigated via mathematical simulation. The correlations between heat transfer and flow resistance performance were fitted with dimensionless numbers. The optimized parameters with performance evaluation criteria (PEC) as the objective were obtained using methods involving computational fluid dynamics and machine learning. The results show the effects of fin tip thickness and fin root thickness on the Nusselt number (Nu), Euler number (Eu), PEC, heat transfer exergy destruction (ExT) and flow exergy destruction (ExP) as obtained via mathematical simulation. A new mathematical correlation is proposed for predicting the Nu and Eu of integral rolled spiral finned tube bundles. Among the four optimization models tested, the random forest regression algorithm was the most accurate algorithm for PEC prediction models. In the studied range, the optimal parameters were a fin tip thickness of 2 mm and a fin root thickness of 3.5 mm. Compared with the initial parameters, when the Reynolds number was 20,380, the PEC increased by 2.53%, the ExP increased by 2.37% and the ExT decreased by 7.96%.

Numerical research on vibration and heat transfer performance of a conical spiral elastic bundle heat exchanger with baffles

To improve the comprehensive heat transfer performance of elastic tube bundle heat exchangers, four heat exchanger schemes were proposed based on a conical spiral elastic tube bundle heat exchanger, in combination with an arrangement of pulsating baffles and heat transfer elements. The four heat exchanger schemes include two without baffle, one with the same direction baffles and one with opposite direction baffles. Using a calculation method of bidirectional fluid-solid coupling, the influences of the vibration and heat transfer performance for the four exchanger schemes at various Reynolds numbers were studied. The results display that the two schemes with baffles can enhance the regularity of the fluid movement and the hierarchy of the temperature field distribution. The average amplitudes of the conical spiral elastic tube bundles in the four heat exchanger schemes are increased by 107%, 117%, 1785%, and 1935%, respectively, when the Reynolds number is increased in the range of 1000–9000. The performance evaluation criteria of conical spiral elastic tube bundles in the four heat exchanger schemes are increased by 1.89%, 2.68%, 5.71%, and 5.84%, respectively. The schemes with the pulsating baffle installed in heat exchangers can improve the heat transfer performance. Especially, the installation direction of the baffle will affect the comprehensive heat transfer performance of heat exchangers, which has important significance in engineering application.

Investigations on boiling heat transfer characteristics in non-uniform heating U-Tube bundles under rolling motion

The development of the marine environment necessitates more rapid development of marine nuclear power plants, and the U-tube bundle heat exchangers, which exist in marine nuclear power plants, play a significant role in safety and economy. However, the radial non-uniform heating (NUH) brought on by the specific structure of the U-tube complicates the boiling heat transfer characteristics. Additionally, the environment of ocean rolling motion superimposed with NUH makes heat transfer more complex. The paper analyzes the time-averaged and transient heat transfer performance by contrasting the rolling UH experiments and rolling NUH experiments. The result demonstrates that the addition of NUH causes no impact on the influenced degree of rolling motion on the time-averaged temperature characteristics, which means a weakly coupled action of NUH and rolling motion. Based on the independence principle, considering the influencing factor of rolling motion and NUH, the developed correlation for calculating time-averaged heat transfer coefficient (HTC) can capture 96.67% of the experimental data within 15% of the error line, and the average absolute error is about 5.75%. For instantaneous fluctuated characteristics, the addition of NUH has a greater impact on the fluctuated range of water temperature than that of the wall temperature, and the fluctuation introduced by rolling motion is displayed based on the effect of NUH. Besides, the calculating method for determining the HTC fluctuation amplitude under uniform heating and rolling motion conditions also applies to situations in which NUH and rolling motion coexist, and the mean absolute error is around 13.80%, indicating that the coupled action of NUH and rolling motion is less obvious in forced circulation. The fluctuation amplitude of HTC depends more on the thermal parameters and rolling parameters. Finally, a novel computational logic approach was created and proposed to estimate transient HTC under conditions for NUH and rolling motion, which can anticipate the fluctuated amplitude and period of instantaneous HTC under corresponding conditions.

