Conjugate Heat Exchange Research Articles

Preliminary design and modeling of mini channels to enhance heat transfer in a millimetric catalytic combustor

Mini channel solution is used in devices that require a high density of transmitted thermal power as a very large-scale integration design in computer systems and compact exchangers. Furthermore, the mini channels are extensively investigated in the literature for turbulent and laminar regimes. In this project, different configurations of mini channels have been studied to enhance heat transfer, using simulations with a commercial multi-physics code. Thanks to the results of the models, more promising configurations with 3D printing technique may be built. The project challenge is improving convective thermal power extracted by the exhaust gases of a mini-catalytic combustor. The combustor feeds six modules for thermoelectric power production (TEMs). As the first step, three different mini channel geometries have been chosen; the first one with 19 channels with rectangular cross-section, the second one with 6 channels with a convergent profile, and the latter with 2 channels with a fractal branching geometry. Simulations started from studying fluid dynamic to investigate the velocity field at the exit of the mini channels. The analysis has been extended by adding the conjugate heat exchange between fluid and combustor wall. The results show an increase in heat exchange compared to the base case for all configurations, with a maximum value for the 19 mini channels configuration.

MODES OF ADHESIVE BONDING LAYER FORMATION DURING ROLL MOTION IN THE PROCESS OF SHEET LAMINATION

Novel technologies for the synthesis and processing of materials and products based on these materials, such as additive manufacturing, require the use of mathematical modeling to study the accompanying physical and chemical phenomena. These technologies include sheet lamination, which today has a variety of applications and numerous modifications. This article presents a model of adhesion layer formation in the creation of multilayer composites, which is controlled by a hot roller moving along the surface of the top layer. The dynamics of reagent heating depends on the thermal properties and sizes of all layers, the temperature of the roller, and its pressure against the surface, with characteristic times and spatial scales for different processes. The transition to dimensionless variables allowed the identification of dimensionless complexes to determine different modes of development of the adhesion layer synthesis process. Based on the combustion theory and non-equilibrium thermodynamics, new criteria of the process initiation and transition to a quasi-stationary mode were elaborated. The behavior of various thermodynamic quantities was analyzed in detail, including heat storage in the heated layer, enthalpy excess, and entropy production. The reaction zone is wide in most cases. The dependences of the half-conversion time on the model parameters are plotted for a finite layer size. This is a new characteristic for sheet lamination technology. A controlled mode, a fast bulk synthesis mode, a slow transformation mode maintained by the heat stored in the inert materials, and a quasi-stationary mode are identified. Criteria were established for different modes of reactions under conjugate heat exchange conditions.

Numerical Study of the Two-Stage Process of Synthesis of a Composite Controlled by a Moving Source under Conditions of Conjugate Heat Exchange

Numerical Study of the Two-Stage Process of Synthesis of a Composite Controlled by a Moving Source under Conditions of Conjugate Heat Exchange

Modelling and Analysis of Viscoelastic and Nanofluid Effects on the Heat Transfer Characteristics in a Double-Pipe Counter-Flow Heat Exchanger

This study computationally investigates the heat transfer characteristics in a double-pipe counter-flow heat-exchanger. A heated viscoelastic fluid occupies the inner core region, and the outer annulus is filled with a colder Newtonian-Fluid-Based Nanofluid (NFBN). A mathematical model is developed to study the conjugate heat transfer characteristics and heat exchange properties from the hot viscoelastic fluid to the colder NFBN. The mathematical modelling and formulation of the given problem comprises of a system of coupled nonlinear partial differential Equations (PDEs) governing the flow, heat transfer, and stress characteristics. The viscoelastic stress behaviour of the core fluid is modelled via the Giesekus constitutive equations. The mathematical complexity arising from the coupled system of transient and nonlinear PDEs makes them analytically intractable, and hence, a recourse to numerical and computational methodologies is unavoidable. A numerical methodology based on the finite volume methods (FVM) is employed. The FVM algorithms are computationally implemented on the OpenFOAM software platform. The dependence of the field variables, namely the velocity, temperature, pressure, and polymeric stresses on the embedded flow parameters, are explored in detail. In particular, the results illustrate that an increase in the nanoparticle volume-fraction, in the NFBN, leads to enhanced heat-exchange characteristics from the hot core fluid to the colder shell NFBN. Specifically, the results illustrate that the use of NFBN as the coolant fluid leads to enhanced cooling of the hot core-fluid as compared to using an ordinary (nanoparticle free) Newtonian coolant.

