Hoseon Yoo

NUMERICAL INVESTIGATION ON THE BIFURCATIVE NATURAL CONVECTION IN A HORIZONTAL CONCENTRIC ANNULUS

Steady state two-dimensional natural convective heat transfer in a horizontal cylindrical annulus was studied by solving the governing equations based on the primitive variables. Emphasis was put on the occurrence of multiple solutions and on the determination of the bifurcation points at which those multiple solutions begin to branch out. The multicellular flow patterns from the results of melting processes in an isothermally heated horizontal cylinder for high Rayleigh numbers were used as initial fields. This approach succeeded in finding new bifurcation points to a tetracellular solution for a set of parameter variables identical to those used in previous studies. Close examination of flow pattern transitions near the bifurcation point was also conducted. It was found that the mechanisms of flow transition are different depending on the critical Rayleigh number of the bifurcation point.

A refined similarity solution for the multicomponent alloy solidification

Abstract This paper deals with a refined similarity solution for the solidification of ternary or higher-order multicomponent alloys. The present approach not only retains the existing features of binary systems such as temperature–solute coupling, shrinkage-induced flow, solid–liquid property differences and finite back diffusion, but also is capable of handling a multicomponent alloy without restrictions on the partition coefficient and microsegregation parameter. A new exact solution to the energy equation including the convection term in the pure liquid region is successfully derived, which allows the present analysis to cover a high initial superheating. For an alloy of K-solute species, governing equations in the mushy region reduce to (K+2) simultaneous ordinary differential equations via similarity transform, which are to be solved along with the closed-form temperature profiles in the solid and liquid regions. A linearized correction scheme adopted in the solution procedure facilitates to determine the solidus and liquidus position stably. Good agreements are found in comparison with the numerical predictions and available simplified similarity solutions for binary and ternary alloys.

FEASIBILITY OF ICE-SLURRY APPLICATION TO THE DISTRICT COOLING SYSTEM IN KOREA

The district-cooling system (DCS) has been in service in Sang-am in Seoul, Korea since 2005. The capacity of the DCS facility in Sang-am was 111 Gcal/h in 2011, and an additional 63 Gcal/h capacity is planned for installation by 2025. However, the cooling demand has increased due to unexpected high-rise building blocks, and the required facility capacity is expected to be 101 Gcal/h. Adding a new building plan to the existing plant is difficult. This study centers on a feasibility study for the new requirement under the restrictions of existing pipelines, limited space and regulations on the use of electric-driven chillers in Korea, etc. The precise estimation of the diversity factor is essential to determine the required capacities. To this end, each building in the district area was categorized, and the cooling loads were measured for the summer seasons of 2010 and 2011. The large energy capacity of an ice-slurry can potentially increase the cooling capacity in existing plants while maintaining the same flow rate and pumping power. Thus introducing an ice-slurry is expected to be a potential solution to the significantly increased cooling load under the restriction of existing pipeline system without requiring increases in pipe size or system flow rates.

A Modified T-Method Simulation for Multi-fan Duct Systems

In this study, we introduce the Modified T-Method which can be applied to the prediction of the flow distribution in multi-fan duct systems. Introduction of individual flow routes enables the division of a complicated multi-fan ductwork into a set of simultaneous single-fan subsystems. The present method was tested on the single-fan duct system presented in the ASHRAE handbook. The test system consisted of several branches and a fan. Our results showed good agreement with those published. Also, we tried another approach, in which the symmetry of a two-fan duct system was investigated. Verification was accomplished successfully by the analysis of a two-fan duct system that yielded physical consistency. In order to evaluate the utility and capability of the title method, a bathroom ventilation system in a 20-storey residential building was selected as an example of an application. Under the typical design conditions, the air flow rate of each exhaust fan at the balance point was predicted successfully and it was shown that such information can lead to an engineering estimate for the system performance as a whole. While some deficiencies in ventilation were found in bathrooms on the lower floors with 6mmAq-rated exhaust fans, they disappeared over the building as a whole by using fans of enhanced static pressures, 7 and 8mmAq. Finally, the work has demonstrated that this method is likely to be useful for the design of multi-fan duct systems.

