Recuperative Heat Exchanger Research Articles

Impact of the type of heat exchanger on the characteristics of low-temperature thermoacoustic heat engines

Thermoacoustic technologies are considered an effective solution for harnessing low-temperature heat, whether from waste or renewable sources. However, in practice, developing and implementing high-performance thermoacoustic systems is a complex challenge. In real waste heat recovery systems, heat exchange between thermoacoustic engines (TAEs) and external heat sources is facilitated by auxiliary systems, such as circulation loops that include recuperative heat exchangers. It is evident that the upper power limit of a TAE is constrained by the thermal performance of its internal heat exchangers. This article investigates the energy transfer processes between the internal recuperative heat exchangers of a TAE and its matrix, and presents a mathematical model describing the interactions between the matrix and the heat exchangers. The model accounts for the effects of temperature inhomogeneities on the surface of the recuperative heat exchangers, which influence the temperature distribution within the TAE matrix. The use of liquid-gas recuperative heat exchangers in TAEs introduces temperature heterogeneity within the components of the thermoacoustic core. This study shows that such temperature variations can reduce the matrix gain factor for additional acoustic energy by a factor of 1.1 to 1.3. Therefore, when designing low-temperature thermoacoustic systems, it is essential to consider the type and characteristics of the heat exchangers used. The analysis indicates that recuperative heat exchangers can reduce the efficiency of thermoacoustic machines, whether they function as engines or refrigerators.

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.

Fluid-thermal characteristics of solar-exhaust gas powered dryer: An experimental validation

A new method of supplementing heat energy in a solar-exhaust gas greenhouse dryer has been experimentally evaluated in this paper. An internal combustion engine using diesel produced exhaust gas which was channeled to a hybrid recuperative heat exchanger (HRHE) for energy recovery. Fluid and thermal characteristics of the dryer were reported for three modes of drying: solar mode (SM), solar-exhaust gas mode (SEGM), and exhaust gas mode (EGM). Consequently, the dryer room air temperature were found as: 14.82–58.46 °C, 34.49–61.97 °C and 25.75–30.77 °C respectively. Moisture evaporated as a result of temperature variations were computed as: 0–20.8 g, 0–17.79 g and 0–22.33 g respectively. Fluid characteristics of exhaust gas included: average density of 0.7218 kg/m3, volumetric flow rates from 4.2×10−3 – 1.67×10−2 m3/s, maximum residence time of 418 s in HRHE, mass flow rates from 11.32 to 45.07 kg/h, velocity in connectors ranging from 2.14 to 8.52 m/s, velocity in tubes from 0.035 to 0.14 m/s, Reynolds number ranging from 2681 to 10674 in connectors and 344–1368 in tubes, Nusselt number ranging from 14 to 43 in connectors and constant at 3.66 in tubes. Available energy in exhaust gas was found in the range of 2082.32–16002.5 kJ/h, corresponding to temperatures of 197.19–359.82 °C as a result of engine speeds varied from 750 to 2500 rpm. Kinetic energy in exhaust gas increased with increased velocity for both tubes and connectors to a maximum of 39.79 kJ/h in connectors and 1.289×10−2 kJ/h in tubes. The significance of this study is in promotion of faster product drying in SEGM and slower drying for delicate products requiring low heat energy supply in EGM.

