Exchange Surface Research Articles

Hydrothermal analysis of hybrid nanofluid flow inside a shell and double coil heat exchanger; Numerical examination

In a shell-and-coil heat exchanger, employing a double coil instead of a simple coil causes more heat transfer compared to a simple coil because the double coil improves heat exchange through enhanced fluid flow and turbulence, enabling a longer tube length in constrained locations.. In addition to reducing fouling and the requirement for maintenance, the helical shape may also provide the benefit of a self-cleaning action. Ensuring a more even flow distribution also maximizes the heat exchanger's surface area use and reduces low flow zones. Applications needing compact solutions without compromising performance especially benefit from this design's versatility, allowing customization to meet unique operational demands. In numerous industrial applications, helical coils offer substantial benefits in terms of effectiveness, upkeep, and design freedom.In the present work, heat transfer and fluid flow in a shell-and-coil tube heat exchanger equipped with a double helical coil are evaluated numerically. The employed heat transfer fluids are water-based hybrid nanofluids, including TiO2-Mgi/water and AG-HEG/Water. The obtained outcomes are compared with the results of pure water. The range of considered Reynolds numbers is 500 – 2000. This work consists of two sections. In the first part, the impact of the type of hybrid nanofluid on the hydrothermal behaviour of the proposed heat exchanger is analysed. The volume concentration of the nanoparticles here is ϕ1 = ϕ2 = 0.3. In the second section, the best hybrid nanofluid is selected according to the results of the first section; then, the impact of the nanoparticles' volume concentration on the thermal performance of the proposed heat exchanger is evaluated. Three various nanoparticle volume concentrations, including ϕ1 = ϕ2 = 0.1, ϕ1 = ϕ2 = 0.3, and ϕ1 = ϕ2 = 0.5, are considered, and the results are compared with those of the pure water case (ϕ1 = ϕ2 = 0).The obtained results show that amongst the different heat transfer fluids considered, the Water/MgO-TiO2 model shows the most outstanding value of thermal performance in all the studied Reynolds numbers. Also, the obtained numerical results show that the use of the proposed double coil tube leads to an increase in the heat transfer rate.

EXPERIMENTAL ANALYSIS OF THE INFLUENCE OF FAN SPEED ON FROST GENERATION AND ITS IMPACT ON A HEAT PUMP

The phenomenon of frost on the heat exchanger surfaces penalizes the performance of the heat pump, one of the operating parameters that influences the evaporation temperature, this is transferred to the formation of frost, and its impact is the amount of air passing through the evaporator, in that sense, an air-water heat pump has been experimented at different fan speeds and environmental conditions inside a climatic chamber. The results allow to obtain different parameters to characterize the frost formation process and to make an effective comparison between them. The lower the fan speed, the lower the air flow rate, the lower the heating time, and the higher the number of defrost cycles required for the same environmental conditions. In this way, some correlations could be developed to predict how the frosting process is affected by the fan speed in more conditions that are not tested. The aim is to determine to what level frost formation can be suppressed by achieving different fan speeds whether it is technically and economically feasible and what implications imposes on the heat pump performance.

Steerable droplet precise bouncing on a superhydrophobic surface with superhydrophilic stripes

The precise rebound of a droplet upon hitting a solid surface has garnered significant attention in recent years due to its critical applications in self-cleaning, printing industries, and the design of heat exchanger surfaces, among others. This study introduces an innovative approach that combines femtosecond laser processing with a high-temperature stearic acid modification to create surfaces that feature superhydrophilic (SHL) stripes on a superhydrophobic substrate. By controlling the offset distance between the droplet's impact point and the SHL stripe, we achieved a directional and precise rebound of the droplets. Our findings indicate that the lateral displacement of the droplet increases with the offset distance and always tilts toward the direction of the SHL stripe. This study also incorporates numerical simulations to validate the findings, shedding light on the energy conversion mechanisms at the liquid–solid interface during the impact, particularly during the retraction phase. This discovery is significant for more accurately predicting the specific landing spots of rebounding droplets.