Feasibility Study for Ice-Slurry to District Cooling System in Korea

The district-cooling system (DCS) was in service in Sang-am in Seoul, Korea after 2005. The facility capacity of the DCS in Sang-am is 111Gcal/h at 2011 and 63Gcal/h of facility is planned to install till 2025. However, the cooling demand is increased due to new high-rise building blocks, and the required facility capacity is expected to be 101Gcal/h. A difficulty comes from building new plan in the existing plant. This paper is on the feasibility study for the new requirement under the restrictions of existing pipeline, limited space and regulation on the usage of electric driven chiller. Precise estimation of the diversity factor is essential to determine the required capacities. For this, every building in the area was categorized and the cooling loads were measured for the summer seasons of 2010 and 2011. The large energy capacity of ice slurry can potentially increase the cooling capacity in existing plants while maintaining the same flow rate and pumping power. Thus under the restriction of existing pipeline system, introducing ice slurry is expected as potential solution to the significantly increased cooling load without requiring increases in pipe size or system flow rates.Copyright

A STUDY ON THE FAN EFFICIENCY DECREASE ON THE BACKWARD FLOW IN AN AXIAL FAN WITH ADJUSTABLE PITCH BLADE

In this study, a numerical study has been carried out to analyze the factors of the efficiency decrease at backward flow situation in an axial fan with adjustable blades. The analysis is done with the pitch angle of 36° on the forward flow and of -26° on the backward flow. The numerical results show that the air flow rates of the pitch angle of 36° and -6° are calculated to be 285 cubic meter per min (CMM) and 212 CMM, respectively. The results are similar to the experimental results by Chang et al.1 The results had the maximum error of 10.6% compared with the experimental results. The fan efficiency decrease is caused by the fact that the axial fan used for this study was designed for the forward flow. As the results, the pitch angle of -26° has induced the recirculation around the impeller blade, impeller cover and downstream. This recirculation caused the large decrease in total pressure coefficient. This turned out to be the main cause of the efficiency decrease.

A FIELD APPLICATION CASE OF THE DIRECT ICE SLURRY TRANSPORTING SYSTEM FOR DISTRICT COOLING

In order to investigate the feasibility of a direct ice slurry transporting system for the purpose of district cooling, a case study of field application is performed. The research aims include the field measurement of ice packing factor, the performance of coldness delivery, and the branching characteristics of ice slurry. Two representative types of pipe branch are dealt with in this work. For the slurry flow with ice volume fraction of 0.16 or less, the pipe blocking due to aggregation is not observed. Based on the time-wise variation of temperature in the storage tank, a calculating method of ice packing factor is newly developed, which seems to be useful when the brine concentration is unknown. It is confirmed that the mass flow rate of ice slurry per unit cooling load is markedly reduced with increasing the ice content. The pumping power also decreases, but remains unchanged for high ice fractions. The distribution of ice particle before and after branching shows a good uniformity within the range of 5% difference, but yields a unique trend depending on the flow rate.

Air diffusion performance index of high sidewall line diffuser in a room with perimeter heating load

Purpose – The purpose of this study is to evaluate the effect of the perimeter heating load about the air diffusion performance index (ADPI) and propose a selection guide for proper line diffuser when perimeter heating load exists.Design/methodology/approach – A high sidewall line diffuser was installed into a test room following the ISO standard 5219. The velocity and temperature distributions and the ADPI values are obtained numerically with 12 cases when the air flow rates are 425, 730 and 950 CMH and the heating loads are 0, 25, 50 and 75 percent of the total heating load, 2,143 W.Findings – The ADPI decreases according to the increases of the flow rate on every heat load ratio of the present study except 0.75 with the line diffuser installed at the high sidewall. And we produced ADPI curves when the sidewall heating load exists or not in the same air flow rate and supply conditions. From that, it is possible to assume that the throw length guarantees more than 80 percent of ADPI with heating load on ...

REALIZATION OF HIGH IPF ICE SLURRY FOR DISTRICT COOLING

The Production of high IPF (ice packing factor) ice slurry was realized. The ice slurry maker which can efficiently produce ice slurry under ice particles in-flowing condition was revised. We removed the stagnant region at the top of the ice slurry maker, thereby high IPF ice slurry was realized. The IPF controller with 6 mm diameter holes could not control IPF in a pipe. This is because ice particles in ice slurry flow exist homogeneously not only at the upper part but also at the bottom part. We changed the hole size at the surface of the IPF controller by means of fine meshes, which, then, increased the IPF in the pipe by 70% when the mesh size was 80 μm or less.