Performance Of Heat Exchanger Research Articles

Design and Optimization of Cross-Corrugated Triangular Ducts with Trapezoidal Baffles Based on Response Surface Methodology and CFD

Plate heat exchangers are widely used in the Heating, Ventilation, and Air Conditioning (HVAC) field. Cross-corrugated triangular ducts are commonly employed in plate heat exchangers. Inserting baffles into the cross-corrugated triangular ducts can improve the heat transfer performance of the plate heat exchangers. This study focuses on intricate interdependencies among the flow channel apex angle, the trapezoidal baffle inclination angle, baffle position, and Reynolds number (Re) on heat transfer and pressure drop using response surface methodology (RSM) and computational fluid dynamic (CFD). To identify the factors that maximize the Nusselt number (Nu) and minimize friction factor (f), the RSM is used to design factors, conduct numerical studies, and establish regression equations. The results show that the apex angle, baffle angle, X-direction position, and Re have significantly affected Nu and f. Compared to a non-baffled channel with the same apex angle and Re conditions, the optimized channel enhances heat transfer by 1.54 times and has an almost identical pressure drop. The inclined baffle significantly enhances comprehensive performance at low Re. The synergistic effect of the heat transfer and pressure drop is most optimal when the apex angle of the flow channel is 90°, the trapezoidal baffle inclination angle is 52.5°, and the Re is 1000, with the baffle position at 0.625H in the X-direction.

Assessment of a hybrid solar water heating system: environmental impact and economic viability

ABSTRACT Conventional methods of space heating are not only expensive but also environmentally harmful, as they emit greenhouse gases that contribute to global warming. To tackle this issue, a hybrid solar water heating system assisted by a solar evacuated tube collector has been developed at the Madhav Institute of Technology and Science in Gwalior, India. Experiments conducted from November to February 2022–23 (winter season) revealed promising results. Throughout the winter season, the system achieved a daily average effective solar heat collection of 2.85 kWh, with a daily average collector efficiency of 45.5%. The average coefficient of performance (COP) of the solar water heat exchanger was 3.5. Additionally, the system demonstrated an energy payback time of 1.2 years, shorter than that of conventional systems. Over its lifetime, the developed winter heating system is estimated to mitigate 127.9 tonnes of CO2 emissions, earning a carbon credit of $1408. Economic analysis further revealed a favorable benefit-to-cost ratio for the hybrid system, ranging from 3.13 to 3.36, with a payback period varying from 5.03 to 4.47 years for an inflation rate of 6–13%. These findings affirm the feasibility and efficacy of a solar-powered hybrid active winter heating system.

Evaluating tidal impact on ground source heat exchanger performance under fluctuating groundwater levels

Coastal regions undergo dynamic changes due to tidal forces, posing distinct challenges for groundwater management and utilization of ground source heat pump (GSHP) systems. This study examines the impact of tidal fluctuations on groundwater levels and the performance of ground heat exchangers (GHEs), specifically focusing on the suitability of horizontal directional drilling (HDD) technology in high-tidal zones. Itoman City in Okinawa Prefecture, Southern Japan, serves as an ideal location for this study due to its complex coastal hydrogeological conditions and increasing interest in renewable energy solutions. Employing a comprehensive approach involving field experiments, FEFLOW software simulations, and model validation, we aim to elucidate the intricate relationship between tidal dynamics, groundwater behavior, and GHE performance. Furthermore, our investigation assesses the effectiveness of HDD-GHEs installed in highly tidal-affected zones to determine the optimal installation depth for this type of GHE in coastal regions. According to our research, the layer at a depth of 30 m is most affected by tidal influence and fluctuations in groundwater levels in coastal regions. This layer is the most suitable for installing horizontal HDD-GHEs, with an average heat exchange rate of about 89 W/m. This rate is approximately four and two times higher than the depths above and below this layer, respectively, due to groundwater saturation and flow in the formation. Furthermore, we noticed that the heat exchange rate increases from the top to the bottom of this layer, but the extent of this increase diminishes as the depth increases. This is because deeper depths are less affected by tidal fluctuations in groundwater levels.