A Screening Apparatus for Comparing the Fouling Resistance of Heat Exchanger Surfaces

AbstractFouling is a challenge in many processes, especially in heat exchangers and evaporators. Even though many treatments have been developed to mitigate fouling on metallic surfaces, no standardized method exists to compare equipment based on its antifouling performance. In this work, a screening apparatus is presented, which allows to compare samples of treated metallic surfaces based on thermal fouling resistance. A parameter screening and a ranking of three differently treated metallic surfaces using two model substances, namely, whey protein concentrate (WPC) and calcium sulfate, are demonstrated.

Review on Absorption Refrigeration Technology and Its Potential in Energy-Saving and Carbon Emission Reduction in Natural Gas and Hydrogen Liquefaction

With the requirement of energy decarbonization, natural gas (NG) and hydrogen (H2) become increasingly important in the world’s energy landscape. The liquefaction of NG and H2 significantly increases energy density, facilitating large-scale storage and long-distance transport. However, conventional liquefaction processes mainly adopt electricity-driven compression refrigeration technology, which generally results in high energy consumption and carbon dioxide emissions. Absorption refrigeration technology (ART) presents a promising avenue for enhancing energy efficiency and reducing emissions in both NG and H2 liquefaction processes. Its ability to utilize industrial waste heat and renewable thermal energy sources over a large temperature range makes it particularly attractive for sustainable energy practices. This review comprehensively analyzes the progress of ART in terms of working pairs, cycle configurations, and heat and mass transfer in main components. To operate under different driven heat sources and refrigeration temperatures, working pairs exhibit a diversified development trend. The environment-friendly and high-efficiency working pairs, in which ionic liquids and deep eutectic solvents are new absorbents, exhibit promising development potential. Through the coupling of heat and mass transfer within the cycle or the addition of sub-components, cycle configurations with higher energy efficiency and a wider range of operational conditions are greatly focused. Additives, ultrasonic oscillations, and mechanical treatment of heat exchanger surfaces efficiently enhance heat and mass transfer in the absorbers and generators of ART. Notably, nanoparticle additives and ultrasonic oscillations demonstrate a synergistic enhancement effect, which could significantly improve the energy efficiency of ART. For the conventional NG and H2 liquefaction processes, the energy-saving and carbon emission reduction potential of ART is analyzed from the perspectives of specific power consumption (SPC) and carbon dioxide emissions (CEs). The results show that ART integrated into the liquefaction processes could reduce the SPC and CE by 10~38% and 10~36% for NG liquefaction processes, and 2~24% and 5~24% for H2 liquefaction processes. ART, which can achieve lower precooling temperatures and higher energy efficiency, shows more attractive perspectives in low carbon emissions of NG and H2 liquefaction.

Justification of the method for calculating the calibration sleeve for the production of thermoplastic pipes

The task of improving the quality of the production of pipes for engineering networks of various purposes with forecasting performance takes on special importance in the face of increased demand in modern conditions. A significant influence on the solution of this problem is found in the organization of heat exchange in the calibrated device. Based on the theoretical analysis of heat exchange during pipe calibration, an improved method is proposed, which consists of calculating the temperature field in the pipe wall during the movement of the pipe billet through different zones of the calibrator with controlled thermal resistance between the surfaces of the polymer pipe and the calibrator. This approach makes it possible to determine the intensity of heat removal, depending on the state of the pipe material during solidification at a given speed of movement of the workpiece, taking into account the change in the thermophysical properties of the pipe material and to more accurately determine the rational heat exchange surface in the calibrator, which is expressed in the length of its sleeve. The proposed method was approved when calculating the calibrator during the production of a polyethylene pipe with a diameter of 500 mm and a wall thickness of 30 mm with a pipe production speed of 8 m/min. in which the calculated length is 862 mm, the working length of the sleeve is 880 mm, the convergence is 98 %.

