Ccm Camilo Rindt

Characterization of MgSO4 Hydrate for Thermochemical Seasonal Heat Storage

Water vapor sorption in salt hydrates is one of the most promising means for compact, low loss, and long-term storage of solar heat in the built environment. One of the most interesting salt hydrates for compact seasonal heat storage is magnesium sulfate heptahydrate MgSO4 ·7H 2O. This paper describes the characterization of MgSO4 ·7H 2 Ot o examine its suitability for application in a seasonal heat storage system for the built environment. Both charging (dehydration) and discharging (hydration) behaviors of the material were studied using thermogravimetric differential scanning calorimetry, X-ray diffraction, particle distribution measurements, and scanning electron microscope. The experimental results show that MgSO4 ·7H 2O can be dehydrated at temperatures below 150° C, which can be reached by a medium temperature (vacuum tube) collector. Additionally, the material was able to store 2.2 GJ/ m 3 , almost nine times more energy than can be stored in water as sensible heat. On the other hand, the experimental results indicate that the release of the stored heat is more difficult. The amount of water taken up and the energy released by the material turned out to be strongly dependent on the water vapor pressure, temperature, and the total system pressure. The results of this study indicate that the application of MgSO4 ·7H 2O at atmospheric pressure is problematic for a heat storage system where heat is released above 40° C using a water vapor pressure of 1.3 kPa. However, first experiments performed in a closed system at low pressure indicate that a small amount of heat can be released at 50° C and a water vapor pressure of 1.3 kPa. If a heat storage system has to operate at atmospheric pressure, then the application of MgSO4 ·7H 2O for seasonal heat storage is possible for space heating operating at 25° C and a water vapor pressure of 2.1 kPa. DOI: 10.1115/1.4000275

A numerical analysis of steady flow in a three-dimensional model of the carotid artery bifurcation

A finite element approximation of steady flow in a rigid three-dimensional model of the carotid artery bifurcation is presented. A Reynolds number of 640 and a flow division ratio of about 50/50, simulating systolic flow, was used. To limit the CPU- and I/O-times needed for solving the systems of equations, a mesh-generator was developed, which gives full control over the number of elements into which the bifurcation is divided. A mini-supercomputer, based on parallel and vector processing techniques, was used to solve the system of equations. The numerical results of axial and secondary flow compare favorably with those obtained from previously performed laser-Doppler velocity measurements. Also, the influence of the Reynolds number, the flow division ratio, and the bifurcation angle on axial and secondary flow in the carotid sinus were studied in the three-dimensional model. The influence of the interventions is limited to a relatively small variation in the region with reversed axial flow, more or less pronounced C-shaped axial velocity contours, and increasing or decreasing axial velocity maxima.

On the wake structure behind a heated horizontal cylinder in cross-flow

This paper describes a numerical and experimental study of the effect of heat input on the behaviour of the vortices shed from a horizontal cylinder in a horizontal cross-flow. The Reynolds number ( Re D ) is fixed at 75, while the Richardson number ( Ri D ) is varied between 0 and 1 (corresponding to forced and mixed convection, respectively). In this parameter regime the wake consists of a double row of alternately shed vortices. A rather unexpected effect of the induced heat is the downward motion of the shed vortex structures. Detailed experiments and numerical simulations show that this effect is caused by the difference in strength between the two vortex rows. An analysis of the vorticity sources present during the formation process shows that the thermally induced baroclinic vorticity production is mainly responsible for this.

A NUMERICAL AND EXPERIMENTAL ANALYSIS OF THE FLOW FIELD IN A TWO-DIMENSIONAL MODEL OF THE HUMAN CAROTID ARTERY BIFURCATION

Axial velocities were measured in an enlarged, two-dimensional, rigid model of the carotid artery bifurcation by means of a laser-Doppler anemometer, under both steady and unsteady flow conditions. Also a numerical model was developed, based on the finite element approximation of the Navier-Stokes and continuity equations. From this study it appeared that the numerically predicted velocities agree well with the experimentally obtained values. Besides, the bifurcation hardly influenced the upstream flow in the main branch (common carotid artery), high velocity gradients were observed at the divider walls of the daughter branches (internal and external carotid arteries) and large zones with reversed flow were present near the nondivider walls of these branches. For steady flow the maximal diameter of this zone at the entrance of the internal carotid artery (carotid sinus) was about 25% of the local diameter of this branch. For unsteady flow this zone was absent during the initial phase of flow acceleration and maximal at the end of flow deceleration with a maximal diameter of about 50% of the local diameter of the carotid sinus.

Direct and large-eddy simulation of the transition of two- and three-dimensional plane plumes in a confined enclosure

We investigate here, the free convection flow induced by a line heat source in a confined geometry. The buoyancy forcing of this flow can be characterized by a Rayleigh number, Ra, which is chosen in the range where an intermittent spatial transition from laminar to turbulent flow takes place. The objective of the study is to explore this flow with help of numerical simulations. We restrict ourselves to the case of an air flow with Ra=1010. For the numerical simulation techniques, we employ Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES). With help of DNS we consider first, a 2D representation of this flow at a resolution of 1952 which is found to be sufficient to represent the heat source and its resulting flow. Next, we consider the 3D case at a resolution of 1953. The 3D simulation reveals a symmetrical time mean recirculation which covers the domain above the heat source. This large scale circulation is driven by the small scale laminar plume generated by the heat source and which breaks down into turbulence. The flow is found to be essentially 3D, especially near the top wall. No clear turbulent inertial range is present. A LES for the same flow has been carried out at a resolution of 453. The comparison of the les results with the DNS data has been used to investigate the performance of several sub-grid models. It turns out that simple equilibrium sub-grid models perform fairly well in estimating the statistics of the flow.

