Kemal Tuzla

Parametric effects of particle size and gas velocity on cluster characteristics in fast fluidized beds

An important characteristic of fast fluidized beds (FFB) is the tendency for the solid particles to aggregate into clusters. Such clusters can strongly affect operational characteristics such as particle holdup, pressure drop, wall heat transfer, and axial mixing. This paper presents results of an experimental study of cluster characteristics, as determined by capacitance-probe measurements of instantaneous local solid concentrations in a 15-cm diameter FFB. Results are presented that indicate the parametric effects of particle size (dp) and superficial gas velocity (Vg) on the duration time, occurrence frequency, time-fraction of existence, and solid concentration in clusters.

Solid mass fluxes in circulating fluidized beds

Abstract Experimental measurements of local solid mass flux were made in the riser component of two different circulating fluidized beds (CFBs). Tests were conducted in both a lab-scale riser measuring 5 cm i.d. and 2.7 m in height and a pilot-scale riser of 15 cm i.d. and 10.8 m height. FCC catalyst particles with a Sauter mean diameter of 68 μm and sand particles with mean diameters of 125 μm and 276 μm were used in the tests. The results showed the local time-average solid mass flux to vary with radial position, elevation, total solid mass flow, superficial gas velocity and mean particle diameter. Based on the solid mass flux and local solid concentration measurements, and on video studies, a physical description of the solid flow phenomena in CFBs is presented.

Falling film evaporation of single component liquids

Abstract Heat transfer coefficients for evaporation of single-component liquids in falling films were experimentally measured over an extended range of parameters. By using two different fluids (propylene glycol and water), over a range of absolute pressures, it was possible to extend the existing data base by an order of magnitude in Prandtl numbers. These new data indicate that existing models and correlations were inadequate for fluids with Prandtl numbers greater than five. New models were developed for both the wavy laminar and the turbulent regimes. An exponential interpolation paradigm between the regimes enabled prediction of the evaporative heat transfer coefficient over the entire range of Reynolds and Prandtl numbers with an average deviation of less than 10% from the experimental data.

Transient dynamics of solid concentration in downer fluidized bed

Abstract This paper presents new results from experiments performed in a 15 cm diameter down-flow fast-fluidized bed. Tests were conducted at room temperature and near atmospheric pressure, with 125 μm glass beads. Superficial gas velocities range from 0 to 6 m/s. A needle capacitance probe was used to obtain measurement of instantaneous solids concentration ( e s ) in a small sampling volume of approximately 0.1 cm 3 . With the downer operating at steady-state, the capacitance probe provided a time-trace of the local solid fraction, indicating that e s was generally less than 0.01. However the measurements indicated that the local concentration did have excursions to significantly higher concentrations for short durations. This behavior is indicative of clusters in the down-flow fast-fluidized bed. While clusters are known to be a dominant feature of upward fast fluidization in risers, their presence has been questioned for solid/gas down flows. Sample data are presented to indicate the density, duration, and time fraction of clusters observed in this downer fluidized bed. Preliminary comparisons are made to the limited data available for comparable operating conditions in a riser.

Freeboard heat transfer in high-temperature fluidized beds

Abstract Average heat-transfer coefficients were measured for horizontal tubes located in freeboard region of air-fluidized beds. The tests were carried out at 300, 500, and 750 °C bed temperatures and atmospheric pressure. Silica sand and limestone with 465 to 1400 μm mean diameters were used as fluidizing particles. Based on the experimental results, a general heat-transfer correlation for horizontal tubes in freeboard were developed which represented the data with an average deviation of 29%.

Convective film boiling in a rod bundle: axial variation of nonequilibrium evaporation rates

Abstract An experimental study of convective film boiling (post-CHF) is carried out for two-phase steamwater flow in a nine-rod bundle test section. Measurements of wall heat flux, wall superheat, and vapor superheat are obtained as a function of axial distance from the critical heat flux (quench front) for a range of flow rates and flow qualities. These data permit the evaluation of the effective evaporation ratio (fraction of total heat input causing net vapor generation) as a function of test conditions and axial distance from the quench front. The results indicate the existence of a ‘near region’ with significant evaporation, followed by a far region where the effective evaporation ratio decreases to less than 10% of the total heat input. While the wall and vapor superheats measured in these rod bundle experiments differ in magnitude from those obtained in earlier single tube experiments, qualitative agreement in the axial behavior of the post-CHF process is observed. These data are presented in the hope of aiding attempts to develop improved mechanistic models for convective film boiling heat transfer.

