Heat transfer of gas–liquid mixture in micro-channel heat sink

The main objective of the present investigation is to study heat transfer in parallel micro-channels of 0.1 mm in size. Comparison of the results of this study to the ones obtained for two-phase flow in “conventional” size channels provides information on the complex phenomena associated with heat transfer in micro-channel heat sinks. Two-phase flow in parallel micro-channels, feeding from a common manifold shows that different flow patterns occur simultaneously in the different micro-channels: liquid alone (or single-phase flow), bubbly flow, slug flow, and annular flow (gas core with a thin liquid film, and a gas core with a thick liquid film). Although the gas core may occupy almost the entire cross-section of the triangular channel, making the side walls partially dry, the liquid phase always remained continuous due to the liquid, which is drawn into the triangular corners by surface tension. With increasing superficial gas velocity, a gas core with a thin liquid film is observed. The visual observation showed that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. The probability of appearance of different flow patterns should be taken into account for developing flow pattern maps. The dependence of the Nusselt number, on liquid and gas Reynolds numbers, based on liquid and gas superficial velocity, respectively, was determined in the range of Re_LS = 4–56 and Re_GS = 4.7–270. It was shown that an increase in the superficial liquid velocity involves an increase in heat transfer (Nu_L). This effect is reduced with increasing superficial gas velocity, in contrast to the results reported on two-phase heat transfer in “conventional size” channels.     

Population balance model of heat transfer in gas–solid particulate systems

A population balance model is derived for heat transfer processes in gas–solid systems with intensive motion of particles in order to describe the temperature distribution of particulate phase. The model involves collisional particle–particle and particle–wall heat transfers, and continuous gas–particle, gas–wall and wall–liquid environment heat transfer processes. Collisional heat transfers are characterised by collision frequencies and random heat exchange parameters with general probability distributions with support [0, 1], describing the heat transfer efficiency between the colliding solid bodies. An infinite hierarchy of moment equations, describing the time evolution of moments of the temperature of particle population is derived from the population balance equation, which can be closed at any order of moments. The properties of the model and the effects of parameters are examined by numerical experiments using the second order moment equation model of a spatially homogeneous fluidized bed.     

Mitigation of ice crystallization fouling in stationary and circulating liquid–solid fluidized bed heat exchangers

Liquid–solid fluidized bed heat exchangers are attractive ice crystallizers since they are able to mitigate ice crystallization fouling and exhibit high heat transfer coefficients. Experiments show that the fouling removal ability of stationary fluidized beds increases with decreasing bed voidage (95–80%) and increasing particle size (2–4 mm). The removal of ice crystallization fouling appears to be more effective in circulating fluidized beds, especially at high circulation rates. Fouling removal is realized by both particle–wall collisions and pressure fronts induced by particle–particle collisions. A comparison between ice crystallization experiments and impact characteristics shows that the removal rate is proportional to the impulse exerted on the wall. A model based on these phenomena is discussed and predicts the transition temperature difference for ice crystallization fouling in both stationary and circulating fluidized beds with an average absolute error of 9.2%.     

Heat transfer model for gas–liquid slug flows under constant flux

This paper investigates the mechanisms leading to enhanced heat and/or mass transfer rates in two-phase non-boiling slug flows. The problem is analyzed in a minichannel geometry subjected to a constant heat flux boundary. Local Nusselt numbers, obtained using Infrared thermography are analyzed in both entrance and fully developed flow regions. These novel measurements highlight the physics governing slug-flow heat transfer and results indicate that optimized slug geometries can yield up to an order of magnitude heat transfer enhancement. Finally, based on the physics identified, a heat transfer model is developed which is also applicable to similar mass transfer problems.     

Heat transfer in a reaction–diffusion system with a moving heat source

A general formulation is presented for a moving boundary problem in which heat is generated at the boundary due to an exothermic reaction involving a species which diffuses into a dispersed phase from an external medium of finite volume. The speed of the moving boundary is prescribed based on the solution of the mass diffusion problem and an analysis is presented of the thermal dynamics of the system. The set of equations describing heat transport leads to a Green’s function type problem with time dependent boundary conditions and the Galerkin finite element method is employed to develop a numerical solution. Transformations are introduced to freeze the moving boundary and partition the domain for ease of computation, and an iterative scheme is defined to satisfy the heat flux jump boundary condition and match the temperature field across the moving boundary. The numerical results are used to set the limits of applicability of an analytical perturbation solution. Essential aspects of thermal dynamics in the system are described and parametric regions resulting in a local temperature hot spot are delineated. Computed contour plots describing thermal evolution are presented for different combinations of parameter values. These may be of utility in the prediction of thermal development, for control and avoidance of hot spot formation, and in physical parameter estimation.     