Research on the heat transfer performance of an improved elastic tube bundle heat exchanger under fluid-induced vibration

The heat transfer performance (HTP) of an improved elastic tube bundle (IETB) was analyzed using a bi-directional fluid-structure interaction calculation method. The vibration-enhanced HTP of heat exchangers with different inlet velocities (Uin) and row number N were studied. The results show that the amplitude gradually increases as the Uin increase with a maximum increase of 309.98%. The amplitude with fixed shell side length (FL-heat exchanger) is generally higher than that with variable shell side length (VL-heat exchanger) under different row numbers. The average amplitude of the three directions increased by 4.55%, 11.54%, and 7.41%, respectively. The heat transfer coefficient (h) is directly proportional to Uin, and the comprehensive HTP gradually decreases with the increase of Uin. With the increase in the row numbers, h generally showed a downward trend. For the VL-heat exchanger, h decreases as the row number increases, which increases from N=6 to N=9, and h decreases by 13.85%. For the FL-heat exchanger, h first increases and then gradually decreases. The comprehensive HTP also maintained the same trend. Compared VL-heat exchanger and FL-heat exchanger, reducing the spacing between tube rows increases heat transfer per unit volume, leading to improved comprehensive HTP of the IETB heat exchanger.

Numerical analysis of vibration response of elastic tube bundle of heat exchanger based on fluid structure coupling analysis

Abstract A tube bundle heat exchanger is a typical heat exchange equipment that exchanges heat between two fluids with different temperatures. Through this equipment, one fluid can be cooled down and another fluid can be heated up to meet their respective needs. The equipment is widely used in chemical, petroleum, pharmaceutical, energy, and other industrial sectors, and is one of the indispensable and important equipments in chemical production. To improve the heat transfer performance and service life of the heat exchanger, a numerical analysis of the vibration response of the elastic tube bundle in the heat exchanger based on fluid–structure coupling analysis is proposed. Using the weak coupling method of fluid–structure coupling, the vibration response of multiple rows of elastic tube bundles induced by shell side fluid in a heat exchanger with different tube row spacing and different tube row numbers is studied numerically, and the effects of shell side fluid and tube side fluid on the vibration response of elastic tube bundles are compared and analyzed. The results show that the maximum relative error of monitoring point amplitude is 43.36% when H = 40 mm and 10.17% when H = 70 mm. For connection IV, the maximum relative error of monitoring point amplitude is 31.71% when H = 40 mm and 24.08% when H = 70 mm. This is because when H is small, the interaction between rows of tube bundles is strong, so the amplitude changes violently with the number of the tube bundle. The step-by-step calculation strategy of rough calculation and actuarial calculation proposed in this article can greatly reduce the calculation time and improve the calculation efficiency.

Numerical study of different Pr number medium turbulent cross flow in staggered tube bundle

Tube bundle heat exchangers have been recognized as the most efficient means of thermal exchange. The staggered row method can enhance the performance of a heat exchanger. However, it results in a large pressure drop loss. As working media with high thermal conductivities, liquid metals can potentially replace conventional working media. In this study, for working fluids with low Prandtl number cross flowing through a staggered tube bundle, the turbulent Prandtl number model of cross flow was verified. The unsteady characteristics of the two working fluids traversing through the tube bundle were analyzed. The effects of molecular thermal diffusion on the total heat transfer capacity of the two working fluids during heat transfer were compared, along with the changes in the circumferential heat transfer characteristics of the tube bundles under different Reynolds numbers. The ζ factor was then proposed to analyze the non-uniformity in the heat transfer characteristics of a single tube. An empirical correlation describing the heat transfer non-uniformity at the fully developed region and inlet region and involving parameters such as working properties and flow state was proposed based on computational fluid dynamics data. Finally, the performance evaluation criteria(PEC) factor was used to analyze the comprehensive performance of the two heat transfer fluids. The results revealed that the comprehensive heat transfer performance of mercury was 60–70 times that of conventional working fluids.

Numerical study of the cold finned-tube bundle heat exchanger under natural convection for a novel passive displacement cooling system

In a passive displacement cooling (PDC) system, conventional finned-tube heat exchangers are being employed and their performance is not optimal under natural convection. In this study, an experimental set-up has been established to validate the numerical model for finned-tube heat exchanger optimization under natural convection. The numerical difference between the predicted and measured data is less than 3.4%, showing that the numerical model could predict the heat transfer and flow rate accurately under natural convection. Subsequently, the effects of fin spacing, coil configuration, tube row number, and fin depth on the coil performance under natural convection have been investigated. The results show that the one-row coil performs better than the two-row coil for a PDC system with 8-10 fins per inch (FPI), despite the smaller heat transfer area. Compared with the conventional two-row circular-tube coil with 10FPI, the one-row circular-tube coil improves the mass flow rate and heat transfer rate by around 29.1% and 20.0% respectively. The findings enable a more responsive and energy-efficient PDC system for green buildings.