C.W.M. van der Geld

Turbulence modification and heat transfer enhancement by inertial particles in turbulent channel flow

We present results of direct numerical simulation of turbulence modification and heat transfer in turbulent particle-laden channel flow and show an enhancement of the heat transfer and a small increase in the friction velocity when heavy inertial particles with high specific heat capacity are added to the flow. The simulations employ a coupled Eulerian-Lagrangian computational model in which the momentum and energy transfer between the discrete particles and the continuous fluid phase are fully taken into account. The effect of turbophoresis, resulting in an increased particle concentration near a solid wall due to the inhomogeneity of the wall-normal velocity fluctuations, is shown to be responsible for an increase in heat transfer. As a result of turbophoresis, the effective macroscopic transport properties in the region near the walls differ from those in the bulk of the flow. To support the turbophoresis interpretation of the enhanced heat transfer, results of simulations employing no particle-fluid coupling and simulations with two-way coupling at considerably lower specific heat, or considerably lower particle concentration are also included. The combination of these simulations allows distinguishing contributions to the Nusselt number due to mean flow, turbulent fluctuations and explicit particle effects. We observe an increase in Nusselt number by more than a factor of two for heavy inertial particles, which is the net result of a decrease in heat transfer by turbulent velocity fluctuations and a much larger increase in heat transfer stemming from the mean temperature difference between the fluid and the particles close to the walls.

Heat transfer, condensation and fog formation in crossflow plastic heat exchangers

In this paper heat transfer of air-water-vapour mixtures in plastic crossflow heat exchangers is studied theoretically and experimentally. First, a model for heat transfer without condensation is derived, resulting in a set of classical differential equations. Subsequently, heat transfer with wall condensation and fog formation are considered in some detail. Separate attention is paid to the heat transfer and condensation of pure steam in the heat exchanger. Finally, the experiments performed are reported and the results compared with the models presented. From this comparison it can be learnt that the models are well able to predict the rate of heat transfer and phenomena such as condensation and fog formation.

On the motion of a spherical bubble deforming near a plane wall

Equations of motion are derived for an expanding spherical bubble in potential flow near a plane wall using the Lagrange-Thomson method and an extended Rayleigh dissipation function to account for the drag. This method is shown to yield the same acceleration of the bubble center as that obtained using the Lagally theorem. An extended Rayleigh-Plesset equation is derived to describe deformation in the vicinity of a plane wall, and expressions relating the drag force to the distance from the wall and the bubble growth rate are derived. The solution method for the velocity potential can also be applied to the case of non-spherical deformation.Equations of motion are derived for an expanding spherical bubble in potential flow near a plane wall using the Lagrange-Thomson method and an extended Rayleigh dissipation function to account for the drag. This method is shown to yield the same acceleration of the bubble center as that obtained using the Lagally theorem. An extended Rayleigh-Plesset equation is derived to describe deformation in the vicinity of a plane wall, and expressions relating the drag force to the distance from the wall and the bubble growth rate are derived. The solution method for the velocity potential can also be applied to the case of non-spherical deformation.

Endovenous Simulated Laser Experiments at 940 nm and 1470 nm Suggest Wavelength-Independent Temperature Profiles

BACKGROUND EVLA has proven to be very successful, but the optimum methods for energy delivery have still not been clarified. A better understanding of the mechanism of action may contribute to achieving a consensus on the best laser method and the most effective use of laser parameters, resulting in optimal clinical outcomes, maintaining high success rates with minimal adverse events. The aim of this study is to assess the impact of wavelength, pullback speed and power level on the endovenous temperature profile in an experimental setting. METHODS In an experimental setting, temperature measurements were performed using thermocouples. The experimental set-up consisted of a transparent box in which a glass tube was fixed. Different laser parameters (wavelength and power) and 2 different pullback speeds (2 and 5 mm/s) were used. Thermocouples were placed at different distances from the fiber tip. Validity of the experimental setting was assessed by performing the same temperature measurements using a stripped varicose vein. The maximal temperature rise and the time span that the temperature was above collagen denaturation temperature were measured. RESULTS The experiments showed that decreasing the pullback speed (2 mm/s) and increasing the power (up to 14 W) both cause higher maximal temperature and a longer time above the temperature for collagen denaturation. The use of different laser wavelengths (940 or 1470 nm) did not influence the temperature profile. CONCLUSION The results of our experiments show that wavelength on its own has not been demonstrated to be an important parameter to influence the temperature profile.