PARAMETRIC EVALUATION OF HEAT RECOVERY IN A RECUPERATIVE HEAT EXCHANGER

As urban centers grow and environmental regulations become more stringent, the complexity of integral systems within vehicles, aircraft, and other urban essentials escalates. A pivotal response to this challenge involves achieving enhanced energy efficiency and environmental appeal while maintaining cost-effectiveness. In this context, mathematical modeling, coupled with simulation and optimization techniques, emerges as a pivotal tool. This approach yields favorable outcomes with modest initial investments, contrasting with the resource-intensive nature of purely experimental design. Amongst the fundamental components, heat exchangers find widespread use, facilitating thermal exchange between fluids across diverse applications. Consequently, meticulous design and parametric optimization of these devices to attain peak performance and optimal energy efficiency are imperative, aligning with evolving environmental and energy trends. The simulation of such systems operates within an expansive range of operational and geometric parameters. These encompass mass flow rates, line pressures, pipe diameters, and pipe placements. However, excessive parameter combinations can render optimization computationally infeasible, necessitating judicious simplifications. Striking a balance between precision and computational efficiency, reduced-order models present a valuable intermediary solution. These models, situated between low- and high-order methods, offer robust mathematical representations without significant precision compromises. Thus, reduced-order models, which constitute an intermediate approach when compared to low- and high-order methods, can be used as a mathematical modeling tool without a significant loss of precision in the results. The present work presents an optimization and parametric analysis of a recuperative heat exchanger using a reduced-order approach employing the volume element model (VEM) as a discretization method, which is capable of providing accurate results at low costs. computational. The Laws of Conservation of Mass and Energy are applied to volume elements in combination with empirical correlations in order to quantify the quantities of interest, such as the convection heat transfer coefficient and temperature distribution. A parametric analysis was performed in order to observe the behavior of entropy generation in order to find its minimum points. The mass flow of water varied from 0.001 kg/s to 0.0085 kg/s with the mass flow of hot gases and the mass flow rate of gas was held constant in three stages, namely: 0.14 kg/s; 0.2 kg/s; and 0.3 kg/s. The local minimum was obtained for each of the three gas mass flow rate considerations, 8.53 W/K, 8.78 W/K, and 9.20 W/K respectively.

LateSearching for the most energy-efficient composition of a mixture of dimethyl ether and carbon dioxide as an air conditioning system refrigerantr

BACKGROUND: Carbon dioxide can be an alternative refrigerant for vapor compression refrigeration systems, particularly air conditioning systems (ACSs). However, its use suffers from the increased pressure in the refrigeration circuit. To solve this, for its reduction, a mixture of CO2 with a substance that has significantly lower pressures under the same conditions can be developed, for example, dimethyl ether (DME), which has zero GWP and ODP, is inexpensive and readily available. The study of DME, in particular, was conducted by the Department E4 “Refrigeration and Cryogenic Engineering, Air Conditioning and Life Support Systems” of N.E. Bauman Moscow State Technical University. DME is moderately toxic and flammable.
 AIM: This study aims to investigate the possible use of a mixture of DME and carbon dioxide for energy-efficient application in ACSs using commercially available compressors for R410A.
 METHODS: Comparative calculation analysis of the characteristics of a simple one-stage vapor compression cycle was performed using R410A and a mixture of DME and CO2 using the calculation packages Mathcad 15, Aspen HYSYS v. 10, SOLKANE8, and REFPROP.
 RESULTS: 1. The cycle on pure DME is the most effective in terms of the coefficient of performance: ε = 5.63 at an ambient air temperature of 26°C, ε = 3.07 at 40°C. 2. It is necessary to consider the influence of temperature glides, the average value of which ranges from 10°C to 30°C depending on the concentration of components. 3. At DME/CO2 ratios of 40/60% and 60/40% (in mole fraction), the discharge pressure corresponds to the discharge pressure in the R410A cycle, with 39.62 bar at an ambient temperature of 26°C and 37 bar at 40°C, respectively.
 CONCLUSIONS: An environmentally friendly mixture of DME and CO2 with low GWP and zero ODP is developed. An increase in the percentage of DME in the mixture increases the coefficient of performance and reduces the pressure range, and at the same time, significant temperature glides arise, which affects the installation efficiency, namely, the transition to a cycle with a receiver tank, i.e., with a recuperative heat exchanger between the fluorinated refrigerant flow leaving the evaporator and the fluorinated refrigerant flow leaving the condenser. The developed mixture is less efficient than the R410A refrigerant in terms of the coefficient of performance and discharge pressure. However, it is possible to further consider a mixture of DME and CO2 with concentrations of 40% and 60%, respectively, as a replacement for the R410 refrigerant because there is a correspondence of discharge pressures for serial compressors (about 40 bar); however, it is necessary to keep in mind the flammability risk of the mixture.