Heat transfer study in a rotor–stator system air-gap with an axial inflow

Discoidal rotor–stator systems are nowadays sometimes used in electrical wind generator. The cooling of such a system is a major problem due to the fact that high electrical losses are dissipated for relatively low rotational speed, responsible of the cooling. A new cooling solution is then investigated in this paper. So, this paper presents an experimental study of the local heat transfers on the rotor surface in the air-gap of a discoidal rotor–stator system, in which an air jet comes through the stator and impinges the rotor. To determine the surface temperatures, measurements were taken on the rotor, using an experimental technique based on infrared thermography. A thermal balance equation was used to identify the local convective heat transfer coefficient. The influence of the axial Reynolds number Re_j and the rotational Reynolds number Re was measured and compared with the data available in the literature. Local convective heat transfer coefficients were obtained for an inter-disk dimensionless spacing interval G ranging from 0.01 to 0.16 for Re_j between 0 and 41,666 and for Re between 20,000 and 516,000. The rotating disk can thus be divided into zones: one dominated by the air jet near the center of the rotor and one affected by both the air jet and rotation. Even though these two zones are not located in the same place on the disk, the heat transfers with non-zero impinging jets appear to be continuously improved compared to those with no jets. Critical radii over the rotor surface are identified and correlations are given.     

Coal steam-gasification model in chemical regime to produce hydrogen in a gas–solid moving bed reactor using nuclear heat

A theoretical model of the coal gasification with steam in a chemical moving bed reactor is developed. A very high temperature nuclear reactor provides the energy for accomplishing the endothermic gasification reactions. The model is developed in chemical regime giving information about the temperature profiles, the coal conversion and the specific productivity in the gasification process. For the heat transfer, the three resistances have been included (conduction, convection and radiation). The results have shown that the coal conversion in the heterogenic water “shift” reaction is smaller than the ones in mass transfer control. The gas velocity can change strongly the conversion, for velocities under 0.06 m/s, for bigger quantities the change is negligible. An increase in the gas temperature at the reactor input leads to an increase in the coal conversion; an increase in the temperature from 900 K to 1300 K increases the conversion 40%.     

Heat transfer enhancement in cool-thermal discharge systems from ice melting with time–velocity variations

A mathematical model for the cool-thermal discharge systems which produce chilled air by melting ice to supply on-peak daytime air-conditioning needs under time-velocity variations and external recycle was developed. The model resolves the moving liquid–solid interface problem with the integral boundary-layer analysis method. For specified target of the outlet air temperature, time variation of the air velocity can be theoretically predicted for the given ambient air inlet temperature functions. The heat transfer efficiency can be improved by employing the external recycle design under the expense of power consumption. The analysis indicates that the recycle effect is more significant for the case with higher outlet air temperature target. Besides, optimal reflux ratio exists for the analyzed system.     

Surface segregation of Pd–Cu alloy in various gas atmospheres

Pd–Cu alloys have been investigated as promising candidates for hydrogen separation membranes. Surface segregation influences the long-term performance of these membranes since their catalytic effect is mainly controlled by the surface composition. In the present research, surface segregation of Pd-40 at.% Cu alloy in vacuum and various gas atmospheres (H₂, CO and CO₂) was investigated with both XPS and LEISS probing different depths below the surface. Adsorption of H₂ and CO on the surface has a significant impact and the surface segregation trend can be reversed as compared to segregation in vacuum, however, CO₂ has almost no influence on the segregation behaviour. A thermodynamic model is also presented to explain these phenomena and to understand surface segregation behaviour of binary alloys in various gas atmospheres. The results can be considered as basic guidelines to design novel alloys for hydrogen separation membranes and predict their long-term performance under actual working conditions.     

Heat transfer in pipe flow of a Johnson–Segalman fluid

A mathematical model for flow and heat transfer has been presented for Johnson–Segalman fluid in a pipe. Employing homotopy analysis method, the developed equations are solved analytically. The velocity and temperature fields are obtained. The dependence of the flow quantities on the material properties and Brinkman number is determined.     

Prevention of fouling and scaling in stationary and circulating liquid–solid fluidized bed heat exchangers: Particle impact measurements and analysis

Fluidized particles in liquid–solid fluidized bed heat exchangers are able to remove deposits from the walls and thus to prevent fouling or scaling. This fouling prevention ability is believed to depend strongly on the frequency and force of particle–wall collisions. This paper presents piezoelectric measurements of impacts on the wall in both stationary and circulating fluidized beds of various particle sizes and bed voidages. Two types of impacts were measured, namely by collisions of particles on the sensor and by liquid pressure fronts induced by particle–particle collisions close to the sensor. The characteristics of both impact types are used to analyze the total impulse and energy exerted by impacts on the wall for various fluidized beds.     

