Numerical Simulation of Capillary Convection with Encapsulated Phase-Change Particles

This study presents preliminary numerical simulations of forced convection with encapsulated phase-change material (EPCM) particles suspended in water, flowing through a square cross-section duct with top and bottom isoflux surfaces. The EPCM particles fit snugly in the channel, mimicking the flow of red blood cells and plasma in alveolar capillaries. Results for particles of different diameters show the snug fitting to yield enhanced heat transfer. The numerical results also seem to indicate the existence of an optimum number of particles for maximum heat transfer coefficient, a result of the interplay of flow circulation and heat transfer competition as the number of particles is changed in the channel.     

Plasma microcirculation in alveolar capillaries: Effect of parachute-shaped red cells on gas exchange

The effects of plasma microcirculation speed and red blood cell (RBC) shape on CO exchange in alveolar capillaries is numerically investigated. In this study, blood is modeled as an inhomogeneous, two-constituent substance, with one of the constituents being the blood plasma, treated as a homogeneous Newtonian fluid, and the other being the RBCs, treated as discrete, suspended particles (i.e., solid bodies) flowing with the plasma. The RBC shape and blood speed effects on the alveolar diffusion process is observed not only by the variations in the iso-lines along the alveolar capillary but also by changes in the lung diffusing capacity. The simulations include varying the capillary blood speed from 1.0 to 10 mm/s and the capillary hematocrit from 3% to 55%. The RBC shape effect is established by comparing results obtained for circular RBCs to results for which the RBCs have a more realistic, parachute shape when flowing with the plasma. Results reveal the parachute-shaped RBCs yield higher lung diffusing capacity compared to the circular ones. Moreover, blood flow can increase the lung diffusing capacity by almost 50% in certain cases, with the effect being more predominant at high blood speed and low hematocrit. Finally, the effect of blood speed for parachute-shaped RBCs is quantified in terms of percentage-increase lung diffusing capacity and presented in a simple predictive equation.     

Onset of secondary flow and enhancement of heat transfer in horizontal convergent and divergent channels heated from below

Experiments for the onset and development of the buoyancy driven secondary air flow and enhancement of heat transfer in a horizontal convergent and a divergent channel have been carried out. The bottom wall of the channel is horizontal and heated uniformly, while the top wall is insulated and inclined with respect to the horizontal plane so as to create a convergence angle of 3° for the convergent channel, or a divergence angle of 3° for the divergent channel. The aspect ratio (width to height) and the ratio of channel length to height at the entrance of the channel is 6.67 and 15, respectively. The Reynolds number ranges from 200 to 2000, the buoyancy parameter, Gr/Re², from 2.5 to 907 and Pr of the air flow is 0.7. Flow structure inside the channel is visualized by injecting smoke at the inlet flowing along the bottom wall. The onset of secondary flow appearing as transverse instability wave and onset of initial protrusion of the bottom heated layer are identified. Secondary flow structures observed are somewhat different from the case in the parallel-plate channel. This is attributed to the destabilization effect of the deceleration in the divergent channel which results in a much earlier initiation of secondary flow and more pronounced enhancement in the heat transfer, and the stabilization effect of the acceleration in the convergent channel which results in a much later initiation of the secondary flow and less pronounced enhancement in the heat transfer. However, the deceleration flow in the divergent channel and the acceleration in the convergent make the average Nusselt numbers approach the results of the parallel-plate channel. Correlation results for the onset of the secondary flow and enhancement of the heat transfer will be presented and discussed.     

Heat Transfer in Coiled Square Tubes for Laminar Flow of Slurry of Microencapsulated Phase Change Material

Passive heat transfer enhancement using a slurry of microencapsulated phase-change material (MEPCM) flowing in a laminar regime through a coiled duct of square cross section was evaluated. The phase-change material is n-octadecane. The flow behavior and heat transfer performance of water and MEPCM suspensions in various configurations (conical spiral, in-plane spiral, and helical spiral) of coiled tubes of square cross section was investigated. The results are compared with those for water as the base fluid flowing through a straight tube. A computational fluid dynamics (CFD) approach is used to simulate the laminar flow of water with MEPCM suspension in these geometries. The liquid suspension properties are expressed as functions of the volumetric concentration of MEPCM particles and the temperature. Improved heat transfer performance was obtained as the concentration of MEPCM suspension increased from 1 to 10%. However, the overall performance in terms of the pumping power consumed for unit heat transferred worsened.     

