Latent thermal energy storage (LTES) employing phase change materials (PCMs) offers a promising solution for thermal management in various applications, compensating for the intermittent and unstable characteristics of several thermal energy sources, such as solar energy. However, the inherently low thermal conductivity of PCMs hinders their heat transfer efficiency, resulting in extended charging and discharging times. This limitation can be addressed either by enhancing the thermal conductivity of the PCM or by optimizing the storage system geometry. In this study, two LTES configurations, finned and finless units based on bar-and-plate technology, were tested under different conditions of mass flow rate (100, 150, 200 kg h⁻¹) and heat transfer fluid (HTF) inlet temperature (46, 49, 52 °C), corresponding to temperature difference (∆T_thermal) of 3, 6 and 9 °C. To the best of the authors' knowledge, the bar-and-plate technology has been only marginally addressed in the context of LTES systems, and no comprehensive experimental investigations are currently available in the literature. The PCM employed, a paraffin wax (RT42), has a melting temperature ra nge between 38.2 °C and 42.5 °C. Results demonstrated that the finned unit reduced the melting time by up to 84 % compared to the finless configuration. At ∆T_thermal = 9 °C and a mass flow rate of 200 kg h⁻¹, the charging process was completed within 2 hours for the finned unit versus about 8 hours for the finless unit. Moreover, for the finned unit, increasing ∆T_thermal from 3 °C to 9 °C resulted in a 28–50 % decrease in melting time, while an increase in the mass flow rate from 100 to 200 kg h⁻¹ shortened melting time by about 35 %. As a further step, the experimental data were used to validate a resistance-capacitance numerical model of the LTES unit, providing a valuable tool for LTES optimization and design according to specific application requirements. Unlike other available calculation methods, the developed model accounts for the explicit incorporation of fin geometry and PCM material in equivalent conductivities (PCM-fin composite) to capture the directional heat transfer pathways. Moreover, a parametric study was carried out to analyze the effect of fin parameters on melting time and energy storage.

Ultrasound vector Doppler techniques for three-dimensional (3-D) blood velocity measurements are currently limited by low temporal resolution and high computational cost. In this paper, an efficient 3-D high-frame-rate vector Doppler method, which estimates the displacements in the frequency domain, is proposed. The novel method extends to 3-D an approach so far proposed for two-dimensional (2-D) velocity measurements by approximating the (x, y, z) displacement of a small volume through the displacements estimated for the 2-D regions parallel to the y and x directions, respectively. The new method was tested by simulation and experiments for a 3.7 MHz, 256-element, 2-D piezoelectric sparse spiral array. Simulations were also performed for an equivalent 7 MHz Capacitive Micromachined Ultrasonic Transducer spiral array. The results indicate performance (bias ± standard deviation: 6.5 ± 8.0) comparable to the performance obtained by using a linear array for 2-D velocity measurements. These results are particularly encouraging when considering that sparse arrays were used, which involve a lower signal-to-noise ratio and worse beam characteristics with respect to full 2-D arrays.

Current 3-D Vector Flow Imaging (VFI) techniques are typically limited by either low temporal resolution and/or high computational cost. In this work, we illustrate a 3-D high frame rate VFI method, which efficiently splits the 3-D (x, y, z) displacement estimation in two separate 2-D VFI estimations operated in the frequency domain. The echo-data received after the transmission of plane waves are beamformed and high-pass filtered before being VFI processed. The new method was experimentally tested by using the ULA-OP 256 research system connected to a 3 MHz, 256-element, 2-D spiral array. Phantom experiments were conducted in steady laminar flow conditions for different probe-to-flow angles. For the tested conditions, the mean relative bias was 11.5% with a standard deviation of 10.9%. The 2-step approach applied to the 2-D VFI technique makes the new 3-D VFI method accurate and computationally efficient.