Numerical and experimental investigation of the gyroid heat exchanger

This paper presents a study of a novel type of heat exchanger (HE) whose core is built based on a Triply Periodic Minimal Surface structure. The core of this exchanger is built as a periodic structure based on a gyroid-type lattice and is manufactured by laser powder-bed fusion technology. This solution is distinguished not only by an exceptionally favorable ratio of the heat exchange surface area to the volume occupied but also by a unique geometry that additionally turbulates the flow and intensifies the heat exchange process. This article contains the results of numerical analyses of the entire exchanger under different operating conditions and the results of analyses of small fragments of the core filled with cells of different sizes. Numerical analyzes of the lattice-type exchanger are performed on the basis of the experimentally validated numerical model. The objective of the study is to determine the performance of the gyroid HE under different operational conditions and select the best elementary cell size per exchanger core for the assumed operating conditions. The printed HE was compared with a plate HE that was 30% larger, although the lattice one managed to achieve 10.5% higher values in on average Number of Transfer Units (NTU) and on average 5% higher temperature effectiveness (TE) in the studied range of flow parameters.

Potentiometric titration study of CaCO3 scaling control with non-water soluble polyetherimide and sulfonated polyetherimide films

Automatic potentiometric titration (APT) is a chemical analysis method to study the crystallization of CaCO3 in solution. We demonstrate how APT is also valuable for studying CaCO3 crystallization using water-insoluble polymer films, ideal for studying surfaces exposed to Ca2+ and CO32− ions with the capacity to remove Ca2+ and/or susceptible to scale formation, equipment surfaces, ion exchange surfaces or desalination membranes, etc. Polyetherimide (PEI) and sulfonated polyetherimide (SPEI) films were prepared by solvent casting technique, and their surface studied using FESEM, XPS, Z potential, and contact angle. APT assays were performed with polymer films, using a carbonate buffer (pH 9), dosed with CaCl2, and monitoring free Ca2+. The APT curves with PEI showed no significant difference compared to the control, while the presence of SPEI film affected the kinetics of CaCO3 formation. This was evidenced by changes in the slope and peak maximum of the APT curves. FTIR, XRD, and FESEM of the polymeric films and the CaCO3 after the APT assays showed control over the types of CaCO3 polymorphs. We show that the SPEI film is a good candidate to delay the formation of CaCO3 in water, through the adsorption of Ca2+ ions and the adsorption of pre-nucleation clusters of CaCO3.

ПРИНЦИПЫ СОЗДАНИЯ ДИСКРЕТНЫХ ПРОТЕКТОРНЫХ ПОКРЫТИЙ ДЛЯ ТЕПЛООБМЕННЫХ ПОВЕРХНОСТЕЙ

The paper proposes the principles of creating discrete tread coatings for heat exchange surfaces. Protective metallized coatings allow you to protect heat exchange equipment from corrosion and in-crease its service life

IMPROVEMENT OF THE METHODOLOGY FOR CALCULATING THE HYDRODYNAMICS OF COILED HEAT EXCHANGERS FOR CRYOGENIC PLANTS OPERATING ON THE J-T CYCLE

The widespread use of cryogenic plants operating on the J-T (Joule-Thomson) cycle is associated with low operating costs, reliability, and long service life. One of the main elements of the plant is a recuperative heat exchanger made in the form of a single- or multi-layer coiled heat exchange surface. The ability to compensate for temperature and mechanical stresses due to the twisted design ensures long-term and trouble-free operation of the heat exchange equipment. The importance of determining the characteristics of the hydrodynamics of the flow of liquids or gases is primarily associated with a conscious choice of methods for solving heat and mass transfer problems, the use of certain methods of process intensification, and optimization of equipment design. The purpose of creating efficient equipment is to determine the maximum heat transfer rate at moderate values of hydraulic resistance. The analysis of known empirical dependencies does not provide a definitive answer regarding the development of a generalized methodology for calculating Hampson-type microheat exchangers used in cryogenic installations. The aim of this work is to improve the methodology for calculating the hydrodynamics of coiled heat exchangers by modifying the calculated correlations. This is possible by introducing appropriate corrections to them that take into account the influence of the geometric characteristics of the tube bundle on its resistance. The experimental study of hydrodynamic processes during forced gas convection in a coiled heat exchanger made it possible to establish the dependence of the Euler number Eu on the main geometric characteristics of the heat exchanger: the relative coil pitch, the gap between the heat exchanger tube and the outer and inner surfaces of the body. Based on the research results, the corrections in the dimensionless form eкр and eз, which are used to perform variational calculations of the structures of coiled heat exchangers located in annular channels, were determined in order to optimize their geometric characteristics. Bibl. 19, Fig. 5.

