Problem Of Heat Exchange Research Articles

Solution of the conjugate heat exchange problem for conical heat exchangers

RELEVANCE The article is devoted to the development of new designs of heat exchangers and evaluating the efficiency of their operation. According to the authors, currently of particular interest are conical heat exchangers of the "pipe-in-pipe" type, so the object of research in this paper is a heat exchanger in the form of a truncated cone based on a spring-twisted channel. The introduction of the proposed heat exchange elements and apparatuses into the industry requires additional research. To assess the effectiveness of the application of the considered TA, it is proposed to evaluate its performance in comparison with conical and cylindrical TA based on a smooth-walled pipe. Due to this, two hypotheses will be tested at once: the use of a conical coil heat exchanger is more efficient than a cylindrical one, and the replacement of a smooth-walled pipe with a spring-twisted channel increases the efficiency of the heat exchanger.OBJECT. The aim of the research is to develop a method for setting and solving the conjugate heat exchange problem for a heat exchanger in the form of a truncated cone with a heat exchange element in the form of a spring-twisted channel, analyze the results obtained and evaluate the efficiency in comparison with conical and cylindrical heat exchangers based on smooth-walled heat exchange elements. METHODS. For the numerical solution of the conjugate heat transfer problem, the FEM implemented by means of Ansys was used Fluent. RESULTS. The main results consist in the fact that the authors developed a model and algorithm for calculating conical coil heat exchangers of the pipe-in-pipe type, implemented in the Ansys program, and determined the thermal and hydrodynamic parameters of coil devices. A comparison of coil heat exchangers was also made: a conical one based on a spring-twisted channel with a conical one and a cylindrical one with a smooth-walled heat exchange element. The calculation results showed that replacing a smooth-walled pipe with a spring-wound channel significantly increases the efficiency of heat exchange equipment. CONCLUSION. The significance of the obtained results lies in the possibility of using modern, more efficient and compact heat exchange equipment for technological needs and in the justification of this choice. Thus, with equal initial data, conical heat exchangers are more efficient than cylindrical ones with a heat exchange element in the form of a smooth tube, since they need a smaller heat exchange surface to achieve the necessary thermal and hydrodynamic parameters. The results of calculations prove the prospects of using a conical TA based on a spring-twisted channel and show the need for further study of the influence of the geometric characteristics of both the coil itself and the heat exchange element on the thermal and hydrodynamic characteristics of the proposed HE.

ГИДРОДИНАМИКА И ТЕПЛООБМЕН ПРИ ТЕЧЕНИИ ПЛЕНКИ КОНДЕНСАТА ПО ВЕРТИКАЛЬНОЙ ПОВЕРХНОСТИ

The article considers the problem of hydrodynamics and heat exchange of a laminarly flowing condensate film over a vertical heat-removing surface during condensation of stationary steam. In this case, the velocity and temperature fields are determined under boundary conditions on the heat-removing surface and on the outer boundary of the condensate film. Obtained equations are used for calculation the thickness of a laminarly flowing film under the action of gravitational forces and the heat transfer coefficient during condensation of stationary steam

THE IMPACT OF SOLAR RADIATION ON THE TEMPERATURE REGIME OF A ROOM IN WINTER

The article is devoted to the computational studies of the air-thermal state of office premises taking into account the effect of solar radiation coming through window openings. The study was conducted for a room with two windows using two heating devices installed under them. The air-temperature regime of premises in winter, characterized by the highest thermal energy consumption for heat supply, is considered. The study is based on the solution of a three-dimensional nonlinear heat transfer problem described by a system of equations of turbulent momentum and energy transfer. The k-e turbulence model is used to close this system. The results of numarical modeling of the physical situation under study are presented. The research data on the features of the air-thermal state of the premises under solar radiation conditions are given. The results of a comparative analysis of the air-temperature conditions of office premises corresponding to the solution of the specified heat exchange problems in the presence and absence of solar radiation are presented and the effects of solar radiation on the structure of the air flow and the thermal state of the premises are established. It is shown that in the presence of solar radiation, the air flow picture and the character of the temperature fields in the premises change significantly. In particular, in these conditions, the increase in the average temperature of the premises for the studied period is 2.5 °С. The possibility of a certain reduction of the load on the heating system in the presence of solar radiation is noted. Bibl. 17, Fig. 5.

