Computational fluid dynamics (CFD) is commonly used to visualize and understand complicated fluid flow and heat transfer in many industries. It is imperative to validate the CFD computer models in order to avoid costly design choices where experimentation cannot be used to ratify the predictions of computer models. Assessments of CFD computer models in the literature conclude that significant errors occur in computer model predictions of fluid flow influenced by buoyancy forces. The Experimental Fluid Dynamics Laboratory at Utah State University constructed a wind tunnel with which to perform experiments on buoyancy induced fluid flow. The experiments measured the heat transfer and fluid velocity occurring in the buoyant flows to be used to validate computer models. Additional experimental measurements at the inlet and around the walls from each experiment allowed the computer models to simulate the fluid flow with realistic boundary conditions.For this study, four experiments were performed, including two cases where the buoy- ancy influence was significant, and two where it was not. For each set of two cases, one experiment was performed where the heat transfer occurred from a wall of the wind tunnel held at constant temperature and in the other experiment the wall temperature fluctuated axially. This study used the experimental data to validate computer models available in the general purpose CFD software STAR-CCM+, including the k − ε models: realizable two- layer, standard two-layer, standard low-Re, v2 − f, the k − ω models from Wilcox and Menter, and the Reynolds stress transport and Spalart–Allmaras models. The k − ε stan- dard low-Re model was found most capable overall of predicting the fluid flow and heat transfer that occurred in the flows where the buoyancy influence was significant. For the experimental cases where the buoyancy influence was less significant, the validation results were inconsistent.

No description available.

No description available.

Due to the recent advancement in computer capability, numerical modelling starts to play an important role in making predictions and improving the understanding of physics in the studies of convective heat transfer to supercritical fluids. Many computational studies have been carried out in recent years to assess the ability of different turbulence models in reproducing the experimental data. The performance of these turbulence models varied significantly in predicting the heat transfer at supercritical pressures, especially for the phenomena of heat transfer deterioration (HTD). The results of these studies showed that the accuracy of different turbulence models was also dependent on the flow conditions. It is still necessary to test these turbulence models against newly available experimental data before the final conclusion can be drawn. In this work computational simulations on convective heat transfer of carbon dioxide (CO2) and water (H2O) at supercritical pressures flowing upward in vertical circular pipes have been carried out using the commercial code STAR-CCM+. Detailed comparisons are made between five turbulence models, including AKN low-Reynolds model by Abe et al. (AKN), Standard low-Reynolds k-ε model by Lien et al. (SLR), k-ω model by Wilcox (WI), SST k-ω model by Menter (SST), and the Reynolds Stress Transport (RST) model, against two independent experiments, i.e., water data by Watts (1980) and the recently published carbon dioxide data by Zahlan (2013). The performance of k-ε models with a two-layer approach, and that of k-ε models with wall-functions are also investigated. For the CO2 study, where wall temperatures in most cases are above the pseudo-critical temperature (Tpc), RST model is found both qualitatively and quantitatively better than other turbulence models in predicting the wall temperatures when HTD occurs. The RST model while superior, predicted HTD at higher heat fluxes as compared to experiments. The wall temperature trends predicted by SST and WI models are very similar to that predicted by RST, except that they start to predict HTD at even higher heat fluxes than RST, and the peak temperatures are overestimated significantly. Because RST and k-ω models (SST and WI) predict the HTD at higher heat fluxes as compared to experiments, often in literature they are overlooked. Rather CFD users should conduct sensitivity analyses on heat flux, and quite often as a result qualitatively excellent agreement can be observed in some of these models. The low-Reynolds turbulence models, i.e., SLR and AKN, tended to over-predict the wall temperature after the onset of first temperature peak, because the turbulence production predicted by these models failed to regenerate. The wall temperatures for these models did not show recovery after deterioration until the bulk temperature is close to Tpc, while experimentally recovery happened well upstream of this location. The k-ε models with two-layer approach, and the k-ε models with wall-functions both failed to predict the HTD in all cases. For the H2O study, where the wall temperatures in most cases are below the pseudo-critical temperature, the SLR model performed the best among all turbulence models in reproducing the experimental data. AKN model was also able to qualitatively predict the observed HTD, however, not as well as SLR. SST and RST models, on the other hand, under-predicted the buoyancy effect even at the lowest mass fluxes and hence did not adequately predict deterioration. In a few high-heat-flux cases with wall temperatures above Tpc, all the turbulence models show consistent response to that discussed in the CO2 study, i.e. RST model is quantitatively better than other turbulence models. Nevertheless, the wall temperature peaks predicted by RST model is very different from that observed experimentally, i.e. the measured peaks are much milder and more flattened than the predicted ones. All the turbulence models including RST overestimate the wall temperatures significantly when Tb<Tpc<Tw. The sensitivity studies of mesh parameters, user-defined fluid properties, turbulent Prandtl number, gravitational orientation, and various boundary conditions (e.g. heat flux, mass flux, pressure, and inlet temperature) have also been carried out, aiming to ensure the reliability of the obtained results, and to gain a deeper insight into the physics of heat transfer deterioration in supercritical fluids. Detailed mechanistic studies of HTD have been carried out for both the CO2 and H2O simulations using different turbulence models (RST, SST, and SLR) in various flow conditions. The radial distribution of fluid properties and turbulence at various axial locations provides direct evidence of the mechanisms involved near the locations of deterioration. The buoyancy effect is found to be responsible for the observed HTD in both experiments (i.e., when gravity forces are removed no deterioration is observed). The buoyancy force exerted on the near-wall low-density layer modifies the velocity profile (thus shear stress distribution) in a way that greatly reduces the near-wall turbulence production, resulting in the impairment of heat transfer. In the CO2 study where the wall temperature exceeds the Tpc in a very short distance from the inlet, the “entrance effect” is found to play a more important role in initially impairing the turbulence production. However, this effect is not observed in cases where wall temperature is below Tpc, which is attributed to the weaker density variation below Tpc. / Thesis / Master of Applied Science (MASc)