Thermoviscous liquid conjugate heat exchange simulation in cylindrical channel

The article is concerned with the flow of a thermoviscous fluid in a cylindrical channel for various types of viscosity functions in the joint solution of the problem of conjugate heat exchange between the channel and the surrounding material. A model of a cylindrical channel with a thermoviscous liquid flowing inside at a given velocity and viscosity is considered. The methodology of the process consists in applying a system of model equations that describes the dynamics of thermoviscous fluid flow processes based on the laws of phase energy conservation, the continuity equation, the Navier-Stokes momentum conservation equation taking into account conjugate heat transfer and the heat transfer equation for a solid. Numerical implementation of model representations was carried out using the Comsol Multiphysics digital package. Velocity and temperature profiles were obtained in the cross section of the channel. The influence of the viscosity function on the flow profile is shown, as well as the quantitative characteristics of the flow and heat transfer. An analysis was made of the dependence of the nature of the velocity and temperature profiles on the viscosity of the liquid and the type of dependence of viscosity on temperature. The result of the work done is the found features and regularities that arise in the process of conjugate heat transfer of a thermoviscous liquid in a cylindrical channel when comparing the linear and exponential dependences of viscosity on temperature. The analysis of the obtained dependences of viscosity and temperature in the flow profile, the value of the maximum flow rate was carried out. It is shown that viscous friction causes a local increase in temperature near the pipe walls, followed by an increase in fluid velocity due to a decrease in viscosity. The results obtained are basic for the further development of this topic and will be used to solve new problems in the field of modeling the thermoviscous fluid flow and conjugate heat transfer.

Validation of RANS Turbulence Models for the Conjugate Heat Exchange Problem

This paper addresses problems of mathematical modeling of heat exchange processes in the pre-nozzle volume of a solid propellant rocket engine with a charge with starlike cross-section and a recessed hinged nozzle. Methods of mathematical modeling are used to solve the quasi-stationary spatial conjugate problem of heat exchange. An analysis is made of the influence of RANS turbulence models on the flow structure in the flow channels of the engine and on the computed heat flow distributions over the surface of the recessed nozzle. Methods of mathematical modeling are used to solve the quasi-stationary spatial conjugate problem of heat exchange. Results of validation of RANS turbulence models are presented using well-known experimental data. A comparison of numerical and experimental distributions of the heat-transfer coefficient over the inlet surface of the recessed nozzle for the engine with a cylindrical channel charge is made for a primary choice of turbulence models providing a qualitative agreement between calculated and experimental data. By analyzing the results of numerical modeling of the conjugate problem of heat exchange in the combustion chamber of the solid propellant engine with a starlike channel, it is shown that the SST $k-\omega$ turbulence model provides local heat-transfer coefficient distributions that are particularly close to the experimental data.

