• Energy Research

Nowadays, limited energy resources face ever-growing demands of the modern world. One engineering approach to mitigate this problem which has received considerable attention in recent years is using latent heat thermal storage (LHTS) systems, a significant opportunity which is provided by phase change materials (PCMs). In the present study, a numerical investigation was devoted to estimate the simultaneous freezing and melting processes of a double-layer PCM in terms of heat transfer and fluid flow phenomena. A double-pipe cylindrical channel with two compartments, A and B, was considered for locating two PCMs of RT28 and RT35 in various arrangements. The inner and outer walls were exposed to both hot and cold heat transfer fluids (HHTFs and CHTFs, respectively) beginning with solid or liquid initial state, which led to solid–liquid phase change process through PCMs. The numerical simulation was handled by a two-dimensional finite volume method (FVM) with a fixed Rayleigh number of 106 in which conduction and convection heat transfer mechanisms are taken into account. The effects of employing double-layer PCM and their arrangements, inner and outer walls’ boundary conditions, and initial statuses of PCMs are discussed, and the details of the compared results are shown in the form of temperature and liquid fraction variations over time.

MR Elastography (MRE) is a relatively novel imaging technique using conventional MRI methods to assess the mechanical properties of tissues. In time-harmonic MRE, a Rayleigh, or proportional, Damping (RD) model incorporates attenuation behavior proportionally related to both elastic and inertial forces, thus providing a more sophisticated description of the elastic energy dissipation occurring in the biological tissue. The overall damping ratio can be extracted from the combined effect of these two components, while an additional measure, called Rayleigh Composition, can be calculated by the ratio between the two components. Thus, RD elastography is capable of not only reconstructing the viscoelastic properties of the material, but also providing additional information about damping behavior and structure. A 3D subzone based reconstruction algorithm using a RD material model has been developed and optimized to reconstruct the viscoelastic properties, damping behavior and elastic energy attenuation mechanism of tissue-simulating damping phantoms across multiple frequencies. Results have shown that all three iterative reconstructed parameters are in relatively close agreement for both the tofu and gelatin materials in both phantom configurations across the frequency range. Preliminary results from in-vivo healthy brain are also presented and discussed.

Abstract This paper presents a Computational Fluid Dynamics (CFD) analysis of a novel free-piston Stirling cryocooler that uses a pair of metal diaphragms to seal and suspend the displacer. The diaphragms allow the displacer to move without rubbing or moving seals. When coupled to a metal diaphragm pressure wave generator, the system produces a complete Stirling cryocooler with no rubbing parts in the working gas space. Initial modelling of this concept using the Sage modelling tool indicated the potential for a useful cryocooler. A proof-of-concept prototype was constructed and achieved cryogenic temperatures. A second prototype was designed and constructed using the experience gained from the first. The prototype produced 29 W of cooling at 77 K and reached a no-load temperature of 56 K. The diaphragm’s large diameter and short stroke produces a significant radial component to the oscillating flow fields inside the cryocooler which were not modelled in the one-dimensional analysis tool Sage that was used to design the prototypes. Compared with standard pistons, the diaphragm geometry increases the gas-to-wall heat transfer due to the higher velocities and smaller hydraulic diameters. A Computational Fluid Dynamics (CFD) model of the cryocooler was constructed to understand the underlying fluid-dynamics and heat transfer mechanisms with the aim of further improving performance. The CFD modelling of the heat transfer in the radial flow fields created by the diaphragms shows the possibility of utilizing the flat geometry for heat transfer, reducing the need for, and the size of, expensive heat exchangers. This paper presents details of a CFD analysis used to model the flow and gas-to-wall heat transfer inside the second prototype cryocooler, including experimental validation of the CFD to produce a robust analysis.