ROTARY SOOT BLOWING TREATMENT INCREASES EFFECTIVENESS OF ECONOMIZER CAPACITY OF 58.5 TPH AT PT SBI INDONESIA

In an industrial era that increasingly prioritizes energy efficiency and environmental sustainability, optimizing the use of energy in the production process is crucial. One of the key aspects of achieving this goal is utilizing waste heat to improve the overall efficiency of the system. In this context, the use of economizers has become a major focus in the boiler industry. PT Surya Borneo Industri (SBI), as an industrial entity committed to energy efficiency and environmental sustainability, has adopted the latest technology in a bid to improve the performance of their boiler system. One of the latest innovations they have implemented is the use of rotary soot blowing, an automated device designed to clean the heat exchanger surfaces in the economizer. This study aims to determine the effectiveness value of the economizer before and after the rotary soot blowing treatment on the PT SBI boiler system. Through a series of field observations and data analysis, this research reveals the potential benefits of using rotary soot blowing in improving thermal efficiency, reducing energy losses, and contributing to the sustainability of industrial operations. The results obtained showed that the economizer effectiveness value before the rotary soot blowing treatment was 55, 54, 55, and 54%, and after the rotary soot blowing treatment, the effectiveness value increased to 59, 60, 61, and 60%. This is due to maintenance activities, namely cleaning dust or soot attached to the economizer pipes.

Development of a Prototype Non-volatile Heating Circuit with Pulsating Mode

The emergence of global energy crisis makes energy saving technology become the key technology of sustainable development. Enhanced heat transfer technology can improve the efficiency of various heat transfer equipment and reduce the weight and volume, which plays a vital role in energy saving and sustainable economic development. In recent years, based on theoretical and experimental research, Chinese researchers and technicians have developed many new technologies to strengthen heat transfer, such as cross-scaled elliptic tube, discontinuous double-inclined inner rib tube, spiral groove tube, spiral groove tube, inner fin tube, twisted elliptic tube and so on. Among them, the pulsating heat transfer technology is an efficient and energy-saving means that can significantly strengthen the convective transport operation and improve the heat transfer effect. Through fluid pulsation or the vibration of the heat transfer pipe, the technology can remove the dirt deposited on the heat exchange surface, reduce the thermal resistance of the dirt, and improve the performance and life of the heat exchanger. It is widely used in the cooling of electronic components, automobile turbines, and the utilization of atomic energy in Marine environment. Firstly, the construction scheme of the experimental device is proposed, and the working principle of the experimental device is described in detail. The complex impedance, frequency function, amplitude-frequency characteristic and phase-frequency characteristic are obtained by mathematical transformation of power supply circuit. Finally, the frequency response of the circuit is constructed.

Numerical study on heat transfer characteristics of multiple air jet impinging at lower jet to plate spacing

For multi-nozzle jet impinging, the effects of impinging distance (Z/D), streamwise spacing (X/D) and spanwise spacing (Y/D) on heat transfer characteristics should be considered. The heat transfer characteristics for multi-nozzle jet impinging structure with different nozzle spacings were investigated by numerical method. When the impinging distance is constant (Z/D = 0.2), the effects of Reynolds number (Re = 12,000–28,000), streamwise spacing (X/D = 3, 4, 6, 8, 10), spanwise spacing (Y/D = 3, 4, 6, 8, 10) and impinging nozzle arrangement (in-line structure and staggered structure) are considered. The Standard k-ε model was chosen for simulating after comparison to the relevant experimental values. The influences of nozzle spacing and the Reynolds number on the heat transfer characteristics were examined. The Colburn factor j and friction factor f were investigated in relation to Re. The performance criteria η of the Reynolds number ranging from 12,000–28,000 was calculated. The calculations indicated that the jet impinging heat transfer coefficient is highest under the structure of X/D = 3 and Y/D = 3. Compared to the in-line arrangement, the staggered arrangement has a higher average Nusselt number. This is because the turbulent kinetic energy of the staggered structure on the impinging target surface is larger and the heat exchange efficiency is higher. However, the gap between the average Nusselt numbers of the staggered and in-line nozzle arrangements decreases with an increase in Reynolds number. Given the same heat exchange surface, the average Nusselt number is highest under the X/D = 8 and Y/D = 8 arrangements. This is because the larger the impinging nozzle spacing, the fewer the number of nozzles. Therefore, the greater the speed of the gas reaching the nozzle outlet, the greater the average Nu of the target surface.