Numerical analysis of the effect of wave amplitude on thermohydraulic performance in a heat exchanger with sinusoidal wavy channels

With the increasing application of heat exchangers, the research has become increasingly comprehensive. Previous studies mainly focused on comparing the amplitudes to investigate the performance of heat exchangers, while the study of variable amplitude channel structures was limited. This study investigated the thermal-hydraulic performance of seven different sinusoidal channels with inclination angles of 20° and 30°, and their combinations. Using the CFD solver, simulations were conducted to assess the heat transfer capabilities of sinusoidal channels under various Reynolds numbers and different amplitude conditions, compared to straight channels and single amplitude sinusoidal channels. The results indicated that within one period, both the pressure drop and temperature difference exhibited a ripple trend of gradually decreasing amplitude. When evaluating the comprehensive performance of the channels, three trends emerged across the seven models, with the performance of the cold fluid and the hot fluid within the same group showing opposite trends. This suggested that more variations could enhance the overall performance of the hot fluid while reducing the performance of the cold fluid. For structures with the same amplitude, in terms of amplitude variation frequency, models with six periods per group exhibited higher overall heat transfer performance than those with three periods per group, with an average increase of 36.37%. For the arrangement of amplitudes (taking the hot fluid inlet as an example), configurations with a smaller amplitude at the inlet outperformed those with a larger amplitude, indicating that in combined amplitude models, a structure with a larger group and a smaller amplitude at the inlet (taking the hot fluid as an example) should be designed.

Experimental study of graphene-based nanofluid dispersions stability for efficient heat transmission within a concentric tube heat exchanger

This study undertook the experimental enhancement of dispersion stability in graphene-based nanofluids to boost the performance of concentric counterflow heat exchangers. Emphasis was placed on the characterization and preparation of graphene suspensions by evaluating their dispersion properties in various solvents: water, ethanol, acetone, and dimethylformamide (DMF). The graphene powder underwent comprehensive analysis using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR). Subsequently, graphene suspensions were prepared and uniformly mixed using both a magnetic stirrer and an ultrasonic vibrator. The investigation confirmed that solvent type significantly influences the stability and solubility of the suspensions. Solutions of reduced graphene oxide (rGO) in water, ethanol, and DMF exhibited remarkable long-term stability, contrasting with those in acetone. Water emerged as the superior solvent for developing the thermal transport fluid. The addition of graphene resulted in a conductivity increase of approximately 48.15% at φ = 0.5% concentration, while the best thermal efficiency was attained at φ = 0.35% and a flow rate of 3 L/min. Furthermore, at lower flow rates, it is advisable to avoid higher concentrations to maintain optimal thermal efficiency.

Comprehensive numerical investigation of the effect of various baffle design and baffle spacing on a shell and tube heat exchanger

This article explores the significance of Shell and Tube Heat Exchangers (STHX), with a focus on various baffle types such as Segmental, Double segmental, Helical, Disc-and-Doughnut, Clamping anti-vibration, and Flower-B. In this study, the commercial software ANSYS FLUENT is used to simulate the problem. The study evaluates their impact on heat exchanger performance using parameters like overall heat transfer coefficient (OHT), pressure drop (PD), and Performance Evaluation Criteria (PEC). Clamping anti-vibration, Flower-B, Helical, and Double segmental baffles exhibit reductions in shell side PD by 14%-28%. Despite OHT reductions in these designs, Disc-and-Doughnut baffles demonstrate a higher OHT (21%-24%) compared to Segmental baffles. Based on PEC values, Disc-and-Doughnut baffles are identified as the most favorable. Further investigation into different numbers of Disc-and-Doughnut baffles reveals increased OHT (17%-29%), PD (58%-59%), and cold water temperature (0.5%-1%). The STHX with Disc-and-Doughnut 10 baffles displays the highest PEC value, establishing it as the most effective STHX in the simulation.

