• Energy Research

Abstract This research investigates the melting rate of a phase change material (PCM) in the presence of Rayleigh–Benard convection. A scaling analysis is conducted for the first time for such a problem, which is useful to identify the parameters affecting the phase change rate and to develop correlations for the solid–liquid interface location and the Nusselt number. The solid–liquid interface and flow patterns in the liquid region are analyzed for PCM in a rectangular enclosure heated from bottom. Numerical and experimental results both reveal that the number of Benard cells is proportional to the ratio of the length of the rectangular enclosure over the solid–liquid interface location (i.e.,, the liquified region aspect ratio). Their effect on the local heat flux is also analyzed as the local heat flux profile changes with the solid–liquid interface moving upward. The variations of average Nusselt number are obtained in terms of the Stefan number, Fourier number, and Rayleigh number. Eventually, the experimental and numerical data are used to develop correlations for the solid–liquid interface location and average Nusselt number for this type of melting problems.

Abstract This research study investigated the melting process of a phase change material (PCM), heated from the bottom, under Neumann boundary conditions, in the presence of Rayleigh-Benard convection. The problem was numerically simulated for a range of domain sizes ( 1 × 1 − 8 × 8 c m 2 ) and heat fluxes ( 0.5 − 2 W / c m 2 ) using the enthalpy porosity technique, which is mostly applicable in cooling heat exchangers of electronic devices. Scaling analysis and the numerical results were employed to find the relationship between the Nusselt number and the solid-liquid interface location with other dimensionless parameters and develop correlations to predict the Nusselt number and the solid-liquid interface location for this type of melting problem. The results of this research could be used to better understand the heat transfer regimes formed during melting in an enclosure heated from the bottom and predict the Nusselt number and the solid-liquid interface location. It was found that the temperature and Nusselt number fluctuations are initiated in the coarsening subregime when the Rayleigh number was not larger than 106 and thus may not occur for small-sized domains. It can be concluded the coarsening and turbulent subregimes may not occur in small enclosures, while the melting process mostly occurs during turbulent subregiem in large enclosures. The numerical results revealed the fastest advance of heat transfer rate and consequently, solid-liquid interface occur during the formation of Benard cells when the Rayleigh number was on the order of 104. The Nusselt number and solid-liquid interface location correlations developed in this study were later validated using two new cases of numerical data. The results revealed that these correlations could accurately predict the Nusselt number and solid-liquid interface location.

Abstract In this paper forced convection turbulent nanofluid flow is numerically investigated to analyze the effects of different types of nanoparticles with different nanoparticle parameters in a fully detached ribbed channel. The bottom wall of the channel is kept at a constant temperature while the upper wall is thermally insulated. The continuity, momentum and energy equations were discretized and solved by the Finite Volume Method (FVM). The influence of different types of nanoparticles (Al 2 O 3 , CuO, SiO 2 , and ZnO) with nanoparticle concentration (1% to 4%) and nanoparticle diameter (20 nm to 50 nm) suspended in a water as a base fluid is studied on the heat transfer enhancement, friction factor and pressure drop. The Reynolds number was in the range of 10,000 to 50,000 in a rectangular channel having mounted rectangular ribs on its bottom wall with clearance ratio of 0.1. The results indicate that the highest heat transfer enhancement is achieved with SiO 2 nanofluid and the friction factor did not considerably change with using different types of nanoparticles in the base fluid. Furthermore, increment of nanoparticle concentration or Reynolds number has a positive impact on heat transfer enhancement due to the increment of the velocity and thermal conductivity of the mixture. However, a rise of nanoparticle diameter decreases the heat transfer enhancement due to stronger Brownian motion even at lower nanoparticle diameter.

Forced convective heat transfer of turbulent flow in a two-dimensional channel mounted triangular and trapezoidal obstacles in upper wall and bottom wall arranged with periodic grooves is numerically studied. Continuity, momentum and energy equations are discretized with second order upwind method is applied to solve the equations. (RNG) k-ε model is implemented to predict the thermo-hydraulic performance of the flow. A thick of 3mm made up by aluminum is implemented for channel walls that the bottom and upper walls are heated with a uniform heat flux. The thermo-hydraulic effects of shapes and positions of obstacles mounted on upper wall referred to the bottom ribbed and grooved wall of the channel as well as its thermal enhancement factorare tested in a Reynolds number range of 3000 to 5000 with engine oil as working fluid. The numerical results demonstrate that combination of trapezoidal obstacles arrays of the upper wall placed against of ribs array of the bottom wall reveals highest thermal enhancement factor due to trapezoidal obstacles with increasing height in flow direction not only lead the flow to the bottom grooved wall but also the flow osculate surface of the obstacle and restart the thermal boundary layer with lowest friction factor compared to other cases.

Abstract This paper experimentally analyzes the effects of nanoparticles on the melting time of Nano-enhanced Phase Change Materials (NePCMs) when Rayleigh-Benard convection is involved. To understand the effects of nanoparticles on the phase change rate reported in many studies, the changes in the parameters affecting the heat transfer are studied. These parameters are identified using scaling analysis on the heat transfer of NePCM in an enclosure heated from the bottom. Al2O3 nanoparticles are dispersed in a PCM (coconut oil) with different concentrations. Several experiments are conducted on these samples to understand whether the effects of nanoparticles on the phase change rate change at different experimental conditions. The results show that the melting rate may increase, decrease or remain unchanged with adding a certain concentration of nanoparticles if the heat transfer conditions change. Adding nanoparticles to a PCM is not recommended in an enclosure heated from the bottom at high Grashof numbers due to the greater Grashof number reduction, although adding nanoparticles to a PCM may reduce the melting time at small Grashof numbers due to the improvement in the thermal conductivity. With the experimental data and scaling analysis, correlations are developed to estimate the effects of nanoparticles on the melting rate and the solid-liquid interface location in the presence of Rayleigh–Benard convection.

Abstract Energy storage is critically important for intermittent renewable sources such as solar or wind. This paper presents a numerical study on a shell and tube thermal energy storage unit using a common organic phase change material (PCM) – paraffin wax. To overcome the problem of slow charging due to low thermal conductivity of paraffin wax, this research applies a multiscale heat transfer enhancement technique, with circular plate fins on outer surface of the heat transfer fluid (HTF) tube and highly conductive nanoparticles (Al2O3) dispersed in the PCM on the shell side. The novelty of this research is that by simultaneous application of two enhancement methods, we are able to analyze the interactions between the two, which was not possible in previous studies on separate technique. A computational fluid dynamics (CFD) model is developed to simulate melting of the PCM with the following parameters: nanoparticle concentration ϕ from 0 to 4 vol%; fin angle α from −45° to 45°, and pitch p from 45 to 65 mm. The obtained numerical data was analyzed with a traditional method and a statistical response surface method (RSM). The latter represents another novelty of this research. The RSM analysis shows that fin angle and nanoparticle concentration are two significant parameters in affecting the PCM melting, but pitch of the fins does not show noticeable effect. Numerical results demonstrate that adding nanoparticles in the PCM does not accelerate the charging process; on the contrary it leads to longer charging time and lower overall heat transfer rate due to reduction of natural convection in the melted PCM. A strong interaction is also found between these two significant parameters, for example the charging time considerably increases when nanoparticles are added at α = −45°, but this effect is less pronounced when α = 45°. Positive fin angles are found to be favorable for PCM melting due to enhanced natural convection with strong local vorticities formed below the fins. A moderate fin angle of 35° leads to the shortest charging time among all studied cases. These new findings can be valuable in design of PCM units for renewable energy storage, waste heat recovery, or thermal management in engineering systems.