Ash fouling characteristic analysis and prediction for pillow plate heat exchanger in waste heat recovery based on attentive-feature decision algorithm

Ash fouling on heat exchanger surfaces in waste heat recovery and utilization is an inevitable result of solid fuel combustion and particle deposition, affecting the efficiency, availability and operating cost of an energy utilization system. Fouling prediction plays a critical role in reasonable design and efficient operation of heat exchangers. However, credible monitoring and accurate prognostic towards fouling condition always remain a challenge, largely due to the complex flue gas component and abominable service environment of heat exchangers. In this paper, a novel ash fouling prediction method combined with Effective Particle Size Filtering and Multistep Attentive-Feature Decision algorithm (EF-MAFD) for Pillow Plate Heat Exchanger (PPHE) is proposed. First, a numerical fouling model with Discrete Phase Model (DPM) combined particle deposition and removal model is developed. Validation work is conducted on Nusselt number, pressure drop coefficient and normal restitution coefficient with existing experimental data. The effect of geometrical configuration of PPHE and condition on the fouling characteristic is then evaluated with this model. Next, the EF-MAFD ash fouling prediction model is established, in which the initial particle size distribution is transformed into an effective diameter innovatively and two indicators evaluating the fouling state, namely critical fouling resistance and critical fouling time are defined and predicted. The results indicate that fouling mainly occurs downstream of the welding spots and shows an asymptotic growing trend over time. PPHE with smaller diameter of welding spot, channel height and pitch ratio possess anti-fouling potential. In comparison to existing methods, the newly developed EF-MAFD method has the capacity to deal effectively with the complex particle distribution and provides the most satisfying prediction ability with coefficient of determination (R2) of 0.93 and mean absolute prediction error (MAPE) of 7.8 %. The proposed method could be utilized as a reliable tool to support fouling condition-based maintenance and design for various types of heat exchangers operating in fly ash conditions.

Effect of grain size variation on strontium sorption to heterogeneous aquifer sediments

Strontium-90 (90Sr) is a major contaminant at nuclear legacy sites. The mobility of 90Sr is primarily governed by sorption reactions with sediments controlled by high surface area phases such as clay and iron oxides. Sr2+ adsorption was investigated in heterogeneous unconsolidated aquifer sediments, analogous to those underlying the UK Sellafield nuclear site, with grainsizes ranging from gravels to clays. Batch sorption tests showed that a linear Kd adsorption model was applicable to all grainsize fractions up to equilibrium [Sr] of 0.28 mmol L−1. Sr2+ sorption values (Kd; Langmuir qmax) correlated well with bulk sediment properties such as cation exchange capacity and surface area. Electron microscopy showed that heterogeneous sediments contained porous sandstone clasts with clay minerals (i.e. chlorite) providing an additional adsorption capacity. Therefore, gravel corrections that assumed that the > 2 mm fractions are inert were not appropriate and underestimated Kd(bulk) adsorption coefficients. However, Kd (<2 mm) values measured from sieved sediment fractions, were effectively adjusted to within error of Kd (bulk) using a surface area dependant gravel correction based on particle size distribution data. Amphoteric pH dependent Sr2+ sorption behaviour observed in batch experiments was consistent with cation exchange modelling between pH 2–7 derived from the measured cation exchange capacities. Above pH 7 model fits were improved by invoking a coupled cation exchange/surface complexation which allowed for addition sorption to iron oxide phases. The overall trends in Sr2+ sorption (at pH 6.5–7) produced by increasing solution ionic strength was also reproduced in cation exchange models. Overall, the results showed that Sr2+ sorption to heterogeneous sediment units could be estimated from Kd (<2 mm) data using appropriate gravel corrections, and effectively modelled using coupled cation exchange and surface complexation processes.

