Tomer Rozenfeld

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

MODELLING OF HIGH-SPEED JET COOLING ON MICROSCALE

In the present work, experimental and numerical studies were performed in order to investigate the cooling performance of a single-phase flow in micro-channel/slot-jet system. A three-dimensional numerical model of jet cooling was developed and implemented using commercial software ANSYS Fluent. The 3D conjugate conduction/convection heat transfer in the micro-channel simulations were used to complement experiments and to obtain detailed flow patterns of the jet, temperature, and heat flux distribution on the heater area, and fluid temperature distribution. The model has been verified in a preliminary study where its time-step and grid independency was established and validated vs. experiments.

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.