Solar–gas solid sorption heat pump

Solid sorption short cycle heat pump (⩽10 kW) which uses physical adsorption and is of interest to the space and domestic application is designed and tested. This heat pump has a very short (12 min), nonintermittent, two adsorber heat recovery cycles with an active carbon fiber as a sorbent bed and ammonia as a working fluid. It has two energy sources: solar and gas flame. The system management consists only in actuating the special type valves to change the direction of the heating circuit and water valves to change the water cooling circuit.     

Co–gasification of coal/biomass blends in 50 kWe circulating fluidized bed gasifier

This paper reports gasification of coal/biomass blends in a pilot scale (50 kWe) air-blown circulating fluidized bed gasifier. Yardsticks for gasification performance are net yield, LHV and composition and tar content of producer gas, cold gas efficiency (CGE) and carbon conversion efficiency (CCE). Net LHV decreased with increasing equivalence ratio (ER) whereas CCE and CGE increased. Max gas yield (1.91 Nm³/kg) and least tar yield (5.61 g/kg of dry fuel) was obtained for coal biomass composition of 60:40 wt% at 800 °C. Catalytic effect of alkali and alkaline earth metals in biomass enhanced char and tar conversion for coal/biomass blend of 60:40 wt% at ER = 0.29, with CGE and CCE of 44% and 84%, respectively. Gasification of 60:40 wt% coal/biomass blend with dolomite (10 wt%, in-bed) gave higher gas yield (2.11 Nm³/kg) and H₂ content (12.63 vol%) of producer gas with reduced tar content (4.3 g/kg dry fuel).     

Thermal stochastic collision model in turbulent gas–solid pipe flows

New thermal stochastic particle collision model in gas–solid flow in a riser is developed. The simulation is based on four-way coupling of phases considering inter-particle collision and heat transfer. It is shown that the limitation of excessive computational time in Eulerian–Lagrangian simulation of gas–solid flows for the high loading ratios is eliminated by using the stochastic particle collision model. The simulation results demonstrate that the predictions of the developed thermal stochastic particle collision modem are in good agreement with those obtained by the direct particle collision model and the available experimental data. The new stochastic modeling is used and nearly dense gas–solid flow is simulated for high loading ratios up to eight and the results are presented and discussed.     

Flow and heat transfer in a micro-cylindrical gas–liquid Couette flow

Analytic solutions for the gas and liquid velocity and temperature distribution are determined for steady state one-dimensional microchannel cylindrical Couette flow between a shaft and a concentric cylinder. The solution is based on the continuum model and takes into consideration the velocity slip and temperature jump in the gaseous phase defined by the Knudsen number range of 0.001 < Kn < 0.1. The two fluids are assumed immiscible. The gas layer is adjacent to the shaft which rotates with angular velocity ω_s and is thermally insulated. The outer cylinder rotates with angular velocity ω_o and is maintained at uniform temperature. The governing parameters are identified and the effects of the Knudsen number and accommodation coefficients on the velocity and temperature profiles, reduction in the overall temperature rise due to the gas layer, the Nusselt number and shear reduction are examined. It was found that the required torque to rotate the liquid in the annular space is significantly reduced by introducing a thin gas layer adjacent to the shaft. Also, reduction in shaft temperature is enhanced through a combination of high energy accommodation coefficient and low momentum accommodation coefficients. Results also indicate that the gas layer becomes more effective in reducing the shaft temperature when the housing angular velocity is much larger than the shaft angular velocity.     

Heat transfer analysis for particle–fluid suspension thermomagnetohydrodynamic peristaltic flow with Darcy–Forchheimer medium

This theoretical analysis explores the effect of heat and mass transfer on particle–fluid suspension for the Rabinowitsch fluid model with the stiffness and dynamic damping effects through Darcy–Brinkman–Forchheimer porous medium. In this study, we also incorporate slip and transverse magnetic field effects. Using low Reynolds number, to neglect inertial forces and to keep the pressure constant during the flow, channel height is used largely as compared with the ratio of length of the wave. A numerical technique is used to solve flow governing system of differential equations. Particular attention is paid to viscous damping force parameter, stiffness parameter, and rigidity parameter; also, the numerical data for thermal profile, momentum, and concentration distribution are presented graphically. Outcomes are deliberated in detail for different fluid models (thinning, thickening, and viscous models). It is found that velocity profile increases for greater values of viscous damping effect and stiffness and rigidity parameter for shear thinning, but conflicting comportment is showed for thickening nature model. Viscous dissipation effects increases the thermal profile for all cases of fluid models. The scope of the present article is valuable in explaining the blood transport dynamics in small vessels while considering the important wall features with chemical reaction characteristics. The current analysis has extensive applications in biomedical engineering field, that is, peristaltic pumps.     