Heat exchange under the longitudinal movement of wet steam in finning heat exchangers

The paper is devoted to the study on hydrodynamics and heat exchange of two-phase medium. While designing technological equipment, when the wet steam is used as the operating medium, the features of the interaction between liquid drops and the heat exchange surface are not considered in most cases. In full, this applies to steam turbines operating on the wet steam whose moisture content depends on the primary and secondary removal of liquid drops from the separation blocks. Purpose. Improving the method of calculation of recuperative heat exchangers, if wet steam is used as the operating medium. Methodology. It is based on the analysis of the physical model of moving the two-phase medium in the heat and mass exchange conditions, considering the design characteristics of the heat transfer surface. Findings. The correlation of critical values of two-phase flow parameters was obtained to determine the lower boundary of the process of plucking the drops from the liquid film depending on the irrigation density, geometric characteristics of the channel and physical properties of the liquid and gas. Correlations were obtained for pipes with longitudinal finning as the Π-shaped profile, based on which we recommend optimizing the geometric characteristics of longitudinal finning. Originality. Determining the limit modes of secondary removal formation during the movement of a two-phase medium in separation devices and the features of heat and mass transfer of wet steam in finning recuperative heat exchangers. Practical value. The presented results make it possible to optimize the design of recuperative heat exchangers with longitudinal Π-shaped finning.

Achievements and problems in studying the mechanism of thermal potential transfer regulation between liquids

The relevance of the study is determined by its focus on one of the key problems of the Ukrainian economy, namely the problem of energy resources, which is a necessary step in the development of resource management and planning strategies in the context of modern challenges and instability in the energy market. The purpose of the study is to determine the hydrodynamic and mass-heat transfer characteristics of the process of mixing liquids with different thermal potentials, and to investigate the influence of design factors on the intensity and efficiency of the flow jet. Experimental methods were used to specify the hydrodynamic and mass-heat transfer parameters of the liquid mixing process, and modelling was used to analyse the influence of design factors and develop a scientifically based methodology for calculating new device designs. An approach is obtained that allows solving two problems simultaneously: to decrease energy costs through their rational operation and to reduce environmental pollution, avoiding harmful emissions that occur during fuel combustion. The functional scheme of the test stand is described, the research methodology is presented, and statistical modelling of the influence of parameters on the length of the liquid jet is carried out. Studies of the energy efficiency of the initial stage of the interaction of thermal potentials during the generation of a liquid jet in a recuperative heat exchanger of mixed action to ensure storage conditions and potential movement in front of the nozzle are presented. A mathematical model for describing the length of a continuous water jet when flowing out of the nozzle is proposed and an analysis of the laminar flow of liquid from the nozzle is performed. The length of the continuous water jet when flowing out of the nozzle is determined and the technical capabilities of the process are identified. The efficiency of installing the tip in front of the nozzle for creating turbulent mixing and the method of controlling the water supply during the mixing of thermal potentials are determined. To control the water supply process, a step-by-step method has been developed that allows gradually increasing or decreasing the length of a continuous water jet through the nozzle. The results of the study provide an opportunity to optimise heat exchangers and mixing machines to increase their efficiency and productivity

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.

Thermal efficiency investigation for organic Rankine cycle and trilateral flash cycle using hydrofluoroether working fluids

The thermal efficiency of the organic Rankine cycle (ORC) is higher than the trilateral flash cycle (TFC) using normal dry working fluids. On the contrary, the thermal efficiency of TFC outperforms the thermal efficiency of ORC by using super dry working fluids at specific conditions. This study aims to investigate thermal efficiency for TFC and ORC cycles using HFE7100 and HFE7500, classified as super dry working fluids. The effect of two techniques was studied on the thermal efficiency of ORC and TFC: the recuperator cycle (RORC and RTFC) with various ratios of effectiveness and the partial evaporator organic Rankine cycle (PE-ORC), with various ratios of vapor quality. The results indicated that the recuperative heat exchanger helps convert the super dry behavior to be like normal dry. Therefore, using HFE7100 required 0.2 effectiveness of recuperative heat exchanger, while for HFE7500, 0.4 effectiveness is needed. A partial evaporator cannot help convert the super dry's behavior to the normal dry fluid completely, while it has the crucial role of moving the intersection point towards critical temperatures (Tcr), which increases the range of heat source temperatures that assist super dry fluids to behave as normal dry fluids. However, PE-ORC can be a good competitor to TFC at vapor quality (x = 0.1, and x = 0.2) and (x = 0.1), and it has higher thermal efficiency than ORC at vapor quality (x = 0.9) and (x = 0.8 and x = 0.9) for HFE7100, and HFE7500 respectively.