This paper presents a numerical method to address function estimation problems in inverse heat transfer problems using parameter estimation approach without prior information on the functional form of the variable to be estimated. Using an inverse analysis, the functional form of a time-dependent heat transfer coefficient is estimated efficiently and accurately. The functional form of the heat transfer coefficient is assumed unknown and the inverse heat transfer problem should be treated using a function estimation approach by solving sensitivity and adjoint problems during the minimization process. Based on proposing a new sensitivity matrix, however, the functional form can be estimated in an accurate and very efficient manner using a parameter estimation approach without the need for solving the sensitivity and adjoint problems and imposing extra computational cost, mathematical complexity, and implementation efforts. In the proposed sensitivity analysis scheme, all sensitivity coefficients can be computed in only one direct problem solution at each iteration. In this inverse heat transfer problem, the body shape is irregular and meshed using a body-fitted grid generation method. The direct heat conduction problem is solved using the finite-difference method. The steepest-descent method is used as a minimization algorithm to minimize the defined objective function and the termination of the minimization process is carried out based on the discrepancy principle. A test case with three different functional forms and two different measurement errors is considered to show the accuracy and efficiency of the used inverse analysis.

Hydrogen storage is a promising technology for storage of renewable energy resources. Despite its high energy density potential, the development of hydrogen storage has been impeded, mainly due to its significant cost. Although its cost is governed mainly by electrical energy expense, especially for hydrogen produced with alkaline water electrolysis, it is also driven by the value of the cell tension. The most common means of electrolyzer improvement is the use of an electrocatalyst, which reduces the energy required for electrochemical reaction to take place. Another efficient means of electrolyzer improvement is to use the Computational Fluid Dynamics (CFD)-assisted design that allows the comprehension of the phenomena occurring in the electrolyzer and also the improvement in the electrolyzer’s efficiency. The designed two-phase hydrodynamics model of this study has been compared with the experimental results of velocity profiles measured using Laser Doppler Velocimetry (LDV) method. The simulated results were in good agreement with the experimental data in the literature. Under the good fit with experimental values, it is efficient to introduce a new physical bubble transfer phenomenon description called “bubble diffusion”.

Abstract Thermophotovoltaics systems involve a thermal-electrical energy conversion process. An energy efficient conversion is desirable for practical applications. In this work, thermal performances and emitter efficiency ( η E ) assessments of two proposed unconventional micro-combustors, i.e. T- and Y-shaped are both experimentally and numerically conducted. Comparison is then made between the proposed combustion systems with the conventional I-shaped micro-combustor with premixed CH4/O2/Ar fueled. The effects of 1) the mass flow rate ( m ˙ ) , 2) the equivalent ratio ( φ ) and 3) the extending depth of the flame holder (H) are numerically investigated using 3D models with structured meshes. The present results show that the thermal performances and the emitter efficiency of both T- and Y-shaped micro-combustors are significantly improved compared to the conventional I-shaped one under same conditions and key construction parameters. Furthermore, it is found that the bifurcation structure (T- and Y-shaped) can reduce the expanding area of the high temperature at the inlet and prevent the wall temperature from dropping too fast at the outlet. When φ is increased to 1.1, the flame position in the T-shaped combustor is found to be shifting away from the outlet of the flame holder, which results in more intensified combustion in the front area. The mean wall temperature shows a great improvement, when an optimal H is chosen. Overall, the Y-shaped combustor presents the best thermal performance with the largest η E of 40.89% and a higher mean wall temperature of 765.1 K, when m ˙ = 4.8 × 10 − 6 k g / s and φ = 0.8 . These findings confirm a significant thermal improvement. This work opens up an energy efficient design of micro-combustors with unconventional structural shapes for thermophotovoltaic applications.

Abstract Geothermal power is an important part of New Zealand's renewable electricity supply due to its attractive cost and reliability. Modular type binary cycle plants have been imported and installed in various geothermal fields in New Zealand, with plans for further expansion. Power output of these plants deteriorates in the summer because plant efficiency depends directly on the geothermal resource and the ambient temperature. As these plants normally use air-cooled condensers, incorporating a water-augmented air-cooled system could improve the power output in summer thereby matching the peak air-conditioning demand. In this work, power generation for the Rotokawa plant was characterized using a similar plant performance and local weather. The improved performance was modelled for retrofit with a wet-cooling system. Maximum generation increase on the hottest day could be 6.8%. The average gain in power over the summer, November–February, was 1.5%, and the average gain for the whole year was 1%. With current binary unit generation capacity at the Rotokawa plant of 35 MW, investment in a water-augmented air-cooled system could provide 2 MW of peak generation on the hottest days. This investment in efficiency is found to compare favourably to other supply options such as solar PV, wind or gas.