Численное моделирование конвекции жидкого магния в аппарате восстановления титана с учетом эффективных граничных условий

We investigate the numerical problem of conjugate heat exchange between liquid magnesium, a steel retort, and the air-cooling zone of the titanium sponge reduction reactor furnace at the initial stages of production. The uneven distribution of heat fluxes on the side surface of the retort significantly affects the formation of convective turbulent flow inside the apparatus. The accurate consideration of thermal boundary conditions will make it possible to adequately solve the problem of heat and mass transfer in the reactor without using model representations of heat flows on the boundaries of the retort. The problem was solved in the axisymmetric nonstationary formulation using the k-ω SST turbulence model. In the numerical simulation, two conceptual solutions of the problem were implemented. The first one involves the end-to-end calculation in the framework of the coupled heat and mass transfer problem, with the calculations performed in all parts of the titanium reduction apparatus and thermal boundary conditions set on its outer boundaries. The second model relies on an effective thermal boundary condition that is applied to the outer boundary of the retort and excludes calculations in the cooling channel when investigating magnesium convection. The first model, due to its greater completeness, is more accurate, but requires considerable computational resources and time. The model with an effective thermal boundary condition, provided that the coefficients in it are selected appropriately, gives results qualitatively and quantitatively close to those obtained through end-to-end calculations. At the same time, such a model is more economical. The paper evaluates the possibility of applying an effective boundary condition. Convective flows of molten magnesium in the retort under different modes of blowing and heating have been studied. The influence of the radiation part of the heat flow on the formation of convective currents is shown.

The effect of the stages of chemical reactions on the composition of the products formed under a dynamic thermal explosion of the mixtures of aluminium and nickel powders

The paper is the first to propose the mathematical model of the process of intermetalliс composite synthesis in the mode of dynamic thermal explosion taking into account the stages of chemical reactions, the rate of which slows down with the increase of the refractory product content. The model takes into account the mutual influence of thermal and chemical processes, as well as the condition of conjugate heat exchange with the reactor walls. Examples of composition evolution in the synthesis process are given. According to the results of the study, diagrams have been compiled for the first time to illustrate changes in the phase composition of the product depending on changes in the composition of the initial pressed material. It is shown that as a result of bulk synthesis a composite of complex composition is formed. It was found that in the studied conditions in the general case there is an inhomogeneous distribution of concentrations.

Numerical Study of the Two-Stage Process of Synthesis of a Composite Controlled by a Moving Source Under Conditions of Conjugate Heat Exchange

Numerical Study of the Two-Stage Process of Synthesis of a Composite Controlled by a Moving Source Under Conditions of Conjugate Heat Exchange

Numerical Simulation of Conjugate Heat Exchange at Gas Flowing in a Well

A mathematical model is offered for conjugate heat exchange when gas flows in the section of a vertical well. The motion of the medium is described using a two-dimensional axisymmetric stationary formulation based on boundary-layer equations. The turbulent gas flowing due to the reservoir energy is considered. Natural gas is taken as a travelling medium. The numerical simulation results are presented in the form of the dependences of the flow temperature and gas density along the radius of the well on the external factors of the heat exchange changing along the well height. The results describe the thermobaric state of the well in the condition of the conjugate heat exchange of the produced natural gas flow and tubed well depending on the external thermal conditions.

Influence of thermal conductivity of the partition wall on non-stationary conjugate natural convective heat exchange and temperature fields in the walls of a rectangular fuel tank

The non-stationary conjugate heat transfer in a rectangular model of a fully filled thin-walled tank, divided by an internal vertical partition wall into two compartments of 1/3 and 2/3 of the tank volume is studied numerically in the conjugate formulation. The outer surfaces of the end walls and the upper wall of the tank are heat-insulated. The lower wall suddenly warms up under the influence of a uniform heat flux. The calculations are performed at the ratio of the thermal conductivity of solid walls to the thermal conductivity of fuel λs/λf = 1041.3 and λs/λf = 1 (low-heat partition wall) and thermal parameters of fuel T1 (Prandtl number Pr = 25.66). The influence of internal vertical partitions wall on the spatial shape of convective flows and the regularities of non-stationary conjugate heat exchange is studied. The temperature fields in the liquid and in the tank walls are calculated.

