Hey-Suk Kim

Numerical and experimental study on the design of a stratified thermal storage system

Abstract The purpose of this study is to figure out the thermal stratification mechanism of a storage tank and thereby to determine optimum design and operating conditions. To this end, a computer program is developed to investigate the fluid flow in a tank, using Patankars SIMPLE algorithm. The validation of program is made successfully by the comparison against experimental data measured with a bench scale facility. Further a systematic investigation has been made in terms of important design and operational parameters such as storage tank size (commercial-scale and bench-scale), loading time, shape of diffuser, turbulence model and inlet velocity or Fr No. Considering the thermal efficiency of storage tank is critically impaired by the effect of flow recirculation and mixing by turbulence, a novel model with minimum mixing between hot and cold water is proposed for the evaluation of the performance of storage system by the assumption of the uniform plug-type flow. This is made by solving only the following governing equation of temperature which has no convective mixing with constant axial velocity, that is, ∂ (ρT) ∂ t +u ∂ (ρT) ∂ x =∇· k c p ∇T Based on a series of parametric investigation, a number of useful results can be drawn. In general the large scale system shows better storage performance than the small system. As the increase of loading time, the degree of stratification lowered due to the increased effect of heat transfer by convection and diffusion via thermal stratification region. And the curved type diffuser shows better performance of thermal stratification compared to the diffuser of flat type. As for the case of large-scale tank, the effect of Fr No., or inlet velocity does not show any significant effect on the thermal stratification compared to the small size one. The calculated results show that the model of ideal type-plug flow can be used as a possible tool for the evaluation of performance of storage system by giving the reference condition of best thermal stratification or least mixing condition. But in the case of full scale system, the difference between ideal and actual cases not significant compared to the case of bench scale. It is considered possibly due to the relatively decreased heat transfer effect by the increased effect of water flow in the full scale system. Two turbulence models, say standard k – e and RNG k – e models, show no visible difference in this study.

A Study for the Thermal Treatment of Dehydrated Sewage Sludge with Gas-Agitated Double Screw Type Dryer

Abstract A specially designed dryer has been developed for the thermal treatment of dehydrated, highly viscous sewage sludge with moisture content more than 80 wt.% by an gas-agitated, double-screw type dryer system. The treatment capacity of sludge dryer was about 100 kg/h. It consists of burner, feeding hopper, hot gas supplying ducts with double screw conveyor, damper and outlets of drying gas and dried material. The sewage sludge was transported by the revolution of the cylinder conveyor together with the tumbling and mixing action of the screw and lifters. The heating of the sludge was made efficient by the combination action of conduction and convection modes together with the gas-agitation process. The conduction occurred across the surface of the combustor cylinder, the convection was made by the flue gas interaction over the sludge and the agitation action through holes of conveyor cylinder into the bottom of the sludge. The number and location of gas-agitation holes and thereby the fraction of drying gas into the agitation holes were evaluated by numerical calculation of turbulent reacting flow. To evaluate the performance of the dryer developed in this study, a series of parametric experiments were performed in terms of important variables. In general drying process occurred successfully even for the highly agglomerating municipal sewage sludge through the first lumped plastic phase to a fine granulate state when the heat and mass balance was matched by adjusting rpm of the conveyor, burner capacity, and the fraction of gas-agitation. Further the avoidance of initial sludge agglomeration was considered to be important by the combination action of screw and lifter. The weight of the sludge was reduced approximately by 60% and therefore the volume of sludge was decreased to approximately 75% of initial volume with the water content of 10–20%. Energy efficiency of dryer was evaluated in the range of 70–75% at the sludge feed rate, 100 kg/h. The results obtained show the development of a robust, drying action applicable to highly viscous and high moisture content sewage sludge by the novel design of conveyor system and gas-agitation process developed in this study.

Study on the validation of the computer fluid dynamics modeling for a continuously flowing water vessel with the industrial SPECT using a radiotracer.

The industrial Single Photon Emission Computerized Tomography (SPECT) system comprised of 30 sets of lead collimated 2 in. NaI(Tl) detectors was built and utilized for measuring the spatial distribution of radiotracers as a function of time at a predefined cross-sectional level of a tank with a diameter of 300 mm and a height of 780 mm. In a stable flow condition, a radiotracer was injected into the upstream of a tank and then the measurement data were processed for the two dimensional concentration mapping of the radiotracers with a time step of 1s. The images were compared with numerical simulation results obtained from the Computational Fluid Dynamics modeling that has been incorporated with several different kinds of turbulence models. The comparison shows one model gives a better result than any others in terms of the similarity of the radiotracer distribution pattern. This study demonstrates the significant possibility of industrial SPECT technology for visualizing flow characteristics in a multi-dimensional way and for validating the CFD modeling.