HEAT TRANSFER OF SOLID–LIQUID PHASE-CHANGE MATERIAL SUSPENSIONS IN CIRCULAR PIPES: EFFECTS OF WALL CONDUCTION

This article considers the problem of conjugate heat transfer in circular pipes with finite heated length to examine the effects of wall conduction on the heat transfer characteristics of solid–liquid phase-change material suspension flow. A mixture continuum approach is adopted in the formulation of the energy equation, with an approximate enthalpy model describing the phase-change process in the phase-change material particles. From numerical simulations via the finite-volume approach, it was found that the conduction heat transfer propagating along the pipe wall results in significant preheating of the suspension flow in the nondirectly heated region upstream of the heated section, where melting of the particles may occur and therefore the contribution of the latent heat transfer to convection heat dissipation over the heated section is markedly attenuated. Contributions of the sensible and latent heat transfer to the total heat transfer rate of the suspension flow over the heated section were delineated quantitatively for various sets of the relevant dimensionless parameters, including the particle volumetric concentration, the modified Stefan number, the Peclet number of suspending fluid, the wall thickness ratio, and the wall-to-fluid thermal conductivity ratio.     

Phase change in the cathode side of a proton exchange membrane fuel cell

A three-dimensional steady state two-phase non-isothermal model which highly couples the water and thermal management has been developed to numerically investigate the spatial distribution of the interfacial mass transfer phase-change rate in the cathode side of a proton exchange membrane fuel cell (PEMFC). A non-equilibrium evaporation-condensation phase change rate was incorporated in the model which allowed supersaturation and undersaturation take place. The most significant effects of phase-change rate on liquid saturation and temperature distributions are highlighted. A parametric study was also carried out to investigate the effects of operating conditions; namely as the channel inlet humidity, cell operating temperature, and inlet mass flow rate on the phase-change rate. It was also found that liquid phase assumption for produced water in the cathode catalyst layer (CL) changed the local distribution of phase-change rate. The maximum evaporation rate zone (above the channel near the CL) coincided with the maximum temperature zone and resulted in lowering the liquid saturation level. Furthermore, reduction of the channel inlet humidity and an increase of the operation temperature and inlet mass flow rate increased the evaporation rate and allowed for dehydration process of the gas diffusion layer (GDL) to take place faster.     

圆管内对流换热过程中潜热型功能流体流动阻力特性的实验研究

实验研究了由正十四烷和尿素甲醛树脂制成的相变微胶囊和水混合制成的潜热型功能流体在流过恒热流圆管进行对流换热时的流动阻力特性,获得了压降随流速的变化关系、摩擦阻力系数和表观黏度随R e的变化关系。并在同样条件下用单相水进行了对比实验。相变微胶囊的加入导致流体流动阻力较单相流体有显著增大。管路中扰动件导致单相流体的流动阻力特性在低R e条件下呈湍流特征;功能流体则呈不同规律,扰动仅导致流动阻力进一步增大,而流动阻力特性仍呈层流特征。     

Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert

Experiments to evaluate heat transfer coefficient and friction factor for flow in a tube and with twisted tape inserts in the transition range of flow with Al₂O₃ nanofluid are conducted. The results showed considerable enhancement of convective heat transfer with Al₂O₃ nanofluids compared to flow with water. It is observed that the equation of Gleninski applicable in transitional flow range for single-phase fluids showed considerable deviation when compared with values obtained with nanofluid. The heat transfer coefficient of nanofluid flowing in a tube with 0.1% volume concentration is 23.7% higher when compared with water at number of 9000. Heat transfer coefficient and pressure drop with nanofluid has been experimentally determined with tapes of different twist ratios and found to deviate with values obtained from equations developed for single-phase flow. A regression equation is developed to estimate the Nusselt number valid for both water and nanofluid flowing in the transition flow Reynolds number range in circular plain tube and with tape inserts. The maximum friction factor with twisted tape at 0.1% nanofluid volume concentration is 1.21 times that of water flowing in a plain tube.     

Fully developed mixed convection flow of a nanofluid through an inclined channel filled with a porous medium

In this paper, the steady fully developed mixed convection flow of a nanofluid in a channel filled with a porous medium is presented. The walls of the channel are heated by a uniform heat flux and a constant flow rate is considered through the channel. The equations of the problem are made non-dimensional and are observed to depend on the dimensionless parameters, namely the mixed convection parameter λ, the Péclet number Pe, the inclination angle of the channel to the horizontal γ and the nanoparticle volume fraction ?. The effects of these parameters on the fluid and heat transfer characteristics are in detail discussed for three different nanofluids as Cu–water, Al₂O₃–water and TiO₂–water.     