Unsteady flow in a rigid 3-D model of the carotid artery bifurcation

In the present study, finite element calculations are performed of blood flow in the carotid artery bifurcation under physiological flow conditions. The numerical results are compared in detail with laser-Doppler velocity measurements carried out in a perspex model. It may be concluded that the numerical model as presented here is well capable in predicting axial and secondary flow of incompressible Newtonian fluids in rigid-walled three-dimensional geometries. With regard to the flow phenomena occurring, a large region with reversed axial flow is found in the carotid sinus opposite to the flow divider. This region starts to grow at peak systole, has its maximal shape at minimal flow rate and totally disappears at the start of the acceleration phase. C-shaped axial velocity contours are formed in the deceleration phase, which are highly influenced by secondary flows. These latter flows are mainly induced by centrifugal forces, flow branching, and tapering of the carotid sinus. Lowering the sinus angle, the angle between the main branch and the carotid sinus, results in a smaller region with reversed axial flow.

Analysis of Fouling in Refuse Waste Incinerators

Gas-side fouling of waste-heat-recovery boilers, caused mainly by the deposition of particulate matter, reduces the heat transfer in the boiler. The fouling as observed on the tube bundles in the boiler of a Dutch refuse waste incinerator varied from thin and powdery for the economizer to thick and sintered for the superheater. Analysis of process data showed that both types of layers resulted in a 27% decrease of the heat transfer coefficient of the bundles. To determine the important mechanisms in the deposition of particles, layers taken from the different bundles are analyzed using electron microscopy. The analysis revealed the existence of a melt in the thick deposit. The melt, giving rise to a liquid phase, increases the sticking efficiency of the deposit and leads to larger deposition rates. For the economizer and the superheater the actual deposition rate is calculated from the change in heat transfer. On the basis of a comparison between the calculated deposition rates and deposition rates to be expected in the case of a pure diffusion and thermophoresis process, it is shown that for both types of deposits inertia-controlled transport is the dominant transport mechanism of particles.

Developing mixed convection in a coiled heat exchanger

Abstract In this paper the development of mixed convection in a helically coiled heat exchanger for Re = 500, Pr = 5 and δ = 1 14 is studied. The influence of buoyancy forces (Gr = ¢O (10 5 )) on heat transfer and secondary flow is analyzed. In the method used the parabolized equations are solved using a finite difference discretization. The code is tested on mixed convection flow in a 90° curved tube of which the results are compared to the results obtained with an elliptical code. For the helically coiled tube a constant wall temperature is considered. It appeared that heat transfer is highly influenced by secondary flow induced by centrifugal and buoyancy forces. For low Grashof numbers a splitting phenomenon of the temperature field is observed due to large secondary velocities, resulting in two separated areas of fluid. For high Grashof numbers the fluid in the coiled pipe becomes almost linearly startified which results in small secondary velocities. A wavy behaviour in the Nusselt number is observed for medium Grashof numbers.

Parameterization of a reactive force field using a Monte Carlo algorithm

Parameterization of a molecular dynamics force field is essential in realistically modeling the physicochemical processes involved in a molecular system. This step is often challenging when the equations involved in describing the force field are complicated as well as when the parameters are mostly empirical. ReaxFF is one such reactive force field which uses hundreds of parameters to describe the interactions between atoms. The optimization of the parameters in ReaxFF is done such that the properties predicted by ReaxFF matches with a set of quantum chemical or experimental data. Usually, the optimization of the parameters is done by an inefficient single‐parameter parabolic‐search algorithm. In this study, we use a robust metropolis Monte‐Carlo algorithm with simulated annealing to search for the optimum parameters for the ReaxFF force field in a high‐dimensional parameter space. The optimization is done against a set of quantum chemical data for MgSO4 hydrates. The optimized force field reproduced the chemical structures, the equations of state, and the water binding curves of MgSO4 hydrates. The transferability test of the ReaxFF force field shows the extend of transferability for a particular molecular system. This study points out that the ReaxFF force field is not indefinitely transferable.

Flow transition behind a heated cylinder

Abstract For more than a century the wake flow behind a cylinder has been the subject of many investigations. Most attention has been focussed on the first instability and the transition towards 3D for the flow around an unheated cylinder. The wake stability for a heated cylinder has until now received very little attention compared to the forced convection case. After a review of the literature on the wake flow behind an unheated and heated cylinder, in this paper the 2D wake behaviour and the 3D flow transition behind a heated cylinder are described. In the analysis performed, the Reynolds number is set around Re D =100 and the Richardson number is varied between Ri D =0 (forced convection case) and Ri D =1.5 (mixed convection case). From the results it is seen that for a relatively small heat input ( Ri D a negative deflection , i.e. downwards, caused by a strength difference between the upper and lower vortices which, in turn, is induced by baroclinic vorticity production. This strength difference also results in a rotational drift of the lower vortex around the upper vortex. For a higher heat input ( Ri D >1) an early 3D transition is observed. Mushroom-type structures appear on top of the upper vortex row. This flow transition is initiated by the occurrence of 3D flow structures at the rear end of the cylinder.