The benefits of a recuperative layout of an ORC-based unit fed by a solar-assisted reservoir operating as a micro-cogeneration plant

Organic Rankine Cycle -based microcogeneration systems that exploit solar sources to generate electricity and domestic hot water simultaneously are promising solutions for reducing CO2 emissions in the residential energy-intensive sector. Such systems can be assisted by thermal energy storage reservoirs in which water is heated by solar energy and cooled by the direct use of thermal energy for domestic hot water production and acting as heat source of the power plant. An interesting plant layout optimisation involves the adoption of a recuperative heat exchanger to preheat the working fluid before it enters the heat recovery vapour generator, which is fed with the same working fluid leaving the expander. Despite the positive effect of recuperative heat exchanger adoption on the efficiency of the entire plant, the impact on electrical energy production at this microscale is not straightforward to assess as the heat source is represented by the water inside the reservoir and it is not a continuous high- or medium-temperature stream. The lack of experimental analyses in the literature on these applications makes this a debatable question.Thus, to fill this gap and quantitatively assess the benefits introduced by the adoption of a recuperative stage, a wide experimental comparison between recuperative and not-recuperative Organic Rankine Cycle-based power unit layout was performed. To support the experimental analysis, a comprehensive theoretical model of the Organic Rankine Cycle-based plant was developed and validated against experimental data.The experimental campaign demonstrated that the recuperative Organic Rankine Cycle-based unit required a lower working fluid flow rate for a given expander inlet temperature and pressure ratio. Consequently, a higher electrical power can be produced, requiring lower thermal power from a heat source. The reduction in the thermal power absorbed from the heat source, produced by a higher temperature incoming working fluid entering the Heat Recovery Vapor Generator, had a positive effect, allowing a longer-in-time working condition with a higher conversion efficiency intrinsically produced by the recuperative heat exchanger. Considering the annual electricity consumption of 2700 kWh of a family living in a city in central Italy, the contribution given by the recuperative Organic Rankine Cycle plant operating with a reservoir fed by 15 m2 of flat solar panels was 330 kWh, whereas without the recuperative stage, the production was 240 kWh. The cost of electricity saved justified the adoption of a recuperative heat exchanger.

Drawn-polymer microchannel heat exchangers with application to recuperative cryocoolers

Polymer microchannel heat exchangers for use in cryocooler applications have been developed. These heat exchangers are manufactured using a thermal drawing process where a bulk polymer preform is heated and stretched. The thermal drawing process results in microchannels with a characteristic dimension of 50–100 µm with an overall length of many meters. The drawn heat exchangers are lightweight, flexible, and have a large surface-area-to-volume ratio. Proof of concept design and testing of a nitrogen and hydrogen Joule-Thomson cryocooler utilizing a Polyetherimide (PEI) drawn-polymer recuperative heat exchanger is presented. These heat exchangers operated leak free at cryogenic temperatures under pressures up to 11 MPa. With a 95 % effective drawn polymer recuperator, the hydrogen cryocooler reached a saturation temperature of 20.5 K and demonstrated a cooling power of 28 mW. Mass flow maldistribution has been identified as a barrier to higher effectiveness performance. Suggestions for improvements and future development are discussed.