Anomalous blow-off limit of methane-air premixed flame in a micro preheated combustor with a flame holder

The present work first reports the combustion characteristics of methane/air premixed fuel mixture (Lewis number Le ≈ 1.0) at different Reynolds numbers (Re) in a micro preheated combustor with a flame holder which can be used as heat source for the micro power generators. Unexpectedly, the previously reported anomalous blow-off of lean H2/CH4/air (Le < 1.0) premixed flame in the mesoscale combustor appears in the present micro combustor, which is observed for the first time. The flame dynamics in the micro combustor also presents other new features comparing with that in the similar mesoscale combustor. Three neighboring cases (Re = 40, 80, and 120) are adopted to reveal the underlying mechanisms of the anomalous blow-off in terms of the flow field, stretch effect, conjugate heat exchange, and preferential transport effect. A better heat recirculation effect results in the increase of blow-off limit from Re = 120 to 80. A worse flow recirculation effect, a larger heat loss ratio to the environment, and a worse preferential transport effect lead to the decrease of blow-off limit from Re = 80 to 40. In the end, the critical standoff distance between the flame root and flame holder presents a second-order function relation with the change of Re.

Anchoring mechanisms of a holder-stabilized premixed flame in a preheated mesoscale combustor

A very lean-premixed, laminar methane–air flame is demonstrated, experimentally, to be stable in a mesoscale combustor with a flame holder. Unlike the anchoring location of the flame tip, the anchoring location of the flame root is practically independent of the equivalence ratio, inlet velocity, and thermal conductivity of the solid wall material. When the mixture becomes leaner, both the flame root and tip can adaptively shift toward the locations with higher temperatures, and additionally, the anchoring temperature of the flame root is higher. Subsequently, by means of the three-dimensional computational mechanics, their anchoring mechanisms are thoroughly analyzed in terms of the flow recirculation, stretch effect, preferential diffusion, and conjugate heat exchange. A recirculation zone or a low-velocity zone formed behind/near the flame holder and combustion chamber wall can assist the balance between the flow velocity and the flame speed for the flame anchoring, and the flame root can adaptively shift to a zone of lower local velocity. The stretch effect is not responsible for the flame root anchoring, but this effect stabilizes the flame tip by increasing the local flame speed near the flame tip. Preferential diffusion significantly promotes the local equivalence ratio near the anchoring location of the flame root, thereby facilitating the stability of this flame root, though it stabilizes the flame tip only slightly. Furthermore, the conjugate heat exchange plays an important role in preheating fuel/air and intensifying combustion, which influences the stabilization of both the flame root and tip. The shorter distance between the flame tip and the combustion chamber wall results in a stronger flame–wall coupling. These results indicate that the anchoring mechanisms for the flame root and tip differ.

Blow-off mechanism of a holder-stabilized laminar premixed flame in a preheated mesoscale combustor

In a previous study, we fabricated a well-designed mesoscale combustor with a flame holder and preheating channels to improve the flame stability under extreme combustion conditions. This combustor can also act as a model for studying the synergistic effect of heat and flow recirculation on the flame dynamics. This paper systematically illustrates and discusses the blow-off mechanisms of a holder-stabilized laminar premixed flame in a preheated mesoscale combustor via three-dimensional numerical simulation. The flame blow-off processes that have been observed experimentally at various Reynolds number (Re) are well reproduced. Pinch-off appears at large Re but is absent at small Re. First, the moving characteristics of the flame root and flame top in the blow-off process at small Re are discussed in detail. Then, the flame pinch-off mechanisms are carefully analyzed in terms of the flow field, conjugate heat exchange, and stretch effect. The results indicate that the influence of conjugate heat exchange, which exists between the flame holder and flame root, is not significant in the flame pinch-off phenomenon, but a larger recirculation zone (which helps to stabilize the flame root) and flame stretch (which splits the flame) both induce flame pinch-off. This work can aid in gaining insight into the flame blow-off dynamics when the effects of heat and flow recirculation work synergistically in many practical combustors.

