Studies on a two-staged micro-combustor for a micro-reformer integrated with a micro-evaporator

A new micro-combustor configuration for a micro fuel-cell reformer integrated with a micro-evaporator is studied experimentally and computationally. The micro-combustor as a heat source is designed for a 10–15 W micro-reformer using the steam reforming method. In order to satisfy the primary requirements for designing a micro-combustor integrated with a micro-evaporator, i.e., stable burning in a small confinement and maximum heat transfer through a wall, the present micro-combustor is a simply cylinder, which is easy to fabricate, but is two-staged (expanding downstream) to control ignition and stable burning. The aspect ratio and wall thickness of the micro-combustor substantially affect ignition and thermal characteristics. For optimized design conditions, a pre-mixed micro-flame is easily ignited in the expanded second-stage combustor, moves into the smaller first-stage combustor, and finally is stabilized therein. The measured and predicted temperature distributions across the micro-combustor walls indicate that heat generated in the micro-combustor is well transferred. Thus, the present micro-combustor configuration can be applied to practical micro-reformers integrated with a micro-evaporator for use with fuel cells.     

Exact analytical solution of a convective diffusion from a wedge to a flow with a first order chemical reaction at the surface

The problem of mass transfer in a fluid flow past a wedge with a chemical reaction of the first order at the surface is investigated. The approach suggested previously by Apelblat [1] for the solution of a mass transfer problem with a first order chemical reaction at the interface in boundary layer flow is generalized for the case of flow past a wedge. The solution of the problem is derived in a closed analytical form.     

Measurements on a PV solar pump equipped with a piston pump with a matching valve

The work on a simple high efficient solar pump equipped with a piston pump with a matching valve, reported at the Solar World Congress in Budapest, has been continued. Quasi-static and dynamic models of the solar pump have been derived with which the operation of the system is simulated. A test rig has been built at ECN in Petten (The Netherlands) to perform some preliminary measurements. These tests show that the piston with a matching valve indeed ensures a good matching of the system components over a wide range of solar insolations. Daily average solar panel maximum power point tracking efficiencies of over 80% were measured. Daily average subsystem efficiencies (solar panel power output to hydraulic power output) measured were about 40%, which is lower than expected. These low efficiencies were caused by substantial power losses in the motor, transmission and pump. The losses can easily be reduced by appropriate design of each component; subsystem efficiencies of 50% should be attainable.     

Thermo-Hydrodynamics of a Helical Coil Heat Exchanger Operated with a Phase-Change Ice Slurry as a Refrigerant

The thermal performance of helical-coil heat exchangers can be significantly enhanced when operated with ice slurry as a phase-change refrigerant. It is essential to also consider the hydrodynamics of ice slurry flow to determine the overall performance of the heat exchanger. This study presents a detailed numerical investigation of the thermo-hydrodynamic performance of a helical coil heat exchanger operated with a laminar and non-Newtonian flow of ethyl-alcohol ice slurry subject to phase change. The Bingham plastic model is used to reflect the non-Newtonian behavior of ice slurry. The phase change of ice slurry is modelled using the enthalpy-porosity method. The pressure drop and heat transfer of ice slurry in a double-turn helical coil are determined in terms of ice mass fraction and Dean number. The results show that an increase in the ice mass fraction and Dean number results in an increase of the heat transfer rate. This is, however, associated with an increase in pressure drop. The entropy generation analysis is introduced to evaluate the overall performance of the heat exchanger, taking into account the opposing effects of heat transfer and pressure drop. It is evident that, at certain ice mass fractions, there exists an optimal value of the Dean number that leads to minimum irreversibility and maximum overall performance.     

Load capacity of a thick-walled cylinder with a radial hole

The paper deals with elastic–plastic analysis of the stress–strain state in the vicinity of a hole in a thick-walled cylindrical pressure vessel. The investigations have been inspired by the phenomenon of ductile fracture observed in a high-pressure reactor. Using finite element calculations, different failure criteria are proposed to aid design and control of high-pressure vessels with piping attachments. They are compared with suggestions of American (ASME) and European (EN) standards. A simple shakedown analysis of the structure is also presented.     

