C.C.M. Rindt

Influence of the Apex Angle of Cone-Shaped Tubes on Particulate Fouling of Heat Exchangers

A two-dimensional (2D) cone shape has been added to the normal circular tubes of heat exchangers to minimize the area of stagnation and to streamline the air flow around the heat exchanger tubes. An experimental setup has been developed to study the influence of the apex angle of the cone-shaped tubes on particulate fouling of heat exchangers. Fouling experiments have been performed in which calcium carbonate particles are injected during the experiments and the deposition of particles on the tubes of the heat exchanger is monitored. Four sets of experiments have been performed, in which normal cylindrical tubes and coned tubes with an apex angle of 60°, 90°, and 120° are examined. It was found that particulate fouling ceased if the apex angle of the cone-shaped tubes is smaller than 90°. The attached cones enhance the flow around the tubes of the heat exchanger, by minimizing the stagnation area and keeping the flow attached to the tubes starting from the tip of the attached cone until separation, such that particles that deposit on the top of the tubes of the heat exchanger can be removed by the air flow.

The influence of the wall temperature on the development of heat transfer and secondary flow in a coiled heat exchanger

Abstract In the present study the development of mixed convective flow is studied in a helically coiled heat exchanger with an axially varying wall temperature for Re = 500, Pr = 5 and δ = 1 14 and compared to the constant wall temperature boundary condition. In the method used the parabolized equations are solved using a finite difference discretization scheme. The influence of buoyancy forces is analyzed on heat transfer and secondary flow. For all Grashof numbers studied it appears that both heat transfer, quantified by the Nusselt number, and secondary flow, quantified by the relative kinetic energy, exhibit a wavy behaviour in axial direction. For higher Grashof numbers, however, this phenomenon diminishes for the case with an axially varying wall temperature due to stabilizing stratification effects.

Phase Change Materials and Thermochemical Materials for Large-Scale Energy Storage

Replacing the use of fossil fuels by renewables is of paramount importance not so much because of declining reserves (fossil fuel reserves are estimated abundant for at least over a century) but because of increasing CO\(_2\) emissions which cause irreversible climate changes. To overcome the mismatch between supply and demand of solar heat and electricity, smart combinations of heat pumps and heat storage are currently investigated. A reliable method for heat storage is to use thermochemical (TCM) and phase change materials (PCM). These materials should be tested for energy density, temperature range, corrosion, toxicity, (dis)charge time and longevity. A prototype TCM reactor is built and tested for hot water generation. Using zeolite 13X as TCM, it is shown that tap water temperatures of 45 \(^\circ \mathrm{C}\) can be obtained. Using optical microscopy, the hydration and dehydration process of TCM material can be observed, as well as the phase transitions of PCMs. It is also argued that computational molecular modelling methods provide a powerful tool for both TCM and PCM material synthesis.

Numerical analysis of an optical method to determine temperature profiles in hot glass melts

In the present study a technique is developed to determine temperature profiles in hot glass melts, using intensity measurements performed at various wavelengths in the infrared spectrum. To that end an analytical model is developed which describes the internal energy transfer in a glass layer and the spectral intensity emerging from the glass layer. The spectral intensity so calculated is confronted with a measured spectral intensity to reconstruct the temperature profile in the glass layer. Because the temperature reconstruction from the measured spectral intensity is an ill posed inverse problem, Tikhonov regularization and the L-curve method are used to determine a meaningful temperature distribution in the glass layer. In order to gain insight into the accuracy of the temperature measurement method a sensitivity analysis is made using numerical simulations. The influence of random noise on the measurement signal and systematic errors in the properties is investigated. Furthermore, the temperature reconstruction of linear, logarithmic and parabolic temperature distributions is analysed.

Direct and Large-Eddy Simulation of Transient Buoyant Plumes: A Comparison with an Experiment Prandtl Numbers

In the present study transitional thermal plumes are examined by means of numerical simulations and experiments. The objective of the research is the determination whether a Large-Eddy Simulation can be applied in these cases and what subgrid scale model should be used. The Prandtl number is 5.82. Results are obtained in terms of the topology of a two dimensional slice of the temperature field and the local temperature and velocity as function of time. Experimentally the topology of the temperature field was obtained using liquid crystals suspended in water; the local temperature was measured with a thermocouple. Numerically, two-and three-dimensional calculations were performed, using the subgrid turbulent kinetic energy model, as used by Nieuwstadt [10], and the Smagorinsky [12] model. In the two dimensional case also direct numerical simulations were performed.

First-Principles Study of Chemical Mixtures of CaCl2 and MgCl2 Hydrates for Optimized Seasonal Heat Storage

Chloride-based salt hydrates form a promising class of thermochemical materials (TCMs), having high storage capacity and fast kinetics. In the charging cycles of these hydrates however hydrolysis might appear along with dehydration. The HCl produced during the hydrolysis degrades and corrodes the storage system. Our GGA-DFT results show that the enthalpy charge during proton formation (an important step in hydrolysis) is much higher for CaCl2·2H2O (33.75 kcal/mol) than for MgCl2·2H2O (19.55 kcal/mol). This is a strong indicator that hydrolysis can be minimized by appropriate chemical mixing of CaCl2 and Mg Cl2 hydrates, which is also confirmed by recent experimental studies. GGA-DFT calculations were performed to obtain and analyze the optimized structures, charge distributions, bonding indicators and harmonic frequencies of various chemical mixtures hydrates and compared them to their elementary salts hydrates. We have further assessed the equilibrium products concentration of dehydration/hydrolysis of the chemical mixtures under a wide range of operating conditions. We observed that chemical mixing leads to an increase of the onset hydrolysis temperature with a maximum value of 79 K, thus increasing the resistance against hydrolysis with respect to the elementary salt hydrates. We also found that the chemical mixing of CaCl2 and MgCl2 hydrates widens the operating dehydration temperature range by a maximum value of 182 K (CaMg2Cl6·2H2O) and lowers the binding enthalpy with respect to the physical mixture by ≈65 kcal/mol for TCM based heat storage systems.