Identification of the Characteristics of Thermal Engineering Materials Under Conditions of Nonstationary Heating With Changes in Pressure by Solving Inverse Heat Exchange Problems. 1. Development of Computing Algorithms

Identification of the Characteristics of Thermal Engineering Materials Under Conditions of Nonstationary Heating With Changes in Pressure by Solving Inverse Heat Exchange Problems. 1. Development of Computing Algorithms

Unveiling the underlying physics of two-phase boiling heat transfer enhancement through shearing of coalescing bubbles

The upcoming energy scarcity problem has driven research toward developing energy-efficient two-phase heat exchangers essential for various cooling applications. This research is rooted in the principles of pool boiling, essential for effective heat transfer in various heat exchangers. A well-known reported problem in heat exchangers is the dry-out phenomena of heated surfaces due to bubble coalescence. To tackle this undesirable problem, an innovative technique has been introduced in this study, which involves the shearing of bubbles through liquid jet impingement over the heated surface. The study has been carried out in a two-dimensional domain numerically, in the wall superheat range of 9–16 K. To study the underlying physics involved in this pool boiling phenomenon, the bubble dynamics parameters such as departure frequency, bubble diameter, cold spot (bubble base) temperature, and vapor volume fraction have been analyzed. The results show that with the jet shearing effect, a maximum enhancement of 25% in heat transfer rate is observed at higher wall superheat. The investigation also highlights that the liquid jet enhances vapor volume fraction, indicating enhanced steam generation, particularly an enhancement of 27% observed at elevated wall superheat. An early onset necking effect is also observed with the shearing effect, which leads to the formation of smaller bubbles with higher departure frequencies. This study is a benchmark to the fundamental physics of enhancing two-phase heat transfer through bubble shearing, offering promising insights for energy conservation in two-phase heat exchanger design, particularly within the context of pool boiling.

Crystallization Fouling on Different Surfaces under Static and Dynamic Experiments

Crystallization fouling is a typical problem in heat exchangers, which leads to the deterioration of heat transfer performance and energy loss. Surface modification and fluid shear are important factors to control crystal nucleation, aggregation, growth, and removal. This study performed static fouling experiments of calcium sulfate crystallization on patterned polymethyl methacrylate for different stages and real-time dynamic fouling experiments on metal surfaces. The grooves provide nucleation sites, and the nuclei prefer aggregation, forming blossoming-like crystals. After washing the fouled sample with distilled water, the size of blossoming-like crystals decreases and it is difficult to fully clean blossoming-like crystals compared with dispersed crystals. The fouling performance for the second time after the cleaning demonstrates that the existent crystals create nucleation sites, leading to a higher deposition rate than that of the first fouling. Furthermore, the fouling morphology is flaky gypsum on the metal surfaces under the dynamic experiment and fouling removal caused by the fluid shear is obvious. The deposition mass per unit area on metal materials under dynamic experiment in descending order is 0.0353 mg/mm2 of copper, 0.0154 mg/mm2 of titanium, 0.0146 mg/mm2 of aluminum, and 0.0070 mg/mm2 of stainless steel, respectively, which is significant to the mitigation of crystallization fouling on different materials.