Operating conditions of the furnaces has the major effect on the quality of steel during steel production process. Furnaces also are the biggest energy consumer in the whole production process  which make them a center of concern, in order to get to the most optimized condition through both energy and quality aspects. Soaking pit furnaces are for heating steel ingots before rolling, in order to provide convenient conditions for ingots for further procedures. These batch furnaces are characterized by heat and temperature conditions that vary in time. The structure permits rapid heating of the metal inside the furnace, since heat is supplied over the entire surface of the ingot. One serious problem that these furnaces might contain is the existence of non-uniform temperature gradient inside the chamber that causes different temperature distribution on the ingots surface which leads to a bad surface quality of them, considering further rolling process. As the first step through obtaining the best temperature gradient inside the chamber, is to ensure the exact temperature condition in the current running procedure. In here as the first step through the problem solving of these furnaces, temperature profile, radiation profile and other effective parameters are investigated with the aid of CFD software. The simulation is done by ICEM and FLUENT programs for geometry and mesh designing, and modeling in respect. Modeling is based on four main steps:         I.            Modeling of the furnace chamber geometry and applying appropriate mesh style with ICEM       II.            Modeling the chamber with fluent, and taking the results (case 0)     III.            Modeling of six cases with different excess air, in order to investigate the best λ magnitude     IV.            Modeling of six cases with different burner capacities in order to investigate its affection on combustion parameters

