Experiments on melting of slush ice in a horizontal cylindrical capsule

Abstract

Melting phenomena are related to a wide variety of engineering fields: purification of metals, welding, electroslag melting, thawing of moist soil, and latent heat-of-fusion thermal-energy storage are a few of important applications which have motivated research in this area. Melting is a phase transformation process that is accompanied by absorption of thermal energy. The essential feature of the systems that exhibit melting phenomena is the existence of a liquid–solid interface that separates the two phases containing different thermophysical properties and the absorption of thermal energy at the interface. The major problem in melting is thus to determine transport phenomena of the latent and sensible heat of the system. There is quite a large body of literature concerning a variety of such problems in engineering as well as in the applied sciences. Recent reviews summarize prior work in this area [1–8]. In previous studies pertaining to melting of a solid contained in a confined vessel, consideration has been given to either to a melting solid which is constrained to prevent its possible movement owing to gravity or to a solid which is free to fall under gravity [8]. In the first case, the melting solid is maintained at a fixed position inside the vessel throughout the melting process, then is completely surrounded by the liquid melt, and the energy needed for the melting is transported from the heating wall to the solid–liquid interface by free convection within the liquid melt. In the second case, the solid is free to respond to the net force acting on it. If the solid phase has higher density than the liquid phase the solid sinks to the bottom of the vessel. On the contrary, if the solid phase is lighter than the liquid phase the unmelted solid is drawn by buoyancy to the top of the vessel. In either case, a region of close-contact melting arises between the solid and the heating wall. Utilization of thermal-energy storage system for air conditioning has recently evoked energy saving and normalizing the requirement level of the electric power supply [9, 10]. For conventional ice-storage systems using a pure ice system as a phase change material (PCM) it has been pointed out [11–14]that the melting heat transfer performance may be reduced with time because the melting ice surface is separated more far from the heat transfer wall. Marked attention has recently been given to a slush ice [15, 16], which is essentially a mixture of fine ice particles and aqueous binary solution, as a new PCM in place of common ice owing to demand of both high efficient ice producing and regulate handling of melting heat transfer performance as well as transportability. With respect to the melting heat transfer performance in releasing the cold thermal energy from the slush ice, the melting characteristics of a quiescent slush ice around a horizontal heated tube [17]and in a rectangular capsule with a vertical heated wall [18]as well as with a top heated wall [18]were determined experimentally. In addition, a solid–gas–liquid three-phase fluidized bed heat exchange system [19]and a direct contact heat-exchange system, in which the slush ice is operated as the fluidized bed, was proposed for releasing the cold thermal energy. Detailed basic data on the melting characteristics of the slush ice are highly required for developing high efficient heat exchanger using the slush ice. However, there seems to be a variety of unknown characteristics on the melting mechanism as well as the melting behavior for releasing both efficiently and regulatively the cold thermal energy from the slush ice. This paper reports a study on the melting characteristics of slush ice in a horizontal cylindrical capsule. In order to inspect the local heat transfer coefficient distribution in circumference direction in detail, the capsule wall was electrically heated to be a constant heat flux condition. Experimental runs were carried out to investigate the effects of heat flux and initial concentration of aqueous binary solution on both the melting behavior and the heat transfer characteristics of the slush ice. Photographs of flow patterns are presented, and dependence of the double-diffusion on the flow structure is discussed. The results obtained form present study have great importance and usefulness not only for design of the high efficient heat exchanger using the slush ice, but also for operations of the thermal energy storage system.