Experimental investigation of the effects of graphene nano particle added water fluid on heat transfer in a smooth tube with double inserts

In this study, experimental studies were carried out to increase the performance of heat exchangers. The effects of two techniques (inserts and nanofluid) on thermohydraulic performance was investigated simultaneously. The twisted tape and the helically wire coil were originally created. The two separation ratios (0.1 and 0.05), the eight twisted tape pitch ratios (1.9/2.3/2.7/3 and 2.1/2.4/2.8/3.2), the four coil wire pitch ratios (0.8/1.2/1.5/1.6) and three weight fractions (0.5/0.75/1 %) for graphene-water nanofluids were used with the Reynolds number ranging from 5000 to 26,000. Experiments in which a coiled wire coil and twisted tape are used together have been found to increase both the heat transfer rate and the friction factor compared to experiments using distilled water in an empty tube. According to the results of all experiments, it was found that the highest heat transfer rate and friction factor were obtained with the model containing graphene-water fluid used at a weight fraction of 1 %. The highest performance evaluation criteria is found as 2.5 b y using the graphene-water mixture model with a wall separation ratio of 0.05, a twisted tape ratio of 2.1, a wire coil ratio of 0.8 and a 1 % weight fraction among all combinations.

Assessing the use of PEC and jf methods for strengthening the evaluation of new heat exchangers

The new type of heat exchanger uses the micro heat pipe as the main heat transfer material. Compared with the traditional heat exchanger of the same kind, the performance of the new type of heat exchanger has significantly improved. By studying the evaluation method of strengthening the performance of the heat exchanger, the performance of the heat exchanger can be further optimized. Most of the current literature study the heat transfer coefficient of the heat exchanger, but there are many influencing factors. Therefore, the evaluation lacks accuracy, and there is no clear research description of fin strengthening performance of the heat exchanger. In this paper, the PEC and jf methods were used in evaluating the enhanced performance of a new type of heat exchanger. Multiple performance indexes were used as evaluation factors and the characteristics of flow and heat transfer were analyzed by studying the working medium, heat transfer coefficient and pressure drop in the heat exchanger. The study, therefore, proposed an optimization direction and the results proved that PEC and jf methods can be used to evaluate the performance of the new heat exchanger efficiently, and the two conclusions were similar. The fins were more capable to improve the performance of heat exchanger when >31000.

Numerical analysis on heat transfer performance of spiral elastic copper tube heat exchanger with helical defectors

Based on the flow-induced vibration mechanism, three kinds of spiral elastic copper tube heat exchangers were proposed to improve the comprehensive heat transfer performance, involving the spiral elastic copper tube heat exchanger with no defector, the spiral elastic copper tube heat exchanger with forward helical deflector and the spiral elastic copper tube heat exchanger with reverse helical deflector. A both-way fluid–structure coupling method was utilized to investigate the effects of helical defectors and their arrangements on the vibration-enhanced heat transfer performance of the spiral elastic copper tubes at different inlet velocities by numerical simulation. The results indicate that: when the inlet velocity ranges from 0.1 m/s to 0.7 m/s, the heat exchanger with reverse helical deflector possesses better heat transfer performance than the other two heat exchangers. For the heat exchangers with no defector and forward helical deflector, the flow-induced vibration always promotes the comprehensive heat transfer performance. Compared with the heat exchanger with no defector, the comprehensive heat transfer performance of the heat exchanger with reverse helical deflector is higher 4.02 % averagely, while that of the heat exchanger with forward helical deflector is lower 29.34 % averagely. Furthermore, a structural optimization is presented to improve the existing disadvantages of the heat exchanger with reverse helical deflector, after structural optimization, the average amplitude of the heat exchanger with forward helical deflector is decreased by 3.26 % averagely and the heat transfer performance is raised to a maximum of 26.24 %, which are of great significance to extend the service life and improve the energy efficiency of heat exchanger.