Transient cooling of a room with a chilled ceiling

Using passive systems or solar energy for space cooling is highly conditional upon the heat transfer potential between the building components and the interior space. This paper presents a simplified theoretical model that describes the evolution of temperature in a room that is cooled by a chilled ceiling in the presence of a heat source on the floor. Using arguments of dynamic similarity, the theoretical model is confirmed with scale-model experiments run in a rectangular tank. The scale model uses a metallic heat exchanger to simulate the chilled ceiling and an electric heater as the heat source. The greatest heat transfer from the fluid towards the ceiling occurs immediately with the simultaneous activation of the heater and the chilled ceiling. As time elapses and the heat transfer decreases, the room cools more slowly and its temperature tends asymptotically towards equilibrium. This is achieved when the heat loss through the ceiling equals the heat supply from the heat source. The theoretical model can predict when a chilled ceiling, acting as transient a heat sink, will achieve a comfortable temperature in a room with an internal heat source.     

Thermal Green's Functions for a Heat Source in a Piezoelectric Solid with a Parabolic Boundary

Using the Stroh formalism combined with the analytical continuation principle of Muskhelishvili, the Green's functions for a line heat source in a piezoelectric solid with a parabolic boundary are obtained in closed form. The obtained Green's functions not only satisfy all the given boundary conditions, but also ensure the displacement and electric potential to be single-valued. As special cases, the solutions for a piezoelectric half-plane are also presented, and they are shown to be consistent with previous works.     

Solidification of a PEG 1500-epoxy nanocomposite around a horizontal pipe

A PCM-epoxy phase change material composite (polyethylene glycol 1500-epoxy) was developed as heat storage building element for houses with low energy consumption. The PCM component, polyethylene glycol 1500, was integrated in an epoxy matrix, and presents a phase change interval of 34-42 °C and an enthalpy of solidification of 103.411 kJ/kg. Experiments on solidification were conducted using a Plexiglas test cell filled with the PEG 1500-epoxy nanocomposite material (P15-E) for further implementation of this material in buildings. The forwarded solidification model assumes negligible convection in the liquid region and predicts the time for radial formation of two regions: a mushy-zone and a solid, annular one around the pipe during solidification. The heat transfer during solidification can be also characterized by the time evolution of both liquid and solid radial fronts. The model was analytically solved using Megerlin approximation concerning “solidification with mushy zone”, with the third order condition at the external frontier. The experimental values are in agreement with the calculated theoretical curves.     

Exploration of heat transfer effect in a magnetohydrodynamics flow of a micropolar fluid between a porous and a nonporous disk

An analysis is carried out for the flow characteristics of a conducting micropolar fluid. The fluid was passed in between two parallel disks of infinite radii. The novelty of the study is to consider one of the disks as porous and the other one as nonporous, and the external magnetic field is applied along the transverse direction of the flow. The flow phenomena for the polar fluid characterized by the magnetic effect in conjunction with the temperature equation reduce to a set of coupled nonlinear ordinary differential equations using the requisite transformations and nondimensionalization. An analytical approach such as the variation parameter method is employed to tackle the system efficiently. To emphasize the effect of various physical parameters contributing to the flow phenomena, that is, non-zero tangential slip, Reynolds number, Prandtl number, magnetic parameter, and material parameter on the flow profiles of axial and radial velocities, the microrotation and temperature profiles are presented graphically. To validate the simulated results, a comparison with established results is made, and it is concluded that both are in good correlation.     

Simulation of a shock tube with a small exit nozzle

Shock tubes are frequently used to rapidly heat up reaction mixtures to study chemical reaction mechanisms and kinetics in the field of combustion chemistry [1]. In the present work, the flow field inside a shock tube with a small nozzle in the end plate has been investigated to support the analysis of reacting chemical mixtures with an attached mass spectrometer and to clarify whether the usual assumptions for the flow field and the related thermodynamics are fulfilled. In the present work, the details of the flow physics inside the tube and the flow out of the nozzle in the end plate have been investigated. Due to the large differences in the typical length scales and the large pressure ratios of this special device, a very strong numerical stiffness prevails during the simulation process. Second-order ROE numerical schemes have been employed to simulate the flow field inside the shock tube. The simulations were performed with the commercial code ANSYS Fluent [2]. Axial-symmetric boundary conditions are employed to reduce the consumption of CPU time. A density-based transient scheme has been used and validated in terms of accuracy and efficiency. The simulation results for pressure and density are compared with analytical solutions. Numerical results show that a density-based numerical scheme performs better when dealing with shock-tube problems [5]. The flow field near the nozzle is studied in detail, and the effects of the nozzle to pressure and temperature variations inside the tube are investigated. The results show that this special shock-tube setup can be used to study high-temperature gas-phase chemical reactions with reasonable accuracy.     