Heat transfer enhancement using Al2O3–water nanofluid for an electronic liquid cooling system

We have experimentally investigated the behaviour and heat transfer enhancement of a particular nanofluid, Al₂O₃ nanoparticle–water mixture, flowing inside a closed system that is destined for cooling of microprocessors or other electronic components. Experimental data, obtained for turbulent flow regime, have clearly shown that the inclusion of nanoparticles into distilled water has produced a considerable enhancement of the cooling block convective heat transfer coefficient. For a particular nanofluid with 6.8% particle volume concentration, heat transfer coefficient has been found to increase as much as 40% compared to that of the base fluid. It has also been found that an increase of particle concentration has produced a clear decrease of the heated component temperature. Experimental data have clearly shown that nanofluid with 36 nm particle diameter provides higher heat transfer coefficients than the ones of nanofluid with 47 nm particle size.     

Cocurrent turbulent mixed convection heat and mass transfer in falling film of water inside a vertical heated tube

A detailed numerical study has been conducted in order to analyse the combined buoyancy effects of thermal and mass diffusion on the turbulent mixed convection tube flows. Numerical results for air-water system are presented under different conditions. A low Reynolds number k-ε turbulent model is used with combined heat and mass transfer analysis in a vertical heated tube. The local heat fluxes, Nusselt and Sherwood numbers are reported to obtain an understanding of the physical phenomena. Predicted results show that a better heat transfer results for a higher gas flow Reynolds number Re, a higher heat flux q_w or a lower inlet water flow Γ₀. Additionally, the results indicate that the convection of heat by the flowing water film becomes the main mechanism for heat removal from the wall.     

Forced flow evaporative cooling of a volumetrically heated porous layer

In this paper, the temperature rise and pressure drop experienced by an evaporating coolant flowing through a volumetrically heated porous layer have been studied experimentally. Experimental data for the temperature distribution and the two-phase pressure drop along the direction of flow is obtained for water flowing through layers of inductively heated steel particles. Spherical steel particles varying in size from 590 to 4763 μm are used to form porous layers in 5 and 10 cm dia. glass jars. In these experiments the data are obtained for layer depths varying from 9 to 81 cm, volumetric heat generation rate varying from 1.44 to 44.0 W/cm³ and the mass flow rate of water varying from 510 to 18200 kg/m² h.A theoretical model for the temperature profile in the liquid region and the two phase region has been made and is found to compare well with the measurements. Vapor channels are observed to form in porous layers of particle diameter less than 1600 μm. Separate semi-theoretical models have been developed for the two phase pressure drop in particles with diameter less than and greater than 1600 μm.     

The use of phase-change coatings for estimating the heat transfer characteristics in a rotating disc system

Phase-change coatings have been applied to the axial-clearance rotor-stator cavity for the estimation of the transient heat transfer characteristics of the surface of the rotating disc. The tests were conducted for an air mass flow coefficient C_w = 1220, a gap ratio G = 0.1, an axial-clearance ratio G_ca = 0.05 and for rotational Reynolds numbers of Re? = 1 × 10⁵ and 2 × 10⁵. The phase-change coating used had a melting point of 38°C. From a video recording of the transient movement of the melt-line on the rotor (coated with the phase-change material) blown with heated air, it was possible to compute the heat transfer coefficients. The data reduction was made using the ‘semi-infinite slab’ approximation to the governing one-dimensional transient heat conduction equation.     

The effect of axial conduction on heat transfer in a liquid microchannel flow

Analysis is presented for conjugate heat transfer in a parallel-plate microchannel. Axial conduction in the fluid and in the adjacent wall are included. The fluid is a constant property liquid with a fully-developed velocity distribution. The microchannel is heated by a uniform heat flux applied to the outside of the channel wall. The analytic solution is given in the form of integrals by the method of Green’s functions. Quadrature is used to obtain numerical results for the local and average Nusselt number for various flow velocities, heating lengths, wall thicknesses, and wall conductivities. These results have application in the optimal design of small-scale heat transfer devices in areas such as biomedical devices, electronic cooling, and advanced fuel cells.     

A Recuperative Solar Collector

This investigation is a theoretical and experimental study of a recuperative type solar water heater where water is allowed to pass through the two upper transparent covers in channel flow before flowing through the tubes of the collector. The water in the channel will be first heated by solar insolation absorbed in the fluid layer and by the upper heat losses from the absorber plate before being heated in the main collector tubes. The performance of this collector was analytically studied under various environmental and operating conditions, in addition, an experimental apparatus was built, commissioned and tested. The analytical model, confirmed by experimental results, indicated that the proposed design was superior to the conventional solar water heater within a certain range of mass flow rates and environmental conditions.     