Anti-Fouling Rotating Polymer-Based Heat Exchanger for Zero Liquid Discharge Humidification-Dehumidification Desalination

The objective of this investigation is to assess the performance of various heat exchangers for application in a novel solar-powered zero liquid discharge humidification-dehumidification desalination system. In this study four heat exchangers (HX) having two different flow configurations namely counter flow (cf), cross flow (cr) made up of three different materials namely high-density polyethylene (HDPE), aluminum (Al), and Polypropelyne (PP) were compared in terms of their effectiveness and overall heat transfer coefficient under varying salinity levels (up to 10%) and mixing ratios (0.22-0.45). At a mixing ratio of 0.2 and 0% salinity, PP-HXcr and Al-HXcf exhibited similar effectiveness (∼85%), surpassing that of HDPE-HXcf (∼65%). Despite PP-HXcr's lower thermal conductivity in comparison to Al-HXcf, comparable effectiveness was achieved due to the superior flow distribution in PP-HXcr. Further investigations focused on the impact of salinity on heat exchanger performance. At 3.5% salinity, all heat exchangers experienced a decline in effectiveness and heat transfer coefficient (HTC), with Al-HXcf experiencing a more pronounced decrease compared to PP-HXcr. The higher thermal conductivity of Al-HXcf led to greater salt accumulation, while PP-HXcr demonstrated minimal fouling. As the experiment progressed, fouling increased for all heat exchangers, with the Al-HXcf being practically ineffective at 10% salinity with an effectiveness below 10%. To address the issue of fouling, a rotating cross-flow heat exchanger (RPP-HXcr) was introduced. While the effectiveness of the PP-HXcr drops from 85% to approximately 60% with increasing salinity from 0% to 10%, the RPP-HXcr demonstrates only a marginal decline in effectiveness with increasing salinity. For instance, at absolute humidity of 0.22 when the salinity is increased from 0% to 10%, the effectiveness of RPP-HXcr only drops from 83% to 77%. This exceptional performance was attributed to the continuous contact between the rotating tubes and the incoming feed, effectively preventing fouling and ensuring sustained efficiency. Rotating cross-flow heat exchanger (RPP-HXcr) is introduced and validated as a potentially reliable solution for mitigating fouling, as it demonstrates sustained efficiency and minimal performance degradation across different salinity conditions.

Comparative Analysis of Influencing Factors on Heat Extraction Performance of Multi-Type Shallow Borehole Heat Exchangers

In this paper, the influence of design parameters and geological parameters on the heat extraction capacity of common coaxial type, single U-type, and double U-type borehole heat exchangers(BHEs) in shallow ground source heat pump system is analyzed, and the heat extraction performance of different types of BHEs is further compared. The results show that with the increase in borehole depth and ground temperature gradient, the outlet temperature of the three types increases at the end of the heating season, and the single U-type BHE increases most significantly. The increase in inlet flow rate will decrease the outlet temperature of the three BHEs, and the U-type is the most obvious decrease. The increase in inlet temperature will reduce the temperature difference between inlet and outlet of the three types BHEs. Given the same design parameters, coaxial-type BHE obtains the maximum heat.

Effect of polyvinylpyrrolidone and lithium chloride composite desiccant-coated heat exchangers on dehumidification studies

