Thermosyphon assisted melting of PCM inside a rectangular enclosure:A synergistic numerical approach

Enhancing the thermosiphon-driven discharge of a latent heat thermal storage system used in a personal cooling device

Abstract: Personal cooling devices reduce energy loads by allowing buildings to operate with elevated setpoint temperatures, without compromising on the occupant comfort. One such novel technology called the Roving Comforter (RoCo) uses a compact R134a based vapor compression system for cooling. Following its cooling operation, during which waste heat from the condensing refrigerant is stored in a phase change material (PCM), a two-phase loop thermosiphon is used to discharge (solidify) the PCM to enable its next operation. The transient operation of this thermosiphon is the focus of the present article. Use of a PCM as the storage medium provides high energy density due to the ability to store thermal energy as latent heat during the phase transition; however, the discharge rate is limited by the low thermal conductivity of the PCM. Insertion of a graphite foam within the PCM can increase the rate of discharge and decrease the downtime of the cooling device. Since graphite enhancement involves a tradeoff between improving the discharge time at the expense of PCM volumetric latent heat, the impact of graphite foam density on the PCM discharge rate is investigated by using a Modelica-based transient model of the thermosiphon. The semi-empirical model, which uses relevant heat transfer coefficient and pressure drop correlations for both refrigerant and airside heat transfer, captures the complex phenomena involving simultaneous phase change of the refrigerant and the PCM. The graphite enhanced PCM selected from this analysis results in a 51% reduction in the discharge time with addition of only 5% to the thermal storage weight, without compromising the required cooling time.

Systematic review on the use of heat pipes in latent heat thermal energy storage tanks

Abstract: This systematic review presents and discusses the previous research about hybrid devices which combine latent thermal energy storage (TES) technology and heat pipes. A bibliometric analysis of this issue shows how hybrid systems have globally grown popularity during time, providing details about the main researchers and research centres on this particular field. Then, the identified papers are assessed and categorized in two main sections, the experimental research carried out, and the numerical modelling of hybrid systems. Experimental research is later classified regarding the operating temperature range, and their final application. Numerical studies are also further categorized, accordingly to how heat pipes were modelled in this case. This review points out the lack of experimental studies at high temperatures, especially when many simulations extended their models (validated at low temperatures) to higher temperature designs. The paper provides details about the research performed, so the gap for future investigations can be spotted.

Two-stage heat transfer characteristics of constrained melting inside an isothermally heated horizontal cylinder

Abstract: An experimental and numerical investigation has been performed on the quantitative determination of two-stage heat transfer characteristics of constrained melting process of phase change material (PCM) inside a horizontal cylinder. Melting experiments are conducted with paraffin in a cylinder subjected to different boundary temperatures. The solid-liquid interface and temperature field of PCM are captured using photography and infrared technique, respectively, which are then used to validate the numerical models established based on the enthalpy method. A scale analysis is performed to estimate the order of magnitude of the influence factors. With the validated models, the melting processes of PCM inside a horizontal cylinder covering a wide range of Rayleigh number (Ra), Stefan number (Ste) and Fourier number (Fo) are simulated and analyzed carefully. It is found that the transition point between the conduction and convection stages depends greatly on the Rayleigh number while has little relation with the Stefan number. Particularly, three important Rayleigh numbers are identified: when Ra 3.3 × 107, the PCM melted in the convection stage is more than 90%, so the natural convection dominates the melting process; when Ra = 5.3 × 104, the PCM melted in conduction and convection stages are both equal to 50%. For the conduction stage, the heat transfer characteristics depend only on the dimensionless melting time (FoSte), while for the convection stage, the heat transfer characteristics depend not only on the Fourier and Stefan numbers, but also on the Rayleigh number. Based on the numerical and the scale analysis results, the correlation for the dimensionless transition melting time (FoSte)tr (which can be used to distinguish between the conduction stage and convection stage) as well as the correlations of liquid fraction (α) and Nusselt number (Nu) for the conduction and convection stages, respectively, are proposed. These correlations, with a wide application range of Rayleigh and Stefan numbers, agree well with the experimental and theoretical results from both present study and open literature, and can well predict the two-stage heat transfer characteristics of constrained melting in a horizontal cylinder.

Applications of coupling thermosyphons with phase change materials: A review

Abstract: As energy demands and environmental contaminants are facing significant challenges, advanced techniques and high-efficient thermal systems are extensively explored to protect the environment and save energy. Thermosyphons (TSs) are widely conducted as outstanding passive thermal transport devices, which can have high effective thermal conductivities. Nowadays, thermosyphons have been recognized in conjunction with phase change materials (PCMs) to improve the performance of the latent heat thermal energy storage systems. The primary purpose of the integration is to embed the high-efficient thermal conductor into the thermal storage unit to transfer and reserve a significant value of heat efficiently. This review documents applications of coupling TSs and PCMs, general design procedures, and numerical analysis of heat transfer for the systems based on the thermal network approach. Various applications are sorted and compared to reveal the potential advantages of the integration of TSs with PCMs. Moreover, this review also discusses the opportunities and challenges based on a comprehensive framework for the benefit of designers and engineers to implement the combination of TSs and PCMs.

Convection Heat Transfer

Abstract: Fundamental Principles Laminar Boundary Layer Flow Laminar Duct Flow External Natural Convection Internal Natural Convection Transition to Turbulence Turbulent Boundary Layer Flow Turbulent Duct Flow Free Turbulent Flows Convection with Change of Phase Mass Transfer Convection in Porous Media.

A fixed grid numerical modelling methodology for convection-diffusion mushy region phase-change problems

Abstract: An enthalpy formulation based fixed grid methodology is developed for the numerical solution of convection-diffusion controlled mushy region phase-change problems. The basic feature of the proposed method lies in the representation of the latent heat of evolution, and of the flow in the solid-liquid mushy zone, by suitably chosen sources. There is complete freedom within the methodology for the definition of such sources so that a variety of phase-change situations can be modelled. A test problem of freezing in a thermal cavity under natural convection is used to demonstrate an application of the method.

High temperature latent heat thermal energy storage using heat pipes

Abstract: A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.

Heat pipe with PCM for electronic cooling

Abstract: This article experimentally investigates the thermal performances of a heat pipe with phase change material for electronic cooling. The adiabatic section of heat pipe is covered by a storage container with phase change material (PCM), which can store and release thermal energy depending upon the heating powers of evaporator and fan speeds of condenser. Experimental investigations are conducted to obtain the system temperature distributions from the charge, discharge and simultaneous charge/discharge performance tests. The parameters in this study include three kinds of PCMs, different filling PCM volumes, fan speeds, and heating powers in the PCM cooling module. The cooling module with tricosane as PCM can save 46% of the fan power consumption compared with the traditional heat pipe.