Yoram Kozak

Analysis of a Latent Heat Storage Device With Radial Fins

Phase-change materials (PCMs) can store large amounts of heat without significant change of their temperature during the phase-change process. This effect may be utilized in thermal energy storage, especially for solar-thermal power plants. In order to enhance the rate of heat transfer into PCMs, one of the most common methods is the use of fins which increase the heat transfer area that is in contact with the PCM.The present work deals with a latent heat thermal storage device that uses a finned tube with an array of radial fins. A heat transfer fluid (HTF) flows through the tube and heat is conducted from the tube to the radial fins that are in contact with the bulk of the PCM inside a cylindrical shell. The thermal storage charging/discharging process is driven by a hot/cold HTF inside the tube that causes the PCM to melt/solidify.The main objective of the present work is to demonstrate that close-contact melting (CCM) can affect the storage unit performance. Accordingly, two different types of experiments are conducted: with the shell exposed to ambient air and with the shell submerged into a heated water bath. The latter is done to separate the PCM from the shell by a thin molten layer, thus enabling the solid bulk to sink. The effect of the solid sinking and close-contact melting on the fins is explored. It is found that close-contact melting shortens the melting time drastically.Accordingly, two types of models are used to predict the melting rate: numerical CFD model and analytical/numerical close-contact melting model. The CFD model takes into account convection in the melt and the PCM property dependence on temperature and phase. The analytical/numerical CCM model is developed under several simplifying assumptions. Good agreement is found between the predictions and corresponding experimental results.Copyright

Development of a Hybrid PCM-Air Heat Sink

The present study deals with the transient thermal management of electro-optical equipment using the phase-change materials (PCMs). These materials can absorb large amounts of heat without significant rise of their temperature during the melting process. This effect is attractive for using in the passive thermal management of portable electro-optical systems, particularly those where the device is intended to operate in the periodic regime, or where the relatively short stages of high power dissipation are followed by long stand-by periods without a considerable power release. In the present work, a so-called hybrid heat sink is developed. The heat sink is made of aluminum. The heat is dissipated on the heat sink base, and then is transferred by thermal conduction to the PCM and to a standard forced-convection air heat sink cooled by an attached fan. The whole system may be initially at some constant temperature which is below the PCM melting temperature. Then, power dissipation on the heat sink base is turned on. As heat propagates within the heat sink, some part of it is absorbed by the PCM causing a delay in the temperature growth at the heat sink base. Alternatively, the steady-state conditions may be such that the base temperature is below the PCM melting temperature, meaning that all the heat generated on the heat sink base is transferred to the cooling air. Then, the fan is turned off reducing the heat transfer to the ambient air, and the heat is absorbed into the PCM resulting in its melting. In both cases, the time that it will take the heat sink base to approach some specified maximum allowed temperature is expected to be longer than that without the PCM.Copyright

Enhanced Melting for Transient Thermal Management

The present study deals with transient thermal management using phase change materials (PCMs). These materials can absorb large amounts of heat without significant rise of their temperature during the melting process. This effect is attractive for passive thermal management, particularly where the device is intended to operate in a periodic regime, or where the relatively short stages of high power dissipation are followed by long stand-by periods without a considerable power release. Heat transfer in PCMs, which have low thermal conductivity, can be enhanced by fins that enlarge the heat transfer area. However, when the PCM melts, a layer of liquid is growing at the fins creating an increasing thermal resistance that impedes the process.The present work aims to demonstrate that performance of a latent-heat thermal management unit may be considerably affected by achieving a so-called close-contact melting (CCM), which occurs when the solid phase is approaching a heated surface, and only a thin liquid layer is separating between the two. Although CCM was extensively studied in the past, its possible role in finned systems has been revealed only recently by our group. In particular, it depends heavily on the specific configuration of the fins.In the present work, close-contact melting is modeled analytically for a geometry which includes two symmetrically inclined fins. A quasi-steady approach is used for calculating the rate of melting based on the force and energy balances.The results are expressed in terms of the time-dependent melt fraction and Nusselt number, showing their explicit dependence on the Stefan and Fourier numbers. Moreover, the approach used in the present study may be applied to other geometries in which the heated surface is not horizontal or where there are a number of heated surfaces or fins.Copyright

Heat Transfer Enhancement in Latent Heat Storage Units

In the present work, latent heat thermal storage devices that use an array of fins are studied experimentally and theoretically. The thermal storage charging/discharging process is driven by a hot/cold heat transfer fluid (HTF) flowing inside the tube from which heat is conducted to the fins that are in contact with the bulk of the PCM inside a cylindrical shell, causing the PCM to melt/solidify. The present study focuses on a case in which the envelope of the unit is exposed to a heated environment and close-contact melting takes place on the upper surface of the fins. It is demonstrated that close-contact melting affects noticeably the melting rate and shortens the melting time considerably.

Detailed Numerical Modeling of a Hybrid PCM-Air Heat Sink

The ability of phase-change materials (PCMs) to absorb large amounts of heat without significant rise of their temperature during the melting process may be utilized in thermal energy storage and passive thermal management. This paper deals with numerical modeling of a hybrid PCM-air heat sink, in which heat may be either absorbed by the PCM stored in compartments with conducting walls, or dissipated to the air using fins, or both.Under the assumptions of perfect insulation (except for the air fins), identity and symmetry between all PCM channels, and negligible 3-D boundary effects, a 2-D model of the problem for half a PCM compartment of the heat sink is solved, saving calculation time and yet taking into account the essential physical phenomena. A commercial program, ANSYS Fluent, is used in order to solve the governing conservation equations. Phase-change is solved using the enthalpy-porosity method. PCM-air interface is modeled using the volume-of-fluid (VOF) approach. The model takes into account natural convection in the liquid PCM and air, volume change, phase- and temperature-dependence of thermal properties, and PCM-air interface interaction.Various scenarios for the hybrid heat sink operation are simulated and compared. The difference in the melting patterns is analyzed for the cases of heating with and without the fan operating. The solidification process with the fan operating is also simulated. It is shown that the VOF model enables simulating realistic void formation in the solidification process.Copyright

Analysis of a Hybrid PCM-Air Heat Sink

Phase-change materials (PCMs) can absorb large amounts of heat without significant rise of their temperature during the melting process. This effect may be utilized in thermal energy storage and passive thermal management. In order to enhance the rate of heat transfer into PCMs, various techniques have been suggested, like fins, metal and graphite-compound matrices, dispersed high-conductivity particles inside the PCM, and micro-encapsulation.The present work deals with a hybrid PCM-air heat sink. The heat is dissipated on the heat sink base, and may be either absorbed by the PCM stored in compartments with conducting walls, or dissipated to the air using fins, or both. The heat sink is made of aluminum 6061. Eicosane (C20H42, 96% purity, nominal melting temperature 36.7°C) is used as the PCM. In order to exclude the effect of sensible heating below the melting temperature, a controllable environment is used. The latter is created in a programmable forced-circulation oven.A simplified thermal model is developed for a conservative estimation of temperature growth of the heat sink base. The results of this model are compared to the experimental results. The relative contributions of heat accumulation, both by latent and sensible heat, and of heat removal by air are presented and discussed.© 2012 ASME