Cooling systems are necessities that remain energy intensive, particularly in tropical regions with elevated humidities and temperatures. Conventional vapor-compression air-conditioners are often inefficient because the sensible and latent heat loads are intrinsically coupled. To realize more energy-efficient cooling, solid desiccant-coated devices can be introduced to mitigate the latent heat load. Among such devices, desiccant-coated heat exchangers (DCHEs) are attractive for enabling nearly isothermal dehumidification. Hence, desiccants with high sorption capacity and rapid kinetics are essential. Solid polymeric desiccants are well-positioned to meet these requirements but comprehensive studies on the dehumidification performance of solid polymeric desiccant-coated heat exchangers are currently lacking. In this study, we investigated the potential of polyvinylpyrrolidone (PVP) and lithium chloride (LiCl) composites as desiccant materials at both material and system levels in terms of gravimetric water uptake. Specifically, LiCl is a hygroscopic salt bound by the moisture-sorbing PVP matrix. This work also demonstrates how the Elovich model accurately predicted the PVP-LiCl composite’s sorption kinetics. The results show that the optimum PVP-LiCl composite ratio is 66.7 wt%, with an equilibrium sorption capacity of 280 % at 75 % RH and 25 °C, compared to 29 % for silica gel under the same condition. With much superior sorption kinetics and capacity, the PVP-LiCl-coated prototype heat exchanger demonstrated a moisture removal capacity eight times higher than a silica gel-coated one under dynamic flow towards energy savings in air-conditioning systems.

Experimental investigation on frosting performance of a microchannel heat exchanger in heat pump system for electric vehicles

Frosting of outdoor heat exchangers deteriorates the heat transfer performance of the electric vehicle (EV) heat pump system. This study conducted frost experiments on the outdoor heat exchanger of an EV heat pump air-conditioning system, considering different flow configurations, tube arrangements, and frontal air volume. The purpose of the study was to establish the optimal tube arrangements and flow configurations for minimizing frost formation and maximizing system performance. The results indicate that a three-flow configuration outperforms a two-flow setup under frosting conditions. Specifically, a flow sequence of 14-21-35 increases the heat capacity by 15.05 %–20.64 % compared to that of the reverse flow order of 35-21-14. The flow sequence a-b-c represents the number of flat tubes along the refrigerant flow process. Additionally, the coefficient of performance (COP) increases by 1.19 %–3.55 %. Higher airflow reduces frost formation. A significant correlation exists between the refrigerant distribution within the heat exchanger and the corresponding frost distribution on the surface of the microchannel heat exchanger. The frost formation rate was slower for the flat tube vertical arrangement. Furthermore, horizontal tube arrangements were more conductive than vertical ones, providing a more uniform temperature distribution and higher heat transfer rates (an increase of up to 2.7% in heat transfer and 3.4% in heat capacity). The COP of the vertical arrangement was consistently higher than that of the horizontal arrangement by 1.4 %–3 %.

Influence of the twisting and nano fluids on performance of a triangular double tube heat exchanger

Abstract The hydraulic and thermals performance of flow in a double-tube heat exchanger with an inner twisted triangle tube of length L = 1 m has been studied numerically. Equations for energy, turbulence, and Navier–Stokes were used to model the fluid flow and heat transfer in a double-tube heat exchanger with an inner twisted triangle tube. The Reynolds number range was 5000–30,000 for nanofluid (water-Al2O3) as a working fluid under a turbulent flow regime. The considered configuration was examined by varying the volume concentration of nanofluid while keeping the twist ratio constant (Tr = 5). Different cases of volume concentration of nanofluid are triggered (0.05 %, 1 %, 2.5 %, and 4 %, respectively). The numerical outcomes of a double-twisted tube heat exchanger with an inner triangle-twisted tube are validated with the obtainable numerical data. The governing equations were solved with ANSYS Fluent 18.2. The findings reveal that the larger volume concentration of nanofluid (4 %) gives a greater Nusselt number and vice versa due to the increased thermal conductivity of the nanofluid in the double twist tube heat exchanger compared with a plain tube. However, the nanofluid Nusselt number grows with the rise of both the Reynolds number and volume concentration (Ø), and the best value of the Nusselt number is 210 when Reynolds number = 30,000 and Ø = 4 %. The effectiveness of the heat exchanger grows with an increase in the concentration of the nanofluid and a reduction in the torsion ratio compared to the normal tube. The effectiveness of the double-tube heat exchanger with an inner twist triangle tube increases to the highest value of 0.38 at Reynolds 30,000 and Ø = 4 %.