Behavior of a sodium-sulfur cell with a dynamic sulfur electrode

A sodium-sulfur cell was constructed with sodium polysulfide circulating in a narrow annulus around a β-alumina tube. The absence of carbon mat in the electrode reduced the current collector surface area appreciably, localized the electrochemical reactions, and ultimately led to film formation on the electrode. The results are consistent with existing models and interpretations of sulfur electrode behavior. Film formation on the electrode is responsible for the rather poor electrical characteristics of the cell.     

Thermal Dissolution of a Spherical Particle with a Moving Boundary

A numerical problem for mass and energy transport considering thermodynamic equilibrium is solved around a spherical particle in the absence of hydrodynamic effects in a binary solution. The analysis includes the radial convective term generated due to the differences on density between the solid and liquid phases. Because the transport of mass and energy compete in the process, how the rate of dissolution is affected by compositional diffusivity or thermal diffusivity is distinguished. The partial differential equations are discretized with the finite difference method in space, and the resulting set of ordinary differential equations in time is solved by the method of lines. The numerical solution for the thermal dissolution of a spherical particle in a binary melt is compared with heat-balance integral method for small times of the process. The solutions are found to agree for conditions close to the dissolution regime.     

Free convection in a wavy cavity filled with a porous medium

The steady-state free convection inside a cavity made of two horizontal straight walls and two vertical bent-wavy walls and filled with a fluid-saturated porous medium is numerically investigated in the present paper. The wavy walls are assumed to follow a profile of cosine curve. The horizontal walls are kept adiabatic, while the bent-wavy walls are isothermal but kept at different temperatures. The Darcy and energy equations (in non-dimensional stream function and temperature formulation) are solved numerically using the Galerkin Finite Element Method (FEM). Flow and heat transfer characteristics (isothermal, streamlines and local and average Nusselt numbers) are investigated for some values of the Rayleigh number, cavity aspect ratio and surface waviness parameter. The present results are compared with those reported in the open literature for a square cavity with straight walls. It was found that these results are in excellent agreement.     

Local convective heat exchanges from a rotor facing a stator

The local convective heat transfer from a rotor with a 310 mm outer radius is studied experimentally at a distance of 3 mm from a coaxial crown-shaped stator with a 176 mm inner radius and a 284 mm outer radius. The experimental technique is based on the use of a thermally thick rotor heated from behind by infrared radiation. The local heat flux distribution from the rotor surface is identified by resolving the Laplace equation by finite difference method using the experimental temperature distribution as boundary conditions. The tests are carried out with the single rotor and the stator/rotor system for local rotational Reynolds numbers ranging from 2.0·10⁴ to 1.47·10⁶ and thus sweeping across the laminar, transition and turbulent flow regimes. The local and mean Nusselt numbers for the single disc are compared with those obtained experimentally for the stator/rotor system. The flow structure in the space between the rotor and the stator is analysed by Particule Image Velocimetry.     

Coupling a two-temperature model and a one-temperature model at a fluid-porous interface

We study a convective heat transfer problem in a fluid-porous domain in the case of the local thermal non-equilibrium assumption (LTNE). The issue of this study is to determine appropriate boundary conditions to model heat transfer, while using models with a different number of equations: a two-temperature model in the homogeneous porous region versus a one-temperature model in the free region. To proceed, a two-step up-scaling approach is used, which has the particularity to provide closed jump relations depending on intrinsic characteristic of the interface. Thus, the use of jump or continuity conditions depend only on the interface location inside the fluid-porous transition region. The pertinence of the approach is illustrated on a 2D convective heat transfer problem considering a solid heat source in the porous medium.     