Performance assessment of hybrid recuperative heat exchanger for diesel engine generated exhaust gas

In this study, the use of supplemental heat energy from exhaust gas of a stationary diesel engine was assessed to explore a new method of drying black nightshade seeds in a solar-exhaust gas greenhouse dryer. The energy recovery potential of a hybrid recuperative heat exchanger (HRHE) was demonstrated with the objective of utilizing the recovered energy from an engine on milling operations to heat a fluid stream of drying air. The results show that 4.45 kW of thermal energy was available in exhaust gas of a diesel engine operated at 2500 rpm when mass flow rate was 45.07 kg/h at a temperature of 357.36 °C. The rate of heat utilized for solar-exhaust gas mode ranged from 40.49 to 685.94 J/m2.s and from 21.69 to 668.11 J/m2.s in exhaust gas mode of drying. The heat exchanger raised the dryer temperature by an average of 11.78 °C when temperature differences between inside and outside were compared in solar-exhaust gas mode of drying. The average hourly rise in temperature inside the dryer was 8.04 °C with a minimum rise of 3.7 °C and a maximum of 9.41 °C when exhaust gas was utilized to provide heat energy. The performance of the solar-exhaust gas greenhouse dryer improved when thermal energy was used as a supplement in drying and as a result the drying time for black nightshade seeds was significantly reduced from 11 h in solar mode to 10 h in solar-exhaust gas mode of drying. Moreover, the seeds were dried for 14 h when exhaust gas mode of drying was performed without utilizing solar energy. The percent internal uncertainty for experimental measurements of relative humidity (γ) was 4.1% and 17.5% for temperature (T) observations. The three proposed thermal models for temperatures and moisture evaporation performed better with low RMSEs in exhaust gas mode compared to the other modes of drying.

Simultaneous working fluid and expander selection method for reaching low‐threshold technology organic Rankine cycle (ORC) design

AbstractMaximizing the utilization of an available source serves as the ideal approach, provided that only technical factors are considered. For sources with low heat flux, however, cost‐effective solutions are more suitable due to the minimal net power generated, regardless of the effectiveness of the energy conversion. In such cases, utilizing low‐threshold technology may be the most fitting solution, the layout of these cycles should be simple and inexpensive. In the case of organic Rankine cycles (ORCs)‐based power cycles, this means the omission of superheaters or recuperative heat exchangers and the use of simple expanders and small heat exchangers. Simplifying the design, however, requires additional considerations about the elementary steps of the cycle. This work presents a procedure to select favorable working fluids for ORC while considering the expander's internal efficiency. The criteria for favourability is to have a nonideal expansion process starting and ending in (or very near) saturated vapor states to avoid problems related to wetness/dryness between the given maximal and minimal expansion temperatures. It is demonstrated that the design can be simplified under the simultaneous working fluid and expander selection method presented in this study, regardless of the type and isentropic efficiency of the expander. The resulting methodology applies the novel classification of working fluids using the sequences of their characteristic points on temperature‐entropy space. The proposed approach is illustrated with a case study finding optimal working fluid for an ORC system fitted to industrial waste heat, a low‐temperature geothermal, and a cryogenic heat source.

Numerical and experimental study on a tube-in-tube heat exchanger working at liquid-hydrogen temperature with a large temperature span

With the advantages of no moving parts at low temperature and a flexible and simple structure, the precooled Joule–Thomson cryocooler working at liquid-hydrogen temperature has the potential to be used in the liquid-hydrogen zero-boil-off storage system. The recuperative heat exchanger is one of the key components of Joule–Thomson cryocoolers, and it is usually a tube-in-tube heat exchanger (T-THX). In this study, a numerical model of a T-THX working from room temperature to liquid-hydrogen temperature was developed. Based on the requirement that the heat-exchanger efficiency should be at least 97%, a T-THX was designed and tested. The performance of the T-THX with a temperature span of 280 K was obtained at pressure of 0.539–0.982 MPa and mass flow rate of 4.57–25.81 mg/s. The heat-exchanger efficiency varied from 95.83% to 97.50%, by which the feasibility of the numerical model was preliminarily verified. To improve the calculation accuracy, the T-THX model was modified. Compared with further experimental results, using the modified T-THX model, the relative deviation of the outlet temperature was within 0.3% and the deviation of the heat-exchanger efficiency was less than 0.05%.