МОДЕЛИРОВАНИЕ ДИНАМИКИ ТЕПЛОВОГО РЕЖИМА ТУПИКОВЫХ ГОРНЫХ ВЫРАБОТОК С ИСТОЧНИКОМ ВЫДЕЛЕНИЯ ТЕПЛОТЫ

Актуальность исследования обусловлена необходимостью снижения температуры воздуха в тупиковых горных выработках при ведении работ, связанных с выделением большого количества теплоты. Разработка способов нормализации термодинамических параметров рудничного воздуха в забоях на основе построения прогнозных математических моделей изменения теплового режима является необходимым условием обеспечения безопасности труда в горнодобывающей промышленности. Цель: получение зависимостей для определения скорости повышения температуры воздуха в тупиковой выработке при известных величинах интенсивности выделения теплоты и скорости её отведения вентиляционной струёй. Объекты: тупиковые выработки и окружающий породный массив. Методы: решение задачи нестационарного теплообмена рудничного воздуха и породного массива в сопряжённой постановке с помощью преобразований Лапласа; оценка безопасности по газовому фактору интенсификации выноса теплоты из горной выработки путём увеличения подачи воздуха в забой за счёт частично-повторного использования воздуха. Результаты. Проведён анализ проблемы отвода теплоты, выделяющейся при работе оборудования в замкнутом пространстве горных выработок. Показано, что процессов поглощения теплоты породным массивом и её отведения за счёт проветривания может быть недостаточно для обеспечения нормативных значений температуры воздуха. Разработана математическая модель сопряжённого теплообмена воздуха и породного массива, позволяющая прогнозировать повышение температуры с течением времени в зависимости от длины выработки и суммарной интенсивности выделения теплоты в ней. Установлено, что теплообменные процессы продолжительностью в несколько часов могут моделироваться в приближении малых времён, что значительно упрощает расчётные зависимости. Произведён расчёт динамики теплового режима при характерном для шахтных условий наборе физических параметров задачи, результаты которого явились обоснованием сделанного приближения и подтвердили опасность быстрого увеличения температуры воздуха при недостаточной скорости выноса теплоты из выработки вентиляционной струёй. Доказано, что при необходимости дополнительного отведения теплоты можно применять источники тяги высокой производительности с частично-повторным использованием исходящего воздуха без риска увеличения содержания газов в тупиковых выработках.

A non-monotonic blow-off limit of micro-jet methane diffusion flame at different tube-wall thicknesses

In order to provide guideline for choosing a suitable tube-wall thickness (δ) for the micro-jet methane diffusion flame, the effect of tube-wall thickness on the blow-off limit is investigated via numerical simulation in the present work. The results show that the blow-off limit of micro-jet methane diffusion flame firstly increases and then decreases with the increase of tube-wall thickness. Subsequently, the underlying mechanisms responsible for the above non-monotonic blow-off limit are discussed in terms of the flow filed, strain effect and conjugate heat exchange. The analysis indicates that the flow field is insignificant for the non-monotonic blow-off limit. A smaller strain effect can induce the increase of the blow-off limit from δ=0.1 to 0.2 mm, and a worse heat recirculation effect can induce the decrease of the blow-off limit from δ=0.2 to 0.4 mm. The non-monotonic blow-off limit is mainly determined by the heat loss of flame to the tube-wall and the performance of tube-wall on preheating unburned fuel. The smallest heat loss of flame to the tube-wall and the best performance of tube-wall on preheating unburned fuel result in the largest blow-off limit at δ=0.2 mm. Therefore, a moderate tube-wall thickness is more suitable to manufacture the micro-jet burner.