Матричное моделирование и диагностика состояния регенеративных подогревателей высокого давления

Increasing the efficiency of heat exchangers is a promising area of energy saving, which is one of the priority areas of the development of science, engineering, and technology in the Russian Federation. Improvement of heat and mass transfer in regenerative heaters and condensers of turbine units of thermal power plants can significantly increase the operating efficiency of power equipment. Diagnostics of the condition and improvement of operating modes of the high-pressure regenerative heater which are an integral part of regenerative heating systems for feedwater of turbine units, are urgent scientific and practical tasks. To develop a model of a high-pressure regenerative heater, the methodology of matrix formalization of the description of heat and mass transfer processes has been used. To solve problems of diagnosing the state of heating surfaces of heat exchange equipment, mathematical programming methods have been applied. Within the framework of the matrix formalization methodology, an approach to modeling and solving diagnostic problems of multi-stage heat exchangers considering the phase transition in coolants has been developed. The sensitivity of the resulting model to input signals has been studied. Solutions of the problem of diagnosing the state of a high-pressure regenerative heater have been obtained and analyzed. Analysis of the obtained calculated results has shown an adequate description using the model of the real heat exchange process for the analyzed heater and the possibility to use the proposed model for monitoring and operational diagnostics of power equipment. The proposed approach allows us to formulate and solve inverse problems of diagnosing the state of equipment of heat exchange installations for various purposes.

Determination of the thermal state of a block gravel filter during its transportation along the borehole

Purpose. Development of a methodology for determining the thermal state of block inverse gravel filters manufactured using low-temperature technology during their transportation in a well based on computer and mathematical modeling. Methods. The study of hydrodynamic processes occurring during the transportation of the filter in the borehole, as well as the calculation of thermal fields in the filter body, was performed using the Ansys Fluent software package. To determine the effective thermophysical characteristics of the filter, the approaches of the heat transfer theory in porous media were applied. To investigate the thermal conditions of the filter at heat exchange with well fluid, the use of analytical solution of the heat conduction problem in a cylindrical shell, taking into account the properties of porous dispersed water-saturated medium, is proposed. Findings. The methodology for calculating the thermal state of a gravel filter during its operation in a well has been developed. For estimation of the filter surface temperature, the expression obtained on the basis of the analytical solution of the heat conduction equation, taking into account the characteristics of the porous filter medium, is proposed. Hydrodynamic and thermal fields in the borehole and the filter body during the filter transportation process in the borehole have been obtained. Originality. For the first time, the problem of heat exchange of a gravel inversion filter, manufactured by low-temperature technology in a well is considered. The influence of hydrothermal conditions in the well on the process of filter heating during its transportation in the well is shown. The hydrodynamic fields during the flow of the drilling mud around the inverse gravel filter are determined. Practical implications. The proposed approaches and results of the study allow to determine and can be used in the development of technological regulations for the use of block gravel filters produced by low-temperature technology for the equipment of hydrogeological wells. The methodology for modelling the process of two-phase inversion transition of aggregate state of the binding agent during transportation of inverted block-type gravel filter during well construction has been developed.

Asymptotic Modeling of Optical Fibres: Annular Capillaries and Microstructured Optical Fibres

Microstructured optical fibres (MOFs) are a new type of optical fibres that possess a wide range of optical properties and many advantages over common optical fibres. Those are provided by unique structures defined by a pattern of periodic or quasi-periodic arrangement of air holes that run through the fibre length. In recent years, MOFs have opened up new possibilities in the field of optics and photonics, enabling the development of advanced devices and novel optical systems for different applications. The key application areas of MOFs vary from telecommunications and high-power energy transmission to quantum optics and sensing. The stack-and-draw method is a standard manufacturing technique for MOFs, where a preform is first manually created and then drawn in a sophisticated furnace into a fibre with the required final dimensions and position of the air holes. During the manufacturing process, experimenters can control only a few parameters, and mathematical models and numerical simulations of the drawing process are highly requested. They not only allow to deepen the understanding of physical phenomena occurring during the drawing process, but they also accurately predict the final cross-section shape and size of the fibre. In this manuscript, we assume thermal equilibrium between the furnace and the fibre and propose a functional form of the fibre temperature distribution. We utilise it with asymptotic mass, momentum, and evolution equations for free surfaces already available in the literature to describe the process of fibre drawing. By doing so, the complex heat exchange problem between the fibre and the furnace need not be solved. The numerical results of the whole asymptotic model overall agree well with experimental data available in the literature, both for the case of annular capillaries and for the case of holey fibres.