THE MULTIFACTORIALITY OF THE PROCEDURE FOR OPTIMIZING THE DESIGN OF A TWISTED HEAT EXCHANGER LOCATED IN AN ANNULAR CHANNEL DURING LAMINAR MOTION

Cryogenic units that operate on the J-T (Joule-Thomson) cycle have found wide application due to the relatively simple design of the main elements, low operating costs, reliability and long service life. To a large extent, the efficiency of the installation depends on the choice of the coolant, the schematic solution, as well as on the design of the recuperative heat exchanger. The ability to compensate for temperature and mechanical stresses due to the twisted structure ensures long-term and trouble-free operation of the heat exchange equipment. The main goal of many studies was to determine the influence of thermophysical properties of the coolant, its mode parameters and geometric characteristics of the surface on heat exchange and hydrodynamics. On the basis of the results of the research, the optimization of the structure was carried out and empirical dependencies were given for calculating its parameters. The analysis of the presented empirical dependencies does not give a final answer regarding the development of a generalized method of calculating microheat exchangers of the Hampson type used in cryogenic installations. It is proposed to calculate the geometric characteristics of this type of heat exchangers, taking into account the influence on the intensity of heat exchange not only of the mode parameters of the heat carrier and the working fluid, but also of the design characteristics of the equipment, namely: the diameter and length of the pipe, the diameter of the coil, the gap between the heat exchange surface and the outer and inner casings, step characteristics of the coil. The work implements the methodology of calculation and optimization of the design of a twisted heat exchanger located in an annular channel with forced convection and laminar flow regime. The main emphasis is made on taking into account the change in the intensity of heat exchange along the length of the heat exchange surface, the length of the initial heat section is determined. An experimental study of heat exchange processes during forced convection of gas in a coiled heat exchanger in the laminar mode of movement of the heat carrier made it possible to establish the dependence of the heat transfer coefficient aк on the main geometric characteristics of the heat exchanger: the relative pitch of the coil, the gap between the heat exchange pipe and the outer and inner surfaces of the housing. On the basis of research results, dimensionless corrections were determined, with the help of which variational calculations of twisted heat exchanger structures located in annular channels are performed in order to optimize their geometric characteristics. Bibl. 31, Fig. 1.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ КАПИЛЛЯРНОЙ СТРУКТУРЫ ТЕРМОСИФОНА НА ЕГО ТЕПЛОВУЮ МОЩНОСТЬ С ТЕПЛОНОСИТЕЛЯМИ R134A, R410A, R407C

Thermosiphon (TC) is an evaporative-condensing heat exchange device, where the circulation of the working fluid (intermediate coolant) is carried out due to gravitational forces. There is no porous wick in thermosiphons, it is replaced by grooves of various geometric shapes. Structurally, thermosiphons are made in the form of hermetically sealed and elongated cylindrical vessels, the inner volume of which is filled with a working fluid. Liquid heat carriers are used as the working fluid, which can perform an aggregate-phase transition at operating temperatures observed during operation in a recuperative heat exchanger. In this article, author is talking about comparing the limits of thermal power of thermosiphons operating at operating temperatures of ventilation and air conditioning systems. At the same time, thermosiphons use freons R134a, R410a, R407c as the working medium, and the capillary structure of thermosiphons is represented in the form of grooves of the following types: a Ω –shaped groove, a rectangular groove and a triangular groove. For comparison, a thermosiphon with an outer diameter of 8 mm is used. The dependences of the thermal power on the operating temperature for all types of limitations of the heat transfer capacity of thermosiphons are presented, depending on the capillary structure used and the working fluid. The analysis of the best capillary structure of thermosiphons when using freons as a working body is carried out.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ КАПИЛЛЯРНОЙ СТРУКТУРЫ ТЕРМОСИФОНА НА ЕГО ТЕПЛОВУЮ МОЩНОСТЬ С ТЕПЛОНОСИТЕЛЯМИ R134A, R410A, R407C