Optimization study on acid condensation and anti-corrosion performance of 3-D finned tube heat exchangers

Low-temperature acid corrosion on flue gas heat exchanger surfaces seriously reduces the stability, safety, and economy of equipment operation. Efficient strategies are urgently required to improve the anti-corrosion performance of heat exchangers. In present paper, a three-dimensional heat and mass transfer model integrating the acid dew point, acid condensation rate, and solution concentration was developed to predict the corrosion characteristics on the surfaces of 3-D finned tube heat exchangers. Firstly, a comparative analysis of acid condensate characteristics for 3-D finned tubes with different configurations was performed. The results showed that the elliptical tube effectively improves the distribution uniformity of acid condensate rate and promotes anti-corrosion performance. Then, the effects of five operating parameters (gas temperature, gas velocity, water vapor concentration, acid vapor concentration and wall temperature) on overall corrosion characteristics were systematically examined in terms of acid dew point, condensation rate and solution concentration using single-factor and orthogonal analysis, and the effect order of each factor was determined. Furthermore, optimization analysis of anti-corrosion performance was conducted based on the orthogonal results, and the optimum working conditions (water vapor concentration below 11.8% and wall temperature of 347K to 355K) are recommended. This research can provide theoretical guidance for the anti-corrosion designation and optimization of 3-D finned tube exchangers.

Heat exchange characteristics between u-shaped butted well and reservoir

The geothermal resources in Shandong are relatively abundant. However, due to geological conditions, the high pressure of formation water reinjection makes it difficult and costly to achieve a 1:1 ratio of formation water extraction/reinjection during geothermal resource extraction. This causes waste of formation water resources and does not meet environmental protection requirements. The U-shaped butted well heat exchanger at the bottom of the well can achieve “heat extraction without water extraction”, effectively solving the problems of difficult formation water reinjection, pipeline reservoir corrosion, and reservoir damage in Shandong. Therefore, to explore the heat exchange characteristics between the U-shaped butted well heat exchanger at the bottom of the well and the reservoir, it is necessary to study various parameters of the heat exchanger operation. To better reflect the heat transfer status between the butted well and the soil, FLUENT software was used for simulation. Through simulation, the operation diagram of the generator after the comprehensive renovation was analyzed, guiding the operation of the U-shaped butted well heat exchanger.

Image-based finite element modeling of air flow and thermal transport in Al-fiber-sintered porous materials

In this study, the pressure drop and heat transfer in a heat-transfer tube filled with a sintered porous medium comprising Al fibers were investigated using computational fluid dynamics (CFD) simulations. We reconstructed the sintered fibrous porous structure and generated a computational mesh using X-ray computed tomography data, and the simulated pressure drop and heat transfer agreed well with those reported by Enoki et al. [2021]. The differences in both the form coefficient and the permeability between two different samples can be explained from the differences in both the specific solid surface area and the porosity. Further, the CFD simulations indicated that thermal conduction of the Al solid phase enhanced the heat exchange between the air and Al fibers. To examine the effect of solid-phase thermal conduction, we generated a model of a heat-transfer tube having an ideal wire mesh structure, in which we introduced the interfacial thermal conductivity between Al fibers and the inner tube wall as additional factors. We also performed CFD simulations with four different shell-region thicknesses and found that even a very narrow gap of 5 to 10 μm heavily affected the heat-exchange performance because of the low thermal conductivity of the shell region.

Experimental investigation and optimization of heat pipe heat exchanger performance under two distinct heat transport mediums through TOPSIS approach