Improving the Cramér-Rao bound with the detailed fluctuation theorem.

In some nonequilibrium systems, the distribution of entropy production p(Σ) satisfies the detailed fluctuation theorem (DFT) p(Σ)/p(-Σ)=exp(Σ). When the distribution p(Σ) shows a time dependence, the celebrated Cramér-Rao (CR) bound asserts that the mean entropy production rate is upper bounded in terms of the variance of Σ and the Fisher information with respect to time. In this paper we employ the DFT to derive an upper bound for the mean entropy production rate that improves the CR bound. We show that this new bound serves as an accurate approximation for the entropy production rate in the heat exchange problem mediated by a weakly coupled bosonic mode. The bound is saturated for the same setup when mediated by a weakly coupled qubit.

Investigation of zirconium (Zr) coated heat exchanger surface for the enhancement of heat transfer and retardation of mineral fouling

BackgroundFouling is a major problem in many industrial heat exchangers as in these devices water has been used for heat exchanging purposes. With the commencement of foulants deposition on heat exchanger surface the performance of heat exchangers is deteriorated due to lower thermal conductivity of the deposits formed on the heat exchanger surface. This study investigated the heat transfer and mitigation of CaCO3 fouling by applying zirconium coating layer on heat exchanger surface. MethodsZirconium was chosen as a coating material because of its surface adhesion and one of the best corrosion resistance materials. The Physical Vapor Deposition approach was implemented to develop a coating layer on heat exchanger surface. Heat transfer was investigated at numerous temperatures and flow rates to examine the impact of coating on heat transfer. Fouling experiments were conducted to examine the CaCO3 accumulation on the uncoated SS316L and Zirconium coated surfaces. After the fouling experiments, the coupon was disassembled from the test segment for the investigation of surface texture, after fouling effect and the morphology of deposits was assessed by using Field Emission Scanning Electron Microscopy (FESEM). Significant findingsResults reveal that the Zirconium coated surface declined the fouling deposition and improved the heat transfer.

Modeling of the flow and heat exchange in pipes with turbulators of viscous heat carriers in the laminar region, as well as in the transition to turbulent flow

Objective. Mathematical modeling of heat transfer in pipes with turbulators for viscous heat carriers at Reynolds numbers characteristic of laminar and transient flow regimes is carried out by the calculation method. The solution of the heat exchange problem for semicircular cross-section flow turbulators based on multiblock computing technologies based on the solution of the Reynolds equations (closed for the transient mode using the Menter shear stress transfer model) and the energy equation (on multi-scale intersecting structured grids) by the factorized finite-volume method (FCOM) was considered.Method. The calculation was carried out on the basis of a theoretical method based on the solution of the Reynolds equations, closed for transient modes using the Menter shear stress transfer model, and the energy equation on multiscale intersecting structured grids (FCOM), by a factorized finite-volume method.Result. Both local and averaged characteristics of the flow and heat exchange in pipes with turbulators for a viscous coolant for laminar and transient flow modes of the coolant were obtained using the FCOM method in the work, which made it possible to determine for these modes the levels of heat exchange intensification that satisfactorily correlate with the existing experiment.Conclusion. The calculated relative hydraulic resistance for low turbulators increases quite slightly, and for medium-altitude turbulators reaches 2÷2.5 to the critical Reynolds number, and subsequently it increases up to 3 times; for high turbulators, the relative hydraulic resistance increases up to 4 times even before the transition flow regime is reached, after which it increases up to 4.5 times. The calculated relative isothermal intensified heat exchange under the laminar flow regime of a viscous coolant for relatively high turbulators increases almost 2 times; for relatively medium heights of turbulators — almost one and a half, and for low relative heights, the intensification of heat exchange is insignificant.

Solution of an Inverse Boundary Value Problem of Heat Exchange for a Hollow Cylinder

Solution of an Inverse Boundary Value Problem of Heat Exchange for a Hollow Cylinder

Bound for the moment generating function from the detailed fluctuation theorem.