Modeling the evaporation of sessile multi-component droplets

We extended a mathematical model for the drying of sessile droplets, based on the lubrication approximation, to binary mixture droplets. This extension is relevant for e.g. inkjet printing applications, where ink consisting of several components are used. The extension involves the generalization of an established vapor diffusion-limited evaporation model to multi-component mixtures. The different volatilities of the liquid components generate a composition gradient at the liquid-air interface. The model takes the composition-dependence of the mass density, viscosity, surface tension, mutual diffusion coefficient and thermodynamic activities into account. This leads to a variety of effects ranging from solutal Marangoni flow over deviations from the typical spherical cap shape to an entrapped residual amount of the more volatile component at later stages of the drying. These aspects are discussed in detail on the basis of the numerical results for water-glycerol and water-ethanol droplets. The results show good agreement with experimental findings. Finally, the accuracy of the lubrication approximation is assessed by comparison with a finite element method.

Combustion of PMMA, PE, and PS in a ramjet

Abstract The combustion behavior of polymethylmethacrylate (PMMA), polyethylene (PE), and polystyrene (PS) with air was investigated in a connected pipe test facility; spectroscopy showed the presence of OH, C2, and CH and temperatures between 1300 and 3000 K during combustion. Particular attention was focused on regression rate and combustion efficiency and the role of temperature and soot production. The present investigation gives an understanding of the most important phenomena that control (or emanate from) the combustion of a cylindrical solid fuel with a rearward facing step, and this has application for solid fuel ramjets, the safe burning of toxic waste, and hot gas generators. The results can be summarized as follows:

Measurement and prediction of solid sphere trajectories in accelerated gas flow

Abstract Trajectories are measured and compared with computed trajectories of solid particles with a diameter of 1–2 mm in downward gas flow near a solid cylinder with a diameter, d c , of 25 mm. The Reynolds number based on d c has been varied from 3000–13,000. The particle Reynolds numbers, based on the relative velocity | U G − U p |, ranged from 0 to 2000. Of all forces other than gravity, drag is dominant, although the pressure gradient and added mass forces for Re c > 10,000 have the same order of magnitude. The Basset force can be neglected. The correlation [3], originally derived by Sridhar and Katz (1995) for the lift force coefficient of bubbles, has been found to be appropriate for freely moving solid particles with shear number less than 0.04.

Extension of local front reconstruction method with controlled coalescence model

The physics of droplet collisions involves a wide range of length scales. This poses a challenge to accurately simulate such flows with standard fixed grid methods due to their inability to resolve all relevant scales with an affordable number of computational grid cells. A solution is to couple a fixed grid method with subgrid models that account for microscale effects. In this paper, we improved and extended the Local Front Reconstruction Method (LFRM) with a film drainage model of Zang and Law [Phys. Fluids 23, 042102 (2011)]. The new framework is first validated by (near) head-on collision of two equal tetradecane droplets using experimental film drainage times. When the experimental film drainage times are used, the LFRM method is better in predicting the droplet collisions, especially at high velocity in comparison with other fixed grid methods (i.e., the front tracking method and the coupled level set and volume of fluid method). When the film drainage model is invoked, the method shows a good qual...

Design improvements of a shell and tube heat exchanger based on practical experience and numerical analysis

It is shown that the performance, maintenance and lifespan of a vertical shell and tube evaporator critically depends on the geometry near the outlet. Several different geometries are examined both theoretically and experimentally, utilizing some heat exchangers of Shell Chemie B.V. Wear mechanisms are located, recognized and quantified with the aid of an eddy current measuring technique. In situ measurements during operation are performed in order to verify the 2-D numerical modelling.