This paper presents a comprehensive thermal performance analysis of the heat pipe heat exchanger by varying the input process parameters and optimizing the best condition for waste heat recovery through the TOPSIS technique. In this work, the heat pipe is constructed with a copper tube and segmented into evaporator and condenser sections carrying hot and cold fluids with a half-fill ratio. The heat pipe is oriented in 0° horizontally with the wick material made from stainless steel and works based on the capillary principle. The efficiency of the HPHE is evaluated by varying the input operational parameters, namely heat input, mass flow rate of hot fluid, inlet temperature of hot fluid, and mass flow rate of cold fluid, on response variables, specifically thermal resistance, energy balance ratio, universal heat transfer coefficient, and effectiveness. The experiment was designed for four factors at three levels as per Taguchi’s L27 orthogonal array, and the best optimising condition was determined by employing the multi-objective technique for order preference by similarity to ideal solution (TOPSIS) approach. The best input condition found was A3B1C1D3, in which the heat input was at 90 W, the mass flow rate of hot fluid was at 0.2 kg min−1, the mass flow rate of cold fluid was at 0.1 kg min−1, and the inlet temperature was at 60 °C. The effect of process parameters and their significance on output responses was analysed by executing an analysis of variance (ANOVA) statistical method. The mathematical regression model was generated, and validation was made to investigate the prediction error percentage.

Experimental Study on Ultra-Low Concentration Methane Regenerative Thermal Oxidation

As a major coal country, China faces the issue of significant gas emissions during the coal mining process. This study aims to improve the utilization efficiency of mine gas, reduce greenhouse gas emissions, and promote the low-carbon and green transformation of the coal industry. A 10 kW gas regenerative thermal oxidizer (RTO) experimental system was constructed. The effects of initial methane concentration, low-temperature flue gas proportion, and operating load on combustion temperature, methane oxidation rate, high-temperature flue gas energy, and system thermal efficiency were studied. The results show that when the combustion temperature is below 600 °C, the CH4 combustion reaction cannot proceed effectively, and the system temperature continuously decreases and cannot be maintained stably. The experiment determines the stable operating methane concentration range of the RTO. In this experimental system, the lower limit of the initial methane concentration is 0.28%, corresponding to an 86% methane oxidation rate. As the initial methane concentration decreases, the combustion temperature also decreases, and the methane oxidation rate follows suit. The higher the low-temperature flue gas proportion, the higher the combustion temperature, and the system’s thermal efficiency and output heat decrease with the increase in the low-temperature flue gas proportion. This experiment explores multiple factors affecting regenerative thermal oxidation, providing a basis for ensuring the safe and stable operation of the system and its optimization. Improving the thermal insulation and heat exchange performance of the storage body can expand the lower limit of the initial methane concentration, thereby increasing the stability and thermal efficiency of the system.

Impact of groundwater seepage on thermal performance of capillary heat exchangers in subway tunnel lining

The metro tunnels are currently facing challenges associated with thermal pollution, which significantly affects their operational safety and energy consumption. The subway source heat pump system (SSHPS) with front-end tunnel lining capillary heat exchangers (CHEs) is an effective technology to address this problem. The heat transfer characteristics of SSHPS have been previously investigated; however, there is a lack of research on the thermal performance of tunnel lining CHEs under groundwater seepage. This study established a numerical simulation model of tunnel lining CHEs based on a demonstration project of SSHPS. Subsequently, the influence of groundwater seepage on the heat transfer characteristics of tunnel lining CHEs was comprehensively analyzed. The findings suggest that an increase in groundwater seepage velocity leads to the initial expansion and subsequent contraction of the thermal influence range caused by CHEs. Simultaneously, the performance of CHEs is enhanced with increasing flow velocity. When the seepage velocity increases from 0 to 1 × 10−4 m/s, the heat exchange capacity per meter and efficiency of the CHEs during the cooling season increase by 450 % and 446 %, respectively. During the heating season, they experience a growth of 320 % and 310 %, respectively. In addition, the heat exchange performance of CHEs is also influenced by the relative flow direction between the fluid within CHEs and the groundwater. The counter flow arrangement exhibits a 7.7 % higher efficiency of heat exchange during the cooling season compared to the parallel flow arrangement, and a 3.9 % higher efficiency of heat exchange during the heating season. Therefore, it is highly recommended to evaluate groundwater seepage conditions during the design phase of SSHPS in order to enhance the performance of tunnel lining CHEs. This study serves as a valuable theoretical reference for the design of tunnel lining CHEs.