A famous consequence of the detailed fluctuation theorem (FT), p(Σ)/p(-Σ)=exp(Σ), is the integral FT 〈exp(-Σ)〉=1 for a random variable Σ and a distribution p(Σ). When Σ represents the entropy production in thermodynamics, the main outcome of the integral FT is the second law, 〈Σ〉≥0. However, a full description of the fluctuations of Σ might require knowledge of the moment generating function (MGF), G(α):=〈exp(αΣ)〉. In the context of the detailed FT, we show the MGF is lower bounded in the form G(α)≥B(α,〈Σ〉) for a given mean 〈Σ〉. As applications, we verify that the bound is satisfied for the entropy produced in the heat exchange problem between two reservoirs mediated by a weakly coupled bosonic mode and a qubit swap engine.

Heat exchange mechanism analysis and structural parameter optimization for series-combined microchannel heat sinks

This study presents a heat exchange mechanism analysis and an optimization of structural parameters of a built-in series combined microchannel heat dissipation platform, aiming to address the problems of low heat exchange and high pumping power caused by the mismatch in the selection of the structural parameters of the microchannel heat sink (MHS). A numerical calculation model was built in Fluent, and the temperature difference, pressure difference, and their ratio between the inlet and outlet of the MHS were used as the evaluation indicators. Using an orthogonal test, the influence of multiple parameters on the heat exchange effect was investigated. It is found that the shape of the microchannel is the main factor affecting the heat exchange phenomena, and the optimal combination of parameters such as the height, width, and length of the rectangular microchannel is obtained by a genetic algorithm. Physical transformation and heat exchange experiments were carried out. It was highlighted that the comprehensive performance of the optimized cooling system has been improved to varying degrees under different inlet flow conditions. At an inlet flow rate of 2.0 m3 h−1, the pressure difference and pumping power consumption are reduced by 24.9%, and the maximum temperature difference and heat dissipation power are increased by 21.0%. An experimental study on the heat exchange effect of MHSs with different numbers of serial pieces was carried out. With a number of MHSs of 15, the heat exchange is found to be sufficient, and the maximum heat exchange estimated was 9289.52 W.

Methods of Simplifying Optimal Control Problems, Heat Exchange and Parametric Control of Oscillators

Methods of simplifying optimal control problems by decreasing the dimension of the space of states are considered. For this purpose, transition to new phase coordinates or conversion of the phase coordinates to the class of controls is used. The problems of heat exchange and parametric control of oscillators are given as examples: braking/swinging of a pendulum by changing the length of suspension and variation of the energy of molecules’ oscillations in the crystal lattice by changing the state of the medium (exposure to laser radiation). The last problem corresponds to changes in the temperature of the crystal.

ТЕОРЕТИЧНІ ДОСЛІДЖЕННЯ ВИЗНАЧЕННЯ КРИТИЧНИХ УМОВ МІКРОКЛІМАТУ ПРИ АВАРІЙНИХ ВІДКЛЮЧЕННЯХ ТЕПЛОПОСТАЧАННЯ З УРАХУВАННЯМ КОНСТРУКТИВНИХ ОСОБЛИВОСТЕЙ БУДИНКІВ