Condensing ejectors utilize the beneficial thermodynamics of condensation to produce an exiting static pressure that can be in excess of either entering static pressure. The phase change process is driven by both turbulent mixing and interphase heat transfer. Semi-empirical models can be used in conjunction with computational fluid dynamics (CFD) to gain some understanding of how condensing ejectors should be designed and operated. The current work describes the construction of a multidimensional simulation capability built around an Eulerian pseudo-fluid approach. The transport equations for mass and momentum treat the two phases as a continuous mixture. The fluid is treated as being in a non-thermodynamic equilibrium state, and a modified form of the homogenous relaxation model (HRM) is employed. This model was originally intended for representing flash-boiling, but with suitable modification, the same ideas could be used for condensing flow. The computational fluid dynamics code is constructed using the open-source OpenFOAM library. Fluid properties are evaluated using the REFPROP database from NIST, which includes many common fluids and refrigerants. The working fluids used are water and carbon dioxide. For ejector flow, simulations using carbon dioxide are more stable than with water. Using carbon dioxide as the working fluid, the results of the validation simulations show a pressure rise that is comparable to experimental data. It is also observed that the flow is near thermodynamic equilibrium in the diffuser for these cases, suggesting that turbulence effects present the greatest challenge in modeling these ejectors.

Flow with separation and reattachment has been encountered in many thermo-fluidic devices. Although it causes energy loss due to pressure drops, it is sometimes intentionally used for heat transfer enhancement. To improve the performance of heat exchangers, understanding the details of such complicated flow and thermal structures is very important. Therefore, attention was paid in this study to a representative typical simple model that can generate separating and reattaching flow called backward-facing step (BFS) flow, the fundamental flow and thermal characteristics of a 3-D BFS flow have been investigated experimentally and a flow modification was also made by numerical simulation aimed to promote the heat transfer enhancement. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

This thesis examines the time resolved heat transfer to drops rolling or sliding along inclined, subcritical heated non-wetting surfaces. Results were experimentally obtained using IR imaging for a smooth hydrophobic surface and post as well as rib structured superhydrophobic surfaces of varying solid fraction (f_s = 0.06 - 0.5). Tests were performed at varying inclination angle (α = 10, 15, 20, and 25°), drop volume (12, 20, 30, and 40 μL), and surface temperature (T_w = 50, 65, and 80 °C). Rib structured superhydrophobic surfaces were explored for drops moving parallel and perpendicular to the rib structures. The findings indicate that transient heat transfer is predominantly influenced by the surface’s solid fraction and the velocity of the drops, with a secondary dependence on drop volume. Surfaces with low solid fraction show a significant reduction in initial heating rate (up to 80% reduction) to the drop, when compared with that of the smooth surface. The drop velocity depends on surface solid fraction and inclination angle, with drop volume exerting smaller influence. Rib structured surfaces impact heat transfer by enhancing heat transfer rate for drops that move along the rib direction compared with drops that move perpendicular to the ribs. The difference is likely due to increased drop velocity that exists for the parallel rib orientation.

No description available.

Thermosyphon transient operation was numerically modeled. The numerical model presented in this work overcame the limitations of previous studies by including transient conduction in the vessel wall, shear stress between the rising vapor and the falling film in the thermosyphon, the influence of the mass in the liquid pool in the evaporator, and by using a more refined and accurate numerical grid. Unique to this model was the accounting for temporal changes in the effective length of the vapor space due to the expanding and contracting of non-condensable gases in the vapor space. The model assumed quasi-steady one-dimensional vapor flow, transient one-dimensional flow in the falling liquid film, and transient behavior in the liquid pool in the evaporator. The model also assumed transient two-dimensional conduction in the thermosyphon wall. Using fundamental principles, the governing equations used in the numerical model were developed and then written in finite difference form. The finite difference forms of the governing equations were integrated using an explicit scheme. A sensitivity study was performed and found that the numerical model was accurate to 4%. An experiment was also conducted to validate the numerical model. The experiment used three distinct transient heat loads to simulate gradual, moderate and sharp increases in temperature. The uncertainty of the experiment was shown to be 2.3%. The temperatures from the numerical model were then compared to those measured during the physical experiment to determine the validity of the numerical model. The model was further exercised to develop a useful engineering relationship that can be used to predict the transient performance of a thermosyphon.