Phase change process with free convection in a circular enclosure: numerical simulations

A numerical study of unsteady natural convection flow during freezing of water in a circular enclosure is presented. Mathematical model for phase change is based on apparent capacity method formulation and the governing equations are discretized on a fixed grid by means of finite element method. Water’s temperature is initially higher than its freezing temperature. Then, the temperature of the enclosure’s boundary is dropped to a temperature lower than freezing temperature. Ice forms at the enclosure boundary while natural convection flow is induced in the liquid region. Calculations have been made for the rate of change of solid fraction and temperature distributions, for conduction and conduction plus convection modes of heat transfer, and density inversion near freezing temperature phenomenon of water is considered. High resolution capturing of solid/liquid moving boundary as well as the details of flow structure is presented. The results indicate that the effect of natural convection is dominant over conduction if the Rayleigh number is higher than 5 × 10⁶ and relatively insignificant if the Rayleigh number is less than 1 × 10⁶.     

Thermal Insulation for a Pipe Using Phase Change Material

This research studies the effectiveness of phase change material (PCM) as a thermal insulation for a pipe. The proposed PCM insulation can be used for a pipe when the operating time is limited. The objective of using PCM is to utilize its latent heat from fusion to minimize heat loss from the pipe by absorbing and storing it to be discharged later to the pipe. The finite element method is employed to solve the problem, and both conduction and natural convection of liquid PCM are considered modes of heat transfer. The effectiveness of the PCM insulation is evaluated by comparing its thermal performance with insulation without phase change. Both time-dependent and time-independent boundary conditions are examined. For the time-independent case, the PCM insulation reduces the heat loss from the pipe for a significant amount of time if the Rayleigh number is low. For the time-dependent case, heat loss is effectively reduced with the PCM insulation for a significant amount of time. The high resolution capturing of the solid/liquid moving boundary and the details of flow structure are also presented.     

Electrophoretic deposition of graphene oxide on carbon brush as bioanode for microbial fuel cell operated with real wastewater

Microbial fuel cell (MFC) is a promising technology for simultaneous wastewater treatment and energy harvesting. The properties of the anode material play a critical role in the performance of the MFC. In this study, graphene oxide was prepared by a modified hummer's method. A thin layer of graphene oxide was incorporated on the carbon brush using an electrophoretic technique. The deoxygenated graphene oxide formed on the surface of the carbon brush (RGO-CB) was investigated as a bio-anode in MFC operated with real wastewater. The performance of the MFC using the RGO-CB was compared with that using plain carbon brush anode (PCB). Results showed that electrophoretic deposition of graphene oxide on the surface of carbon brush significantly enhanced the performance of the MFC, where the power density increased more than 10 times (from 33 mWm^?2 to 381 mWm^?2). Although the COD removal was nearly similar for the two MFCs, i.e., with PCB and RGO-CB; the columbic efficiency significantly increased in the case of RGO-CB anode. The improved performance in the case of the modified electrode was related to the role of the graphene in improving the electron transfer from the microorganism to the anode surface, as confirmed from the electrochemical impedance spectroscopy measurements.     

Forced convection cooling enhancement for rectangular blocks using a wavy plate

Heat transfer enhancement using a wavy plate in a channel containing heated blocks is numerically studied. The finite-element method is utilized to solve the problem. The blocks simulate an electronic package with a high thermal dissipation rate. The considered assembly consists of a channel formed by two plates with heated blocks attached to both internal walls and a wavy plate installed at the centerline of the channel. The wavy plate enhances heat transfer from the blocks through the modification of the flow pattern in the channel. The effect of the Reynolds number, waviness of the wavy plate, and blocks' spacing on the Nusselt number and maximum temperature of the blocks is investigated. Heat transfer enhancement of the blocks with a wavy plate is evaluated by comparing their thermal characteristics to blocks with a zero waviness plate. The results show that the wavy plate enhances heat flow out of the blocks and reduces their temperature up to 23%.     

PCM thermal control unit for portable electronic devices: experimental and numerical studies

This paper investigates the effectiveness of a thermal control unit (TCU) for portable electronic devices by performing experimental and numerical analyses. The TCU objective is to improve thermal management of electronic devices when their operating time is limited to a few hours. It is composed of an organic phase change material (PCM) and a thermal conductivity enhancer (TCE). To overcome the relatively low thermal conductivity of the PCM, a TCE is incorporated into the PCM to boost its conductivity. The TCU structure is complex, and modeling an electronic device with it requires time and effort. Hence, this research develops approximate, yet effective, solutions for modeling the TCU, which employ effective thermo-physical properties. The TCU component properties are averaged and a single TCU material is considered. This approach is evaluated by comparing the numerical predictions with the experimental results. The numerical model is then used to study the effect of important parameters that are experimentally expensive to examine, such as the PCM latent heat, Stefan number, and heat source power. It is shown that the TCU can provide a reliable solution to portable electronic devices, which avoids overheating and thermally-induced fatigue, as well as a solution which satisfies the ergonomic requirement.     

Thermal Analysis of a Pipe Insulation with a Phase Change Material: Material Selection and Sizing

This research investigates the thermal characteristics of a thermal insulation for a pipe with a phase change material (PCM) for an unsteady operating condition. A layer of the PCM located at the inner surface of the insulation is aimed to minimize the heat loss from the pipe by absorbing and storing the heat loss in the form of latent and sensible heats. A convection boundary condition is applied at the inner and outer surfaces of the insulation, and one-dimensional finite element method is utilized to solve the problem. The effectiveness of the insulation with the PCM is evaluated by comparing the heat loss to insulation without a PCM. The effect of the PCM type, the PCM layer thickness, and temperature cycle of the inner surface is studied. The results indicate that heat loss is reduced significantly when the PCM layer is used for a significant amount of time, and the heat loss is reduced more when the quantity of the PCM is increased. The temperature cycle has an insignificant influence on the thermal performance of the insulation with the PCM.     

Thermal analysis of a building brick containing phase change material

This paper presents the thermal analysis of a building brick containing phase change material (PCM) to be used in hot climates. The objective of using the PCM is to utilize its high latent heat of fusion to reduce the heat gain by absorbing the heat in the bricks through the melting process before it reaches the indoor space. The considered model consists of bricks with cylindrical holes filled with PCM. The problem is solved in a two-dimensional space using the finite element method. The thermal effectiveness of the proposed brick-PCM system is evaluated by comparing the heat flux at the indoor surface to a wall without the PCM during typical working hours. A paramedic study is conducted to assess the effect of different design parameters, such as the PCM's quantity, type, and location in the brick. The results indicate that the heat gain is significantly reduced when the PCM is incorporated into the brick, and increasing the quantity of the PCM has a positive effect. PCM cylinders located at the centerline of the bricks shows the best performance.     

Thermal analysis of a channel containing multiple heated obstacles with localized heat generations

This paper studies conjugate heat transfer for two-dimensional, developing flows over an array of multiple rectangular obstacles with localized heat generations. The paper focuses on the spatial distributions of Nusselt number and temperature along the solid/fluid interface, as well as the maximum temperature of the obstacles. The results are compared to uniform heat generation configuration to validate the approach of approximating the local heat generation as uniform. A finite element technique is utilized to solve the governing equations, along with boundary conditions for a wide range of Reynolds numbers and obstacles' thermal conductivities. At certain conditions, the numerical results showed that the uniform heat generation approach could lead to a significant analysis error. When the obstacles' thermal conductivity and Reynolds number are low, the uniform heat generation approach becomes invalid. The average Nusselt number and maximum temperature of the obstacles for the localized heat generation configuration are fully documented.