Purpose. Analysis and research of determining critical microclimate conditions in emergency situations in heat supply systems, taking into account external climatic conditions and structural features of buildings, which are caused by the acceleration and cheapening of construction processes thanks to the introduction of the latest technologies. Methodology. Theoretical and experimental research based on fundamental knowledge in the field of thermal processes and methods of solving heat exchange problems, modelling of dynamic processes, methods and analysis of random processes, methods of mathematical statistics and forecasting. Results. On the basis of the conducted research, the regularities of the influence of the environment on the maintenance of the microclimate conditions of the premises, depending on the structural features of buildings and structures in emergency situations in heat supply systems, have been established. The limit conditions for achieving critical conditions of the microclimate of premises as a result of accidents in heat supply systems have been determined, which makes it possible to predict the limit conditions for eliminating the consequences of emergency situations and to optimize the activities of repair and rescue services, which are especially relevant in connection with the failure of outdated heat supply systems and combat operations on territory of the country. Scientific novelty. Determining the patterns of environmental influence on the maintenance of indoor microclimate conditions in connection with the structural features of buildings and structures in emergency situations in heat supply systems, which allows controlling the heat supply mode to ensure certain indoor microclimate conditions. Practical significance. Improving the forecasting of the limit conditions for liquidation of the consequences of emergency situations in heat supply systems allows optimizing the activities of repair and rescue services, especially in connection with the failure of outdated heat supply systems and combat operations on the territory of the country.

Mathematical Modelling of Heat Transfer in Pipes with Turbulators in the Laminar Region and in the Transition to Turbulent Flow

The article presents an analysis of mathematical modeling of heat transfer in pipes with turbulators at low Reynolds numbers characteristic of laminar and transient flow modes of heat carriers. The solution of the heat exchange problem for semicircular cross-section flow turbulators based on multi-block computing technologies based on the solution of Reynolds equations closed using the Menter shear stress transfer model and the energy equation (on multi-scale intersecting structured grids) by the factorized finite-volume method is considered. Both local and averaged characteristics of the flow and heat transfer to a pipe with a system of turbulators were obtained, which made it possible to determine the levels of heat exchange intensification for these regime ranges.

Brownian motion with radioactive decay to calculate the dynamic bulk modulus of gases saturating porous media according to Biot theory

We present a new stochastic simulation method for determining the long-wavelength effective dynamic bulk modulus of gases, such as ambient air, saturating porous media with relatively arbitrary microgeometries, i.e., simple enough to warrant Biot’s simplification that the fluid and solid motions are quasi-incompressible motions at the pore scale. The simulation method is based on the mathematical isomorphism between two different physical problems. One of them is the actual Fourier heat exchange problem between gas and solid in the context of Biot theory. The other is a diffusion-disintegration-controlled problem that considers Brownian motion of diffusing particles undergoing radioactive-type decay in the pore volume and instant decay at the pore walls. By appropriately choosing the decay time and the diffusion coefficient, the stochastic algorithm we develop to determine the average lifetime of the diffusing particles, directly gives the effective apparent modulus of the saturating fluid. We show how it leads to purely geometric stochastic constructions to determine a number of geometrical parameters. After validating the algorithm for cylindrical circular pores, its power is illustrated for the case of fibrous materials of the type used in noise control. The results agree well with a model of the effective modulus with three purely geometric parameters of the pore space: static thermal permeability divided by porosity, static thermal tortuosity, and thermal characteristic length.

Identification of the Effect of Mass Flow Rate and Hydrogen to Hydrocarbon Ratio on the Thermal Performance of a Shell and Tube Heat Exchanger – An Industrial Case Study

In various industrial processes such as petroleum refineries, crude oil must be heated to the required temperature. Here a study of a heat exchanger problem of a catalytic naphtha reforming unit of an SKIKDA refinery (RA1K) is carried out. In this unit the feed (naphtha and recycle gas) is required to enter the first reactor of the reaction section at 471℃, while the feed inlet temperature at the reactor is only 450℃. This problem appeared after starting the unit with a mass flow of 60% of the naphtha. The essential device for heating the charge before entering the reactor is shell-and-tube heat exchanger. In the present study, the Kern method is used to check the heat exchanger in the design and experimental cases. The Aspen HYSYS software has been used to study the influence of various naphtha mass flow rates on the thermal performance of a heat exchanger. The outlet feed temperature was examined for each mass flow rate of naphtha (i.e., 60, 70, 80, 90 and 100%). The simulation results show the important role of the studied parameters in the thermal performance enhancement of heat exchanger, where the case of a mass flow of 60% of the naphtha, the temperature 471℃, provided for by the design, is obtained with an H2/HC ratio of 4.68.