Dynamic and magnetic properties of excited magnons in tetragonal fcc [formula omitted]/SiO multilayer alloys

In this paper, we investigate by numerical lattice simulations the magnetic and dynamic properties of excited magnons in d transition metal alloys FexPt1−x/SiO using Green functions and second quantification techniques. The sandwich structure consists of magnetic (FePt) and non-magnetic (SiO) layers. The corresponding quantum Heisenberg Hamiltonian includes exchange interactions, magnetocrystalline anisotropy, surface anisotropy and magnetic field. The existence of the surface magnons is demonstrated by the surface creation factor η, which is in good agreement with the measured lifetime data. Surface magnons are characterized by their excitation spectrum, susceptibility of creation, creation gap, permitted band for creation, and lifetime, which are different from volume magnons properties. The susceptibility of creation analysis is compatible with experimental values, and the calculated lifetimes are in the order of femtoseconds (fs) reproducing the analytical behavior of the measured values along the lattice symmetry axis. The surface magnons are found to be more confined in time and space than those excited in volume, indicating the presence of short-range spin correlations. Magnetization per spin versus temperature is also calculated for different iron concentrations x and found to be in excellent agreement with the measured values. Also, the response of the system to an external magnetic field is shown for xFe=0.29 with various temperature values, and it agrees well with the experience.

Enhancing performance in solar air channels: A numerical analysis of turbulent flow and heat transfer with novel shaped baffles

Solar air heaters are emerging as a promising solution for sustainable heating, as they harness abundant solar energy to provide heat for a variety of applications. However, their optimum efficiency remains a major challenge. Deflectors have significant potential to improve the performance of solar air heaters by redirecting airflow and optimizing heat transfer, making them more efficient and more suitable for large-scale adoption. This article examines the performance of a new baffle design aimed at improving heat transfer in the channel by increasing the exchange surface between fluid and solid surface. It consists of a “≠” -shaped baffle mounted on the top and bottom walls of the channel. An in-depth analysis of the spacing between these baffles was carried out to assess their impact on the thermal and aerodynamic properties of the solar air channel. Computational Fluid Dynamics (CFD) was used to perform the calculations, using the Finite Volume Method (FVM) in conjunction with the SIMPLE algorithm. All studies are carried out using the CFD code Fluent. Crucial steps in this research include studying the influence of these new baffles at different Reynolds numbers, ranging from 15000 to 35000, as well as varying the separation distance between the two baffles, chosen periodically (DBaffle/2, 3DBaffle/4, DBaffle, 5DBaffle/4 and 3DBaffle/2). Some of the numerical results were validated with the available experimental data and showed satisfactory agreement. The study identified an optimal case for the new baffles, by reducing the distance between the baffles, an improvement in system performance was observed, by increasing the flow strength and widening the recycling cells, which has an impact on dynamic behavior and heat transfer. The presence of this new baffle design, with minimal separation distance between them, demonstrated a significant thermal improvement, with a thermal improvement factor increasing by 44% compared to the simple channel without baffles, thus highlighting the effectiveness of the new solar air channel with “≠” -shaped baffles.

Dependencies for determining heat transfer coefficients and air resistance for heat exchangers with tubular-plate heat exchange surface