Heat transfer enhancement via liquid–liquid phase separation

The heat transfer and flow phenomena during phase separation of partially miscible liquid solvent system were investigated experimentally. The experiments were conducted with a three components system which has an upper critical solution temperature, using critical and off-critical compositions of the solvent mixtures. The convective heat transfer rates were studied for laminar flow in a small diameter horizontal tube and for free convection from its outer surface. It was found that with phase separation the forced convective heat transfer can be augmented by up to 130% compared to heat transfer rates obtained in single phase flow (without phase separation). However, for low quenching rate and depth associated mainly with experiments conducted with critical compositions, deterioration of the heat transfer rates was observed. The free convection heat transfer coefficients were found to be augmented up to 100%. Macro- and micro-flow visualization were also conducted to follow the flow phenomena during the phase separation, and the mechanisms responsible to the heat transfer enhancement are discussed.     

Analysis of a pressurized solid oxide fuel cell–gas turbine hybrid power system with cathode gas recirculation

A pressurized solid oxide fuel cell–gas turbine hybrid system (SOFC–GT system) has been received much attention for a distributed power generation due to its high efficiency. When considering an energy management of the system, it is found that a heat input is highly required to preheat air before being fed to the SOFC stack. The recirculation of a high-temperature cathode exhaust gas is probably an interesting option to reduce the requirement of an external heat for the SOFC–GT system. This study aims to analyze the pressurized SOFC–GT hybrid system fed by ethanol with the recycle of a cathode exhaust gas via a simulation study. Effect of important operating parameters on the electrical efficiency and heat management of the system is investigated. The results indicate that an increase in the operating pressure dramatically improves the system electrical efficiency. The suitable pressure is in a range of 4–6 bar, achieving the highest system electrical efficiency and the lowest recuperation energy from the waste heat of the GT exhaust gas. In addition, it is found that the waste heat obtained from the GT is higher than the heat required for the system, leading to a possibility of the SOFC–GT system to be operated at a self-sustainable condition. Under a high pressure operation, the SOFC–GT system requires a high recirculation of the cathode exhaust gas to maintain the system without supplying the external heat; however, the increased recirculation ratio of the cathode exhaust gas reduces the system electrical efficiency.     

Reactivity tests of the water–gas shift reaction on fresh and used fluidized bed materials from industrial DFB biomass gasifiers

The dual fluidized bed gasification process, offers various advantages for biomass gasification as well as the utilization of other solid feedstocks. In order to improve the knowledge of the reactions in fluidized bed gasifier, different types of bed material used in the gasifier were tested in a micro-reactivity test rig. It has been previously observed that during long-term operation, the surface of the bed material used (calcined olivine) undergoes a modification that improves catalytic activity. The main reaction of interest is the water–gas shift reaction. Olivine taken from long-term operation at the 8 MW biomass gasifier at Güssing (Austria), fresh olivine as a reference, and calcite, which is commonly used for enhancing in-bed catalytic tar reduction, were tested using the micro-reactivity test rig. Tests were carried out at temperatures of 800, 850, and 900 °C and space velocities of 40,000 to 50,000 h⁻¹ were applied. CO conversions of up to 61.5% were achieved for calcite. Used olivine showed a similar behavior, representing a large improvement compared to fresh olivine, which had CO conversion rates of less than 20%.     

Slagging behavior and mechanism of high-sodium–chlorine coal combustion in a full-scale circulating fluidized bed boiler

The contents of chlorine and sodium in Xinjiang Shaerhu (SEH) coal are extremely high, leading to severe slagging. In this paper, the slag was sampled from a circulating fluidized bed (CFB) boiler purely burning SEH coal, to analyze the slagging mechanism based on the characterization of morphology and composition. The results show a three-layer structure for the slag sampled from the buried heat-exchanger in the dense-phase zone of the CFB boiler. The inner layer close to the heat-exchanger is NaCl, which enhances the adhesion of ash particles, while the middle layer and the outer layer are mainly composed of Ca₂Al₂SiO₇ and other Si–Al materials. In comparison, the slag sampled from the refractory wall shows a molten state without a layered structure and mainly composed of NaCl, NaAlSiO₄, Ca₂Al₂SiO₇, and CaSiO₃. The effect of mixing bed material, on the ash melting and release of chlorine and sodium was further conducted, which indicates that the mixing of bed material has no significant effect on the release of chlorine(Cl) and sodium(Na) but highly affects the melting temperature and compositions. The ash fusion temperature reaches the lowest with a 50% mixing ratio of bed material, which is 120 °C lower than that of SEH coal ash. This study can provide better guidance for controlling severe slagging, from the combustion of high Na and Cl coal in industrial furnaces.