Vapor generation rate model for dispersed drop flow

Abstract A comparison of predictions of existing nonequilibrium post-CHF heat transfer models with the recently obtained rod bundle data has been performed. The models used the experimental conditions and wall temperatures to predict the heat flux and vapor temperatures at the location of interest. No existing model was able to reasonably predict the vapor superheat and the wall heat flux simultaneously. Most of the models, except Chen-Sundaram-Ozkaynak, failed to predict the wall heat flux, while all of the models could not predict the vapor superheat data or trends. A recently developed two-region heat transfer model, the Webb-Chen two-region model, did not give a reasonable prediction of the vapor generation rate in the far field of the CHF point. A new correlation was formulated to predict the vapor generation rate in convective dispersed droplet flow in terms of thermal-hydraulic parameters and thermodynamic properties. A comparison of predictions of the two-region heat transfer model, with the use of a presently developed correlation, with all the existing post-CHF data, including single-tube and rod bundle, showed significant improvements in predicting the vapor superheat and tube wall heat flux trends.

Convective boiling in a rod bundle: transverse variation of vapor superheat temperature under stabilized post-CHF conditions

Abstract Transverse variations of vapor superheat temperatures across a rod bundle are measured for convective boiling of water in the post-CHF regime. Significant differences in superheat (up to 120°C) are observed across the flow subchannels. The steepness of this transverse superheat profile decreases with increasing vapor Reynolds number.

Thermal Analysis of Encapsulated Phase Change Materials for Energy Storage

Concentrating solar power technology is recognized as an attractive option for solar power. A major limitation however is that solar power is available for only about 2,000 hours a year in many places. Therefore it is critical to find ways to store solar thermal energy for the off hours and it is better to store the energy at high temperatures. The present work deals with certain aspects of storing solar thermal energy at high temperatures with phase change materials (PCM) in the range of 275°C to 425°C. NaNO3 is selected as a phase change material encapsulated by stainless steel. The objective is the storage of hundreds mega-watt-hours equivalent of solar energy in systems using encapsulated phase change materials (EPCM). Numerical predictions of conduction and phase change processes are reported here in the form of transient temperature profiles in the PCM and encapsulation materials of EPCM capsules for convective boundary conditions outside EPCM. The time for heating and melting during charging (storage of thermal energy into encapsulated phase change material) and the time for cooling and solidification during discharging (discharge/retrieval of thermal energy) are predicted for NaNO3 PCM in encapsulation. For a temperature range of about 125°C around melting/freezing temperature of the PCM the time it takes to melt/freeze the PCM during storage/retrieval is much longer than the time it takes for diffusion (sensible heat) storage alone. Depending on the properties of the PCM, the energy associated with the latent heat of melting can be a significant leading to smaller thermal energy storage systems and lower costs. As can be expected, the time for heat transfer is much shorter for liquid heat transfer fluids compared to those for gaseous heat transfer fluids that transport the energy to the EPCM.Copyright

Characteristics of liquid–wall contact in post-CHF flow boiling

Abstract In several industrial applications, highly heated metal channels are quenched by advancing fronts of cold liquid. The transient heat transfer in the vicinity of the advancing quench front governs the dynamics of the overall process, controlling the rate of quench front advancement. Most analyses to date assume that the hot metal in front of the quench front is above the Leidenfrost temperature and cannot sustain direct liquid contacts. This investigation sought to experimentally assess the validity of the no-contact assumption, and to record specific characteristics of such contacts if they occur. In order to detect and record potential liquid contacts in the vicinity of an advancing quench front, a special rapid-response probe was utilized. Tests were carried out with water at atmospheric pressure and over a range of wall superheats and quench front propagation velocities. Analysis of the experimental data provided information on the probability, duration, and time fraction of liquid contacts as functions of wall superheat and distance from the quench front.