Effect of thermal condition of solid wall on the stabilization of a preheated and holder-stabilized laminar premixed flame

A novel combustor with a flame holder and preheating channels was developed to investigate the synergistic effect of heat and flow recirculation on combustion characteristics. This paper mainly focuses on the thermal condition of flame holder and combustion chamber wall (CCW) and their interaction on flame stabilization. Based on the distribution characteristics of temperature and flow fields, it can be deduced that the interaction between the flame holder and CCW can further increase the incoming fresh mixture temperature and decrease the height of recirculation zone, which subsequently affect the anchoring locations and temperatures of the flame root and top. To be more specific, the flame holder and CCW synergistically improve the preheating temperature of fresh mixture, making it higher than simple linear superposition. Moreover, the anchoring location of the flame root is mainly determined by the heat exchange of flame holder, but CCW matters more significantly for the flame top. Additionally, the anchoring temperature of the flame root is higher than that of the flame top. This study can help us to gain insights into the effect of conjugate heat exchange on flame stability when the effect of heat recirculation and flow recirculation works synergistically in practical combustors (like aero-engine).

Effect of conjugate heat exchange of flame holder on laminar premixed flame stabilization in a meso-scale diverging combustor

The present work investigates the effect of heat exchange of flame holder on the premixed flame stabilization in a diverging channel through numerical simulation. When the heat exchange of flame holder exists, the flame can remain stable and presents a “V” shape. A majority of the heat flux that enters the flame holder is used to preheat the incoming fresh mixture, which can induce and enhance the initiation of reactions. Moreover, three different zones are found and defined, including the free reaction zone, the quenching zone, and the enhanced reaction zone. However, when the heat exchange of flame holder is absent, the flame pulsates up and down instead of remaining stable, and shows a cone “Λ” shape. Subsequently, the pulsating mechanisms of flame are quantitatively analyzed in terms of local flow velocity, the angle between flow direction and flame front, and the stretch rate. The pulsation is a dynamic process where the flame speed self-adjusts to match the normal component of flow velocity. In summary, the present study indicates that the heat exchange of flame holder can significantly suppress the flame instability and improve the flame anchoring performance.

Numerical simulation of conjugated heat exchange in the turbulent motion of fluid in an oil well

The mathematical model of conjugate heat exchange is proposed for the turbulent movement of reservoir oil in a vertical well section. The motion of the medium is described using a two-dimensional axisymmetric stationary formulation and boundary layer equations. The movement of the turbulent flow of reservoir oil due to reservoir energy is presented. The liquid medium is a mixture of reservoir oil with dissolved gas and formation water. The results of the numerical modeling are presented in the form of dependences of the changing flow rate, temperature, and mass fraction of paraffin deposits that occur along the full vertical extent of the well. The results obtained describe the thermobaric state of the well under the condition of conjugate heat exchange between the fluid flow and the production pipe.

Numerical investigation of the local extinction and re-ignition mechanisms of premixed flame in a micro combustor with a flame holder and preheating channels

A novel combustor with a flame holder and preheating channels, which takes advantage of heat and flow recirculation, was developed to improve the stability of the micro flame in our previous work. The results demonstrated that the lean methane/air premixed flame could be stabilized within a wide operating range. Certainly, the flame blows off when the equivalence ratio of the fuel mixture becomes too small or too big or the Reynolds number is too large. The present work reports and analyzes the blow-off dynamics of a methane/air laminar premixed flame at a relatively large Reynolds number under the synergistic effect of heat and flow recirculation. Interestingly, a flame with local extinction and re-ignition appears, which is first found in the micro combustor. The underlying mechanisms of this interesting phenomenon are systematically discussed in terms of the flow recirculation effect, stretch effect, conjugate heat exchange, and preferential transport effect. We find that the counter flow in the recirculation zone and stretch effect induce the local extinction of the flame. The smaller heat loss from the residual flame to the flame holder and the better preferential transport effect contribute to the re-ignition of the fuel mixture. The preheating effect on the unburned fuel mixture is not expected to be insignificant for the local extinction and re-ignition phenomenon. These results mean that the generated mechanism of the flame with local extinction and re-ignition in the micro combustor is different from that in the conventional combustors.