Heat-transfer, pressure-drop and performance relationships for in-line, staggered, and continuous plate heat exchangers

Basic heat-transfer and pressure-drop results for laminar airflow through arrays of in-line or staggered plate segments have been determined from numerical solutions of the fluid flow and energy equations. The results depend on only a single dimensionless parameter which encompasses the relevant geometrical and fluid flow quantities. Application of the results was made to compare the performance of the two types of segmented-plate arrays with each other and with the parallel-plate channel. At constant mass flow rate and constant heat-transfer surface area, the heat-transfer effectiveness . of the segmented arrays is appreciably higher than that of the parallel-plate channel, both in the range of small and intermediate effectiveness values. In addition, for a fixed heat duty corresponding to an intermediate value and for a constant mass flow, the heat-transfer area of the segmented arrays is only about a third of that for the parallel-plate channel. Under constant pumping power and constant surface area conditions, the heat transfer for the segmented arrays exceeds that for the parallel-plate channel when the effectiveness of the latter is less than 0.65–0.75. Under most conditions, the staggered array yields better performance than the in-line array, but situations are identified where the reverse is true.     

Numerical investigation of nonuniform transverse magnetic field effects on the flow and heat transfer of magnetic nanofluid in a sintered porous channel

In this paper, fluid flow and convective heat transfer of a ferrofluid (water and 4 vol% Fe₃O₄) in sintered Aluminum porous channel, which is subjected to a nonuniform transverse magnetic field have been studied. The numerical simulations supposed an ordinary cubic and staggered arrangement organized by uniformly sized particles with a small contact area for the porous media and constant heat flux at the surface of the microchannel. A wire, in which the electric current passes creates a nonuniform magnetic field, which is perpendicular to the flow direction. To do this simulation, the control volume technique and the two‐phase mixture model have been employed. The results show that the obtained local heat transfer coefficient on the channel surface increased with increasing mass flow rate and decreased slightly along the axial direction. Moreover, exerting the above‐mentioned magnetic field increases the Nusselt number that enhances the heat transfer rate while it has no effect on the pressure drop along the channel.     

Numerical investigation on the thermohydraulic performance of a shell-and-double concentric tube heat exchanger using nanofluid under the turbulent flow regime

The heat transfer characteristics of water-based Al₂O₃ nanofluid flowing through the annulus-side of a shell-and-double concentric tube heat exchanger (SDCTHEX) are investigated numerically. The temperature-dependent thermophysical properties of the nanofluid and pure water were used. The heat exchanger is analyzed considering conjugate heat transfer from hot oil flowing in the shell and the inner tube to the nanofluid flowing in the annulus formed between the concentric tubes. The overall performance is assessed based on the thermohydraulic performance. The overall thermohydraulic performance of the SDCTHEX, expressed in terms of the ratio of the overall heat transfer rate to the overall pressure drop with the nanofluid flowing in the annulus, is lower than that obtained with water when compared at constant hot fluid mass flow rates and at different inner tube diameters.     

Interfacial heat transfer during microdroplet evaporation on a laser heated surface

A comprehensive experimental and numerical investigation on water microdroplet impingement and evaporation is presented from the standpoint of phase-change cooling technologies. The study investigates microdroplet impact and evaporation on a laser heated surface, outlining the experimental and numerical conditions necessary to quantify the interfacial thermal conductance (G) of liquid-metal interfaces during two-phase flow. To do this, continuum-level numerical simulations are conducted in parallel with experimental measurements facilitating high-speed photography and in-situ time-domain thermoreflectance (TDTR). During microdroplet evaporation on laser heated Al thin-films at room temperature, an effective interfacial thermal conductance of G_eff = 6.4 ± 0.4 MW/m² is measured with TDTR. This effective interfacial thermal conductance (G_eff) is interpreted as the high-frequency (ac) interfacial heat transfer coefficient measured at the microdroplet/Al interface. Also on a laser heated surface, fractal-like condensation patterns form on the Al surface surrounding the evaporating microdroplet. This is due to the temperature gradient in the Al surface layer and cyclic vapor/air convection patterns outside the contact line. Laser heating, however, does not significantly increase the evaporation rate beyond that expected for microdroplet evaporation on isothermal Al thin-film surfaces.     

Bounds for the optimal conditions of forced convective flows inside multiple channels whose plates are heated by a uniform flux

Laminar forced flow moving inside a bundle of parallel-plate channels placed in a specified volume is normally sustained by a fixed pressure difference. In a typical channel, the velocity and the temperature of the coolant develop together from free-stream conditions at the inlet into fully developed regimes at the outlet. Heating of the parallel plates can occur by applying a uniform temperature or a uniform heat flux bilaterally or unilaterally. The goal of this paper is to extend an order-of-magnitude procedure for the optimization of a bundle of parallel-plate channels heated isothermally to a more difficult situation involving isoflux heating.