Problem. The article analyzes existing generalized dependencies for determining convective heat transfer coefficients and air resistance coefficients for tubular-plate surfaces widely used in the production of modern charge air coolers and radiators. It is established that practical application of such dependencies, in some cases, is associated with significant errors revealed during experimental checks. Consequently, there is a need to adjust such dependencies for corridor bundles of flat-oval tubes with transverse grouped finning by flat fins. Goal. The aim of this study is to correct experimental dependencies for heat exchange and resistance. Numerical coefficients are adjusted on this basis, while the structure of dimensional dependencies remains unchanged. Simultaneously, the study is conducted for further improvement of the methodology for obtaining similar dependencies. Methodology. Adjustment of generalized dependencies for determining convective heat transfer coefficients and air resistance coefficients for tubular-plate heat exchange surfaces is carried out experimentally. Results. New dependencies are proposed for determining heat transfer coefficients and air resistance coefficients for tubular-plate heat exchange surfaces. Their accuracy and efficiency are investigated over a wide range of regimes. Originality and practical value. Unlike known dependencies of a similar nature, the proposed dependencies are provided in a single complex that should be used in modern heat exchanger calculation methods. It is established that the maximum error in determining air temperature according to the proposed dependence does not exceed 0.3 °C for the entire investigated range, and the error in determining aerodynamic resistance does not exceed 8 %, indicating high accuracy of modeling. Such accuracy ensures the possibility of applying the proposed dependencies for engineering calculations.

Experimental investigation on macroscopic fracture and microstructural evolution of granite under high-temperature steam action

This study conducted simulated experiments on the thermal fracturing of granite under non-uniform stress conditions using the method of injecting high-temperature and high-pressure steam. Additionally, a comprehensive investigation was carried out on the thermal fracturing characteristics of granite after steam treatment, as well as the distribution patterns of cracks. The results show that: Firstly, the process of fracturing granite using high-temperature and high-pressure steam as a medium can be divided into five stages. The threshold temperature for thermal fracturing of granite is 260 ℃, at which point a large number of cracks develop or form interconnected networks. Secondly, under high-temperature conditions, the tensile strength of granite is greatly reduced. The temperature at which brittle fracture occurs in granite during the experiment was found to be 427 ℃ at a steam pressure of 10.6 MPa. Thirdly, under the influence of high-temperature and high-pressure steam, granite not only generates primary cracks along the direction perpendicular to the minimum principal stress but also produces a certain number of secondary cracks in other directions. The crack propagation direction can be determined from the velocity and permeability distribution maps of fractured granite. Finally, the fracture surface of granite exhibits significant brittle characteristics. The fracture morphology mainly includes transgranular and intergranular fractures. In the experiment, the fracture surface also showed fragmented and fatigue-induced fracture features. The fractal dimension of micro-morphological features can be used to assess the extent of thermal fracturing at different locations on the fracture surface. Through macroscopic fracturing experiments and comprehensive analysis of theory, wave velocity, and micro-morphology, the study obtained relevant crack propagation patterns and the formation of cracks in different directions. This is of practical significance in the formation of interconnected spatial fracture networks, thereby helping to increase the thermal exchange surface area of artificial reservoirs and improve the efficiency of geothermal exploitation.

Comprehensive evaluation on deposition and heat transfer performance of the mixed tube bundle based on orthogonal experimental study

Deposition on heat exchange surfaces is an urgent problem to be solved to achieve efficient utilization of energy in thermal equipment. To mitigate deposition and the resultant heat attenuation, this work proposes a mixed tube bundle strategy of inserting attached cylinders behind the tube bundle, which can effectively meet the design requirements of high anti-deposition performance, high heat transfer efficiency, and low flow resistance. An integrated dynamic model for deposition is developed to elucidate the effect of ash particles on deposition, flow, and heat transfer characteristics. Based on the inverse distance weighting interpolation, a dynamic mesh smoothing technique is employed to simulate the evolving deposition layer on the heat transfer surface. In addition, the L16(43) orthogonal experimental design method is adopted to study the influence of the arrangement angle, diameter, and distance to the primary bundle of the attached cylinder on the overall performance, and determine the optimal scheme. The results show that deposition in the cases using the mixed tube bundle strategy has significantly decreased. The weakening effects of the attached cylinder on the deposition of small-medium particles and large-sized particles are due to the inhibition of vortex development in the gap of tube bundle and the deviation of the motion trajectory, respectively. The novel design exhibits the lowest deposition mass Mdep = 1.12 g and the best comprehensive performance PEC = 1.14 under the condition of the arrangement angle of 50°, the diameter of 10 mm, and the distance between the primary bundle of 5 mm. The obtained results can provide reference schemes and guidance for the optimization of the design of heat exchange equipment and accessory devices.