An inverted flag with the free leading edge and fixed trailing edge has been widely adopted in an energy harvesting system due to its highly unstable characteristics in a flow. In the present study, the non-zero inclination angle is set on the fixed trailing edge of the inverted flag to increase its instability and improve the energy harvesting performance. The effects of the bending rigidity and the inclination angle on the energy harvesting efficiency are numerically analyzed where the interaction between the flag and the surrounding fluid is considered by using an immersed boundary method. The inverted flag shows five flapping motions depending on the bending rigidity and the inclination angle: straight, symmetric, asymmetric, biased, and the over flapping modes. The mode change is observed from the straight mode to the flapping mode by increasing the inclination angle from the zero to non-zero degree, which is favorable in terms of the energy harvesting performance. The optimal efficiency is obtained by the inverted flag at the inclination angle of around 40°–45° corresponding to the biased flapping mode. In the biased flapping mode, the strain energy is continuously produced without a period where energy production drops to zero. The strain energy is quantitatively scaled based on a vortex formation that consists of factors associated with the kinematics of the inverted flag.

The critical review presented here exclusively covers the studies on battery thermal management systems (BTMSs), which utilize heat pipes of different structural designs and operating parameters as a cooling medium. The review paper is divided into five major parts, and each part addresses the role of heat pipes in BTMS categorically. Experimental studies, numerical analyses, combined experimental and numerical investigations, optimum utilization of a phase-change material (PCM) with a heat pipe (HP), oscillating heat pipe (OHP), and micro heat pipes combined with PCM for Li-ion BTMS using heat pipes are presented. The usage of HP’s and PCM can keep the temperature of the battery system in the desirable limit for a longer duration compared to other traditional and passive methods. More emphasis is made on how one can achieve a suitable cooling system design and structure, which may tend to enhance the energy density of the batteries, improve thermal performance at maximum and minimum temperature range. Arrangement of battery cells in a pack or module, type of cooling fluid used, heat pipe configuration, type of PCM used, working fluid in a heat pipe, and surrounding environmental conditions are reviewed. According to the study, the battery's effectiveness is significantly influenced by temperature. The usage of flat HPs and heat sink proves to be the best cooling method for keeping the battery working temperature below 50 °C and reduces the heat sink thermal resistance by 30%. With an intake temperature of 25 °C and a discharge rate of 1 L per minute, an HP that uses water as a coolant is also effective at regulating battery cell temperature and maintaining it below the permissible 55 °C range. Using beeswax as a PCM in HPs reduces the temperature of BTMS by up to 26.62 °C, while the usage of RT44 in HPs reduces the temperature of BTMS by 33.42 °C. The use of fins along with copper spreaders drastically decreases the temperature capability of HPTMS by 11 °C. MHPA shows excellent performance in controlling the battery temperature within 40 °C. The effective thermal management can be done by incorporating heat pipe alone or by coupling with liquid cooling or metal plate. However, extensive and extended research is required to improve thermal management to safely and effectively use the battery for day-to-day applications.

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The heat transfer system, including an inclined inverted flag that plays a role of a vortex generator, is proposed in the present study. A two-dimensional simulation is performed to analyze the effects of the inclination angle and the bending rigidity of the inverted flag on thermal performance. To consider the fluid-flexible body-thermal interaction, an immersed boundary method is adopted. The four regimes are observed depending on the inclination angle and the bending rigidity, i.e., large amplitude flapping (LAF), small amplitude flapping (SAF), deflected (D), and straight (S) modes. The SAF and LAF modes are observed to be favorable in terms of the heat transfer efficiency, which considers the heat flux and mechanical energy loss. A scaling analysis is performed to explain the correlation between the flapping kinematics and the thermal quantities. A scaling parameter is newly defined based on the momentum transfer to the inverted flag due to a vortical impulse and shows a proportional relation to the mean drag force with a slope of 0.166. The heat transfer efficiency is observed to be proportional and inversely proportional to the parameter in the SAF and LAF modes, respectively. The optimized heat transfer system is obtained at the angle of 12{degree sign} and the bending rigidity of 0.7, where the efficiency is enhanced up to 112.8% over the baseline flow.