Thermosiphon (TC) is an evaporative-condensing heat exchange device, where the circulation of the working fluid (intermediate coolant) is carried out due to gravitational forces. There is no porous wick in thermosiphons, it is replaced by grooves of various geometric shapes. Structurally, thermosiphons are made in the form of hermetically sealed and elongated cylindrical vessels, the inner volume of which is filled with a working fluid. Liquid heat carriers are used as the working fluid, which can perform an aggregate-phase transition at operating temperatures observed during operation in a recuperative heat exchanger. In this article, author is talking about comparing the limits of thermal power of thermosiphons operating at operating temperatures of ventilation and air conditioning systems. At the same time, thermosiphons use freons R134a, R410a, R407c as the working medium, and the capillary structure of thermosiphons is represented in the form of grooves of the following types: a Ω –shaped groove, a rectangular groove and a triangular groove. For comparison, a thermosiphon with an outer diameter of 8 mm is used. The dependences of the thermal power on the operating temperature for all types of limitations of the heat transfer capacity of thermosiphons are presented, depending on the capillary structure used and the working fluid. The analysis of the best capillary structure of thermosiphons when using freons as a working body is carried out.

Modeling of heat exchange processes in a condenser of a pyrolysis bioenergy plant

The article presents the results of mathematical modeling of heat exchange processes in the condenser of a pyrolysis plant, in order to identify the optimal parameters for cooling the biomass pyrolysis steam. A mathematical model of the thermal balance of the condenser of a pyrolysis plant has been developed for the calculation, selection and thermal analysis of the working cycle of the condenser, which allows to obtain the values of the optimal parameters of the temperature regime. The technological scheme of a pyrolysis bioenergy plant with a recuperative heat exchanger, which simultaneously provides hot water, gaseous and liquid biofuels to the consumer, is presented. The analysis of the obtained results shows that the water temperature at the outlet of the condenser of the pyrolysis plant is on average in the range of 60-70°C and suitable for heat supply systems of an autonomous consumer.

Thermal mode of the condenser of a pyrolysis bioenergy plant with recuperation of secondary thermal energy

The article describes the technological scheme and the principle of operation of a pyrolysis bioenergy plant (PBEP) with a recuperative heat exchanger, which simultaneously provides hot water, gaseous and liquid biofuels to the consumer. The results of mathematical modeling of heat exchange processes in the condenser of a pyrolysis plant are presented in order to identify optimal thermal parameters for cooling the biomass pyrolysis steam. A mathematical model of the heat balance of a “mixing-displacement” type condenser (along the length of the coil) of a pyrolysis plant has been developed for calculating, selecting and thermotechnical analysis of the working cycle of the condenser, which allows to obtain the values of optimal parameters of the temperature regime. The simulation results show that the considered variants of pipes with diameters of 15 mm and 20 mm fully meet the technological requirements of condenser cooling.

MELCOR integrated severe accident code application to safety assessment of high-temperature gas-cooled reactors

MELCOR is an integrated thermal hydraulics, accident progression and source term code for reactor safety analysis that has been developed at Sandia National Laboratories for the United States Nuclear Regulatory Commission (U.S. NRC) since the early 1980 s. Though MELCOR originated as a light water reactor (LWR) code, development and modernization efforts over the past decades have expanded its application space to non-LWR reactor concepts. Current MELCOR development efforts have been focused on providing the U.S. NRC with the analytical capabilities to support regulatory readiness for licensing non-LWR technologies under Strategy 2 of the NRC’s near-term Implementation Action Plans. Beginning with the Next Generation Nuclear Project (NGNP), MELCOR has undergone a range of enhancements to provide analytical capabilities for modeling the spectrum of advanced non-LWR concepts. In this paper, we describe the generic plant model developed to demonstrate MELCOR capabilities to perform high-temperature gas reactor (HTGR) safety evaluations. This model represents a TRi-structural ISOtropic particle fuel (TRISO) pebble bed HTGR with a primary system rejecting heat to a recuperative heat exchanger. Surrounding the reactor vessel is a reactor cavity contained within a confinement volume cooled by the Reactor Cavity Cooling System (RCCS). A range of demonstration calculations are performed to evaluate the plant response and MELCOR capabilities to characterize a range of accident conditions. The accidents selected for evaluation consider a range of degraded and failed modes of operation for key safety functions providing reactivity control, primary system heat removal and reactor vessel decay heat removal, and confinement cooling.