Dongsoon Jang

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.

A Numerical Study for Effective Operation of MSW Incinerator for Waste of High Heating Value by the Addition of Moisture Air

Stoker type incinerator is one of the most popular one used as municipal solid waste (MSW) incineration because, in general, it is quite suitable for large capacity and need no preprocessing facility. Nowadays, however, since the combustible portion of incoming MSW increases together with the decrease of the moisture content due to prohibition of directly burying food waste in landfill, the heating value of waste is remarkably increasing in comparison with the early stage of incinerator installation. Consequently, the increased heating value in incinerator operation causes a number of serious problems such as reduction of waste amount to be burned due to the boiler heat capacity together with the significant NO generation in high temperature environment. Therefore, in this study, a series of numerical simulation have been made as parameters of waste amount and the fraction of moisture in air stream in order to investigate optimal operating condition for the resolution of the problems associated with the high heating value of waste mentioned above. In specific, a detailed turbulent reaction flow field calculation with NO model was made for the full scale incinerator of D city. To this end, the injection method of moisturized air as oxidizer was intensively reviewed by the addition of moisture water amount from 10% and 20%. The calculation result, in general, showed that the reduction of maximum flame temperature appears consistently due to the combined effects of the increased specific heat of combustion air and vaporization heat by the addition of water moisture. As a consequence, the generation of NOx concentration was substantially reduced. Further, for the case of 20% moisture amount stream, the afterburner region is quite appropriate in temperature range for the operation of SNCR. This suggests the SNCR facility can be considered for reoperation. which is not in service at all due to the increased heating value of MSW.

A Study on the Optimum Design of Multiple Screw Type Dryer for Treatment of Sewage Sludge

The purpose of this study is to investigate basically the mechanism of heat transfer by the resolution of complex fluid flow inside a sophisticated designed screw dryer for the treatment of sewage sludge by using numerical analysis and experimental study. By doing this, the result was quite helpful to obtain the design criteria for enhancing drying efficiency, thereby achieving the optimal design of a multiple screw type dryer for treating inorganic and organic sludge wastes. One notable design feature of the dryer was to bypass a certain of fraction of the hot combustion gases into the bottom of the screw cylinder, by the fluid flow induction, across the delicately designed holes on the screw surface to agitate internally the sticky sludges. This offers many benefits not only in the enhancement of thermal efficiency even for the high viscosity material but also greater flexibility in the application of system design and operation. However, one careful precaution was made in operation in that when distributing the hot flue gas over the lump of sludge for internal agitation not to make any pore blocking and to avoid too much pressure drop caused by inertial resistance across the lump of sludge. The optimal retention time for rotating the screw at 1 rpm in order to treat 200 kg/hr of sewage sludge was determined empirically about 100 minutes. The corresponding optimal heat source was found to be 150,000 kcal/hr. A series of numerical calculation is performed to resolve flow characteristics in order to assist in the system design as function of important system and operational variables. The numerical calculation is successfully evaluated against experimental temperature profile and flow field characteristics. In general, the calculation results are physically reasonable and consistent in parametric study. In further studies, more quantitative data analyses such as pressure drop across the type and loading of drying sludge will be made for the system evaluation in experiment and calculation.

A Study of the Effect of the Permeability and Selectivity on the Performance of Membrane System Design

Manufacturing membrane materials with high selectivity and permeability is quite desirable but practically not possible, since the permeability and selectivity are usually inversely proportional. From the viewpoint of reducing the cost of CO2 capture, module performance is even more important than the performance of membrane materials itself, which is affected by the permeance of the membrane (P, stagecut) and selectivity (S). As a typical example, when the mixture with a composition of 13% CO2 and 87% of N2 is fed into the module with 10% stage cut and selectivity 5, in the 10 parts of the permeate, CO2 represents 4.28 parts and N2 represents 5.72 parts. In this case, the CO2 concentration in the permeate is 42.8% and the recovery rate of CO2 in this first separation appears as 4.28/13 = 32.9%. When permeance and selectivity are doubled, however, from 10% to 20% and from 5 to 10, respectively, the CO2 concentration in the permeant becomes 64.5% and the recovery rate is 12.9/13 = 99.2%. Since in this case, most of the CO2 is separated, this may be the ideal condition. For a given feed concentration, the CO2 concentration in the separated gas decreases if permeance is larger than the threshold value for complete recovery at a given selectivity. Conversely, for a given permeance, increasing the selectivity over the threshold value does not improve the process further. For a given initial feed gas concentration, if permeance or selectivity is larger than that required for the complete separation of CO2, the process becomes less efficient. From all these considerations, we can see that there exists an optimum design for a given set of conditions.

Numerical Study on the Thermal NOx Reduction by Addition of Moisture in LNG Flame

Abstract : A computer program is developed for the prediction of NO generation by the addition of water moisture and water electrolysis gas in LNG-fired turbulent reacting flow. This study is the first part to deal with the moisture effect on NO genera-tion. In this study, parametric investigation has been made in order to see the reduction of thermal NO as a function of amount of moisture content in a LNG-fired flame together with the swirl and radiation effect. First of all, calculation results show that the flame separation together with the NO concentration separation are observed by the typical flow separation due to strong swirl flow. With a fixed amount of air, the increased amount of water moisture from 0 to 10% by 2% interval shows the decrease of NO concentration and flame temperature at exit are from 973℃ and 139 ppm to 852℃ and 71 ppm. The radiation effects on the generation on NO appears more dominant than swirl strength over the range employed in this study. However, for the strong swirl flow employed in this study, the flow separation cause the relatively high NO concentration observed near exit after peak concentration in the front side of the combustor.