Transient rotating flow over a moving surface with a magnetic field

The transient flow and heat transfer on a moving surface in a rotating fluid in the presence of a magnetic field have been investigated. The unsteadiness in the flow field has been introduced by the sudden change in the surface velocity or the fluid angular velocity. The parabolic partial differential equations governing the unsteady flow and heat transfer have been solved by using an implicit finite-difference scheme in combination with the quasilinearization technique. The computations have been carried out from the initial steady state to the final steady state. The effects of the sudden change in the surface velocity on the flow and heat transfer are found to be more significant than those of the impulsive change in the angular velocity of the fluid. When the surface velocity is suddenly reduced, the surface shear stress is found to vanish in a small time interval after the start of the impulsive motion, but it does not imply flow separation. The surface shear stress for the primary flow increases with the magnetic field and the fluid angular velocity, but the surface heat transfer decreases. The surface shear stress for the secondary flow increases with the angular velocity of the fluid, but decreases with increasing magnetic field.     

Performance of a Hybrid Desiccant Cooling System in a Residential Environment

A series of field tests of hybrid desiccant cooling systems was conducted during July and August 2012. The temperature and humidity of supply and return air and power and heat consumption were monitored, and data were transferred in real time through the internet. The performance of the cooling system (i.e., cooling capacity and the coefficient of performance [COP]) was evaluated from measured data and variations with regard to outdoor conditions were analyzed. The cooling capacity decreased with an increase of outdoor temperature, while it increased as outdoor humidity increased. The COP also increased with an increase in outdoor humidity. However, the dependence of the COP on outdoor temperature was rather weak.     

Effects of a transverse,horizontal magnetic field on natural convection of a paramagnetic fluid in a cube

The present work is concerned with magnetic convection of a paramagnetic fluid in a cubic enclosure heated and cooled from the sidewalls. The influence of a 10-T transverse magnetic field on the convection mode of a paramagnetic fluid and the heat transfer rate was investigated numerically and experimentally, and compared with gravitational natural convection. The present study clearly shows that natural convection can be enhanced and the direction of the convective flow can be changed when using a strong magnetic field in terrestrial conditions.     

Unsteady natural convection from a horizontal annulus filled with a porous medium

The unsteady natural convection flow from a horizontal cylindrical annulus filled with a non-Darcy porous medium has been studied. The unsteadiness in the problem arises due to the impulsive change in the wall temperature of the outer cylinder. The Navier–Stokes equations along with the energy equation governing the unsteady natural convection flow have been solved by the finite-volume method. The effect of time variation on the heat transfer is more pronounced only in a small time interval immediately after the start of the impulsive motion and the steady state is reached after certain time. The results show that the annulus completely filled with a porous medium has the best insulating effectiveness. Convection in the horizontal annulus is confined mostly at top and bottom regions. Hence, only these regions should be insulated. In case of annulus partially filled with a porous material, insulating the region near the outer cylinder is more effective than insulating the region near the inner cylinder. The effect of Darcy number on the heat transfer is more pronounced than that of the Grashof number.     

Bioconvection in a squeezing flow of a micropolar fluid in a horizontal channel

The bioconvection flow of an incompressible micropolar fluid containing microorganisms between two infinite stretchable parallel plates is considered. A mathematical model, with a fully coupled nonlinear system of equations describing the total mass, momentum, thermal energy, mass diffusion, and microorganisms is presented. The governing equations are reduced to a set of nonlinear ordinary differential equations with the help of suitable transformations. The resulting nonlinear ordinary differential equations are linearized using successive linearization method, and the resulting system of linear equations is solved using the Chebyshev collocation method. The detailed analysis illustrating the influences of various physical parameters, such as the micropolar coupling number, squeezing parameter, the bioconvection Schmidt number, Prandtl numbers, Lewis number, and bioconvection Peclet number on the velocity, microrotation, temperature, concentration and motile microorganism distributions, skin friction coefficient, Nusselt number, Sherwood number, and density number of motile microorganism, is examined. The influence of the squeezing parameter is to increase the dimensionless velocities and temperature and to decrease the local Nusselt number and local Sherwood number. The density number of motile microorganism is decreasing with squeezing parameter, bioconvection Lewis number, bioconvection Peclet number, and bioconvection Schmidt number.